Fatigue Analysis in ASME B 31 3 Piping

Fatigue Analysis in ASME B 31. 3 Piping Nigel Marsh Chief Pipe Stress Analysis Engineer Worley Parsons Australia © Intergraph 2014

Fatigue Analysis in ASME B 31. 3 Piping Content ¢ ASME B 31. 3 ¢ Typical Fatigue – Offshore Oil and Gas ¢ Fatigue approach using pressure vessel code ¢ Fatigue evaluation example © Intergraph 2014

ASME B 31. 3 ¢ Stresses Types £ £ £ ¢ Sustained Occasional Displacement stress range At fittings and discontinuities SIF’s increase stresses for weld system © Intergraph 2014

ASME B 31. 3 ¢ Stress range factor corrects for cycles ¢ ASME B 31. 3 now limits Sc, Sh to 138 MPa © Intergraph 2014

Typical Fatigue – Offshore Oil & Gas ¢ Conductors and Risers £ £ Wave and current move conductor relative to the platform Conductor guide gaps limit relative movement Wave motion causes high cycle fatigue loading in piping Wellhead vertical growth low cycle fatigue loading in piping © Intergraph 2014

Typical Fatigue – Offshore Oil & Gas ¢ Bridge piping £ £ £ Platforms can move relative to each other Bridge pinned one end, sliding other Piping must absorb relative movement Variable amplitude high cycle fatigue Thermal expansion low cycle fatigue © Intergraph 2014

Typical Fatigue – Offshore Oil & Gas ¢ FPSO’s £ £ Ship hull motion and acceleration causes high cycle fatigue loading in piping Many permutations of the different motions and accelerations Variable amplitude high cycle fatigue Thermal expansion low cycle fatigue © Intergraph 2014

Fatigue approach using pressure vessel code ¢ ¢ This discussion is limited to static analysis only, not dynamic analysis Identify fatigue loadings on piping. £ £ £ £ ¢ ¢ Thermal Pressure Wave loading Movements Accelerations FPSO cargo loading/unloading Operational / Transport Identify cycles for each load. Wave induced loading may be defined for a design condition with the majority of cycles at a significantly less load © Intergraph 2014

Fatigue approach using pressure vessel code ¢ ¢ ¢ Build CAESAR II model including fatigue loadings Build load cases in CAESAR II to reflect loads Decide which pressure vessel code to use to evaluate fatigue Select fatigue curve for welded system Design Fatigue Factor £ £ Factor of safety for uncertainties in fatigue analysis Some operating companies specify EN 13445 requires inspection at 20% of fatigue life, DFF 5. 0 DNV offshore structural code has requirements that are sometimes used © Intergraph 2014

Pressure vessel codes ¢ ¢ ¢ EN 13445 Unfired pressure vessels - Part 3: Design EN 13445 uses equivalent stress range ( 2 x shear stress ) Load cases in CAESAR II must be the stress range © Intergraph 2014

CAESAR II fatigue load case ¢ ¢ Fatigue load (FAT) cases in CAESAR II calculate the “Stress Intensity” CAESAR II Config file, “SIFs and Stresses” £ £ ¢ ¢ Max 3 D Shear – Tresca Von. Mises – Maximum distortion energy theory Removes corrosion allowance to calculate stresses Refer to CAESAR II users guide stress formulation. © Intergraph 2014

CAESAR II fatigue load case ¢ Refer to CAESAR II users guide stress formulation. © Intergraph 2014

EN 13445 ¢ 18. 10. 1. 2 Classification of weld details to be assessed using equivalent stress range - Table 18 -4. © Intergraph 2014

EN 13445 ¢ Classification of weld details for supports © Intergraph 2014

EN 13445 © Intergraph 2014

EN 13445 ¢ Correction factors £ £ £ Deviations from design shape (Ovality, weld misalignment) Material thickness (typically > 25 mm) Temperature Corrosion NDT Elastic / plastic behaviour © Intergraph 2014

