Process Equipment for Offshore Viscous Crude Handling: Captain Field

Abstract

Abstract The Captain Field in the UK Sector of the North Sea is an accumulation of relatively high density, high viscosity, low temperature, low GOR crude oil, all factors which in themselves present challenges in the design of oil and gas processing equipment. The selection of an FPSO-based production facility, with its inherent motion characteristics, compounded these challenges. The Captain Field facility design has successfully incorporated several novel features to address the oil, water and gas processing requirements. Introduction The Captain Field is located in UKCS Block 13/22a, 130 km northeast of Aberdeen, in 104 metres of water. The oil accumulation is heavy and viscous by North Sea standards, being 19.3°API and 88 cP at a relatively low reservoir temperature and pressure of 30°C and 93 bara, and with a gas/oil ratio of 23 sm3/m3 (130 scf/stb).@ The fluid properties and offshore environment require innovative horizontal drilling, well completions, and artificial lift technology for economically successful field development. Extended reach, horizontal wells are proposed for all oil producers and water injectors, to be drilled from two drilling centres. Downhole electric submersible pumps will deliver a capacity of 20,000 per well. The initial development comprises a Wellhead Protector Platform (WPP) tied back 1.5 km to a Floating Production Storage and Offloading vessel (FPSO), from which export quality crude will be shuttle tanker. Later stages of development will likely utilise subsea or additional wellhead platform tie-backs to the FPSO. Process design for the Captain field has had to address several significant concerns not normally encountered in offshore processing applications. The principal problems which have been addressed in the facility design are in the primary three phase separation. Although the separator vessels are some of the largest in the North Sea, implementation ofconventional separator design parameters would have required even larger vessels, exceeding practical limits. Separator internals have been designed to minimise effects of external motion acceleration forces on the vessels. Submerged liquid inlets to "water wash", the incoming crude are employed to enhance efficiency of separation of the liquid phases. Because of the extremely high foaming tendency of the Captain crude, a novel "foam handling package" has been developed. This process allows treatment of the foam outside of the primary separator, rather than trying to accommodate the foam within an oversized separator vessel. Also, overhead spray headers have been installed in the primary separators in an experimental attempt to mechanically break foamaccumulations. And finally, the process separation train has been designed to accommodate the "mousse effect" (gas carry-under in the outlet oil stream) noted in Captain and other North Sea heavy crudes. These factors and others have been successfully combined into an integrated process facility designed specifically for the Captain field.