NORSOK STANDARD N-004 Rev. 3, February 2013

Transcription

1 NORSOK STANDARD N-004 Rev. 3, February 013 Design o steel structures This NORSOK standard is developed with broad petroleum industry participation by interested parties in the Norwegian petroleum industry and is owned by the Norwegian petroleum industry represented by The Norwegian Oil Industry Association (OLF) and Federation o Norwegian Manuacturing Industries (TBL). Please note that whilst every eort has been made to ensure the accuracy o this NORSOK standard, neither OLF nor TBL or any o their members will assume liability or any use thereo. Standards Norway is responsible or the administration and publication o this NORSOK standard. Standards Norway Telephone: Strandveien 18, P.O. Box 4 Fax: N-136 Lysaker [email protected] NORWAY Website: Copyrights reserved

2 NORSOK standard N-004 Rev. 3, February 013 FOREWORD 3 INTRODUCTION 3 1 SCOPE 4 NORMATIVE AND INFORMATIVE REFERENCES 4.1 Normative reerences 4. Inormative reerences 4 3 TERMS, DEFINITIONS, ABBREVIATIONS AND SYMBOLS Terms and deinitions 4 3. Abbreviations Symbols 6 4 GENERAL PROVISIONS 9 5 STEEL MATERIAL SELECTION AND REQUIREMENTS FOR NON-DESTRUCTIVE TESTING Design class Steel quality level Welding and non-destructive testing (NDT) 11 6 ULTIMATE LIMIT STATES General Ductility Tubular members General Axial tension Axial compression Bending Shear Hydrostatic pressure Hoop buckling Ring stiener design Material actor Tubular members subjected to combined loads without hydrostatic pressure Axial tension and bending Axial compression and bending Interaction shear and bending moment Interaction shear, bending moment and torsional moment Tubular members subjected to combined loads with hydrostatic pressure Axial tension, bending, and hydrostatic pressure Axial compression, bending, and hydrostatic pressure Tubular joints General Joint classiication Strength o simple joints General Basic resistance Strength actor Q u Chord action actor Q Design axial resistance or X and Y joints with joint cans Strength check Overlap joints Ringstiened joints Cast joints Strength o conical transitions General Design stresses Equivalent design axial stress in the cone section Local bending stress at unstiened junctions Hoop stress at unstiened junctions Strength requirements without external hydrostatic pressure Local buckling under axial compression 35 NORSOK standard Page 1 o 64

3 NORSOK standard N-004 Rev. 3, February Junction yielding Junction buckling Strength requirements with external hydrostatic pressure Hoop buckling Junction yielding and buckling Ring design General Junction rings without external hydrostatic pressure Junction rings with external hydrostatic pressure Intermediate stiening rings Design o plated structures Design o cylindrical shells Design against unstable racture General Determination o maximum acceptable deect size 39 7 SERVICEABILITY LIMIT STATES 40 8 FATIGUE LIMIT STATES General Methods or atigue analysis 41 9 ACCIDENTAL DAMAGE LIMIT STATES General Check or accidental actions 4 10 REASSESSMENT OF STRUCTURES General 4 11 BIBLIOGRAPHY 43 1 COMMENTARY 44 Annex A (Normative) Design against accidental actions 60 Annex K (Normative) Special design provisions or jackets 1 Annex L (Normative) Special design provisions or ship shaped units 158 Annex M (Normative) Special design provisions or column stabilized units 01 Annex N (Normative) Special design provisions or tension leg platorms 41 NORSOK standard Page o 64

4 NORSOK standard N-004 Rev. 3, February 013 Foreword The NORSOK standards are developed by the Norwegian petroleum industry to ensure adequate saety, value adding and cost eectiveness or petroleum industry developments and operations. Furthermore, NORSOK standards are, as ar as possible, intended to replace oil company speciications and serve as reerences in the authorities regulations. The NORSOK standards are normally based on recognised international standards, adding the provisions deemed necessary to ill the broad needs o the Norwegian petroleum industry. Where relevant, NORSOK standards will be used to provide the Norwegian industry input to the international standardisation process. Subject to development and publication o international standards, the relevant NORSOK standard will be withdrawn. The NORSOK standards are developed according to the consensus principle generally applicable or most standards work and according to established procedures deined in NORSOK A-001. The NORSOK standards are prepared and published with support by The Norwegian Oil Industry Association (OLF) and Federation o Norwegian Manuacturing Industries (TBL). NORSOK standards are administered and published by Standards Norway. Annex A, K, L, M and N are normative parts o this NORSOK standard. Introduction This NORSOK standard is intended to ulil PSA regulations relating to design and outitting o acilities etc. in the petroleum activities. The design principles ollow the requirements in ISO NORSOK standard Page 3 o 64

5 NORSOK standard N-004 Rev. 3, February SCOPE This NORSOK standard speciies guidelines and requirements or design and documentation o oshore steel structures. This NORSOK standard is applicable to all type o oshore structures made o steel with a speciied minimum yield strength less or equal to 500 MPa. For steel with higher characteristic yield strength, see Clause 1. NORMATIVE AND INFORMATIVE REFERENCES The ollowing standards include provisions and guidelines which, through reerence in this text, constitute provisions and guidelines o this NORSOK standard. Latest issue o the reerences shall be used unless otherwise agreed. Other recognized standards may be used provided it can be shown that they meet or exceed the requirements and guidelines o the standards reerenced below..1 Normative reerences API RP T, Planning, Designing and Constructing Tension Leg Platorms DNV, Rules or classiication o ships DNV, Classiication Note 30.7 DNV-OS-C101, Design o oshore Steel Structures DNV-RP-C01, Buckling Strength o Plated Structures DNV-RP-C03, Fatigue Strength Analysis o Oshore Steel Structures DNV-RP-C0, Buckling o Shells DNV-OS-C50, Oshore Concrete Structures ISO 19900, Petroleum and natural gas industries General requirements or oshore structures ISO 1990, Petroleum and natural gas industries Fixed steel oshore structures NORSOK G-001, Soil investigation NORSOK M-001, Materials selection NORSOK M-101, Structural steel abrication NORSOK M-10, Material data sheets or structural steel NORSOK N-001, Structural design NORSOK N-003, Action and action eects NORSOK N-006, Assessment o structural integrity or existing oshore load-bearing structures NORSOK Z-001, Documentation or operation NS 3481, Soil investigation and geotechnical design or marine structures NS-EN , Eurocode 3: Design o steel structures - Part 1-1: General rules and rules or buildings NS-EN , Eurocode 3: Design o steel structures - Part 1-5: Plated structural elements NS-EN , Eurocode 3: Design o steel structures - Part 1-8: Design o joints VMO Standards As deined in DNV-OS-H101, Marine Operations, General. Inormative reerences See Clause TERMS, DEFINITIONS, ABBREVIATIONS AND SYMBOLS For the purposes o this NORSOK standard, the ollowing terms, deinitions and abbreviations apply. 3.1 Terms and deinitions can verbal orm used or statements o possibility and capability, whether material, physical or casual 3.1. may NORSOK standard Page 4 o 64

6 NORSOK standard N-004 Rev. 3, February 013 verbal orm used to indicate a course o action permissible within the limits o the standard Norwegian petroleum activities petroleum activities where Norwegian regulations apply operator company or an association which through the granting o a production licence is responsible or the day to day activities carried out in accordance with the licence petroleum activities oshore drilling, production, treatment and storage o hydrocarbons principal standard standard with higher priority than other similar standards NOTE Similar standards may be used as supplements, but not as alternatives to the principal standard shall verbal orm used to indicate requirements strictly to be ollowed in order to conorm to the standard and rom which no deviation is permitted, unless accepted by all involved parties should verbal orm used to indicate that among several possibilities one is recommended as particularly suitable, without mentioning or excluding others, or that a certain course o action is preerred but not necessarily required 3. Abbreviations ALS API BSI CTOD DAF DC DFF DFI DNV EA ECCS FE FEM FLS FPSO HF IMO ISO LF MDS MPI NDT NMD NPD RAO RCS ROV accidental limit state American Petroleum Institute British Standards Institution crack tip opening displacement dynamic ampliication actor design class design atigue actor design, abrication and installation Det Norske Veritas environmental actions European Convention or Constructional Steelwork inite element inite element method atigue limit state loating production storage and oloading high requency International Maritime Organisation International Organisation or Standardisation low requency material data sheet magnetic particle inspection non-destructive testing Norwegian Maritime Directorate Norwegian Petroleum Directorate response amplitude operator Recognised Classiication Society remotely operated vehicle NORSOK standard Page 5 o 64

7 NORSOK standard N-004 Rev. 3, February 013 SDOF SQL SCF TLP ULS WF single degree o reedom steel quality level stress concentration actor tension leg platorm ultimate limit state wave requency 3.3 Symbols A cross sectional area, accidental action, parameter, ull area o the brace/chord intersection A c cross sectional area o composite ring section A e eective area A w cross sectional area o web B hoop buckling actor C rotational stiness, actor C e critical elastic buckling coeicient C h elastic hoop buckling strength actor C m reduction actor C my, C mz reduction actor corresponding to the member y and z axis respectively C 0 actor D outer diameter o chord, cylinder diameter at a conical transition, outside diameter D c diameter to centeroid o composite ring section D e equivalent diameter D j diameter at junction D max maximum measured diameter D min minimum measured diameter D nom nominal diameter D s outer cone diameter E Young s modulus o elasticity, MPa G shear modulus I, I z, I s moment o inertia I c moment o inertia or ring composite section I ch moment o inertia I ct moment o inertia o composite ring section with external hydrostatic pressure and with eective width o lange I e eective moment o inertia I p polar moment o inertia K a actor L length, distance L 1 distance to irst stiening ring in tubular section L c distance to irst stiening ring in cone section along cone axis L r ring spacing M pl,rd design plastic bending moment resistance M Rd design bending moment resistance M red,rd reduced design bending moment resistance due to torsional moment M Sd design bending moment M T,Sd design torsional moment M T,Rd design torsional moment resistance M y,rd design in-plane bending moment resistance M y,sd in-plane design bending moment M z,rd design out-o-plane bending moment resistance M z,sd out-o-plane design bending moment N c,rd design axial compressive resistance N can,rd design axial resistance o can N cg,rd axial compression resistance o a grouted, composite member N cl characteristic local buckling resistance N cl characteristic local buckling resistance N cl,rd design local buckling resistance N E Euler buckling strength N E,dent Euler buckling strength o a dented tubular member, or buckling in-line with the dent elastic Euler buckling load o a grouted, composite member N Eg NORSOK standard Page 6 o 64

8 NORSOK standard N-004 Rev. 3, February 013 N Ey,N Ez N Sd N t,rd N x,rd N x,sd N y,rd N y,sd P Sd Q Q Q g Q u Q yy Q R R d R k S d S k T T c T n V Rd V Sd W Z a a i a m a u b b e c d d 0, d 1, d e c ch,rd cj cl cl,rd clc cle cr d E Epx, Epy Ey, Ez h he hec h,rd k, kx, ky, kp m m,rd m,red mh,rd th,rd y y,b y,c Euler buckling resistance corresponding to the member y and z axis respectively design axial orce design axial tension resistance design axial resistance in x direction design axial orce in x direction design axial resistance in y direction design axial orce in y direction design lateral orce actor chord action actor chord gap actor strength actor angle correction actor geometric actor, tubular joint geometry actor radius, radius o chord, radius o conical transition design resistance characteristic resistance design action eect characteristic action eect thickness o tubular sections, thickness o chord chord-can thickness nominal chord member thickness design shear resistance design shear orce elastic section modulus plastic section modulus stiener length, crack depth, weld throat, actor initial crack depth maximum allowable deect size calculated crack depth at unstable racture actor eective width actor diameter o tubular cross section, brace diameter distance lange eccentricity characteristic axial compressive strength design axial compressive strength in the presence o hydrostatic pressure corresponding tubular or cone characteristic compressive strength characteristic local buckling strength design local buckling strength, design local buckling strength o undamaged cylinder local buckling strength o conical transition characteristic elastic local buckling strength critical buckling strength design yield strength Euler buckling strength Euler buckling strength corresponding to the member y and z axis respectively Euler buckling strength corresponding to the member y and z axis respectively characteristic hoop buckling strength elastic hoop buckling strength or tubular section elastic hoop buckling strength or cone section design hoop buckling strength characteristic buckling strength characteristic bending strength design bending strength reduced bending strength due to torsional moment design bending resistance in the presence o external hydrostatic pressure design axial tensile resistance in the presence o external hydrostatic pressure characteristic yield strength characteristic yield strength o brace characteristic yield strength o chord NORSOK standard Page 7 o 64

9 NORSOK standard N-004 Rev. 3, February 013 g gap h height i radius o gyration i e eective radius o gyration k, k g,k l, k buckling actor l, l L length, element length l e eective length p Sd design hydrostatic pressure r radius, actor t thickness t c cone thickness t e eective thickness o chord and internal pipe o a grouted member coeicient, angle between cylinder and cone geometrical coeicient, actor actor actor BC additional building code material actor partial actor or actions M resulting material actor M0 material actor or use with EN M1 material actor or use with EN M material actor or use with EN and EN actor hoop buckling actor angle c the included angle or the compression brace t the included angle or the tension brace reduced slenderness, column slenderness parameter e reduced equivalent slenderness s reduced slenderness, shell slenderness parameter coeicient, geometric parameter Poisson s ratio rotational stiness actor a,sd design axial stress in member ac,sd design axial stress including the eect o the hydrostatic capped end axial stress at,sd design axial stress in tubular section at junction due to global actions equ,sd equivalent design axial stress within the conical transition h,sd design hoop stress due to the external hydrostatic pressure hc,sd design hoop stress at unstiened tubular-cone junctions due to unbalanced radial line orces hj,sd net design hoop stress at a tubular cone junction j,sd design von Mises equivalent stress m,sd design bending stress mc,sd design bending stress at the section within the cone due to global actions mlc,sd local design bending stress at the cone side o unstiened tubular-cone junctions mlt,sd local design bending stress at the tubular side o unstiened tubular-cone junctions mt,sd design bending stress in tubular section at junction due to global actions my,sd design bending stress due to in-plane bending mz,sd design bending stress due to out-o-plane bending p,sd design hoop stress due to hydrostatic pressure tot,sd total design stress q,sd capped end axial design compression stress due to external hydrostatic pressure T,Sd shear stress due to design torsional moment, x, y actors actor NORSOK standard Page 8 o 64

10 NORSOK standard N-004 Rev. 3, February GENERAL PROVISIONS All relevant ailure modes or the structure shall be identiied and it shall be checked that no corresponding limit state is exceeded. In this NORSOK standard the limit states are grouped into: 1. Serviceability limit states. Ultimate limit states 3. Fatigue limit states 4. Accidental limit states For deinition o the groups o limit states, reerence is made to ISO The dierent groups o limit states are addressed in designated clauses o this NORSOK standard. In general, the design needs to be checked or all groups o limit states. The general saety ormat may be expressed as: Sd R d where S d = Sk R d = R k M Design action eect Design resistance S k = Characteristic action eect = partial actor or actions R k = Characteristic resistance M = Material actor In this NORSOK standard the values o the resulting material actor are given in the respective clauses. General requirements or structures are given in NORSOK N-001. Determination o actions and action eects shall be according to NORSOK N-003. The steel abrication shall be according to the requirements in NORSOK M-101. (4.1) NORSOK standard Page 9 o 64

11 NORSOK standard N-004 Rev. 3, February STEEL MATERIAL SELECTION AND REQUIREMENTS FOR NON-DESTRUCTIVE TESTING 5.1 Design class Selection o steel quality and requirements or inspection o welds shall be based on a systematic classiication o welded joints according to the structural signiicance and complexity o joints. The main criterion or decision o DC o welded joints is the signiicance with respect to consequences o ailure o the joint. In addition the geometrical complexity o the joint will inluence the DC selection. The selection o joint design class shall be in compliance with Table 5-1 or all permanent structural elements. A similar classiication may also be used or temporary structures. Table 5-1 Classiication o structural joints and components Design Class 1) Joint complexity ) Consequences o ailure DC1 DC DC3 DC4 DC5 High Low High Low Any Applicable or joints and members where ailure will have substantial consequences 3) and the structure possesses limited residual strength. 4). Applicable or joints and members where ailure will be without substantial consequences 3) due to residual strength. 4). Applicable or joints and members where ailure will be without substantial consequences. 3) Notes: 1) Guidance or classiication can be ound in Annex K, L, M and N. ) High joint complexity means joints where the geometry o connected elements and weld type leads to high restraint and to triaxial stress pattern. E.g., typically multiplanar plated connections with ull penetration welds. 3) Substantial consequences in this context means that ailure o the joint or member will entail - danger o loss o human lie; - signiicant pollution; - major inancial consequences. 4) Residual strength means that the structure meets requirements corresponding to the damaged condition in the check or accidental damage limit states, with ailure in the actual joint or component as the deined damage. See Clause Steel quality level Selection o steel quality level or a structural component shall normally be based on the most stringent DC o joints involving the component. Through-thickness stresses shall be assessed. The minimum requirements or selection o steel material are ound in Table 5-. Selection o a better steel quality than the minimum required in design shall not lead to more stringent requirements in abrication. The principal standard or speciication o steels is NORSOK M-10, Material data sheets or rolled structural steel. Material selection in compliance with NORSOK M-10 assures toughness and weldability or structures with an operating temperature down to -14 C. Cast and orged steels shall be in accordance with recognised standards. I steels o higher speciied minimum yield strength than 500 MPa or greater thickness than 150 mm are selected, the easibility o such a selection shall be assessed in each case. Traceability o materials shall be in accordance with NORSOK Z-001. NORSOK standard Page 10 o 64

12 NORSOK standard N-004 Rev. 3, February 013 Table 5- Correlation between design classes and steel quality level Design Class Steel Quality Level I II III IV DC1 X DC (X) X DC3 (X) X DC4 (X) X DC5 X (X) = Selection where the joint strength is based on transerence o tensile stresses in the through thickness direction o the plate. See Clause Welding and non-destructive testing (NDT) The required racture toughness level shall be determined at the design stage or joints with plate thickness above 50 mm when or joints in design class DC1, DC or DC3. The extent o non-destructive examination during abrication o structural joints shall be in compliance with the inspection category. The selection o inspection category or each welded joint shall be in accordance with Table 5-3 and Table 5-4 or joints with low and high atigue utilisation respectively. Welds in joints below 150 m water depth should be assumed inaccessible or in-service inspection. The principal standard or welder and welding qualiication, welding perormance and non-destructive testing is NORSOK M-101. Table 5-3 Determination o inspection category or details with low atigue utilisation 1) Design Class DC1 and DC DC3 and DC4 DC5 Type o and level o stress and direction in relation to welded joint. Inspection category ) Welds subjected to high tensile stresses transverse to the weld. 6) A Welds with moderate tensile stresses transverse to the 7) weld and/or high shear stresses. 3) B Welds with low tensile stresses transverse to the weld 8) and/or moderate shear stress. 4) C Welds subjected to high tensile stresses transverse to the 6) weld. 3) B Welds with moderate tensile stresses transverse to the 7) weld and/or high shear stresses. 4) C Welds with low tensile stresses transverse to the weld 8) and/or moderate shear stress. 5) D All load bearing connections. Non load bearing connections. Notes: 1) Low atigue utilisation means connections with calculated atigue lie longer than 3 times the required atigue lie (design atigue lie multiplied with the DFF). ) It is recommended that areas o the welds where stress concentrations occur be marked as mandatory inspection areas or B, C and D categories as applicable. 3) Welds or parts o welds with no access or in-service inspection and repair should be assigned inspection category A. 4) Welds or parts o welds with no access or in-service inspection and repair should be assigned inspection category B. 5) Welds or parts o welds with no access or in-service inspection and repair should be assigned inspection category C. 6) High tensile stresses mean ULS tensile stresses in excess o 0.85 o design stress. 7) Moderate tensile stresses mean ULS tensile stresses between 0.6 and 0.85 o design stress. 8) Low tensile stresses mean ULS tensile stresses less than 0.6 o design stress. D E NORSOK standard Page 11 o 64

13 NORSOK standard N-004 Rev. 3, February 013 Table 5-4 Determination o inspection category or details with high atigue utilisation 1) Design Class DC1 and DC DC3 and DC4 Direction o dominating principal stress Inspection category 3) Welds with the direction o the dominating dynamic principal stress transverse to the weld ( between 45and 135) Welds with the direction o the dominating dynamic principal stress in the direction o the weld ( between - 45and 45) Welds with the direction o the dominating dynamic principal stress transverse to the weld ( between 45and 135) Welds with the direction o the dominating dynamic principal stress in the direction o the weld ( between - 45and 45) A ) B 4) B 4) C 5) DC5 Welds with the direction o the dominating dynamic principal stress transverse to the weld ( between 45and 135) Welds with the direction o the dominating dynamic principal stress in the direction o the weld ( between - 45and 45) Notes: 1) High atigue utilisation means connections with calculated atigue lie less than 3 times the required atigue lie (design atigue lie multiplied with the DFF). ) Butt welds with high atigue utilisation and SCF less than 1.3 need stricter NDT acceptance criteria. Such criteria need to be developed in each case. 3) For joints in inspection categories B, C or D, the hot spot regions (regions with highest stress range) at welds or areas o welds o special concern shall be addressed with individual notations as mandatory or selected NDT methods. 4) Welds or parts o welds with no access or in-service inspection and repair should be assigned inspection category A. 5) Welds or parts o welds with no access or in-service inspection and repair should be assigned inspection category B. D E NORSOK standard Page 1 o 64

14 NORSOK standard N-004 Rev. 3, February ULTIMATE LIMIT STATES 6.1 General This clause gives provisions or checking o ULSs or typical structural elements used in oshore steel structures, where ordinary building codes lack relevant recommendations. Such elements are tubular members, tubular joints, conical transitions and some load situations or plates and stiened plates. For other types o structural elements NS-EN or member design and NS-EN or other joints than tubular joints apply. See also sub clause and 6.7. The material actor is 1.15 or ULSs unless noted otherwise. The material actors according to Table 6-1 shall be used i NS-EN and NS-EN are used or calculation o structural resistance: Table 6-1 Material actors Type o calculation Material actor 1) Value Resistance o Class 1, or 3 cross-sections M Resistance o Class 4 cross-sections M Resistance o member to buckling M Resistance o net section at bolt holes M 1.3 Resistance o illet and partial penetration welds M 1.3 Resistance o bolted connections M 1.3 1) Symbols according to NS-EN and NS-EN For resistance check where other material actors are used than given in Table 6-1 the recommended material actor in NS-EN , NS-EN and NS-EN should be multiplied by an additional building code material actor BC =1.05 The ultimate strength o structural elements and systems should be evaluated by using a rational, justiiable engineering approach. The structural analysis may be carried out as linear elastic, simpliied rigid plastic, or elastic-plastic analyses. Both irst order or second order analyses may be applied. In all cases, the structural detailing with respect to strength and ductility requirements shall conorm to the assumption made or the analysis. When plastic or elastic-plastic analyses are used or structures exposed to cyclic loading (e.g. wave loads) checks shall be carried out to veriy that the structure will shake down without excessive plastic deormations or racture due to repeated yielding. A characteristic or design cyclic load history needs to be deined in such a way that the structural reliability in case o cyclic loading (e.g. storm loading) is not less than the structural reliability or ULSs or non-cyclic actions. It should be checked as a minimum that the structure will carry all loads throughout the entire storm comprising the ULS environmental condition. In case o linear global beam analysis combined with the resistance ormulations set down in this NORSOK standard, shakedown can be assumed without urther checks. 6. Ductility It is a undamental requirement that all ailure modes are suiciently ductile such that the structural behaviour will be in accordance with the anticipated model used or determination o the responses. In general all design procedures, regardless o analysis method, will not capture the true structural behaviour. Ductile ailure modes will allow the structure to redistribute orces in accordance with the presupposed static model. Brittle ailure modes shall thereore be avoided or shall be veriied to have excess resistance compared to ductile modes, and in this way protect the structure rom brittle ailure. The ollowing sources or brittle structural behaviour may need to be considered or a steel structure: 1. Unstable racture caused by a combination o the ollowing actors: - brittle material; NORSOK standard Page 13 o 64

15 NORSOK standard N-004 Rev. 3, February a design resulting in high local stresses; - the possibilities or weld deects.. Structural details where ultimate resistance is reached with plastic deormations only in limited areas, making the global behaviour brittle, e.g. partial butt weld loaded transverse to the weld with ailure in the weld. 3. Shell buckling. 4. Buckling where interaction between local and global buckling modes occur. In general a steel structure will be o adequate ductility i the ollowing is satisied: 5. Material toughness requirements are met, and the design avoids a combination o high local stresses with possibilities o undetected weld deects. 6. Details are designed to develop a certain plastic delection e.g. partial butt welds subjected to stresses transverse to the weld is designed with excess resistance compared with adjoining plates. 7. Member geometry is selected such that the resistance does not show a sudden drop in capacity when the member is subjected to deormation beyond maximum resistance. An unstiened shell in crosssection class 4 is an example o a member that may show such an unavourable resistance deormation relationship. For deinition o cross-section class see NS-EN Local and global buckling interaction eects are avoided. 6.3 Tubular members General The structural strength and stability requirements or steel tubular members are speciied in this section. The requirements given in this section apply to un stiened and ring stiened tubulars having a thickness t 6 mm, D/t < 10 and material meeting the general requirements in Clause 5. In cases where hydrostatic pressure are present, the structural analysis may proceed on the basis that stresses due to the capped-end orces arising rom hydrostatic pressure are either included in or excluded rom the analysis. This aspect is discussed in Clause 1. In the ollowing subclauses, y and z are used to deine the in-plane and out-o-plane axes o a tubular member, respectively. The requirements assume the tubular is constructed in accordance with the abrication tolerances given in NORSOK M-101. The requirements are ormulated or an isolated beam column. This ormulation may also be used to check the resistance o rames and trusses, provided that each member is checked or the member orces and moments combined with a representative eective length. The eective length may in lieu o special analyses be determined according to the requirements given in this chapter. Alternatively the ULSs or rames or trusses may be determined on basis o non-linear analyses taking into account second order eects. The use o these analyses requires that the assumptions made are ulilled and justiied. Tubular members subjected solely to axial tension, axial compression, bending, shear, or hydrostatic pressure should be designed to satisy the strength and stability requirements speciied in 6.3. to Tubular members subjected to combined loads without hydrostatic pressure should be designed to satisy the strength and stability requirements speciied in Tubular members subjected to combined loads with hydrostatic pressure should be designed to satisy the strength and stability requirements speciied in The equations in this section are not using an unique sign convention. Deinitions are given in each paragraph Axial tension Tubular members subjected to axial tensile loads should be designed to satisy the ollowing condition: N Sd where N t,rd A M y (6.1) NORSOK standard Page 14 o 64

16 NORSOK standard N-004 Rev. 3, February 013 N Sd = design axial orce (tension positive) y = characteristic yield strength A = cross sectional area M = Axial compression Tubular members subjected to axial compressive loads should be designed to satisy the ollowing condition: N Sd N c,rd A M c (6.) where N Sd = design axial orce (compression positive) c = characteristic axial compressive strength M = see In the absence o hydrostatic pressure the characteristic axial compressive strength or tubular members shall be the smaller o the in-plane or out-o-plane buckling strength determined rom the ollowing equations: c c [ ] 0.9 cl E cl kl i cl E cl or 1.34 or 1.34 where cl = characteristic local buckling strength = column slenderness parameter E = smaller Euler buckling strength in y or z direction E = Young s modulus o elasticity, MPa k = eective length actor, see l = longer unbraced length in y or z direction i = radius o gyration The characteristic local buckling strength should be determined rom: y c l y or y c l c le or y c l y or y cle c le and cle C E e t D cle cle (6.3) (6.4) (6.5) (6.6) (6.7) (6.8) NORSOK standard Page 15 o 64

17 NORSOK standard N-004 Rev. 3, February 013 where cle = characteristic elastic local buckling strength C e = critical elastic buckling coeicient = 0.3 D = outside diameter t = wall thickness y For c le the tubular is a class 4 cross section and may behave as a shell. Shell structures may have a brittle structure ailure mode. Reerence is made to 6.. For class 4 cross sections increased M values shall be used according to Equation (6.) Bending Tubular members subjected to bending loads should be designed to satisy the ollowing condition: M Sd M Rd mw M where M Sd = design bending moment m = characteristic bending strength W = elastic section modulus M = see section The characteristic bending strength or tubular members should be determined rom: Z yd m y or W Et D Z Et W y y m y m yd Et Z W where W = elastic section modulus 4 4 D (D t) = 3 D Z = plastic section modulus y cle = D (D t) 6 y D Et yd y Et E (6.9) (6.10) (6.11) (6.1) For the tubular is a class 4 cross section and may behave as a shell. Shell structures may have a brittle structure ailure mode. Reerence is made to 6.. For class 4 cross sections increased M values shall be used according to Equation (6.) Shear Tubular members subjected to beam shear orces should be designed to satisy the ollowing condition: NORSOK standard Page 16 o 64

18 NORSOK standard N-004 Rev. 3, February 013 V Sd V Rd A y 3 M (6.13) where V Sd = design shear orce y = yield strength A = cross sectional area M = 1.15 Tubular members subjected to shear rom torsional moment should be designed to satisy the ollowing condition: M T,Sd M T,Rd I p y D 3 where M T,Sd = design torsional moment M 4 4 I p = polar moment o inertia = D (D t) Hydrostatic pressure 3 (6.14) Hoop buckling Tubular members subjected to external pressure should be designed to satisy the ollowing condition: p,sd p, Sd h,rd p Sd D t h M (6.15) (6.16) where h = characteristic hoop buckling strength p, Sd = design hoop stress due to hydrostatic pressure (compression positive) p Sd = design hydrostatic pressure M = see I out-o-roundness tolerances do not meet the requirements given in NORSOK M-101, guidance on calculating reduced strength is given in Clause 1. h y, 0.4 he h 0.7 y or.44 y he 0.55 y y h he, or.44 (6.17) he he y y (6.18) or 0.55 (6.19) The elastic hoop buckling strength, he, is determined rom the ollowing equation: he C h E t D where C h = 0.44 t/dor 1.6D/t = 0.44 t/d (D/t) 3 / 4 or 0.85D/t < 1.6D/t = 0.737/( ) or 1.5 < 0.85D/t (6.0) NORSOK standard Page 17 o 64

19 NORSOK standard N-004 Rev. 3, February 013 = 0.80 or < 1.5 and where the geometric parameter,, is deined as: L L D D t and = length o tubular between stiening rings, diaphragms, or end connections Ring stiener design The circumerential stiening ring size may be selected on the ollowing approximate basis: I c he tl rd 8E (6.1) where I c = required moment o inertia or ring composite section L r = ring spacing D = diameter (see Note or external rings) Notes: 1. Equation (6.1) assumes that the yield strength o the stiening ring is equal to or greater than that o the tubular.. For external rings, D in Equation (6.1) should be taken to the centroid o the composite ring. 3. An eective width o shell equal to 1.1 D t may be assumed as the lange or the composite ring section. 4. Where out-o-roundness in excess o tolerances given in NORSOK M-101 is permitted, larger stieners may be required. The bending due to out-o-roundness should be specially investigated. Local buckling o ring stieners with langes may be excluded as a possible ailure mode provided that the ollowing requirements are ulilled: h 1.1 t E w y and b t 0.3 E y where h = web height t w = web thickness b = hal the width o lange o T-stieners t = thickness o lange Local buckling o ring stieners without langes may be excluded as a possible ailure mode provided that: h t 0.4 E w y Torsional buckling o ring stieners with langes may be excluded as a possible ailure mode provided that: 3.5 h b h E 10 r y Check or torsional buckling o the stiener shall be made according to DNV RP-C0 /13/ Material actor The material actor, M, is given as: NORSOK standard Page 18 o 64

20 NORSOK standard N-004 Rev. 3, February 013 M M M s or or 0.5 or s s 0.5 s (6.) where s c,sd cl c p,sd h h (6.3) where cl is calculated rom Equation (6.6) or Equation (6.7) whichever is appropriate and h rom Equation (6.17), Equation (6.18), or Equation (6.19) whichever is appropriate. y (6.4) y c,and h cle he cle and he is obtained rom Equation (6.8), and Equation (6.0) respectively. p,sd is obtained rom Equation (6.16) and c,sd NSd A M y,sd M W N Sd is negative i in tension. z,sd (6.5) Tubular members subjected to combined loads without hydrostatic pressure Axial tension and bending Tubular members subjected to combined axial tension and bending loads should be designed to satisy the ollowing condition at all cross sections along their length: N N Sd t,rd where 1.75 M y,sd M M Rd z,sd 1.0 M y,sd = design bending moment about member y-axis (in-plane) M z, Sd = design bending moment about member z-axis (out-o-plane) N Sd = design axial tensile orce (6.6) I shear or torsion is o importance, the bending capacity M Rd needs to be substituted with M Red,Rd calculated according to subclause or Axial compression and bending Tubular members subjected to combined axial compression and bending should be designed to satisy the ollowing condition accounting or possible variations in cross-section, axial load and bending moment according to appropriate engineering principles: N N Sd c,rd 1 M Rd C mym N 1 N y,sd Sd Ey C mzm N 1 N and at all cross sections along their length: z,sd Sd Ez (6.7) NORSOK standard Page 19 o 64

21 NORSOK standard N-004 Rev. 3, February 013 N N Sd cl,rd M y,sd M M Rd z,sd 1.0 (6.8) where N N Sd = design axial compression orce C my, C mz = reduction actors corresponding to the member y and z axes, respectively N Ey, N Ez = Euler buckling strengths corresponding to the member y and z axes, respectively cl,rd c A l M design axial local buckling resistance N Ey EA kl i y (6.9) N Ez EA kl i z (6.30) k in Equation (6.9) and Equation (6.30) relate to buckling in the y and z directions, respectively. These actors can be determined using a rational analysis that includes joint lexibility and side-sway. In lieu o such a rational analysis, values o eective length actors, k, and moment reduction actors, C m, may be taken rom Table 6-. All lengths are measured centreline to centreline. Table 6- Eective length and moment reduction actors or member strength checking Structural element k (1) C m Superstructure legs - Braced 1.0 (a) - Portal (unbraced) k () (a) Jacket legs and piling - Grouted composite section 1.0 (c) - Ungrouted jacket legs 1.0 (c) - Ungrouted piling between shim points 1.0 (b) Jacket braces - Primary diagonals and horizontals 0.7 (b) or (c) -K-braces (3) 0.7 (c) - Longer segment length o X-braces (3) 0.8 (c) Secondary horizontals 0.7 (c) Notes: 1. C m values or the cases deined in Table 6- are as ollows: (a) 0.85 (b) or members with no transverse loading, C m = M 1,Sd/M,Sd, where M 1,Sd/M,Sd is the ratio o smaller to larger moments at the ends o that portion o the member unbraced in the plane o bending under consideration. M 1,Sd/M,Sd is positive when the number is bent in reverse curvature, negative when bent in single curvature. (c) or members with transverse loading, C m = N Sd/N E, or 0.85, whichever is less, and N E = N Ey or N Ez as appropriate.. Use eective length alignment chart in Clause At least one pair o members raming into the a K- or X-joint shall be in tension i the joint is not braced out-o-plane. For X-braces, when all members are in compression, the k-actor should be determined using the procedures given in Clause The eective length and C m actors given in Table 6- do not apply to cantilever members and the member ends are assumed to be rotationally restrained in both planes o bending. NORSOK standard Page 0 o 64

22 NORSOK standard N-004 Rev. 3, February Interaction shear and bending moment Tubular members subjected to beam shear orce and bending moment should be designed to satisy the ollowing condition provided that the direction o the shear orce and the moment vectors are orthogonal within 0: M M V Sd Sd Sd 1.4 or 0. 4 Rd VRd VRd V (6.31) M Sd VSd 1.0 or 0. 4 M V Rd Rd (6.3) Interaction shear, bending moment and torsional moment Tubular members subjected to beam shear orce, bending moment and torsion should be designed to satisy the ollowing condition provided that the direction o the shear orce and the moment vectors are orthogonal within 0: M M M Sd Red,Rd M Sd Red,Rd where V 1.4 V Sd Rd VSd or 0. 4 V VSd 1.0 or 0. 4 V M Red,Rd = m,red = T,Sd = d = W m,red M m 1 3 M R y M T,Sd t T,Sd d R = radius o tubular member M = see Rd Rd (6.33) Tubular members subjected to combined loads with hydrostatic pressure The design provisions in this section are divided into two categories. In Method A it is assumed that the capped-end compressive orces due to the external hydrostatic pressure are not included in the structural analysis. Alternatively, the design provisions in Method B assume that such orces are included in the analysis as external nodal orces. Dependent upon the method used in the analysis, the interaction equations in either Method A or Method B should be satisied at all cross sections along their length. It should be noted that the equations in this section are not applicable unless Equation (6.15) is irst satisied. For guidance on signiicance o hydrostatic pressure, see Clause 1. NORSOK standard Page 1 o 64

23 NORSOK standard N-004 Rev. 3, February Axial tension, bending, and hydrostatic pressure Tubular members subjected to combined axial tension, bending, and hydrostatic pressure should be designed to satisy the ollowing equations in either Method A or Method B at all cross sections along their length. Method A ( a,sd is in tension) In this method, the calculated value o member axial stress, a,sd, should not include the eect o the hydrostatic capped-end axial stress. The capped-end axial compression due to external hydrostatic pressure, q,sd, can be taken as 0.5 p, Sd. This implies that, the tubular member takes the entire capped-end orce arising rom external hydrostatic pressure. In reality, the stresses in the member due to this orce depend on the restraint provided by the rest o the structure on the member. The stress computed rom a more rigorous analysis may be substituted or 0.5. (a) For a,sd q,sd (net axial tension condition) p, Sd a,sd where th, Rd mh, Rd th,rd q,sd my,sd mh, Rd mz,sd 1.0 (6.34) a,sd = design axial stress that excludes the eect o capped-end axial compression arising rom external hydrostatic pressure (tension positive) q,sd = capped-end design axial compression stress due to external hydrostatic pressure (= 0.5 ) (compression positive) p,sd my,sd = design in plane bending stress mz,sd = design out o plane bending stress th,rd = design axial tensile resistance in the presence o external hydrostatic pressure which is given by Equation (6.35): y M [ B B 0.3B] (6.35) mh,rd = design bending resistance in the presence o external hydrostatic pressure which is given by Equation (6.36): m M [ B M = see and Equation (6.37): B p,sd h,rd, 5 4 h y B 1.0 B 0.3B] (b) For a,sd < q,sd (net axial compression condition) a,sd cl,rd q,sd my,sd mh, Rd mz,sd 1.0 (6.36) (6.37) (6.38) (6.39) NORSOK standard Page o 64

24 NORSOK standard N-004 Rev. 3, February 013 cl,rd cl M (6.40) where cl is ound rom Equation (6.6) and Equation (6.7). he When c,sd 0.5 and cle 0.5 he M, Equation (6.41) should also be satisied: c,sd cle M 0.5 he 0.5 he M M he M p,sd 1.0 (6.41) in which c, Sd m,sd q,sd a, Sd ; c,sd should relect the maximum combined compressive stress. m,sd M z,sd M W y,sd M = see Method B ( ac,sd is in tension) In this method, the calculated member axial stress, ac,sd, includes the eect o the hydrostatic capped-end axial stress. Only Equation (6.4) needs to be satisied: ac,sd th,rd where my,sd mh, Rd mz,sd 1.0 (6.4) ac,sd = design axial stress that includes the eect o the capped-end compression arising rom external hydrostatic pressure (tension positive) Axial compression, bending, and hydrostatic pressure Tubular members subjected to combined compression, bending, and hydrostatic pressure should be proportioned to satisy the ollowing requirements at all cross sections along their length. Method A ( a,sd is in compression) a,sd ch,rd a,sd where 1 cl,rd mh,rd q,sd C my 1 my,sd my,sd a,sd Ey mh, Rd mz,sd C mz mz,sd a,sd Ez (6.43) (6.44) a,sd = design axial stress that excludes the eect o capped-end axial compression arising rom external hydrostatic pressure (compression positive) NORSOK standard Page 3 o 64

25 NORSOK standard N-004 Rev. 3, February 013 Ey E kl i y (6.45) Ez E kl i z (6.46) ch,rd = design axial compression strength in the presence o external hydrostatic pressure which is given by the ollowing equations: ch,rd 1 cl M [ q,sd cl 1.1 q,sd cl ] or 1.34 (1 cl q,sd ) 1 (6.47) and ch,rd where 0.9 cl M When 0.5 also be satisied., he c,sd and he M M = see Method B ( ac,sd is in compression) (a) or ac,sd > q,sd ac,sd cl,rd When ac,sd satisied. ch,rd q,sd 1 my,sd mh,rd mh, Rd or 1.34 (1 cl cle 0.5, Equation (6.41), in which c, Sd m,sd q,sd a, Sd C 1 mz,sd my my,sd ac,sd 1.0 Ey q,sd C 1 mz mz,sd ac,sd Ez q,sd q,sd ) 1 (6.48) (6.49), should (6.50) (6.51) ac,sd = design axial stress that includes the eect o capped-end axial compression arising rom external hydrostatic pressure (compression positive) 0.5 he c,sd and M cle he 0.5, Equation (6.41), in which c, Sd m,sd ac, Sd M M, should also be NORSOK standard Page 4 o 64

26 NORSOK standard N-004 Rev. 3, February 013 (b) or ac,sd q,sd For ac,sd q,sd, Equation (6.51) should be satisied. When satisied. 0.5 he c,sd and M M = see cle M he 0.5, Equation (6.41), in which c, Sd m,sd ac, Sd M, should also be 6.4 Tubular joints General The ollowing provisions apply to the design o tubular joints ormed by the connection o two or more members. Terminology or simple joints is deined in Figure 6-1. Figure 6-1 also gives some design requirements with respect to joint geometry. The gap or simple K-joints should be larger than 50 mm and less than D. Minimum distances or chord cans and brace stubs should not include thickness tapers. Figure 6-1 Detail o simple joint Reductions in secondary (delection induced) bending moments or inelastic relaxation through use o joint elastic stiness may be considered. In certain instances, hydrostatic pressure eects may be signiicant Joint classiication Joint classiication is the process whereby the axial orce in a given brace is subdivided into K, X and Y components o actions, corresponding to the three joint types or which resistance equations exist. Such subdivision normally considers all o the members in one plane at a joint. For purposes o this provision, brace planes within ±15º o each other may be considered as being in a common plane. Each brace in the plane can have a unique classiication that could vary with action condition. The classiication can be a NORSOK standard Page 5 o 64

27 NORSOK standard N-004 Rev. 3, February 013 mixture between the above three joint types. Once the breakdown into axial components is established, the resistance o the joint can be estimated using the procedures in Figure 6- provides some simple examples o joint classiication. For a brace to be considered as K-joint classiication, the axial orce in the brace should be balanced to within 10 % by orces in other braces in the same plane and on the same side o the joint. For Y-joint classiication, the axial orce in the brace is reacted as beam shear in the chord. For X-joint classiication, the axial orce in the brace is carried through the chord to braces on the opposite side. Additional explanation o joint-classiication is ound in Clause 1. NORSOK standard Page 6 o 64

28 NORSOK standard N-004 Rev. 3, February 013 Figure 6- Classiication o simple joints NORSOK standard Page 7 o 64

29 NORSOK standard N-004 Rev. 3, February Strength o simple joints General The validity range or application o the equations deined in is as ollows: g 0.6 (or K joints) D The equations can be used or joints with geometries which lie outside the validity ranges, by taking the usable strength as the lesser o the capacities calculated on the basis o: a) actual geometric parameters, b) imposed limiting parameters or the validity range, where these limits are inringed. The above geometry parameters are deined in Figure 6-3 to Figure 6-6. D T crown Figure 6-3 t T Figure 6-4 t crown saddle L Deinition o geometrical parameters or T- or Y-joints d d D d D D T Deinition o geometrical parameters or X-joints t T d D D T t T NORSOK standard Page 8 o 64

30 NORSOK standard N-004 Rev. 3, February 013 BRACE A da d B A d A D B d B D T A t A g t B B BRACE B D A t A T D T B t B T Figure 6-5 Deinition o geometrical parameters or K-joints d Figure 6-6 A A d A D t A T B B Deinition o geometrical parameters or KT-joints Basic resistance Tubular joints without overlap o principal braces and having no gussets, diaphragms, grout, or stieners should be designed using the ollowing guidelines. The characteristic resistances or simple tubular joints are deined as ollows: N A M Rd Rd t A A T yt Q sin M M A u Q yt d Q uq sin t B AB B d B B where N Rd = the joint design axial resistance M Rd = the joint design bending moment resistance y = the yield strength o the chord member at the joint M = 1.15 BC For joints with joint cans, N Rd shall not exceed the resistance limits deined in C C t C d C D g g T D d B D t B T (6.5) (6.53) C C d C D t C T NORSOK standard Page 9 o 64

31 NORSOK standard N-004 Rev. 3, February 013 For braces with axial orces with a classiication that is a mixture o K, Y and X joints, a weighted average o N Rd based on the portion o each in the total action is used to calculate the resistance Strength actor Q u Q u varies with the joint and action type, as given in Table 6-3. Joint Classiication Table 6-3 Values or Q u Brace action Axial Tension Axial Compression In-plane Bending K Y min X The ollowing notes apply to Table 6-3: (a) Q is a geometric actor deined by: Q 0.3 or Q 1.0 or 0.6 (b) Q g is a gap actor deined by: Q g Q 1. g Q g min.8 g g or 0.05, but Q g 1. 0 D D or Qg 65 t where T y,b y,c g D Q y,b = yield strength o brace (or brace stub i present) y,c = yield strength o chord (or chord can i present) Q g = linear interpolated value between the limiting values o the above expressions or g D Chord action actor Q Q is a design actor to account or the presence o actored actions in the chord. Q 1.0 C1 a,sd y C my,sd 1.6 y C The parameter A is deined as ollows: 3 A Out-o-plane bending (6.54) NORSOK standard Page 30 o 64

32 NORSOK standard N-004 Rev. 3, February 013 A my,sd 1.6 y a,sd mz,sd y (6.55) where a,sd = design axial stress in chord, positive in tension my,sd = design in-plane bending stress in chord, positive or compression in the joint ootprint mz,sd = design out-o-plane bending stress in chord y = yield strength C 1, C, C 3 = coeicients depending on joint and load type as given in Table 6-4 Table 6-4 Values or C 1, C and C 3 Joint type C 1 C C 3 K joints under balanced axial loading T/Y joints under brace axial loading X joints under brace axial tension loading 1) 0.9 = 1.0 X joints under brace axial compression loading 1) 0.9 = All joints under brace moment loading ) Linear interpolated values between = 0.9 and = 1.0 The average o the chord loads and bending moments on either side o the brace intersection should be used in Equations (6.54) and (6.55). The chord thickness at the joint should be used in the above calculations Design axial resistance or X and Y joints with joint cans For Y and X joints with axial orce and where a joint can is speciied, the joint design resistance should be calculated as ollows: N Rd where r T T c n 1- r N can, Rd (6.56) N can,rd = N Rd rom Equation (6.5) based on chord can geometric and material properties, including Q calculated with respect to chord can T n = nominal chord member thickness T c = chord can thickness r = L c /.5 D or joints with L = 1.5 D c or joints with > 0.9 L c = eective total length. Figure 6-7 gives examples o calculation o L c In no case shall r be taken as greater than unity. NORSOK standard Page 31 o 64

33 NORSOK standard N-004 Rev. 3, February 013 Figure 6-7 Examples o chord length L c Strength check Joint resistance shall satisy the ollowing interaction equation or axial orce and/or bending moments in the brace: N N Sd Rd M M y,sd y,rd M M z,sd z,rd 1 where N Sd = design axial orce in the brace member N Rd = the joint design axial resistance M y,sd = design in-plane bending moment in the brace member M z,sd = design out-o-plane bending moment in the brace member M y,rd = design in-plane bending resistance M z,rd = design out-o-plane bending resistance Overlap joints (6.57) Braces that overlap in- or out-o-plane at the chord member orm overlap joints. Joints that have in-plane overlap involving two or more braces may be designed using the simple joint provision o with the ollowing exemptions and additions: a. Shear parallel to the chord ace is a potential ailure mode and should be checked. b does not apply to overlapping joints. c. I axial orces in the overlapping and through braces have the same sign, the combined axial orce representing that in the through brace plus a portion o the overlapping brace orces should be used to check the through brace intersection capacity. The portion o the overlapping brace orce can be NORSOK standard Page 3 o 64

34 NORSOK standard N-004 Rev. 3, February 013 calculated as the ratio o cross sectional area o the brace that bears onto the through brace to the ull area. d. For both in-plane or out-o-plane bending moments, the combined moment o the overlapping and through braces should be used to check the through brace intersection capacity. This combined moment should account or the sign o the moments. The overlapping brace should also be checked on the basis o the chord having through brace properties. Further, through brace capacity shall be checked or combined axial and moment loading in the overlapping brace. In this instance the Q associated with the through brace shall be used. Joints with out-o-plane overlap may be assessed on the same general basis as in-plane overlapping joints, except that axial orce resistance should normally revert to that or Y joints Ringstiened joints Design resistance o ringstiened joints shall be determined by use o recognised engineering methods. Such methods can be elastic analyses, plastic analyses or linear or non-linear inite element analyses. See Clause Cast joints Cast joints are deined as joints ormed using a casting process. They can be o any geometry and o variable wall thickness. The design o a cast joint requires calibrated FE analyses. An acceptable design approach or strength is to limit the stresses ound by linear elastic analyses to the design strength y M using appropriate yield criteria. Elastic peak stresses may be reduced ollowing similar design principles as described in Clause 1, Comm Strength o conical transitions General The provisions given in this section are or the design o concentric cone rusta between tubular sections. They may also be applied to conical transitions at brace ends, where the junction provisions apply only to the brace-end transition away rom the joint Design stresses Equivalent design axial stress in the cone section. The equivalent design axial stress (meridional stress) at any section within the conical transition can be determined by the ollowing equations: equ,sd ac,sd mc,sd where ac,sd 4 s (D cos N t s c Sd mc,sd cos)t M t c Sd c cos) t c equ,sd = equivalent design axial stress within the conical transition ac,sd = design axial stress at the section within the cone due to global actions (6.58) (6.59) (6.60) NORSOK standard Page 33 o 64

35 NORSOK standard N-004 Rev. 3, February 013 mc,sd = design bending stress at the section within the cone due to global actions D s = outer cone diameter at the section under consideration t c = cone thickness (to be equal or larger than the thickness o the smaller tubular) = the slope angle o the cone (see Figure 6-8) N Sd and M Sd are the design axial orce and design bending moment at the section under consideration. ac,sd and mc,sd should be calculated at the junctions o the cone sides. CL. CL. t 0.5D L 1 b e L c t c Figure 6-8 Cone geometry Local bending stress at unstiened junctions In lieu o a detailed analysis, the local bending stress at unstiened tubular-cone junctions can be estimated by the ollowing equation: ml, Sd 0.85 D t j ( at,sd mt, Sd )tan ml,sd = local design bending stress D j = cylinder diameter at junction t = tubular member wall thickness at,sd = design axial stress in tubular section at junction due to global actions mt,sd = design bending stress in tubular section at junction due to global actions (6.61) Hoop stress at unstiened junctions The design hoop stress at unstiened tubular-cone junctions due to unbalanced radial line orces may be estimated rom: hc, Sd D j 0.45 ( at,sd mt, Sd )tan t (6.6) NORSOK standard Page 34 o 64

36 NORSOK standard N-004 Rev. 3, February 013 At the smaller-diameter junction, the hoop stress is tensile (or compressive) when ( at,sd + mt,sd ) is tensile (or compressive). Similarly, the hoop stress at the larger-diameter junction is tensile (or compressive) when ( at,sd + mt,sd ) is compressive (or tensile) Strength requirements without external hydrostatic pressure Local buckling under axial compression For local buckling under combined axial compression and bending, Equation (6.63) should be satisied at all sections within the conical transition. equ,sd clc M (6.63) where clc = local buckling strength o conical transition For conical transitions with slope angle < 30, clc can be determined using Equation (6.6) to Equation (6.8) with an equivalent diameter, D e, at the section under consideration. D e Ds cos (6.64) For conical transitions o constant wall thickness, it would be conservative to use the diameter at the larger end o the cone as D s in Equation (6.64) Junction yielding Yielding at a junction o a cone should be checked on both tubular and cone sides. This section only applies when the hoop stress, hc,sd, is tensile. For net axial tension, that is, when equ,sd is tensile, equ,sd hc, Sd hc,sd equ,sd y M but equ For net axial compression, that is, when equ,sd is compression, where equ,sd hc, Sd hc,sd equ,sd y M or checking stresses on the tubular side o the junction equ,sd = at, Sd mt, Sd ac, Sd mc,sd = or checking stresses on the cone side o the junction cos y = corresponding tubular or cone yield strength. y M (6.65) (6.66) The local bending stress mlt,sd at a junction o a cone should be limited according to the ollowing or equ,sd at both sides o the junction: NORSOK standard Page 35 o 64

37 NORSOK standard N-004 Rev. 3, February 013 ml,sd 1.5 y M 1 equ,sd y (6.67) M = see using N Sd, M y,sd and M z,sd and geometry or the part with the largest D/t ratio Junction buckling This section only applies when the hoop stress, hc,sd, is compressive. In the equations, hc,sd denotes the positive absolute value o the hoop compression. For net axial tension, that is, when equ,sd is tensile, a b a b 1.0 (6.68) where: a equ,sd b y M hc,sd hj M and is Poisson s ratio o 0.3 and is deined in Equation (6.38) For net axial compression, that is, when equ,sd is compressive, and equ,sd hc,sd hj clj M M where clj = corresponding tubular or cone characteristic axial local compressive strength (6.69) (6.70) (6.71) (6.7) hj can be determined using Equation (6.17) to Equation (6.19) with he = 0.40E t/d j and corresponding y. The local bending stress mlt,sd at a junction o a cone should be limited according to the ollowing or equ,sd at both sides o the junction: NORSOK standard Page 36 o 64

38 NORSOK standard N-004 Rev. 3, February 013 ml,sd 1.5 y M 1 equ,sd y (6.73) M = see using N Sd, M y,sd and M z,sd and geometry or the part with the largest D/t ratio Strength requirements with external hydrostatic pressure Hoop buckling Un stiened conical transitions, or cone segments between stiening rings with slope angle < 30, may be designed or hoop collapse by consideration o an equivalent tubular using Equation (6.44) or Equation (6.51) as appropriate. The eective diameter is D/cos, where D is the diameter at the larger end o the cone segment. The equivalent nominal axial stress should be used to represent the axial stress in the design. The length o the cone should be the maximum distance between adjacent rings or ring-stiened cone transitions or the maximum unstiened distance, including the prismatic members on both ends o the cone, or un stiened cone transition. Note that the prismatic members on both ends o a ring-stiened cone should be checked against the stipulations in Junction yielding and buckling The net design hoop stress at a tubular-cone junction is given by the algebraic sum o hc,sd and h,sd, that is hj, Sd where hc,sd h,sd h,sd = design hoop stress due to the external hydrostatic pressure, see eq. (6.16) (6.74) When hj,sd is tensile, the equations in should be satisied by using hj,sd instead o hc,sd. When hj,sd is compressive, the equations in should be satisied by using hj,sd instead o hc,sd Ring design General A tubular-cone junction that does not satisy the above criteria may be strengthened either by increasing the tubular and cone thicknesses at the junction, or by providing a stiening ring at the junction Junction rings without external hydrostatic pressure I stiening rings are required, the section properties should be chosen to satisy both the ollowing requirements: A I c c td y td jd 8E where j ( c a,sd ( a,sd m, Sd m, Sd )tan )tan a,sd = larger o at,sd and ac,sd. m,sd = larger o mt,sd and mc,sd. D c = diameter to centroid o composite ring section. See Note 4 in (6.75) (6.76) NORSOK standard Page 37 o 64

