Trender inne FoU utmatting av offshorekonstruksjoner

Transcription

1 Trender inne FoU utmatting av offshorekonstruksjoner Agnes Marie Horn 8 november 2010

2 Innledning Arktiske forhold Tøyningsbasert utmatting Forskning og utvikling innen utmatting av offshorekonstruksjoner 2

3 Arctic / cold climate Harsh environment; Climatic conditions such as low temperatures, harsh weather and ice Vulnerability of marine and coastal environment with respect to potential emissions, operational discharges or accidental leaks from offshore activities Remoteness of arctic offshore fields from infrastructure and support bases Permafrost 3

4 Extreme average annual temperature according to ISO ºC -45ºC/+28ºC -38ºC/+34ºC -30ºC -46ºC -31ºC -31ºC -26ºC -39ºC -24ºC -26ºC -25ºC 4

5 Projects in Arctic and Cold Climate Region Endicott Trans Alaska pipeline Hibernia Sakhalin I Molipaq Terra Nova Snøhvit LNG Kashagan Yield strength (MPa) Technical requirements for different offshore projects Shtokman -40 CTOD test temperature C Future? Sakhalin 1 Orlan Sakhalin -50

6 Design codes The arctic area impact on design, construction and operation will affect the design solution Offshore; DNV Design of offshore steel structures OS-C101 Eurocode 3: Design of steel structures part 1-10 Norsok N-004 Design of Steel structures ISO Arctic Offshore Structure Pipelines DNV OS-F101 ISO Petroleum and natural gas industries, Pipeline transportation systems ASME 31.3, 31,4 or 31,8 Production of linepipe - ISO API 5L 6

7 Material challenges, test temperature ISO (Fixed Steel Offshore Structured) specifies the LAST for determination of Charpy-V impact test temperature. Temp =LAST minus -0ºC, -10ºC or -30ºC. The requirement is toughness values of approximately 10% of the yield. ASM 31.3, Charpy testing at C below the lowest anticipated service temperature, with a requirement in the range of J. For high pressure pipelines the requirement is up to 81 J (average values). DNV-OS-F101, the Charpy testing temperatures for C-Mn steel linepipe, are T 0 = T min 20 ºC for risers and gas pipelines and T 0 = T mi 10 ºC for liquid pipelines. Requirement is approximately 10% of of the material yield stress API RP 2N requirement; material requirements according to of API 2A-WSD) Charpy testing at C below the low est anticipated service temperature, with a requirement in the range of J ISO Arctic structure Charpy testing temperatures from 0 C to -30 C below LAST Norsok M-001 Charpy-V test temperaturens from 0ºC to -40ºC Test temperature as low as -80 to -90 C! 7

8 ISO Arctic Offshore Structure General requirements Arctic grade steels shall be used to achieve the ductility and toughness required for proper performance Fatigue; Fatigue shall be given particular attention at cold temperatures. Responses of the structure to cyclic ice actions that have the potential to cause the fatigue failure of structural members and welded connections shall be considered. Attention should be given to the validity of S-N curves with respect to low temperature application. 8

9 Fracture Control in Structures Fundamental design approach involves fracture mechanics - To prevent fracture, keep driving force below resistance force - In practice, this can be done either by - reducing factors that affect the driving force, e.g. - applied stress, constraint, crack size - or by increasing the resistance force, i.e. material fracture toughness - selecting material with sufficient fracture toughness at the temperature, strain (loading) rate, and constraint level appropriate for the structure 9

10 Lake Carling bulk carrier (1992) 10

11 Fatigue at low temp Fatigue failure develops in three stages; crack initiation, crack propagation and failure. Low temperatures can have both positive and negative effect on fatigue resistance, depending on the specific load conditions. Evaluations of fatigue take into account both the toughness and the strength of the material. Fatigue crack growth rates will be slower at lower temperatures leading to better fatigue performance (da/dn)/e~ K) Higher yield strength due to low temperatures will limit local deformation in highstress areas, hence reducing the risk of initiating cracks. At the same time, brittle material will have a higher risk of cracking (i.e. initiation of sharp defects), which can reduce the fatigue capacity 11

12 Example; 9% nickel steel Confirm that fatigue behavior at cryogenic temperature was at least as good as that at room temperature. specimens were fatigue tested at -165 C prior work had shown that that 9% nickel weldments could be safely analyzed by the usual S-N curves for carbon steel weldments when at room temperature Comparison of the results showed that the cryogenic condition improved the life noticeably. finding was not unexpected based on literature evidence showing reduction in crack growth rate at lower temperatures for this particular material. Fatigue test data of nine percent nickel steel at ambient temperature and cryogenic temperature (Gioielli and Zettlemoyer 2008) 12

