The majority of oilfield Wellhead and Tree equipment has been designed with guidance from codes API 6A and 17D. However, their design methods are not the most appropriate for the new High Pressure High Temperature (HPHT) applications; equipment rated above 15 ksi (103 MPa) Working Pressure and/or above 350 °F (177 °C). This paper discusses the limitations of established design methods and presents more suitable methods for HPHT applications.
FEA is well established as a stress analysis method for use in conventional Pressure Vessel design; however it is not so well established for load bearing interfaces. This leaves a gap in our Design Methods, since load bearing interfaces are intrinsic to Wellhead Equipment Pressure Vessel design. Intrinsic because many of our Pressure Vessels are “capped” by hangers and connectors instead of flanges; if a hanger Load Shoulder fails then the Pressure Vessel above it has failed.
Unique to the oilfield are infrequent but extremely high loads. These loads are much higher than the Working Condition and in most cases they stem from field testing and emergency situations. If the established ASME methods are used for these cases certain projects may not be viable.