39 NORSOK standard N-004 Rev. 3, February 013 A c = cross-sectional area o composite ring section I c = moment o inertia o composite ring section In computing A c and I c, the eective width o shell wall acting as a lange or the composite ring section may be computed rom: b e 0.55( D t j D t j c ) (6.77) Notes: 1. For internal rings, D j should be used instead o D c in Equation.(6.76). For external rings, D j in Equation (6.75) and Equation (6.76)should be taken to the centroid o the composite ring Junction rings with external hydrostatic pressure Circumerential stiening rings required at the tubular-cone junctions should be designed such that the moment o inertia o the composite ring section is equal to or greater than the sum o Equation (6.76) and Equation (6.79): I ct I where c I ch hec Ich he D j tl1 16E t cl c cos (6.78) (6.79) where I ct = moment o inertia o composite ring section with external hydrostatic pressure and with eective width o lange computed rom Equation (6.77) D j = diameter o tubular at junction. See Note 4 in L c = distance to irst stiening ring in cone section along cone axis 113. L 1 = distance to irst stiening ring in tubular section 113. he = elastic hoop buckling strength or tubular hec = he or cone section treated as an equivalent tubular D e = larger o equivalent diameters at the junctions Notes: 3. A junction ring is not required or hydrostatic collapse i Equation (6.15) is satisied with he computed using C h equal to 0.44 (cos)(t/d j ) in Equation (6.0), where D j is the tubular diameter at the junction. 4. For external rings, D j in Equation (6.79) should be taken to the centroid o the composite ring, except in the calculation o L Intermediate stiening rings I required, circumerential stiening rings within cone transitions may be designed using Equation (6.1) with an equivalent diameter equal to D s /cos, where D s is the cone diameter at the section under consideration, t is the cone thickness, L is the average distance to adjacent rings along the cone axis and he is the average o the elastic hoop buckling strength values computed or the two adjacent bays. 3 D j t D t 3 e NORSOK standard Page 38 o 64

40 NORSOK standard N-004 Rev. 3, February Design o plated structures Design o plated structures such as stiened panels shall be done according to Part 1 o DNV-RP-C01 /14/. Design o plates may also be done according to NS-EN or cases where these standards give recommendations e.g. web o plate girders. Capacity checks according to this standard may imply utilisation o the plate into the post linear range and their capacity against dynamic loads should be considered. See also sub-clause Design o cylindrical shells Unstiened and ring stiened cylindrical shells subjected to axial orce, bending moment and hydrostatic pressure may be designed according to 6.3. For more reined analysis o cylindrical shells or cylindrical shells with other stiening geometry or loading, DNV-RP-C0 /13/ shall be used. The susceptibility to less avourable post-critical behaviour associated with more slender geometry and more and /or larger stress components may be expressed through the reduced slenderness parameter,, which is deined in DNV-RP-C0 /13/. The resulting material actor or design o shell structures is given as M M M s or s 0.5 or 0.5 s or 1.0 s is deined in DNV-RP-C0 /13/. s Design against unstable racture General (6.80) Normally brittle racture in oshore structures is avoided by selecting materials according to Clause 5 and with only acceptable deects present in the structure ater abrication. Unstable racture may occur under unavourable combinations o geometry, racture toughness, welding deects and stress levels. The risk o unstable racture is generally greatest with large material thickness where the state o deormation is plane strain. For normal steel qualities, this typically implies a material thickness in excess o 40 mm to 50 mm, but this is dependent on geometry, racture toughness, weld deects and stress level. See /8/ or guidance on the use o racture mechanics. I relevant racture toughness data is lacking, material testing should be perormed Determination o maximum acceptable deect size The design stress shall be determined with load coeicients given in NORSOK N-001. The maximum applied tensile stress, accounting also or possible stress concentrations, shall be considered when calculating the tolerable deect size. Relevant residual stresses shall be included in the evaluation. Normally, a structure is designed based on the principle that plastic hinges may develop without giving rise to unstable racture. In such case, the design nominal stress or unstable racture shall not be less than the yield stress o the member. The characteristic racture toughness, K Ic,(or, alternatively K c, J c, J Ic, CTOD c ), shall be determined as the lower 5% ractile o the test results. The design racture toughness shall be calculated rom Equation (6.81). K Icd where K Ic M M = 1.15 or members where ailure will be without substantial consequences (6.81) NORSOK standard Page 39 o 64

41 NORSOK standard N-004 Rev. 3, February 013 M = 1.4 or members with substantial consequences Notes: 1. Substantial consequences in this context means that ailure o the joint will entail: Danger o loss o human lie; Signiicant polution; Major inancial consequences.. Without substantial consequences is understood ailure where it can be demonstrated that the structure satisy the requirement to damaged condition according to the Accidental Limit States with ailure in the actual joint as the deined damage. The design values o the J-integral and CTOD shall be determined with a saety level corresponding to that used to determine the design racture toughness. For example, the design CTOD is ound rom the ollowing ormula: CTOD cd CTOD M c (6.8) The maximum deect size likely to remain undetected (a i ) shall be established, based on an evaluation o the inspection method, access or inspection during abrication, abrication method, and the thickness and geometry o the structure,. When determining the value o a i consideration shall be given to the capabilities o the inspection method to detect, localise, and size the deect. The maximum allowable deect size ( a m ), shall be calculated on the basis o the total stress (or corresponding strains) and the design racture toughness. It shall be shown that a i < a m. For a structure subjected to atigue loading, the crack growth may be calculated by racture mechanics. The initial deect size shall be taken as a i. The inal crack size, a u, shall be determined with the atigue load applied over the expected lie time. It shall be veriied that a u <a m. 7 SERVICEABILITY LIMIT STATES General requirements or the serviceability limit states are given in NORSOK N FATIGUE LIMIT STATES 8.1 General In this NORSOK standard requirements are given in relation to atigue analyses based on atigue tests and racture mechanics. Reerence is made to DNV-RP-C03 /15/or more details with respect to atigue design. The aim o atigue design is to ensure that the structure has an adequate atigue lie. Calculated atigue lives can also orm the basis or eicient inspection programmes during abrication and the operational lie o the structure. The design atigue lie or the structure components should be based on the structure service lie speciied by the operator. I no structure service lie is speciied by the operator, a service lie o 15 years shall be used. A short design atigue lie will imply shorter inspection intervals. To ensure that the structure will ulil the intended unction, a atigue assessment, supported where appropriate by a detailed atigue analysis should be carried out or each individual member which is subjected to atigue loading. It should be noted that any element or member o the structure, every welded joint and attachment, or other orm o stress concentration, is potentially a source o atigue cracking and should be individually considered. The number o load cycles shall be multiplied with the appropriate actor in Table 8-1 beore the atigue analysis is perormed. NORSOK standard Page 40 o 64

42 NORSOK standard N-004 Rev. 3, February 013 Table 8-1 Classiication o structural components based on damage consequence Design atigue actors Not accessible or inspection and repair or in the splash zone Accessible or inspection, maintenance and repair, and where inspections or maintenance is planned Below splash zone Above splash zone Substantial 10 3 consequences Without substantial 3 1 consequences Substantial consequences in this context means that ailure o the joint will entail a) danger o loss o human lie; b) signiicant pollution; c) major inancial consequences. Without substantial consequences is understood ailure where it can be demonstrated that the structure satisy the requirement to damaged condition according to the ALSs with ailure in the actual joint as the deined damage. Welds in joints below 150 m water depth should be assumed inaccessible or in-service inspection. In project phases where it is possible to increase atigue lie by modiication o structural details, grinding o welds should not be assumed to provide a measurable increase in the atigue lie. 8. Methods or atigue analysis The atigue analysis should be based on S-N data, determined by atigue testing o the considered welded detail, and the linear damage hypothesis. When appropriate, the atigue analysis may alternatively be based on racture mechanics. I the atigue lie estimate based on atigue tests is short or a component where a ailure may lead to substantial consequences, a more accurate investigation considering a larger portion o the structure, or a racture mechanics analysis, should be perormed. For calculations based on racture mechanics, it should be documented that the planned in-service inspections accommodate a suicient time interval between time o crack detection and the time o unstable racture. See also 6.8. Reerence is made to /15/ or more details. All signiicant stress ranges, which contribute to atigue damage in the structure, should be considered. The long term distribution o stress ranges may be ound by deterministic or spectral analysis. Dynamic eects shall be duly accounted or when establishing the stress history. 9 ACCIDENTAL DAMAGE LIMIT STATES 9.1 General The accidental damage limit states should be checked or accidental loads and abnormal environmental loads (return period year). See NORSOK N-003 or details. The material actor is 1.0 or ALSs unless noted otherwise. For tubular members and conical sections the material actor in ALS shall be taken as the value calculated rom but divided by The material actors according to Table 9-1shall be used i NS-EN and NS-EN is used or calculation o structural resistance. NORSOK standard Page 41 o 64

43 NORSOK standard N-004 Rev. 3, February 013 Table 9-1 Material actors Type o calculation Material actor 1) Value Resistance o Class 1, or 3 cross-sections M0 1.0 Resistance o Class 4 cross-sections M1 1.0 Resistance o member to buckling M1 1.0 Resistance o net section at bolt holes M 1.1 Resistance o illet and partial penetration welds 1.1 Resistance o bolted connections 1.1 1) Symbols according to NS-EN and NS-EN For resistance check where other material actors are used than given in Table 9-1, the recommended material actor in NS-EN , NS-EN and NS-EN should be multiplied by an additional building code material actor BC = Check or accidental actions The structure shall be checked or all ALSs or the design accidental actions deined in the risk analysis. The structure shall according to NORSOK N-001 be checked in two steps: a) Resistance o the structure against design accidental actions b) Post accident resistance o the structure against environmental actions. Should only be checked i the resistance is reduced by structural damage caused by the design accidental actions The overall objective o design against accidental actions is to achieve a system where the main saety unctions are not impaired by the design accidental actions. In general the ailure criteria to be considered, should also be deined in the risk analyses, see Clause 1. The design against accidental actions may be done by direct calculation o the eects imposed by the actions on the structure, or indirectly, by design o the structure as tolerable to accidents. Examples o the latter are compartmentation o loating units which provides suicient integrity to survive certain collision scenarios without urther calculations. The inherent uncertainty o the requency and magnitude o the accidental loads, as well as the approximate nature o the methods or determination o accidental action eects, shall be recognised. It is thereore essential to apply sound engineering judgement and pragmatic evaluations in the design. I non-linear, dynamic inite elements analysis is applied, it shall be veriied that all behavioural eects and local ailure modes (e.g. strain rate, local buckling, joint overloading, joint racture) are accounted or implicitly by the modelling adopted, or else subjected to explicit evaluation. Typical accidental actions are impact rom ship collisions, impact rom dropped objects, ire, explosions. The dierent types o accidental actions require dierent methods and analyses to assess the structural resistance. Design recommendations or the most common types o accidental actions are given in Annex A. 10 REASSESSMENT OF STRUCTURES 10.1 General Reassessment o existing structures should be made according to Norsok N-006 NORSOK standard Page 4 o 64

44 NORSOK standard N-004 Rev. 3, February BIBLIOGRAPHY /1/ API Recommended Practice A-WSD Twenty irst edition including Errata and supplement 1, and 3. October 007 // Odland, J.: Improvements in design Methodology or Stiened and Unstiened Cylindrical Structures. BOSS 88. Proceedings o the International Conerence on behaviour o Oshore Structures. Trondheim, June /3/ Frieze, P.A., Hsu, T. M., Loh, J. T. and Lotsberg, I.: Background to Drat ISO Provisions on Intact and Damaged Members. BOSS, Delt, /4/ Smith, C. S., Kirkwood, W. and Swan, J. W.: Buckling Strength and Post-Collapse Behaviour o Tubular Bracing Members Including Damage Eects. Procs. nd International Conerence on the Behaviour o Oshore Structures, BOSS 1979, London, August /5/ Livesley, R. K.: The Application o an Electronic Digital Computer to Some Problems o Structural Analysis. The Structural Engineer, January /6/ American Institute o Steel Construction (AISC), Manual o Steel Construction Load and Resistance Factor Design, Vol. 1, nd ed., 1994 /7/ Boardman, H. C.: Stresses at Junctions o Two Right Cone Frustums with a Common Axis,The Water Tower, Chicage Bridge and Iron Company, March /8/ BS 7910:005 Guide to methods or assessing the acceptability o laws in metallic structures British Standard /9/ Faulkner, D.: Design Against Collapse or Marine Structures, Advances in Marine Technology, Trondheim, 1979 /10/ Dow, R.S. and Smith, C.S.: Eects o Localised Imperections on Compressive Strength o Long Rectangular plates, Journal o Constructional steel Research, vol. 4, pp , 1984 /11/ ULTIGUIDE. DNV, SINTEF, BOMEL. Best Practice Guidelines or Use o Non-linear Analysis Methods in Documentation o Ultimate Limit States or Jacket Type Oshore Structures. /1/ A. Stacey, J.V. Sharp, N.W. Nichols. "Static Strength Assessment o Cracked Tubular Joints", OMAE, Florence /13/ DNV RP-C0, Buckling o Shells, Det Norske Veritas. /14/ DNV RP-C01, Buckling Strength o Plated Structures, Det Norske Veritas 00 /15/ DNV RP-C03, Fatigue Strength Analysis o Oshore Steel Structures /16/ Dier, A.F, Smedley; P., Solland, G., Bang, H.: New data on the capacity o x-joints under tension and implications or codes, OMAE Estoril, Portugal /17/ Pecknold, D.A.; Ha, C.C.; Mohr,W.C.: Chord stress eects on ultimate strength o DT tubular joints, OMAE , /18/ Pecknold, D.; Marshall, P.; Bucknell, J.: New API RPA tubular joint strength design provisions, OTC 17310, 005 Oshore Technology Conerence, Houston Texas. NORSOK standard Page 43 o 64

45 NORSOK standard N-004 Rev. 3, February COMMENTARY This clause provides additional guidance and background to selected clauses o this NORSOK standard. Comm. 1 Scope In general the provisions o this NORSOK standard are developed, tested and calibrated or steel with traditional stress/strain relations. Design o structures made rom steel material with higher yield strength may require additional or dierent design checks due to, among other the ollowing eects: 9. - usually a higher yield to tensile strength ratio implying less strain hardening; larger elastic delection beore reaching resistance limit, which is important where second-order eects play a role, usually reduced weld overmatch leading to increased risk o ailure in weld material; reduced maximum elongation. Comm. 4 General provisions The notation R k and S k shall be read as indicative or a resulting characteristic resistance and action eect respectively. In the general case the design resistance, R d, will be a unction o several parameters where the material parameter y should be divided by M, and S d will be derived as a summation o dierent characteristic actions multiplied with dierent partial coeicients. The groups o limit states, applied in this NORSOK standard, are deined according to ISO In contrast, NS-ENV and most national building codes are treating limit states associated with ailure due to accidental loads or atigue as ultimate limit states. Whilst atigue and ailure rom accidental loads are characteristically similar to ultimate limit states, it is convenient to distinguish between them due to dierent load actors or ULS, FLS and ALS. FLS and ALS may thereore be regarded as subgroups o ULS. Comm. 5.1 Design class The check or damaged condition according to the ALS imply that the structure with damage is checked or all characteristic actions, but with all saety actors set to unity. With regard to material selection the associated damage is brittle ailure or lamellar tearing. I the structure subjected to one such ailure is still capable o resisting the characteristic loads, the joint or component should be designated DC 3 or DC 4. As an example one can consider a system o our cantilever beams linked by a transverse beam at their ree ends. A material ailure at the supports in one o the beams will only reduce the resistance with 5 % (assuming suicient strength in the transverse beam). Since the structure without damage need to be checked with partial saety actors according to ULSs, such a system will prove to satisy ALS, and design class DC 3 or DC 4 dependent upon complexity will apply. A similar system o only two cantilever beams will normally not ulill the ALS criterion to damage condition and DC 1 or DC will apply. Comm. 5. Steel quality level Steel materials selected in accordance with the tabled SQL and as per MDS in NORSOK M-10 are assured to be o the same weldability within each SQL group, irrelevant o the actual material thickness or yield strength chosen. Requirements or SQL I are more severe than those or SQL II. This is achieved through the dierent optional requirements stated in each MDS or each SQL. The achievement o balanced weldability is reached mainly by the ollowing two means: requirement to higher energy absorption at toughness testing or yield strength above 400 MPa; lowering the test temperature or qualiication testing, relecting the dierences between SQL IIand I and stepwise increases in material thickness. For high strength material o 40 MPa and above, the MDSs assume the same minimum yield strength irrelevant o material thickness o plates. This is normally achieved through adjustments in chemical composition, but without jeopardising the required weldability. Improved properties in the trough thickness direction (SQL I) should be speciied or steel materials where ailure due to lamellar tearing will mean signiicant loss o resistance. Alternatively the plate can be tested ater welding to check that lamellar tearing has not taken place. NORSOK standard Page 44 o 64

46 NORSOK standard N-004 Rev. 3, February 013 Comm. 6 Ultimate limit states Comm. 6.1 General NS-EN uses the ollowing deinition o ULSs: "ULSs are those associated with collapse, or with other orms o structural ailure which may endanger the saety o people. States prior to structural collapse which, or simplicity, are considered in place o the collapse itsel are also classiied and treated as ULSs ". For structures designed according to this NORSOK standard structural ailures which will imply signiicant pollution or major inancial consequences should be considered in addition to human saety. All steel structures behave more or less non-linear when loaded to their ultimate limit. The ormulae or design resistance in this NORSOK standard or similar codes and standards are thereore developed on the basis that permanent deormation may take place. Traditionally, oshore structures are analysed by linear methods to determine the internal distribution o orces and moments, and the resistances o the cross-sections are checked according to design resistances ound in design codes. The design ormulae oten require deormations well into the in-elastic range in order to mobilise the prescribed resistances. However, no urther checks are considered necessary as long as the internal orces and moments are determined by linear methods. When non-linear analysis methods are used, additional checks o ductility and repeated yielding, are to be perormed. The check or repeated yielding is only necessary in case o cyclic loading, e.g. wave loads. The check or ductility requires that all sections subjected to deormation into the in-elastic range should deorm without loss o the assumed load-bearing resistance. Such loss o resistance can be due to racture, instability o cross-sectional parts or member buckling. The design codes give little guidance on this issue, with exception or stability o cross-sectional parts in yield hinges, which will normally be covered by requirement to cross-sectional class 1, see e.g. NS-EN The check against repeated yielding is oten reerred to as the check that the structure can achieve a stable state called shakedown. In the general case it is necessary to deine a characteristic cycling load and to use this load with appropriate partial saety actors. It should be checked that yielding only take place in the irst ew loading cycles and that later load repetitions only cause responses in the linear range. Alternatively a low cycle atigue check can be perormed and substitute the check or shakedown. I non-linear analyses are applied, it shall be checked that the analysis tool and the modelling adopted represent the non-linear behaviour o all structural elements that may contribute to the ailure mechanism with suicient accuracy, see /11/). Stiness, resistance and post ultimate behaviour (i applicable) should be represented, including local ailure modes such as local buckling, joint overload, joint racture, etc. Use o non-linear analysis methods may result in more structural elements being governed by the requirements to the SLS and additional SLS requirements may be needed compared with design using linear methods. The ollowing simpliications are valid or design with respect to the ultimate limit states, provided that ductile ailure modes can be assumed: 1. - built in stresses rom abrication and erection may be neglected when abrication requirements according to NORSOK M-101 are met;. - stresses rom dierential temperature under normal conditions (e.g. sun heating) may be neglected; 3. - the analytical static model need not in detail represent the real structure, e.g. secondary (delection induced) moments may be neglected or trusses. I the design is based on other parts o NS-EN-1993 than part 1-1- and 1-8 it is recommended to multiply the recommended material actor in the code i not given in Table 6-1 by an additional building code modiication actor BC = Comm. 6. Ductility Steel structures behave generally ductile when loaded to their limits. The established design practise is based on this behaviour, which is beneicial both with respect to the design and perormance o the structure. For a ductile structure, signiicant delections may occur beore ailure and thus give a collapse warning. Ductile structures also have larger energy absorption capabilities against impact loads. The possibility or the structure to redistribute stresses lessens the need or an accurate stress calculation during design as the NORSOK standard Page 45 o 64

47 NORSOK standard N-004 Rev. 3, February 013 structure may redistribute orces and moments to be in accordance with the assumed static model. This is the basis or use o linear analyses or ULS checks even or structures, which behave signiicantly non-linear when approaching their ultimate limit states. Comm General This subclause has been developed speciically or circular tubular shapes that are typical o oshore platorm construction. The types o tubulars covered include abricated roll-bent tubulars with a longitudinal weld seam, hot-inished seamless pipes, and ERW pipes that have undergone some types o post-weld heat treatment or normalisation to relieve residual stresses. Relie o the residual stresses is necessary to remove the rounded stress-strain characteristic that requently arises rom the ERW orm o manuacture. The limit o D/t applies to tubular members that are part o a rame system. The ormulas may be used or more slender members i the member is not a part o a structural system, e.g. a tubular buoyancy tank. The recommendations in this subclause are basically in accordance with ISO Comm Axial compression Tubular members subjected to axial compression are subject to ailure due to either material yielding, overall column buckling, local buckling, or a combination o these ailure modes. The characteristic equation or column buckling is a unction o, a normalised orm o column slenderness parameter given by ( cl / E ) 0.5 ; where cl is the local buckling strength o the cross-section and E is the Euler buckling strength or a perect column. For members with two or more dierent cross sections, the ollowing steps can be used to determine the resistance: 1. Determine the elastic buckling load, N E, or the complete member, taking into accounts the end restraints and variable cross-section properties. In most cases, the eective length actor o the member needs to be determined.. The design compressive resistance, N cr,rd, is given by N N c,rd N cl M 0.9 N N N cl E or N N cl E 1.34 E cl c,rd or M NE N (1.1) (1.) in which N cl = smallest characteristic local axial compressive strength o all the cross sections = cl A cl = as given by Equation (6.6) or Equation (6.7) A = cross-sectional area In design analysis, a member with variable cross sections can be modelled with several prismatic elements. For each prismatic element, added length and/or input eective length actor are used to ensure that the design compressive resistance is correctly determined. The theoretical value o C x or an ideal tubular is 0.6. However, a reduced value o C x = 0.3 is recommended or use in Equation (6.8) to account or the eect o initial geometric imperections within tolerance limits given in NORSOK M-101. A reduced value o C x = 0.3 is also implicit in the limits or y / cle given in Equation (6.6) or Equation (6.7). Short tubular members subjected to axial compression will ail either by material yielding or local buckling, depending on the diameter-to-thickness (D/t) ratio. Tubular members with low D/t ratios are generally not subject to local buckling under axial compression and can be designed on the basis o material yielding, i.e., the local buckling stress may be considered equal to the yield strength. However, as the D/t ratio increases, the elastic local buckling strength decreases, and the tubular should be checked or local buckling. NORSOK standard Page 46 o 64

48 NORSOK standard N-004 Rev. 3, February 013 Un stiened thin-walled tubular subjected to axial compression and bending are prone to sudden ailures at loads well below the theoretical buckling loads predicted by classical small-delection shell theory. There is a sudden drop in load-carrying resistance upon buckling o such members. The post-buckling reserve strength o tubular members is small, in contrast to the post-buckling behaviour o lat plates in compression, which usually continue to carry substantial load ater local buckling. For this reason, there is a need or more conservatism in the deinition o buckling load or tubulars than or most other structural elements. The large scatter in test data also necessitates a relatively conservative design procedure. The large scatter in test data is partly caused by initial imperections generated by abrication. Other actors o inluence are boundary conditions and built-in residual stresses, (see /3/ and /4/). Some experimental evidence indicates that inelastic local buckling may be less sensitive to initial imperections and residual stresses than elastic local buckling, see /3/. Thereore, in order to achieve a robust design, it is recommended to select member geometry such that local buckling due to axial orces is avoided. The characteristic equations are developed by screening test data and establishing the curve at 95 % success at the 50 % conidence level, which satisies the ollowing conditions: 1) it has a plateau o material characteristic yield strength over the range 0 y / cle 0.17, ) it has the general orm o Equation (6.7), 3) it converges to the elastic critical buckling curve with increasing member slenderness ratio, 4) the dierence between the mean minus standard deviations o test data and the developed equations is minimum. The local buckling data base contains 38 acceptable tests perormed by several dierent investigators, see /3/. A comparison between test data and the characteristic local buckling strength equation, Equation (6.6) to Equation (6.8) was made. The developed equations have the bias o 1.065, the standard deviation o 0.073, and the coeicient o variation o The elastic local buckling stress ormula recommended in Equation (6.8) represent one-hal o the theoretical local buckling stress computed using classical small-delection theory. This reduction accounts or the detrimental eect o geometric imperections. Based on the test data shown in /3/, this reduction is considered to be conservative or tubulars with t 6 mm and D/t < 10. Oshore platorm members typically all within these dimensional limits. For thinner tubulars and tubulars with higher D/t ratios, larger imperection reduction actors may be required, see /13/. The local buckling database limits the applicability o the nominal strength equations to D/t<10 and t 6mm. Reerence /13/ provides guidance or the design o tubular members beyond these dimensional limits. Comm Hoop buckling Un-stiened tubular members under external hydrostatic pressure are subject to elastic or inelastic local buckling o the shell wall between the restraints. Once initiated, the collapse will tend to latten the member rom one end to the other. Ring-stiened members are subject to local buckling o the shell wall between rings. The shell buckles between the rings, while the rings remain essentially circular. However, the rings may rotate or warp out o their plane. Ring-stiened tubular members are also subject to general instability, which occurs when the rings and shell wall buckle simultaneously at the critical load. It is desirable to provide rings with suicient overstrength to prevent general instability. Reerence is made to /14/. For tubular members satisying the maximum out-o-roundness tolerance o 1 %, the hoop buckling strength is given by Equation (6.17) to Equation (6.19). For ring-stiened members, Equation (6.17) to Equation (6.19) gives the hoop buckling strength o the shell wall between the rings. To account or the possible 1 % out-o roundness, the elastic hoop buckling stress is taken as 0.8 o the theoretical value calculated using classical small delection theory. That is, C h =0.44t/D, whereas the theoretical C h =0.55t/D. In addition, the remaining C h values are lower bound estimates. For members with out-o-roundness greater than 1 %, but less than 3 %, a reduced elastic hoop buckling strength, he, should be determined, see /3/. The characteristic hoop buckling strength, h, is then determined using the reduced he. Reduced he he 0.8 NORSOK standard Page 47 o 64

49 NORSOK standard N-004 Rev. 3, February 013 where = geometric imperection actor D max D min = D D D max 0.01 D min nom nom = out-o-roundness (%) where D max and D min are the maximum and minimum o any measured diameter at a cross section and D nom the nominal diameter. Comm Ring stiener design The ormula recommended or determining the moment o inertia o stiening rings, Equation (6.1), provides suicient strength to resist buckling o the ring and shell even ater the shell has buckled between stieners. It is assumed that the shell oers no support ater buckling and transers all its orces to the eective stiener section. The stiener ring is designed as an isolated ring that buckles into two waves (n=) at a collapse pressure 0 % higher than the strength o the shell. The eect o ring stieners on increased axial resistance or large diameter/thickness ratios is not accounted or in this section. Reerence is made to /13/ or guidance on this. Comm Tubular members subjected to combined loads without hydrostatic pressure This subclause describes the background o the design requirements in 6.3.8, which covers un-stiened and ring-stiened cylindrical shell instability mode interactions when subjected to combined axial and bending loads without hydrostatic pressure In this subclause and 6.3.9, the designer should include the second order rame moment or P- eect in the bending stresses, when it is signiicant. The P- eect may be signiicant in the design o un-braced deck legs, piles, and laterally lexible structures. Comm Axial tension and bending This subclause provides a resistance check or components under combined axial tensile load and bending. The interaction equation is modiied compared with ISO/DIS 1990 as the term or axial load is rised in the power o Comm Axial compression and bending This subclause provides an overall beam-column stability check, Equation (6.7), and strength check, Equation (6.8), or components under combined axial compression orce and bending. Use o -unctions (see /5/) implies that exact solutions are obtained or the considered buckling problem. General eective buckling lengths have been derived using the -unctions incorporating end lexibility o the members. The results or an X-brace with our equal-length members are shown in Figure 1-1 to Figure 1-3 as a unction o the load distribution in the system Q/P and o the end rotational stiness. P is the maximum compression orce. The non-dimensional parameter is given as: CL EI where C is the local rotational stiness at a node (accounting or local joint stiness and stiness o the other members going into the joint) and I is the moment o inertia o the tubular member. Normally L reers to center-line-to-center-line distances between nodes. The results in Figure 1- indicates or realistic end conditions ( > 3) or single braces, i.e., when Q/P = 1, a k-actor less than 0.8 is acceptable provided end joint lexibilities are not lost as the braces become ully loaded. Experimental results relating to rames seem to conirm that 0.7 is acceptable. For X-braces where the magnitude o the tension brace load is at least 50 % o the magnitude o the compression brace load, NORSOK standard Page 48 o 64

50 NORSOK standard N-004 Rev. 3, February 013 i.e., when Q/P < -0.5, and the joints remain eective, k = 0.45 times the length L is supported by these results, whilst 0.4 seems justiied based on experimental evidence. Figure 1- and Figure 1-3 provide eective length actors or X-braces when the longer segment is equal to 0.6 times the brace length and 0.7 times the brace length, respectively. To estimate the eective length o a un-braced column, such as superstructure legs, the use o the alignment chart in Figure 1-4 provides a airly rapid method or determining adequate k-values. For cantilever tubular members, an independent and rational analysis is required in the determination o appropriate eective length actors. Such analysis shall take ull account o all large delection (P-) eects. For a cantilever tubular member, C m =1.0. The use o the moment reduction actor (C m ) in the combined interaction equations, such as Equation (6.7), is to obtain an equivalent moment that is less conservative. The C m values recommended in Table 6- are similar to those recommended in /6/. NORSOK standard Page 49 o 64

51 NORSOK standard N-004 Rev. 3, February 013 Figure 1-1 Eective length actors or a X-brace with equal brace lengths NORSOK standard Page 50 o 64

52 NORSOK standard N-004 Rev. 3, February 013 Figure 1- brace length Eective length actors or a X-brace with the shorter segment equal to 0.4 times the NORSOK standard Page 51 o 64

53 NORSOK standard N-004 Rev. 3, February 013 Figure 1-3 brace length Eective length actors or a X-brace with the shorter segment equal to 0.3 times the NORSOK standard Page 5 o 64

54 NORSOK standard N-004 Rev. 3, February 013 Figure 1-4 Alignment chart or eective length o columns in continuous rames Comm Tubular members subjected to combined loads with hydrostatic pressure This subclause provides strength design interaction equations or the cases in which a tubular member is subjected to axial tension or compression, and/or bending combined with external hydrostatic pressure. Some guidance on signiicance o hydrostatic pressure may be ound rom Figure 1-5 or a given water depth and diameter/thickness ratio. NORSOK standard Page 53 o 64

55 NORSOK standard N-004 Rev. 3, February 013 Figure 1-5 Reduction in bending resistance o members due to external pressure, ( M = 1.5, y = 350 MPa) The design equations are categorised into two design approaches, Method A and Method B. The main purpose o providing two methods is to acilitate tubular member design by the two common analyses used by designers. Either design method is acceptable. In the limit when the hydrostatic pressure is zero, the design equations in this section reduce to those given in In both methods the hoop compression is not explicitly included in the analysis, but its eect on member design is considered within the design interaction equations. For both design methods, the hoop collapse design check stipulated in shall be satisied irst. Method A should be used when the capped-end axial compression due to external hydrostatic pressure is not explicitly included in the analysis, but its eect is accounted or while computing the member utilisation ratio. Method B should be used when the capped-end axial compression due to external hydrostatic pressure is included explicitly in the analysis as nodal loads. The explicit application o the capped-end axial compression in the analysis allows or a more precise redistribution o the capped-end load based on the relative stiness o the braces at a node. The two methods are not identical. However, since redistribution o the capped-end axial compression in Method B is minimal because o similar brace sizes at a node, the dierence between the two methods should be small. Comm Method A ( a,sd is in tension) Axial tension, bending, and hydrostatic pressure The actual member axial stress, or the net axial stress, is estimated by subtracting the ull capped-end axial compression rom the calculated axial tension, a,sd. The net axial tensile stress, ( a,sd - q,sd ), is then used in Equation (6.34), which is a linear tension-bending interaction equation. There are mainly three eects due to the presence o external hydrostatic pressure: 1) a reduction o the axial tension due to the presence o a capped-end axial compression, ) a reduction o the axial tensile strength, th,rd, caused by the hoop compression, and 3) a reduction o the bending strength, mh,rd, caused by the hoop compression. As demonstrated in /3/, the axial tension-hydrostatic pressure interaction is similar to the bending-hydrostatic pressure interaction. The reduced axial tensile and bending strengths, as given by Equation (6.35) and Equation (6.36), were derived rom the ollowing ultimate-strength interaction equations: Combined axial tension and hydrostatic pressure: NORSOK standard Page 54 o 64

56 NORSOK standard N-004 Rev. 3, February 013 a,sd d p,sd h,rd p,sd h,rd a,sd d 1.0 d y M (1.3) m,sd m,rd p,sd h,rd p,sd h,rd m,sd m, Rd 1.0 m,rd m M (1.4) To obtain the axial tensile and bending strengths rom the above two equations, the a,sd and m,sd terms are represented by th and mh, respectively, which are given by the positive roots o the quadratic equations. When the calculated axial tensile stress is greater than or equal to the capped-end axial compression ( i.e. a,sd q,sd ) the member is subjected to net axial tension. For this case, the member yield strength, y, is not replaced by a local buckling axial strength. When the calculated axial tensile stress is less than the capped-end axial compression ( i.e. a,sd < q,sd )the member is subjected to net axial compression and to a quasi-hydrostatic pressure condition. (A member is subjected to a pure hydrostatic pressure condition when the net axial compressive stress is equal to the capped-end axial stress, i.e. a,sd = 0.) Under this condition there is no member instability. Hence or this case, in which a,sd q,sd, the cross-sectional yield criterion [Equation (6.39)] and the cross-sectional elastic buckling criterion [Equation (6.41)] need to be satisied. Method B ( ac,sd is in tension) In this method the member net axial stress is the calculated value, ac,sd, since the eect o the capped-end axial compression is explicitly included in the design analysis. Thereore, the calculated axial tensile stress, ac,sd, can be used directly in the cross-sectional strength check, as given in Equation (6.4). Comm Axial compression, bending, and hydrostatic pressure Method A ( a,sd is in compression) The capped-end axial compression due to hydrostatic pressure does not cause buckling o a member under combined external compression and hydrostatic pressure. The major contribution o the capped-end axial compression is earlier yielding o the member in the presence o residual stresses and additional external axial compression. The earlier yielding in turn results in a lower column buckling strength or the member, as given in Equation (6.47). When there is no hydrostatic pressure (i.e. q,sd = 0) Equation (6.47) reduces to the in-air case, see Equation (6.3). It is incorrect to estimate the reduced column buckling strength by subtracting the capped-end axial compression rom the in-air buckling strength calculated by Equation (6.3). This approach assumes that the capped-end axial compression can cause buckling and actually the reduced strength can be negative or cases where the capped-end axial compression is greater than the in-air buckling strength. For the stability check [see Equation (6.43)], the calculated axial compression, a,sd, which is the additional external axial compression, is used. The eect o the capped-end axial compression is captured in the buckling strength, ch,rd, which is derived or hydrostatic conditions. For strength or cross-sectional yield check [see Equation (6.44)], the net axial compression o the member is used. In addition, the cross-section elastic buckling criterion [see Equation (6.41)] need to be satisied. Method B ( ac,sd is in compression) In this method the calculated axial stress, ac,sd, is the net axial compressive stress o the member since the capped-end axial compression is included in the design analysis. For the stability check [see Equation (6.50)], the axial compression to be used with the equation is the component that is in additional to the pure hydrostatic pressure condition. Thereore, the capped-end axial compression is subtracted rom the net axial NORSOK standard Page 55 o 64

57 NORSOK standard N-004 Rev. 3, February 013 compressive stress in Equation (6.50). For the strength check [see Eq. (6.51)], the net axial compressive stress is used. When the calculated axial compressive stress is less than the capped-end axial compression, the member is under a quasi-hydrostatic pressure condition. That is, the net axial compression is less than the capped-end axial compression due to pure hydrostatic pressure. Under this loading, the member can not buckle as a beam-column. O course, hoop collapse is still a limit state. For this case, in which ac,sd q,sd, only the yield criterion o Equation (6.51) needs to be satisied. Comm 6.4 Tubular joints Reasonable alternative methods to the requirements in this standard may be used or the design o joints. Test data and analytical techniques may be used as a basis or design, provided that it is demonstrated that the resistance o such joints can be reliably estimated. The recommendations presented here have been derived rom a consideration o the characteristic strength o tubular joints. Characteristic strength is comparable to lower bound strength. Care should thereore be taken in using the results o limited tests programs or analytical investigations to provide an estimate o joint resistance. Consideration shall be given to the imposition o a reduction actor on the calculation o joint resistance to account or a small amount o data or a poor basis or the calculation. Analytical or numerical techniques should be calibrated and benchmarked to suitable test data. The ormulas in this subclause are based upon API RP-A /1/ and paper by Dier et al. /16/. Comm General Detailing practice Joint detailing is an essential element o joint design. For simple tubular joints, the recommended detailing nomenclature and dimensioning are shown in Figure 6-1. This practice indicates that i an increased wall thickness o chord or special steel, is required, it should extend past the outside edge o incoming bracing a minimum o one quarter o the chord diameter or 300 mm, whichever is greater. Short chord can lengths can lead to a downgrading o joint resistance. The designer should consider speciying an increase o such chord can length to remove the need or resistance downgrading, see An increased wall thickness o brace or special steel, i required, should extend a minimum o one brace diameter or 600 mm, whichever is greater. Neither the cited chord can nor brace stub dimension includes the length over which thickness taper occurs. The minimum nominal gap between adjacent braces, whether in- or out-o-plane, is normally 50 mm. Care should be taken to ensure that overlap o welds at the toes o the joint is avoided. When overlapping braces occur, the amount o overlap should preerably be at least d/4 (where d is diameter o the through brace) or 150 mm, whichever is greater. This dimension is measured along the axis o the through member. Where overlapping o braces is necessary or preerred, the brace with the larger wall thickness should be the through brace and ully welded to the chord. Further, where substantial overlap occurs, the larger diameter brace should be speciied as the through member. This brace require an end stub to ensure that the thickness is at least equal to that o the overlapping brace. Comm Joint classiication Case (h) in Figure 6- is a good example o the actions and classiication hierarchy that should be adopted in the classiication o joints. Replacement o brace actions by a combination o tension and compression orce to give the same net action is not permitted. For example, replacing the orce in the horizontal brace on the let hand side o the joint by a compression orce o 1000 and tension orce o 500 is not permitted, as this may result in an inappropriate X classiication or this horizontal brace and a K classiication or the diagonal brace. Special consideration should be given to establish the proper gap i a portion o the action is related to K-joint behaviour. The most obvious case in Figure 6- is (a), or which the appropriate gap is between adjacent braces. However, i an intermediate brace exists, as in case (d), the appropriate gap is between the outer loaded braces. In this case, since the gap is oten large, the K-joint resistance could revert to that o a Y- joint. Case (e) is instructive in that the appropriate gap or the middle brace is gap 1, whereas or the top brace it is gap. Although the bottom brace is treated as 100 % K classiication, a weighted average in NORSOK standard Page 56 o 64

58 NORSOK standard N-004 Rev. 3, February 013 resistance is required, depending on how much o the acting axial orce in this brace is balanced by the middle brace (gap 1) and how much is balanced by the top brace (gap ). Comm Strength actor Qu The Q u term or tension orces is based on limiting the resistance to irst crack. Comm Chord action actor Q The Q actor is similar as in API RP-A /1/ or brace in compression or X-joints. For X joints with the brace in tension modiied values or the Q actor are recommended. The API ormulas are based on test and analyses only with the brace in compression (see /17/ and /18/) and it is judged that the inluence o chord stress on the joint strength will be dierent when the brace is in tension. As test and analytical results are not available the recommended Q actors in the case o X-joints with the brace in tension are established by the ollowing assumptions: For = 1.0 the Q actor or the case o brace in tension is assumed to be mirrored rom the compression case. For < 0.9 it is assumed that when the brace is in tension and the chord stress in compression the Q actor to be close to the case or the brace in compression with tension chord stress. (Mirrored) For the case with the brace in tension and the chord stress in tension the Q actor is assumed as mirror symmetry o the case with the chord stress in compression. See Figure ,8-0,6-0,4-0, 0 0, 0,4 0,6 0,8 1 Pc/Py Figure 1-6 ratios Q actor as a unction o chord load or brace tension and compression or two Comm Ringstiened joints Q- actor 1, 1 0,8 0,6 0,4 0, Compression Beta<0.9 Compression Beta=1.0 Tension Beta<0,9 Tension Beta= 1.0 For ring-stiened joints, the load eects determined by elastic theory will, in general, include local stress peaks. In the ULS check, such peaks may be reduced to mean values within limited areas. The extent o these areas shall be evaluated or the actual geometry. An assumed redistribution o stress should not lead to signiicant change in the equilibrium o the dierent parts o the joint. For example, i an action in a brace is resisted by a shear orce over a ring-stiener with an associated moment, a removal o local stress peaks should not imply signiicant reduction in this moment required or equilibrium. Ring-stiened joints may be designed according to plastic theory provided that all parts o the joint belong to cross section class 1 or (see EN or deinitions). Load eects may be determined by assuming relevant plastic collapse mechanisms. The characteristic resistance shall be determined by recognised methods o plastic theory. The design resistance is determined by dividing the characteristic resistance by M = The resistance may also be determined based on non-linear analysis. The computer programme used or such analysis should be validated as providing reliable results in comparison with other analysis and tests. NORSOK standard Page 57 o 64

59 NORSOK standard N-004 Rev. 3, February 013 Also the type o input to the analysis should be calibrated to provide reliable results. This includes type o element used, element mesh and material description in terms o stress strain relationship. Comm. 6.5 Strength o conical transitions The ormulas are based on ISO 1990, but the inluence o local bending stress is changed in revision 3. Similar to what is assumed in API RP-A /1/ it assumed that the ultimate strength limit need not be limited to yield in extreme ibre when the stresses rom local bending is included. In reality these stresses are not necessary or the connection to be in equilibrium with the external loads when ultimate strength is considered. It is uncertain how important the local bending stresses are or the ultimate capacity o the connection, but is assumed to be well to the conservative side to include the local bending moment according to plastic capacity o the shell wall. It is urthermore assumed that the local bending stresses are not detrimental to the buckling capacity o the tubular or the cone sections. Comm Local buckling under axial compression Platorms generally have a very small number o cones. Thus, it might be more expeditious to design the cones with a geometry such that the axial resistance is equal to that o yield, see Comm Junction yielding The resistance o the junction is checked according to von Mises yield criterion when the hoop stress is tensile. Comm Hoop buckling Hoop buckling is analysed similarly to that o a tubular subjected to external pressure using equivalent geometry properties. Comm Junction yielding and buckling The load eect rom external pressure is directly added to the existing stress at the junction or utilisation check with respect to yielding and buckling. Comm Junction rings without external hydrostatic pressure The resistance o stieners at a junction is checked as a ring where the eective area o the tubular and the cone is added to that o the ring section. Comm Junction rings with external hydrostatic pressure The required moment o inertia is derived as the sum o that required or the junction itsel and that due to external pressure. Comm Intermediate stiening rings Design o intermediate ring stieners within a cone is perormed along the same principles as used or design o ring stieners in tubulars. Comm. 6.7 Design o cylindrical shells A tubular section in air with diameter/thickness ratio larger than 60 is likely to ail by local buckling at an axial stress less than the material yield strength. Based on NS-ENV the upper limit o section class 3, where an axial stress equal yield strength can be achieved, is a D/t ratio o 1150/ y where y is material yield strength in MPa. The resistance o members ailing due to local buckling is more sensitive to geometric imperections than members that can sustain yielding over the thickness and allow some redistribution o local stresses due to yielding. The ailure o such members is normally associated with a descending postcritical behaviour that may be compared more with that o a brittle structure, i. e. the redistribution o load cannot be expected. Structures with this behaviour are denoted as shells. A deinition o a shell structure should not only include geometry and material resistance, but also loading as the axial resistance is reduced NORSOK standard Page 58 o 64

60 NORSOK standard N-004 Rev. 3, February 013 by e. g. increasing pressure. Design equations have been developed to account or dierent loading conditions, see /14/. The background or these design equations is given by //. Comm. 9 Accidental damage limit states Examples o ailure criteria are as ollows: - Critical deormation criteria deined by integrity o passive ire protection. To be considered or walls resisting explosion pressure and shall serve as ire barrier ater the explosion. - Critical delection or structures to avoid damage to process equipment (riser, gas pipe, etc). To be considered or structures or part o structures exposed to impact loads as ship collision, dropped object etc. - Critical deormation to avoid leakage o compartments. To be considered in case o impact against loating structures where the acceptable collision damage is deined by the minimum number o undamaged compartments to remain stable. - o0o - NORSOK standard Page 59 o 64

61 Annex A Rev.3, February 013 DESIGN OF STEEL STRUCTURES ANNEX A DESIGN AGAINST ACCIDENTAL ACTIONS NORSOK standard Page 60 o 64

62 Annex A Rev.3, February 013 CONTENTS A.1 SYMBOLS 63 A. GENERAL 66 A.3 SHIP COLLISIONS 67 A.3.1 General 67 A.3. Design principles 67 A.3.3 Collision mechanics 68 A Strain energy dissipation 68 A.3.3. Reaction orce to deck 69 A.3.4 Dissipation o strain energy 69 A.3.5 Ship collision orces 70 A Recommended orce-deormation relationships 70 A.3.5. Force contact area or strength design o large diameter columns. 71 A Energy dissipation in ship bow 7 A.3.6 Force-deormation relationships or denting o tubular members 73 A.3.7 Force-deormation relationships or beams 74 A General 74 A.3.7. Plastic orce-deormation relationships including elastic, axial lexibility 74 A Bending capacity o dented tubular members 76 A.3.8 Strength o connections 77 A.3.9 Strength o adjacent structure 77 A.3.10 Ductility limits 78 A General 78 A Local buckling 78 A Lateral stability at yield hinges 79 A Tensile Fracture 79 A Tensile racture in yield hinges 80 A.3.11 Resistance o large diameter, stiened columns 8 A General 8 A Longitudinal stieners 8 A Ring stieners 8 A Decks and bulkheads 8 A.3.1 Energy dissipation in loating production vessels 83 A.3.13 Global integrity during impact 83 A.4 DROPPED OBJECTS 84 A.4.1 General 84 A.4. Impact velocity 84 A.4.3 Dissipation o strain energy 86 A.4.4 Resistance/energy dissipation 86 A Stiened plates subjected to drill collar impact 86 A.4.4. Stieners/girders 87 A Dropped object 87 A.4.5 Limits or energy dissipation 87 A Pipes on plated structures 87 A.4.5. Blunt objects 87 A.5 FIRE 88 A.5.1 General 88 A.5. General calculation methods 88 A.5.3 Material modelling 88 A.5.4 Equivalent imperections 89 A.5.5 Empirical correction actor 89 A.5.6 Local cross sectional buckling 89 A.5.7 Ductility limits 89 A General 89 A.5.7. Beams in bending 89 A Beams in tension 90 A.5.8 Capacity o connections 90 NORSOK standard Page 61 o 64

63 Annex A Rev.3, February 013 A.6 EXPLOSIONS 91 A.6.1 General 91 A.6. Classiication o response 91 A.6.3 Failure modes or stiened panels 9 A.6.4 SDOF system analogy 94 A General 94 A.6.4. Dynamic response charts or SDOF system 97 A.6.5 MDOF analysis 99 A.6.6 Classiication o resistance properties 100 A Cross-sectional behaviour 100 A.6.6. Component behaviour 100 A.6.7 Idealisation o resistance curves 101 A.6.8 Resistance curves and transormation actors or plates 101 A Elastic - rigid plastic relationships 101 A.6.8. Axial restraint 103 A Tensile racture o yield hinges 103 A.6.9 Resistance curves and transormation actors or beams 103 A Beams with no- or ull axial restraint 104 A.6.9. Beams with partial end restraint. 108 A Eective lange 110 A Strength o adjacent structure 111 A Strength o connections 111 A Ductility limits 111 A.7 RESIDUAL STRENGTH 11 A.7.1 General 11 A.7. Modelling o damaged members 11 A.7..1 General 11 A.7.. Members with dents, holes, out-o-straightness 11 A.8 REFERENCES 113 A.9 COMMENTARY 114 NORSOK standard Page 6 o 64

64 Annex A Rev.3, February 013 A.1 SYMBOLS A Cross-sectional area A e A s A p A w B C D D Eective area o stiener and eective plate lange Area o stiener Projected cross-sectional area Shear area o stiener/girder Width o contact area Hydrodynamic drag coeicient Diameter o circular sections, plate stiness E Young's Modulus o elasticity, E p E kin E s F A w H I J K l K m K lm L M M P N P T N N Sd N Rd N P R R 0 V W P W a a s a i Plastic modulus Kinetic energy Strain energy Lateral load, total load Shear area o stiener/girder Non-dimensional plastic stiness Moment o inertia, impuls Mass moment o inertia Load transormation actor Mass transormation actor Load-mass transormation actor Beam length Total mass, cross-sectional moment Plastic bending moment resistance Plastic axial resistance Fundamental period o vibration Axial orce Design axial compressive orce Design axial compressive capacity Axial resistance o cross section Resistance Plastic collapse resistance in bending Volume, displacement Plastic section modulus Elastic section modulus Added mass Added mass or ship Added mass or installation NORSOK standard Page 63 o 64

65 Annex A Rev.3, February 013 b b c c c lp c s c Q c w Width o collision contact zone Flange width Factor Axial lexibility actor Plastic zone length actor Shear actor or vibration eigenperiod Shear stiness actor Displacement actor or strain calculation d Smaller diameter o threaded end o drill collar d c Characteristic dimension or strain calculation Generalised load y Characteristic yield strength u Ultimate material tensile strength g Acceleration o gravity, 9.81 m/s h w i k k k 1 ' k1 k Q k l m m s m i m eq m p r r c s s c s e t t d t t w v s v i v t Web height or stiener/girder Radius o gyration Generalised stiness Stiness, equivalent stiness, plate stiness Bending stiness in linear domain or beam Stiness in linear domain including shear deormation Shear stiness in linear domain or beam Temperature reduction o eective yield stress or maximum temperature in connection Plate length, beam length Distributed mass Ship mass Installation mass Equivalent mass Generalised mass Explosion pressure Radius o deormed area Plastic collapse resistance in bending or plate Distance, stiener spacing Characteristic distance Eective width o plate Thickness, time Duration o explosion Flange thickness Web thickness Velocity o ship Velocity o installation Terminal velocity NORSOK standard Page 64 o 64

66 Annex A Rev.3, February 013 w w c w d w x y y el z Deormation, displacement Characteristic deormation dent depth Non-dimensional deormation Axial coordinate Generalised displacement, displacement amplitude Generalised displacement at elastic limit Distance rom pivot point to collision point Plate aspect parameter Cross-sectional slenderness actor Yield strength actor, strain cr y Critical strain or rupture Yield strain Plate eigenperiod parameter Displacement shape unction Reduced slenderness ratio Ductility ratio Poisson's ratio, 0.3 Angle Density o steel, 7860 kg/m 3 w Density o sea water, 105 kg/m 3 Shear stress cr Critical shear stress or plate plugging Interpolation actor Plate stiness parameter NORSOK standard Page 65 o 64

67 Annex A Rev.3, February 013 A. GENERAL This Annex deals with the design to maintain the load-bearing unction o the structures during accidental events. The overall goal o the design against accidental actions is to achieve a system where the main saety unctions o the installation are not impaired. Design Accidental Actions and associated perormance criteria are determined by Quantiied Risk Assessment (QRA), see NORSOK N-003 /1/. In conjunction with design against accidental actions, perormance criteria may need to be ormulated such that the structure or components or sub-assemblies thereo - during the accident or within a certain time period ater the accident - shall not impair the main saety unctions such as: usability o escapeways, integrity o shelter areas, global load bearing capacity. The perormance criteria derived will typically be related to: energy dissipation local strength resistance to deormation (e.g braces in contact with risers/caissons, use o escape ways) endurance o ire protection ductility (allowable strains) - to avoid cracks in components, ire walls, passive ire protection etc. The inherent uncertainty o the requency and magnitude o the accidental loads as well as the approximate nature o the methods or determination analysis o accidental load eects shall be recognised. It is thereore essential to apply sound engineering judgement and pragmatic evaluations in the design. The material actor to be used or checks o accidental limit states is M = 1.0 unless noted otherwise. I determination o resistance is made according to NS-EN , NS-EN or NS-EN , the material actor should be determined as given in subclause 9.1. NORSOK standard Page 66 o 64

68 Annex A Rev.3, February 013 A.3 SHIP COLLISIONS A.3.1 General The ship collision action is characterised by a kinetic energy, governed by the mass o the ship, including hydrodynamic added mass and the speed o the ship at the instant o impact. Depending upon the impact conditions, a part o the kinetic energy may remain as kinetic energy ater the impact. The remainder o the kinetic energy has to be dissipated as strain energy in the installation and, possibly, in the vessel. Generally this involves large plastic strains and signiicant structural damage to either the installation or the ship or both. The strain energy dissipation is estimated rom orce-deormation relationships or the installation and the ship, where the deormations in the installation shall comply with ductility and stability requirements. The load bearing unction o the installation shall remain intact with the damages imposed by the ship collision action. In addition, the residual strength requirements given in Section A.7 shall be complied with. The structural eects rom ship collision may either be determined by non-linear dynamic inite element analyses or by energy considerations combined with simple elastic-plastic methods. I non-linear dynamic inite element analysis is applied all eects described in the ollowing paragraphs shall either be implicitly covered by the modelling adopted or subjected to special considerations, whenever relevant. Oten the integrity o the installation can be veriied by means o simple calculation models. I simple calculation models are used the part o the collision energy that needs to be dissipated as strain energy can be calculated by means o the principles o conservation o momentum and conservation o energy, reer Section A.3.3. It is convenient to consider the strain energy dissipation in the installation to take part on three dierent levels: local cross-section component/sub-structure total system Interaction between the three levels o energy dissipation shall be considered. Plastic modes o energy dissipation shall be considered or cross-sections and component/substructures in direct contact with the ship. Elastic strain energy can in most cases be disregarded, but elastic axial lexibility may have a substantial eect on the load-deormation relationships or components/sub-structures. Elastic energy may contribute signiicantly on a global level. A.3. Design principles With respect to the distribution o strain energy dissipation there may be distinguished between, see Figure A.3-1: strength design ductility design shared-energy design NORSOK standard Page 67 o 64

