I want to perform fatigue analysis of piping systems (B31.3 code used) for off shore bridge piping which is connected between two platforms. I have displacement values (say D1,D2,D3)of bridge supports and platforms due to sea waves and number of cycles of occurance of same (say N1,N2,N3). I have made following load cases in CAESAR-II.

1.W+P1+T1+D1 (OPE)
2.W+P1+T1+D2 (OPE)
3.W+P1+T1+D3 (OPE)
4.W+P1 (SUS)
5.L1-L4 (FAT)......N1 CYCLES
6.L2-L4 (FAT)......N2 CYCLES
7.L3-L4 (FAT)......N3 CYCLES
S-N Curve for materials is as per C-II database of ASME FAT:5-110.1
Now I am checking stresses in case 5,6 &7 and also cummulative usage ratio of together 5,6&7.
Is above method to do fatigue analysis is acceptable? or I am missing something here.

One more question on ASME fatigue curve and above method is: in above method, fatigue load cases include temperature and wave displacements effect but igoring pressure effect. While there will be pressure stress acting all the time. Now as per B31.3, in derivation of allowable stress for displacement strain range, one Sh for primary stress (for W+P)is reserved. Now Do ASME fatigue curves reserve such margin of primary stress in derivation of allowable stress due to fatigue? if not then my above method is questioable for W+P effect in total stress calculation and it's allowable?

Urgent response is highly appreciated as I am working on live project and doing fatigue analysis first time.

The stress from T1 had the same number of cycle like D1,D2,D3 ?
I think T1 cycle number are different, so you cant take the sum stress for allowable cycle calculation.
May be T1 must be seperat load case with his load cycle.

Thanks Ohlinger!
Yes you are right on this as the thermal cycles (7000 cylces) would be less compared to D1,D2 and D3. So may I propose below load cases to consider cycle variation,

1.W+P1+T1+D1 (OPE)
2.W+P1+T1+D2 (OPE)
3.W+P1+T1+D3 (OPE)
4.W+P1+T1 (OPE)
5.W+P1 (SUS)
6.L1-L4 (FAT)......N1 CYCLES....Due to Pure D1 Displacement strain
7.L2-L4 (FAT)......N2 CYCLES....Due to Pure D2 Displacement strain
8.L3-L4 (FAT)......N3 CYCLES....Due to Pure D3 Displacement strain
9.L4-L5 (FAT)......7000 CYCLES..Due to Pure T1 Displacement strain

Now check stress for L6,L7,L8 & L9 and Cummulative usage ratio for all four fatigue cases L6,L7,L8 & L9.

Experts please comment our way forward is correct or missing something here.

Requesting intergraph people for their valuable inputs. Immediate response higly appreciated.
Thanks.

If you dont have other thermal loads ( fast transients, thermal shocks)
or dynamic fluid/gas loads in the process then it is so ok.

May i requst Dave and Richard to confirm our above understanding is inline or not.
Thanks.

Then is in future better your ask not here in the forum and mail direct your question to Dave and Richard ?

Hey ohlinger! Don't be serious.I appreciate your response.
But I am using CAESAR-II for doing fatigue analysis and if at all any extra thgings to be taken into consideration from software point of view or additonal load case then always advisable to get response from Intergraph as they are the developer.
Hope you won't mind!

The ASME curves include a mean stress that, I believe, accommodates your margin for primary stress.
Your load cases do not show changes between operating cases. For example, you do not show L1-L2 or L1-L3 and these may produce higher stress ranges. They too will have their own set of cycles.
Google "rainflow cycle counting example" to learn more.

Aside:
If your question is focused on CAESAR II operation, we will respond. But if it is engineering, we avoid answering questions such as "Is this correct?".

Thanks Dave and Ohlinger for your valuable recommendations.

I would suggest to take advise of an expert senior. The approach is not taking into account of full range also read BS 5500 and look into DNV examples.

PKU,
Load case has stress range calculation. Better you explin with real load cases (real example) for clarification. Thanks.

The following is required:

Wave exceedance data, from project environmental criteria
Relative movement of platforms, under particular wave heights, from Structural Group
Relevant S-N curve for stress location and material
Results of stress calculations for each displacement identified

From wave exceedance data, blocks of waves within a particular height range must be established. Each block of waves must be related to a relative movement, and consequently to a stress range in piping. Note that the stress range is due to the full cycle of displacement. The F2 curve is normally used for piping butt welds.

It is important, particularly when considering smaller wave heights and smaller diameter pipes, to include friction effects at restraints. It may be the case that the force imposed at the bridge end is insufficient to cause the entire loop to move, effectively reducing the loop size. This has the effect of introducing nonlinearity and increasing stress local to the bridge.

The most significant wave directions are clearly those axial to the bridge.
Waves at 45° to the bridge axis may be included, by comparison with axial direction results and a reduction to 0.707 of the axial direction stress range, assuming that wave heights are similar. It may be acceptable to neglect transverse wave directions entirely.

The partial damage ratio, for each wave block, must be established. Partial damage ratios must be summed according to Miner's rule, to give the cumulative damage ratio. The reciprocal of this figure then gives the expected life in years.

To reduce post processing, it may be helpful to incorporate several bridge lines into a single stress calculation.

Duplex, Super Duplex and Stainless pipework should be analyzed using the F2 curve for Carbon Steel pipework. Adjustments shall be made for Young’s modulus in accordance with BS 5500 Annex C Para C.3.2.2.

Corroded wall thickness values shall be used when evaluating fatigue stresses.

Thanks KUMAR !
For your valuable inputs......

Some usefull document regarding the Fatigue Analysis