13 Summary; Arctic Material for Cold Climate Condition Limited recommendations in current design rules and limited field experience from installations in cold climate regions Materials need acceptable toughness properties at the installation and operating temperatures expected in the cold climate site in question. New or improved material grades needs to be developed. In particular, new high strength steels, welding procedures and consumables are required. ISO 19906: Attention should be given to the validity of S-N curves with respect to low temperature application. Painting and coating systems for insulation and corrosion protection need to be developed for cold climate applications 13

14 Strain based methods STATE OF THE ART OF LIFE PREDICTION METHODS 14

15 Notch stress and Strain Approach Low cycle fatigue (typically less than 10 3 cycles) is associated with widespread plasticity in metals; thus, a strain-based parameter should be used for fatigue life prediction in metals and alloys. Testing is conducted with constant strain amplitudes typically at Hz IIW recommendations DNV RP-C203; S-N curves can be linearly extrapolated to fewer cycles for practical use in a fatigue assessment 15

16 Strain based fracture mechanics evaluation Current fracture mechanic assessment procedures, e.g. BS 7910, R6, API 579 and API 1104 are not specifically developed to handle strain > 0.5% pipelines have been designed for strain in the magnitude of up to 4 % due to reeling or environmental loads One major challenge is to develop welding procedures that give good toughness requirements and acceptable welding defects and where the integrity of the pipeline still is acceptable. Line Pipe Longitudinal yield strength Longitudinal tensile strength Elongation Uniform elongation (Uel) Y/T Ratio Toughness Weld Metal Yield strength overmatch Elongation Toughness 16

17 Loads introducing high strain to the pipe Potential environmental load conditions push pipeline design beyond conventional design limit, i.e. strain loads <0.5%. fault crossings and ice gouges from riches and icebergs permafrost or channel crossings subject to ice movement Fault crossings may result in localized displacement which should be accommodated with a strain based design approach S e is m ic F a u lt Discontinuous permafrost may result in frost heave Frost and/or Heave thaw settlement fa u lt frozen soil Ice lenses Thaw Settlement frozen soil d is p la c e m e n t Frozen soil Unfrozen soil Frozen soil 17

18 Strain based fracture mechanics evaluation Two JIP s are currently running; Fracture Control JIP: Sintef and DNV, DNV-RP-F108 PRCI (Pipeline Research Council International) Several pipelines have been installed or are under construction in cold climate areas; Trans Alaska pipelines and Sakhalin. 18

19 Summary Strain based fracture mechanics Strain based fracture mechanics is a quite new field within offshore and shipping, it is of great importance that a common applicability for a strain approach is standardised into a practical design procedure. Fracture mechanics evaluation requires detailed knowledge of the material in question like: - yield and tensile stress, - residual stress, - material toughness PIPE SENT 300mm mm 400 mm FRACTURE TOUGHNESS [J, K, CTOD] SENB (a/w = 0.3) SENB (a/w = 0.5) CT (a/w = 0.5) 900mm mm GEOMETRY / CONSTRAINT [T,Q,M] Facture toughness compared by the constraint level for different specimens, Nyhus et al (2001) CWPT specimens geometry and test machine (Fairchild et al 2007) 19

20 Forskning og utvikling innen utmatting av offshorekonstruksjoner Resultater fra ISSC 2009 Utmattingsrelaterte JIP er i regi av DNV 20

21 Aims of ISSC The International Ship and Offshore Structures Congress (ISSC) is a forum for the exchange of information by experts undertaking and applying marine structural research. The aim of the ISSC is to further understanding in the various disciplines underpinning marine structural design, production and operation through internationally collaborative endeavors. 21

22 ISSC2012 Working groups I.1 Environment I.2 Loads II.I Quasi Static Response II.2 Dynamic Response III.1 Ultimate Strength III.2 Fatigue and Fracture IV.1 Design Principles and Criteria IV.2 Design Methods V.1 Damage Assessment after Accidental Events V.2 Natural Gas Storage and Transportation V.3 Materials and Fabrication Technology V.4 Ocean Wave & Wind Energy Utilization V.5 Naval Vessels V.6 Arctic Technology V.7 Impulsive Pressure Loading and Response Assessment V.8 Yacht Design Working Group: Trends in Offshore Research and Technology 22

