API 17TR12 – Consideration of External Pressure in the Design of Deepwater Subsea Equipment

This paper presents an overview of API 17 Technical Report TR12: Consideration of External Pressure in the Design and Pressure Rating of Subsea Equipment, a technical report prepared by a work group under the auspices of the API Subcommittee 17 (API SC17). This subject is of significant interest for current deepwater subsea projects where well shut-in pressures may only be slightly higher than 15K API Rated Working Pressure (RWP), and progressing to 20K API RWP equipment results in significant cost increases and/or delivery delays. At first glance, it seems logical that the high external seawater pressure in deep water (3000 – 4500 psi) could offset a significant portion of the internal pressure, keeping the differential pressure acting on the equipment to 15K equivalent, or less. This would allow for a higher well shut-in pressure than the equipment API RWP with consideration to the external hydrostatic pressure. This approach has been taken with 10K equipment on several medium water depth projects in the past (with special qualification testing). However, recent regulatory rulings and technical studies have called this practice into question. There are several technical issues discussed in this paper, some subtle, that must be evaluated and resolved before attempting to use equipment above its API RWP based on the presence of external hydrostatic pressure (from ambient seawater pressure or from the fluid column in risers or flowlines). API 17TR12 contains detailed guidelines on these issues, and proposes methods for evaluation and qualification that should be used when considering the effects of hydrostatic external pressure on API RWP.