A Comparative Study of Pressure Vessel Design Using Two Different Asme Methodologies

Abstract

Abstract As offshore production moves to higher pressure reservoirs, water injection, gas lift and gas injection swivel designs need to follow the trend. To accommodate these higher pressures, an internal study was launched to reassess the capacity of a 12" toroidal swivel called VHP swivel. A first attempt in this requalification was performed using the ASME Div. 3 Part 5 "elastic stress analysis" method. Even though being widely used in the pressure vessel industry, the stress linearization method rapidly showed its limitations. Indeed, above 830 barg, the deformed swivel geometry was producing highly non-linear stress profiles which invalidated the method's results. On the other hand, the "elastic plastic stress analysis" method gave higher acceptance pressure levels but required advanced modeling skills. The use of sub-models, programed failure criterion verification tools, mesh refinement and converge criteria control were mandatory for complying with the code requirements. Numerical results confirmed that the VHP maximum operation pressure could be extended from 830 barg to 1020 barg, but also gave reliance in using this methodology as it properly predicted the swivel behavior. Indeed the FE predictions were confirmed by calibrating the numerical results against the actual swivel deformations through monitoring of the deformations during live testing. Finally, the completion of the VHP swivel test program, simulating 25 years of a recent offshore North Sea yaw cycle motions, confirmed the robustness of the overall design approach. This paper presents the analysis performed as part of the requalification of the VHP swivel at a maximum operating pressure of 1020 barg.