23 CH. 2: Stress Approaches NOMINAL STRESS APPROACH; is the stress calculated in the sectional area under consideration HOT SPOT STRESS APPROACH defines the fatigue strengths directly by the local stress at the crack initiation point EFFECTIVE NOTCH STRESS APPROACH NOTCH STAIN APPROACH predicts the life for a crack at the root of a notch The uncertainties in calculation and measurement of structural stress concentration factors are stressed by Lotsberg et al (2007c), who compared the results obtained by FE analyses with those derived by on-site measurement. A round robin was carried out within the Network of Excellence MARSTRUCT (Fricke et al, 2007a) with the objective to quantify the uncertainties related to modelling, stress evaluation and identification of sources of scatter of weld root cracking of fillet-welded structures. They provided design recommendations for effective notch stress calculation. 23

24 CH. 3: Probabilistic Approaches and Mulitiaxial fatigue The fatigue damage in marine structures is a stochastic process with large uncertainties associated with both the loading and the resistance side. Muhammed and Stacey (2008) examined the factors affecting the uncertainties in the fatigue life prediction of welded joints and showed that fatigue life predictions for offshore structures are mainly dominated by uncertainties in the estimation of nominal stress and stress concentration factor. Multiaxial fatigue procedures are not used to a large extent in ship design Ninic et al (2007) present a multi-axial fatigue damage function that search of the most critical plane where crack occurrence and initial growth is assumed to take place. Despite that most structures are subjected to multiaxial stresses most design rules only consider the first principal stress. 24

25 4 FACTORS INFLUENCING FATIGUE Mean stresses Residual stresses Thickness of the structural joints Corrosive environments and temperature of the surroundings Design Fabrication and methods for improving fatigue performance Sensitivity of the material 25

26 New Steel with Improved Crack Growth Properties (FCA, FMDP Steels) It is often assumed that the fatigue crack growth properties of structural steels fall in a common scatterband like in BS 7910 Studies have shown that improvement of microstructure of steel can leads to an increased life for the component in question It is recommended that more details need to be introduced in the ECA procedures for fatigue crack growth with the advent of steels with improved crack growth properties DNV deltar i en JIP i Korea som er i ferd med å avsluttes med å etablere utmattingsdata ved lave temperaturer (RT, og -163 grader C) for SUS 304, 9% Nickel og Al legering. Fracture crack growth test results in atmospheric environment, Katsumoto et al (2005) 26

27 CH. 6: Fatigue Design Methods for Ship and Offshore Structures DNV recommended practice Fatigue Design of Offshore Steel Structures, DNV- RP-C203 was issued in April 2008 European standard for the design of Fixed Steel Offshore Structures EN ISO was issued in January 2008 Bureau Veritas Rules for the Classification of Offshore Units (2007), Fatigue Check of Structural Details 27

28 CH. 7 PRACTICAL EXAMPLES AND A BENCHMARK STUDY As more economic and reliable design is required, benchmark studies have been performed to justify upgrading of current fatigue design S-N curves, to provide a better basis for design, or to propose improved methods. Girth welds in pipelines; usually UK Class F2. Maddox et al (2006) performed full scale fatigue tests on a range of girthwelded pipes, and results supported the use of UK Class C. Haagensen et al (2007) introduced artificial welding defects of approximately 2ⅹ50mm on the pipe, and the results supported the use of the E design curve 28

29 CH. 8: OPERATING EXPERIENCE AND LIFE EXTENSION OF SHIPS AND OFFSHORE STRUCTURES Several units have been on location for a significant part of their intended service life or beyond, inspection, maintenance and repair strategies are necessary to ensure safe operation, Van der Cammen (2008) Johansen et al (2006) discuss aspects regarding life cycle management. Fast development of monitoring technique opens new possibilities to ensure that the unit operates within design conditions, May et al (2008); acoustic emission, on-line monitoring and sensors The tremendous amount of monitoring information gives a challenge to analyse this data and to arrive at meaningful conclusions. 29

30 DNV utmattingsrelaterte JIP er som kjøres /starter opp høsten 2010 DNV arbeider sammen med Statoil om å gi bedre veiledning om forbedringsfaktorer ved sliping av sveisetå. Fullskala laboratorieforsøk av doblingsplater som vil bli avsluttet i løpet av året. DNV er avslutter i høst en JIP på groutforbindelser i vindmølletårn som har vist seg å ha en langt mindre styrke enn tidligere antatt. Resultatene herfra har gitt grunnlag for bekymring av dynamisk kapasitet også av grouta forbindelser med "shear keys" som er påkjent av reversibel dynamisk belastning. DNV lanserer (høst 2010) en JIP på groutforbindelser med skjærkile som er utsatt for reversibel dynamisk last, kontakt; [email protected] DNV lanserer (høsten 2010) en JIP på sur service ECA for bestemmelse av akseptkriterier 30

31 Safeguarding life, property and the environment 31