69 Annex A Rev.3, February 013 Energy dissipation Ductile design Shared-energy design Strength design ship installation Relative strength - installation/ship Figure A.3-1 Energy dissipation or strength, ductile and shared-energy design Strength design implies that the installation is strong enough to resist the collision orce with minor deormation, so that the ship is orced to deorm and dissipate the major part o the energy. Ductility design implies that the installation undergoes large, plastic deormations and dissipates the major part o the collision energy. Shared energy design implies that both the installation and ship contribute signiicantly to the energy dissipation. From calculation point o view strength design or ductility design is avourable. In this case the response o the «sot» structure can be calculated on the basis o simple considerations o the geometry o the «rigid» structure. In shared energy design both the magnitude and distribution o the collision orce depends upon the deormation o both structures. This interaction makes the analysis more complex. In most cases ductility or shared energy design is used. However, strength design may in some cases be achievable with little increase in steel weight. A.3.3 A Collision mechanics Strain energy dissipation The collision energy to be dissipated as strain energy may - depending on the type o installation and the purpose o the analysis - be taken as: Compliant installations E s 1 (m s a Fixed installations E 1 s )v s s (ms a s )vs Articulated columns 1 (m i v i 1 v s ms a 1 m a v i 1 v s a s ) msz 1 J Es s m s = ship mass a s = ship added mass s i (A.3.1) (A.3.) (A.3.3) NORSOK standard Page 68 o 64

70 Annex A Rev.3, February 013 v s = impact speed m i = mass o installation a i = added mass o installation v i = velocity o installation J = mass moment o inertia o installation (including added mass) with respect to eective pivot point z = distance rom pivot point to point o contact In most cases the velocity o the installation can be disregarded, i.e. v i = 0. The installation can be assumed compliant i the duration o impact is small compared to the undamental period o vibration o the installation. I the duration o impact is comparatively long, the installation can be assumed ixed. Jacket structures can normally be considered as ixed. Floating platorms (semi-submersibles, TLP s, production vessels) can normally be considered as compliant. Jack-ups may be classiied as ixed or compliant. A.3.3. Reaction orce to deck In the acceleration phase the inertia o the topside structure generates large reaction orces. An upper bound o the maximum orce between the collision zone and the deck or bottom supported installations may be obtained by considering the platorm compliant or the assessment o total strain energy dissipation and assume the platorm ixed at deck level when the collision response is evaluated. Figure A.3- A.3.4 Model or assessment o reaction orce to deck Dissipation o strain energy The structural response o the ship and installation can ormally be represented as load-deormation relationships as illustrated in Figure A.3-3. The strain energy dissipated by the ship and installation equals the total area under the load-deormation curves. dw s Figure A.3-3 Collision response E s,s Ship R s R i E s,i Installation Model dw i Dissipation o strain energy in ship and platorm NORSOK standard Page 69 o 64

71 Annex A Rev.3, February 013 E s E s,s E s,i w s,max R sdw s 0 0 w i,max R dw i i (A.3.4) As the load level is not known a priori an incremental procedure is generally needed. The load-deormation relationships or the ship and the installation are oten established independently o each other assuming the other object ininitely rigid. This method may have, however, severe limitations; both structures will dissipate some energy regardless o the relative strength. Oten the stronger o the ship and platorm will experience less damage and the soter more damage than what is predicted with the approach described above. As the soter structure deorms the impact orce is distributed over a larger contact area. Accordingly, the resistance o the strong structure increases. This may be interpreted as an "upward" shit o the resistance curve or the stronger structure (reer Figure A.3-3). Care should be exercised that the load-deormation curves calculated are representative or the true, interactive nature o the contact between the two structures. A.3.5 Ship collision orces A Recommended orce-deormation relationships Force-deormation relationships or a supply vessels with a displacement o 5000 tons are given in Figure A.3-4 or broad side -, bow-, stern end and stern corner impact or a vessel with stern roller. The curves or broad side and stern end impacts are based upon penetration o an ininitely rigid, vertical cylinder with a given diameter and may be used or impacts against jacket legs (D = 1.5 m) and large diameter columns (D = 10m). The curve or stern corner impact is based upon penetration o an ininitely rigid cylinder and may be used or large diameter column impacts. In lieu o more accurate calculations the curves in Figure A.3-4 may be used or square-rounded columns. For supply vessels and merchant vessels in the range o tons displacement, the orce deormation relationships given in Figure A.3-5 may be used or impacts against jacket legs with diameter 1.5 m.5 m. The orce deormation relationships given are or conventional supply ship without e.g. bow reinorcements or operations in ice. Impact orce (MN) Figure A.3-4 Stern corner Bow Broad side D = 10 m = 1.5 m Stern end D = 10 m = 1.5 m Indentation (m) Recommended-deormation curve or beam, bow and stern impact D D NORSOK standard Page 70 o 64

72 Annex A Rev.3, February 013 Figure A.3-5 Force -deormation relationship or bow with and without bulb ( dwt) The curve or bow impact is based upon collision with an ininitely rigid, plane wall and may be used or large diameter column impacts, but should not be used or signiicantly dierent collision events, e.g. impact against tubular braces. For beam -, stern end and stern corner impacts against jacket braces all energy shall normally be assumed dissipated by the brace, reer Comm. A Force [MN] b a 0 b a Figure A.3-6 Force -deormation relationship or tanker bow impact (~ dwt) Force-deormation relationships or tanker bow impact is given in Figure A.3-6 or the bulbous part and the superstructure, respectively. The curves may be used provided that the impacted structure (e.g. stern o loating production vessels) does not undergo substantial deormation i.e. strength design requirements are complied with. I this condition is not met interaction between the bow and the impacted structure shall be taken into consideration. Non-linear inite element methods or simpliied plastic analysis techniques o members subjected to axial crushing shall be employed /3/, /5/. A.3.5. Bulb orce Deormation [m] 1 10 Contact dimension [m] 6 0 b 4 10 Force superstructure a Force contact area or strength design o large diameter columns. The basis or the curves in Figure A.3-4 is strength design, i.e. limited local deormations o the installation at the point o contact. In addition to resisting the total collision orce, large diameter columns have to resist Force [MN] a b Deormation [m] Contact dimension [m] NORSOK standard Page 71 o 64

73 Annex A Rev.3, February 013 local concentrations (subsets) o the collision orce, given or stern corner impact in Table A.3-1 and stern end impact in Table A.3-. Table A.3-1 Local concentrated collision orce -evenly distributed over a rectangular area. Stern corner impact Contact area Force (MN) a(m) b (m) b a Table A.3- Local concentrated collision orce -evenly distributed over a rectangular area. Stern end impact Contact area Force (MN) a (m) b(m) I strength design is not aimed or - and in lieu o more accurate assessment (e.g. nonlinear inite element analysis) - all strain energy has to be assumed dissipated by the column, corresponding to indentation by an ininitely rigid stern corner. A Energy dissipation in ship bow For typical supply vessels bows and bows o merchant vessels o similar size (i.e tons displacement), energy dissipation in the ship bow may be taken into account provided that the collapse resistance in bending or the brace, R 0, see Section A.3.7, is according to the values given in Table A.3-3. The igures are valid or normal bows without ice strengthening and or brace diameters less than 1.5 m. The values should be used as step unctions, i.e. interpolation or intermediate resistance levels is not allowed. I contact location is not governed by operation conditions, size o ship and platorm etc., the values or arbitrary contact location shall be used. (see also Comm. A.3.5.3) Table A.3-3 Energy dissipation in bow versus brace resistance Contact location Energy dissipation in bow i brace resistance R 0 > 3 MN > 6 MN > 8 MN > 10 MN Above bulb 1 MJ 4 MJ 7 MJ 11 MJ First deck 0 MJ MJ 4 MJ 17 MJ First deck - oblique brace 0 MJ MJ 4 MJ 17 MJ Between 'cstle/irst deck 1 MJ 5 MJ 10 MJ 15 MJ Arbitrary loaction 0 MJ MJ 4 MJ 11 MJ In addition, the brace cross-section must satisy the ollowing compactness requirement y t 1.5 D 0.5 actor 3 where actor is the required resistance in [MN] given in Table A.3-3. b a (A.3.5) NORSOK standard Page 7 o 64

74 Annex A Rev.3, February 013 See Section A.3.6 or notation. I the brace is designed to comply with these provisions, special care should be exercised that the joints and adjacent structure is strong enough to support the reactions rom the brace. A.3.6 Force-deormation relationships or denting o tubular members The contribution rom local denting to energy dissipation is small or brace members in typical jackets and should be neglected. The resistance to indentation o unstiened tubes may be taken rom Figure A.3-7. Alternatively, the resistance may be calculated rom Equation (A.3.6): Figure A.3-7 R R c t D R c y 4 t B c1 1. D 1.95 c B 3.5 D k 1.0 N k 1.0 N k 0 Resistance curve or local denting c w d kc1 D Sd Rd 0. N N Sd Rd 0. N 0. N N 0.6 N Sd Rd Sd Rd 0.6 (A.3.6) NORSOK standard Page 73 o 64

75 Annex A Rev.3, February 013 N Sd = design axial compressive orce N Rd = design axial compressive resistance B = width o contact area w d = dent depth The curves are inaccurate or small indentation, and they should not be used to veriy a design where the w d dent damage is required to be less than D A.3.7 Force-deormation relationships or beams A General The response o a beam subjected to a collision load is initially governed by bending, which is aected by and interacts with local denting under the load. The bending capacity is also reduced i local buckling takes place on the compression side. As the beam undergoes inite deormations, the load carrying capacity may increase considerably due to the development o membrane tension orces. This depends upon the ability o adjacent structure to restrain the connections at the member ends to inward displacements. Provided that the connections do not ail, the energy dissipation capacity is either limited by tension ailure o the member or rupture o the connection. Simple plastic methods o analysis are generally applicable. Special considerations shall be given to the eect o : elastic lexibility o member/adjacent structure local deormation o cross-section local buckling strength o connections strength o adjacent structure racture A.3.7. Plastic orce-deormation relationships including elastic, axial lexibility Relatively small axial displacements have a signiicant inluence on the development o tensile orces in members undergoing large lateral deormations. An equivalent elastic, axial stiness may be deined as 1 K 1 K node EA (A.3.7) K node = axial stiness o the node with the considered member removed. This may be determined by introducing unit loads in member axis direction at the end nodes with the member removed. Plastic orce-deormation relationship or a central collision (midway between nodes) may be obtained rom : Figure A.3-8 or tubular members Figure A.3-9 or stiened plates in lieu o more accurate analysis. The ollowing notation applies: 4c1M P R 0 plastic collapse resistance in bending or the member, or the case that contact point is at mid span w w non-dimensional deormation c 1 w c 4c1Kw c c ya non-dimensional spring stiness c 1 = or clamped beams c 1 = 1 or pinned beams NORSOK standard Page 74 o 64

76 Annex A Rev.3, February 013 D w c characteristic deormation or tubular beams 1.WP w c A characteristic deormation or stiened plating W P = plastic section modulus = member length For non-central collisions the orce-deormation relationship may be taken as the mean value o the orcedeormation curves or central collision with member hal-length equal to the smaller and the larger portion o the member length, respectively. For members where the plastic moment capacity o adjacent members is smaller than the moment capacity o the impacted member the orce-deormation relationship may be interpolated rom the curves or pinned ends and clamped ends: R R clamped 1 R pinned (A.3.8) where actual R MP 4 (A.3.9) actual R 0 = Plastic resistance by bending action o beam accounting or actual bending resistance o adjacent members R actual 0 Pj 4M P M M M i Pj,i P P1 M P (A.3.10) M i = adjacent member no i, j = end number {1,} (A.3.11) M Pj,i = Plastic bending resistance or member no. i. Elastic, rotational lexibility o the node is normally o moderate signiicance NORSOK standard Page 75 o 64

77 Annex A Rev.3, February 013 6,5 6 5,5 R/R 0 5 4,5 4 3,5 3,5 c , k Bending & membrane Membrane only F (collision load) w k 1,5 1 0,5 Figure A.3-8 R/R 0 5 4,5 4 3,5 3,5 1,5 1 0,5 0 Figure A.3-9 A ,5 1 1,5,5 3 3,5 4 Deormation w Force-deormation relationship or tubular beam with axial lexibility c 1 0 0,5 1 1,5,5 3 3,5 4 Deormation 0.5 w 0. Force-deormation relationship or stiened plate with axial lexibility. Bending capacity o dented tubular members The reduction in plastic moment capacity due to local denting shall be considered or members in compression or moderate tension, but can be neglected or members entering the ully plastic membrane state. Conservatively, the lat part o the dented section according to the model shown in Figure A.3-10 may be assumed non-eective. This gives: k Bending & membrane Membrane only F (collision load) w k NORSOK standard Page 76 o 64

78 Annex A Rev.3, February 013 M M M red P P cos D y w arccos 1 D t 1 sin d (A.3.1) w d = dent depth as deined in Figure A Figure A.3-10 A.3.8 Reduction o moment capacity due to local dent Strength o connections Provided that large plastic strains can develop in the impacted member, the strength o the connections that the member rames into has to be checked. The resistance o connections should be taken rom ULS requirements in this standard or tubular joints and Eurocode 3 or NS347 or other joints. For braces reaching the ully plastic tension state, the connection shall be checked or a load equal to the axial resistance o the member. The design axial stress shall be assumed equal to the ultimate tensile strength o the material. I the axial orce in a tension member becomes equal to the axial capacity o the connection, the connection has to undergo gross deormations. The energy dissipation will be limited and rupture has to be considered at a given deormation. A sae approach is to assume disconnection o the member once the axial orce in the member reaches the axial capacity o the connection. I the capacity o the connection is exceeded in compression and bending, this does not necessarily mean ailure o the member. The post-collapse strength o the connection may be taken into account provided that such inormation is available. A.3.9 Strength o adjacent structure The strength o structural members adjacent to the impacted member/sub-structure must be checked to see whether they can provide the support required by the assumed collapse mechanism. I the adjacent structure ails, the collapse mechanism must be modiied accordingly. Since, the physical behaviour becomes more complex with mechanisms consisting o an increasing number o members it is recommended to consider a design which involves as ew members as possible or each collision scenario. NORSOK standard Page 77 o 64

79 Annex A Rev.3, February 013 A.3.10 Ductility limits A General The maximum energy that the impacted member can dissipate will ultimately - be limited by local buckling on the compressive side or racture on the tensile side o cross-sections undergoing inite rotation. I the member is restrained against inward axial displacement, any local buckling must take place beore the tensile strain due to membrane elongation overrides the eect o rotation induced compressive strain. I local buckling does not take place, racture is assumed to occur when the tensile strain due to the combined eect o rotation and membrane elongation exceeds a critical value. To ensure that members with small axial restraint maintain moment capacity during signiicant plastic rotation it is recommended that cross-sections be proportioned to Class 1 requirements, deined in Eurocode 3 (EN ). Initiation o local buckling does, however, not necessarily imply that the capacity with respect to energy dissipation is exhausted, particularly or Class 1 and Class cross-sections. The degradation o the crosssectional resistance in the post-buckling range may be taken into account provided that such inormation is available, reer Comm. A For members undergoing membrane stretching a lower bound to the post-buckling load-carrying capacity may be obtained by using the load-deormation curve or pure membrane action. A Local buckling Circular cross-sections: Buckling does not need to be considered or a beam with axial restraints i the ollowing condition is ulilled: 14c c1 where y D t 35 d c y 1 3 axial lexibility actor c c 1 c d c = characteristic dimension = D or circular cross-sections c 1 = or clamped ends = 1 or pinned ends c = non-dimensional spring stiness, reer Section A = the smaller distance rom location o collision load to adjacent joint (A.3.13) (A.3.14) (A.3.15) I this condition is not met, buckling may be assumed to occur when the lateral deormation exceeds w d c 1 c 1 14c 1 3 c 1 y dc For small axial restraint (c < 0.05) the critical deormation may be taken as (A.3.16) NORSOK standard Page 78 o 64

80 Annex A Rev.3, February 013 w d c 3.5 c y 3 1 d c (A.3.17) Stiened plates/ I/H-proiles: In lieu o more accurate calculations the expressions given or circular proiles in Eq. (A.3.16) and (A.3.17) may be used with d c = characteristic dimension or local buckling, equal to twice the distance rom the plastic neutral axis in bending to the extreme ibre o the cross-section = h height o cross-section or symmetric I proiles = h w or stiened plating (or simplicity) For langes subjected to compression;.5 3 b b t 35 t 35 y For webs subjected to bending A h w t w 35 h w t w 35 y y y b = lange width t = lange thickness h w = web height t w = web thickness class 1 cross-sections class and class 3 cross-sections class 1 cross-sections class and class 3 cross-sections Lateral stability at yield hinges (A.3.18) (A.3.19) (A.3.0) (A.3.1) The compressed part o I or H beams needs to be laterally supported to avoid instability. Unless more accurate investigations are undertaken, H and I beams may be considered to be stable when a yield hinge is ormed i the length between lateral supports are less than: L0,*b A y A EA 0.33A b = lange width A = lange area A w = web area Tensile Fracture w (A.3.) The degree o plastic deormation or critical strain at racture will show a signiicant scatter and depends upon the ollowing actors: NORSOK standard Page 79 o 64

81 Annex A Rev.3, February 013 material toughness presence o deects strain rate presence o strain concentrations The critical strain or plastic deormations o sections containing deects need to be determined based on racture mechanics methods. (See chapter 6.5.) Welds normally contain deects and welded joints are likely to achieve lower toughness than the parent material. For these reasons structures that need to undergo large plastic deormations should be designed in such a way that the plastic straining takes place away outside the weld. In ordinary ull penetration welds, the overmatching weld material will ensure that minimal plastic straining occurs in the welded joints even in cases with yielding o the gross cross section o the member. In such situations, the critical strain will be in the parent material and will be dependent upon the ollowing parameters: stress gradients dimensions o the cross section presence o strain concentrations material yield to tensile strength ratio material ductility Simple plastic theory does not provide inormation on strains as such. Thereore, strain levels should be assessed by means o adequate analytic models o the strain distributions in the plastic zones or by nonlinear inite element analysis with a suiciently detailed mesh in the plastic zones. When structures are designed so that yielding take place in the parent material, the ollowing value or the critical average strain in axially loaded plate material may be used in conjunction with nonlinear inite element analysis or simple plastic analysis cr where: t y t = plate thickness = length o plastic zone. Minimum 5t y = yield stress in MPa A Tensile racture in yield hinges (A.3.3) When the orce deormation relationships or beams given in Section A.3.7. are used rupture may be assumed to occur when the deormation exceeds a value given by w d c c 1 4cwccr 1 1 c c1 where the ollowing actors are deined; Displacement actor c w 1 c c 1 lp 1 1 c 3 plastic zone length actor lp 4 1 W W P y cr d c (A.3.4) (A.3.5) NORSOK standard Page 80 o 64

82 Annex A Rev.3, February 013 c lp cr 1 y cr 1 y W W W W P P H H 1 (A.3.6) axial lexibility actor c c 1 c (A.3.7) non-dimensional plastic stiness Ep H E 1 E cr cr y y c 1 = or clamped ends = 1 or pinned ends c = non-dimensional spring stiness, reer Section A.3.7. l 0.5l the smaller distance rom location o collision load to adjacent joint W = elastic section modulus W P = plastic section modulus cr = critical strain or rupture y y E yield strain y = yield strength cr = strength corresponding to cr The characteristic dimension shall be taken as: d c = D diameter o tubular beams = h w twice the web height or stiened plates = h height o cross-section or symmetric I-proiles For small axial restraint (c < 0.05) the critical deormation may be taken as w d c c w cr (A.3.8) (A.3.9) The critical strain cr and corresponding strength cr should be selected so that idealised bi-linear stress-strain relation gives reasonable results. See Commentary. For typical steel material grades the ollowing values are proposed: Table A.3-4 Proposed values or e cr and H or dierent steel grades Steel cr H grade S 35 0 % 0.00 S % S % NORSOK standard Page 81 o 64

83 Annex A Rev.3, February 013 A.3.11 Resistance o large diameter, stiened columns A General Impact on a ring stiener as well as midway between ring stieners shall be considered. Plastic methods o analysis are generally applicable. A Longitudinal stieners For ductile design the resistance o longitudinal stieners in the beam mode o deormation can be calculated using the procedure described or stiened plating, section A.3.7. For strength design against stern corner impact, the plastic bending moment capacity o the longitudinal stieners has to comply with the requirement given in Figure A.3-11, on the assumption that the entire load given in Table A.3-1 is taken by one stiener. 3 Figure A.3-11 A Required bending capacity o longitudinal stieners Ring stieners In lieu o more accurate analysis the plastic collapse load o a ring-stiener can be estimated rom: F 0 where Plastic bending capacity (MNm) 4 M w c D P W P w c = characteristic deormation o ring stiener A e (A.3.30) D = column radius M P = plastic bending resistance o ring-stiener including eective shell lange W P = plastic section modulus o ring stiener including eective shell lange A e = area o ring stiener including eective shell lange Eective lange o shell plating: Use eective lange o stiened plates, see Chapter 6. For ductile design it can be assumed that the resistance o the ring stiener is constant and equal to the plastic collapse load, provided that requirements or stability o cross-sections are complied with, reer Section A A Distance between ring stieners (m) Decks and bulkheads Calculation o energy dissipation in decks and bulkheads has to be based upon recognised methods or plastic analysis o deep, axial crushing. It shall be documented that the collapse mechanisms assumed yield a realistic representation o the true deormation ield. NORSOK standard Page 8 o 64

84 Annex A Rev.3, February 013 A.3.1 Energy dissipation in loating production vessels For strength design the side or stern shall resist crushing orce o the bow o the o-take tanker. In lieu o more accurate calculations the orce-deormation curve given in Section A.3.5. may be applied. For ductile design the resistance o stiened plating in the beam mode o deormation can be calculated using the procedure described in section A Calculation o energy dissipation in stringers, decks and bulkheads subjected to gross, axial crushing shall be based upon recognised methods or plastic analysis, eg. /3/ and /5/. It shall be documented that the olding mechanisms assumed yield a realistic representation o the true deormation ield. A.3.13 Global integrity during impact Normally, it is unlikely that the installation will turn into a global collapse mechanism under direct collision load, because the collision load is typically an order o magnitude smaller than the resultant design wave orce. Linear analysis oten suices to check that global integrity is maintained. The installation should be checked or the maximum collision orce. For installations responding predominantly statically the maximum collision orce occurs at maximum deormation. For structures responding predominantly impulsively the maximum collision orce occurs at small global deormation o the platorm. An upper bound to the collision orce is to assume that the installation is ixed with respect to global displacement. (e.g. jack-up ixed with respect to deck displacement) NORSOK standard Page 83 o 64

85 Annex A Rev.3, February 013 A.4 DROPPED OBJECTS A.4.1 General The dropped object action is characterised by a kinetic energy, governed by the mass o the object, including any hydrodynamic added mass, and the velocity o the object at the instant o impact. In most cases the major part o the kinetic energy has to be dissipated as strain energy in the impacted component and, possibly, in the dropped object. Generally, this involves large plastic strains and signiicant structural damage to the impacted component. The strain energy dissipation is estimated rom orce-deormation relationships or the component and the object, where the deormations in the component shall comply with ductility and stability requirements. The load bearing unction o the installation shall remain intact with the damages imposed by the dropped object action. In addition, the residual strength requirements given in Section A.7 shall be complied with. Dropped objects are rarely critical to the global integrity o the installation and will mostly cause local damages. The major threat to global integrity is probably puncturing o buoyancy tanks, which could impair the hydrostatic stability o loating installations. Puncturing o a single tank is normally covered by the general requirements to compartmentation and watertight integrity given in ISO and NORSOK N-001. The structural eects rom dropped objects may either be determined by non-linear dynamic inite element analyses or by energy considerations combined with simple elastic-plastic methods as given in Sections A.4. - A.4.5. I non-linear dynamic inite element analysis is applied all eects described in the ollowing paragraphs shall either be implicitly covered by the modelling adopted or subjected to special considerations, whenever relevant. A.4. Impact velocity The kinetic energy o a alling object is given by E kin 1 mv or objects alling in air and, E kin 1 m av or objects alling in water. a = hydrodynamic added mass or considered motion For impacts in air the velocity is given by v gs s = travelled distance rom drop point v = v o at sea surace (A.4.1) (A.4.) (A.4.3) For objects alling rectilinearly in water the velocity depends upon the reduction o speed during impact with water and the alling distance relative to the characteristic distance or the object. NORSOK standard Page 84 o 64

86 Annex A Rev.3, February 013 Figure A.4-1 Velocity proile or objects alling in water The loss o momentum during impact with water is given by m v 0 t d F t dt F(t) = orce during impact with sea surace Ater the impact with water the object proceeds with the speed v v 0 (A.4.4) Assuming that the hydrodynamic resistance during all in water is o drag type the velocity in water can be taken rom Figure A.4-1 where v s t c g m wv C A w m a C A w d p d p a v t 1 m wv g1 m = terminal velocity or the object = characteristic distance w = density o sea water C d = hydrodynamic drag coeicient or the object in the considered motion m = mass o object A p = projected cross-sectional area o the object V = object displacement The major uncertainty is associated with calculating the loss o momentum during impact with sea surace, given by Equation (A.4.4). However, i the travelled distance is such that the velocity is close to the terminal velocity, the impact with sea surace is o little signiicance. Typical terminal velocities or some typical objects are given in Table A.4-1 NORSOK standard Page 85 o 64

87 Annex A Rev.3, February 013 Table A.4-1 Terminal velocities or objects alling in water. Item Mass [kn] Terminal velocity [m/s] Drill collar Winch, Riser pump BOP annular preventer Mud pump Rectilinear motion is likely or blunt objects and objects which do not rotate about their longitudinal axis. Barlike objects (e.g. pipes) which do not rotate about their longitudinal axis may execute lateral, damped oscillatory motions as illustrated in Figure A.4-1. A.4.3 Dissipation o strain energy The structural response o the dropped object and the impacted component can ormally be represented as load-deormation relationships as illustrated in Figure A.4-. The part o the impact energy dissipated as strain energy equals the total area under the load-deormation curves. E s E s,o E s,i w max R odw o 0 o, wi,max 0 R dw i i (A.4.5) As the load level is not known a priori an incremental approach is generally required. Oten the object can be assumed to be ininitely rigid (e.g axial impact rom pipes and massive objects) so that all energy is to be dissipated by the impacted component.. Figure A.4- Dissipation o strain energy in dropped object and installation I the object is assumed to be deormable, the interactive nature o the deormation o the two structures should be recognised. A.4.4 A Resistance/energy dissipation Stiened plates subjected to drill collar impact The energy dissipated in the plating subjected to drill collar impact is given by E sp dw o E s,o Object R m 1 i 0.48 K m R o R i E s,i Installation dw i (A.4.6) NORSOK standard Page 86 o 64

88 Annex A Rev.3, February 013 K y t d r 6c 1 c 6.5 d r y = characteristic yield strength c e d.51 r R = contact orce or cr m t : stiness o plate enclosed by hinge circle reer Section A or cr i p = mass o plate enclosed by hinge circle m = mass o dropped object p = mass density o steel plate d = smaller diameter at threaded end o drill collar r = smaller distance rom the point o impact to the plate boundary deined by adjacent stieners/girders, reer Figure A.4-3. For validity range o design ormula see Commentary. Figure A.4-3 A.4.4. Deinition o distance to plate boundary Stieners/girders In lieu o more accurate calculations stieners and girders subjected to impact with blunt objects may be analysed with resistance models given in Section A.6.9. A Dropped object Calculation o energy dissipation in deormable dropped objects shall be based upon recognised methods or plastic analysis. It shall be documented that the collapse mechanisms assumed yield a realistic representation o the true deormation ield. A.4.5 A Limits or energy dissipation Pipes on plated structures The maximum shear stress or plugging o plates due to drill collar impacts may be taken as cr r u A u t d = ultimate material tensile strength Blunt objects r For stability o cross-sections and tensile racture, reer Section A r (A.4.7) NORSOK standard Page 87 o 64

89 Annex A Rev.3, February 013 A.5 FIRE A.5.1 General The characteristic ire structural action is temperature rise in exposed members. The temporal and spatial variation o temperature depends on the ire intensity, whether or not the structural members are ully or partly enguled by the lame and to what extent the members are insulated. Structural steel expands at elevated temperatures and internal stresses are developed in redundant structures. These stresses are most oten o moderate signiicance with respect to global integrity. The heating causes also progressive loss o strength and stiness and is, in redundant structures, accompanied by redistribution o orces rom members with low strength to members that retain their load bearing capacity. A substantial loss o load-bearing capacity o individual members and subassemblies may take place, but the load bearing unction o the installation shall remain intact during exposure to the ire action. In addition, the residual strength requirements given in Section A.7 shall be complied with. Structural analysis may be perormed on either individual members subassemblies entire system The assessment o ire load eect and mechanical response shall be based on either simple calculation methods applied to individual members, general calculation methods, or a combination. Simple calculation methods may give overly conservative results. General calculation methods are methods in which engineering principles are applied in a realistic manner to speciic applications. Assessment o individual members by means o simple calculation methods should be based upon the provisions given in Eurocode 3 Part 1.. //. Assessment by means o general calculation methods shall satisy the provisions given in Eurocode 3 Part1., Section 4.3. In addition, the requirements given in this section or mechanical response analysis with nonlinear inite element methods shall be complied with. Assessment o ultimate strength is not needed i the maximum steel temperature is below 400 o C, but deormation criteria may have to be checked or impairment o main saety unctions. A.5. General calculation methods Structural analysis methods or non-linear, ultimate strength assessment may be classiied as stress-strain based methods stress-resultants based (yield/plastic hinge) methods Stress-strain based methods are methods where non-linear material behaviour is accounted or on ibre level. Stress-resultants based methods are methods where non-linear material behaviour is accounted or on stress-resultants level based upon closed orm solutions/interaction equation or cross-sectional orces and moments. A.5.3 Material modelling In stress-strain based analysis temperature dependent stress-strain relationships given in Eurocode 3, Part 1., Section 3. may be used. For stress resultants based design the temperature reduction o the elastic modulus may be taken as k E, according to Eurocode 3. The yield stress may be taken equal to the eective yield stress, y, The temperature reduction o the eective yield stress may be taken as k y,. Provided that above the above requirements are complied with creep does need explicit consideration. NORSOK standard Page 88 o 64

90 Annex A Rev.3, February 013 A.5.4 Equivalent imperections To account or the eect o residual stresses and lateral distortions compressive members shall be modelled with an initial, sinusoidal imperection with amplitude given by Elasto-plastic material models * e Y E i z Elastic-plastic material models: * e Wp Y i Y W E z E 0 0 W P AI = 0.5 or ire exposed members according to column curve c, Eurocode 3 i = radius o gyration z 0 = distance rom neutral axis to extreme ibre o cross-section W P = plastic section modulus W = elastic section modulus A = cross-sectional area I = moment o inertia e * = amplitude o initial distortion = member length The initial out-o-straightness should be applied on each physical member. I the member is modelled by several inite elements the initial out-o-straightness should be applied as displaced nodes. The initial out-o-straightness shall be applied in the same direction as the deormations caused by the temperature gradients. A.5.5 Empirical correction actor The empirical correction actor o 1. has to be accounted or in calculating the critical strength in compression and bending or design according to Eurocode 3, reer Comm. A.5.5. A.5.6 Local cross sectional buckling I shell modelling is used, it shall be veriied that the sotware and the modelling is capable o predicting local buckling with suicient accuracy. I necessary, local shell imperections have to be introduced in a similar manner to the approach adopted or lateral distortion o beams I beam modelling is used local cross-sectional buckling shall be given explicit consideration. In lieu o more accurate analysis cross-sections subjected to plastic deormations shall satisy compactness requirements given in Eurocode 3 : class 1 : Locations with plastic hinges (approximately ull plastic utilization) class : Locations with yield hinges (partial plastiication) I this criterion is not complied with explicit considerations shall be perormed. The load-bearing capacity will be reduced signiicantly ater the onset o buckling, but may still be signiicant. A conservative approach is to remove the member rom urther analysis. Compactness requirements or class 1 and class cross-sections may be disregarded provided that the member is capable o developing signiicant membrane orces. A.5.7 A Ductility limits General The ductility o beams and connections increase at elevated temperatures compared to normal conditions. Little inormation exists. A.5.7. Beams in bending In lieu o more accurate analysis requirements given in Section A.3.10 are to be complied with. NORSOK standard Page 89 o 64

91 Annex A Rev.3, February 013 A Beams in tension In lieu o more accurate analysis an average elongation o 3% o the member length with a reasonably uniorm temperature can be assumed. Local temperature peaks may localise plastic strains. It is considered to be to the conservative side to use critical strains or steel under normal temperatures. See A and A A.5.8 Capacity o connections In lieu o more accurate calculations the capacity o the connection can be taken as: R k y, R 0 where R 0 = capacity o connection at normal temperature k y, = temperature reduction o eective yield stress or maximum temperature in connection NORSOK standard Page 90 o 64

92 Annex A Rev.3, February 013 A.6 EXPLOSIONS A.6.1 General Explosion loads are characterised by temporal and spatial pressure distribution. The most important temporal parameters are rise time, maximum pressure and pulse duration. For components and sub-structures the explosion pressure shall normally be considered uniormly distributed. On global level the spatial distribution is normally nonuniorm both with respect to pressure and duration. The response to explosion loads may either be determined by non-linear dynamic inite element analysis or by simple calculation models based on Single Degree O Freedom (SDOF) analogies and elastic-plastic methods o analysis. I non-linear dynamic inite element analysis is applied all eects described in the ollowing paragraphs shall either be implicitly covered by the modelling adopted or subjected to special considerations, whenever relevant In the simple calculation models the component is transormed to a single spring-mass system exposed to an equivalent load pulse by means o suitable shape unctions or the displacements in the elastic and elastic-plastic range. The shape unctions allow calculation o the characteristic resistance curve and equivalent mass in the elastic and elastic-plastic range as well as the undamental period o vibration or the SDOF system in the elastic range. Provided that the temporal variation o the pressure can be assumed to be triangular, the maximum displacement o the component can be calculated rom design charts or the SDOF system as a unction o pressure duration versus undamental period o vibration and equivalent load amplitude versus maximum resistance in the elastic range. The maximum displacement must comply with ductiliy and stability requirements or the component. The load bearing unction o the installation shall remain intact with the damages imposed by the explosion loads. In addition, the residual strength requirements given in Section A.7 shall be complied with. A.6. Classiication o response The response o structural components can conveniently be classiied into three categories according to the duration o the explosion pressure pulse, t d, relative to the undamental period o vibration o the component, T: Impulsive domain - t d /T< 0.3 Dynamic domain < t d /T < 3 Quasi-static domain - 3 < t d /T Impulsive domain: The response is governed by the impulse deined by I t d 0 F t dt (A.6.1) Hence, the structure may resist a very high peak pressure provided that the duration is suiciently small. The maximum deormation, w max, o the component can be calculated iteratively rom the equation I m eq 0 w max R w dw where R(w) = orce-deormation relationship or the component m eq = equivalent mass or the component (A.6.) NORSOK standard Page 91 o 64

93 Annex A Rev.3, February 013 Quasi-static-domain: The response is governed by the peak pressure and the rise time o the pressure relative to the undamental period o vibration. I the rise time is small the maximum deormation o the component can be solved iteratively rom the equation: w max 1 F max 0 w max R w dw (A.6.3) I the rise time is large the maximum deormation can be solved rom the static condition F max R(w max ) (A.6.4) Dynamic domain: The response has to be solved rom numerical integration o the dynamic equations o equilibrium. A.6.3 Failure modes or stiened panels Various ailure modes or a stiened panel are illustrated in Figure A.6-1. Suggested analysis model and reerence to applicable resistance unctions are listed in Table A.6-1. Application o a Single Degree o Freedom (SDOF) model in the analysis o stieners/girders with eective lange is implicitly based on the assumption that dynamic inter action between the plate lange and the proile can be neglected. NORSOK standard Page 9 o 64

94 Annex A Rev.3, February 013 Beam collapse plate elastic Figure A.6-1 Plastic deormation o plate Girder collapse beam elastic- plate elastic or plastic Failure modes or two-way stiened panel Beam collapse plate plastic (i) (ii) Girder and beam collapse plate elastic (i) or plastic (ii) NORSOK standard Page 93 o 64

95 Annex A Rev.3, February 013 Table A.6-1 Failure mode Elastic-plastic deormation o plate Stiener plastic plate elastic Stiener plastic plate plastic Girder plastic stiener and plating elastic Girder plastic stiener elastic plate plastic Girder and stiener plastic plate elastic Girder and stiener plastic plate plastic Analysis models Simpliied analysis model SDOF SDOF SDOF SDOF SDOF MDOF MDOF Resistance models Section A.6.8 Stiener: Section A Plate: Section A Stiener: Section A Plate: Section A.6.8 Girder: Section.A Plate: Section A.6.8 Girder: Section A Plate: Section A.6.8 Girder and stiener: Section A Plate: Section A.6.8 Girder and stiener: Section A Plate: Section A.6.8 Comment Elastic, eective lange o plate Eective width o plate at mid span. Elastic, eective lange o plate at ends. Elastic, eective lange o plate with concentrated loads (stiener reactions). Stiener mass included. Eective width o plate at girder mid span and ends. Stiener mass included Dynamic reactions o stieners loading or girders Dynamic reactions o stieners loading or girders By girder/stiener plastic is understood that the maximum displacement w max exceeds the elastic limit w el A.6.4 A SDOF system analogy General Biggs method: For many practical design problems it is convenient to assume that the structure - exposed to the dynamic pressure pulse - ultimately attains a deormed coniguration comparable to the static deormation pattern. Using the static deormation pattern as displacement shape unction, i.e. w x, t xyt the dynamic equations o equilibrium can be transormed to an equivalent single degree o reedom system: m y ky (t) (x) = displacement shape unction y(t) = displacement amplitude m mx dx M = generalized mass x i i ( t) q( t) dx = generalized load k EI i F i i i, xx x dx = generalized elastic bending stiness k 0 = generalized plastic bending stiness k N x (ully developed mechanism), x dx = generalized membrane stiness (ully plastic: N = N P ) m = distributed mass M i = concentrated mass q = explosion load F i = concentrated load (e.g. support reactions) NORSOK standard Page 94 o 64

96 Annex A Rev.3, February 013 x i = position o concentrated mass/load x i x i The equilibrium equation can alternatively be expressed as: (K m, um u K m,cm c )y l l k(y)y F(t) (A.6.5) where K K K K K m,u l m, u = load-mass transormation actor or uniorm mass K l K m,c l m, c = load-mass transormation actor or concentrated mass K l m,u m,c l i m(x) M M M i c u dx = mass transormation actor or uniorm mass = mass transormation actor or concentrated mass q(x)dx Kl l F Fi i i K F Mu mdx M M = total concentrated mass F F = load transormation actor or uniormly distributed load l = load transormation actor or concentrated load c i i = total uniormly, distributed mass qdx = total load in case o uniormly distributed load F = total load in case o concentrated load i i k k e = equivalent stiness k l The natural period o vibration or the equivalent system in the linear resistance domain is given by T m k K l m,u M u K k e l m,c M c (A.6.6) The response, y(t), is - in addition to the load history - entirely governed by the total mass, load-mass actor and the equivalent stiness. For a linear system, the load mass actor and the equivalent stiness are constant k 1 = k e. The response is then alternatively governed by the eigenperiod and the equivalent stiness. For a non-linear system, the load-mass actor and the equivalent stiness depend on the response (deormations). Non-linear systems may oten conveniently be approximated by equivalent bi-linear or trilinear systems, see Section A.6.7. In such cases the response can be expressed in terms o (see Figure A.6-10 or deinitions): k 1 = equivalent stiness in the initial, linear resistance domain y el = displacement at the end o the initial, linear resistance domain NORSOK standard Page 95 o 64

97 Annex A Rev.3, February 013 T = eigenperiod in the initial, linear resistance domain and, i relevant, k 3 = normalised equivalent resistance in the third linear resistance domain. Equivalent stinesses are given explicitly or can be derived rom load-deormation relationships given in Section A.6.9. I the response is governed by dierent mechanical behaviour relevant equivalent stinesses must be calculated. For a given explosion load history the maximum displacement, y max, is ound by analytical or numerical integration o equation (A.6.6). For standard load histories and standard resistance curves maximum displacements can be presented in design charts. Figure A.6- shows the normalised maximum displacement o a SDOF system with a bi- (k 3 =0) or trilinear (k 3 > 0) resistance unction, exposed to a triangular pressure pulse with zero rise time. When the duration o the pressure pulse relative to the eigenperiod in the initial, linear resistance range is known, the maximum displacement can be determined directly rom the diagram as illustrated in Figure A ,1 Figure A.6- F max /R el = Impulsive asymptote, k 3 =0.k 1 ymax/yel or system F(t) t d Static asymptote, k 3 =0.k 1 Elastic-perectly plastic, k 3 =0 td/t or system k 3 =0.1k 1 k 3 =0.k 1 0, t d /T Maximum response a SDOF system to a triangular pressure pulse with zero rise time. Design charts or systems with bi- or tri-linear resistance curves subjected to a triangular pressure pulse with dierent rise time are given in Figures A A6-7. Baker's method The governing equations (A.6.1) and (A.6.) or the maximum response in the impulsive domain and the quasi-static domain may also be used along with response charts or maximum displacement or dierent F max /R el ratios to produce pressure-impulse (F max,i) diagrams - iso-damage curves - provided that the NORSOK standard Page 96 o 64

98 Annex A Rev.3, February 013 maximum pressure is known. Figure A.6-3 shows such a relationship obtained or an elastic-perectly plastic system when the maximum dynamic response is y max /y el =10. Pressure-impulse combinations to the let and below o the iso-damage curve represent admissible events, to the right and above inadmissible events. The advantage o using iso-damage diagrams is that "back-ward" calculations are possible: The diagram is established on the basis o the resistance curve. The inormation may be used to screen explosion pressure histories and eliminate those that obviously lie in the admissible domain. This will reduce the need or large complex simulation o explosion scenaries Impulsive Figure A.6-3 A.6.4. Iso-damage curve or ymax/yel = 10. Triangular pressure pulse. Dynamic response charts or SDOF system Transormation actors or elastic plastic-membrane deormation o beams and one-way slabs with dierent boundary conditions are given in Table A.6- Maximum displacement or a SDOF system exposed to dierent pressure histories are displayed in Figures A A.6-7. The response o the system is based upon the resistance in the linear range, k e =k 1, where the equivalent stiness is determined rom the elastic solution to the actual system. I the displacement shape unction changes as a non-linear structure undergoes deormation the transormation actors change. In lieu o accurate analysis an average value o the combined load-mass transormation actor can be used:. K 7 Pressure F/R average lm Iso-damage K elastic lm Pressure Impulse I/(RT) 1 K plastic lm = y max /y el ductility ratio Since is not known a priori iterative calculations may be necessary. (A.6.7) NORSOK standard Page 97 o 64

99 Annex A Rev.3, February R el /F max =0.05 =0.1 = 0.3 = 0.5 = 0.6 = 0.7 R el /F max = 0.8 = = 1.0 y max/ y el = 1.1 = 1. = Figure A y max/ y el k 3 = 0 k 3 = 0.1k 1 k 3 = 0.k 1 k 3 = 0.5k 1 Dynamic response o a SDOF system to a triangular load (rise time=0) R el /F max =0.05 =0.1 Figure A.6-5 Dynamic response o a SDOF system to a triangular load (rise time=0.15t d ) t d /T F F max t d R R el k 3 = 0.5k 1 =0.k 1 =0.1k 1 = 1.0 = = 1. = 1.5 F R k 3 = 0.5k 1 =0.k 1 =0.1k 1 k 3 = 0 k 3 = 0.1k 1 F max R el k 3 = 0.k 1 k 1 k 3 = 0.5k td t d y el y t d /T = 0.3 k 1 y el = 0.5 = 0.6 y = 0.7 R el /F max = 0.8 = 0.9 NORSOK standard Page 98 o 64

100 Annex A Rev.3, February R el /F max =0.05 =0.1 = 0.3 = 0.5 = 0.6 = 0.7 R el /F max = y max/ y el = Figure A.6-6 Dynamic response o a SDOF system to a triangular load (rise time=0.30t d ) y max/ y el Figure A.6-7 Dynamic response o a SDOF system to a triangular load (rise time=0.50t d ) A.6.5 MDOF analysis k 3 = 0 k 3 = 0.1k 1 k 3 = 0.k 1 k 3 = 0.5k 1 SDOF analysis o built-up structures (e.g. stieners supported by girders) is admissible i t d /T F F max 0.30td R el /F max =0.05 =0.1 = 0.3 t d R R el k 3 = 0.5k 1 =0.k 1 =0.1k 1 k 1 y el y = 1.0 = 1.1 = 1. = = 1.0 = 1.1 = 1. F R k 3 = 0.5k 1 =0.k 1 =0.1k 1 = 1.5 k 3 = 0 k 3 = 0.1k 1 F max R el k 3 = 0.k 1 k 1 k 3 = 0.5k td t d y el y t d /T = 0.5 = 0.6 = 0.7 R el /F max = 0.8 = 0.9 NORSOK standard Page 99 o 64

101 Annex A Rev.3, February the undamental periods o elastic vibration are suiciently separated - the response o the component with the smallest eigenperiod does not enter the elastic-plastic domain so that the period is drastically increased I these conditions are not met, then signiicant interaction between the dierent vibration modes is anticipated and a multi degree o reedom analysis is required with simultaneous time integration o the coupled system. A.6.6 A Classiication o resistance properties Cross-sectional behaviour Moment elasto-plastic Figure A.6-8 Curvature Bending moment-curvature relationships Elasto-plastic : The eect o partial yielding on bending moment accounted or Elastic-perectly plastic: Linear elastic up to ully plastic bending moment The simple models described herein assume elastic-perectly plastic material behaviour. Any strain hardening may be accounted or by equivalent (increased) yield stress. A.6.6. R Figure A.6-9 Component behaviour k k R R R k k 3 k 1 k 1 k k 1 1 Elastic w w w w Resistance curves Elastic-plastic (determinate) Elastic-plastic (indeterminate) Elastic-plastic with membrane Elastic: Elastic material, small deormations Elastic-plastic (determinate): Elastic-perectly plastic material. Statically determinate system. Bending mechanism ully developed with occurrence o irst plastic hinge(s)/yield lines. No axial restraint Elastic-plastic (indeterminate): Elastic perectly plastic material. Statically indeterminate system. Bending mechanism develops with sequential ormation o plastic hinges/yield lines. No axial restraint. For simpliied analysis this system may be modelled as an elastic-plastic (determinate) system with equivalent initial stiness. In lieu o more accurate analysis the equivalent stiness should be determined such that the area under the resistance curve is preserved. Elastic-plastic with membrane: Elastic-perectly plastic material. Statically determinate (or indeterminate). Ends restrained against axial displacement. Increase in load-carrying capacity caused by development o membrane orces. NORSOK standard Page 100 o 64

102 Annex A Rev.3, February 013 A.6.7 Idealisation o resistance curves The resistance curves in clause A.6.6 are idealised. For simpliied SDOF analysis the resistance characteristics o a real non-linear system may be approximately modelled. An example with a tri-linear approximation is illustrated in Figure A R k 3 R el k =0 Figure A.6-10 Representation o a non-linear resistance by an equivalent tri-linear system. In lieu o more accurate analysis the resistance curve o elastic-plastic systems may be composed by an elastic resistance and a rigid-plastic resistance as illustrated in Figure A Figure A.6-11 A.6.8 Elastic A k 1 w el w + = Rigid-plastic Construction o elastic-plastic resistance curve Resistance curves and transormation actors or plates Elastic - rigid plastic relationships In lieu o more accurate calculations rigid plastic theory combined with elastic theory may be used. In the elastic range the stiness and undamental period o vibration o a clamped plate under uniorm lateral pressure can be expressed as: r = k 1 w = resistance-displacement relationship or plate center D k1 4 s = plate stiness 4 ts T = natural period o vibration D Elastic-plastic with membrane NORSOK standard Page 101 o 64

103 Annex A Rev.3, February t D E = plate bending stiness 1 1 The actors and are given in Figure A Figure A.6-1 Coeicients and. In the plastic range the resistance o plates subjected to uniorm pressure can be taken as: p p c p p c w 3 1 w w1 ( 3 3w Pinned ends : w w t w t 0 r Clamped ends : r c c s /s 6 y t 1 y t s 3 ( ) s 1) s w 1 w 1 = plate aspect parameter = plate length s = plate width t = plate thickness r c = plastic resistance in bending or plates with no axial restraint 0 (A.6.8) NORSOK standard Page 10 o 64

104 Annex A Rev.3, February 013 Resistance [r/rc] l/s = Figure A.6-13 A Relative displacement w Plastic load-carrying capacities o plates as a unction o lateral displacement Axial restraint Unlike stieners no simple method with a clear physical interpretation exists to quantiy the eect o lexible boundaries or a continuous plate ield under uniorm pressure. Full axial restraint may probably be assumed. At the panel boundaries assumption o ull axial restraint is non-conservative. In lieu o more accurate calculation the axial restraint may be estimated by removing the plate and apply a uniormly distributed unit in-plane load normal to the plate edges. The axial stiness should be taken as the inverse o the maximum in-plane displacement o the long edge. The relative reduction o the plastic load-carrying capacity can calculated according to the procedure described in Section A.6.9. or a beam with rectangular cross-section (plate thickness x unit width) and length equal to stiener spacing, using the diagram or =. For a plate in the middle o a continuous, uniormly loaded panel a high degree o axial restraint is likely. A non-dimensional spring actor c= 1.0 is suggested. I membrane orces are likely to develop it has to be veriied that the adjacent structure is strong enough to anchor ully plastic membrane orces. A Tensile racture o yield hinges In lieu o more accurate calculations the procedure described in Section A may be used or a beam with rectangular cross-section (plate thickness x unit width) and length equal to stiener spacing. A.6.9 Resistance curves and transormation actors or beams Provided that the stieners/girders remain stable against local buckling, tripping or lateral torsional buckling stiened plates/girders may be treated as beams. Simple elastic-plastic methods o analysis are generally applicable. Special considerations shall be given to the eect o: Elastic lexibility o member/adjacent structure Local deormation o cross-section Local buckling Strength o connections Strength o adjacent structure Fracture NORSOK standard Page 103 o 64

105 Annex A Rev.3, February 013 A Beams with no- or ull axial restraint Equivalent springs and transormation actors or load and mass or various idealised elasto-plastic systems are shown in Table A.6-.For more than two concentrated loads, use values or uniorm loading. Shear deormation may have a signiicant impact on the elastic lexibility and eigenperiod o beams and girders with a short span/web height ratio (L/h w ), notably or clamped ends. The eigenperiod and stieness in the linear domain including shear deormation may be calculated as: T and k m k K l m,u M u K k ' 1 GA w, k Q c ' Q k1 k Q L 1 where l m,c M c 1 c s r L g 1 E G A A w (A.6.9) (A.6.10) c s c s c s L E G A A w r g k Q k 1 k 1 k e M ps M pm = 1.0 or both ends simply supported = 1.5 or one end clamped and one end simply supported = 1.5 or both ends clamped = length o beam/girder = elastic modulus = shear modulus = total cross-sectional area o beam/girder = shear area o beam/girder = radius o gyration = shear stiness or beam/girder = bending stiness in the linear domain according to Table A.6- = beam stiness due to bending and shear = equivalent bending in the elasto-plastic domain = plastic bending capacity o beam at support = plastic bending capacity o beam at midspan and regardless o rotational boundary conditions the ollowing values may be used c Q = 8 or uniormly distributed loads c Q = 4 or one concentrated loads = 6 or two concentrated loads c Q The dynamic reactions according to Table A.6- become increasingly inaccurate or loads with short duration and/or high magnitudes. NORSOK standard Page 104 o 64

106 Annex A Rev.3, February 013 Table A.6- Transormation actors or beams with various boundary and load conditions. Load case Resistance domain Load Factor K l Mass actor Concentrated mass K m Unior m mass Load-mass actor Concentrated mass K lm Uniorm mass Maximum resistance R el Linear stiness Dynamic reaction V k 1 F=pL Elastic M p 384EI L 5L R 011. F L Plastic bending 8M p Rel 01. F L F L/ L/ F/ F/ L/3 L/3 L/3 Plastic membrane Elastic Plastic bending Plastic membrane 4 M p L Elastic Plastic bending Plastic membrane 6M p Rel 00. F L N P L 4 M p 48EI L 3 L 4N P L 6M p 56. 4EI L 3 L 6N P L N P y max L 078. R 08. F 075. R 05. F el N P y max L 055. R 0. 05F 3N P y max L NORSOK standard Page 105 o 64

107 Annex A Rev.3, February 013 Load case Resistance domain Load Factor K l Mass actor Concentrated mass K m Unior m mass Load-mass actor Concentrated mass K lm Uniorm mass Maximum resistance R el Linear stiness k 1 Equivalent linear stiness k e Dynamic reaction V F=pL Elastic M ps 384EI L 3 L 0.36R F L Elastoplastic bending Mps M 384EI Pm L 5L R 011. F el Plastic bending Mps MPm Rel 01. F L L/ F L/ Plastic membrane Elastic Plastic bending Plastic membrane Mps MPm Rel 05. F L Elastic Elastoplastic bending Plastic bending Plastic membrane M ps MPm L q = explosion load per unit length (q = ps or stieners, q = p or girders) EI 1.5M ps 0. l M 5 ps M pm m1 3 4N P L 4 Mps M 19EI Pm 3 L L 9M ps L 6 M ps MPm L 4N P L 60EI 3 L 56.4EI 3 L 6N P L 1 N p y max L 071. R 01. F N P y max L 0.48R 0.0F 0.5R 0.0F el 0.5R 0.0F el 3N P y max L NORSOK standard Page 106 o 64

