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

2. 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

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

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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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.

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

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

14. EN13445
Classification of weld details for supports

15. EN13445

16. EN13445

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

18. EN13445
Simplified counting method

19. EN13445
Simplified counting method

20. EN13445
Simplified counting method

21. 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

22. 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

23. Bridge Piping Example Loadings
Thermal expansion; -10° to 100°C, 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

24. 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

25. Bridge Piping Example CAESAR II model Topsides Bridge (Sliding end)
North
Fixed point in model
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26. Bridge Piping Example CAESAR II fatigue allowable stress
Fatigue data to include correct factors

27. 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+P Operating temp
2 (OPE) W+T2+P Max design temp
3 (OPE) W+T 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)

28. CAESAR II output reports
Single fatigue load case
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29. CAESAR II output reports
Cumulative damage report
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30. 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
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31. Bridge Piping Example
CAESAR II results
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33. Bridge Piping Example Elastic - Plastic check
ASME B31.3 displacement stress range check
EN13445
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34. 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

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