Fatigue Analysis

Hi

now i am analyzing a new well head flow line for a offshore platform for doing the fatigue anlysis which temperature is to be considerd (for checking maximun stress range)shall i take operating temp or installation ambiant temperature.
because based on the full expansion stress range the fatigue life is calculated as per PD5500.

Hello My friend,

For fatigue analysis, you should specify extream hot and cold conditions as well as anticipated cycles during system life time. Then build FAT load cases. There are many similar posts regarding fatigue issue in this forum

It is probable that thermal cycles ofthe flowline are insignificant in fatigue terms. Wave induced movement of the wellhead might be a much more serious fatigue concern.

Hello Kaaliappan,

Typical load cases setup are as follows:

L1=W+T1+P1(OPE)
L2=WAV1(OCC)
L3=WIN1(OCC)
L4=L1+L2+L3(OCC)(ALG. COMB.)
L5=L1-L2-L3(OCC)(ALG. COMB.)
L6=L4-L5(FAT)(ALG. COMB.)
L7=L1+L2+L3(OPE)(SIGNMAX)(SUPPORT LOAD CHECK ONLY)
L8=L1+L2+L3(OPE)(SIGNMIN)(SUPPORT LOAD CHECK ONLY)

Hi Moverz

As u told the wave movements are more serious in fatigue my doubt is whether to consider this wave movements with operating case(wave displacement in well head + operating temperature)beacuse this will be the worst case


give me the suggestion
Regards
Kaaliappan

If you used the Miner's Rule approach as desctibed in DnV Rules, you could segregate the partuial damage assigned to thermal and wave cycles. For max (gross) stress you should use the worst case.

BTW .... you are asking complex questions in this forum .... do you not have a more senior engineer to offer guidance ? Are you sure you are not going to engineer a big accident ?