108 Annex A Rev.3, February 013 Load case Resistance domain Load Factor K l Mass actor Concentrated mass K m Unior m mass Load-mass actor Concentrated mass K lm Uniorm mass Maximum resistance R el Linear stiness k 1 Equivalent linear stiness k 1 Dynamic reaction V F=pL Elastic M ps 185EI L 3 L V1 06. R 01. F V 043. R 019. F 160 V 1 L V Elastoplastic bending Mps M 384EI 039. R 011. F Pm L 5 3 L MPs L Plastic bending Mps M 038. R 01. F Pm 0 L M L Ps V 1 V 1 m F L/ L/ F/ F/ L/3 L/3 L/3 V V Plastic membrane Elastic Elastoplastic bending Plastic bending Plastic membrane Mps M 075. R 05. F Pm 0 L M L Elastic Elastoplastic bending Plastic bending Plastic membrane EI 1.5M ps 0. M M 5 3 l ps pm Mps 3MPm 05. Rel 00. F 0 M L m 3 EI M ps 0. M 3M 5 3 l ps pm 4N P L 16M Ps 107EI 3L 3 L Mps MPm L 48EI 3 L 4N P L 6M Ps 13EI 3 L L Mps 3 MPm 56EI L 3 L L 6N P L N P y max L V1 05. R 007. F V 054. R 014. F 078. R 08. F M L Ps Ps N P y max L V R 017. F V 033. R 033. F 0. 55R 0. 05F M L Ps Ps 3N P y max L NORSOK standard Page 107 o 64

109 Annex A Rev.3, February 013 A.6.9. Beams with partial end restraint. Relatively small axial displacements have a signiicant inluence on the development o tensile orces in members undergoing large lateral deormations. Equivalent elastic, axial stiness may be deined as 1 1 k k node EA (A.6.11) k node = axial stiness o the node with the considered member removed. This may be determined by introducing unit loads in member axis direction at the end nodes with the member removed. Plastic orce-deormation relationship or a beam under uniorm pressure may obtained rom Figure A.6-14, Figure A.6-15 or Figure A.6-16 i the plastic interaction between axial orce and bending moment can be approximated by the ollowing equation: M M P N 1 1 N P (A.6.1) In lieu o more accurate analysis = 1. can be assumed or stiened plates and H or I beams. For members with tubular section 8c1 y WP R 0 = plastic collapse resistance in bending or the member. = member length w w c = non-dimensional deormation w c c 1 w c WP A = characteristic beam height or beams described by plastic interaction equation (A.6.1). 4c kw 1 c y A = non-dimensional spring stiness c 1 = or clamped beams c 1 = 1 or pinned beams W P = Plastic section modulus or the cross section o the beam W z A = plastic section modulus or stiened plate or s e t > A s P g s A A st = total area o stiener and plate lange A e s A s t = eective cross-sectional area o stiener and plate lange, s e z g = distance rom plate lange to stiener centre o gravity. A s = stiener area s = stiener spacing s e = eective width o plate lange see A NORSOK standard Page 108 o 64

110 Annex A Rev.3, February R/R0 4 3 = 1. Bending & membrane Membrane only F (explosion load) c k w k ,5 1 1,5,5 3 3,5 4 Deormation w Figure A.6-14 Plastic load-deormation relationship or beam with axial lexibility (=1.) R/R c = k ,5 1 1,5,5 3 3,5 4 Bending & membrane Membrane only F (explosion load) k Deormation w Figure A.6-15 Plastic load-deormation relationship or beam with axial lexibility (=1.5) w NORSOK standard Page 109 o 64

111 Annex A Rev.3, February = R/R c k Bending & membrane Membrane only F w (explosion load) k Figure A.6-16 Plastic load-deormation relationship or beam with axial lexibility (=) For members where the plastic moment capacity o adjacent members is smaller than the moment capacity o the exposed member the orce-deormation relationship may be interpolated rom the curves or pinned ends and clamped ends: R R where clamped actual R M P 8 1 R pinned (A.6.13) (A.6.14) actual R 0 = Collapse load in bending or beam accounting or actual bending resistance o adjacent members R actual Pj 0 8M P 4M M M i Pj,i P P1 4M P (A.6.15) M ; i = adjacent member no i, j = end number {1,} (A.6.16) M Pj,i = Plastic bending moment or member no. i. Elastic, rotational lexibility o the node is normally o moderate signiicance A ,5 1 1,5,5 3 3,5 4 Eective lange Deormation w In order to analyse stiened plate as a beam the eective width o the plate between stiener need to be determined. The eective width needs to be reduced due to buckling and/or shear lag. NORSOK standard Page 110 o 64

112 Annex A Rev.3, February 013 Shear lag eects may be neglected i the length is more than.5 times the width between stieners. For guidance see Commentary. Determination o eective lange due to buckling can be made as or buckling o stiened plates see DNV- RP-C01. The eective width or elastic deormations may be used when the plate lange is on the tension side. In most cases the lange will experience varying stress with parts in compression and parts in tension. It may be unduly conservative to use the eective width or the section with the largest compression stress to be valid or the whole member length. For continuous stieners it will be reasonable to use the mean value between eective width at the section with the largest compression stress and the ull width. For simple supported stieners with compression in the plate it is judged to be reasonable to use the eective width at midspan or the total length o the stiener. A Strength o adjacent structure The adjacent structure must be checked to see whether it can provide the support required by the assumed collapse mechanism or the member/sub-structure A Strength o connections The capacity o connections can be taken rom recognised codes The connection shall be checked or the dynamic reaction orce given in Table A.6-. For beams with axial restraint the connection should also be checked or lateral - and axial reaction in the membrane phase: I the axial orce in a tension member exceeds the axial capacity o the connection the member must be assumed disconnected. I the capacity o the connection is exceeded in compression and bending, this does not necessarily mean ailure o the member. The post-collapse strength o the connection may be taken into account provided that such inormation is available. A Ductility limits Reerence is made to Section A.3.10 The local buckling criterion in Section A and tensile racture criterion given in Section A may be used with: d c = characteristic dimension equal to twice the distance rom the plastic neutral axis in bending to the extreme ibre o the cross-section c = non-dimensional axial spring stiness calculated in Section A.6.9. Alternatively, the ductility ratios Table A.6-3 Boundary conditions Cantilevered Pinned Fixed y y max el in Table A.6-3 may be used. Ductility ratios beams with no axial restraint Load Concentrated Distributed Concentrated Distributed Concentrated Distributed Cross-section category Class 1 Class Class NORSOK standard Page 111 o 64

113 Annex A Rev.3, February 013 A.7 RESIDUAL STRENGTH A.7.1 General The second step in the accidental limit state check is to veriy the residual strength o the installation with damage caused by the accidental load. The check shall be carried out or unctional loads and design environmental loads. The partial saety actor or loads and material can be taken equal to unity. Ater the action o the accidental load an internal, residual stress/orce ield remains in the structure. The resultant o this ield is zero. In most cases the residual stresses/orces have a minor inluence on the residual capacity and may be neglected. Any detrimental eect o the residual stress on the ultimate capacity should, however, be subject to explicit consideration. I necessary, the residual stresses should be included in the analysis. I non-linear FE analysis is used, the residual stress ield can be conveniently included by perorming integrated analysis: 1) Application o design accidental loading ) Removal o accidental load by unloading 3) Application o design environmental load A.7. A.7..1 Modelling o damaged members General Compressive members with large lateral deormations will oten contribute little to load-carrying and can be omitted rom analysis. A.7.. Members with dents, holes, out-o-straightness Tubular members with dents, holes and out-o-straightness, reerence is made to Chapter 10. Stiened plates/beams/girders with deormed plate lange and out-o-straightness, reerence is made to Chapter 10. NORSOK standard Page 11 o 64

114 Annex A Rev.3, February 013 A.8 REFERENCES /1/ NORSOK Standard N-003 Action and Action Eect // NS-EN Eurocode 3: Design o Steel structures Part 1-. General rules - Structural ire design /3/ Amdahl, J.: Energy Absorption in Ship-Platorm Impacts, UR-83-34, Dept. Marine Structures, Norwegian Institute o Technology, Trondheim, /4/ SCI 1993: Interim Guidance Notes or the Design and Protection o Topside Structures against Explosion and Fire /5/ Amdahl, J.: Mechanics o Ship-Ship Collisions- Basic Crushing Mechanics.West Europene Graduate Shcool o Marine Technology, WEGEMT, Copenhagen, 1995 NORSOK standard Page 113 o 64

115 Annex A Rev.3, February 013 A.9 COMMENTARY Comm. A.3.1 General For typical installations, the contribution to energy dissipation rom elastic deormation o component/substructures in direct contact with the ship is very small and can normally be neglected. Consequently, plastic methods o analysis apply. However, elastic elongation o the hit member as well as axial lexibility o the nodes to which the member is connected, have a signiicant impact on the development o membrane orces in the member. This eect has to be taken into account in the analysis, which is otherwise based on plastic methods. The diagrams in Section A.3.7. are based on such an approach. Depending on the structure size/coniguration as well as the location o impact elastic strain energy over the entire structure may contribute signiicantly. Comm. A.3. Design principles The transition rom essentially strength behaviour to ductile response can be very abrupt and sensitive to minor changes in scantlings. E.g. integrated analyses o impact between the stern o a supply vessel and a large diameter column have shown that with moderate change o (ring - and longitudinal) stiener size and/or spacing, the energy dissipation may shit rom predominantly platorm based to predominantly vessel based. Due attention should be paid to this sensitivity when the calculation procedure described in Section A.3.5 is applied. Comm. A Recommended orce-deormation relationship The curve or bow impact in Figure A.3-4 has been derived on the assumption o impacts against an ininitely rigid wall. Sometimes the curve has been used erroneously to assess the energy dissipation in bow-brace impacts. Experience rom small-scale tests /3/ indicates that the bow undergoes very little deormation until the brace becomes strong enough to crush the bow. Hence, the brace absorbs most o the energy. When the brace is strong enough to crush the bow the situation is reversed; the brace remains virtually undamaged. On the basis o the tests results and simple plastic methods o analysis, orce-deormation curves or bows subjected to (strong) brace impact were established in /3/ as a unction o impact location and brace diameter. These curves are reproduced in Figure A.9-1. In order to ulil a strength design requirement the brace should at least resist the load level indicated by the broken line (recommended design curve). For braces with a diameter to thickness ratio < 40 it should be suicient to veriy that the plastic collapse load in bending or the brace is larger than the required level. For larger diameter to thickness ratios local denting must probably be taken into account. Normally sized jacket braces are not strong enough to resist the likely bow orces given in Figure A.9-1, and thereore it has to be assumed to absorb the entire strain energy. For the same reasons it has also to be assumed that the brace has to absorb all energy or stern and beam impact with supply vessels. NORSOK standard Page 114 o 64

116 Annex A Rev.3, February 013 Impact orce [MN] 1 8 Recommended design curve or brace impact Between a stringer (D= 1.0 m) On a stringer (D= 0.75 m) 4 Between stringers (D= 0.75) m 0 Figure A.9-1 Load-deormation curves or bow-bracing impact /3/ Comm. A.3.5. Force contact area or strength design o large diameter columns. Figure A Indentation [m] Distribution o contact orce or stern corner/large diameter column impact Figure A.9- shows an example o the evolution o contact orce intensity during a collision between the stern corner o a supply vessel and a stiened column. In the beginning the contact is concentrated at the extreme end o the corner, but as the corner deorms it undergoes inversion and the contact ceases in the central part. The contact area is then, roughly speaking, bounded by two concentric circles, but the distribution is uneven. The orce-deormation curves given in Figure A.3-4 relate to total collision orce or stern end - and stern corner impact, respectively. Table A.3-1 and Table A.3- give the anticipated maximum orce intensities (local orce and local contact areas, i.e. subsets o the total orce and total area) at various stages o deormation. The basis or the design curves is integrated, non-linear inite element analysis o stern/column impacts. NORSOK standard Page 115 o 64

117 Annex A Rev.3, February 013 The inormation given in A.3.5. may be used to perorm strength design. I strength design is not achieved numerical analyses have shown that the column is likely to undergo severe deormations and absorb a major part o the strain energy. In lieu o more accurate calculations (e.g. non-linear FEM) it has to be assumed that the column absorbs all strain energy. Comm. A Energy dissipation in ship bow. The requirements in this paragraph are based upon considerations o the relative resistance o a tubular brace to local denting and the bow to penetration o a tubular beam. A undamental requirement or penetration o the brace into the bow is, irst - the brace has suicient resistance in bending, second - the cross-section does not undergo substantial local deormation. I the brace is subjected to local denting, i.e. undergoes lattening, the contact area with the bow increases and the bow inevitably gets increased resistance to indentation. The provisions ensure that both requirements are complied with. shows the minimum thickness as a unction o brace diameter and resistance level in order to achieve suicient resistance to penetrate the ship bow without local denting. It may seem strange that the required thickness becomes smaller or increasing diameter, but the brace strength, globally as well as locally, decreases with decreasing diameter. Local denting in the bending phase can be disregarded provided that the ollowing relationship holds true: D 1 (A.9.1) 0.14 t c D 1 Figure A.9-4 shows brace thickness as a unction o diameter and length diameter ratio that results rom Equation (A.9.1). The thickness can generally be smaller than the values shown, and still energy dissipation in the bow may be taken into account, but i Equation (A.9.1) is complied with denting does not need to be urther considered. The requirements are based upon simulation with LS-DYNA or penetration o a tube with diameter 1.0 m. Great caution should thereore be exercised in extrapolation to diameters substantially larger than 1.0 m, because the resistance o the bow will increase. For brace diameters smaller than 1.0 m, the requirement is conservative. Thickness [mm] ,6 0,8 1 1, 1,4 Diameter [m] y = 35 MPa, 6 MN y = 35 MPa, 3 MN y = 355 MPa, 6 MN y = 355 MPa, 3 MN y = 40 MPa, 6 MN y = 40 MPa, 3 MN Figure A.9-3 Required thickness versus grade and resistance level o brace to penetrate ship bow without local denting NORSOK standard Page 116 o 64

118 Annex A Rev.3, February Thickness [mm] ,6 0,8 1 1, 1,4 Diameter [m] L/D =0 L/D =5 L/D =30 Figure A.9-4 Brace thickness yielding little local deormation in the bending phase Comm. A General I the degradation o bending capacity o the beam cross-section ater buckling is known the load-carrying capacity may be interpolated rom the curves with ull bending capacity and no bending capacity according to the expression: R(w) R M 1 (w) R M 0 (w)(1 ) R MP 1 (w) R M P 0 (w) R R R M M 1 P P P = Collapse load with ull bending contribution = Collapse load with no bending contribution P,red (w 0) (A.9.) M P,red = Plastic collapse load in bending with reduced cross-sectional capacities. This has to be updated along with the degradation o cross-sectional bending capacity. Comm. A Tensile racture in yield hinges The rupture criterion is calculated using conventional beam theory. A linear strain hardening model is adopted. For a cantilever beam subjected to a concentrated load at the end, the strain distribution along the beam can be determined rom the bending moment variation. In Figure A.9-5 the strain variation, cr Y, is shown or a circular cross-section or a given hardening parameter. The extreme importance o strain hardening is evident; with no strain hardening the high strains are very localised close to the support (x = 0), with strain hardening the plastic zone expands dramatically. On the basis o the strain distribution the rotation in the plastic zone and the corresponding lateral deormation can be determined. I the beam response is aected by development o membrane orces it is assumed that the membrane strain ollows the same relative distribution as the bending strain. By introducing the kinematic relationships or beam elongation, the maximum membrane strain can be calculated or a given displacement. NORSOK standard Page 117 o 64

119 Annex A Rev.3, February 013 Figure A.9-5 Axial variation o maximum strain or a cantilever beam with circular cross-section Adding the bending strain and the membrane strain allows determination o the critical displacement as a unction o the total critical strain. Figure A.9-6 shows deormation at rupture or a ully clamped beam as a unction o the axial lexibility actor c. w/d Figure A.9-6 /D = 30 c= 0 = 0.05 = 0.5 = cr y /D = 0 c = 0 = 0.05 = 0.5 = 1000 Maximum deormation or a tubular ully clamped beam. (H=0.005) The plastic stiness actor H is determined rom the stress-strain relationship or the material.. The equivalent linear stiness shall be determined such that the total area under the stress-strain curve up to the critical strain is preserved (The two portions o the shaded area shall be equal), reer Figure A.9-7. It is unconservative and not allowable to use a reduced eective yield stress and a plastic stiness actor as illustrated in Figure A.9-8. NORSOK standard Page 118 o 64

120 Annex A Rev.3, February 013 cr H E cr H E E E cr cr Figure A.9-7 Determination o plastic stiness H E Figure A.9-8 Erroneous determination o plastic stiness Comm. A Stiened plates subjected to drill collar impact The validity or the energy expression A.4.6 is limited to 7 < r/d < 41, t/d < 0. and m i /m < The ormula neglect the local energy dissipation which can be added as E loc In case o hit near the plate edges the energy dissipation will be low and may lead to unreasonable plate thickness. The ailure criterion used or the ormula is locking o the plate. In many cases locking may be acceptable as long as the alling object is stopped. I the design is based on a hit in the central part o a plate with use o the smaller diameter in the treaded part in the calculations, no penetration o the drill collar will take place at any other hit location due to the collar o such dropped objects. Comm. A.5.1 General For redundant structures thermal expansion may cause buckling o members below C. Forces due to thermal expansion are, however, purely internal and will be released once the member buckles. The net eect o thermal expansion is thereore oten to create lateral distortions in heated members. In most cases these lateral distortions will have a moderate inluence on the ultimate strength o the system. As thermal expansion is the source o considerable inconvenience in conjunction with numerical analysis it would tempting to replace its eect by equivalent, initial lateral member distortions. There is however, not suicient inormation to support such a procedure at present. Comm. A.5.5 Empirical correction actor In Eurocode 3 an empirical reduction actor o 1. is applied in order to obtain better it between test results and column curve c or ire exposed compressive members. In the design check this is perormed by multiplying the design axial load by 1.. In non-linear analysis such a procedure is, impractical. In nonlinear space rame, stress resultants based analysis the correction actor can be included by dividing the yield compressive load and the Euler buckling load by a actor o 1.. (The inluence o axial orce on members stiness is accounted or by the so-called Livesly s stability multipliers, which are unctions o the Euler buckling load.) In this way the reduction actor is applied consistently to both elastic and elasto-plastic buckling. The above correction actor comes in addition to the reduction caused by yield stress and elastic modulus degradation at elevated temperature i the reduced slenderness is larger than 0.. NORSOK standard Page 119 o 64

121 Annex A Rev.3, February 013 Comm. A.6. Classiication o response Equation (A.6.) is derived using the principle o conservation o momentum to determine the kinetic energy o the component at the end o the explosion pulse. The entire kinetic energy is then assumed dissipated as strain energy. Equation (A.6.3) is based on the assumption that the explosion pressure has remained at its peak value during the entire deormation and equates the external work with the total strain energy. In general, the explosion pressure is not balanced by resistance, giving rise to inertia orces. Eventually, these inertia orces will be dissipated as strain energy. Equation (A.6.4) is based on the assumption that the pressure increases slowly so that the static condition (pressure balanced by resistance) applies during the entire deormation. Comm. A.6.4 SDOF system analogy The displacement at the end o the initial, linear resistance domain y el will generally not coincide with the displacement at irst yield. Typically, y el represents the displacement at the initiation o a plastic collapse mechanism. Hence, y el is larger than the displacement at irst yield or two reasons: i. Change rom elastic to plastic stress distribution over beam cross-section ii. Bending moment redistribution over the beam (redundant beams) as plastic hinges orm Comm. A.6.4. Dynamic response charts or SDOF system Figure A.6-3 is derived rom the dynamic response chart or a SDOF system subjected to a triangular load with zero rise time given in Figure A.6-4. In the example it is assumed that rom ductility considerations or the assumed mode o deormation a maximum displacement o ten times elastic limit is acceptable. Hence the line y y allow el y y max el 10 represents the upper limit or the displacement o the component. From the diagram it is seen that several combinations o pulses characterised by F max and t d may produce this displacement limit. Each intersection with a response curve (e.g. k 3 = 0) yields a normalized pressure F R F R max and a normalised impulse I RT el 1 F R max el T t d 1 R F el max t d T By plotting corresponding values o normalised impulse and normalised pressure the iso-damage curve given in Figure A.6-3 is obtained. Comm. A Eective lange For deormations in the elastic range the eective width (shear lag eect) o the plate lange, s e, o simply supported or clamped stieners/girders may be taken rom Figure A.9-9 NORSOK standard Page 10 o 64

122 Annex A Rev.3, February 013 Figure A.9-9 Eective lange or stieners and girders in the elastic range Comm. A Ductility limits The table is taken rom Reerence /4/. The values are based upon a limiting strain, elasto.plastic material and cross-sectional shape actor 1.1 or beams and 1.5 or plates. Strain hardening and any membrane eect will increase the eective ductility ratio. The values are likely to be conservative. o0o NORSOK standard Page 11 o 64

123 Annex K Rev. 3 February 013 DESIGN OF STEEL STRUCTURES ANNEX K SPECIAL DESIGN PROVISIONS FOR JACKETS NORSOK standard Pge 1 o 64

124 Annex K Rev. 3 February 013 CONTENTS Page K.1 GENERAL 15 K.1.1 Introduction 15 K.1. Deinitions 15 K.1.3 Design or non-operational phases 15 K.1.4 Design or operational phases 15 K. STRUCTURAL CLASSIFICATION 16 K..1 Structural classiication 16 K.3 DESIGN ACTIONS 130 K.3.1 General 130 K.3. Permanent action 130 K.3.3 Variable action 130 K.3.4 Deormation action 130 K.3.5 Environmental action 130 K General 130 K.3.5. Wave and current action 131 K Wind action 131 K Earthquake action 131 K Ice 131 K.3.6 Accidental action 131 K.3.7 Fatigue actions 13 K.3.8 Combination o actions 13 K.3.9 Vortex shedding 13 K.3.10 Wave slamming 13 K.4 GLOBAL RESPONSE ANALYSES 133 K.4.1 General 133 K.4. Dynamic eects 133 K.4.3 Analysis modelling 133 K.4.4 Design conditions 134 K General 134 K.4.4. In-place ULS analysis 134 K Fatigue analysis 134 K Accidental analysis 137 K Earthquake analysis 138 K Installation analysis 138 K.5 SPECIAL DESIGNS 139 K.5.1 Member design 139 K.5. Tubular connections 139 K.5.3 Grouted connection 139 K General 139 K.5.3. Failure o the grout to pile connection due to interace shear rom axial load and torsional moment (ULS and ALS) 140 K Check o compressive stresses at the lower end o the grout due to bending moment and shear in the pile (ULS and ALS) 143 K Fatigue o the grouted connection or alternating interace shear stress due to K axial load and bending moment in the pile (FLS) 144 Fatigue o the grout due compression and shear stresses at the lower end o the grout due to bending moment and shear in the pile (FLS) 145 K Fatigue check due to torsion 146 K Requirements to ribbed steel reinorcement 146 K Considerations on in-service inspection 147 K.5.4 Cast items 147 K.6 FOUNDATION DESIGN 148 K.6.1 General 148 K Design principles 148 K.6.1. Soil investigation 148 NORSOK standard Pge 13 o 64

125 Annex K Rev. 3 February 013 K.6. Piled oundation 149 K.6..1 Axial pile resistance 149 K.6.. Lateral pile resistance 150 K.6..3 Foundation response analysis 150 K.6..4 Installation 150 K.6..5 Pile atigue 151 K.6..6 Foundation simulation or jacket atigue analysis 151 K.6.3 Skirted oundation 151 K General 151 K.6.3. Foundation capacity 151 K Skirt penetration 15 K Skirted oundation structural design 153 K.6.4 On-bottom stability 154 K.7 DOCUMENTATION REQUIREMENTS FOR THE DESIGN BASIS & DESIGN BRIEF 155 K.7.1 General 155 K.7. Design basis 155 K.7.3 Design brie 155 K.8 REFERENCES 157 NORSOK standard Pge 14 o 64

126 Annex K Rev. 3 February 013 K.1 GENERAL K.1.1 Introduction Annex K This Annex to NORSOK N-004 is intended to give guidance on how jacket steel structures should be designed according to the provisions o relevant NORSOK standards. It is intended to give guidance to how the various standards should be applied or jacket structures, how important parameters should be selected and to give additional requirements especially relevant or jacket structures. K.1. Jacket: Bottle leg: Deinitions Foundation pile: Pile cluster: Bucket oundation: Multilegged jacket: K.1.3 A welded tubular space rame structure consisting o vertical or battered legs supported by a lateral bracing system. The jacket is designed to support the topside acilities, provide supports or conductors, risers, and other appurtenances and serve as a template or the oundation system. Leg section with larger diameter to thickness ratio used as buoyancy compartment during installation and to acilitate eective pile cluster design. Steel tubular driven into the soil and ixed to the jacket structure or transere o global actions. Pile sleeves or oundation piles arranged in groups with shear connections to jacket legs. Basis or mudmats and skirts. Steel plate construction integrated to bottom o jacket leg penetrating into soil or ixing platorm to ground. A jacket with more than 4 legs. Design or non-operational phases The jacket shall be designed to resist actions associated with conditions that will occur during stages rom abrication yard to the stage where the platorm is ready or operation at inal location. Such stages are transportation at the abrication yard and load-out, sea transportation, installation, mating and hook-up. Abandonment o the jacket shall be planned or in design. K.1.4 Design or operational phases The jacket shall be designed to resist actions caused by gravity, wind, waves and current, earthquake and accidental actions that may occur during its service lie. Each mode o operation o the platorm, such as drilling, production, work over or combinations thereo should be considered. The atigue requirements as given in Section 8 shall be ulilled. NORSOK standard Pge 15 o 64

127 Annex K Rev. 3 February 013 K. STRUCTURAL CLASSIFICATION K..1 Structural classiication Selection o steel quality level and requirements or inspection o welds shall be based on a systematic classiication o welded joints according to the structural signiicance and complexity o joints. The main criterion or decision o design class (DC) o welded joints is the signiicance with respect to joint complexity and the consequences o ailure o the joint. In addition the stress predictability (complexity) will inluence the DC selection. Classiication o DC or a typical 4-legged jacket is given in Table K.-1 and Figure K.-1. A practical limit or water depths or which in-service inspection can be perormed by use o diver technology may be set to 150 m. Consequently, an upgrade o one DC class should be speciied or connections below 150 m water depths. NORSOK standard Pge 16 o 64

128 Annex K Rev. 3 February 013 Table K.-1 Typical design classes in jackets Joint/ Component Design Class DC Steel QualityL evel S.Q.L Inspection Category Note a) Comments Legs and main bracing system Leg nodes & cones / Butt welds (I) or II A or B Note c), Note d) Legs nodes & cones / Longitud. welds B Leg strakes / Butt welds II A or B Note c), Note d) Leg strakes / Longitudinal welds B Lit-nodes / complex Lit-nodes / simple 1 I (I) or II A or B A or B Nodes in vertical bracing / Butt welds Nodes in vertical bracing / Longitudinal 4 II B C welds Vertical bracing / Butt welds Vertical bracing / Longitud. welds 4 II B C Bottle leg / Butt welds Bottle leg / Longitudinal welds II A or B B Horizontal bracing / All welds Nodes horizontal bracings / All welds 4 III B or C B or C Watertight diaphragms / All welds Ring stieners, main nodes / All welds Ring stieners bottle leg / All welds II (I) or II (I) or II A or B A A Other stiening / All welds 5 III D or E Foundation system Mudmat & yoke plate incl. stiening / All 4 III C Note e) welds Skirts & bucket oundation plates incl. 4 III C stiening / All welds Shear plates / All welds Pile sleeves / All welds 4 4 III III C C Note e) Note e) Pile sleeve catcher, cone & spacers 5 IV D or E Piles, top part / Butt welds Piles, top part / Longitud. welds Piles, remaining / All welds 4 III A Note e) Riser guides / All welds J-tubes & supports / All welds Conductor support / All welds Caissons & support / All welds Outitting / Butt welds Appurtenances and outitting steel 4 III B or C Note b) 4 III 4 III 5 IV 4 or 5 III or IV D or E Note b) Outitting / Part-Pen. & Fillets Notes: a) Local areas o welds with high utilisation shall be marked with rames showing areas or mandatory NDT when partial NDT are selected. Inspection categories depending on access or in-service inspection and repair. b) Outitting structures are normally o minor importance or the structural saety and integrity. However, in certain cases the operational saety is directly inluenced by the outitting and special assessment is required in design and abrication. A typical example is guides and supports or gas risers. c) I multilegged jacket with corner legs supporting a oundation systems; Upper part o corner legs and inner legs: DC 4, III, B or C Lower part o corner legs: DC, II, A or B d) I multilegged jacket where each leg supporting a oundation system: DC 4, III, B or C e) I one or two pile(s) per leg: DC, II. NORSOK standard Pge 17 o 64

129 Annex K Rev. 3 February 013 When the design class is deined the material shall be selected according to Section 5. The drawings shall indicate the inspection category or all welds according to Section 5. Figure K.-1 Typical design classes. 4-legged jacket with pileclusters. NORSOK standard Pge 18 o 64

130 Annex K Rev. 3 February 013 Table K.- summarises the minimum design class and steel quality level or dierent parts o a typical seabed support and protection structure o moderate size. Severe criticality and severe atigue utilisation may lead to more stringent assessment and requirements. Table K.- Typical design classes in subsea structures Joint/ Component Design Class DC Steel Qualitylevel S.Q.L Note a) Main structure Inspection Category Note b) Comments Corner legs Bumper rame DC 4 III C or B Bottom rame DC 4 III C or B Padeye and padeye welds DC 4 or III or II A or B Bracing and brace welds DC 5 IV C or B Pipes DC 5 IV C or B Hinge plate, butt welds Stieners, part pen & illets Mudmat & skirts, butt welds Part pen & illets Plate girders incl. stieners Butt welds Part pen & illets DC 5 IV B C Foundation DC 4 III B C DC 5 IV Ventilation & reinorcements DC 5 IV D Hatch Pipes DC 5 IV D or C Beams, butt welds Stieners,part pen & illets DC 5 IV C D Hinges (plates & bolt) DC 5 III C or B All other steel Outitting, butt welds DC 5 IV D or E Note c Outitting, part. pen & illets DC 5 IV E Note c Notes: a) In cases with through thickness stress occur Z-quality steel should be evaluated, as an alternative additional testing may be perormed b) Local areas o welds with high utilisation shall be marked with rames showing areas or mandatory NDT when partial NDT are selected. c) Outitting structures are normally o minor importance or the structural saety and integrity. However, in certain cases the operational saety is directly inluenced by the outitting and special assessment is required in design and abrication. The actual MDS selection should be in compliance with M-10. C D NORSOK standard Pge 19 o 64

131 Annex K Rev. 3 February 013 K.3 DESIGN ACTIONS K.3.1 General Characteristic actions shall be used as reerence actions in the partial coeicient method. Design actions are in general deined in NORSOK N-003. Applicable action categories and combinations o actions relevant or jacket design are given in this Section. K.3. Permanent action Permanent actions are actions that will not vary in magnitude, position or direction during the period considered. Permanent actions relevant or jacket design are: mass o jacket incl. piles above mudline mass o appurtenances supported by the jacket and/or topside mass o permanently installed topside such as accommodation, drilling and production equipment buoyancy and hydrostatic pressure rom sea water mass o marine growth For design o a jacket, the characteristic value o permanent actions representing the mass o steel to be designed shall include contingency actors. The contingency plan shall relect the uncertainties in the weight and material estimates as expressed by the stage in the calculations process. At the detail design stage, the characteristic values o permanent actions are to be veriied by accurate measurements (weight control) or calculated based upon accurate data. K.3.3 Variable action Variable actions on deck areas are actions that may vary in magnitude, position and direction during the period under consideration. Normally, a jacket design is based upon budget weight o topside and a speciied envelope o centre o gravity to account or variable actions on deck areas. K.3.4 Deormation action Deormation actions are actions due to deormations applied to the structure. Deormation actions relevant or jacket design are: pre-stressing temperature barge delections dierential settlements uneven seabed ield subsidence eects K.3.5 K Environmental action General The characteristic value o an environmental action is the maximum or minimum value (whichever is the most unavourable) corresponding to an action eect with a prescribed probability o exceedance. The environmental conditions should be described using relevant data or the relevant period and areas in which the jacket is to be abricated, transported, installed and operated. The long-term variation o environmental phenomena such as wind, waves and current should be described by recognised statistical distributions relevant to the environmental parameter considered. Inormation on the joint probability o the various environmental actions may be taken into account i such inormation is available and can be adequately documented. Details o environmental actions are ound in NORSOK N-003. NORSOK standard Pge 130 o 64

132 Annex K Rev. 3 February 013 K.3.5. Wave and current action Wave actions may be described either by deterministic or statistical methods. Normally, a deterministic analysis method is used or jacket design. In deterministic design analysis based on regular wave ormulation, the wave is described by wave period, wave height and water depth. Stokes 5 th order wave kinematic theory shall be used. In stochastic wave description, the short-term irregular sea states are described by means o wave energy spectra which are normally characterised by signiicant wave height (H s ), and average zero-up-crossing period (T z ), or spectral peak period (T p ). Wave and current actions on the jacket structure are normally calculated by use o Morison's equation. Member drag (C D ) and inertia (C M ) coeisients shall be established according to NORSOK N-003. Shielding eects may be taken into account where the presence o such eects can be adequately documented, (NORSOK N-003). Where elements are closely grouped (e.g. conductors), solidiication eects shall be considered. The design current velocity and proile is to be selected using site speciic statistics. The current applied in design shall include both tidal and wind induced eects. The current proile is normally deined up to still water level. In combination with waves, the proile shall be adjusted such that the lux o the current is kept constant. K Wind action Extreme values o wind speeds are to be expressed in terms o most probable largest values with their corresponding recurrence periods. Design o jacket and oundation shall be based upon the 1 hour extreme mean wind deined at a reerence height o 10m above still water level. The 1 minute wind gust parameter shall be used when wind action is applied in combination with the wave, and the wave action being the dominating action. Relevant scaling methods to obtain averaged wind speeds and wind height proiles are given in NORSOK N K Earthquake action For earthquake analyses, ground motions may be deined either as response spectra or time histories. The selection o method depends on the actual problem being considered. For deep-water jacket structures with long undamental periods and signiicant soil-structure interaction eects, time-history analysis is normally recommended. I inelastic material behaviour is considered, time-history methods are normally necessary. When perorming time-history earthquake analysis, the response o the structure/oundation system shall be analysed or a representative set o time histories. In areas with small or moderate earthquake activity, the ULS check may be omitted and the earthquake analysis may be limited to the ALS check only. K Ice I the structure is to be located in an area where snow or icing may accumulate, icebergs, or sea-ice may develop or drit, actions rom such phenomena shall be taken into account where relevant, re. NORSOK N K.3.6 Accidental action Accidental actions are actions related to abnormal operation or technical ailure. Details o accidental actions are ound in NORSOK N-003. Accidental actions relevant or jacket design are: impact rom vessel dropped object rom crane handling pool ire at sea extreme environmental actions The determination o accidental actions is normally to be based on a risk analysis. NORSOK standard Pge 131 o 64

133 Annex K Rev. 3 February 013 K.3.7 Fatigue actions Repetitive actions, which may lead to possible signiicant atigue damage, shall be evaluated. The ollowing listed sources o atigue actions shall, where relevant, be considered: waves (including those actions caused by slamming and variable (dynamic) pressures). wind (especially when vortex induced vibrations may occur ), e.g. during abrication, installation and operation. currents (especially when vortex induced vibrations may occur ). The eects o both local and global dynamic response shall be properly accounted or when determining response distributions related to atigue actions. K.3.8 Combination o actions Action coeicients and combinations o actions or the dierent limit states are in general given in NORSOK N-003. The Serviceability Limit State (SLS) need not normally to be considered in jacket design i not special unctional requirements are speciied. Such unctional requirements could be distance to other acilities (bridge connections, drilling rig) etc. K.3.9 Vortex shedding Vortex-induced action and vibrations with regard to wind, current and waves shall be investigated and taken into account where relevant. Any contribution to atigue damage rom vortex shedding shall be added to damage rom drag and inertia actions rom the wave, (NORSOK N-003). Relevant hot-spot locations may be considered. K.3.10 Wave slamming Horizontal or near-horizontal members that may be subjected to wave slamming shall be designed to resist impact orces caused by such eects. Any contribution to atigue damage rom wave slamming shall be added to damage rom drag and inertia actions rom the wave, (NORSOK N-003). Relevant hot-spot locations may be considered. NORSOK standard Pge 13 o 64

134 Annex K Rev. 3 February 013 K.4 GLOBAL RESPONSE ANALYSES K.4.1 General The selected method o response analysis is dependent on the design condition; ULS, FLS or ALS. Due attention shall be paid to the dynamic behaviour o the structure including possible non-linearities in action and response. K.4. Dynamic eects Dynamic eects are present in all jacket structures. However, the signiicance o the dynamic eects is dependent upon stiness, soil-structure interaction, mass and mass distribution and the structural coniguration. E.g. a complex or wide conigured space rame structure will experience dierent inertia actions than a narrow or column type structure due to inluence on the natural period. For traditional jacket platorms with natural periods less than 3 seconds, the inertial actions may be neglected and a quasi-static analysis will suice provided that the provisions in K4.4 are adhered to. The inertial action is established by a global dynamic analysis. The magnitude o the inertial action can be a direct result o the global dynamic analysis, or can be derived rom the quasi-static analysis and the Dynamic Ampliication Factor (DAF) rom the global dynamic analysis. Wave and other time varying actions should be given realistic representations o the requency content o the action. Time history methods using random waves are preerred. Frequency domain methods may be used or the global dynamic analysis both or ULS and FLS, provided the linearization o the drag orce can be justiied. The random waves should originate rom one or more wave spectra that are plausible conditions or producing the design wave deined shape. K.4.3 Analysis modelling Internal orces in members should be determined by three-dimensional structural analysis. Analytical models utilised in the global analyses o a platorm should adequately describe the properties o the actual structure including the oundation. The non-linear behaviour o axial and lateral soil-oundation system should be modelled explicitly to ensure action-delection compatibility between the structure and the soil-oundation system. The structural members o the ramework may be modelled using one or more beam elements or each span between nodes. The number o beam elements depends on the element ormulation, distribution o actions and potential or local dynamic response. Major eccentricities o action carrying members should be assessed and incorporated in the model. Face-to-ace length may be modelled by sti-end ormulation o the chord. For analyses o the operational design phases (static strength), any corrosion allowance on members should be subtracted in the stiness and stress calculations, but included in the wave action calculations. For atigue analysis, hal the corrosion allowance thickness should be included both or action, stiness and stress calculations. Determination o atigue lives in pile cluster connections should be determined based on stresses derived rom a local inite element analysis. Appurtenances such as conductors, J-tubes and risers, caisson, ladders and stairs, launch box, boat landing, guides and anodes should be considered or inclusion in the hydrodynamic model o the structure. Depending upon the type and number, appurtenances can signiicantly increase the global wave orces. In addition, orces on some appurtenances may be important or local member design. Appurtenances not welded to main structure are generally modelled by non-structural members that only contribute as equivalent wave orces. The increase in hydrodynamic- and gravity actions caused by marine growth shall be accounted or. NORSOK standard Pge 133 o 64

135 Annex K Rev. 3 February 013 K.4.4 Design conditions K General The platorm shall be designed to resist gravity actions, wave, current and wind actions, earthquake and accidental actions that may occur during its service lie. K.4.4. In-place ULS analysis Requirements concerning in-place ULS capacity checks are given in Section 6. Each mode o operation o the platorm, such as drilling, production, work-over or combination thereo shall be considered. The stiness o the deck structure shall be modelled in suicient detail to ensure compatibility between the deck design and the jacket design. Studies should be perormed to establish maximum base shear o wave and current actions or dimensioning o jacket bracings. Maximum overturning moments shall be established or dimensioning o jacket legs and oundation system. Diagonal approach directions normal to an axis through adjacent legs should also be considered in search or maximum leg and oundation reactions. The studies shall be undertaken by varying wave approach directions, wave periods and water depths. Detail design analysis should be based on minimum eight wave approach directions. A reduced number o approach directions may be considered or early stage design analysis or jackets which are symmetric about the two vertical axes. Maximum local member actions may occur or wave positions other than that causing the maximum global orce. Horizontal members close to still water level shall be checked both or maximum vertical and horizontal wave particle velocities. The eect o varying buoyancy shall also be included. The choice o an appropriate design wave theory is to be based on particular considerations or the problem in question. Stokes 5 th order wave theory is normally used or jacket design. However, or low water depth jacket locations, shallow water eects shall be assessed. DAF s should be established by use o time history analyses or structures where the dynamic eects are important. Figure K.4-1 K Recommended wave approach directions or ULS and FLS analysis Fatigue analysis Wave approach direction or ULS and FLS analysis Additional wave approach direction or pile/ leg design General requirements concerning atigue analysis o steel structures are given in Section 8. Special considerations and details o atigue strength analysis are given in Annex C. Fatigue analyses should include all relevant actions contributing to the atigue damage both in nonoperational and operational design conditions. Local action eects due to wave slamming, vortex shedding are to be included when calculating atigue damage i relevant. Whilst jackets in low to moderate water depths are not normally sensitive to dynamic eects, non-linearities associated with wave theory and ree-surace eects may be important. A deterministic analysis is normally recommended or such jackets. In the detail design phase, the deterministic analysis should include eight wave approach directions and at least our wave heights rom each direction. Wave orces should be NORSOK standard Pge 134 o 64

136 Annex K Rev. 3 February 013 calculated or at least ten positions in each wave. In perorming a deterministic wave analysis, due attention should be paid to the choice o wave periods i such data are not explicitly speciied, or can be determined based upon wave scatter diagrams relevant or the location. In lack o site speciic data, the wave periods shall be determined based on a wave steepness o 1/0 (NORSOK N-003). For deep water jackets and jackets where the dynamic eects are important, a atigue analysis in the requency domain (dynamic stochastic analysis) is recommended. Frequency domain action calculations are carried out in order to determine hydrodynamic transer unctions or member orce intensities. The transer unctions are expressed as complex numbers in order to describe the phase lags between the action variable and the incoming wave Studies are to be perormed to investigate wave actions or a range o periods in order to ensure a suicient accuracy o the response. For a stochastic atigue analysis, it is important to select periods such that response ampliications and cancellations are included. Also selection o wave periods in relation to the platorm undamental period o vibration is important. The number o periods included in the analysis should not be less than 30, and be in the range rom T = sec. to at least T = 0 sec. Figure K.4- Deterministic atigue analysis procedure Short crested waves (angular distribution o wave energy) may be taken into account i such eects are present at the location. When applying short crested waves, an increased number o wave approach directions should be used, normally not less than 1. The dynamic analysis can be carried out by a modal superposition analysis, direct requency response analysis or by mode syntehesis techniques. Traditional jackets are drag dominated structures. For drag dominated structures stochastic analysis based upon linear extrapolation techniques may signiicantly underestimate action eects. When the eect o drag orces on the expected atigue damage and the expected extreme responses are to be assessed, Morisontype wave actions are to be based on relevant non-linear models. NORSOK standard Pge 135 o 64

137 Annex K Rev. 3 February 013 I practical, horizontal plan elevations (or complex systems) should be designed to be located away rom the still water level or splash zone. Structure-to-ground connections in the analysis shall be selected to adequately represent the response o the oundations. Structure-to-ground connections may normally be simulated by linear stiness matrices. To linearize the actually non-linear soil response, these matrices should be developed based on a wave height which contributes signiicantly to the atigue damage. The matrices shall account or the correlation between the rotational and translation degrees o reedom, which may be important or proper calculation o atigue lives in the lower part o the structure. Structural components in the jacket shall be classiied according to consequences o ailure and accessibility or inspection and repair as outlined in Section 8. Structures or structural parts located in water depths below 150 m shall be considered as being not accessible or inspection and repair. Fatigue design actors or typical components in jackets are given in Table K.4-1 and Figure K.4-3. Table K.4-1 Fatigue design actors in jackets Classiication o structural components Not based on damage consequence accessible or inspection and repair or in the splash zone Brace/stub to chord welds in main loadtranserring joints in vertical plans, Chord/cone to leg welds, between leg sections, Brace to stub and brace to brace welds in main loadtranserring members in vertical plans, Shear plates and yoke plates incl. stiening Piles and bucket oundation plates incl. stiening Brace/stub to chord welds in joints in horisontal plans, Chord/cone to brace welds and welds between sections in horisontal plans, Appurtenance supports, Anodes, doubler plates, Outitting steel Accessible or inspection, maintenance and repair, and where inspections or maintenance is planned Below splash zone Above splash zone NORSOK standard Pge 136 o 64

138 Annex K Rev. 3 February 013 Figure K.4-3 K Typical atigue design actors in jackets Accidental analysis General requirements concerning the Accidental Limit State (ALS) o steel structures are given in Section 9. The jacket is to be designed to be damage tolerant, i.e. credible accidental damages or events should not cause loss o global structural integrity. NORSOK standard Pge 137 o 64

139 Annex K Rev. 3 February 013 A credible collision or dropped object against a bracing member or a nodal brace joint will reduce the action carrying ability o the member or the joint. Such members or joints are thus to be assumed non-eective when the global strength o the platorm (residual strength) is to be assessed or combinations o design actions in the ALS condition. A credible collision or dropped object against a leg member will normally cause limited damages, and the residual strength o the platorm is to be assessed taking due account o the residual strength o the damaged member or combination o design actions in the ALS condition. Details may be ound in Annex A. Pool ire analyses are normally perormed to establish requirements to the platorm passive and active ire protection system. For consideration o the 10-4 wave action and requirement to air gap see Norsok N-003. K Earthquake analysis General requirements concerning earthquake analyses are given in NORSOK N-003. Deck acilities that may respond dynamically to earthquake excitation (lare boom, derrick) are to be included in the structural model where relevant, in order to account or interaction eects due to correlation between closely spaced modes o vibration. Earthquake analysis may be perormed using response spectrum analysis or time history analysis. When the response spectrum method is used, the design spectrum shall be applied in one o the orthogonal directions parallel to a main structural axis. /3 o the design spectrum shall be applied in the other orthogonal direction and /3 in the vertical direction. The complete quadratic combination (CQC) method should be used or combining modal responses. The square root o the sum o the squares (SRSS) method should be used or combining directional responses. The model used or earthquake analysis should satisactorily simulate the behaviour o the actual structure. The number o vibration modes in the analysis should represent at least 90% o the total response energy o all modes. The number o modes to be included in the analysis is normally to be determined by a parametric study. Normally, 15-0 modes are suicient to ensure an adequate representation o all response quantities. The number o modes may be larger i topside structures such as lare booms and derricks are incorporated in the global model and i their dynamic behaviour is required Topside structures may normally be included in a simpliied manner provided that such modelling will properly simulate the global stiness and mass distributions. Structure-to-ground connections are normally simulated by linear stiness matrices. For jackets located in shallow to moderate water depths, conservative response values are usually achieved by a sti soil assumption. However, or deep water jackets this may not be conservative. Damping properties should be adequately assessed and included in the analysis as appropriate. In the absence o more accurate inormation, a modal damping ratio o 5 % o critical may be used. This value covers material damping and hydrodynamic damping, as well as radiation and hysteretic soil damping. Other damping ratios may be used where substantiated data exists. K Installation analysis Transport and installation design and operation shall comply with the requirements given in the VMO standard. The recurrence periods to be considered or determination o environmental actions shall comply with the requirements given in NORSOK N-001. NORSOK standard Pge 138 o 64

140 Annex K Rev. 3 February 013 K.5 SPECIAL DESIGNS K.5.1 Member design Reerence is made to Section 6 or static strength requirements to tubular members. Buckling actors or X-bracing s may be determined according to procedures which take into account the degree o lateral support urnished to the primary compression member by the cross member (tension member) and member end restraints. Thickness transitions should be made by lushing inside o tubular to achieve good atigue capabilities. Reerence is made to DNV-RP-C03 or atigue requirements to butt welds. K.5. Tubular connections Reerence is made to Section 6 and 8 or static strength and atigue requirements to tubular member connections and conical transitions. K.5.3 K Grouted connection General Grouted pile connections shall be designed to satisactorily transer the design loads rom the pile sleeve to the pile as shown in Figure K.5-1. The grout packer may be placed above or below the lower yoke plate as indicated in Figure K.5-. The connection may be analysed by using a load model as shown in Figure K.5-3. The ollowing ailure modes o grouted pile to sleeve connections need to be considered: Failure o grout to pile interace shear due to axial load and torsional moment (ULS and ALS). Figure K.5-1 Failure o the grout due to compressive stresses at the lower end o the grout due to bending moment and shear orce in the pile (ULS and ALS). Fatigue o the grouted connection or alternating interace shear stress due to axial load and bending moment in the pile (FLS). Fatigue o the grout due to contact pressure and riction orces creating shear stresses at the lower end caused by bending moment and shear orce in the pile (FLS). Jacket Leg Shear Plate Mudline Pile Terms or typical pile-sleeve connections Upper Yoke plate Pile sleeve Lower Yoke plate (Mudmat) NORSOK standard Pge 139 o 64

141 Annex K Rev. 3 February 013 Pile sleeve Pile sleeve Grout Lower Yoke plate Grout Lower Yoke plate Pile Packer Pile Packer Figure K.5- The let igure shows grout termination above Lower Yoke plate and the right igure shows grout termination below Lower Yoke plate The recommendations or check o the above ailure modes or pile sleeve connections with circular hoop or helix curved strings o weld beads or bars denoted shear keys are described in Section K.5.3. to K F,Sd Figure K.5-3 K.5.3. Model or calculation o orces in grouted pile-sleeve connections Failure o the grout to pile connection due to interace shear rom axial load and torsional moment (ULS and ALS) When a grouted connection is subjected to combined axial orce and torsional moment, the interace shear stress shall be taken as the result o the component stresses caused by axial orce and torsional moment calculated at the outer surace o the inner member. The design interace stress due to axial orce, ba,sd, is deined by: where ba,sd P t,sd N D F 1,Sd Sd p V Sd M b,sd M t,sd L e H Upper Yoke plate Lower Yoke plate (K.5.1) NORSOK standard Pge 140 o 64

142 Annex K Rev. 3 February 013 N Sd = design axial orce [N] D p = outside diameter o pile [mm] L e = eective grouted connection length [mm] In calculating the eective grouted connection length, L e, the ollowing non-structural lengths shall be subtracted rom the connection s nominal gross grouted length: 1. Where setting o a grout plug is the primary means o sealing, or is the contingency sealing method in the event o packer ailure, the grout plug length shall be considered as non-structural.. To allow or potential weak interace zones, grout slump, etc. at each end o the connection, the greater o the ollowing grouted lengths shall be considered as non-structural: - two thickness o the grout annulus, t g - one shear key spacing, s, i shear keys are used. 3. Any grouted length that is not certain to contribute eectively to the connection capacity, shall be considered as non-structural (e.g. when shear keys are used, the implications o possible over and under driving o piles shall be considered in relation to the number o shear keys present in the grouted length). 4. For pile-sleeve connections carrying signiicant bending moments a distance o D t below the lower yoke plate should be without shear keys. The design interace transer stress due to torsional moment, bt,sd, is deined by: bt,sd where M D t, Sd p Le M t,sd = design torsional moment on the connection. The combined axial and torsional design interace shear is calculated as: b, Sd ba,sd bt,sd The characteristic interace strength or grout to steel interace with shear keys, is given by: bks 800 h 140 D p s 0.8 C 0.6 s 0.3 ck p p above and (K.5.) (K.5.3) (K.5.4) The characteristic interace transer strength or grout steel interace in the connection without shear keys, is given by: bk 800 C D p 0.6 s 0.3 ck The characteristic interace transer strength or grout matrix shear ailure is given by: h s 0.5 bkg ck (K.5.5) (K.5.6) NORSOK standard Pge 141 o 64

143 Annex K Rev. 3 February 013 where s = shear key spacing (mm) h = shear key height (mm) C s = radial stiness actor = (D p / t p ) + (D s / t s ) -1 + (1 /m)(d g / t g ) -1 ck = characteristic cube strength (MPa) D p = outside diameter o pile (mm) t p = wall thickness o pile (mm) D s = outside diameter o pile sleeve (mm) t s = wall thickness o pile sleeve (mm) D g = outside diameter o grout annulus (mm) t g = thickness o grout annulus (mm) E = Young s modulus o elasticity or steel (MPa) m = steel-grout elastic modular ratio (to be taken as 18 in lieu o actual data) The inherent variability in the test data should be considered when calculating the characteristic strength i the capacity is based on test results. Figure K.5-4 Grouted connection with recommended shear key details Equation (K.5.4), (K.5.5) and (K.5.6) are valid or uncoated tubulars with normal abrication tolerances, where mill scale has been ully removed. The recommendations are valid or the ollowing range: 80 MPa ck 0 MPa 0.10 h/s D p /t p D s /t s D g /t g h/d p 16 D p /s 10 L e /D p 1 Connection with circular hoop shear keys should be checked as ollows: ba,sd bt,sd bks M bk M (K.5.7) (K.5.8) NORSOK standard Pge 14 o 64

144 Annex K Rev. 3 February 013 b,sd bkg M (K.5.9) where M = material actor or interace transer strength equal to.0 or ULS and 1.5 or ALS I the check o torsion stresses according to (K.5.8) is not ulilled the ULS checks can be made assuming redistribution o the pile torsion moment. This can be done by releasing the corresponding degree o reedom or the pile in the model. In addition a FLS check according to K needs to be satisied. A shear key shall be a continuous hoop or a continuous helix. Where hoop shear keys are used, they shall be uniormly spaced, oriented perpendicular to the axis o the tube, and be o the same orm, height and spacing on both the inner and outer tubes. Where helical shear keys are used, the ollowing additional limitation shall be applied: s D p /.5 and the characteristic interace transer strength given by equations (K.5.4) and (K.5.6) shall be reduced by a actor o The possible movements between the inner and the outer steel tubular member during the 4 hour period ater grouting shall be determined or the maximum expected sea states during that time, assuming that the grouted connection does not contribute to the stiness o the system. For oundation pile-to-sleeve connections, this analysis shall be an on-bottom analysis o the structure with ungrouted piles. I the expected relative axial movement at the grout steel interace exceeds 0.035%D p during this period, the movements should be limited by e.g. installation o pile grippers. K Check o compressive stresses at the lower end o the grout due to bending moment and shear in the pile (ULS and ALS) The compressive capacity o the grout shall be checked or orces in the Ultimate Limit States (ULS) and the Accidental limit States (ALS). The compressive contact stress between the steel and the grout will create tensile stresses in the grout due to the shear stresses resulting rom the riction orces at the grout to steel suraces. It is considered acceptable that the grout cracks or tensile stresses during these limit states; however, the grout needs to transer the orces rom the sleeve to the pile throughout the storm that includes the dimensioning environmental load. The ollowing procedure assumes that the speciied mechanical properties o the grout are met at all positions within the area assumed or carrying loads. I there is doubt about the grout quality that can be achieved at these locations, the calculations should be adjusted accordingly or the connection should be itted with reinorcement bars according to K in order to improve the robustness and ductility o the connection. The design contact pressure between steel and grout can be obtained rom p, Sd C A F 1, Sd D 3 p t p Where: F 1,Sd = V M H (See Figure K.5-3 ) C A Sd b, Sd / (K.5.10) = or grout ending above lower yoke plate and without reinorcement steel (See Figure K.5-) = 1 or grout ending a minimum distance = reinorcement steel D NORSOK standard Pge 143 o 64 p t p below lower yoke plate and without = 1 or grout ending above lower yoke plate and with longitudinal reinorcement steel = 0.5 or grout ending a minimum distance = D t reinorcement steel. p p below lower yoke plate and with

