What determines shape and length of a tie-in spool?

[Turbonorman]

Hi,

I am wondering what determines the length and shape of a tie-in spool, drop down spool etc? I know the purpose (accommodate expansion of pipeline) however I have seen so many variations of lengths and shapes (M, Z, L) I don't exactly know why?

It makes sense for length and size of the spool to be determined by the amount of predicted expansion of flowline?
As for shape I have no idea!
Can anyone shed light on this and how they shape and length are determined?

Cheers guys

 

[BigInch]

The dogleg allows axial movement by trading axial stress for bending stress. If you think about it as a fixed end beam supported by two columns where one column shortens in relation to the other and a bending moment is developed in the beam. If the length of the beam is increased, the moment developed by the support displacement is ever smaller and smaller. The length of the dogleg in a pipe must be long enough such that the moments developed by the axial displacement of the mainline is small enough so that the pipe is not overstressed in bending. As bending moment is increased, axial stress is reduced, so it's a trade off.

I hate Windowz 8!!!!