EN 13445 ¢ Simplified counting method © Intergraph 2014

Cumulative Fatigue Damage ¢ ¢ ASME B 31. 3 and vessel codes evaluate cumulative damage in the same way Each Di = ni/Ni is the damage from a fatigue load Where; n = number of applied cycles N = allowed cycles at a given stress © Intergraph 2014

Fatigue Evaluation Example – typical bridge piping connecting two offshore platforms ¢ Identify loads ¢ Build CAESAR II model ¢ Build load cases to calculate fatigue stress ranges ¢ Constant amplitude – Thermal, Pressure ¢ Variable amplitude – platform movements due to waves. £ £ £ ¢ ¢ Use relationship between stress and loading Assume to be linear in this example. Consider nonlinear affects in CAESAR II model Elastic - plastic check Cumulative fatigue damage © Intergraph 2014

Bridge Piping Example ¢ Loadings £ £ ¢ ¢ ¢ Thermal expansion; -10° to 100°C, 1560 cycles Relative platform movement; 100 year return period storm, movements in various directions Wave induced loading simplified in this example Pressure vessel code ; EN 13445 Design Fatigue Factor 3. 0 as the piping is difficult to inspect © Intergraph 2014

Bridge Piping Example CAESAR II model ¢ Temperature T 3 -10°, T 2 100°C ¢ Displacements D 1 North/South ± 350 mm, D 2 East/West ± 700 mm ¢ Apply displacements to support CNodes ¢ Consider nonlinear aspects of model, include friction, exclude guide gaps © Intergraph 2014

Bridge Piping Example CAESAR II model es T sid op ) e end g d i Br liding (S North Fixed point in model © Intergraph 2014

Bridge Piping Example ¢ ¢ CAESAR II fatigue allowable stress Fatigue data to include

Bridge Piping Example CAESAR II load cases ¢ Load case for full fatigue stress range ¢ D 1 & D 2 are half the displacement stress range from movements 1 (OPE) W+T 1+P 1 2 (OPE) W+T 2+P 1 3 (OPE) W+T 3 - Operating temp - Max design temp - Min design temp 4 (OPE) W+D 1+T 1+P 1 5 (OPE) W-D 1+T 1+P 1 6 (OPE) W+D 2+T 1+P 1 7 (OPE) W-D 2+T 1+P 1 8 (SUS) W+P 1 9 (EXP) L 9=L 2 -L 3 10 (FAT) L 10=L 2 -L 3 (1560 cycles) 11 (FAT) L 11=L 4 -L 5 (3. 28 E 7 cycles) 12 (FAT) L 12=L 6 -L 7 (1. 12 E 8 cycles) © Intergraph 2014

CAESAR II output reports ¢ Single fatigue load case © Intergraph 2014

CAESAR II output reports ¢ Cumulative damage report © Intergraph 2014

Bridge Piping Example ¢ Fatigue assessment including wave height distribution Start with assumption that max stress in all fatigue load cases are at same location for all fatigue loads. ¢ The following example is simplified ¢ © Intergraph 2014

Bridge Piping Example CAESAR II results © Intergraph 2014

Bridge Piping Example ¢ Elastic - Plastic check £ £ ASME B 31. 3 displacement stress range check EN 13445 © Intergraph 2014

Summary ¢ ¢ ¢ ¢ ASME B 31. 3 has limitations for high cycle fatigue analysis Define fatigue loadings and associated cycles Select fatigue approach / code Select fatigue curve for piping butt weld Apply fatigue curve correction factors Use CAESAR II to calculate stresses required Evaluate cumulative damage Document, method, assumptions and results © Intergraph 2014

Fatigue Analysis in ASME B 31. 3 Piping Thank You Nigel Marsh Chief Pipe Stress Analysis Engineer Worley Parsons Australia © Intergraph 2014