145 Annex K Rev. 3 February 013 V Sd = Pile resultant shear orce with ULS or ALS load actors as appropriate M b,sd = Pile resultant bending moment with ULS or ALS load actors appropriate The largest principal design stress can be calculated as p, Sd I, Sd (K.5.11) where a riction coeicient between pile and grout should be taken as: 0.7 It should be checked that: where I, Sd cn M (K.5.1) cn 1 ck 0.85 ck ck = characteristic cube strength o grout in MPa M = 1.5 or ULS and 1.5 or ALS (K.5.13) When steel reinorcements are prescribed, the reinorcements should meet requirement given in K K General Fatigue o the grouted connection or alternating interace shear stress due to axial load and bending moment in the pile (FLS) The maximum axial loads in piles o jacket platorms will normally be in compression or the case where permanent and variable loads add to the loads rom environmental actions. However, as the capacity o grouted connections exposed to cyclic loads where the axial load changes rom compression to tension during the load cycle are known to be signiicantly less than the static capacity it is necessary to check the conditions where the axial load in the pile is in tension. As research data on the long-term capacity o grouted connections is scarce especially on eects rom moment, there are no established methods or capacity assessment or pile-sleeve connections exposed or cyclic loads. Until adequate test data is available the ollowing simpliied atigue check is proposed: Calculate the maximum pile axial tension load (P t, Sd ) or 100 year loading with load actor = 1.0 considering minimum permanent and variable loads. Calculate the axial tension capacity as: P, Rd 0.3 D bks M P L e C PMred (K.5.14) where: bks = characteristic interace strength or grout to steel D p = outside diameter o pile L e = eective grouted connection length, see K.5.3. M = material actor =.0 or pile clusters with one or two piles and 1.5 or pile clusters with 3 or more piles i the integrity o the connection can be conirmed by inspection ater severe storms. C PMred = Reduction actor or alternating moment. C PMred =1-, where: (K.5.15) NORSOK standard Pge 144 o 64

146 Annex K Rev. 3 February 013 M PEnv,Sd, = bending moment rom 100 year environmental loads with load actor = 1.0 M re = W* 0.001*E E = Youngs modulus o the steel in the pile W = Elastic section modulus o the pile The stiness representation o the soil should represent the best estimate or cyclic loading. Check i P t,sd P, Rd (K.5.16) I equation (K.5.16) is not ulilled, the capacity rom riction created by the orces rom the minimum corresponding bending moment and shear orce in the pile according to the calculation model shown in Figure K.5-3 can be added to obtain larger capacity. The reaction orces can be calculated as: F 1, Cor F F, Cor 3, Cor V Cor M M Cor M / H t, Cor Cor / D p / H (K.5.17) V Cor and M Cor are the minimum shear and bending moment that correspond with the tensile axial orce P t,sd and M t,cor is the corresponding maximum torsional moment to be taken by the pile. Then the net riction orce can be derived as F where:, Rd Check i P t,sd K = F F F F 1, Cor, Cor 3, Cor (K.5.18) coeicient o riction between pile and grout or the contact area at the upper and lower yoke plates = 0.4 or this assessment, Rd P, Rd (K.5.19) Fatigue o the grout due compression and shear stresses at the lower end o the grout due to bending moment and shear in the pile (FLS) The stress variations in the grout at the lower end is caused by cyclic bending moment and shear orce in the pile giving compression stress in the grout and shear stress due to the riction caused by the sliding between the pile and the grout. For a grouted connection without steel reinorcement, one should limit these riction stresses such that they will not exceed the tensile capacity more than once during the lie o the platorm. This is considered to be achieved i the ollowing requirements are met based on calculation o 100 year return period environmental loads in combination with permanent loads. For atigue the action eects are derived with a load actor =1.0. The tensile stress in the grout can be calculated as p, Sd II, Sd where a riction coeicient between pile and grout should be taken as. The design criterion reads: (K.5.0) NORSOK standard Pge 145 o 64

147 Annex K Rev. 3 February 013 II, Sd tk M (K.5.1) where tk = the characteristic tensile strength according to DNV-OS-C50 determined rom splitting tensile testing o the actual grout. M = 1.5 = material actor used in atigue assessment or the grout. I equation (K.5.1) is not ulilled, the capacity can be increased by introduction o longitudinal and circumerential reinorcements. The atigue capacity o the reinorced grouted section is or ordinary jacket pile-sleeve connections judged to be acceptable i the requirements stated in K are met. K Fatigue check due to torsion In cases where the ULS check according to K.5.3. ails in the check or interace shear stresses due to torsional moments and redistribution o the moment is assumed, the ollowing check need to be carried out. This check is valid or connections with hoop shear keys. The return period or the load should be taken as 100 year and the load actor = 1.0. where btenv100, Sd btenv100,sd M tenv100,sd bk M t Env100, Sd D p L e (K.5.) (K.5.3) = Design torsional moment rom the 100 year return period environmental actions I this check is not satisied it is recommended to increase the capacity against torsional moments e.g. by introduction o shear keys transverse to the direction o the interaces shear stresses. K Requirements to ribbed steel reinorcement I the grout strength and durability is increased by introduction o ribbed steel reinorcement bars, the ollowing requirements to the steel reinorcement are assumed or the ormulas given in this Annex: The steel reinorcements should be arranged in the longitudinal direction o the sleeve starting rom the lower end o the grout over a length o minimum.5 D t in case or grout ending above A s Where lower yoke plate and 5.0 D p t p p p or the case when the grout extend below the lower yoke plate. The longitudinal reinorcements should be placed as close as possible to the pile wall. Each reinorcement bar should be bent and welded to the sleeve at the top and bottom or the ull yield capacity o the bar. Longitudinal bar spacing should be in the range o 0.5 to.0 times the grout thickness. Ring bars to be arranged to secure the position o the longitudinal bars throughout all phases and should as a minimum meet ordinary code requirements to reinorcements or shrinkage (e.g. DNV- OS-C50). The required area o the longitudinal reinorcement can be calculated as 0.5 p, Sd sd t g b (K.5.4) NORSOK standard Pge 146 o 64

148 Annex K Rev. 3 February 013 p,sd to be calculated according to Equation (K.5.10) using C A or the case with reinorcements. t g = thickness o grout b = distance between longitudinal reinorcement bars sd = sy / M = design strength o the reinorcement bars sy = characteristic yield stress o the reinorcement bars M = 1.0 = material actor or reinorcement bars A s = area o reinorcement bar = riction coeicient = 0.6 K Considerations on in-service inspection Generally the experience with grouted pile-sleeve connections is good and regular in-service inspections have not been considered to be necessary. However, it should be kept in mind that the long term perormance o grouted sleeve connections is uncertain. An eective in-service inspection o these connections is usually diicult to execute as there is limited access. Especially i the requirements o this standard are not met it should be considered to implement measures at the design and construction phases that enable easy inspections to check that the connection is unctioning as intended. Presently, there are ew adequate inspection methods or grouted pile-sleeve connections available, but it is expected that more suited inspection methods will be developed in the time to come. K.5.4 Cast items Cast joints are deined as joints ormed using a casting process. They can be o any geometry and o variable wall thickness. The design o a cast joint is normally done by use o FE analyses. An acceptable design approach or strength is to limit stresses in the joint due to actored actions to below yielding o the material using appropriate yield criteria. Additionally, the FE analyses should provide SCF s or atigue lie calculations. Oten, this design process is carried out in conjunction with the manuacturer o the cast joints. Fatigue requirements to cast nodes are given in DNV-RP-C03. NORSOK standard Pge 147 o 64

149 Annex K Rev. 3 February 013 K.6 FOUNDATION DESIGN K.6.1 General K Design principles The principles o limit state design and the partial coeicient ormat shall be used. A general description o the method and the deinition o the limit state categories are given in the ISO Standard /1/. Material actors are to be applied to the soil shear strength as ollows: or eective stress analysis, the tangent to the characteristic riction angle is to be divided by the material actor, M or total shear stress analysis, the characteristic undrained shear strength is to be divided by the material actor, M For soil resistance to axial and lateral loading o piles, material actors are to be applied to the characteristic resistance as described in K Material actors to be used are speciied in subsection K.6..1 and K.6.. and K K.6.1. Soil investigation Generally, the extent o site investigations and the choice o investigation methods are to take into account the type, size and importance o the structure, uniormity o soil, seabed conditions and the actual type o soil deposits. The area to be covered by site investigations is to account or positioning and installation tolerances. The soil investigation should give basis or a complete oundation design, comprising evaluations or piled oundation o On-bottom stability o unpiled structure Lateral pile resistance Axial pile resistance Pile/soil/structure interaction Pile driveability predictions For skirted oundations, the soil investigation should give basis or evaluations o Foundation stability Soil/structure interaction Skirt penetration Settlements Site investigations are to provide relevant inormation about the soil to a depth below which possible existence o weak ormations will not inluence the saety or perormance o the structure. Site investigations are normally to comprise o the ollowing type o investigations: site geology survey including assessment o shallow gas. topography survey o the seabed geophysical investigations or correlation with borings and in-situ testing soil sampling with subsequent laboratory testing in-situ tests, e.g. cone penetration tests The soil investigation should provide the ollowing type o geotechnical data: data or soil classiication and description shear strength data and deormation properties, as required or the type o analysis to be carried out in-situ stress conditions Further details and requirements related to the soil investigation are given in Norsk Standard NS3481 // and NORSOK G-001. NORSOK standard Pge 148 o 64

150 Annex K Rev. 3 February 013 K.6. K.6..1 Piled oundation Axial pile resistance General Soil resistance against axial pile loads is to be determined by one, or a combination o, the ollowing methods: load testing o piles semi-empirical pile resistance ormulae based on pile load test data. The soil resistance in compression is to be taken as the sum o accumulated skin riction on the outer pile surace and resistance against pile tip. In case o open-ended pipe piles, the resistance o an internal soil plug is to be taken into account in the calculation o resistance against pile tip. The equivalent tip resistance is to be taken as the lower value o the plugged (gross) tip resistance or the sum o skin resistance o internal soil plug and the resistance against pile tip area. The soil plug may be replaced or reinorced by a grout plug or equivalent in order to achieve ully plugged tip resistance. The submerged weight o the pile below mudline should be taken into account. For piles in tension, no resistance rom the soil below pile tip is to be accounted or when the tip is located in cohesionless soils. Examples o detailed calculation procedures may be ound in /3/, /4/,/5/ and /9/. The relevance o alternative methods should be evaluated related to actual design conditions. The chosen method should as ar as possible have support in a data base which its the actual design conditions related to soil conditions, type and dimensions o piles, method o installation, type o loading etc. When such an ideal it is not available, a careul evaluation o important deviations between data base and design conditions should be perormed and conservative modiications to selected methods should be made. Eect o cyclic loading Eects o repeated loading are to be included as ar as possible. In evaluation o the degradation, the inluence o lexibility o the piles and the anticipated load history is to be included. Cohesive soils For piles in mainly cohesive soils, the skin riction is to be taken equal to or smaller than the undrained shear strength o undisturbed clay within the actual layer. The degree o reduction depends on the nature and strength o clay, method o installation (e.g. driven or drilled/grouted), time eects, geometry and dimensions o pile, load history and other actors. Especially, the time required to achieve ull consolidation ater installation o driven piles should be considered. Design conditions that may occur prior to ull consolidation should be checked or reduced resistances. The unit tip resistance o piles in mainly cohesive soils may be taken as 9 times the undrained shear strength o the soil near the pile tip. Cohesionless soil For piles in mainly cohesionless soils the skin riction may be related to the eective normal stresses against the pile surace by an eective coeicient o riction between the soil and the pile element. Examples o recommended calculation methods and limiting skin riction values may be ound in /3/. The calculation procedures given in /9/ or cohesionless soils should not be used beore considerably more extensive documentation is available or without a careul evaluation o limiting values. The unit tip resistance o piles in mainly cohesionless soils may be calculated by means o conventional bearing capacity theory, taking into account a limiting value which may be governing or long piles /3/. Material actors For determination o design soil resistance against axial pile loads in ULS design, a material actor, M =1.3 is to be applied to all characteristic values o soil resistance, e.g. to skin riction and tip resistance. NORSOK standard Pge 149 o 64

151 Annex K Rev. 3 February 013 For individual piles in a group lower material actors may be accepted, provided that the pile group as a whole is designed with the required material actor. A pile group in this context is not to include more piles than those supporting one speciic leg. Group eects should be accounted or when relevant, as urther detailed in K.6... K.6.. Lateral pile resistance When lateral soil resistance governs pile penetrations, the design resistance is to be checked within the limit state categories ULS and ALS, using ollowing material actors applied to characteristic resistance: M = 1.3 or ULS condition M = 1.0 or ALS condition For calculation o pile stresses and lateral pile displacements, the lateral soil reaction is to be modelled using characteristic soil strength parameters, with the soil material actor M =1.0 The non-linear mobilisation o soil resistance is to be accounted or. The eect o cyclic loading should be accounted or in the lateral load-delection (p-y) curves. Recommended procedures or calculation o p-y curves may be ound in /3/, /4/. Scour The eect o local and global scour on the lateral resistance should be accounted or. The scour potential may be estimated by sediment transport studies, given the soil particle sizes, current velocity etc. However, experience rom nearby similar structures, where available, may be the most important guide in deining the scour criteria. Group eects When piles are closely spaced in a group, the eect o overlapping stress zones on the total resistance o the soil is to be considered or axial as well as lateral loading o the piles. The increased displacements o the soil volume surrounding the piles due to pile-soil-pile interaction and the eects o these displacements on interaction between structure and pile oundation is to be considered. In evaluation o pile group eects, due considerations should be given to actors as: pile spacing pile type soil strength and deormation properties soil density pile installation method. K.6..3 Foundation response analysis The pile responses should preerably be determined rom an integrated pile/soil/structure analysis, accounting or the soils non-linear response and ensuring load-delection compatibility between the structure and the pile/soil system. Such an analysis is normally carried out with characteristic soil strength parameters. K.6..4 General Installation The structure shall be documented to have adequate oundation stability ater touchdown, as well as beore and ater piling, when subjected to the environmental and accidental actions relevant during this period. Pile stress check prior to driving The pile should be checked with respect to yield and buckling (ULS) in the maximum possible inclined position or a design condition o maximum relevant equipment weight (e.g. hammer weight), plus pile sel-weight. Current loads and possible dynamic eects should be accounted or. NORSOK standard Pge 150 o 64

152 Annex K Rev. 3 February 013 Pile driving It should be demonstrated by calculations that the indented driving equipment is capable o driving the pile to target penetration within the pile reusal criteria speciied and without damaging the piles. The soil resistance during driving used in the driveability analysis may account or the gradual reduction in the skin riction caused by driving. The set-up eects leading to increased skin riction ater stop o driving should be taken into account or realistic duration o halts that may occur during driving. Dynamic stresses caused by pile driving are to be assessed based upon recognised criteria or by using wave equation analysis. The sum o the dynamic driving stresses and the static stresses during the driving process is not to exceed the speciied minimum yield strength. Allowance or under- or over-drive to account or uncertainties in pile-driving predictions should be considered. K.6..5 Pile atigue The pile atigue damage should be evaluated and demonstrated to be within the requirements. Considering the pile as a structural component with no inspection access and a substantial consequence o damage, a atigue design actor o 10 should be applied, as urther outlined in Section K It should be noted that the pile atigue damage rom the pile driving operation normally contributes substantially to the total atigue damage, which is the sum o the partial damages rom the pile driving and the environmental action during the design service lie. Appropriate S-N-data or double-sided butt welds in piles is the D-curve. The D-curve or air may be applied or the pile driving sequence. The D-curve or seawater shall be used or the long term wave actions expected during the service lie o the platorm. Provisions or use o the D-curve is that the weld be made in the shop and that all weld runs are made up with the pile in the horizontal position by use o submerged arc welding technology, and that 100% MPI is perormed. The atigue design actor should be applied both to the pile driving damage and to the environmental damage. K.6..6 Foundation simulation or jacket atigue analysis For a jacket atigue analysis, the pile/soil system may be simulated by pile stiness matrices generated rom independent pile/soil analysis. To account or the soil non-linearity, the pile analysis or derivation o pile atigue stiness matrices should be perormed at a representative atigue load level. K.6.3 K Skirted oundation General Skirted oundations are an alternative to pile oundations or jacket structures. The base area can be o any shape and the skirts can be corrugated or plane. The skirts are penetrating the soil and contribute to increase the oundation capacity with respect to horizontal as well as vertical orces. Also, uplit may be resisted by the skirted oundation to a variable degree depending on the soil and loading conditions. The skirted oundation may be used in both cohesive and cohesionless soils. The design or the main dimensions o the oundation should consider both the soil and the structural behaviour during operation as well as during installation. Possibility o inhomogenity o the soil over the area and local seabed variations should be accounted or. K.6.3. Foundation capacity In calculation o the oundation capacity one should consider the simultaneously acting vertical and horizontal orces and overturning moment acting on the skirted oundation. The acting orces should be determined rom a soil structure interaction analysis. The stinesses used to simulate the oundation should account or the non-linearity o the soil and be compatible with the load level o the soil reactions obtained rom the analysis. This may require iterative analyses unless an adequate ully integrated analysis tool is used. NORSOK standard Pge 151 o 64

153 Annex K Rev. 3 February 013 The determination o the oundation capacity should account or the act that most soils, even sandy soils, behave undrained or the short duration o a wave action, and that the cyclic loading eects may have signiicant eect on the undrained strength. The determination o oundation capacity should preerably be based on cyclic strengths derived by combining relevant storm load histories or the loads on the skirted oundations with the load dependent cyclic strengths determined rom undrained cyclic soil tests. Depending on the reliability o determining cyclic strengths or the soil in question the use o large-scale ield tests should be evaluated. Special concerns or clay In clay the soil can normally be considered ully undrained or all the accumulated eects o a design storm, and cyclic strengths can be determined based on recognised principles, as e.g. given in /4/, and described in more detail in /7/. It is important that the cyclic strength is determined or a realistic relation between average and cyclic shear stresses, representative or the storm load history on the skirted oundation. Special concerns or sand Even in sand, the transient type o loading rom one wave will result in an undrained or partly undrained response in the soil. I the sand is dense dilatancy eects would lead to high shear resistance or a single transient loading, resulting in high capacity or any type o loading, including uplit. However, when the soil is exposed to repeated loading in an undrained state, accumulated pore pressures will result. Such pore pressures can ultimately lead to a true ailure or less dense sands, or a state o initial liqueaction or cyclic mobility or dense sands. In the latter case, large cyclic soil strains and corresponding displacements o the oundation may occur beore the soil dilates and higher resistance is obtained. It is recommended that the deinition o ailure is based on an evaluation o tolerable cyclic or total strains in the soil. Even though the sandy soils will be undrained or partly undrained rom the transient loads rom each wave, there may be a considerable dissipation o excess pore pressures through the duration o a storm. The combined eects o pore pressure build up rom cyclic loading and dissipation may be accounted or. It should be aimed at deining an equivalent load history with a limited duration (as compared to the duration o the storm) or which the soil can be assumed to be basically undrained. An example o a calculation procedure is described in /10/. This load history may be used in deining cyclic shear strengths o the soil or calculation o cyclic capacities, or scaled directly as loads onto large-scale tests i such tests are perormed. The general ailure mechanisms or skirted oundations in sand are described in more detail in part o /6/, together with description o the design approach used or a jacket on dense sand. Material actors The minimum values o the material actors should be those speciied by NORSOK N-001, i.e. M =1.5 or ULS design and M =1.0 or ALS design. There are elements in the design process or oundation stability design o a skirted oundation that may contribute to greater uncertainties than or other type o oundations. This especially relates to deining the cyclic strengths in sandy soils. Depending on the conservatism used in the design it may be necessary to increase the material actors generally recommended by NORSOK N-001. It is recommended that the sensitivity o the main assumptions made in the design process is investigated beore such a decision is made. K Skirt penetration For installation o the skirted oundations the ollowing design aspects should be considered A suicient penetrating orce to overcome the soil resistance against penetration should be provided by a combination o weight and suction. The suction should be applied in a controlled manner to prevent excessive soil heave inside the skirted oundation The skirted oundation should be designed to prevent buckling due to the applied suction Channelling NORSOK standard Pge 15 o 64

154 Annex K Rev. 3 February 013 Penetration resistance In calculation o penetration resistance, there are several uncertainties related to spatial variations in soil conditions; may be dierent at dierent corners general uncertainties in determination o basic soil parameters uncertainties in calculation o penetration resistance rom basic soil parameters The total range o expected resistances should be documented. The upper bound resistance will be governing or design o ballast and/or suction system and or structural design o the oundation. Depending o the conservatism applied in choice o parameters and method o calculation to cover up or the uncertainties listed above, saety actors may have to be applied to the calculated resistances. In clayey soils it can be considered that the resistance is unaected by the applied suction, and the resistance can be calculated as or skirts penetrated by weight only. Skirt penetration resistances may be determined through correlations with cone penetration resistances as given in /4/, relevant basically or overconsolidated clays. Alternatively the skin riction may be related to the remoulded shear strength o the clay, and tip resistance may be related to the intact shear strength through conventional bearing capacity actors. In sand the penetration resistance will be eectively reduced by applied suction. The reduction is caused by general decrease o eective stresses inside the skirted oundation reducing the skin riction large gradients in eective stresses combined with large inwards gradients in the low below the skirt tip level, which triggers an inward bearing ailure beneath the skirt tip and thus signiicantly reduces the otherwise in sand dominating tip resistance. The general governing mechanisms are described in /7/ and /8/. The reduction eect o suction on penetration resistance is dependent both on the geometry o the skirted oundations and on the soil stratiication. When penetrating sand layers that are overlain by a clay layer, signiicantly higher suction will be required than in homogeneous sand. Reliable universal calculation methods or calculation o penetration resistance in sand when suction is applied are not generally available. The estimation penetration resistance should thus to a high extent be based on empirical evidence rom relevant conditions, taking account or the eects described above. Control o soil heave The control o soil heave should be based on a detailed monitoring o penetration, tilt, heave o soil plug, suction pressures, etc. As long as there is a net weight rom the jacket acting on the skirted oundation, the amount o soil heave inside the skirted oundation is normally controlled by controlling the discharge o water through the pumps. The restraining eects on a jacket with 4 or more legs should be taken into account. By applying suction to one skirted oundation only, the weight rom the jacket on that skirted oundation will decrease. I the weight rom the jacket is reduced to zero, signiicant soil heave may occur. This should be avoided. A penetration procedure should normally be aimed at, where the jacket as ar as possible is penetrated in a level position. K Skirted oundation structural design Reerence is made to Section 6 and 8 or general static strength and atigue requirements. The skirted oundation should be designed or all relevant design conditions including transportation orces orces when lowering the skirted oundations into the water and through the water line orces due to the motions o the jacket when the skirts start penetrating the seabed the suction applied during skirt penetration operational orces The buckling resistance during penetration o the skirts should be checked or various stages o the penetration using calculation models that account or the boundary conditions both at the skirt top as well as within the soil. The design or operational loading conditions should be based on soil reaction distributions that are derived rom realistic soil/structure interaction analyses. Alternatively more simple approaches with obviously conservative soil reaction distributions can be used. NORSOK standard Pge 153 o 64

155 Annex K Rev. 3 February 013 K.6.4 On-bottom stability The oundation system or the jacket temporary on-bottom condition prior to installation o the permanent oundation system shall be documented to have the required oundation stability or the governing environmental conditions as speciied, and or all relevant limit states. NORSOK standard Pge 154 o 64

156 Annex K Rev. 3 February 013 K.7 DOCUMENTATION REQUIREMENTS FOR THE DESIGN BASIS & DESIGN BRIEF K.7.1 General Adequate planning shall be undertaken in the initial stages o the design process in order to obtain a workable and economic structural solution to perorm the desired unction. As an integral part o this planing, documentation shall be produced identiying design criteria and describing procedures to be adopted in the structural design o the platorm. Applicable codes, standards and regulations shall be identiied at the commencement o the design. When the design has been inalised, a summary document containing all relevant data rom the design and abrication and installation phase (DFI) shall be produced. Design documentation (see below) shall, as ar as practicable, be concise, non-voluminous, and, should include all relevant inormation or all relevant phases o the lietime o the unit. General requirements to documentation relevant or structural design are given in NORSOK, N-001, Section 5. K.7. Design basis A Design Basis Document shall be created in the initial stages o the design process to document the basis criteria to be applied in the structural design o the unit. A summary o those items normally to be included in the Design Basis document is included below. Platorm location and main unctionalities, general description, main dimensions and water depth, applicable Regulations, Codes and Standards (including revisions and dates), service lie o platorm, topside interace requirements (including leg spacing, topside weight and c.o.g, appurtenance dimensions and routing), materials, coating and corrosion protection system, environmental and soil data, in-service inspection philosophy installation method, oundation system, system o units K.7.3 Design brie A Design Brie (DB) shall be created in the initial stages o the design process. The purpose o the Design Brie shall be to document the criteria and procedures to be adopted in the design o the jacket structure. The Design Brie shall, as ar as practicable, be concise, non-voluminous, and, should include all relevant limiting design criteria or the relevant design phase. Design Bries shall as a minimum cover the ollowing main design phases and designs: Load-out Transportation Installation Inplace Fatigue Earthquake and accidental Foundation, on-bottom stability and pile driving A summary o those items normally to be included in the Design Brie is included below. NORSOK standard Pge 155 o 64

157 Annex K Rev. 3 February 013 Environmental design criteria Limiting environmental design criteria (including all relevant parameters) or relevant conditions, including : wind, wave, current, earthquake, snow and ice description or relevant annual probability events. Temporary phases Design criteria or all relevant temporary phase conditions including, as relevant : limiting permanent, variable, environmental and deormation action criteria, essential design parameters and analytical procedures associated with temporary phases e.g. or loadout, transportation, liting, installation, on-bottom stability and pile installation and driving, platorm abandonment. Operational design criteria Design criteria or relevant operational phase conditions including : limiting permanent, variable, environmental and deormation action criteria, deck load description (maximum and mimimum) including variation in gravity, designing accidental event criteria (e.g. collision criteria, earthquake, explosion and ire), soil parameters. Global structural analyses A general description o models to be utilised in the global analysis including : description o global analysis model(s) including modelling or wave and current loading, oundation system, description o analytical procedures (including methodology, actors, dynamic representation and relevant parameters). Structural evaluation A general description o the structural evaluation process including : description o local analytical models, description o procedures to be utilised or combining global and local responses, criteria or member and joint code checking, description o atigue analytical procedures and criteria (including design atigue actors, S-N-curves, basis or stress concentration actors (SCF s), etc.). Miscellaneous A general description o other essential design inormation, including : description o corrosion allowances, where applicable, in-service inspection criteria (as relevant or evaluating atigue allowable cumulative damage ratios). NORSOK standard Pge 156 o 64

158 Annex K Rev. 3 February 013 K.8 REFERENCES /1/ International Standard ISO 19900: Petroleum and natural gas industries Oshore structures, General requirements // Norges Standardiseringsorbund, NS3481: Soil investigation and geotechnical design or marine structures, 1989 /3/ American Petroleum Institute, API RPA LRFD: Planning, designing and constructing ixed oshore platorms Load and Resistance Factor Design, 1993 /4/ DNV Class Note 30.4: Foundations, 199 /5/ Nowacki,F., Karlsrud,K., Sparrevik,P.: "Comparison o Recent Tests on OC Clay and Implications or Design". Conerence on Recent Large Scale Fully Instrumented Pile Tests in Clay, The Institute o Civil Engineers, London, June 199. Also published in NGI Publication No /6/ Jardine, R. J and F. C. Chow: New design methods or oshore piles, The Marine Tecnology Directorate Ltd, Publication 96/103 /7/ Andersen, K.H. and Lauritzen, R. Bearing Capacity o Foundations with Cyclic Loads, Journal o Geotechnical Engineering, Vol. 114, No.5, May 1988 /8/ Bye,A., Erbrich,C., Rognlien,B., Tjelta,T.I.: Geotechnical Design o Bucket Foundations, OTC paper 7793, 7 th annual OTC in Houston, May /9/ Tjelta,T.I. : Geotechnical Experience rom the Installation o the Europipe Jacket with Bucket Foundations, OTC paper 7795, 7 th annual OTC in Houston, May /10/ Sparrevik,P. : Suction in Sand New Foundation Technique or Oshore Structures, Proceedings o the 8 th European Young Geotechnical Engineers Conerence, Stara Lesna, Slovakia, Also published in NGI Publication No /11/ Andersen, K.H., Allard, M.A. and Hermstad,J. Centriuge model tests o a gravity platorm on very dense sand; II Interpretation BOSS 94, Seventh International Conerence on the Behaviour o Oshore Structures at MIT July 1994, Volume I Geotechnics. Also published in NGI Publ. No o0o - NORSOK standard Pge 157 o 64

159 Annex L Rev. 3, February 013 DESIGN OF STEEL STRUCTURES ANNEX L SPECIAL DESIGN PROVISIONS FOR SHIP SHAPED UNITS NORSOK standard Page 158 o 64

160 Annex L Rev. 3, February 013 CONTENTS L.1 INTRODUCTION 161 L.1.1 General 161 L.1. Deinitions 161 L. BASIS OF DESIGN 163 L..1 Saety ormat 163 L.. Design criteria 163 L...1 Non-operational phases 163 L... Operational phases 164 L.3 STRUCTURAL CLASSIFICATION AND MATERIAL SELECTION 165 L.3.1 Structural classiication 165 L.3. Material selection 165 L.3.3 Inspection categories 166 L.3.4 Guidance to minimum requirements 166 L.4 DESIGN ACTIONS 171 L.4.1 General 171 L.4. Permanent actions (G) 171 L.4.3 Variable actions (Q) 171 L.4.4 Deormation actions (D) 171 L.4.5 Environmental actions (EA) 17 L Mooring actions 17 L.4.5. Sloshing actions in tanks 17 L Green water eect 173 L Slamming in the ore and at ship 173 L.4.6 Accidental actions (A) 174 L.4.7 Fatigue actions (F) 174 L.4.8 Combination o actions 175 L.5 STRUCTURAL RESPONSE 176 L.5.1 General 176 L.5. Analysis models 177 L.5..1 General 177 L.5.. Global structural model (Model level 1) 177 L.5..3 Cargo tank model (Model level ) 177 L.5..4 Turret analysis (Model level ) 178 L.5..5 Local structural analysis (Model level 3) 178 L.5..6 Stress concentration models (Model level 4) 178 L.5.3 Calculation o wave induced actions 179 L General 179 L.5.3. Transer unctions 179 L Viscous damping 179 L Extreme wave induced responses 179 L Non-linear eects 180 L.6 ULTIMATE LIMIT STATES (ULS) 181 L.6.1 Global strength 181 L General 181 L.6.1. Calculation o global stresses 18 L Calculation o local transverse and longitudinal stresses 18 L Calculation o local pressures 184 L Combination o stresses 184 L Transverse structural strength 185 L Capacity check 185 L.6. Local structural strength 185 L.6.3 Turret and turret area / moonpool 186 L General 186 L.6.3. Structure in way o moonpool opening in the unit hull 186 L Turret structure 186 NORSOK standard Page 159 o 64

161 Annex L Rev. 3, February 013 L.6.4 Topside acilities structural support 187 L.6.5 Transit conditions 187 L.7 FATIGUE LIMIT STATES (FLS) 188 L.7.1 General 188 L.7. Design atigue actors 188 L.7.3 Splash zone 189 L.7.4 Structural details and stress concentration actors 190 L.7.5 Design actions and calculation o stress ranges 190 L Fatigue actions 190 L.7.5. Topside structures 190 L Turret structure 191 L Calculation o global dynamic stress ranges 191 L Calculation o local dynamic stress ranges 191 L Combination o stress components 191 L.7.6 Calculation o atigue damage 191 L General 191 L.7.6. Simpliied atigue analysis 19 L Stochastic atigue analysis 19 L.8 ACCIDENTAL LIMIT STATES (ALS) 193 L.8.1 Dropped objects 193 L.8. Fire 193 L.8.3 Explosion 193 L.8.4 Collision 193 L.8.5 Unintended looding 193 L.8.6 Loss o heading control 193 L.9 COMPARTMENTATION & STABILITY 194 L.9.1 General 194 L.9. Compartmentation and watertight integrity 194 L.9.3 Stability 194 L.10 SPECIAL CONSIDERATION 195 L.10.1 Structural details 195 L.10. Positioning o superstructure 195 L.10.3 Structure in way o a ixed mooring system 195 L.10.4 Inspection and maintenance 195 L.10.5 Facilities or inspection on location 195 L.10.6 Action monitoring 195 L.10.7 Corrosion protection 196 L.11 DOCUMENTATION 197 L.11.1 General 197 L.11. Design basis 197 L.11.3 Design brie 198 L.11.4 Documentation or operation (DFO) 199 L.1 REFERENCES 00 L.13 COMMENTARY 01 NORSOK standard Page 160 o 64

162 Annex L Rev. 3, February 013 L.1 INTRODUCTION L.1.1 General This Annex is intended to provide requirements and guidance to the structural design o ship shaped units constructed in steel, according to the provisions o relevant NORSOK standards. The Annex is intended as the rest o the standard to ulill NPD regulations relating to design and outitting o acilities etc in the petroleum activities /4/ and NORSOK N-001. In addition it is intended that the unit shall ulill technical requirements to standard ship hull design. Thereore reerences to the technical requirements in maritime standards such as NMD, IMO and DNV classiication rules or guidance and requirements to design is also given. The Annex is intended as being generally applicable to all types o conventional ship shaped structures, including the ollowing variants: Floating Production Units (FPU) Floating Storage and Oloading Units (FSU) Floating Production, Storage and Oloading Units (FPSO) Floating Production, Drilling, Storage and Oloading Units (FPDSO). In this Annex the above will collectively be reerred to as units. This Annex is intended to cover several variations with respect to conceptual solutions as listed below: Units intended or production which may be equipped with topside structures, supporting the production acilities. Units intended or storage with storage tanks together with acilities or oloading to shuttle tankers. Units intended or production will normally have a turret installed, while units intended or storage only, may have a buoy installed, replacing the turret. Units that can either be permanently moored on site or have a disconnectable mooring system. In the latter case, the unit may disconnect rom its moorings and leave the site under its own power or assisted by tugs, to avoid certain exceptional events such as extreme storms, icebergs, hurricanes etc. Normally the mooring lines are connected to the turret or buoy, positioned orward o the midship area. Requirements concerning mooring and riser systems other than the interaces with the structure o the units are not explicitly considered in this Annex. The intention o this Annex is to cover units weather vaning by rotating around a turret or a buoy. Fixed spread mooring arrangements (and similar) should be specially considered. L.1. Deinitions Ship Shaped Floating Production Units: A loating unit can be relocated, but is generally located on the same location or a prolonged period o time. Inspections and maintenance can be carried out on location. The Ship Shaped Floating Productions unit may consist o a ship shaped hull, with turret, and production equipment on the deck. Ship Shape Floating Storage and Oloading Units: A loating unit can be relocated, but is generally located on the same location or a prolonged period o time. Inspections and maintenance can be carried out on location. The Ship Shaped Floating Storage and Oloading units normally consist o a ship shaped hull equipped or crude oil storage. The crude oil may be transported to shore by shuttle tankers via an oloading arrangement. Ship Shaped Floating Production, Storage and Oloading Units: A loating unit can be relocated, but is generally located on the same location or a prolonged period o time. Inspections and maintenance are carried out on location. The Ship Shaped Floating Production, Storage and Oloading unit normally consists o a ship shaped hull, with turret, and production equipment on the deck. The unit is equipped or crude oil storage. The crude may be transported to shore by shuttle tankers via an oloading arrangement. Ship Shaped Floating Production, Drilling, Storage and Oloading Units: NORSOK standard Page 161 o 64

163 Annex L Rev. 3, February 013 A loating unit can be relocated, but is generally located on the same location or a prolonged period o time. Inspections and maintenance are carried out on location. The Ship Shaped Floating Production, Drilling, Storage and Oloading unit normally consists o a ship shaped hull, with turret, and production and drilling equipment on the deck. The unit is equipped or crude oil storage. The crude may be transported to shore by shuttle tankers via an oloading arrangement. Turret: A device providing a connection point between the unit and the combined riser- and mooring- systems, allowing the unit to rotate around the turret (weather vane) without twisting the risers and mooring lines. RCS: Recognised Classiication Society NORSOK standard Page 16 o 64

164 Annex L Rev. 3, February 013 L. BASIS OF DESIGN L..1 Saety ormat Generally, the design o units shall be based upon the partial actor design methodology as described in NORSOK N-001, based on direct calculated actions as described in L.5. To ensure that experiences with traditional ships are taken into account in the design, units shall as a minimum, ulil the relevant technical requirements given in the DNV Rules or Ship, Pt.3 Ch.1, /3/. Due consideration should be made with respect to the relevance o actions speciied, taking into account possible increase in action level due to the dierences in operation o the unit compared to normal trading ships. L.. Design criteria The design o units should as a minimum, comply with the technical requirements given in the DNV Rules or Ships, Pt.3 Ch.1, /3/. In cases where the unit is registered in a speciic country (as though it was a merchant ship), the relevant requirements o the lag state authority shall additionally be complied with. A unit may be designed to unction in a number o operational modes, e.g. transit, operation and survival. Limiting design criteria or going rom one mode to another shall be clearly established and documented when relevant. In these operational modes, the design criteria normally relate to consideration o the ollowing items: Intact strength covering transit, temporary conditions (e.g. installation), extreme operating conditions and survival conditions (relevant only i normal operation o the unit is terminated when limiting weather conditions or the extreme operating condition are exceeded). Structural strength in damaged condition Compartmentation and stability I it is intended to dry-dock the unit, the bottom structure shall be suitable strengthened to withstand such actions. L...1 Non-operational phases The structure shall be designed to resist relevant actions associated with conditions that may occur during all relevant stages o the lie cycle o the unit. Such stages may include: abrication, site moves, sea transportation, installation, and, decommissioning. Structural design covering marine operation construction sequences shall be undertaken in accordance with NORSOK, N-001. Marine operations should be undertaken in accordance with the requirements stated in the VMO standard. Sea Transportation A detailed transportation assessment shall be undertaken which includes determination o the limiting environmental criteria, evaluation o intact and damage stability characteristics, motion response o the global system and the resulting, induced actions. The occurrence o slamming actions on the structure and the eects o atigue during transport phases shall be evaluated when relevant. Satisactory compartmentation and stability during all loating operations shall be ensured. Technical requirements to structural strength and stability as stated in relevant parts o the DNV Rules or Ships, /3/, e.g. Pt.3 Ch.1 and Ch. 4 should be complied with or transit conditions where such rules are ound to be applicable. Installation Installation procedures o oundations (e.g. drag embedded anchors, piles, suction anchor or dead weights etc.) shall consider relevant static and dynamic actions, including consideration o the maximum environmental conditions expected or the operations. NORSOK standard Page 163 o 64

165 Annex L Rev. 3, February 013 Installation operations shall consider compartmentation and stability, and, dynamic actions on the mooring system(s). The actions induced by the marine environment involved in the operations and the orces exerted by the positioning equipment, such as airleads and padeyes, shall be considered or local strength checks. Decommissioning Abandonment o the unit shall be planned or at the design stage. However, decommissioning phases or ship shaped units are normally not considered to provide design action conditions or the unit and may normally be disregarded at the design phase. L... Operational phases The unit shall be designed to resist all relevant actions associated with the operational phases o the unit. Such actions include ; Permanent actions (see L.4.) Variable actions (see L.4.3) Deormation actions (see L.4.4) Environmental actions (see L.4.5) Accidental action (see L.4.6) Fatigue actions (see L.4.7) Each mode o operation, such as production, work over, or, combinations thereo, shall be considered. The unit may be designed to unction in a number o operational modes, e.g. operational and survival. Limiting design criteria or going rom one mode o operation to another mode o operation shall be clearly established and documented. NORSOK standard Page 164 o 64

166 Annex L Rev. 3, February 013 L.3 STRUCTURAL CLASSIFICATION AND MATERIAL SELECTION L.3.1 Structural classiication Selection o steel quality, and requirements or inspection o welds, shall be based on a systematic classiication o welded joints according to the structural signiicance and the complexity o the joints/connections as documented in Chapter 5. In addition to in-service operational phases, consideration shall be given to structural members and details utilised or temporary conditions, e.g. abrication, liting arrangements, towing arrangements, etc. Basic Considerations Structural connections in ship shaped units designed in accordance with this Annex will normally not all within the categorisation criteria relevant or Design Class DC1 or DC. In particular, relevant ailure o a single weld, or element, should not lead to a situation where the accidental limit state damaged condition is not satisied. Structural connections will thereore be categorised within classiication groups DC3, DC4 or DC5. Consideration shall be given to address areas where through thickness tensile properties may be required. Special consideration shall be given to ensure the appropriate inspection category or welds with high utilisation in atigue i the coverage with standard local area allocation is insuicient. Examples o typical design classes applicable to ship shaped units are stated below. These examples provide minimum requirements and are not intended to restrict the designer in applying more stringent requirements should such requirements be desirable. Typical locations - Design class : DC3 Structure in way o moonpool Structure in way o turret, airlead and winches supporting structure Highly stressed areas in way o main supporting structures o heavy substructures and equipment e.g. anchor line airleads, cranes substructure, gantry, topside support stools, lare boom, helicopter deck, davits, hawser brackets or shuttle tanker, towing brackets etc. DC3 areas may be limited to local, highly stresses areas i the stress gradient at such connections is large. Typical locations - Design class : DC4 In general all main loadbearing structural elements not described as DC3, such as: Longitudinal structure Transverse bulkheads Transverse rames an stringers Turret structure Accommodation Typical locations - Design class : DC5 Non main loadbearing structural elements, such as: Non-watertight bulkheads, Tween decks Internal outitting structure in general other non-loadbearing components L.3. Material selection Material speciications shall be established or all structural materials utilised in a ship shaped unit. Such materials shall be suitable or their intended purpose and have adequate properties in all relevant design conditions. Material selection shall be undertaken in accordance with the principles given in NORSOK M When considering criteria appropriate to material grade selection, adequate consideration shall be given to all relevant phases in the lie cycle o the unit. In this connection there may be conditions and criteria, other than those rom the in-service, operational phase, that provide the design requirements in respect to the NORSOK standard Page 165 o 64

167 Annex L Rev. 3, February 013 selection o material. (Such criteria may, or example, be design temperature and/or stress levels during marine operations.) Selection o steel quality or structural components shall normally be based on the most stringent Design Class o the joints involving the component. Requirements to through-thickness strength shall be assessed. The evaluation o structural resistance shall include relevant account o variations in material properties or the selected material grade. (e.g. Variation in yield stress as a unction o thickness o the base material). L.3.3 Inspection categories Welding, and the extent o non-destructive examination during abrication, shall be in accordance with the requirements stipulated or the appropriate 'inspection category' as deined in NORSOK, M-101. The lower the extent o NDT selected, the more important is the representativeness o the NDT selected and perormed. The designer should thereore exercise good engineering judgement in indicating priorities o locations or NDT, where variation o utility along welds is signiicant. L.3.4 Guidance to minimum requirements The ollowing igures illustrate minimum requirements to selection o Design Class and Inspection Category or typical structural conigurations o ship shaped units. The indicated Design Class and Inspection Categories should be regarded as guidance o how to apply the recommendations in chapter 5. Figure L.3-1 Example o typical design classes and inspection categories or butt welds in the midship area Key: 1. [DCn,..] Design Class, n. [.., N] Inspection Category, N Notes: 1. Inspection draught see L.7.3 Transverse buttweld DC4, B - External above min. inspection draught, and internal. A - External below min. inspection draught. Longitudinal buttweld DC4, C - External below min. inspection draught. D - External above min. inspection draught, and internal. 1) NORSOK standard Page 166 o 64

168 Annex L Rev. 3, February 013. The selection o the inspection categories is made on the ollowing assumptions with reerence to Table 5.3 and 5.4: Transverse buttwelds, external above min. insp. draught and internal High atigue, (re. Table 5.4), dominating dynamic principal stress transverse to the weld. Transverse buttwelds, below min. insp. draught High atigue, (re. Table 5.4), dominating dynamic principal stress transverse to the weld. No access or in-service inspection and repair. Longitudinal buttwelds, external above min insp. draught and internal Low atigue, low tensile stress transverse to the weld. Longitudinal buttwelds, below min. insp. draught Low atigue, low tensile stress, transverse to the weld. No access or in-service inspection and repair. NORSOK standard Page 167 o 64

169 Annex L Rev. 3, February 013 C L DC4, B Welds between transverse rames, stringer/girder and internal structure in general DC4, D. DC4, B DC4, B DC4, B DC4, B DC4, C - External below min. inspection draught D - External above min. inspection draught and internal (typ. longitudinal weld) Typ. transverse rame bracket Similar or stringers on transverse bulkheads MPI Mandatory in way o toe Figure L.3- Example o typical design classes and inspection categories or the transverse rames in the midship area Key: 1. [DCn,..] Design Class, n. [.., N] Inspection Category, N Notes: 1. Inspection drat see L.7.3. The Design Classes and Inspection Categories should be applied along the whole length o the welds 3. For welds o longitudinal girders and stringers MPI should be mandatory in way o transverse rame intersection. NORSOK standard Page 168 o 64

170 Annex L Rev. 3, February 013 MPI. Mandatory MPI Mandatory DC4, C 1) DC4 D ) MPI Mandatory Figure L.3-3 Example o typical design classes and inspection categories longitudinal stieners connection to transverse rames / bulkheads Key: 1. [DCn,..] Design Class, n. [.., N] Inspection Category, N Notes: 1. The selected inspection class is relevant or a high atigue loading with principal stresses parallel to the weld and assuming in-service access (see table 5.4). The selected inspection class is relevant or a low atigue joint with low tensile stresses perpendicular to the weld NORSOK standard Page 169 o 64

171 Annex L Rev. 3, February 013 MPI Mandatory ) Longitudinal DC4 C - External below min. inspection draught 1), 3) D - External above min. inspection draught and internal DC4, C ) DC4, D MPI Mandatory in way o toe (typical) ) Transverse rame/ Bulkhead Figure L.3-4 Example o design classes and inspection categories longitudinal stieners connection to transverse rames / bulkheads Key: 1. [DCn,..] Design Class, n. [.., N] Inspection Category, N Notes: 1. Inspection drat see L.7.3. The proposed inspection categories are relevant or high atigue loading, or longitudinal stiener in the sideshell. 3. The proposed inspection categories are relevant or low atigue loading with low tensile stresses transverse to the weld, and no access or repair below min. inspection draught. NORSOK standard Page 170 o 64

172 Annex L Rev. 3, February 013 L.4 DESIGN ACTIONS L.4.1 General Characteristic actions shall be used as reerence actions in the partial coeicient method. Design actions relevant or ship shaped units are in general deined in NORSOK N-003. Guidance concerning action categories relevant or ship shaped unit designs are given in the ollowing. Design action criteria dictated by operational requirements shall be ully considered. Examples o such requirements may be: drilling (i applicable), production, workover and combination thereo, consumable re-supply procedures and requency, maintenance procedures and requency, possible action changes in extreme conditions. L.4. Permanent actions (G) Permanent actions are actions that will not vary in magnitude, position or direction during the period considered. Permanent actions relevant or ship shaped unit designs are: 'lightmass' o the unit including mass o permanently installed modules and equipment, such as accommodation, helideck, cranes, drilling (i applicable) and production equipment, L.4.3 hydrostatic pressures resulting rom buoyancy, pretension in respect to mooring, drilling (i applicable) and production systems (e.g. mooring lines, risers etc.). Variable actions (Q) Variable actions are actions that may vary in magnitude, position and direction during the period under consideration. Except where analytical procedures or design speciications otherwise require, the value o the variable actions utilised in structural design should normally be taken as either the lower or upper design value, whichever gives the more unavourable eect. Variable actions on deck areas or local, primary and global design are stated in NORSOK, N-003. For global design the actor to be applied to primary design actions (as stated NORSOK, N-001) is not, however, intended to limit the variable load carrying capacity o the unit in respect to non-relevant, variable load combinations. Global load cases should be established based upon 'worst case', representative variable load combinations, where the limiting global criteria is established by compliance with the requirements to intact and damage hydrostatic and hydrodynamic stability. Global design action conditions, and associated limiting criteria, shall be documented in the stability manual or the unit, see L.9. Variations in operational mass distributions (including variations in tank illing conditions) shall be adequately accounted or in the structural design. Local design actions or all decks shall be documented on a load-plan. This plan shall clearly show the design uniorm and concentrated actions or all deck areas or each relevant mode o operation. Dynamic actions resulting rom low through air pipes during illing operations shall be adequately considered in the design o tank structures. L.4.4 Deormation actions (D) Deormation actions are actions due to deormations applied to the structure. Deormation actions resulting rom construction procedures and sequences (e.g. as a result o welding sequences, orced alignments o structure etc.) shall be accounted or when relevant. Other relevant deormation action eects may include those resulting rom temperature gradients, as or example: when hot-oil is stored in the cargo tanks, radiation rom laring operations. Delection imparted to topside structure due to G, Q or EA actions acting on the ship hull, shall not be considered as deormation actions. NORSOK standard Page 171 o 64

173 Annex L Rev. 3, February 013 L.4.5 Environmental actions (EA) Environmental actions are actions caused by environmental phenomena. The characteristic value o an environmental action is the maximum or minimum value (whichever is the most unavourable) corresponding to an action eect with a prescribed probability o exceedance. Environmental conditions shall be described using relevant data or the relevant period and areas in which the ship shaped unit is to be abricated, transported, installed and operated. The long-term variation o environmental phenomena such as wind, waves and current shall be described by recognised statistical distributions relevant to the environmental parameter considered, (NORSOK N-003). Inormation on the joint probability o the various environmental actions may be taken into account i such inormation is available and can be adequately documented. Consideration shall be given to responses resulting rom the ollowing listed environmental induced action eects that may be relevant or the design o a ship shaped unit: wave actions acting on the hull, (including variable sea pressure) wind actions, current actions, temperature actions, snow and ice actions. dynamic stresses or all limit states (e.g. with respect to atigue see L.4.7) rigid body motion (e.g. in respect to air gap and maximum angles o inclination), sloshing, slamming (e.g. on bow and bottom in ore and at ship) green water on the deck vortex induced vibrations (e.g. resulting rom wind actions on structural elements in a lare tower), wear resulting rom environmental actions at mooring and riser system interaces with hull structures, For ship shaped units with traditional, catenary mooring systems, earthquake actions may normally be ignored. Further considerations in respect to environmental actions are given in NORSOK, N-003. Analytical considerations with regard to evaluation o global response resulting rom environmental action components are stated in L.5. L Mooring actions A unit may be kept on station by various methods, depending on speciic site criteria and operational goals. These methods may include several dierent types o station-keeping systems such as internal and submerged turret systems, external turret, CALM buoy, ixed spread mooring and dynamic positioning. Each mooring system coniguration will impose actions into the hull structure which are characteristic to that system. These actions shall be addressed in the structural design o the unit, and combined with other relevant action components. Only mooring arrangements, which allows weather vaning o the unit, are covered by this annex. L.4.5. Sloshing actions in tanks Merchant ships are normally operated so that cargo tanks are typically either nearly ull or nearly empty. For a loating production unit, in general, no restrictions should be imposed on partly illing o cargo tanks, and some cargo tanks may be partially illed most o the time. For partially illed tanks, the phenomenon o resonant liquid motion (sloshing) inside the tank should be considered. Sloshing is deined as a dynamic magniication o internal pressures acting on the boundaries o cargo tanks and on internal structure within the tank, to a level greater than that obtained rom static considerations alone. Sloshing occurs i the natural periods o the luid and the vessel motions are close to each other. Major actors governing the occurrence o sloshing are: tank dimensions tank illing levels structural arrangements inside the tank (wash bulkheads, web rames etc.) transverse and longitudinal metacentric height (GM) vessel draught natural periods o ship and cargo in roll (transverse) and pitch (longitudinal) modes. NORSOK standard Page 17 o 64

174 Annex L Rev. 3, February 013 The pressure ields created by sloshing o the cargo/ballast may be considered, according to the requirements given in DNV-OS-C10 /1/. The DNV Rules dierentiate between ordinary sloshing loads (non-impact) and sloshing impact actions. To arrive at 100 year return period impact pressures, the 10-8 values should be multiplied by a actor 1.15 (See Commentary). For units to be operated in severe environments, direct calculation o sloshing pressures using site-speciic criteria, should be considered. L Green water eect The green water eect is the overtopping by water in severe wave conditions. Signiicant amounts o green water will inluence the deck structural design, accommodation superstructure, equipment design and layout, and may induce vibrations in the hull. Normally the orward part o the deck and areas at o midship will be most severe exposed to green water. Short wave periods are normally most critical. Appropriate measures should be considered to avoid or minimize green water eects on the ship structure and topside equipment. These measures include bow shape design, bow lare, breakwaters and other protective structure. Adequate drainage arrangements shall be provided. Exposed structural members or topside equipment on the weather deck shall be designed to withstand the loads induced by green water. For (horizontal) weather deck structural members this load will mainly be the hydrostatic load rom the green water. For horizontal or vertical topside members this load will mainly be the dynamic action rom the green water rushing over the deck. In lack o more exact inormation, or example rom model testing, relevant technical requirements o the DNV-OS-C10 /1/ may be applied. L Slamming in the ore and at ship Slamming is impact rom waves which give rise to an impulsive pressure rom the water, resulting in a dynamic transient action (whipping) o the hull. The most important locations to be considered with respect to slamming are the orward bottom structure, the bow lare and accommodation structure in the ore ship. Slamming orces should be taken into account in the turret design when relevant. Other locations on the hull e.g. exposed parts o the at ship, which may be subject to wave impacts should be considered in each separate case. The requency o occurrence and severity o slamming are signiicantly inluenced by the ollowing: vessel draught, hull geometrical orm, site environment, heading, orward speed (including current) position o superstructure. The eects o slamming on the structure shall be considered in design particularly with regard to enhancement o global hull girder bending moments and shear orces induced by slamming, local strength aspects and limitations to ballast drat conditions. In lack o more exact inormation, or example rom model testing, relevant requirements o the DNV Rules or Classiication o Ships, /3/, may be applied: or bottom slamming in the ship ore body, Part 3, Chapter 1, Section 6, Bottom Structures, H00, Strengthening against slamming or bow lare slamming in the ship ore body, Part 3, Chapter 1, Section 7, Side Structures, E100, Strengthening against bow impact. or bottom slamming in the ship at body, Part 3, Chapter 1, Section 7, Side Structures, E00, Stern slamming For bottom slamming in the ship ore body, in order to account or: 100 year return period Main wave direction head waves Long crested waves NORSOK standard Page 173 o 64

