Fatigue Assessment. CSR Harmonisation. Content. Industry Presentation

Transcription

1 CSR Harmonisation Fatigue Assessment Industry Presentation September 2012 Philippe Baumans & Åge Bøe Project Management Team (PMT) Content Basis acceptance criteria Fatigue loads Loading condition for fatigue assessment Fatigue strength criteria, hot spot stress approach Simplified method for longitudinal stiffeners Very fine mesh finite element analysis (t n50 x t n50 mesh) Fatigue assessment by screening (50x50 mesh) Design standard 2

2 Fatigue evaluation - Acceptance criteria Calculated fatigue life should equal or exceed the design fatigue life: T F T DF Design fatigue life T DF is not to be taken less than 25 years. Acceptance criteria expressed as life and not Miner sum, D 1.0, since Miner sum D is to be based on design life T D = 25 years independent on design fatigue life. September 2012 CSR-H Buckling 3 Fatigue evaluation - Acceptance criteria Acceptance criteria, same as CSR-OT and CSR-BC: Design life of 25 years in North Atlantic environment. Ship speed: ¾ V design for fatigue loads = average speed in 25 years (5 knots for extreme loads = minimum speed to maintain manoeuvring) September 2012 Fatigeu Assessment 4

3 Selected dynamic load cases for fatigue Selection of 5 EDWs for fatigue loads at 10-2 level: HSM: head sea EDW maximizing VBM amidships HSA: head sea EDW maximizing A Z at FP FSM OST BSP BSR OSA HSM FSM: following sea EDW maximizing VBM amidships HSA BSR: beam sea EDW maximizing roll motion BSP: beam sea EDW maximizing waterline pressure at amidships OST BSP BSR OSA Az OST: oblique sea EDW maximizing torsional moment at ¼L OSA: oblique sea EDW maximizing pitch acceleration HSA and OSA are covered by HSM. Stress range computed for each EDW Damage is evaluated based in the highest stress range: Ps Std 5 Weibull distributed stress range Damage calculation is based on the assumption of long term Weibull distributed stress ranges. Stress range is taken at 10-2 probability level with shape parameter equal to 1.0. f 0 = 0.85 : Factor taking into account time in seagoing operations. 6

4 Loading conditions for fatigue assessment Oil tankers Fraction of time in each loading condition for oil tankers 7 Loading conditions for fatigue assessment Bulk carriers BC-A, empty hold Fraction of time in each loading condition for bulk carriers BC-A, loaded cargo hold BC-B and BC-C September 2012 CSR-H Buckling 8

5 Fatigue strength - Failure modes Included: cracks at weld toe and free plate edges Excluded: crack at weld root is covered by design standards 9 Hot spot at weld toe and at free edge Hot spot in way of free edge (base material) Hot spot in way of weld toe 10

6 Hot spot stress Nominal stress Hot spot stress Notch stress Stress range Hot spot stress: σ HS = K g σ nom Number of cycles Notch stress Hot spot stress Nominal stress S-N curves 11 S-N curves Fatigue evaluation CSR-H applies the DEn S-N curves B, C and D, with modification on slope value for B and C curve. CSR-H curves DEn curves 12

7 S-N curve - Corrosive environment 13 Stress corrections Welded joints Mean stress correction factor, f mean, i(j) Thickness effect, f thick 14

8 Fatigue assessment Fatigue damage calculation Elementary damage: (all loading conditions, both environment) D α( j) N = K Δσ m D E( j) 1 2 R m FS,( j) μ Γ + m / ξ ( j) ( ln N ) ξ Combined damage (all loading conditions) Total damage: Sum of all loading conditions 15 Fatigue assessment Fatigue analysis methodology Miner-Palmgren damage summation used to calculate damage ratio in all relevant loading conditions for the predominate dynamic load case. Total fatigue damage, sum of all loading conditions: Fatigue life calculation (design life T D =25 years), T F T DF : 16

9 Fatigue assessment Hot spot stress concept Hot spot stress range Simplified stress analysis Nominal stress (beam theory) x Tabulated SCF Very fine mesh FE stress analysis (t n50 x t n50 ) Very fine mesh FE model Screening Fatigue Assessment (50 x 50 mesh) ) Nominal stress (Fine mesh FE model) x Tabulated η Hot spot S-N curve Fatigue life calculation 17 Simplified stress analysis d Fatigue assessment Hot spot stress concept Stresses are based on: t n50 Correction factor for HG stresses and FE stresses: f c = 0.95 Screening fatigue assessment Very fine mesh FE stress analysis 18

10 Simplified stress analysis for longitudinal stiffeners Mandatory for all longitudinal stiffener end connection for all cargo area, at web-frame, at transverse bulkhead and at swash bulkhead: The hot spots stress is a combination of: Hull girder bending: Relative deflections in way of transv. bhd: Local stiffener bending: 19 Simplified stress analysis for longitudinal stiffeners Stress concentration factors at top of longitudinal stiffener: Stress concentration factors ( K g ): σ HS = K g σ nom Where K g = K a is given for axial loads due to global bending K g = K b K n is given for lateral loads due to stiffener bending K b : stress concentration factor for local bending K n : additional factor for unsymmetrical stiffener profile (warping of stiffener) 20

11 Very fine mesh finite element analysis Mandatory for specific details, OT and BC Required if fatigue screening fails Required if detail design standard is not followed Modeling requirement: t n50 x t n50 mesh 4 node elements with improved in-plane bending 8 node recommended at steep stress gradients Model extension in t n50 x t n50 : at least 10 elements in all directions 21 Mandatory details: Details to be assessed Very fine mesh FE analysis Oil tanker Bulk carrier 22

12 Screening fatigue assessment Fine mesh FE model (50x50 mm) Nominal membrane stress Tabulated η Hot spot stress Stress correction factors: Mean stress effect Thickness effect Fatigue damage calculation Damage criteria, T F T DF OK Damage criteria, T F < T DF Very fine mesh fatigue analysis (t n50 x t n50 ) 23 Details to be assessed Screening fatigue assessment Mandatory details, fatigue screening assessment based on 50x50 mesh: 24

13 Detail design standard Stiffener-frame connections Scallops in way of block joints Hopper knuckle connection Horizontal stringer heel Bulkhead connection to lower/upper stool Bulkhead connection to inner bottom Toe of hold frame Hatch corner 25 Weld improvement - Post weld treatment Benefit factor 2.2 on fatigue life; a stress reduction factor of 1.3, applicable for transverse butt joints and cruciforms, not longitudinal stiffener end connections Minimum requirement without treatment: T F T DF / 1.47 corresponding to 17 years for T DF = 25 years; same as CSR-OT Inside bulk cargo holds T F 25 years Reduced thickness effect exponent ( n ) given for improved welds TIG dressing Peening Burr grinding 26

14 Fatigue - Summary Changes from current CSR: S-N curve for corrosive environment introduced, damage calculated for both environments, in air and corrosive Surface finishing factors for base material are introduced Reference stresses are calculated at a probability level of 10-2 with shape parameter fixed to 1.0 Thickness effect exponent depends on structural detail and is in the range of Mean stress effect is revised with lower bound in compression = 0.3, at zero mean stress = 0.9 Definition of hot spot stress in very fine mesh FE analysis is improved Screening criteria based on fine mesh (50x50) introduced Design standard for critical areas given 27