Subsea Pipeline End Expansion Calculation

[steven tran]

Dear Expert,
Now i'm work with 250 km subsea pipeline project. In the middle of pipeline, there are one piggable wye (wye tie-in). The wye is put in wye protection frame and the elevation between wye and seabed is 1.5 m. So i use a goose neck to connect between pipeline and wye.
when calculating end expansion for this project, i confuse between two method:
1) Consider pipeline free end at Central process platform (CPP) and land fall point (LFP), ignore the discontinue by wye
2) Separate pipeline into two section: section 1: from CPP to wye, section 2: from wye to LFP.

If i calculating end expansion with method 2, i have to make expansion spool before and after wye. However, i've refer some similar project and see that, they ignore the wye and don't make the expansion spool at wye.

Could you give me some advice in this problem?

Thanks.

Steven

[tokiero]

Dear Steve,
In my opinion you can ignore pigging tee, if deformations (or stress) allows you to do.
If not you should use compensators after and before tee.
Usually, as u have seen well, you dont need any compensator, just because the pipeline can self-compensate some deformations in elastic region of stress-strain diagram.
Regards,
J.V.

[steven tran]

Dear J.V,
Thanks for your opinion.
If we consider the end of pipeline and wye is free end, the expansion length is about 0.7m and force acting on the goose neck and make it over stress.
You can see the figure i attached with the post for the position of pipeline and wye.
i don't think the end of pipeline is free, because it is prevented by wye (wye fix at its location).
How about your opinion?

Thanks and Regards,
Nam

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[tokiero]

Dear Steve,
Its not exactly the idea of deformation and elongation. I mean your explanation, as I may see of course.
Because, if you have a free-ended pipeline - there is no deformations (stress) at all, only movements at endpoint.
Only if you will have fixed end you will have some stress due to thermal exapnsion or pressure changes.
Thats just theory of deformations.

According to your case, we should considering whole pipeline as a mechanical system. Each pipeline has some bends and ellbows - these things will work as compensator for thermal elongations. The same situation with the gooseneck.

I can not suggest you any solutions, cause I dont know whole situation. I mean: diameter, length, profile and other nice things about your pipeline.
But in general the rule is: if you want to avoid movements at free end - you should compensate expansion. If you want to avoid stress - you should allow free movements. If you have a fixed end - you should check stress at connection points.

Regards,
J.>.

[steven tran]

Dear J.V,
Paramter of pipeline at Piggable wye is:
Diameter: 711 mm
Wall Thickness: t=19.1 mm
Concrete thickness: t=90 mm, density=3040 kg/m3
internal corrosion allowance: 2.5 mm
exteranal corrosion : 3.2 mm, density =940kg/m3
concrete coating cut back length: 375 mm,
field joint coating: HDPUF, density:1025 kg/m3
Total pipeline length: 250 km
Wye at KP 115.
End expansion at WYE: 677 mm, anchor length: 3940 m
In attachment is description about conncection bettwen WYE-GOOSENECK-LINE PIPE.
The WYE can slide in protection frame.
Do you think are there still expansion force at connection flange (goose neck-pipline)?
As you mention, to avoid stress, i should allow free movement. The WYE can slide but two force of expansion acting 2 side. Do we still consider is slide? or it fixed?
Now i'm trying to proof that stress at GOOSENECK-WYE will be overstress.
I already model WYE-GOOSENECK-LINE PIPE in Autopipe by put thermal displacement load at 2 side of line pipe.
One point with thermal displacement load (677 mm) at line pipe and distance between this point and connection flange (linepipe-goosneck) is 150 m.
Other point (symmetrical) with thermal displacement load (677 mm) at line pipe and distance between this point and connection flange (linepipe-goosneck) is 150 m.
The result is over stress.
Can you advice me to model this one?

Thanks and Regards,
Steven

[steven tran]

Dear J.V,
Do you think the expansion force acting on the goose neck from 2 side at the same time?

Thanks,
Steven

[tokiero]

Dear Steve,

According to your explanation, I should say that you have a well balanced system, in terms of acting forces. In my opinion your construction is self-sufficient, just because the expansion forces will act on both sides of wye, and both pipelines are more or less similar.
In your shoes, Id considered wye as fixed. And the reason is the length of pipeline bigger than self-anchoring length in both cases.
So, it is not necessary to load both sides with 677 mm of displacement. I expect the displacement 5-10 times less, taking into account self-anchoring length and pipeline geometry.
Please, check everything before making a decision. Remember, that simulation code has not your expirience and imagination of whole system, that you have.

Regards,
JV.

[steven tran]

Dear JV,
Back to theory of expansion, which element (pressure or temperature or both pressure and temperature) is the reason to make expansion displacment at end of pipeline-goose neck?
Last time I calculate the displacement at end of pipeline (pipeline-gooseneck) by taking into account pressure and temperature, and the results the displacment is 677 mm.
However, i just think again, i realize pressure do not effect the end-cap force at end of pipeline (connection pipeline-gooseneck) because the pipeline and gooseneck is just create a 15 deg, not 90 deg when we assume to calculate in expansion calculation.
If i consider the thermal displacment affect by only temperature, the displacemnt at the end of pipeline only 30 mm and when i check in Autopipe, it not over stress.

Could you advice me about the expansion?

Thanks and Regards,
Steven

[tokiero]

Dear Steve,

You're absolutely right.

Regards,
J.V.