175 Annex L Rev. 3, February 013 the expression or coeicient c in Part 3, Chapter 1, Section 6, Bottom Structures, H00, Strengthening against slamming, should be changed to: 1T BF c L For bow lare slamming, in order to account or: 100 year return period Main wave direction head waves Long crested waves the expression or bow lare slamming pressures in the ship ore body, Part 3, Chapter 1, Section 7, Side Structures, E103, are to be multiplied by a actor: 5 ac 1 L The applicable speed V is not to be taken less than 8 knots. For bottom slamming in the ship at body, in order to account or: 100 year return period Main wave direction head waves Long crested waves the expression or stern slamming pressure in Part 3, Chapter 1, Section 7, Side Structures, E03, are to be multiplied by a actor: 5 ac 1 L The resulting slamming impact pressures or bow impact pressures shall be multiplied by a actor i they are to be applied as a mean pressure over a larger area (applicable or global structural evaluation). L.4.6 Accidental actions (A) Accidental actions are actions related to abnormal operation or technical ailure. In the design phase particular attention should be given to layout and arrangements o acilities and equipment in order to minimise the adverse eects o accidental events. Risk analyses shall be undertaken to identiy and assess accidental events that may occur and the consequences o such events. Structural design criteria or the ALS condition are identiied rom the risk analyses. Generally, the ollowing ALS events are those required to be considered in respect to the structural design o a ship shaped unit : dropped objects (e.g. rom crane handling), ire, explosion, collision, unintended looding and counterlooding situations, and, abnormal wave events. The counterlooding situations shall consider the possible loading conditions ater an unintended looding has occurred, to compensate or the heeling resulting rom such looding. Further considerations in respect to accidental actions are given in Chapters 9 and Annex A. L.4.7 Fatigue actions (F) Repetitive actions, which may lead to possible signiicant atigue damage, shall be evaluated. In respect to a ship shaped unit, the ollowing listed sources o cyclic response shall, where relevant, be considered: waves (including those actions caused by dynamic pressure, sloshing / slamming and variable buoyancy), wind (especially when vortex induced vibration may occur), mechanical vibration (e.g. caused by operation o machinery), NORSOK standard Page 174 o 64

176 Annex L Rev. 3, February 013 crane actions, ull / empty variation o illing level in cargo tanks (low cycle). The eects o both local and global dynamic response shall be properly accounted or when determining response distributions o repetitive action eects. L.4.8 Combination o actions Action coeicients and action combinations or the design limit states are, in general, given in NORSOK, N Structural strength shall be evaluated considering all relevant, realistic action conditions and combinations. Scantlings shall be determined on the basis o criteria that combine, in a rational manner, the eects o relevant global and local responses or each individual structural element. A suicient number o load conditions shall be evaluated to ensure that the most probable largest (or smallest) response, or the appropriate return period, has been established. (For example, maximum global, characteristic responses or a ship shaped unit may occur in environmental conditions that are not associated with the characteristic, largest, wave height. In such cases, wave period and associated wave steepness parameters are more likely to be governing actors in the determination o maximum/minimum responses.) NORSOK standard Page 175 o 64

177 Annex L Rev. 3, February 013 L.5 STRUCTURAL RESPONSE L.5.1 General The structure shall as a minimum comply with the technical requirements o the DNV Rules or Ship, /3/, Pt.3 Ch.1, see L..1, as applicable to a normally trading ship. In addition, the dynamic actions to which the hull may be subjected will vary rom those associated with seagoing trading ships, and will require assessment. This assessment may be based on results o model testing and/or by suitable direct calculation methods o the actual wave actions on the hull at the service location, taking into account relevant service related actors. When the design is based on direct calculations, the sotware used should have been veriied by model testing. Suicient action conditions or all anticipated pre-service and in-service conditions shall be determined and analysed to evaluate the most unavourable design cases or the hull girder global and local strength analysis. For units intended or multi ield developments the most onerous still water and site speciic environmental actions shall be considered or the design. The action conditions shall be stated in the Operating Manual, see L The units shall comply with requirements taking into account site speciic service. Factors which may inluence the hull actions and shall be accounted or, include the ollowing: Site speciic environmental conditions Eect o mooring system Long term service at a ixed location Seas approaching predominantly rom a narrow sector ahead Zero ship speed Range o operating action conditions Tank inspection requirements Dierent return period requirements compared with normal trading tankers Site speciic environment; or ship shaped units o normal orm, strength standards set by RCS are based on criteria relating to a world-wide trading pattern. Eect o mooring system; static and dynamic mooring and riser actions can be substantial. Their eects on the hull girder longitudinal bending moments and shear orces shall be accounted or in the design calculations. Long term service at a ixed location; seagoing ships would generally spend a proportion o their time in sheltered water conditions. Permanently moored units would normally remain on station all the time and disconnectable units would only move o station in certain conditions and would generally remain in the local area. In addition the expectation o the ield lie may be in excess o 0 years. This shall be accounted or e.g. in the design o corrosion protection systems. Seas approaching rom a narrow sector ahead; or seagoing ships in severe weather, steps would generally be taken to minimise the eects o such conditions such as altering course or alternative routing. Moored production/storage units cannot take avoiding action and will weathervane due to the combined eects o waves, wind and current, resulting in a greater proportion o waves approaching rom bow sector directions. Zero ship speed; the eect o orward ship speed would enhance static predictions o hull bending moments and shear orces and other actors such as slamming. Due to zero orward speed this enhancement would not apply to moored production/storage units. Range o operating action conditions; ocean going tankers traditionally have a airly limited range o operational action conditions and would typically be ully loaded or in ballast. Due to their oil storage capability moored units however potentially have an almost unlimited range o action conditions. These eectively cover a ull spectrum o cases rom ballast through intermediate conditions to ully loaded returning to ballast via oloading. Tank inspection requirements; ocean-going ships would generally be taken to dry dock or periodic survey and repair. Moored units can be inspected on station. Thus a ull range o conditions covering each tank empty in turn, and tank sections empty should be addressed as appropriate or necessary access or inspection and maintenance. These conditions should be combined with appropriate site-speciic environmental actions. NORSOK standard Page 176 o 64

178 Annex L Rev. 3, February 013 Dierent return period requirements compared with normal trading tankers; normal ship class rule requirements are based on providing adequate saety margins based on a 0 years return period. This standard speciies that the design shall be based on data having a return period o 100 years or the site speciic action conditions. Taking into consideration the conditions listed above, calculations shall be carried out to address all design action conditions including ully loaded, intermediate operating conditions, minimum loaded condition, inspection conditions with each tank empty in turn, etc. Still water bending moment and shear orce distributions shall be calculated or each case and stated in the Operating Manual, see L L.5. Analysis models L.5..1 General Analysis models relevant or in-place ULS and FLS are described in this chapter. The inite element modelling o the hull structure should be carried out according with principles given in the ollowing. Four typical modelling levels are described below. Other equivalent modelling procedures may also be applied. L.5.. Global structural model (Model level 1) In this model a relatively coarse mesh extending over the entire hull should be used. The overall stiness o the primary members o the hull shall be relected in the model. The mesh should be ine enough to satisactorily take account o the ollowing: Vertical hull girder bending including shear lag eects Vertical shear distribution between ship side and bulkheads Stress distribution around large penetrations such as the moonpool Horizontal hull girder bending including shear lag eects Termination o longitudinal members The models should be used or analysing global wave response and still water response where ound relevant. L.5..3 Cargo tank model (Model level ) The cargo tank analysis shall be used to analyse local response o the primary hull structural members in the cargo area, due to relevant internal and external action combinations. The extent o the structural model shall be decided considering structural arrangements and action conditions. The mesh ineness shall be decided based on the method o action application in the model. The model should normally include plating, stieners, girders, stringers, web-rames and major brackets. For units with topside the stiness should be considered in the tank modelling. In some cases the topside structure should be included in the model. The cargo tank model may be included in the global structural model, see L.5... The inite element model should normally cover the considered tank, and one hal tank outside each end o the considered tank. The eect o non-structural elements in the topside may introduce additional stiness and should be considered. Conditions o symmetry should as ar as possible, be applied at each end o the inite element model. The model should be supported vertically by distributed springs at the intersections o the transverse bulkheads with ship sides and longitudinal bulkheads. The spring constants shall be calculated or each longitudinal bulkhead and ship sides based on actual bending and shear stiness and or a model length o three tanks. I horizontal unbalanced loads are applied to the model in transverse direction e.g. heeling conditions, horizontal spring supports should be applied at the intersections o all continuous horizontal structural members. The ollowing basic actions are normally to be considered or the ULS condition: Cargo and ballast loading, static and dynamic External sea pressure, static and dynamic Topside loading, vertical static and dynamic and also horizontal acceleration actions These ULS responses are to be combined with global stresses, see L.5... The ollowing basic actions are normally to be analysed in this model or the FLS condition: Dynamic cargo and ballast actions NORSOK standard Page 177 o 64

179 Annex L Rev. 3, February 013 External sea pressure range Topside loading, vertical dynamic and also horizontal acceleration actions These FLS responses are to be combined with global stresses, see L.5... L.5..4 Turret analysis (Model level ) A inite element model should be made o the turret structure. The model should relect the geometry and stiness o the structure. A model with relatively coarse mesh may be used. In cases where the stress distribution in the moonpool is dependent o the relative stiness between unit hull and turret a model comprising both the unit hull and the turret should be made. Modelling o interace areas may require iner mesh and use o gap elements etc. to describe the actual stress low. Following actions should normally be considered: Static and dynamic actions rom the turret itsel Mooring actions Riser actions The analysis should cover both the ULS and the FLS condition. L.5..5 Local structural analysis (Model level 3) The purpose o the local structural analysis is to analyse laterally loaded local stieners (including brackets) subject to large relative deormations between girders. The ollowing typical areas should be given particular attention: Longitudinal stieners between transverse bulkhead and the irst rame at each side o the bulkhead Vertical stieners at transverse bulkheads with horizontal stringers in way o inner bottom and deck connection Horizontal stieners at transverse bulkheads with vertical stringers in way o inner side and longitudinal bulkhead connection Corrugated bulkhead connections Local structural models may be included in the cargo tank analysis or run separately with prescribed boundary deormations or boundary orces rom the tank model. L.5..6 Stress concentration models (Model level 4) For atigue assessment, ine element mesh models shall be made or critical stress concentration details, or details not suiciently covered by stress concentration actors given in recognised standards, see Annex C. In some cases detailed element mesh models may be necessary or ultimate limit state assessment in order to check maximum peak stresses and the possibility o repeated yielding. The ollowing typical areas may have to be considered: Hopper knuckles in way o web rames Topside support stools Details in way o the moonpool Other large penetrations in longitudinal loadbearing elements Longitudinal bulkhead terminations Stiener terminations Other transition areas when large change in stiness occur The size o the model should be o such extent that the calculated stresses in the hot spots are not signiicantly aected by the assumptions made or the boundary conditions. Element size or stress concentration analyses is to be in the order o the plate thickness. Normally, shell elements may be used or the analysis. The correlation between dierent actions such as global bending, external and internal luid pressure and acceleration o the topside should be considered in the atigue assessment. For urther details reerence is made to DNV Classiication Note 30.7, /11/. The correlation between dierent actions such as global bending, external and internal luid pressure and acceleration o the topside should be considered in the atigue assessment. NORSOK standard Page 178 o 64

180 Annex L Rev. 3, February 013 L.5.3 Calculation o wave induced actions According to Recognised Classiication Society Rules, normal trading ships are designed according to the ollowing environmental criteria: North Atlantic wave condition, 0 year return period Short crested sea Same probability or all wave directions relative to the ship Speciic requirements or weather vaning units covered by this Annex are given below. L General Global linear wave induced actions such as bending moments and shear orces should be calculated by using either strip theory or three dimensional sink source (diraction) ormulation. Strip theory is a slender body theory and is not recommend when the length over beam ratio is less than 3. Generally, the most importance global responses are midship vertical bending moments and vertical shear orces in the ore and at body o the unit and the associated vertical bending moments. These responses should be calculated or head sea conditions. Horizontal and torsional moments may be o interest depending on the structural design, alone and in combination with other action components. The calculation o wave induced actions may ollow the ollowing steps: Calculation o the relevant transer unctions (RAOs) Calculation o the 100 year linear response Evaluation o non-linear eects When a 3-D diraction program is used, the hydrodynamic model shall consist o suicient number o acets. In general, the acets should be suiciently modelled to describe the unit in a propitiate way. The size o the acets shall be determined with due consideration to the shortest wave length included in the hydrodynamic analysis. Smaller acets should be used in way o the water surace. The mass model shall be made suiciently detailed to give centre o gravity, roll radius o inertia and mass distribution as correct as practically possible. L.5.3. Transer unctions The ollowing wave induced linear responses should be calculated: Motions in six degrees o reedom. Vertical bending moment at a suicient number o positions along the hull. The positions have to include the areas where the maximum vertical bending moment and shear orce occur and at the turret position. The vertical wave induced bending moment shall be calculated with respect to the section s neutral axis. Horizontal and torsional moment i applicable. The linear transer unctions shall be calculated by either strip or 3D sink source (diraction) theory. The responses shall be calculated or a suicient number o wave periods in the range rom 4 35 seconds. For short crested sea at least seven directions in a sector ± 90º relative to head sea, with maximum 30º interval, should be considered. L Viscous damping I roll resonance occurs within the range o wave periods likely to be encountered, the eect o non-linear viscous roll damping should be taken into account. The damping coeicients that are derived within linear potential theory relect the energy loss in the system due to generation o surace waves rom the ship motions. However, in the case o roll motion some special treatment is necessary. Viscous eects, such as the production o eddies around the hull, will mainly act as a damping mechanism, especially at large roll angles, and these eects should be included. Furthermore, the eects rom roll damping devices, like bilge keels, should be evaluated. The roll damping shall be evaluated or the return period in question. The sea state in question may be considered when the damping is calculated. L Extreme wave induced responses Extreme wave induced responses shall be long term responses. The standard method or calculation o the 100-year wave induced long term responses (e.g. vertical moment and shear orce) shall use a scatter diagram in combination with a wave spectrum (e.g. Jonswap) and RAOs.Short term predictions o the responses are to be calculated or all signiicant wave heights (H s ) and peak (T p ) or zero up-crossing (Tz) periods combinations within the scatter diagram. A Weibull distribution is itted to the resulting range o the NORSOK standard Page 179 o 64

181 Annex L Rev. 3, February 013 responses against probability o occurrence. This Weibull distribution is used to determine the response with a probability o occurrence corresponding to once every 100 years (100 year return period). The short crested nature o the sea may be taken into account as a wave spreading unction as given in NORSOK N-003. The method described above is considered most accurate or estimating the 100-year value or the response in question. A short-term analysis based on the predicted 100-year wave height with corresponding T z, will give comparable values i the ollowing criteria are satisied: The scatter diagram should be well developed with wave steepness approaching a constant value or the most extreme sea states The maximum wave induced response shall occur in a short term sea sate which is retraceable rom the scatter diagram with a 100 year wave height steepness The Weibull it to predict the response shall be good it with low residual sum (deviations rom the regressed line) It should be noted that the method o calculating the 100-year wave induced response or a short-term sea state based upon the predicted 100 year wave height with corresponding single value T z or T p does not recognise that there are a range o possible T z.(t p ). Thereore the range o possible T z (T p ) within the 100- year return period should be investigated. The range o sea states with a 100 year return period should be ound by developing a 100 year contour line rom the scatter diagram. The wave bending moment should be calculated or several sea states on this contour line in order to ind the maximum value. L Non-linear eects Linear calculations as described above do not dierentiate between sagging and hogging responses. The non-linearities shall be taken into account or the vertical bending moments and shear orces. The non-linear eects come rom integration o the wave pressure over the instantaneous position o the hull relative to the waves, with the inclusion o bottom slamming, bow lare orces and deck wetness. These eects generally result in a reduction in hogging response and an increase in the sagging response (midship moment and shear ore in the ore body) compared with the linear response. NORSOK standard Page 180 o 64

182 Annex L Rev. 3, February 013 L.6 ULTIMATE LIMIT STATES (ULS) L.6.1 Global strength L General Ultimate strength capacity check shall be perormed or all structure contributing directly to the longitudinal and transverse strength o the ship. Structure to be checked is all plates and continuous stieners including the ollowing structure: Main deck, bottom and inner bottom Ship side, inner ship side and longitudinal bulkheads Stringers and longitudinal girders Foundations o turret and topside structure Transverse bulkheads Transverse web rames Global actions on the hull girder shall be calculated by direct wave analyses, see L.5.3. Longitudinal wave bending moments, shear orces and dynamic external sea pressures should be calculated by a wave load analysis. Design accelerations should also be based on a wave load analysis. For unconventional designs torsional eects may also be o importance. The design moments, orces, pressures and accelerations shall be calculated or a representative number o sections. Stillwater shear orces and bending moments should be calculated by direct analyses, taking in account all relevant loading conditions. Internal static and dynamic pressures can be calculated by simpliied ormulas, see L Local stresses should be calculated by FE-analyses o relevant parts o the ship, see L.5.. All relevant load conditions should be taken in account. The hull girder strength shall be evaluated or relevant combinations o still water bending moment and shear orce, and wave induced bending moment and shear orce. The wave-induced bending moments and shear orces shall be calculated by means o an analysis carried out utilising the appropriate statistical site speciic environmental data. Relevant non-linear action eects shall be accounted or, see L.5.3. The ollowing design ormat shall be applied: S M S + w M w < M G / M S Q S + w Q w < Q G / M (L.6.1) (L.6.) where: M G = Characteristic bending moment resistance o the hull girder calculated as an elastic beam M S = Characteristic design still water bending moment based on actual cargo and ballast conditions M W = Characteristic wave bending moment. Annual probability o exceedance o Q G = Characteristic shear resistance o the hull girder calculated as an elastic beam Q S = Characteristic design still water shear orce based on actual cargo and ballast conditions Q W = Characteristic wave shear orce. Annual probability o exceedance o M = Material actor S = Permanent and variable action actor W = Environmental action actor The action actors shall be in accordance with NORSOK N-001, see also Table L.6-1: NORSOK standard Page 181 o 64

183 Annex L Rev. 3, February 013 Table L.6-1 Action S W combinations a b For combination o actions an action coeicient o 1.0 shall be applied or permanent actions where this gives the most unavourable response. The action coeicient or environmental actions may be reduced to 1.15 in action combination b, when the maximum stillwater bending moment represents between 0 and 50 % o the total bending moment. This reduction is applicable or the entire hull, both or shear orces and bending moments. The action coeicient or permanent actions in action combination a, may be reduced to 1., i actions and responses are determined with great accuracy e.g. limited by the air-pipe height, and external static pressure to well deined draught. Gross scantlings may be utilised in the calculation o hull structural resistance, provided a corrosion protection system in accordance with NORSOK N-001, is maintained. The buckling resistance o the dierent plate panels shall be considered according to Section 8. The global and local longitudinal stress components shall be combined in an appropriate manner with transverse stress and shear stress as relevant, see L L.6.1. Calculation o global stresses Global longitudinal stresses should be calculated by FE-analysis, see L.5.. For parallel parts o the midship, a simpliied calculation o the hull girder section modulus can be perormed. For units with moonpool / turret, a FE-analysis shall be carried out to describe the stress distribution in way o the openings, in particular in way o deck and bottom, and at termination o longitudinal strength elements. Global shear stresses shall be calculated considering the shear low distribution in the hull. L Calculation o local transverse and longitudinal stresses Local transverse and longitudinal stresses shall be superimposed with the global longitudinal stresses. Local stresses should be taken rom a FEM analysis o a typical cargo area, see L.5.. Phase inormation should be taken into account when available. The FEM analysis shall include extreme hogging and sagging conditions as described in Figure L.6-1. All relevant variations in tank illing should be considered in the analysis and relected in the Operation Manual. The ollowing stress components can be ound rom the FEM analysis, and should be combined with other stress components as described in L.6.1.5: Transverse stresses in webrames Double shell and double bottom stresses Local shear stresses in panels NORSOK standard Page 18 o 64

184 Annex L Rev. 3, February 013 TYPICAL LOADS AT MIDSHIPS SAGGING CONDITION TYPICAL LOADS AT MIDSHIPS HOGGING CONDITION Figure L.6-1 Typical extreme sagging and hogging conditions NORSOK standard Page 183 o 64

185 Annex L Rev. 3, February 013 L Calculation o local pressures Sea pressure : The dynamic sea pressure should be taken rom a 3D hydrodynamic wave analysis. Action coeicients according to L shall be considered. Tank pressure: The internal tank pressure shall be calculated in accordance with NORSOK N-003. L Combination o stresses Total longitudinal design stress in the structure can be calculated as: x, Total = x,global + x, Local (L.6.3) Total transverse design stress in the structure is ound directly rom the FE-analysis: y, Total = y, Local + y, Global (L.6.4) y, Global is normally relevant or the moonpool / turret area, and may be neglected in parallel parts o the midship. Total design shear stress in the structure can be calculated as: x, Total = Global + Local (L.6.5) The combination o stresses should take into account actual stress directions and phase. However, i phase inormation is limited or uncertain, the maximum design value or each component may be combined as a worst-case scenario. Combination o typical stress components is shown in Figure L.6-. NORSOK standard Page 184 o 64

186 Annex L Rev. 3, February 013 Figure L.6- Typical stress components acting on the hull beam. L Transverse structural strength Transverse strength reers to the ability o the hull to resist lateral pressure and racking actions in combination with longitudinal action eects. This resistance is provided by means o transverse bulkheads, web rames, girders and stringers. Transverse strength should be evaluated using a inite element model o a speciic portion o the hull, and the eects o process equipment deck actions should be included. The transverse strength shall meet the requirements given in L.6.1. The lateral pressure due to waves should be predicted using the hydrodynamic motion analysis approach, see L.5.3. L Capacity check Capacity checks shall be perormed in accordance with Chapter 6. L.6. Local structural strength Local structure not taking part o the overall strength o the unit may be designed in accordance with technical requirements given in the DNV Rules or Ships, Pt.3 Ch.1, /3/. Such structure may be in the ollowing areas: Fore Ship. At Ship. Superstructure e.g. Process Deck and Accommodation. For evaluation o slamming, sloshing and green sea eects, see L.4.5., L and L NORSOK standard Page 185 o 64

187 Annex L Rev. 3, February 013 L.6.3 Turret and turret area / moonpool L General The ollowing areas shall be considered as relevant, with respect to structural response rom the mooring actions, combined with other relevant actions: Structure in way o moonpool opening in the ship hull. Turret structure including support, towards the unit hull. Structure in way o loading buoy support. Gantry structure including support. L.6.3. Structure in way o moonpool opening in the unit hull The structural strength shall be evaluated considering all relevant, realistic action conditions and combinations, see L.4. In particular action combinations due to the ollowing shall be accounted or in the design: Turret bearing reactions. Overall hull bending moments and shear orces. Internal and external pressure actions, covering the intended range o draughts and action conditions, including non-symmetric cases as applicable. Finite element analyses shall be carried out. Particular attention shall be given to critical interaces. Functional limitations (bearing unction) and structural strength shall be evaluated. Continuity o primary longitudinal structural elements should be maintained as ar as practicable in way o the turret opening. Reductions in hull section modulus shall be kept at a minimum and compensation shall be made where necessary. L Turret structure A FEM analysis o the turret structure shall be perormed, see L.5..4, demonstrating that the structural strength o the turret is acceptable. The structural strength shall be evaluated considering all relevant, realistic action conditions and combinations, see L.4. In particular, action combinations due to the ollowing actions (as applicable) shall be accounted or in the design: Unit motion induced accelerations Mooring actions Riser actions Turret bearing reactions (calculated based on all the relevant actions on the turret) Moonpool deormations (based on hull bending moments and shear orces) Internal and external pressure actions, covering the intended range o draughts and load conditions including non-symmetric cases as applicable. Filling o void spaces to be accounted or (i relevant). Local actions rom equipment and piping system (weight, thermal expansion, mechanical actions) Green seas Wave slamming Boundary conditions or the model shall relect the true coniguration o the interace towards the unit. Local analyses, (model level 3 & 4, see L.5..5 and L.5..6) shall be perormed or structural areas, which are critical or the structural integrity o the turret. The ollowing list contains typical areas which should be considered: Structure in vicinity o riser connection(s) Riser hang-o structure Structure in way o airleads Hang-o structure or anchor line Local structure transerring bearing reactions Chain lockers Pipe supports (single supports and complex structures) Equipment supports Foundation or transer system (especially or swivel solutions) Liting appliances and pad-eyes including structure in way o these NORSOK standard Page 186 o 64

188 Annex L Rev. 3, February 013 L.6.4 Topside acilities structural support The structural strength o topside acilities structural support shall be evaluated considering all relevant action conditions and combinations o actions. Scantlings shall be determined on the basis o criteria, which combine, in a rational manner, the eects o global and local responses or each structural element. The ollowing actions shall be considered: Permanent actions (weight o structures, process and drilling equipment, piping etc.) Live actions (equipment unctional actions related to liquid, machinery, piping reaction orces, helicopter, cranes etc.) Wave actions Wave accelerations (inertia actions) Hull delections due to tank illing, wave, temperature dierences etc. Wind Snow and ice Green sea L.6.5 Transit conditions For sel-propelled units, requirements given by the RCS (and the lag state) or seagoing ships cover the transit condition. Due consideration should be made with respect to dynamic actions, in particular due to roll motion, on the topside equipment. NORSOK standard Page 187 o 64

189 Annex L Rev. 3, February 013 L.7 FATIGUE LIMIT STATES (FLS) L.7.1 General General requirements and guidance concerning atigue criteria are given in Chapter 8, and Annex C. Evaluation o the atigue limit states shall include consideration o all signiicant actions contributing to atigue damage both in non-operational and operational design conditions. The minimum atigue lie o the unit (beore the design atigue actor is considered) should be based upon a period o time not being less than the planned lie o the structure. Local eects, or example due to : slamming, sloshing, vortex shedding, dynamic pressures, and, mooring and riser systems, shall be included in the atigue damage assessment when relevant. Calculations carried out in connection with the atigue limit state may be undertaken without deducting the corrosion additions, provided a corrosion protection system in accordance with NORSOK, N-001 is maintained. In the assessment o atigue resistance, relevant consideration shall be given to the eects o stress raisers (concentrations) including those occurring as a result o : abrication tolerances, (including due regard to tolerances in way o connections o large structural sections), cut-outs, details at connections o structural sections (e.g. cut-outs to acilitate construction welding). L.7. Design atigue actors Criteria related to Design Fatigue Factors, are given in the Chapter 8. When determining the appropriate Design Fatigue Factor or a speciic atigue sensitive location, consideration shall be given to the ollowing : Consideration o economic consequence o ailure may indicate the use o larger design actors than those provided or as minimum actors. The categorisation : Accessible / Above splash zone is, intended to reer to atigue sensitive locations where the possibility or close-up, detailed inspection in a dry and clean condition exists. When all o these requirements are not ulilled, the relevant design atigue actor should be considered as being that appropriate or Accessible / Below splash zone, or, No access or in the splash zone as relevant to the location being considered. Evaluation o likely crack propagation paths (including direction and growth rate related to the inspection interval), may indicate the use o a dierent Design Fatigue Factor than that which would be selected when the detail is considered in isolation, such that : Where the likely crack propagation indicates that a atigue ailure, rom a location satisying the requirements or a 'Non-substantial' consequence o ailure, may result in a 'Substantial' consequence o ailure, such atigue sensitive location is itsel to be deemed to have a 'Substantial' consequence o ailure. Where the likely crack propagation is rom a location satisying the requirement or a given Access or inspection and repair category to a structural element having another access categorisation, such location is itsel to be deemed to have the same categorisation as the most demanding category when considering the most likely crack path. For example, a weld detail on the inside (dry space) o a submerged shell plate should be allocated the same Fatigue Design Factor as that relevant or a similar weld located externally on the plate see Figure L.7-1. NORSOK standard Page 188 o 64

190 Annex L Rev. 3, February 013 Figure L.7-1 Example illustrating considerations relevant or selection o design atigue actors (DFF s) in locations considered to have Non-substantial consequence o ailure. Notes: 1. Due to economic considerations (e.g. cost o repair to an external underwater structural element) the DFF s assigned a value o in Figure L.7-1 may be considered as being more appropriately assigned a value o 3.). The unit may be considered as accessible and above the splash zone (DFF = 1.0), see Annex C, i the survey extent e.g. given or main class (see DNV Ship Rules Pt.7 Ch.) is ollowed i.e. drydocking or inspection and maintenance every 5 years. L.7.3 Splash zone The deinition o splash zone as given in NORSOK N-003, relates to a highest and lowest tidal reerence. For ship shaped units, or the evaluation o the atigue limit state, reerence to the tidal datum should be substituted by reerence to the draught that is intended to be utilised when condition monitoring is to be undertaken. The requirement that the extent o the splash zone is to extend 5 m above and 4 m below this draught may then be applied. (For application o requirements to corrosion addition, however, the normal operating draught should generally be considered as the reerence datum). I signiicant adjustment in draught o the unit is possible in order to provide or satisactory accessibility in respect to inspection, maintenance and repair, account may be taken o this possibility in the determination o the Design Fatigue Factors. In such cases however, a margin o minimum 1 meter in respect to the minimum inspection draught should be considered when deciding upon the appropriate Design Fatigue Factor in relation to the criteria or Below splash zone as opposed to Above splash zone. Where drat adjustment possibilities exist, a reduced extent o splash zone may be applicable. Consideration should be given to operational requirements that may limit the possibility or ballasting / deballasting operations. When considering utilisation o Remotely Operated Vehicle ( ROV) inspection consideration should be given to the limitations imposed on such inspection by the action o water particle motion (e.g. waves). The practicality o such a consideration may be that eective underwater inspection by ROV, in normal sea conditions, may not be achievable unless the inspection depth is at least 10 metres below the sea surace. NORSOK standard Page 189 o 64

191 Annex L Rev. 3, February 013 L.7.4 Structural details and stress concentration actors Fatigue sensitive details in the FPSO should be documented to have suicient atigue strength. Particular attention should be given to connection details o the ollowing: Integration o the mooring system with hull structure Main hull bottom, side and decks Main hull longitudinal stiener connections to transverse rames and bulkheads Main hull attachments; seats, supports etc. Openings in main hull Transverse rames Flare tower Riser interaces Major process equipment seats Selections o local details and calculations o stress concentration actors may be undertaken in accordance with DNV Classiication Note 30.7, /11/. For details not covered in this document, stress concentration actors should be otherwise documented. Detailed inite element analysis may be utilized or determination o SCF s, according to procedure given in DNV Classiication Note 30.7, /11/. L.7.5 L Design actions and calculation o stress ranges Fatigue actions An overview o atigue actions is given in L.4.7. Site speciic environmental data shall be used or calculation o long term stress range distribution. For units intended or multi ield developments the site speciic environmental actions or each ield should be utilised considering the expected duration or each ield. The most onerous environmental actions may be applied or the complete lietime o the unit, as a conservative approach. A representative range o action conditions shall be considered. It is generally acceptable to consider two action conditions, typically: ballast condition and the ully loaded condition, with appropriate amount o time at each condition, normally 50 % or each condition unless otherwise documented. An appropriate range o wave directions and wave energy spreading shall be considered. For weather waning units, and in absence o more detailed documentation, the head sea direction shall be considered with the spreading taken as the most unavourable between cos cos 10, see NORSOK, N-003. Maximum spacing should be 30 degrees. Smaller spacing should be considered around the head-on heading, e.g. 15 degrees. Typically, most unavourable spreading will be cos or responses dominated by beam sea e.g. external side pressure, and cos 10 or responses dominated by head sea e.g. global wave bending moment. The ollowing dynamic actions shall be included in a FLS analysis as relevant: Global wave bending moments External dynamic pressure due to wave and ship motion Internal dynamic pressure due to ship motion Sloshing pressures due to luid motion in tanks or ships with long or wide tanks Loads rom equipment and topside due to ship motion and acceleration L.7.5. Topside structures The ollowing actions shall be considered or the topside structure: Hull deormations due to wave bending moment acting on the hull Wave induced accelerations (inertia actions) Vortex induced vibrations rom wind Vibrations caused by operation o topside equipment Additionally, the ollowing low cycle actions should be considered where relevant or the topside structure: Hull deormations due to temperature dierences Hull deormations due to change in illing condition e.g. ballasting / deballasting The relevance o combining action components is dependent on the structural arrangement o the topside structure. Relevant stress components, both high cycle and low cycle, shall be combined, including phase NORSOK standard Page 190 o 64

192 Annex L Rev. 3, February 013 inormation, when available. I limited phase inormation is available, the design may be based on worstcase action conditions, by combination o maximum stress or each component. L Turret structure The turret structure will normally be exposed to high dynamic action level. The choice o atigue design actor or the turret should relect the level o criticality and the access or inspection at the dierent locations, L.7.. The ollowing actions shall be considered or the atigue design o turret structures: Dynamic luctuations o mooring line tension. Dynamic actions (tension and bending moment) rom risers Varying hydrodynamic pressure due to wave action Varying hydrodynamic pressure due to unit accelerations (including added mass eects) Reactions in the bearing structure due to the other eects Inertia actions due to unit accelerations Fluctuating reactions in pipe supports due to thermal and pressure induced pipe delections Typical critical areas or atigue evaluation listed in L L Calculation o global dynamic stress ranges Global stress ranges shall be determined rom the global hull bending moments. I applicable, both vertical and horizontal bending moments shall be included. Shear lag eects and stress concentrations shall be considered. L Calculation o local dynamic stress ranges Local stress ranges are determined rom dynamic pressures acting on panels, accelerations acting on equipment and topside and other environmental actions resulting in local stresses to part o the structure. Dynamic pressures shall be calculated rom a 3D sink-source wave load analysis. The transer unction or the dynamic pressure could either be used directly to calculate local stress transer unctions and combined with the global stress transer unction, or a long-term pressure distribution could be calculated. As a minimum, the ollowing dynamic pressures components shall be considered: Double hull stresses due to bending o double hull sections between bulkheads Panel stresses due to bending o stiened plate panels Plate bending stresses due to local plate bending For a description o calculation o local stress components, reerence is made to DNV Classiication Note 30.7, /11/. L Combination o stress components Global and local stresses should be combined to give the total stress range or the detail in question. In general, the global and the local stress components dier in amplitude, phase and location. The method o combining these stresses or the atigue damage calculation will depend on the location o the structural detail. A method or combination o actions is given in DNV Classiication Note 30.7, /11/. L.7.6 L Calculation o atigue damage General The basis or determining the acceptability o atigue resistance, with respect to wave actions, shall be appropriate stochastic atigue analyses. The analyses shall be undertaken utilising relevant site speciic environmental data and take appropriate consideration o both global and local (e.g. pressure luctuation) dynamic responses. (These responses do not necessarily have to be evaluated in the same model but the cumulative damage rom all relevant eects should be considered when evaluating the total atigue damage.) Simpliied atigue analyses may orm the basis o a screening process to identiy locations or which a detailed, stochastic atigue analysis should be undertaken. Such simpliied atigue analysis shall be calibrated, see L Local, detailed FE-analysis (e.g. unconventional details with insuicient knowledge about typical stress distribution) should be undertaken in order to identiy local stress distributions, appropriate SCF s, and/or NORSOK standard Page 191 o 64

193 Annex L Rev. 3, February 013 extrapolated stresses to be utilised in the atigue evaluation, see Annex C or urther details. Dynamic stress variations through the plate thickness shall be documented and considered in such evaluations. Explicit account shall be taken o any local structural details that invalidate the general criteria utilised in the assessment o the atigue strength. Such local details may, or example be access openings, cut-outs, penetrations etc. in structural elements. Principal stresses (see Annex C) should be utilised in the evaluation o atigue responses. L.7.6. Simpliied atigue analysis Provided that the provisions stated in L.7.6.1, are satisied, simpliied atigue analysis may be undertaken in order to establish the general acceptability o atigue resistance. In all cases when a simpliied atigue analysis is utilized a control o the results o the simpliied atigue evaluation, compared to the stochastic results, shall be documented to ensure that the simpliied analysis provides or a conservative assessment or all parts o the structure being considered. The atigue damage may be calculated based on a cumulative damage utilizing Miner-Palmgren summation, applying a Weibull distribution to describe the long-term stress range. The stress range shape parameter and the average zero-crossing requency may be taken rom the long-term stress distribution, utilizing the stress transer unction and environmental data or the operating area. A simpliied method o atigue analysis is described in DNV Classiication Note 30.7, /11/. L Stochastic atigue analysis Stochastic atigue analyses shall be based upon recognised procedures and principles utilising relevant site speciic data. Providing that it can be satisactorily documented, scatter diagram data may be considered as being directionally speciic. In such cases, the analyses shall include consideration o the directional probability o the environmental data. Relevant wave spectra shall be utilised. Wave energy spreading may be taken into account i relevant. Structural response shall be determined based upon analyses o an adequate number o wave directions. Generally a maximum spacing o 30 degrees should be considered. Transer unctions should be established based upon consideration o a suicient number o periods, such that the number, and values o the periods analysed : adequately cover the site speciic wave data, satisactorily describe transer unctions at, and around, the wave cancellation and ampliying periods. (Consideration should be given to take account that such cancellation and ampliying periods may be dierent or dierent elements within the structure), and, satisactorily describe transer unctions at, and around, the relevant excitation periods o the structure. The method is described in DNV Classiication Note 30.7, /11/. NORSOK standard Page 19 o 64

194 Annex L Rev. 3, February 013 L.8 ACCIDENTAL LIMIT STATES (ALS) Minimum impact energy levels, action combinations and allowable utilisation levels shall comply with the requirements given in NORSOK, N-003. Relevant actions shall be determined based on a Risk Analysis. L.8.1 Dropped objects The possibility o dropped objects impacting on the structural components o the unit shall be considered in design. Resistance to dropped objects may be accounted or by indirect means, such as, using redundant raming conigurations, and, materials with suicient toughness in aected areas. The masses o the dropped objects rom crane operation to be considered or design o units are normally taken based upon operational hook actions o the platorm crane. Critical areas o dropped objects are to be determined based on crane operation sectors, crane reach and actual movement o actions assuming a drop direction within an angle to the vertical direction o 5 o in air and 15 o in water. L.8. Fire General guidance and requirements concerning accidental limit state events involving ire are given in Chapter 9 and Annex A. L.8.3 Explosion In respect to design considering actions resulting rom explosions one, or a combination o the ollowing main design philosophies are relevant ; Ensure that the probability o explosion is reduced to a level where it is not required to be considered as a relevant design action. Ensure that hazardous locations are located in unconined (open) locations and that suicient shielding mechanisms (e.g. blast walls) are installed. Locate hazardous areas in partially conined locations and design utilising the resulting relatively small overpressures. Locate hazardous areas in enclosed locations and install pressure relie mechanisms (e.g. blast panels) and design or the resulting overpressure. Structural design accounting or large plate ield rupture resulting rom explosion actions should normally be avoided due to the uncertainties o the actions and the consequences o the rupture itsel. Structural support o the blast walls and the transmission o the blast action into main structural members shall be evaluated. Eectiveness o connections and the possible outcome rom blast, such as lying debris, shall be considered. L.8.4 Collision Resistance to unit collisions may be accounted or by indirect means, such as, using redundant raming conigurations, and, materials with suicient toughness in aected areas. Collision impact shall be considered or all elements o the unit, which may be impacted by sideways, bow or stern collision. The vertical extent o the collision zone shall be based on the depth and draught o attending units and the relative motion between the attending units and the unit. To avoid possible penetration o a cargo tank, the side structure o the unit shall be capable o absorbing the energy o a vessel collision with an annual probability o L.8.5 Unintended looding A procedure describing actions to be taken ater relevant unintended looding shall be prepared. Structural aspects related to counterlooding (i relevant) shall be investigated. The unintended looding conditions shall be considered in the turret and topside structure design. L.8.6 Loss o heading control For units normally operated with heading control, either by weather vaning or by thruster assistance, the eect o loss o the heading control shall be evaluated. The loss o heading control condition shall be considered in the topside and turret structure design. NORSOK standard Page 193 o 64

195 Annex L Rev. 3, February 013 L.9 COMPARTMENTATION & STABILITY L.9.1 General In the assessment o compartmentation and stability o a ship shaped unit, consideration shall be given to all relevant detrimental eects, including those resulting rom: environmental actions, relevant damage scenarios, rigid body motions, the eects o ree-surace, and, boundary interactions (e.g. mooring and riser systems). An inclining test should be conducted when construction is as near to completion as practical in order to accurately determine the unit s mass and position o the centre o gravity. Changes in mass conditions ater the inclining test should be careully accounted or. The unit s mass and position o the centre o gravity shall be relected in the unit s Operating Manual. L.9. Compartmentation and watertight integrity General requirements relating to compartmentation o ship shaped units are given in ISO 19900, /6/. Detailed provisions given in the IMO MODU Code relating to subdivision and reeboard should be complied with. Special provisions relating to the Norwegian petroleum activities are given in NORSOK N-001. Additionally, the design criteria or watertight and weathertight integrity as stated in the relevant rules o Recognised Classiication Societies should normally be satisied. The number o openings in watertight structural elements shall be kept to a minimum. Where penetrations are necessary or access, piping, venting, cables etc., arrangements shall be made to ensure that the watertight integrity o the structure is maintained. Where individual lines, ducts or piping systems serve more than one watertight compartment, or are within the extent o damage resulting rom a relevant accidental event, arrangements shall be provided to ensure that progressive looding will not occur. L.9.3 Stability Stability o a ship shaped unit shall satisy the requirements as stated in the IMO MODU Code. Special provisions relating to the Norwegian petroleum activities are given in NORSOK N-001. Adequacy o stability shall be established or all relevant in-service and temporary phase conditions. The assessment o stability shall include consideration o both the intact and damaged conditions. NORSOK standard Page 194 o 64

196 Annex L Rev. 3, February 013 L.10 SPECIAL CONSIDERATION L.10.1 Structural details For the design o structural details consideration should be given to the ollowing: The thickness o internal structure in locations susceptible to excessive corrosion. The design o structural details, such as those noted below, against the detrimental eects o stress concentrations and notches: - Details o the ends and intersections o members and associated brackets. - Shape and location o air, drainage, and lightening holes. - Shape and reinorcement o slots and cut-outs or internals. - Elimination or closing o weld scallops in way o butts, sotening bracket toes, reducing abrupt changes o section or structural discontinuities. Proportions and thickness o structural members to reduce atigue damage due to engine, propeller or wave-induced cyclic stresses, particularly or higher strength steel members. L.10. Positioning o superstructure Living quarters, lieboats and other means o evacuation shall be located in non-hazardous areas and be protected and separated rom production, storage, riser and lare areas. Superstructure shall be located such that a satisactory level o separation and protection is provided or. L.10.3 Structure in way o a ixed mooring system Local structure in way o airleads, winches, etc. orming part o the position mooring system is, as a minimum, to be capable o withstanding orces equivalent to 1.5 times the breaking strength o the individual mooring line. The strength evaluation should be undertaken utilising the most unavourable operational direction o the anchor line. In the evaluation o the most unavourable direction, account shall be taken o relative angular motion o the unit in addition to possible line lead directions. L.10.4 Inspection and maintenance The structural inspection and maintenance programmes shall be consistent with NORSOK N-001. L.10.5 Facilities or inspection on location Inspections may be carried out on location based on approved procedures outlined in a maintenance system and inspection arrangement, without interrupting the unction o the unit. The ollowing matters should be taken into consideration to be able to carry out condition monitoring on location: Arrangement or underwater inspection o hull, propellers, thrusters, rudder and openings aecting the units seaworthiness. Means o blanking o all openings including side thrusters. Use o corrosion resistant materials or shats, and glands or propeller and rudder. Accessibility o all tanks and spaces or inspection. Corrosion protection o hull. Maintenance and inspection o thrusters. Ability to gas ree and ventilate tanks Provisions to ensure that all tank inlets are secured during inspection Measurement o wear in the propulsion shat and rudder bearings. Testing acilities o all important machinery. L.10.6 Action monitoring An on-board loading instrument complying with the requirements o the DNV Ship Rules, /3/ shall be installed in order to be able to monitor still water bending moments and shear orces, and the stability o the unit. The limitations or still water bending moments and shear orces shall be in accordance with maximum allowable stillwater bending moments and shear orces speciied in the Operating Manual. NORSOK standard Page 195 o 64

197 Annex L Rev. 3, February 013 L.10.7 Corrosion protection The corrosion protection arrangement shall be consistent with the reerences listed in NORSOK N-001. NORSOK standard Page 196 o 64

198 Annex L Rev. 3, February 013 L.11 DOCUMENTATION L.11.1 General Adequate planning shall be undertaken in the initial stages o the design process in order to obtain a workable and economic structural solution to perorm the desired unction. As an integral part o this planning, documentation shall be produced identiying design criteria and describing procedures to be adopted in the structural design o the unit. Applicable codes, standards and regulations should be identiied at the commencement o the design. When the design has been inalised, a summary document containing all relevant data rom the design and abrication phase shall be produced. Design documentation (see below) shall, as ar as practicable, be concise, non-voluminous, and, should include all relevant inormation or all relevant phases o the lietime o the unit. General requirements to documentation relevant or structural design are given in NORSOK, N-001, Section 5. L.11. Design basis A Design Basis Document shall be created in the initial stages o the design process to document the basis criteria to be applied in the structural design o the unit. A summary o those items normally to be included in the Design Basis document is included below. Unit description and main dimensions A summary description o the unit, including : general description o the unit (including main dimensions and draughts), main drawings including : main structure drawings (This inormation may not be available in the initial stage o the design), general arrangement drawings, plan o structural categorisation, load plan(s), showing description o deck uniorm (laydown) actions, capacity (tank) plan, service lie o unit, position keeping system description. Rules, regulations and codes A list o all relevant, applicable, Rules, Regulations and codes (including revisions and dates), to be utilised in the design process. Environmental design criteria Environmental design criteria (including all relevant parameters) or all relevant conditions, including : wind, wave, current, snow and ice description or 10-1,10 - and 10-4 annual probability events, design temperatures. Stability and compartmentation Stability and compartmentation design criteria or all relevant conditions including : external and internal watertight integrity plan, lightweight breakdown report, design loadcase(s) including global mass distribution, damage condition waterlines.(this inormation may not be available in the initial stage o the design.) Temporary phases Design criteria or all relevant temporary phase conditions including, as relevant : limiting permanent, variable, environmental and deormation action criteria, procedures associated with construction, (including major liting operations), NORSOK standard Page 197 o 64

199 Annex L Rev. 3, February 013 relevant ALS criteria. Operational design criteria Design criteria or all relevant operational phase conditions including : limiting permanent, variable, environmental and deormation action criteria, designing accidental event criteria (e.g. collision criteria), tank loading criteria (all tanks) including a description o system, with : loading arrangements height o air pipes, loading dynamics, densities, mooring actions. In-service inspection criteria A description o the in-service inspection criteria and general philosophy (as relevant or evaluating atigue allowable cumulative damage ratios). Miscellaneous A general description o other essential design inormation, including : description o corrosion allowances, where applicable. L.11.3 Design brie A Design Brie Document shall be created in the initial stages o the design process. The purpose o the Design Brie shall be to document the intended procedures to be adopted in the structural design o the unit. All applicable limit states or all relevant temporary and operational design conditions shall be considered in the Design Brie. A summary o those items normally to be included in the Design Brie Document is included below. Analysis models A general description o models to be utilised, including description o : global analysis model(s), local analysis model(s), loadcases to be analysed. Analysis procedures A general description o analytical procedures to be utilised including description o procedures to be adopted in respect to : the evaluation o temporary conditions, the consideration o accidental events, the evaluation o atigue actions, the establishment o dynamic responses (including methodology, actors, and relevant parameters), the inclusion o built-in stresses (i relevant), the consideration o local responses (e.g. those resulting rom mooring and riser actions, ballast distribution in tanks as given in the operating manual etc.) Structural evaluation A general description o the evaluation process, including : description o procedures to be utilised or considering global and local responses, description o atigue evaluation procedures (including use o design atigue actors, SN-curves, basis or stress concentration actors (SCF s), etc.). description o procedures to be utilised or code checking. NORSOK standard Page 198 o 64

200 Annex L Rev. 3, February 013 L.11.4 Documentation or operation (DFO) Documentation requirements covering structural design, as applicable in the operational phase o the unit (e.g. the DFI resume), are given in NORSOK Z-001. NORSOK standard Page 199 o 64

201 Annex L Rev. 3, February 013 L.1 REFERENCES /1/ Oshore Installations: Guidance on design, construction and certiication, U.K., Health & Saety Executive, Fourth Edition, HMSO // DNV-OS-C101 Oshore Standard. Design o Oshore Steel Structures, General. DNV October 000. /3/ Rules or Classiication o Ship, latest valid edition, DNV /4/ Regulations relating to design and outitting o acilities etc. in the petroleum activities. Issued by the Norwegian Petroleum Directorate. /5/ Regulations o 0 Dec. 1991, No.878, concerning stability, watertight subdivision and watertight / weathertight closing means on mobile oshore units, Norwegian Maritime Directorate. /6/ ISO Petroleum and Natural Gas Industries Oshore Structures, General Requirements. /7/ Salvesen, N., Tuck, E. O. and Faltinsen, O., "Ship Motions and Sea Loads".Trans. SNAME, Vol 78, /8/ Tanaka, N., "A Study o the Bilge Keels, Part 4, on the Eddy-Making Resistance to the Rolling o a Ship Hull". Japan Society o Naval Architects, Vol. 109, /9/ Torsethaugen, Knut, Model or a Doubly Peaked Spectrum. Sinte Report No. SFT 9604 dated /10/ NORSOK Z-001 Documentation or Operation (DFO) /11/ DNV Classiication Note 30.7: Fatigue Assessment o Ship Structures /1/ DNV-OS-C10 Oshore Standard. Structural Design o Oshore Ships. DNV October 011. NORSOK standard Page 00 o 64

202 Annex L Rev. 3, February 013 L.13 COMMENTARY Comm. L.4.5. Sloshing actions in tanks The non-impact sloshing loads in the DNV Rules are given at annual probability level 10-4 and represent the sloshing loads generally applicable or a FLS structural evaluation. These loads are governed by the inertia orces induced by the liquid in the tank. This means that the Weibull slope parameter applicable or a FLS structural evaluation, will be approximately equal to the Weibull slope parameter o the liquid acceleration in the longitudinal direction or the longitudinal sloshing mode and in the transverse direction or the transverse sloshing mode. Based on analysis o several oshore vessels ranging in length rom m, a Weibull slope parameter h=1.0 has been determined as being appropriate or an FLS structural evaluation o ship shaped oshore units, both in the longitudinal sloshing mode and in the transverse sloshing mode. The nonimpact sloshing loads should be applied on strength members as indicated in DNV Ship Rules Pt.3, Ch.1, Sec.4 C300 Liquid in tanks. The sloshing impact loads in the DNV Rules are given at annual probability level 10-8 (0 year return period) and represent the sloshing impact loads generally applicable or an ULS structural evaluation. In the period , DNV perormed a signiicant amount o sloshing model tests, using irregular excitation, with the main ocus on sloshing impact loads. The duration o the tests was typically -4 hours in model scale. Based on statistical analysis o these model tests, a Weibull slope parameter h1.0 has been determined as being representative or sloshing impact loading. To arrive at 100 year return period impact pressures, the 10-8 values should be multiplied by a actor The impact sloshing loads should be applied on strength members as indicated in DNV Ship Rules Pt.3, Ch.1, Sec.4 C300 Liquid in tanks. A wave which will impose the maximum wave bending moment on the vessel will have a wavelength o the order ships length. Provided that separation between the period o encounter or this wave T e and the natural period T o the luid in the tank is suicient (T0.75T e ), use o a reduced wave bending moment in calculating the allowable stress used in connection with sloshing loads or deck longitudinals is acceptable. The strength calculation or deck longitudinals may then be based on a wave bending moment reduced by 5% relative to the design wave bending moment. The applicable number o cycles in a atigue evaluation will in principle depend on the vessel pitch response period or the longitudinal sloshing mode and on the vessel roll response period or the transverse sloshing mode. A simpliied expression or the applicable response period, both or the longitudinal and transverse sloshing mode, may be ound in DNV Class Note 30.7, Section.1., T resp =4 log 10 (L), where L is the rule ship length. Comm. L Green water eect It is recommended that provisions are made during model testing or suitable measurements to determine design pressures or local structural design. This implies that model tests should be perormed at design draught, or sea states with a spectrum peak period approximately % o the pitch resonance period o the vessel. The vessel model should be equipped with load cells on the weather deck at positions o critical structural members or critical topside equipment. -o0o- NORSOK standard Page 01 o 64

203 Design o steel structures N-004 Annex M Rev.3, February 013 DESIGN OF STEEL STRUCTURES ANNEX M SPECIAL DESIGN PROVISIONS FOR COLUMN STABILIZED UNITS NORSOK standard Page 0 o 64

204 Design o steel structures N-004 Annex M Rev.3, February 013 Contents M.1 Introduction 05 M.1.1 General 05 M.1. Deinitions 05 M.1.3 Non-operational phases 05 M. Structural classiication and material selection 06 M..1 Structural classiication 06 M.. Material selection 07 M..3 Inspection categories 07 M..4 Guidance to minimum requirements 08 M.3 Actions 11 M.3.1 General 11 M.3. Permanent actions 11 M.3.3 Variable actions 11 M.3.4 Deormation actions 11 M.3.5 Environmental actions 1 M.3.6 Accidental actions 1 M.3.7 Repetitive actions 1 M.3.8 Combination o actions 1 M.4 Ultimate limit states (ULS) 14 M.4.1 General 14 M.4. Global models 14 M.4..1 Sea loading model 15 M.4.. Structural model 15 M.4..3 Mass modelling 15 M.4..4 Global analysis 16 M.4.3 Local structural models 17 M.4.4 Superimposing responses 18 M.5 Fatigue limit states (FLS) 0 M.5.1 General 0 M.5. Design atigue actors (DFFs) 0 M.5.3 Splash zone 1 M.5.4 Fatigue analysis M General M.5.4. Simpliied atigue analysis M Stochastic atigue analysis 3 M.6 Accidental limit states (ALS) 5 M.6.1 General 5 M.6. Dropped objects 5 M.6.3 Fire 5 M.6.4 Explosion 5 M.6.5 Collision 6 M.6.6 Unintended looding 8 M.6.7 Abnormal wave events 9 M.6.8 Reserve buoyancy 9 M.7 Air gap 9 M.7.1 General 9 M.8 Compartmentation and stability 9 M.8.1 General 9 M.8. Compartmentation and watertight integrity 30 M.8.3 Stability 30 M.8.4 Damaged condition 30 M.9 Special considerations 30 M.9.1 Structural redundancy 30 M.9. Brace arrangements 30 NORSOK standard Page 03 o 64

205 Design o steel structures N-004 Annex M Rev.3, February 013 M.9.3 Structure in way o a ixed mooring system 31 M.9.4 Structural detailing 31 M.10 Documentation requirements or the "design basis" and "design brie" 31 M.10.1 General 31 M.10. Design basis 31 M.10.3 Design brie 3 M.11 Reerences 33 M.1 Commentary 33 NORSOK standard Page 04 o 64

