External pressures due to hydrostatic heads on subsea equipment which fall under API SC 17 can now be accounted for in the equipment design and rating as a depth-adjusted working pressure (DAWP) with design guidelines from the recently published API 17 TR 12. This can be beneficial for overcoming some challenges in designing the HPHT equipment and can also result in economically efficient and lightweight equipment. However, design guidelines of recently published API 17 TR 8 will need to be followed for designing HPHT subsea equipment. Combining the two design guidelines, it is possible to design or adjust the working pressures based on the working depths for the subsea equipment for HPHT applications.
This paper suggests how to combine the design and validation guidelines of API 17 TR 8 and API 17 TR 12. It recommends the different load cases to be analyzed based on recommendations of API 17 TR 12 and applicable design methods from API 17 TR 8 to be used for those particular load cases. Validation testing requirements pose a challenge when combining these two guidelines, and this paper makes an effort to clarify them according to the component under consideration. It also addresses the fatigue screening and analysis criteria based on the components under consideration.
To demonstrate these design methods and load cases, an example analysis of a 15,000-psi subsea component is presented with results for uprating the internal pressures up to 20,000-psi using external pressures due to different working seawater depths.
API 17 TR 12 refers to API 17 TR 8 for a well shut-in pressure greater than 15,000-psi. In the situation in which subsea equipment requires a slightly higher operating pressure (approximately 10 to 15% of the absolute rated working pressure), the equipment can benefit in utilizing DAWP. This paper also recommends the design and validation methods for situations in which the well shut-in pressure is at or below 15,000-psi but the depth-adjusted working pressure is above 15,000-psi. This situation would typically occur during a bullheading or well kill. For this type of equipment, and in general, potential time and investment can be saved by depth adjusting the working pressures to pressures slightly higher than 15,000-psi instead of developing an absolute-rated 20,000-psi system.