The effect of the external pressure is accounted in the GR load case by AutoPIPE. If we model a subsea straight empty pipe section with two anchors at both the ends (continuously supported by V-stops), the pipe is subject to a traction.
If we model the above reported situation, but with an anchor at only one end, leaving the other one free (continuously supported with V-stops), the pipe has no axial load and shows a negative axial displacement (shortening).
Starting from the latter model, if we consider also a positive temperature gradient in the pipe, the pipe has no axial load, but the resultant axial displacement is the algebrical sum of the contribution of both the temperature (elongation) and the external pressure (shortening). We need to neglect the effect of the external pressure on the axial force and axial displacement, taking at the same time into consideration the effect of the buoyancy and the hydrodynamic loads.
Can we model this situation with AutoPIPE?
External pressure (sea water) effects are included in gravity load case including the end effects which result in an axial force or displacement.
If the user is performing a linear analysis, can you use the results from your thermal load case only for modeling the expansion loop? This way you will not see any axial forces or displacements from the buoyancy load case.
As the axial effects of buoyancy are included in the gravity load case and are dependent on the same variables on which the other buoyancy effects are, it may not be possible to decouple these effects. It could be possible to eliminate the external pressure axial effects by performing a pressure extension analysis using a pressure that is equal to depth of water * density of water * some factor. As the external pressure effects and internal pressure extension effects calculations are performed differently, this will need to be a hit and trial to obtain the factor and to get the pressure which will offset the external pressure axial effects. Then you could combine GR+T1+P1 and get expansion axial effects with buoyancy effects minus buoyancy axial effects. Attached model APC does something similar.
Model Subsea (Underwater) Piping - AutoPIPE
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