206 Design o steel structures N-004 Annex M Rev.3, February 013 M.1 Introduction M.1.1 General This annex provides requirements and guidance to the structural design o column stabilised units, constructed in steel, in accordance with the provisions o this NORSOK standard. The requirements and guidance documented in this annex are generally applicable to all conigurations o column stabilised units, including those with ring (continuous) pontoons, twin pontoons or, multi-ooting arrangements. Such units may be kept on station by either a passive mooring system (e.g. anchor lines), or an active mooring system (e.g. thrusters), or a combination o these methods. Requirements concerning mooring and riser systems are not considered in this annex. A column stabilised unit may be designed to unction in a number o modes, e.g. transit, operational and survival. Limiting design criteria or going rom one mode o operation to another mode o operation shall be clearly established and documented. Such limiting design criteria shall include relevant consideration o the ollowing items: intact condition, structural strength; damaged condition, structural strength; air gap; compartmentation and stability. For novel designs, or unproved applications o designs where limited or no direct experience exists, relevant analyses and model testing, shall be perormed which clearly demonstrate that an acceptable level o saety is obtained. M.1. Deinitions Column stabilised unit: A loating unit that can be relocated. A column stabilised unit normally consists o a deck structure with a number o widely spaced, large diameter, supporting columns that are attached to submerged pontoons. Notes: 1. In the context o this annex the term "column stabilised unit" is oten abbreviated to the term "unit".. The deinition or a column stabilised unit is in accordance with ISO deinition or a "semisubmersible" unit. M.1.3 Non-operational phases The structure shall be designed to resist relevant actions associated with conditions that may occur during all stages o the lie-cycle o the unit. Such stages may include abrication, site moves, mating, sea transportation, installation, decommissioning. Structural design covering marine operation, construction sequences shall be undertaken in accordance with NORSOK N-001. Marine operations should be undertaken in accordance with the requirements stated in the VMO standard. All marine operations shall, as ar as practicable, be based upon well proven principles, techniques, systems and equipment and shall be undertaken by qualiied, competent personnel possessing relevant experience. Structural responses resulting rom one temporary phase condition (e.g. a construction or transportation operation) that may aect design criteria in another phase shall be clearly documented and considered in all relevant design workings. NORSOK standard Page 05 o 64

207 Design o steel structures N-004 Annex M Rev.3, February 013 I it is intended to dry-dock the unit the bottom structure shall be suitably strengthened to withstand such actions. Fabrication Planning o construction sequences and o the methods o construction shall be perormed. Actions occurring in abrication phases shall be assessed and, when necessary the structure and the structural support arrangement shall be evaluated or structural adequacy. Major liting operations shall be evaluated to ensure that deormations are within acceptable levels and that relevant strength criteria are satisied. Mating All relevant action eects incurred during mating operations shall be considered in the design process. Particular attention should be given to hydrostatic actions imposed during mating sequences. Sea transportation A detailed transportation assessment shall be undertaken which includes determination o the limiting environmental criteria, evaluation o intact and damage stability characteristics, motion response o the global system and the resulting, induced actions. The occurrence o slamming actions on the structure and the eects o atigue during transport phases shall be evaluated when relevant. Satisactory compartmentation and stability during all loating operations shall be ensured. All aspects o the transportation, including planning and procedures, preparations, sea-astenings and marine operations should comply with the requirements o the warranty authority. Installation Installation procedures o oundations (e.g. drag embedded anchors, piles, suction anchor or dead weights etc.) shall consider relevant static and dynamic actions, including consideration o the maximum environmental conditions expected or the operations. The actions induced by the marine spread involved in the operations and the orces exerted on the structures utilised in positioning the unit, such as airleads and pad-eyes, shall be considered or local strength checks. Decommissioning Abandonment o the unit shall be planned or in the design stage. Decommissioning phases or column stabilised units are however not normally considered to provide design load conditions or the structure o a unit and may thereore normally be disregarded in the design phase. M. Structural classiication and material selection M..1 Structural classiication Selection o steel quality, and requirements or inspection o welds, shall be based on a systematic classiication o welded joints according to the structural signiicance and the complexity o the joints/connections as documented in Clause 5. In addition to in-service operational phases, consideration shall be given to structural members and details utilised or temporary conditions, e.g. abrication, liting arrangements, towing arrangements, etc. Basic considerations Structural connections in column stabilised units designed in accordance with this annex should normally not all within the categorisation criteria relevant or design class DC1 or DC. In particular, relevant ailure o a single weld, or element, should not lead to a situation where the accidental limit state condition is not satisied. Structural connections will thereore be categorised within classiication groups DC3, DC4 or DC5. Consideration shall be given to address areas where through thickness tensile properties may be required. Special consideration shall be given to ensure the appropriate inspection category or welds with high utilisation in atigue i the coverage with standard local area allocation is insuicient. Examples o typical design classes applicable to column stabilised units are stated below. These examples provide minimum requirements and are not intended to restrict the designer in applying more stringent requirements should such requirements be desirable. NORSOK standard Page 06 o 64

208 Design o steel structures N-004 Annex M Rev.3, February 013 Typical locations - Design class: DC3 Locations in way o connections at major structural interaces (e.g. pontoon/pontoon, pontoon/column, column/deck, and, major brace connections) should generally be classiied as DC3 due to the complexity o the connections and the general diiculty with respect to inspection and repair. Local areas in way o highly stressed complex structural connections (e.g. in way o airleads, riser supports, topside structures, or other similar locations with high complexity) should also be classiied as DC3. DC3 areas may be limited to local, highly stressed areas i the stress gradient at such connections is large. Typical locations - Design class: DC4 Except as provided or in the description or DC3 and DC5 structural categorisation, connections appropriate to general stiened plate ields, including connections o structural elements supporting the plate ields, e.g. girders and stieners, should normally be classiied in Design Class DC4. General brace structural connections, (e.g. butt welds in brace plate ields) should be designated DC4, as should connections o the main supporting structure or topside components and equipment. Typical locations - Design class: DC5 All welds or non-main load bearing structural elements, including non-watertight bulkheads, mezzanine (tween) decks, and, deck superstructures, may be classiied as DC5. The same applies or welds or internal outitting structures in general. M.. Material selection Material speciications shall be established or all structural materials utilised in a column stabilised unit. Such materials shall be suitable or their intended purpose and have adequate properties in all relevant design conditions. Material selection shall be undertaken in accordance with the principles given in Clause 5. When considering criteria appropriate to material grade selection, adequate consideration shall be given to all relevant phases in the lie cycle o the unit. In this connection there may be conditions and criteria, other than those rom the in-service, operational phase, that provide the design requirements in respect to the selection o material. Such criteria may, or example, be design temperature and/or stress levels during marine operations. In the selection o material grades adequate consideration shall be given to the appropriateness o the design temperature including the deinition o such. When considering design temperature related to material selection the applicability that standard NORSOK requirements to abrication o steel structures (see NORSOK M-101) are based upon a minimum design temperature o -14 C, shall be evaluated. Selection o steel quality or structural components shall normally be based on the most stringent DC o the joints involving the component. Consideration shall be given to the presence o tensile stress in the direction o the thickness o the plate when determining the appropriate steel quality. M..3 Inspection categories Welding, and the extent o non-destructive examination during abrication, shall be in accordance with the requirements stipulated or the appropriate "inspection category" as deined in NORSOK M-101. Determination o inspection category should be in accordance with the criteria given in Table 5.3 and Table 5.4. Inspection categories determined in accordance with this NORSOK stanard provide requirements to the minimum extent o required inspection. When considering the economic consequence that repair may entail, or example, in way o complex connections with limited or diicult access, it may be considered prudent engineering practice to require more demanding requirements to inspection than the required minimum. When determining the extent o inspection, and the locations o required NDT, in additional to evaluating design parameters (or example atigue utilisation) consideration should be given to relevant abrication parameters including; positioning o block (section) connections, the possibility that manual welded connections may not achieve the quality that automatic weld locations achieve, etc. The lower the extent o NDT selected, the more important is the representativeness o the NDT selected and perormed. The designer should thereore exercise good engineering judgement in indicating mandatory locations or NDT, where variation o utility along welds is signiicant. NORSOK standard Page 07 o 64

209 Design o steel structures N-004 Annex M Rev.3, February 013 M..4 Guidance to minimum requirements Figure M.-1 to Figure M.-3 illustrate minimum requirements to selection o DC, and inspection category or typical column stabilised unit, structural conigurations. Figure M.-1 Typical "ring" pontoon DCs and inspection categories Key: 1. [DCn,..] Design Class n, N. [.., N] Inspection Category, N Notes: 1. In way o the pontoon/column connection (except in way o brackets), the pontoon deck plate ields will normally be the continuous material. These plate ields should normally be material with documented through-thickness properties (i.e. Steel Quality Level I material).. Shaded areas indicated in the igures are intended to be three-dimensional in extent. This implies that, in way o these locations, the shaded area logic is not only to apply to the outer surace o the connection but is also to extend into the structure. 3. "Major brackets" are considered, within the context o this igure, to be primary, load-bearing brackets supporting primary girders. "Minor brackets" are all other brackets. 4. The inspection categories or general pontoon, plate butt welds and girder welds to the pontoon shell are determined based upon, amongst others: accessibility, atigue utilisation, and dominating stress direction see NORSOK N-004, Table 5.3 and Table 5.4). E.g. Variations in dynamic stress levels across the section o the pontoon and also along the length o the pontoon may alter a general inspection category designation. 5. Major bracket toes will normally be designated as locations with a mandatory requirement to MPI. NORSOK standard Page 08 o 64

210 Design o steel structures N-004 Annex M Rev.3, February 013 Figure M.- Typical brace/column design classes and inspection categories Key: 1. [DCn,..] Design Class n, N. [.., N] Inspection Category, N Notes: 1. In way o the column/brace connection the brace, and brace bracket plate ields, will normally be the continuous material. These plate ields should normally be material with documented through-thickness properties, i.e. Steel Quality Level I material.. Figure M.- illustrates a relatively small brace connected to a column. For large brace connections the extent o the DC3 area may be limited somewhat to that illustrated. NORSOK standard Page 09 o 64

211 Design o steel structures N-004 Annex M Rev.3, February 013 Figure M.-3 Typical column/upper hull (deck box) design classes and inspection categories Key: 1. [DCn,..] Design Class n, N. [.., N] Inspection Category, N Notes: 1. In way o the column/upper hull connection (except in way o brackets) the upper hull deck plate ields will normally be the continuous material. These plate ields should normally be material with documented through-thickness properties, i.e. Steel Quality Level I material.. Shaded areas indicated in the igures are intended to be three-dimensional in extent. This implies that, in way o these locations, the shaded area logic is not only to apply to the outer surace o the connection but is also to extend into the structure. 3. "Major brackets" are considered, within the context o this igure, to be primary, load-bearing brackets supporting primary girders. "Minor brackets" are all other brackets. 4. The inspection categories stated or general upper hull are determined based upon, the assumption that the atigue utilisation may be categorised as being "low", i.e. that Table 5.3 is relevant. This assumption shall be veriied. NORSOK standard Page 10 o 64

212 Design o steel structures N-004 Annex M Rev.3, February 013 M.3 Actions M.3.1 General Characteristic actions are to be used as reerence actions. Design actions are, in general, deined in NORSOK N-003. Guidance concerning action categories relevant or column stabilised unit designs are given in the ollowing. M.3. Permanent actions Permanent actions are actions that will not vary in magnitude, position, or direction during the period considered, and include "light mass" o the unit, including mass o permanently installed modules and equipment, such as accommodation, helideck, drilling and production equipment, hydrostatic pressures resulting rom buoyancy, pretension in respect to mooring, drilling and production systems, e.g. mooring lines, risers etc.. M.3.3 Variable actions Variable actions are actions that may vary in magnitude, position and direction during the period under consideration. Except where analytical procedures or design speciications otherwise require, the value o the variable actions utilised in structural design should normally be taken as either the lower or upper design value, whichever gives the more unavourable eect. Variable actions on deck areas or local, primary and global design are stated in NORSOK N-003, Table 5.1. The actor to be applied to "global design" actions (as stated in Table 5.1) is, however, not intended to limit the variable load carrying capacity o the unit in respect to non-relevant, variable load combinations. Global load cases should be established based upon "worst case", representative variable load combinations, where the limiting global design criteria is established by compliance with the requirements to intact and damage hydrostatic and hydrodynamic stability. See M.1 or a urther discussion on this issue. Global, design load conditions, and associated limiting criteria shall be documented in the stability manual or the unit /1/. Variations in operational mass distributions (including variations in tank load conditions in pontoons) shall be adequately accounted or in the structural design. Design criteria resulting rom operational requirements should be ully considered. Examples o such operations may be drilling, production, workover, and combinations thereo, consumable re-supply procedures, maintenance procedures, possible mass re-distributions in extreme conditions. Local, design deck actions shall be documented on a "load-plan". This plan shall clearly show the design uniorm and concentrated actions or all deck areas, or each relevant mode o operation. Dynamic actions resulting rom low through air pipes during illing operations shall be adequately considered in the design o tank structures. Operational actions imposed by supply boats while approaching, mooring, or, lying alongside the unit should be considered in the design. I endering is itted, the combined ender/structural system is to be capable o absorbing the energy o such actions. The resulting response in the structural system is to satisy the requirements o the considered limit state. A typical design impact action would, or example, be a SLS design consideration o a 5000 tonne displacement vessel with a contact speed o 0.5 m/s. M.3.4 Deormation actions Deormation actions are actions due to deormations applied to the structure. Deormation actions resulting rom construction procedures and sequences (e.g. as a result o mating operations, welding sequences, orced alignments o structure etc.) shall be adequately accounted or. Other relevant deormation action eects may include those resulting rom temperature gradients, as or example: NORSOK standard Page 11 o 64

213 Design o steel structures N-004 Annex M Rev.3, February 013 when hot-oil is stored in a compartment adjacent to the sea; radiation rom laring operations. M.3.5 Environmental actions Environmental actions are actions caused by environmental phenomena. Environmental conditions shall be described using relevant data, or the relevant period and areas in which the unit is to be abricated, transported, installed and operated. All relevant environmental actions shall be considered. Amongst those environmental actions to be considered, in the structural design o a column stabilised unit, are wave actions (including variable pressure, inertia, wave 'run-up', and slamming actions), wind actions, current actions, temperature actions, snow and ice actions. Amongst those environmental action induced responses to be considered, in the structural design o a column stabilised unit, are dynamic stresses or all limit states (e.g. with respect to atigue, see M.3.7), rigid body motion (e.g. in respect to air gap and maximum angles o inclination), sloshing, slamming induced vibrations, vortex induced vibrations (e.g. resulting rom wind actions on structural elements in a lare tower), wear resulting rom environmental actions at mooring and riser system interaces with hull structures. For column stabilised units with traditional, catenary mooring systems, earthquake actions may normally be ignored. Further considerations with respect to environmental actions are given in NORSOK N-003. M.3.6 Accidental actions Accidental actions are actions related to abnormal operation or technical ailure. Risk analyses shall be undertaken to identiy and assess accidental events that may occur and the consequences o such events, see Clause 11. Relevant, structural design events or the ALS condition are identiied rom the risk analyses. The ollowing ALS events are amongst those required to be considered in respect to the structural design o a column stabilised unit: dropped objects (e.g. rom crane handling); ire; explosion; collision; unintended looding; abnormal wave events. Further considerations in respect to accidental actions are given in Clause 11. M.3.7 Repetitive actions Repetitive actions, which may lead to possible signiicant atigue damage, shall be evaluated. The ollowing listed sources o atigue actions shall, where relevant, be considered: waves (including those actions caused by slamming and variable (dynamic) pressures); wind (especially when vortex induced vibrations may occur); currents (especially when vortex induced vibrations may occur); mechanical vibration (e.g. caused by operation o machinery); mechanical loading/unloading (e.g. crane actions). The eects o both local and global dynamic response shall be properly accounted or when determining response distributions related to atigue actions. M.3.8 Combination o actions Load coeicients and load combinations or the design limit states are, in general, given in NORSOK N-001. NORSOK standard Page 1 o 64

214 Design o steel structures N-004 Annex M Rev.3, February 013 Structural strength shall be evaluated considering all relevant, realistic loading conditions and combinations. Scantlings shall be determined on the basis o criteria that combine, in a rational manner, the eects o relevant global and local responses or each individual structural element. A suicient number o load conditions shall be evaluated to ensure that the characteristic largest (or smallest) response, or the appropriate return period, has been established. For example, maximum global, characteristic responses or a column stabilised unit may occur in environmental conditions that are not associated with the characteristic, largest, wave height. In such cases, wave period and associated wave steepness parameters are more likely to be governing actors in the determination o maximum/minimum responses. NORSOK standard Page 13 o 64

215 Design o steel structures N-004 Annex M Rev.3, February 013 M.4 Ultimate limit states (ULS) M.4.1 General General considerations in respect to methods o analysis are given in NORSOK N-003, Clause 9. Analytical models shall adequately describe the relevant properties o actions, stiness, and displacement, and shall satisactorily account or the local and system eects o time dependency, damping, and inertia. It is normally not practical, in design analysis o column stabilised units, to include all relevant actions (both global and local) in a single model. Generally, a single model would not contain suicient detail to establish local responses to the required accuracy, or to include consideration o all relevant actions and combinations o actions (see M.1 or a more detailed discussion o this item). It is oten the case that it is more practical, and eicient, to analyse dierent action eects utilising a number o appropriate models and superimpose the responses rom one model with the responses rom another model in order to assess the total utilisation o the structure. The modelling guidance given in the ollowing thereore considers a proposed division o models in accordance with an acceptable analytical procedure. The procedures described are not however intended to restrict a designer to a designated methodology when an alternative methodology provides or an acceptable degree o accuracy, and includes all relevant action eects. Further, the modelling procedures and guidance provided are intended or establishing responses to an acceptable level o accuracy or inal design purposes. For preliminary design, simpliied models are recommended to be utilised in order to more eiciently establish design responses and to achieve a simple overview o how the structure responds to the designing actions. M.4. Global models The intention o the global analysis model(s) should be to enable the assessment o responses resulting rom global actions. An example o a global analysis model is shown in Figure M.4-1. Figure M.4-1 Example o a global analysis model NORSOK standard Page 14 o 64

216 Design o steel structures N-004 Annex M Rev.3, February 013 M.4..1 Sea loading model Global action eects The environmental action model (e.g. simulating the appropriate wave conditions) shall be based upon site speciic data in accordance with NORSOK N-003, Clause 6. The stochastic nature o wave actions shall be adequately accounted or. For extreme response analysis, the short-crested nature o the sea may be accounted or according to the requirements given in NORSOK N-003, Appropriate wave spectra criteria (as given in NORSOK N- 003, 6...3) based upon site speciic environmental data shall be considered when simulating sea actions. A simple, Morrison element model may normally be utilised in order to ascertain global actions in preliminary design stages. It may be prudent to apply a contingency actor to the global actions resulting rom such a model. A contingency actor o 1. to 1.3 would generally be considered as being appropriate. For inal design a diraction model should be created in order to more accurately establish the design actions. Global viscous damping eects may be included in the model either by the inclusion o Morrison elements, or by the inclusion o relevant terms in a global damping matrix. Viscous eects are non-linear and should thereore be careully considered in requency domain analyses when such eects may be considered as signiicantly aecting the response, e.g. in the evaluation o motion response (in connection with establishing air gap and acceleration parameters). Where hydrostatic actions are modelled explicitly (as opposed to an implicit inclusion rom a diraction model) it shall be ensured that adequate account is given to the inclusion o end-pressure action eects, e.g. in respect to axial actions acting on pontoon structures. Local viscous (drag) action eects For relatively slender members, viscous (drag) local action eects should be accounted or. In such cases, the inclusion o local drag action eects may be undertaken in a number o ways, or example by use o hybrid (diraction and Morrison) models o the structural elements, by only having a Morrison model o selected individual elements, by the inclusion o drag action eects by hand calculations. M.4.. Structural model For large volume, thin-walled structures, three-dimensional inite element structural models created in shell (or membrane) inite elements, are normally required. For space rame structures consisting o slender members a three-dimensional, space rame representation o the structure may be considered adequate. The stiness o major structural connections (e.g. pontoon/column, column/deck) shall be modelled in suicient detail to adequately represent the stiness o the connection, such that the resulting responses are appropriate to the model being analysed. It is preerable that actions resulting rom the sea-loading model are mapped directly onto a structural model. M.4..3 Mass modelling A representative number o global design load conditions, simulating the static load distribution or each draught, should be evaluated in the global model. This may be achieved by the inclusion o a "mass model". The mass model may be an independent model or may be implicitly included in the structural (or sea action) model(s). Usually, only a limited number o global load conditions are considered in the global analysis. The global model may not thereore adequately cover all "worst case" global load distributions or each individual structural element. Procedures shall be established to ensure that the most unavourable load combinations have been accounted or in the design. Pontoon tank loading conditions In respect to global pontoon tank loading arrangements the maximum range o responses resulting rom the most onerous, relevant, static load conditions shall be established. In order to assess the maximum range o stresses resulting rom variations in pontoon tank loading conditions a simpliied model o the structure may be created. This simpliied model may typically be a space rame model o the unit. Responses or relevant load combinations with ull pontoon tank loading conditions (maximum "sagging" condition) and empty tank loading conditions (maximum "hogging" condition) shall be considered. For column stabilised units with non-box type, upper hull arrangements it may also be necessary to investigate conditions with non-symmetric, diagonally distributed tank loading conditions. Response resulting rom NORSOK standard Page 15 o 64

217 Design o steel structures N-004 Annex M Rev.3, February 013 variations in pontoon tank load conditions may normally be considered as providing a signiicant contribution to the designing stress components in the areas o the upper and lower langes o the pontoon deck and bottom plate ields. Such variations in stresses due to ull/empty load combinations o pontoon tanks shall thereore be explicitly accounted or when considering logical combinations o global and local responses in the structural design. M.4..4 Global analysis Model testing shall be undertaken when non-linear eects cannot be adequately determined by direct calculations. In such cases, time domain analysis may also be considered as being necessary. Where non-linear actions may be considered as being insigniicant, or where such actions may be satisactorily accounted or in a linear analysis, a requency domain analysis can be undertaken. Transer unctions or structural response shall be established by analysis o an adequate number o wave directions, with an appropriate radial spacing. A suicient number o periods shall be analysed to adequately cover the site speciic wave conditions, satisactorily describe transer unctions at, and around, the wave "cancellation" and "ampliying" periods, satisactorily describe transer unctions at, and around, the heave resonance period o the unit. Global, wave-requency, structural responses shall be established by an appropriate methodology, or example: a regular wave analysis; a "design wave" analysis; a stochastic analysis. These analytical methods are described in NORSOK N-003, Regular wave analysis: A suicient number o wave periods and wave directions shall be investigated to clearly demonstrate that maximum responses have been determined by the undertaken analysis. A wave directional radial spacing o maximum 15 degrees, should be utilised in a regular wave design analysis. However, it may be considered advisable to investigate with a spacing less than 15 degrees or structure with a substantial consequence o ailure. Appropriate consideration shall be given to the modelling and inclusion o wave steepness characteristics. Design wave analysis: Design waves, utilised to simulate characteristic, global hydrodynamic responses shall be established based upon recognised analytical practices or column stabilised units. Appropriate consideration shall be given to the modelling and inclusion o wave steepness and spectral characteristics. A suicient number o wave periods and wave directions shall be investigated to clearly demonstrate that maximum responses have been determined by the undertaken analysis. In design wave analysis a wave directional radial spacing o maximum 15 degrees or less, should normally be utilised. The ollowing characteristic responses will normally be governing or the global strength o a column stabilised unit: split ("squeeze and pry") orce between pontoons; torsion ("pitch connecting") moment about a transverse horizontal axis; longitudinal shear orce between the pontoons; combined longitudinal shear/split orce between pontoons; longitudinal acceleration o deck mass; transverse acceleration o deck mass; vertical acceleration o deck mass. Particular attention shall be given to the combined action eects o the characteristic responses o split and longitudinal shear orce. Generally, it will be ound that maximum longitudinal shear orces occur on a dierent wave heading than that heading providing maximum response when simultaneous split orces are taken into account. Stochastic analysis: Stochastic methods o analysis are, in principle recognised as providing the most accurate methods or simulating the irregular nature o wave actions. Long term, stress distributions shall be established based upon the relevant site speciic, scatter diagram wave data. In a stochastic design analysis, a wave directional radial spacing o 15 degrees or less, should be utilised. NORSOK standard Page 16 o 64

218 Design o steel structures N-004 Annex M Rev.3, February 013 Responses resulting rom stochastic analytical procedures may however, not be particularly well suited to structural design as inormation concerning the simultaneity o internal orce distribution is generally not available. M.4.3 Local structural models An adequate number o local structural models should be created in order to evaluate response o the structure to variations in local actions, e.g. in order to evaluate pressure acting across a structural section, lay-down loads acting on deck plate ield, or, support point loads o heavy items o equipment. The model(s) should be suiciently detailed such that resulting responses are obtained to the required degree o accuracy. A number o local models may be required in order to ully evaluate local response at all relevant sections. Example o local pressure acting on a pontoon section: An example o considerations that should be evaluated in connection with the load case o local pressures acting on a pontoon section o a column stabilised unit are given below. Typical parameters contributing to variations in internal and external local pressure distributions acting on the pontoon tanks o a cross-section o a column stabilised unit are illustrated in Figure M.4-. For details concerning tank pressure actions reerence should be made to NORSOK N-003, 5.4. A local structural model should be created with the intention o simulating local structural response or the most unavourable combination o relevant actions. When evaluating local response rom the local model the ollowing considerations may apply: The intention o the local model is to simulate the local structural response or the most unavourable combination o relevant actions. Relevant combinations o internal and external pressures or tanks should be considered (or both the intact and damage load conditions). Maximum (and minimum) design pressures (both internal and external) should be investigated. I cross-section arrangements change along the length o the structure, a number o local models may be required in order to ully evaluate local response at all relevant sections. When internal structural arrangements are such that the structure is divided across its section (e.g. there may be a watertight centreline bulkhead or access/pipe tunnel in the pontoon) relevant combinations o tank loadings across the section shall be considered. In most cases, or external plate ields, relevant combinations o external and internal pressure load cases will result in load conditions where the external and internal pressures are not considered to be acting simultaneously. For internal structures the simultaneous action o external and internal pressures load cases may provide or the governing design load case. For internal structures the internal pressure should normally not be considered to act simultaneously on both sides o the structure. Actions are usually applied in the analysis model at the girder level and not at the individual stiener level in order to ensure that local stiener bending is not included in the model response as the stiener bending response is explicitly included in the buckling code check, see 6.5. For transversely stiened structures (i.e. girders orientated in the transverse direction) the local responses extracted rom the local model are normally responses in the structural transverse direction ( y in Figure M.4-3). For structural arrangements with continuous, longitudinal girder arrangements however a longitudinal response will also be o interest. For structural cross-sections without continuous longitudinal girder elements, two-dimensional structural models may be considered as being adequate. For space rame, beam models relevant consideration shall be given to shear lag eects. Whilst this example considers the load case o local pressure acting on a pontoon section o a column stabilised unit, similar types o considerations are applicable in the evaluation o other local responses resulting rom local actions on the column stabilised unit, e.g. lay-down actions. NORSOK standard Page 17 o 64

219 Design o steel structures N-004 Annex M Rev.3, February 013 Figure M.4- stabilised unit M.4.4 Design action parameters acting as pressure on the pontoon tanks o a column Superimposing responses The simultaneity o the responses resulting rom the analysis models described in M.4. and M.4.3 may normally be accounted or by linear superposition o the responses or logical load combinations. Structural utilisation shall be evaluated in accordance with the requirements o Clause 6. When evaluating responses by superimposing stresses resulting rom a number o dierent models, consideration shall be given to the ollowing: It should be ensured that responses rom design actions are not included more than once. For example, when evaluating responses resulting rom tank loading conditions (see M.4..3), it may be necessary to model a load condition, in the simpliied model proposed in M.4..3, with a load case simulating that load case adopted in the global analysis model. In this way, stress variations resulting rom relevant load conditions, when compared that load condition included in the global model may be established. Relevant peak responses may then be included in the capacity evaluation o the combined responses. Continuous, longitudinal structural elements (e.g. stiened plate ields in the pontoon deck, bottom, sides, bulkheads, tunnels etc.) located outside areas o global stress concentrations, may be evaluated utilising linear superposition o the individual responses as illustrated in Figure M.4-3 or a pontoon section. The summation o such responses should then be evaluated against the relevant structural capacity criteria. In locations in way o column connections with pontoon and upper hull structures, global stress components become more dominant and should not be ignored. When transverse stress components are taken directly rom the local structural model an evaluation, that it is relevant to ignore transverse responses rom the global model, shall be carried out. This may normally be undertaken by considering the transverse stresses in the global model along the length o the structural element and ensuring that no additional transverse stress components have been set-up as a result o global stress concentrations, "skew" load conditions, or other interacting structural arrangements. Stiener induced buckling ailure normally tends to occur with lateral pressure on the stiener side o the plate ield. Plate induced buckling ailure normally tends to occur with lateral pressure on the plate side. NORSOK standard Page 18 o 64

220 Design o steel structures N-004 Annex M Rev.3, February 013 Relevant combinations o buckling code checking should thereore include evaluation o the capacity with relevant lateral pressure applied independently to both sides o the plate ield. In order to ensure that local bending stress components resulting rom action acting directly on the stieners ( bs, bp,-see Clause 6) are included in the buckling code check, the lateral pressure should be explicitly included in the capacity check. Unless, or the case in question, there is always a pressure acting over the stiened plate ield being evaluated, the capacity checking should include a buckling check with no lateral pressure in addition to the case with lateral pressure. Figure M.4-3 Combination o stress components or buckling assessment o an individual stiened plate ield in a "typical" pontoon section. NORSOK standard Page 19 o 64

221 Design o steel structures N-004 Annex M Rev.3, February 013 M.5 Fatigue limit states (FLS) M.5.1 General General requirements and guidance concerning atigue criteria are given in Clause 8, and DNV-RP-C03. Evaluation o the atigue limit states shall include consideration o all signiicant actions contributing to atigue damage both in non-operational and operational design conditions. The minimum atigue lie o the unit (beore the DFF is considered) should be based upon a period o time not being less than the planned lie o the structure. Assumptions related to the resistance parameters adopted in the atigue design (e.g. with respect to corrosion protection) (see DNV-RP-C03) shall be consistent with the in-service structure. Local eects, or example due to slamming, sloshing, vortex shedding, dynamic pressures, mooring and riser systems. shall be included in the atigue damage assessment, when relevant. In the assessment o atigue resistance, relevant consideration shall be given to the eects o stress raisers (concentrations) including those occurring as a result o abrication tolerances (including due regard to tolerances in way o connections o large structural sections, e.g. as involved in mating sequences or section joints), cut-outs, details at connections o structural sections, e.g. cut-outs to acilitate construction welding. M.5. Design atigue actors (DFFs) Criteria related to DFFs, are given in Clause 8. Structural components and connections designed in accordance with this annex should normally satisy the requirement to damage condition according to the ALS with ailure in the actual joint deined as the damage. As such DFFs selected in accordance with Table 8-1 should normally all within the classiication: "Without substantial consequences". When determining the appropriate DFF or a speciic atigue sensitive location, consideration shall be given to the ollowing: Consideration that economic consequence o ailure may indicate the use o larger design actors than those provided or as minimum actors. The categorisation: "Accessible/Above splash zone" is, intended to reer to atigue sensitive locations where the possibility or close-up, detailed inspection in a dry and clean condition exists. I any o these requirements are not ulilled, the relevant DFF should be considered as being that appropriate or "Accessible/Below splash zone", or, "No access or in the splash zone" as relevant to the location being considered. Evaluation o likely crack propagation paths (including direction and growth rate related to the inspection interval), may indicate the use o a dierent DFF than that which would be selected when the detail is considered in isolation, such that: Where the likely crack propagation indicates that a atigue ailure, rom a location satisying the requirements or a "Non-substantial" consequence o ailure, may result in a "Substantial" consequence o ailure, such atigue sensitive location is itsel to be deemed to have a "Substantial" consequence o ailure. Where the likely crack propagation is rom a location satisying the requirement or a given "Access or inspection and repair" category to a structural element having another access categorisation, such location is itsel to be deemed to have the same categorisation as the most demanding category when considering the most likely crack path. For example, a weld detail on the inside (dry space) o a submerged shell plate should be allocated the same DFF as that relevant or a similar weld located externally on the plate, see Figure M.5-1. NORSOK standard Page 0 o 64

222 Design o steel structures N-004 Annex M Rev.3, February 013 Implications o the above are that or internal structures, below or above splash zone, the DFF shall normally be 1 where access or inspection and repair is possible. Non-accessible areas, like welds covered with ire prooing and other areas inaccessible or inspection, shall be designed with DFF=3. The hull shell below splash zone shall normally be assigned a DFF=. This applies also to any attachment, internal or external, rom where a atigue crack can grow into the shell. Figure M.5-1 Example illustrating considerations relevant or selection o DFFs in locations considered to have "non-substantial" consequence o ailure M.5.3 Splash zone The deinition o "splash zone" as given in NORSOK N-003, 6.6.3, relates to a highest and lowest tidal reerence. For column stabilised units, or the evaluation o the atigue limit state, reerence to the tidal datum should be substituted by reerence to the draught that is intended to be utilised when condition monitoring is to be undertaken. The requirement that the extent o the splash zone is to extend 5 m above and 4 m below this draught may then be applied. For application o requirements to corrosion addition, however, the normal operating draught should generally be considered as the reerence datum. I signiicant adjustment in draught o the unit is possible in order to provide or satisactory accessibility in respect to inspection, maintenance and repair, account may be taken o this possibility in the determination o the DFFs. In such cases, however, a suicient margin in respect to the minimum inspection draught should be considered when deciding upon the appropriate DFF in relation to the criteria or "Below splash zone" as opposed to "Above splash zone". Where drat adjustment possibilities exist, a reduced extent o splash zone may be applicable. See M.1 or urther details. The entire unit may be regarded as being above the splash zone i the unit is to be regularly dry-docked every 4 years to 5 years. NORSOK standard Page 1 o 64

223 Design o steel structures N-004 Annex M Rev.3, February 013 M.5.4 Fatigue analysis M General The basis or determining the acceptability o atigue resistance, with respect to wave actions, shall be appropriate stochastic atigue analyses. The analyses shall be undertaken utilising relevant site speciic environmental data and take appropriate consideration o both global and local (e.g. pressure luctuation) dynamic responses. These responses do not necessarily have to be evaluated in the same model but the cumulative damage rom all relevant eects should be considered when evaluating the total atigue damage. Simpliied atigue analyses may orm the basis o a "screening" process to identiy locations or which a detailed, stochastic atigue analysis should be undertaken, e.g. at critical intersections. Such simpliied atigue analysis shall be calibrated, see M Local, detailed FE analysis o critical connections (e.g. pontoon/pontoon, pontoon/column, column/deck and brace connections) should be undertaken in order to identiy local stress distributions, appropriate SCFs, and/or extrapolated stresses to be utilised in the atigue evaluation, see DNV-RP-C03 or urther details. Dynamic stress variations through the plate thickness shall be documented and considered in such evaluations. Explicit account shall be taken o any local structural details that invalidate the general criteria utilised in the assessment o the atigue strength. Such local details may, or example be access openings, cut-outs, penetrations etc. in structural elements. Principal stresses (see DNV-RP-C03) should be utilised in the evaluation o atigue responses. M.5.4. Simpliied atigue analysis Provided that the provisions stated in M.5.4.1, are satisied, simpliied atigue analysis may be undertaken in order to establish the general acceptability o atigue resistance. In all cases when a simpliied atigue analysis is utilised a control o the results o the simpliied atigue evaluation, compared to the stochastic results, shall be documented to ensure that the simpliied analysis provides or a conservative assessment or all parts o the structure being considered. Simpliied atigue analysis is particularly well suited or preliminary design evaluation studies. In such cases, a space rame model o the structure may be considered as being adequate. For inal design however the basis model or the "screening" evaluation should be a model o similar (or better) detail to that described in M.4... Simpliied atigue analyses should be undertaken utilising appropriate design parameters that provide or conservative results. Normally a two-parameter, Weibull distribution (see DNV-RP-C03) may be utilised to describe the long-term stress range distribution. In such cases the Weibull shape parameter ( h, see Equation (M.5.1)) should normally have a value between 1.0 and 1.1. n 0 lnn 0 (DFF) 1 h 1 m a n 01 m h 1 m (M.5.1) where n 0 is the total number o stress variations during the lietime o the structure n n0 is the extreme stress range that is exceeded once out o 0 stress variations. The n extreme stress amplitude 0 ampl_n0 is thus given by. h a 1 m DFF m h is the shape parameter o the Weibull stress range distribution is the intercept o the design S-N curve with the log N axis (see, or example, DNV-RP- C03) is the complete gamma unction (see DNV-RP-C03) is the inverse slope o the S-N curve (see DNV-RP-C03) is the DFF Generally, the simpliied global atigue analysis should consider the "F3", S-N class curve (see DNV-RP- C03), adjusted to include any thickness eect, as the minimum basis requirement. Areas not satisying this NORSOK standard Page o 64

224 Design o steel structures N-004 Annex M Rev.3, February 013 requirement are normally to be excluded rom the simpliied atigue evaluation "screening" procedure. This implies that connections with a more demanding S-N atigue class than F3, are not to be applied in the structure, e.g. i overlap connections are applied then the atigue S-N class is to be suitably adjusted. The cumulative atigue damage may then be obtained by considering the dynamic stress variation, Actual ( n 0 ), which is exceeded once out o n 0 cycles [see Equation (M.5.) with the allowable equivalent stress variation, (M.5.1)]. n0, calculated rom Equation (M.5.1). The atigue lie thus obtained is ound rom Equation N Actual N Design n 0 Actual(n ) 0 m (M.5.) where N Actual N Design n 0 Actual n 0 is the actual (calculated) atigue lie is the target atigue lie is described in Equation (M.5.1) is the actual design, dynamic stress variation which is exceeded once out o "n 0 "cycles. When the simpliied atigue evaluation involves utilisation o the dynamic stress responses resulting rom the global analysis (as described in M.4.), the response should be suitably scaled to the return period o the basis, minimum atigue lie o the unit. In such cases, scaling may normally be undertaken utilising the appropriate actor ound rom Equation (M.5.3). where 1 log n h 0 n 0 n100 log n100 n100 is the number o stress variations (e.g. 100 years) appropriate to the global analysis n100 is the extreme stress range that is exceeded once out o 100 [Other parameters are as or Equation (M.5.1) and Equation (M.5.)]. M Stochastic atigue analysis n stress variations (M.5.3) Stochastic atigue analyses shall be based upon recognised procedures and principles utilising relevant site speciic data. Analyses shall include consideration o the directional probability o the environmental data. Providing that it can be satisactorily documented, scatter diagram data may be considered as being directionally speciic. Relevant wave spectra shall be utilised. Wave energy spreading may be taken into account i relevant. Structural response shall be determined based upon analyses o an adequate number o wave directions. Generally a maximum radial spacing o 15 degrees should be considered. Transer unctions should be established based upon consideration o a suicient number o periods, such that the number, and values o the periods analysed adequately cover the site speciic wave data, satisactorily describe transer unctions at, and around, the wave "cancellation" and "ampliying" periods. Consideration should be given to take account that such "cancellation" and "ampliying" periods may be dierent or dierent elements within the structure), satisactorily describe transer unctions at, and around, the relevant excitation periods o the structure. Stochastic atigue analyses utilising simpliied structural model representations o the unit (e.g. a space rame model) may orm the basis o a "screening" process to identiy locations or which a, stochastic atigue analysis, utilising a detailed model o the structure, should be undertaken (e.g. at critical intersections). When space rame, beam models are utilised in the assessment o the atigue strength o large volume structures, (other than or preliminary design studies) the responses resulting rom the beam models should be NORSOK standard Page 3 o 64

225 Design o steel structures N-004 Annex M Rev.3, February 013 calibrated against a more detailed model to ensure that global stress concentrations are included in the evaluation process. NORSOK standard Page 4 o 64

226 Design o steel structures N-004 Annex M Rev.3, February 013 M.6 Accidental limit states (ALS) M.6.1 General General guidance and requirements concerning accidental events are given in Clause 9, and Annex A. Units shall be designed to be damage tolerant (i.e. credible accidental damages, or events, are not to cause loss o global structural integrity) see M.9.1. The capability o the structure to redistribute actions should be considered when designing the structure. In the design phase, attention shall be given to layout and arrangements o acilities and equipment in order to minimise the adverse eects o accidental events. Satisactory protection against accidental damage may be obtained by a combination o the ollowing principles: reduction o the probability o damage to an acceptable level; reduction o the consequences o damage to an acceptable level. Structural design in respect to the ALS shall involve a two-stage procedure considering resistance o the structure to a relevant accidental event, capacity o the structure ater an accidental event. Global structural integrity shall be maintained both during and ater an accidental event. Actions occurring at the time o a design accidental event and thereater shall not cause complete structural collapse, or loss o hydrostatic (or hydrodynamic) stability, see M.8. Requirements to compartmentation and stability in the damage condition are given in M.8.4. When the upper hull (deckbox) structure becomes buoyant in satisying requirements to damage stability /1/, consideration shall be given to the structural response resulting rom such actions. M.6. Dropped objects Critical areas or dropped objects shall be determined on the basis o the actual movement o potential dropped objects (e.g. crane actions) relative to the structure o the unit itsel. Where a dropped object is a relevant accidental event, the impact energy shall be established and the structural consequences o the impact assessed. The impact energy at sea level is normally not to be taken less than 5 MJ or cranes with maximum liting capacity o more than 30 tonnes, however, reduced impact energy may be acceptable or smaller cranes and special purpose cranes. The impact energy below sea level is assumed to be equal to the energy at sea level, unless otherwise documented. Generally, dropped object assessment will involve the ollowing considerations: assessment o the risk and consequences o dropped objects impacting topside, wellhead (e.g. on the sealoor), and saety systems and equipment. The assessment shall identiy the necessity o any local structural reinorcement or protection to such arrangements; assessment o the risk and consequences o dropped objects impacting externally on the hull structure. However, the structural consequences are normally ully accounted or by the requirements or watertight compartmentation and damage stability (see M.8) and the requirement or structural redundancy o slender structural members, see M.9.1. M.6.3 Fire General guidance and requirements concerning accidental limit state events involving ire are given in Clause 9 and Annex A. M.6.4 Explosion In respect to design considering actions resulting rom explosions one, or a combination o the ollowing main design philosophies are relevant: ensure that the probability o explosion is reduced to a level where it is not required to be considered as a relevant design load case; ensure that hazardous locations are located in unconined (open) locations and that suicient shielding mechanisms (e.g. blast walls) are installed; NORSOK standard Page 5 o 64

227 Design o steel structures N-004 Annex M Rev.3, February 013 locate hazardous areas in partially conined locations and design utilising the resulting, relatively small overpressures; locate hazardous areas in enclosed locations and install pressure relie mechanisms (e.g. blast panels) and design or the resulting overpressure. As ar as practicable, structural design accounting or large plate ield rupture resulting rom explosion actions should normally be avoided due to the uncertainties o the actions and the consequences o the rupture itsel. Structural support o blast walls, and the transmission o the blast action into main structural members shall be evaluated when relevant. Eectiveness o connections and the possible outcome rom blast, such as lying debris, shall be considered. M.6.5 Collision Collision shall be considered as a relevant ALS load case or all structural elements o the unit that may be impacted in the event o a collision. The vertical zone o impact shall be based on the depth and draught o the colliding vessel and the relative motion o the vessel and the unit. In the assessment o the collision condition, the ollowing general considerations with respect to structural strength will normally apply: An evaluation shall be undertaken in order to assess the extent o structural damage occurring to the unit at the time o impact. The extent o the local damage resulting rom the collision should be compared to that damage extent implicit in the NMD regulations covering damage stability o the unit /1/. Provided that the extent o local damage does not exceed that damage criteria stated in the NMD regulations, the unit shall satisy the relevant damage stability requirements o the NMD, see also M.8.4. I the extent o the damage exceeds that damage criteria stated in the NMD regulations /1/, an equivalent level o saety to that implicit in the NMD stability regulations should be documented. NMD damaged condition requirements /1/, in respect to structural strength o watertight boundaries (including boundaries required or reserve buoyancy) shall be satisied, see M.6.8. Global structural integrity o the unit ater the collision shall be evaluated. Topside structural arrangements should be designed or the damaged (inclined) condition. Considerations concerning structural evaluation at, and ater, the time o the collision are given below. Damage occurring at the time o collision A structural evaluation shall be perormed in order to document the extent o the local damage occurring to the unit at the time o impact. I a risk analysis shows that the greatest relevant accidental event with regard to collision is a driting vessel at m/s, with a displacement which does not exceed 5000 tonnes, the ollowing kinetic energy occurring at the time o collision may be considered or the structural design: 14 MJ or sideways collision; 11 MJ or bow or stern collision. Local damage assessment may be undertaken utilising sophisticated non-linear analytical tools, however, simpliied analytical procedures will normally be considered as being suicient to evaluate the extent o damage occurring under the action o the collision. Simpliied local damage assessment o the collision event normally involves the ollowing considerations the typical geometry o the supply vessel, together with relevant orce-deormation curves or side, bow and stern impact, documented in Annex A, may normally be utilised; in the local structural strength assessment the side, bow and stern proiles o the supply vessel are progressively "stepped" into the collision zone o the column stabilised unit, see example shown in Figure M.6-1; NORSOK standard Page 6 o 64

228 Design o steel structures N-004 Annex M Rev.3, February 013 Figure M.6-1 Illustration o the bow proile o a supply vessel being "stepped" into the structure o a column stabilised unit E by considering the local geometry o the supply vessel and the impacted structure, relevant orcedeormation curves or the column stabilised unit may be produced; For a given action level the area under the orce-deormation curves represents the absorption o energy. The distribution and extent o the damage results rom the condition o equal collision orce acting on the structures, and that the sum o the absorbed energies equals the portion o the impact energy dissipated as strain energy, i.e. s where Ps, P u s, u E u Ps (s )ds 0 0 P u ( u )d u are the orce in the impacting vessel and the impacted unit respectively, are the deormations in the impacting vessel and the impacted unit respectively. This procedure is illustrated in Figure M.6-. (M.6.1) NORSOK standard Page 7 o 64

229 Design o steel structures N-004 Annex M Rev.3, February 013 Figure M.6- Dissipation o strain energy global structural integrity ater collision Having evaluated the extent o local damage incurred by the relevant collision event (as described above) an assessment o the resulting, global structural capacity (considering environmental actions) shall be undertaken. In such an evaluation the ollowing listed items are relevant: In cases where the impact damage is limited to local damage to the column particular consideration should be given to column/deckbox interaces, and the damaged (impacted) section. For typical column structures a simpliied approach to assess the reduced capacity o the structure in way o the damaged location would be to assume that all the impacted (deormed) area is ineective. When counter-looding is utilised as a means o righting the unit in an accidental event the actions resulting rom such counter looding shall be evaluated. In cases where the impact damage is limited to local damage to a single brace, redundancy requirements given in M.9.1 should adequately cover the required structural evaluation. NMD requirements to watertight boundary, structural strength in the damaged condition (see M.6.8) (including inclined angles resulting rom requirements to reserve buoyancy) should be satisied. In order to avoid risk assessment considerations in respect to implications o structural ailure o topside structures in the inclined condition (e.g. in respect to the possibility o progressive collapse in the event o structural ailure), it is normal practice to consider the structural capacity o topside structural arrangements in the damaged condition. Capacity exceedances may be accepted or local areas provided that adequate account is given to the redistribution o orces. Due to the large heel angles in the damaged condition, the in-plane orce component o the deck box mass may be considerable. Normally part o the deck will be submerged and counteract this orce. The most critical condition is thereore generally the heel angle corresponding to a water plane just below the deck box corner. M.6.6 Unintended looding Considerations in respect to unintended looding are generally system and stability design considerations rather than structural design. Requirements with respect to intact and reserve buoyancy conditions are normally considered to adequately cover any structural strength requirements (see M.6.8) in respect to unintended looding. NORSOK standard Page 8 o 64

230 Design o steel structures N-004 Annex M Rev.3, February 013 M.6.7 Abnormal wave events Air gap considerations with respect to evaluation o the abnormal wave events are given in M.7. Should any part o the structure receive wave impact actions in the abnormal wave condition the consequences o such wave impacts shall be evaluated. M.6.8 Reserve buoyancy Structural strength o watertight boundaries (both internal and external watertight boundaries, including all stieners and girders supporting the plate ields) shall comply with the requirements stated in NMD regulations /1/. M.7 Air gap M.7.1 General Requirements and guidance to air gap analyses or a column stabilised unit are given in NORSOK N-003, Both ULS (10 - ) and ALS (10-4 ) conditions shall be evaluated. In the ULS condition, positive air gap should be ensured. However, wave impact may be permitted to occur provided that it can be demonstrated that such actions are adequately accounted or in the design and that saety to personnel is not signiicantly impaired. See M.1 or more details. Analysis undertaken to document air gap should be calibrated against relevant model test results. Such analysis shall include relevant account o wave/structure interaction eects, wave asymmetry eects, global rigid body motions (including dynamic eects), eects o interacting systems (e.g. mooring and riser systems), maximum/minimum draughts. Column "run-up" action eects shall be accounted or in the design o the structural arrangement in way o the column/deck box connection. These "run-up" actions are to be treated as an environmental load component. However, they need not normally be considered as occurring simultaneously with other environmental responses. For urther considerations in respect to run-up eects, reerence should be made to NORSOK N-003. Evaluation o air gap adequacy shall include consideration o all aected structural items including lieboat platorms, riser balconies, overhanging deck modulus etc.. M.8 Compartmentation and stability M.8.1 General In the assessment o compartmentation and stability o a column stabilised unit, consideration shall be given to all relevant detrimental eects, including those resulting rom environmental actions, relevant damage scenarios, rigid body motions, the eects o ree-surace, boundary interactions (e.g. mooring and riser systems). The mass distribution o a column stabilised unit, including all associated permanent and variable actions, shall be determined to an appropriate accuracy by a relevant procedure. An inclining test should be conducted when construction is as near to completion as practical in order to accurately determine the unit s mass and position o the centre o gravity. Changes in load conditions ater the inclining test should be careully accounted or. Monitoring o the mass and centre o gravity shall be perormed during the entire lie cycle o the unit. A procedure or control o the mass and position o the centre o gravity o the unit shall be incorporated into the design. Note: Special provisions relating to the Norwegian activities NORSOK standard Page 9 o 64

231 Design o steel structures N-004 Annex M Rev.3, February Special provisions relating to subdivision, stability and reeboard or units engaged in Norwegian petroleum activities are given in NORSOK N-001, M.8. Compartmentation and watertight integrity Compartmentation, and watertight and weather tight integrity o a column stabilised unit shall satisy the general requirements as stated in ISO Detailed provisions stated in IMO MODU Code //, should also be satisied. The number o openings in watertight structural boundaries shall be kept to a minimum. Where penetrations are necessary or access, piping, venting, cables etc., arrangements shall be made to ensure that the watertight integrity o the structure is maintained. Where individual lines, ducts or piping systems serve more than one watertight compartment, or are within the extent o damage resulting rom a relevant accidental event, arrangements shall be provided to ensure that progressive looding will not occur. M.8.3 Stability Stability o a column stabilised unit in the operational phase, shall satisy the requirements o the relevant provisions stated in the IMO MODU Code //. Adequacy o stability shall be established or all relevant operational and temporary phase conditions. The assessment o stability shall include consideration o both the intact and damaged conditions. It shall be ensured that the assumed basis or the damage stability design criteria is compatible with accidental events identiied as being relevant or the structure, see M.6 and M.8.4. M.8.4 Damaged condition The dimensioning extent o damage and loss o buoyancy shall be based on a risk analysis, see M.6.1. I such risk analysis shows that the greatest relevant accidental event with regard to collision is a driting vessel with a displacement that does not exceed 5000 tonnes, the extent o damage indicated in the Norwegian Maritime Directorates, Regs. or Mobile Oshore Units, 1991, /1/, may be utilised. In order to ensure adequacy o ballast systems, both with respect to unintentional looding, (see M.6.6) and capacity in the damage condition (see M.6.5), the requirements to ballast systems, stated in the Norwegian Maritime Directorate, Regs. or Mobile Oshore Units, 1991, /1/, concerning ballast systems on mobile oshore units, /3/, Section 11 and Section 1 should be satisied. M.9 Special considerations M.9.1 Structural redundancy Structural robustness shall, when considered necessary, be demonstrated by appropriate analysis. Slender, main load bearing, structural elements shall normally be demonstrated to be redundant in the accidental limit state condition. Considerations with respect to redundancy o bracing systems are given in M.9.. M.9. Brace arrangements For bracing systems (see M.1, Comm. M.9.1) the ollowing listed considerations shall apply: brace structural arrangements shall be investigated or relevant combinations o global and local actions; structural redundancy o slender bracing systems (see M.9.1) shall normally include brace node redundancy (i.e. all bracings entering the node), in addition to individual brace element redundancy; brace end connections (e.g. brace/column connections) shall normally be designed such that the brace element itsel will ail beore the end connection; underwater braces should be watertight and have a leakage detection system to make it possible to detect cracking at an early stage; when relevant (e.g. in the sel-loating, transit condition) the eect o slamming on braces shall be considered. NORSOK standard Page 30 o 64

232 Design o steel structures N-004 Annex M Rev.3, February 013 M.9.3 Structure in way o a ixed mooring system Local structure in way o airleads, winches, etc. orming part o the position mooring system is, as a minimum, to be capable o withstanding orces equivalent to 1.5 times the breaking strength o the individual mooring line. The strength evaluation should be undertaken utilising the most unavourable operational direction o the anchor line. In the evaluation o the most unavourable direction, account shall be taken o relative angular motion o the unit in addition to possible line lead directions. M.9.4 Structural detailing In the design phase particular attention should be given to structural detailing, and requirements to reinorcement in areas that may be subjected to high local stresses, or example: design class 3 (DC3) connections (see M..1); locations that may be subjected to wave impact (including column run-up actions); locations in way o mooring arrangements; locations that may be subjected to accidental, or operational, damage. In way o DC3 connections, continuity o strength is normally to be maintained through joints with the axial stiening members and shear web plates being made continuous. Particular attention should be given to weld detailing and geometric orm at the point o the intersections o the continuous plate ields with the intersecting structure. M.10 Documentation requirements or the "design basis" and "design brie" M.10.1 General Adequate planning shall be undertaken in the initial stages o the design process in order to obtain a workable and economic structural solution to perorm the desired unction. As an integral part o this planning, documentation shall be produced identiying design criteria and describing procedures to be adopted in the structural design o the unit. Applicable codes, standards and regulations should be identiied at the commencement o the design. A summary document containing all relevant data rom the design and abrication phase shall be produced. Design documentation (see below) shall, as ar as practicable, be concise, non-voluminous, and, should include all relevant inormation or all relevant phases o the lietime o the unit. General requirements to documentation relevant or structural design are given in NORSOK N-001, Clause 5. M.10. Design basis A design basis document shall be created in the initial stages o the design process to document the basis criteria to be applied in the structural design o the unit. A summary o those items normally to be included in the design basis document is included below. Unit description and main dimensions A summary description o the unit, including general description o the unit (including main dimensions and draughts), main drawings including: main structure drawings (this inormation may not be available in the initial stage o the design), general arrangement drawings, plan o structural categorisation, load plan(s), showing description o deck uniorm (laydown) actions, capacity (tank) plan. service lie o unit, position keeping system description. Rules, regulations and codes A list o all relevant, and applicable, rules, regulations and codes (including revisions and dates), to be utilised in the design process. Environmental design criteria Environmental design criteria (including all relevant parameters) or all relevant conditions, including NORSOK standard Page 31 o 64

