This paper discusses the design, development and testing of an optimized subsea wellhead annulus seal. Historically annulus seal qualification and validation testing requirements are defined in API 6A and API 17D requiring a PR2F temperature and pressure cyclic test program, and x3 lockdown load cycles. Throughout the operational life of a production subsea wellhead, the annulus seal can be subjected to many start-up and shut-down cycles, leading to substantially more lockdown load cycles on the annulus seal than initially validated (OTC-30784-MS). Traditionally, it has been left up to operators to define their own requirements over and above API.
Operating conditions experienced by the annulus sealing system and the subsea wellhead equipment can be exceptionally challenging. Prior to landing and locking the annulus sealing system in place, the sealing surfaces of both the wellhead system and the annulus seal are subjected to a multitude of operational and environmental conditions such as cuttings/ debris from circulating well fluids, possible damage from lowering the annulus seal through several thousand feet of subsea riser and a BOP stack and misalignment of the seal during installation.
The annulus sealing system of the subsea wellhead equipment is a critical secondary well barrier for well control. Either in exploration or production mode, with the casing cemented in place, the annulus sealing system locks the subsea casing hanger into position, providing a metal-to-metal seal across the annulus of the intermediate casing. The purpose of the annulus sealing system is to ensure it does not unseat and lose integrity in the event there is any pressure from below the seal due to a leak, reduced hydrostatic pressure from above or from thermal expansion during the production life of the well.