Fatigue Analysis in ASME B31.3 Piping Nigel Marsh Chief Pipe Stress Analysis Engineer Worley Parsons Australia

Fatigue Analysis in ASME B31.3 Piping Content ASME B31.3 Typical Fatigue – Offshore Oil and Gas Fatigue approach using pressure vessel code Fatigue evaluation example

ASME B31.3 Stresses Types Sustained Occasional Displacement stress range At fittings and discontinuities SIF’s increase stresses for weld system

ASME B31.3 Stress range factor corrects for cycles ASME B31.3 now limits Sc, Sh to 138MPa

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

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

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

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

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 EN13445 requires inspection at 20% of fatigue life, DFF 5.0 DNV offshore structural code has requirements that are sometimes used

Pressure vessel codes EN13445 Unfired pressure vessels - Part 3: Design EN13445 uses equivalent stress range ( 2 x shear stress ) Load cases in CAESAR II must be the stress range

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

CAESAR II fatigue load case Refer to CAESAR II users guide stress formulation.

EN13445 18.10.1.2 Classification of weld details to be assessed using equivalent stress range - Table 18-4.

EN13445 Classification of weld details for supports

EN13445

EN13445

EN13445 Correction factors Deviations from design shape (Ovality, weld misalignment) Material thickness (typically > 25 mm) Temperature Corrosion NDT Elastic / plastic behaviour

EN13445 Simplified counting method

EN13445 Simplified counting method

EN13445 Simplified counting method

Cumulative Fatigue Damage ASME B31.3 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

Fatigue Evaluation Example 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

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 ; EN13445 Design Fatigue Factor 3.0 as the piping is difficult to inspect

Bridge Piping Example CAESAR II model Temperature T3 -10°, T2 100°C Displacements D1 North/South ±350mm, D2 East/West ±700mm Apply displacements to support CNodes Consider nonlinear aspects of model, include friction, exclude guide gaps

Bridge Piping Example CAESAR II model Topsides Bridge (Sliding end) North Fixed point in model CAU2014 Events

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

Bridge Piping Example CAESAR II load cases Load case for full fatigue stress range D1 & D2 are half the displacement stress range from movements 1 (OPE) W+T1+P1 - Operating temp 2 (OPE) W+T2+P1 - Max design temp 3 (OPE) W+T3 - Min design temp 4 (OPE) W+D1+T1+P1 5 (OPE) W-D1+T1+P1 6 (OPE) W+D2+T1+P1 7 (OPE) W-D2+T1+P1 8 (SUS) W+P1 9 (EXP) L9=L2-L3 10 (FAT) L10=L2-L3 (1560 cycles) 11 (FAT) L11=L4-L5 (3.28E7 cycles) 12 (FAT) L12=L6-L7 (1.12E8 cycles)

CAESAR II output reports Single fatigue load case CAU2014 Events

CAESAR II output reports Cumulative damage report CAU2014 Events

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 CAU2014 Events

Bridge Piping Example CAESAR II results CAU2014 Events

Bridge Piping Example Elastic - Plastic check ASME B31.3 displacement stress range check EN13445 CAU2014 Events

Summary ASME B31.3 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

Fatigue Analysis in ASME B31.3 Piping Thank You Nigel Marsh Chief Pipe Stress Analysis Engineer Worley Parsons Australia