233 Design o steel structures N-004 Annex M Rev.3, February 013 wind, wave, current, snow and ice description or 10-1,10 - and 10-4 annual probability events, design temperatures. Stability and compartmentation Stability and compartmentation design criteria or all relevant conditions including external and internal watertight integrity plan, lightweight breakdown report, design loadcase(s) including global mass distribution, damage condition waterlines (this inormation may not be available in the initial stage o the design). Temporary phases Design criteria or all relevant temporary phase conditions including, as relevant: limiting permanent, variable, environmental and deormation action criteria; procedures associated with construction (including major liting operations); essential design parameters associated with temporary phases (e.g. or mating load cases, mating weldup sequences, crushing tube stiness, or transit phases, transit speed etc.), relevant ALS criteria. Operational design criteria Design criteria or all relevant operational phase conditions including limiting permanent, variable, environmental and deormation action criteria, designing accidental event criteria (e.g. collision criteria), tank loading criteria (all tanks) including a description o system, with loading arrangements, height o air pipes, loading dynamics, densities. mooring actions. Air gap Relevant basis inormation necessary or the assessment o air gap suiciency, including a description o the requirements to be applied in the ULS and ALS conditions, basis model test report (this inormation may not be available in the initial stage o the design). In-service inspection criteria A description o the in-service inspection criteria and general philosophy (as relevant or evaluating atigue allowable cumulative damage ratios). Miscellaneous A general description o other essential design inormation, including description o corrosion allowances, where applicable. M.10.3 Design brie A design brie document shall be created in the initial stages o the design process. The purpose o the design brie shall be to document the intended procedures to be adopted in the structural design o the unit. All applicable limit states or all relevant temporary and operational design conditions shall be considered in the design brie. A summary o those items normally to be included in the design brie document is included below. Analysis models A general description o models to be utilised, including description o global analysis model(s), local analysis model(s), load cases to be analysed. Analysis procedures A general description o analytical procedures to be utilised including description o procedures to be adopted in respect to the evaluation o temporary conditions, the consideration o accidental events, NORSOK standard Page 3 o 64

234 Design o steel structures N-004 Annex M Rev.3, February 013 the evaluation o atigue actions, air gap evaluation (including locations to be considered, damping, inclusion o asymmetry actors, disturbed (radiated) wave considerations, combined motion response), the establishment o dynamic responses (including methodology, actors, and relevant parameters), the inclusion o "built-in" stresses, the consideration o local responses (e.g. those resulting rom mooring and riser actions, ballast distribution in pontoon tanks etc.), the consideration o structural redundancy. Structural evaluation A general description o the evaluation process, including description o procedures to be utilised or considering global and local responses, description o atigue evaluation procedures (including use o DFFs, SN-curves, basis or stress concentration actors (SCFs), etc.), description o procedures to be utilised or code checking. Air gap evaluation A general description o the air gap evaluation procedure, including description o procedures to be utilised or considering air gap suiciency. M.11 Reerences /1/ Regulations o 0 Dec. 1991, No.878, concerning stability, watertight subdivision and watertight/weathertight closing means on mobile oshore units, Norwegian Maritime Directorate. // Code or the Construction and Equipment o Mobile Oshore Drilling Units, 1989, (1989 MODU CODE), International Maritime Organisation, London /3/ Regulations o 0 Dec. 1991, No.879, concerning ballast systems on mobile oshore units, Norwegian Maritime Directorate. /4/ Oshore Installations: Guidance on design, construction and certiication, U.K., Health & Saety Executive, Fourth Edition, HMSO. M.1 Commentary Comm. M.1.1 General The content o this annex is applicable to column stabilised units located at a single site over a prolonged period o time. Normally such units will be engaged in production activities. There may thereore be certain requirements and guidance given within this annex that are not considered as being ully appropriate or "mobile", column stabilised units, e.g. engaged in exploration drilling activities at a location or only a limited period o time. Comm. M.3.3 Variable actions Applicable variable actions acting on deck areas are stated in NORSOK N-003, Table 5.1. This table provides appropriate action actors or local, primary and global design. For loating units, ull application o the global design action actors will lead to a load condition that, in practice, would not be possible to achieve as the total static mass is always required to be in balance with the buoyancy actions. Additionally requirements to stability and compartmentation will urther restrict the practical distribution o variable actions. In the assessment o global structural response the variable action design actors stated in Table 5.1 are not intended to limit the variable load carrying capacity o the unit in respect to non-relevant, variable load combinations. Full application o the global design actors, stated in Table 5.1, to all variable actions would normally imply such a restriction. Global design load conditions should thereore be established based upon "worst case", representative variable load combinations, where the limiting global mass distribution criteria is established taking into account compliance with the requirements to intact and damage hydrostatic and hydrodynamic stability. NORSOK standard Page 33 o 64

235 Design o steel structures N-004 Annex M Rev.3, February 013 These limit global load conditions shall be ully documented in the design basis, see M.10.. Comm. M.4. Global models It is normally not practical to consider all relevant actions (both global and local) in a single model, due to, or example the ollowing listed reasons: Single model solutions do not normally contain suicient structural detailing (or ULS structural assessment, response down to the level o the stress in plate ields between stieners is normally required). Examples o insuicient structural detailing may be: - internal structure is not modelled in suicient detail to establish internal structural response to the degree o accuracy required, - element type, shape or ineness (e.g. mesh size) is insuicient. Single model solutions do not normally account or the ull range o internal and external pressure combinations. Examples o eects that may typically not be ully accounted or include: - internal tank pressure up to the maximum design pressure, - maximum external pressures (e.g. i a "design wave" analytical approach has been adopted, the maximum external pressure height is that height resulting rom the design wave, which is not normally the maximum external pressure that the section may be subjected to), - the ull extent o internal and external pressure combinations, - variations in tank loadings across the section o the pontoon (e.g. i the pontoon is sub-divided into a number o watertight compartments across its section), - load conditions that may not be covered by the global analysis (e.g. damage, inclined conditions). Single model solutions do not normally account or the ull range o "global" tank loading conditions. Examples o global tank loading conditions that may typically not be ully accounted or include - tank loading distributions along the length o the pontoon, - asymmetric tank loadings rom one pontoon as compared to another. Single model solutions may not ully account or all action eects. Examples o load eects that may typically not be ully accounted or include - viscous eects (drag loading) on slender members, - riser interace actions, - thruster actions. Generally, single model solutions that do contain suicient detail to include consideration o all relevant actions and load combinations are normally extremely large models, with a very large number o load cases. It is thereore oten the case that it is more practical, and eicient, to analyse dierent action eects utilising a number o appropriate models and superimposing the responses rom one model with the responses rom another model in order to assess the total utilisation o the structure. Comm. M.5. Design atigue actors (DFFs) I signiicant adjustment in draught is possible in order to provide or satisactory accessibility in respect to inspection, maintenance and repair, a suicient margin in respect to the minimum inspection draught should be considered when deciding upon the appropriate DFFs. As a minimum this margin is to be at least 1 m. However, it is recommended that a larger value be considered especially in the early design stages where suicient reserve should be allowed or to account or design changes in the mass and the centre o mass o the unit. Consideration should urther be given to operational requirements that may limit the possibility or ballasting/deballasting operations. When considering utilisation o ROV inspection consideration should be given to the limitations imposed on such inspection by the action o water particle motion, e.g. waves. The practicality o such a consideration may be that eective underwater inspection by ROV, in normal sea conditions, may not be achievable unless the inspection depth is at least 10 m below the sea surace. Comm. M.6.5 Collision In the damaged, inclined condition (i.e. ater the collision event) the structure shall continue to resist the deined environmental conditions, i.e annual probability o exceedance criteria. In practice, it is not normally considered practicable to analyse this damaged, inclined condition as the deck box structure becomes buoyant (due both to the static angle o inclination in the damaged condition and also due to rigid NORSOK standard Page 34 o 64

236 Design o steel structures N-004 Annex M Rev.3, February 013 body motion o the unit itsel). The global system o loading and response becomes extremely non-linear. Even with exhaustive model testing it would be diicult to ensure that all relevant responses are measured, or all designing sea-state conditions, and directions, or all relevant damage waterlines. Further, it is considered to be extremely unlikely that a 10 - environmental load event would occur beore corrective action had taken place, e.g. righting o the unit to an even keel. NMD requirements to ballast systems or column stabilised units, /3/, (and required in M.8.4) include requirements that the ballast system shall be capable o restoring the unit to an upright position, and an acceptable draught as regards strength, within 3 h ater the collision damage event. On the assumption that the 5000 tonne, m/s, collision criteria is based upon a ree driting supply vessel with a drit velocity equal to H s / (where H s = signiicant wave height), within a 3 h period, a 4 m signiicant wave height may be expected to increase to max. 6.5 m signiicant wave height. Hence, disregarding the damaged structural element itsel, the ULS load case b. may generally be considered as providing more demanding design criteria than the ALS condition with 100- year load event. Additionally, in respect to global response, as soon as the deck box starts to become buoyant the global actions resulting rom the inclined deck box mass rapidly become reduced. With background in the above logic simpliied "engineering" solutions to structural design or the collision event, that do not explicitly require analyses involving 100-year environmental actions in the inclined (heeled) condition, are normally considered as being acceptable. Comm. M.7.1 General In the ULS condition positive clearance between the upper hull (deck box) structure and the wave crest, including relative motion and interaction eects, should normally to be ensured. Localised, negative air gap, may however be considered as being acceptable or overhanging structures and appendages to the upper hull. In such cases ull account o the wave impact orces is to be taken into account in the design. The consequence o wave impact shall not result in ailure o a saety related system, e.g. lieboat arrangements. It is recommended in the design phase to consider operational aspects, including requirements to inspection and maintenance, which may impose criteria to air gap that exceed minimum requirements. In the context o M.7, column run-up actions are not considered as resulting in negative air-gap responses. Comm. M.9.1 Structural redundancy The requirement concerning structural redundancy o a slender structural element is included within this annex or the ollowing listed reasons: in order to maintain compatibility with the requirements to column stabilised stated by the International Association o Classiication Societies; in order to ensure that the structure exhibits ductility ater irst ailure; in order to ensure that a single ailure does not lead to a critical condition. A critical condition may or example be a atigue ailure that has not been accounted or in design calculations due to the act that, in the process o abrication, details built into the structure do not exactly relect those details that considered in the assessment o the design. Typically a member is to be considered as being slender i the reduced (relative) slenderness,, is greater than 0., see o0o- NORSOK standard Page 35 o 64

237 Annex N Rev.3, February 013 DESIGN OF STEEL STRUCTURES ANNEX N SPECIAL DESIGN PROVISIONS FOR TENSION LEG PLATFORMS NORSOK standard Page 36 o 64

238 Annex N Rev.3, February 013 CONTENTS N.1 INTRODUCTION 39 N.1.1 General 39 N.1. Deinitions 39 N.1.3 Description o tendon system 41 N.1.4 Non-operational phases 4 N. STRUCTURAL CLASSIFICATION AND MATERIAL SELECTION 44 N..1 General 44 N.. Structural classiication 44 N..3 Material selection 44 N..4 Inspection categories 45 N..5 Guidance to minimum requirements 45 N.3 DESIGN CRITERIA 47 N.3.1 General 47 N.3. Design principles, tendons 47 N.4 ACTIONS 49 N.4.1 General 49 N.4. Dierent actions 49 N.5 GLOBAL PERFORMANCE 50 N.5.1 General 50 N.5. Frequency domain analysis 50 N.5..1 High requency analyses 50 N.5.. Wave-requency analyses 51 N.5..3 Low-requency analyses 51 N.5.3 Time domain analyses 51 N.5.4 Model testing 51 N.5.5 Action eects in the tendons 5 N.6 ULTIMATE LIMIT STATE (ULS) 53 N.6.1 General 53 N.6. Hull 53 N.6..1 Structural analysis 53 N.6.. Structural design 54 N.6.3 Deck 54 N General 54 N.6.3. Air gap 54 N.6.4 Tendons 55 N Extreme tendon tensions 55 N.6.4. Structural design 55 N.6.5 Foundations 55 N.7 FATIGUE LIMIT STATE (FLS) 56 N.7.1 General 56 N.7. Hull 56 N.7.3 Deck 56 N.7.4 Tendons 56 N.7.5 Foundation 56 N.8 SERVICEABILITY LIMIT STATE (SLS) 57 N.8.1 General 57 N.9 ACCIDENTIAL LIMIT STATE (ALS) 58 N.9.1 General 58 N.9. Hull and deck 58 N.9.3 Tendons 58 N.9.4 Foundations 58 N.10 COMPARTMENTATION & STABILITY 59 N.10.1 General 59 NORSOK standard Page 37 o 64

239 Annex N Rev.3, February 013 N.10. Compartmentation and watertight integrity 59 N.11 DOCUMENTATION REQUIREMENTS FOR THE DESIGN BASIS & DESIGN BRIEF 61 N.11.1 General 61 N.11. Design basis 61 N.11.3 Design brie 6 N.1 REFERENCES 64 NORSOK standard Page 38 o 64

240 Annex N Rev.3, February 013 N.1 INTRODUCTION N.1.1 General This Annex provides requirements and guidance to the structural design o tension leg platorms, constructed in steel, in accordance with the provisions o this NORSOK standard. The requirements and guidance documented in this Annex are generally applicable to all conigurations o tension leg platorms. For novel designs, or unproved applications o designs where limited or no direct experience exists, relevant analyses and model testing, shall be perormed which clearly demonstrate that an acceptable level o saety is obtained which is not inerior to the saety level set orth by this Annex when applied to traditional designs. N.1. Deinitions Terms A Tension Leg Platorm (TLP) is deined as a buoyant installation connected to a ixed oundation by pretensioned tendons. The tendons are normally parallel, near vertical elements, acting in tension, which restrain the motions o the platorm in heavy, pitch and roll. The platorm is compliant in surge, sway and yaw. The TLP tendon system comprises all components associated with the mooring system between, and including the top connection(s) to the hull and the bottom connection(s) to the oundation(s). Guidelines, control lines, umbilicals etc. or tendon service and/or installation are considered to be included as part o the tendon system. The TLP oundation is deined as those installations at, or in, the sealoor which serve as anchoring o the tendons and provides transer o tendon actions to the oundation soil. The TLP hull consists o buoyant columns, pontoons and intermediate structural bracings, as applicable. The TLP deck structure is the structural arrangement provided or supporting the topside equipment. Normally, the deck serves the purpose o being the major structural components to ensure pontoons, columns and deck acting as one structural unit to resist environmental and gravity actions. Ringing is deined as the high requency resonant response induced by transient loads rom high, steep waves. Springing is deined as the high requency resonant response induced by cyclic (steady state) in low to moderate seastates. High Frequency (HF) responses are deined as TLP rigid body motions at, or near heave, roll and pitch eigenperiods. Low Frequency (LF) responses is deined as TLP rigid body motions at, or near surge, sway and yaw eigenperiods. NORSOK standard Page 39 o 64

241 Annex N Rev.3, February 013 DECK/TOPSIDE Figure N.1-1 COLUMN PONTOON FOUNDATION Typical tension leg platorm RIGID RISERS TENDONS WELL TEMPLATE HULL FLEXIBLE RISERS NORSOK standard Page 40 o 64

242 Annex N Rev.3, February 013 N.1.3 Description o tendon system Individual tendons are considered within this Annex as being composed o three major parts: Interace at the platorm Interace at the oundation (sealoor) Link between platorm and oundation Tendon components at the platorm interace shall adequately perorm the ollowing main unctions: Apply, monitor and adjust a prescribed level o tension to the tendon Connect the tensioned tendon to the platorm Transer side actions and absorb bending moments or rotations o the tendon Tendon components providing the link between the platorm and the oundation consist o tendon elements (tubulars, solid rods etc.), termination at the platorm interace and at the oundation interace, and intermediate connections o couplings along the length as required. The intermediate connections may take the orm o mechanical couplings (threads, clamps, bolted langes etc.), welded joints or other types o connections. Tendon components at the oundation interace shall adequately perorm the ollowing main unctions: Provide the structural connection between the tendon and the oundation Transer side actions and absorb bending moments or rotations o the tendon The tendon design may incorporate specialized components, such as: corrosion-protection system components, buoyancy devices, sensors and other types o instrumentation or monitoring the perormance and condition o the tendons, auxiliary lines, umbilicals etc. or tendon service requirements and/or or unctions not related to the tendons, provisions or tendons to be used as guidance structure or running other tendons or various types o equipment, and elastomeric elements. NORSOK standard Page 41 o 64

243 Annex N Rev.3, February 013 TLP hull Flex element Protection Top connector (side entry) Transition piece Tendon body Figure N.1- N.1.4 Typical TLP tendon system Non-operational phases The structure shall be designed to resist relevant actions associated with conditions that may occur during all stages o the lie-cycle o the unit. Such stages may include : abrication, site moves, mating, sea transportation, installation, and, decommissioning. Structural design covering marine operation, construction sequences shall be undertaken in accordance with NORSOK N-001. Marine operations should be undertaken in accordance with the requirements stated in the VMO standard. All marine operations shall, as ar as practicable, be based upon well proven principles, techniques, systems and equipment and shall be undertaken by qualiied, competent personnel possessing relevant experience. Structural responses resulting rom one temporary phase condition (e.g. a construction or transportation operation) that may eect design criteria in another phase shall be clearly documented and considered in all relevant design workings. Fabrication Flex element Foundation Bottom connector (vertical stab-in) Planning o construction sequences and o the methods o construction shall be perormed. Actions occurring in abrication phases shall be assessed and, when necessary the structure and the structural support arrangement shall be evaluated or structural adequacy. Major liting operations shall be evaluated to ensure that deormations are within acceptable levels, and that relevant strength criteria are satisied. NORSOK standard Page 4 o 64

244 Annex N Rev.3, February 013 Mating All relevant action eects incurred during mating operations shall be considered in the design process. Particular attention should be given to hydrostatic actions imposed during mating sequences. Sea transportation A detailed transportation assessment shall be undertaken which includes determination o the limiting environmental criteria, evaluation o intact and damage stability characteristics, motion response o the global system and the resulting, induced actions. The occurrence o slamming actions on the structure and the eects o atigue during transport phases shall be evaluated when relevant. In case o transportation (surace/sub surace) o tendons, this operation shall be careully planned and analysed. Model testing shall be considered. Satisactory compartmentation and stability during all loating operations shall be ensured. All aspects o the transportation, including planning and procedures, preparations, seaastenings and marine operations should comply with the requirements o the Warranty Authority. Installation Installation procedures o oundations (e.g. piles, suction anchor or gravity based structures) shall consider relevant static and dynamic actions, including consideration o the maximum environmental conditions expected or the operations. For novel installation activities (oundations and tendons), relevant model testing should be considered. Tendon stand-o (pending TLP installation) phases shall be considered with respect to actions and responses. The actions induced by the marine spread mooring involved in the operations and the orces exerted on the structures utilised in positioning the unit, such as airleads and padeyes, shall be considered or local strength checks. Decommissioning Abandonment o the unit shall be planned or in the design stage. NORSOK standard Page 43 o 64

245 Annex N Rev.3, February 013 N. STRUCTURAL CLASSIFICATION AND MATERIAL SELECTION N..1 General Selection o steel quality, and requirements or inspection o welds, shall be based on a systematic classiication o welded joints according to the structural signiicance and the complexity o the joints/connections as documented in Chapter 5. In addition to in-service operational phases, consideration shall be given to structural members and details utilised or temporary conditions, e.g. abrication, liting arrangements, towing and installation arrangements, etc. N.. Structural classiication The structural classiication guidance given below assumes that the tendon system is demonstrated to have residual strength and that the TLP structural system satisies the requirements o the ALS condition with ailure o the tendon (or a connection in the tendon) as the deined damage. (See also NORSOK N-004, Table 5-1). I this is not the case then design classes, DC3 and DC4 stated below should be substituted by design classes DC1 and DC and the inspection categories respectively upgraded in accordance with the requirements o NORSOK N-004, Tables 5-3 and 5-4 as applicable. Examples o typical design classes applicable to the hull and deck structures o a TLP are given in Annex M. Examples o typical design classes applicable to the pile oundation structures are given in Annex K. Examples o considerations with respect to structural classiication o tendons, tendon interaces are given below. These examples provide minimum requirements and are not intended to restrict the designer in applying more stringent requirements should such requirements be desirable. Typical locations - Design class : DC3 Locations in way o complex structural connections should be classiied as DC3, such connections may occur at : tendon interaces with the oundation and the TLP hull, and, complex tendon / tendon connections. DC3 areas may be limited to local, highly stressed areas i the stress gradient at such connections is large. Typical locations - Design class : DC4 Except as provided or in the description or DC3 structural categorisation, the ollowing listed connections may appropriately be classiied as being DC4 : simple tendon / tendon connections, interace arrangements outside locations o complex connections including general stiened plate ields (e.g. at hull interace) Consideration o the economic consequence o ailure may however indicate the utilisation o a higher design class than the minimum class as given above. Typical locations - Design class : DC5 DC5 locations are not normally relevant or tendons or tendon interaces. N..3 Material selection Material speciications shall be established or all structural materials utilised in a TLP. Such materials shall be suitable or their intended purpose and have adequate properties in all relevant design conditions. Material selection shall be undertaken in accordance with the principles given in NORSOK M-DP-001. When considering criteria appropriate to material grade selection, adequate consideration shall be given to all relevant phases in the lie cycle o the unit. In this connection there may be conditions and criteria, other than those rom the in-service, operational phase, that provide the design requirements in respect to the selection o material. (Such criteria may, or example, be design temperature and/or stress levels during marine operations.) Selection o steel quality or structural components shall normally be based on the most stringent Design Class o the joints involving the component. NORSOK standard Page 44 o 64

246 Annex N Rev.3, February 013 Through-thickness stresses and low temperature toughness requirements shall be assessed. The evaluation o structural resistance shall include relevant account o variations in material properties or the selected material grade (e.g. Variation in yield stress as a unction o thickness o the base material) N..4 Inspection categories Welding, and the extent o non-destructive examination during abrication, shall be in accordance with the requirements stipulated or the appropriate 'inspection category' as deined in NORSOK, M-101. Determination o inspection category should be in accordance with the criteria given in Table 5-5. The lower the extent o NDT selected, the more important is the representativeness o the NDT selected and perormed. The designer should thereore exercise good engineering judgement in indicating mandatory locations or NDT, where variation o utility along welds is signiicant. N..5 Guidance to minimum requirements Figures N.-1 and N.- illustrate minimum requirements to selection o Design Class, and Inspection Category or typical TLP unit, structural conigurations. Figue N.-1 DC B ) Segment o Column DC A 1) Examples o typical hull porch design classes and inspection categories 1. Inspection Category A is selected in accordance with Table 5.4 assuming high atigue utilisation and principal stresses in the transverse direction.. Inspection Category B is selected in accordance with Table 5.4 assuming high atigue and principal stresses longitudinal to the weld. NORSOK standard Page 45 o 64

247 Annex N Rev.3, February 013 Longitud. weld DC B 1) Welded Girth weld DC A ) Threaded Figure N.- Examples o design classes and inspection categories or typical tendon connections 1. Inspection Category B is selected in accordance with Table 5.4 assuming high atigue utilisation and principal stresses in the longitudinal direction.. Inspection Category A is selected in accordance with Table 5.4 assuming high atigue and principal stresses transverse to the weld. Stricter acceptance criteria as per ootnote in Table 5.4 will apply. NORSOK standard Page 46 o 64

248 Annex N Rev.3, February 013 N.3 DESIGN CRITERIA N.3.1 General The ollowing basic design criteria shall be complied with or the TLP design: The TLP is to be able to sustain all actions liable to occur during all relevant temporary and operating design conditions or all applicable limit states. Direct wave actions on the deck structure should not occur in the ultimate limit-state (ULS). Direct wave actions on the deck structure may be accepted in the ALS condition provided that such actions are adequately included in the design. Operating tolerances shall be speciied and shall be achievable in practice. Normally, the most unavourable operating tolerances shall be included in the design. Active operation shall not to be dependent on high reliability o operating personnel in an emergency situation. Note: Active operation o the ollowing may be considered in an emergency situation, as applicable: Ballast distribution Weight distribution Tendon tension Riser tension N.3. Design principles, tendons Essential components o the tendon system shall be designed by the principle that, as ar as practicable, they are to be capable o being inspected, maintained, repaired and/or replaced. Tendon mechanical components shall, as ar as practicable, be designed ail to sae. Consideration is to be given in the design to possible early detection o ailure or essential components which cannot be designed according to this principle. Certain vital tendon components may, due to their specialised and unproven unction, require extensive engineering and prototype testing to determine: Conirmation o anticipated design perormance Fatigue characteristics Fracture characteristics Corrosion characteristics Mechanical characteristics The tendon system and the securing/supporting arrangements shall be designed in such a manner that a possible ailure o one tendon is not to cause progressive tendon ailure or excessive damage to the securing/supporting arrangement at the platorm or at the oundation. A racture control strategy should be adopted to ensure consistency o design, abrication and in service monitoring assumptions. The object o such a strategy is to ensure that the largest undetected law rom abrication o the tendons will not grow to a size that could induce ailure within the design lie o the tendon, or within the planned in-service inspection interval, within a reasonable level o reliability. Elements o this strategy include: Design atigue lie Fracture toughness Reliability o inspection during abrication In-service inspection intervals and methods Fracture mechanics should be used to deine allowable law sizes, estimate crack growth rates and thus help deine inspection intervals and monitoring strategies. All materials liable to corrode shall be protected against corrosion. Special attention should be given to: Local complex geometries Areas that are diicult to inspect/repair Consequences o corrosion damage NORSOK standard Page 47 o 64

249 Annex N Rev.3, February 013 Possibilities or electrolytic corrosion All sliding suraces shall be designed with suicient additional thickness against wear. Special attention should be given to the ollowing: Cross-load bearings Seals Balljoints Satisactory considerations shall be given to settlement/subsidence which may be a signiicant actor in determining tendon-tension adjustment requirements. NORSOK standard Page 48 o 64

250 Annex N Rev.3, February 013 N.4 ACTIONS N.4.1 General Characteristic actions are to be used as reerence actions. Design actions are, in general, deined in NORSOK N-003. Guidance concerning action categories relevant or TLP designs are given in the ollowing. N.4. Dierent actions All relevant actions that may inluence the saety o the structure or its parts rom commencement o construction to permanent decommissioning should be considered in design. The dierent actions are deined in NORSOK N-003. For the deck and hull o the TLP, the actions are similar to those also described in Annex M. Additional actions on the hull are those rom ringing and springing. The wave actions on the tendons can be described as recommended in NORSOK N-003 or slender structures with signiicant motions. I o importance, the disturbance rom hull and risers on the wave kinematics at the tendon locations should be accounted or. The earthquake actions at the oundation o the tendons are described according to NORSOK N-003. The ollowing actions should be considered: Permanent Actions Variable Actions Deormation Actions Accidental Actions Environmental Actions The environmental actions are to be summarized as: Wind Actions - Mean wind - Dynamic wind - Local wind Wave and current Actions - Actions on slender members - Actions induced by TLP motions - Slamming and shock pressure - Wave diraction and radiation - Mean drit orces - Higher order non-linear wave actions (slowly varying, ringing and springing) - Wave enhancement - Vortex shedding eects Marine growth Snow and ice accumulation Direct ice action (icebergs and iceloes) Earthquake Temperature Tidal eects NORSOK standard Page 49 o 64

251 Annex N Rev.3, February 013 N.5 GLOBAL PERFORMANCE N.5.1 General The selected methods o response analysis are dependent on the design conditions, dynamic characteristics, non-linearities in action and response and the required accuracy in the actual design phase. For a detailed discussion o the dierent applicable methods or global analysis o tension leg platorms, reerence is made to API RP T, /5/. The selected methods o analysis and models employed in the analysis shall include relevant non-linearities and motion-coupling eects. The approximations, simpliications and/or assumptions made in the analysis shall be justiied, and their possible eects shall be quantiied e.g. by means o simpliied parametric studies. During the design process, the methods or analytical or numerical prediction o important system response shall be veriied (calibrated) by appropriate model tests. Model tests may also be used to determine speciic responses or which numerical or analytical procedures are not yet developed and recognized. Relevant motions shall be determined, by relevant analysis techniques, or those applicable design conditions speciied in NORSOK N-003. The basic assumptions and limitations associated with the dierent methods o analysis o global perormance shall be duly considered prior to the selection o the methods. The TLP should be analysed by methods as applicable to column-stabilised units (re. Annex M) when the unit is ree loating. The method o platorm-motion analysis as outlined in this Annex is one approximate method which may be applied. The designer is encouraged also to consider and apply other methods in order to discover the eects o possible inaccuracies etc. in the dierent methods. N.5. Frequency domain analysis Frequency domain HF, WF and LF analyses techniques may be applied or a TLP. Regarding action eects due to mean wind, current and mean wave drit, see NORSOK N-003. For typical TLP geometries and tendon arrangements, the analysis o the total dynamic action eects may be carried out as: A high-requency (HF) analysis o springing A wave-requency (WF) analysis in all six degrees o reedom A low-requency (LF) analysis in surge, sway and yaw The ollowing assumptions are inherent in adopting such on independent analysis approach: The natural requencies in heave, pitch and roll are included in the wave-requency analysis The natural requencies in surge, sway and yaw are included in the low-requency analysis The high and low natural requencies are suicient separate to allow independent dynamic analysis to be carried out The low-requency excitation orces have negligible eect on the wave-requency motions The low-requency excitation orces have a negligible dynamic eect in heave, pitch and roll Tendon lateral dynamics are unimportant or platorm surge/sway motions Typical parameters to be considered or global perormance analyses are dierent platorm drat, tidal eects, storm surges, set down, settlement, subsidence, mispositioning and tolerances. Possible variations in vertical centre o gravity shall also be analysed (especially i ringing responses are important). N.5..1 High requency analyses Frequency domain springing analyses shall be perormed to evaluate tendon and TLP suspectability to springing responses. Recognised analytical methods exist or determination o springing responses in tendons. These methods include calculation o Quadratic Transer Functions or axial tendon (due to sum requency actions on the hull) stresses which is the basis or determination o tendon atigue due to springing. Damping level applied in the springing response analyses shall be duly considered and documented. NORSOK standard Page 50 o 64

252 Annex N Rev.3, February 013 N.5.. Wave-requency analyses A wave-requency dynamic analysis may normally be carried out by using linear wave theory in order to determine irst-order platorm motions and tendon response. First order wave action analyses shall also serve as basis or structural response analyses. Finite wave action eects shall be evaluated and taken into acccount. In linear theory, the response in regular waves (transer unctions) is combined with a wave spectrum to predict the response in irregular seas. The eect o low-requency set-down variations on the WF analysis is to be investigated by analysing at least two representative mean oset positions determined rom the low-requency analysis. Wave approach headings shall be selected with basis in global coniguration (e.g. number o columns). N.5..3 Low-requency analyses A low-requency dynamic analysis could be perormed to determine the slow drit eects at early design stage due to luctuating wind and second order wave actions. Appropriate methods o analysis shall be used with selection o realistic damping levels. Damping coeicients or low-requency motion analyses are important as the low-requency motion may be dominated by resonant response. N.5.3 Time domain analyses For global motion response analyses, a time domain approach will be beneicial. In this type o analyses it is possible to include all environmental action eects and typical non-linear eects such as: Hull drag orces (including relative velocities) Finite wave amplitude eects Non-linear restoring (tendons, risers) Highly non-linear eects such as ringing may also require a time domain analysis approach. Analytical methods exist or estimation o ringing responses. These methods can be used or the early design stage, but shall be correlated against model tests or the inal design. Ringing and springing responses o hull and deck may however be analysed within the requency domain with basis in model test results, or equivalent analytical results. For deep waters a ully coupled time domain analysis o tendons, risers and platorm may be required. A relevant wave spectrum shall be used to generate random time series when simulating irregular wave elevations and kinematics. Simulation length shall be long enough to obtain suicient number o LF maxima (surge, sway, yaw). Statistical convergence shall be checked by perorming sensitivity analyses where parameters as input seed, simulation length, time step, solution technique etc. are varied. Determination o extreme responses rom time domain analyses shall be perormed according to recognised principles. Depending on selected TLP installation method, time domain analyses will probably be required to simulate the situation when the TLP is transerred rom a ree loating mode to the vertical restrained mode. Model testing shall also be considered in this context. N.5.4 Model testing Model testing will usually be required or inal check o TLP designs. The main reason or model testing is to check that analytical results correlate with model tests. The most important parameters to evaluate are: Air-gap First order motions Total oset Set-down WF motions versus LF motions Accelerations Ringing Springing NORSOK standard Page 51 o 64

253 Annex N Rev.3, February 013 The model scale applied in testing shall be appropriate such that reliable results can be expected. A suicient number o seastates need to be calibrated covering all limit states. Wave headings and other variable parameters (water levels, vertical centre o gravity, etc.) need to be varied and tested as required. I HF responses (ringing and springing) shows to be governing or tendon extreme and atigue design respectively, the amount o testing may have to be increased to obtain conidence in results. N.5.5 Action eects in the tendons Action eects in the tendons comprise o steady-state and dynamic components. The steady-state actions may be determined rom the equilibrium condition o the platorm, tendon and risers. Tendon action eects arise rom platorm motions, any ground motions and direct hydrodynamic actions on the tendon. Dynamic analysis o tendon response shall take into account the possibility o platorm pitch, roll and heave excitation (springing and ringing eects). Linearized dynamic analysis does not include some o the secondary wave eects, and may not model accurately extreme wave responses. A check o linear-analysis results using non-linear methods may be necessary. Model testing may also be used to conirm analytical results. Care shall be exercised in interpreting model-test results or resonant responses, particularly or actions due to platorm roll, pitch and heave, since damping may not be accurately modelled. Lit and overturning moment generated on the TLP hull by wind actions shall be included in the tendon response calculations. Susceptibility to vortex induced vibrations shall be evaluated in operational and non-operational phases. Intererence (tendon/riser, tendon/tendon, tendon/hull, tendon/oundation) shall be evaluated or nonoperational as well as the operational phase. NORSOK standard Page 5 o 64

254 Annex N Rev.3, February 013 N.6 ULTIMATE LIMIT STATE (ULS) N.6.1 General General considerations in respect to methods o analysis are given in NORSOK N-003. The TLP hull shall be designed or the loading conditions that will produce the most severe eects on the structure. A dynamic analysis shall be perormed to derive at characteristic largest stresses in the structure. Analytical models shall adequately describe the relevant properties o actions, stiness and displacement, and shall account or the local and system eects o, time dependency, damping and inertia. It is normally not practical, in design analysis o TLP s, to include all relevant actions (both global and local) in a single model. Generally, a single model would not contain suicient detail to establish local responses to the required accuracy, or to include consideration o all relevant actions and combinations o actions. It is oten the case that it is more practical, and eicient, to analyse dierent action eects utilising a number o appropriate models and superimpose the response rom one model with the responses rom another model in order to assess the total utilisation o the structure. For preliminary design, simpliied models are recommended to be utilised in order to more eiciently establish design responses and to achieve a simple overview o how the structure responds to the designing actions. For inal design, a complete three dimensional model o the platorm is required. N.6. Hull The ollowing analysis procedure to obtain characteristic platorm-hull response shall be applied: a) Steady-state analysis o the initial position. In this analysis, all vertical actions are applied (weights, live loads, buoyancy etc.) and equilibrium is achieved taking into account pretension in tendons and risers. b) Steady-state oset In this analysis the lateral steady-state wind, wave-drit and current actions are applied to the TLP resulting in a static oset position with a given set-down. c) Design wave analysis To satisy the need or simultaneity o the responses, a design wave approach, see NORSOK N-003, may normally be used or maximum stress analysis. The merits o the stochastic approach are retained by using the extreme stochastic values o some characteristic parameters in the selection o the design wave and is applied to the platorm in its oset position. The results are superimposed on the steady-state solution to obtain maximum stresses. d) Spectral analysis Assuming the same oset position as described in b) and with a relevant storm spectrum, an analysis is carried out using n wave requencies rom m directions. Traditional spectral analysis methods should be used to compute the relevant response spectra and their statistics. For a TLP hull, the ollowing characteristic global sectional actions due to wave orces shall be considered as a minimum, see also Annex M: Split orces (transverse, longitudinal or oblique sea or odd columned TLP s) Torsional moment about a transverse and longitudinal, horizontal axis (in diagonal or near-diagonal) Longitudinal opposed orces between parallel pontoons (in diagonal or near-diagonal seas) Longitudinal, transverse and vertical accelerations o deck masses It is recommended that a ull stochastic wave action analysis is used as basis or the inal design. Local load eects (e.g. maximum direct environmental action o an individual member, wave slamming loads, external hydrostatic pressure, ballast distribution, internal tank pressures etc.) shall be considered. Additional actions rom e.g high-requency ringing accelerations shall be taken into account. N.6..1 Structural analysis For global structural analysis, a complete three-dimensional structural model o the platorm is required. NORSOK standard Page 53 o 64

255 Annex N Rev.3, February 013 Note: Linear elastic space-rame analysis may be utilised i the torsional moments, or example as resulting rom diagonal seas, are transerred mainly through a sti bracing arrangement. Otherwise, inite-element analysis is required, see also Annex M.. Additional detailed inite-element analyses may be required or complex joints and other complicated structural parts to determine the local stress distribution more accurately and/or to veriy the results o a space-rame analysis, see also Annex M. Where relevant local stress concentrations shall be determined by detailed inite-element analysis or by physical models. For standard details, however, recognised ormulas will be accepted. Supplementary manual calculations or members subjected to local actions may be required where appropriate. I both static and dynamic action contributions are included in one analysis, the results shall be such that the contributions rom both shall be individually identiiable. Local environmental action eects, such as wave slamming and possible wave- or wind-induced vortex shedding, are to be considered as appropriate. N.6.. Structural design Special attention shall be given to the structural design o the tendon supporting structures to ensure a smooth transer and redistribution o the tendon concentrated actions through the hull structure without causing undue stress concentrations. The internal structure in columns in way o bracings should to be designed stronger than the axial strength o the bracing itsel. Special consideration shall be given to the pontoon strength in way o intersections with columns, accounting or possible reduction in strength due to cut-outs and stress concentrations. N.6.3 N Deck General Structural analysis design o deck structure shall ollow the principles as outlined in NORSOK N-004, Annex M. Additional actions (e.g. global accelerations) rom high-requency ringing and springing shall be taken into account when relevant. N.6.3. Air gap Requirements and guidance to air gap analyses or a TLP unit are given in NORSOK N-003. In the ULS condition, positive air gap should be ensured. However, wave impact may be permitted to occur on any part o the structure provided that it can be demonstrated that such actions are adequately accounted or in the design and that saety to personnel is not signiicantly impaired. Analysis undertaken to document air gap should be calibrated against relevant model test results. Such analysis shall include relevant account o: wave/structure interaction eects, wave asymmetry eects, global rigid body motions (including dynamic eects), eects o interacting systems (e.g. riser systems), and, maximum/minimum draughts (setdown, tidal surge, subsidence, settlement eects). Column run-up action eects shall be accounted or in the design o the structural arrangement in way o the column/deckbox connection. These 'run-up' actions should be treated as an environmental action component, however, they need not normally be considered as occurring simultaneously with other environmental responses. Evaluation o air gap adequacy shall include consideration o all aected structural items including lieboat platorms, riser balconies, overhanging deck modulus etc. NORSOK standard Page 54 o 64

256 Annex N Rev.3, February 013 N.6.4 Tendons N Extreme tendon tensions As a minimum the ollowing tension components shall be taken into account: Pretension (static tension at MSL) Tide (tidal eects) Storm surge (positive and negative values) Tendon weight (submerged weight) Overturning (due to current, mean wind/drit load) Setdown (due to current, mean wind/drit load) WF tension (wave requency component) LF tension (wind gust and slowly varying drit) Ringing (HF response) Additional components to be considered are: Margins or abrication, installation and tension reading tolerances. Operational requirements (e.g. operational lexibility o ballasting operations) Allowance or oundation mispositioning Field subsidence Foundation settlement and uplit Bending stresses along the tendon shall be analysed and taken into account in design. For the constraint mode the bending stresses in tendon will usually be low. In case o surace, or subsurace tow (nonoperational phase) the bending stresses shall be careully analysed and taken into account in design. For nearly buoyant tendons the combination o environmental action (axial & bending) and high hydrostatic water pressure may be a governing combination. Limiting combinations o tendon tension and rotations (lex elements) need to be established. For speciic tendon components such as couplings, lex elements, top and bottom connections etc. the stress distribution shall be determined by appropriate inite-element analysis. I tendon tension loss is permitted, tendon dynamic analyses shall be conducted to evaluate its eect on the complete tendon system. Alternatively model tests may be perormed. The reasoning behind this is that loss o tension could result in detrimental eects rom tendon buckling and/or damage to lex elements. N.6.4. Structural design The structural design o tendons shall be carried out according to NORSOK N-001 and N-003 with the additional considerations given in this subsection. When deriving maximum stresses in the tendons relevant stress components shall be superimposed on the stresses due to maximum tendon tension, minimum tendon tension or maximum tendon angle, as relevant. Such additional stress components may be: Tendon-bending stresses due to lateral actions and motions o tendon Tendon-bending stresses due to lexelement rotational stiness Thermal stresses in the tendon due to temperature dierences over the cross sections Hoop stresses due to hydrostatic pressure N.6.5 Foundations Geotechnical ield investigations and careul data interpretation shall orm the basis or geotechnical design parameters. Relevant combinations o tendon tensions and angles shall be analysed or the oundation design. For gravity oundations the pretension shall be compensated by submerged weight o the oundation, whereas the varying actions may be resisted by or example suction and riction. NORSOK standard Page 55 o 64

257 Annex N Rev.3, February 013 N.7 FATIGUE LIMIT STATE (FLS) N.7.1 General Structural parts where atigue may be a critical mode o ailure shall be investigated with respect to atigue. All signiicant actions contributing to atigue damage (non-operational and operational) design conditions shall be taken into account. For a TLP, the eects o springing and ringing resonant responses shall be considered or the atigue limit state. Fatigue design may be carried out by methods based on atigue tests and cumulative damage analysis, methods based on racture mechanics, or a combination o these. General requirements to atigue design are given in Chapter 8 and Annex C. Careul design o details as well as stringent quality requirements to abrication are essential in achieving acceptable atigue strength. It is to be ensured that the design assumptions made concerning these parameters are achievable in practice. The results o atigue analyses shall be ully considered when the in-service inspection plans are developed or the platorm. N.7. Hull Fatigue design o hull structure shall be perormed in accordance to principles given in Annex M. N.7.3 Deck Fatigue design o deckstructure shall be perormed in accordance to principles given in Annex M. N.7.4 Tendons All parts o the tendon system shall be evaluated or atigue. First order wave actions (direct/indirect) will usually be governing, however also atigue due to springing shall be careully considered. HF and WF tendon responses shall be combined realistically. In case o wet transportation (surace/subsurace) to ield these atigue contributions shall be accounted or in design. Vortex Shedding shall be considered and taken into account. This applies to operation and non-operational (e.g. tendon stand-o) phases. Series eects (welds, couplings) shall be evaluated. When racture-mechanics methods are employed, realistic estimates o strains combined with maximum deect sizes likely to be missed with the applicable NDE methods shall be used N.7.5 Foundation Tendon responses (tension and angle) will be the main contributors or atigue design o oundations. Local stresses shall be determined by use o inite element analyses. NORSOK standard Page 56 o 64

258 Annex N Rev.3, February 013 N.8 SERVICEABILITY LIMIT STATE (SLS) N.8.1 General Considerations shall be given to the eect o delections where relevant. The stiness o the structure and structural components shall be suicient to prevent serious vibrations and to ensure sae operation o the platorm. Allowance or wear shall be considered in areas exposed to abrasion (e.g where cross load bearings may move relative to the TLP column). NORSOK standard Page 57 o 64

259 Annex N Rev.3, February 013 N.9 ACCIDENTIAL LIMIT STATE (ALS) N.9.1 General General guidance and requirements concerning accidental events are given in Chapter 9 and Annex A. Units shall be designed to be damage tolerant, i.e. credible accidental damages, or events, should not cause loss o global structural integrity. The capability o the structure to redistribute actions should be considered when designing the structure. In the design phase, attention shall be given to layout and arrangements o acilities and equipment in order to minimise the adverse eects o accidental events. Satisactory protection against accidental damage may be obtained by a combination o the ollowing principles : reduction o the probability o damage to an acceptable level, and, reduction o the consequences o damage to an acceptable level. Structural design in respect to the Accidental damage Limit State (ALS) shall involve a two-stage procedure considering : resistance o the structure to a relevant accidental event, and, capacity o the structure ater an accidental event. Global structural integrity shall be maintained both during and ater an accidental event. Actions occurring at the time o a design accidental event and thereater shall not cause complete structural collapse. Requirements to compartmentation and stability in the damage condition are given in N10. When the upper hull (deckbox) structure becomes buoyant in satisying requirements to damage stability, //, consideration shall be given to the structural response resulting rom such actions. N.9. Hull and deck The most relevant accidental events or hull and deck design are: dropped objects ire explosion collision unintended looding Compartmentation is a key issue or TLP s due to the ine balance between weight, buoyancy and pretensions. See N10.. N.9.3 Tendons The most relevant accidental events or the tendons are: missing tendon tendon looding dropped objects hull compartment(s) looding Missing tendon requires analysis with 100 year environment to satisy the ALS. The same applies to tendon looding, i relevant. For accidental events leading to tendon ailure the possible detrimental eect o the release o the elastic energy stored in the tendon may have on the surrounding structure shall be considered. Dropped object may cause damage to the tendons and in particular the top and bottom connectors may be exposed. Shielding may be required installed. Flooding hull compartments and the eects on design shall be analysed thoroughly. N.9.4 Foundations Accidental events to be considered or the oundations shall as a minimum be those listed or tendons. NORSOK standard Page 58 o 64

260 Annex N Rev.3, February 013 N.10 COMPARTMENTATION & STABILITY N.10.1 General The TLP shall have draught marks pertinent to all phases o operation. The draught marks shall be located orward and at on the platorm and at starboard and port sides. The draught marks shall be clearly visible or operating personnel, unless instruments or continuous draught sensing and reading are provided. When the construction o the platorm is as close to completion as practical, an inclining test shall be undertaken in order to establish the position o the centre o gravity and the light weight. The inormation obtained rom the inclining test is to be continuously updated and adjustments shall be made to account or any items taken on or o the TLP ater such tests. Loading conditions during operating or temporary phases o the TLP shall have loads and centre-o-gravity location within the envelope o maximum/minimum allowable values. Inormation concerning these requirements is to be speciied in the Operating Manual o the TLP. Detailed provisions as stated in IMO MODU code, /4/ shall be satisied. N.10. Compartmentation and watertight integrity Temporary conditions For temporary phases where the tendons are not installed, the platorm may be considered as a columnstabilised unit or which procedures and criteria concerning hydrostatic stability should be taken according to Annex M. For temporary phases where tendons are partly engaged, adequate stability shall be documented. The stability in this condition may be provided by hydrostatic stability, by the tensioned tendons or by a combination. Operating conditions The tension in the tendons is to be suicient to ensure the stability o the platorm in the operating phase, both or the intact and damaged conditions. Monitoring o weight, ballast and pretension shall be perormed. The ollowing minimum accidental looding criteria is normally to be assumed or the ALS condition: a) Any one compartment adjacent to the sea is to be assumed looded, regardless o cause. b) Any one compartment containing sea water piping systems o other sources containing liquids, due to ailure o such arrangements, is to be assumed looded unless it can be adequately documented that such requirement is unwarranted. Watertight integrity Watertight closing appliances are required or those external openings up to the water levels deined by: The water level or an angle o heel equal to the irst intercept in the intact or damage condition, whichever is greater (ree-loating conditions) The water level corresponding to the required air-gap or deck clearance in the ULS condition Where pipe-runs may lead to critical looding scenarios, ull account is to be taken o this possibility in the design o these pipe-runs and the size o the internal spaces they run to or rom. The number o openings in watertight bulkheads and decks are to be kept to a minimum compatible with the design and proper operation o the TLP. Where penetration o watertight decks and bulkheads are necessary or access, piping, ventilation, electrical cables etc., arrangements are to be made to maintain the watertight integrity. Where valves are provided at watertight boundaries to provide watertight integrity, these valves are to be capable o being operated rom a normally manned space. Valve position indicators are to be provided at a remote-control situation. NORSOK standard Page 59 o 64

261 Annex N Rev.3, February 013 Watertight doors and hatches are to be remotely controlled rom a central sae position and are also to be operable locally rom each side o the bulkhead or deck. Indicators are to be provided at the remote-control position to indicate whether the doors are open or closed. Where the tendon elements and the tendon-column annulus are designed as non-looded members in service, such compartments are to be considered as TLP buoyancy compartments with respect to watertight integrity. Special consideration o closing devices (seals) or the tendon/column duct annulus is necessary to ulil requirements as to watertight closure. NORSOK standard Page 60 o 64

262 Annex N Rev.3, February 013 N.11 DOCUMENTATION REQUIREMENTS FOR THE DESIGN BASIS & DESIGN BRIEF N.11.1 General Adequate planning shall be undertaken in the initial stages o the design process in order to obtain a workable and economic structural solution to perorm the desired unction. As an integral part o this planning, documentation shall be produced identiying design criteria and describing procedures to be adopted in the structural design o the unit. Applicable codes, standards and regulations shall be identiied at the commencement o the design. When the design has been inalised, a summary document containing all relevant data rom the design and abrication phase shall be produced. Design documentation (see below) shall, as ar as practicable, be concise, non-voluminous, and, should include all relevant inormation or all relevant phases o the lietime o the unit. General requirements to documentation relevant or structural design are given in NORSOK, N-001, Section 5. N.11. Design basis A Design Basis Document shall be created in the initial stages o the design process to document the basis criteria to be applied in the structural design o the TLP. A summary o those items normally to be included in the Design Basis document is included below. Unit description and main dimensions A summary description o the unit, including : general description o the unit (including main dimensions and draughts), main drawings including : - main structure drawings (This inormation may not be available in the initial stage o the design), - general arrangement drawings, - plan o structural categorisation, - load plan(s), showing description o deck uniorm (laydown) actions, - capacity (tank) plan, service lie o unit, tendon system description. oundation system. Rules, regulations and codes A list o all relevant, applicable, Rules, Regulations and codes (including revisions and dates), to be utilised in the design process. Environmental design criteria Environmental design criteria (including all relevant parameters) or all relevant conditions, including : wind, wave, current, snow and ice description or 10-1,10 - and 10-4 annual probability events water levels (tides, surges, subsidence) design temperatures Stability and compartmentation Stability and compartmentation design criteria or all relevant conditions including : external and internal watertight integrity plan lightweight breakdown report design loadcase(s) including global mass distribution damage condition waterlines.(this inormation may not be available in the initial stage o the design.) NORSOK standard Page 61 o 64

263 Annex N Rev.3, February 013 Temporary phases Design criteria or all relevant temporary phase conditions including, as relevant: limiting permanent, variable, environmental and deormation action criteria procedures associated with construction, (including major liting operations) essential design parameters associated with temporary phases ( e.g. or mating loadcases, mating weldup sequences, crushing tube stiness, or transit phases, transit speed etc.) relevant ALS criteria tendon installation/replacement oundation installation Operational design criteria Design criteria or all relevant operational phase conditions including : limiting permanent, variable, environmental and deormation action criteria designing accidental event criteria (e.g. collision criteria) tank loading criteria (all tanks) including a description o system, with : - loading arrangements - height o air pipes - loading dynamics - densities tendon criteria tendon monitoring philosophy oundation criteria tendon interace criteria (unctionality, stiness etc.) Air gap Relevant basis inormation necessary or the assessment o air gap suiciency, including: a description o the requirements to be applied in the ULS and ALS conditions basis model test report (This inormation may not be available in the initial stage o the design) In-service inspection criteria A description o the in-service inspection criteria and general philosophy (as relevant or evaluating atigue allowable cumulative damage ratios). Miscellaneous A general description o other essential design inormation, including : description o corrosion allowances, where applicable N.11.3 Design brie A Design Brie Document shall be created in the initial stages o the design process. The purpose o the Design Brie shall be to document the intended procedures to be adopted in the structural design o the TLP. All applicable limit states or all relevant temporary and operational design conditions shall be considered in the Design Brie. A summary o those items normally to be included in the Design Brie Document is included below. Analysis models A general description o models to be utilised, including description o : global analysis model(s) local analysis model(s) loadcases to be analysed Analysis procedures A general description o analytical procedures to be utilised including description o procedures to be adopted in respect to : the evaluation o temporary conditions NORSOK standard Page 6 o 64

264 Annex N Rev.3, February 013 the consideration o accidental events the evaluation o atigue actions air gap evaluation, (including locations to be considered, damping, inclusion o asymmetry actors, disturbed (radiated) wave considerations, combined motion response) the establishment o dynamic responses (including methodology, actors, and relevant parameters) the inclusion o built-in stresses the consideration o local responses (e.g. those resulting rom tendon and riser actions, ballast distribution etc.) the consideration o structural redundancy Structural evaluation A general description o the evaluation process, including : description o procedures to be utilised or considering global and local responses description o atigue evaluation procedures (including use o design atigue actors, SN-curves, basis or stress concentration actors (SCF s), etc.) description o procedures to be utilised or code checking Air gap evaluation A general description o the air gap evaluation procedure, including : description o procedures to be utilised or considering air gap suiciency NORSOK standard Page 63 o 64

265 Annex N Rev.3, February 013 N.1 REFERENCES /1/ Regulations o 0 Dec. 1991, No.878, concerning stability, watertight subdivision and watertight/weathertight closing means on mobile oshore units, Norwegian Maritime Directorate. // ISO 19900, Petroleum and Natural Gas Industries-Oshore Structures: General Requirements, International Standard,. /3/ Code or the Construction and Equipment o Mobile Oshore Drilling Units, 1989, (1989 MODU CODE), International Maritime Organisation, London /4/ API RP T Recommended Practice or Planning, Designing, and Constructing Tension Leg Platorms, 3. edition August 01. -o0o- NORSOK standard Page 64 o 64