Assurance Process and Technology for Riserless Interventions on Subambient Subsea Wells

Abstract As deepwater wells age and reservoir pressure drops, wells have the potential to enter a subambient state in which shut in wellhead pressure is below ambient seawater hydrostatic pressure. Standard intervention pressure control and completion equipment is typically designed to prevent positively pressured wellbore fluids from flowing to sea, not prevent the sea from flowing into the well, with pressure integrity evaluated based on net internal pressure, not net external pressure. This poses a multitude of risks ranging from developing hydrates in pressure control equipment to uncontrolled flow of seawater into wellbores or flowlines. Managing subsea riserless intervention operations in this state requires additional assurance at both an equipment component level and an operational execution level to maintain well control and execute operations safely. A formal assurance process was developed to establish the integrity capability of subsea riserless intervention equipment under significant subambient well conditions ( 2,000 psi net external pressure). This was a collaborative effort between the operator, intervention vessel contractor, and original equipment manufacturers. Equipment was evaluated at both a component and system level to verify capability, and operating parameters and procedures were revised to maintain well control and well integrity under net external pressure conditions. The results of this process were formally documented to allow risks and safeguards to be clearly communicated to offshore personnel executing well intervention operations. Multiple significantly subambient riserless well interventions have been executed by BP in the Gulf of Mexico and West Africa to-date. Multiple equipment options and intervention methods have been developed into standard operations, resulting in significant production uplift with no major incidents seen in operations relating to seawater ingress or intervention equipment failure due to net external pressure. As part of this process, BP applied for and received the first BSEE approval to allow consideration of external seawater hydrostatic pressure to develop a depth adjusted working pressure for subsea intervention equipment.