TAQA`s Otter field in the U.K. sector of the North Sea has been operating since 2002 with in-well electrical submersible pumps (ESPs) to boost production. It is predicted that the field will need some means of artificial lift to kick off oil production as the reservoir pressure decreases. A subsea multiphase boosting system was integrated into the existing facilities to provide a new, reliable, and highly energy-efficient pressure boosting solution for the Otter field. With gas cap from the wells, the main challenge for the multiphase pump (MPP) system was to generate sufficient differential pressure with only gas entering the pump station inlet. A second challenge was to avoid overheating the system with only gas flowing from wells.
A novel liquid extractor was developed, built, and tested in Bergen, Norway. Internal chambers in the liquid extractor separate gas and liquid fractions of the fluid. The liquid extractor is included in the pump station, together with a flow mixer, MPP, recirculation choke, and valves. The design of the liquid extractor ensures that liquid can be recirculated by the choke back to the pump inlet. The mixture of gas from wells and recirculated liquid allows the pump to generate sufficient differential pressure.
Testing of the entire pump station prior to subsea installation confirmed that the liquid was maintained in the station and the overall heat loss from the station was sufficient to prevent overheating during fluid recirculation. Hence, the new liquid extractor ensures sufficient time to allow for startup of a nonproducing field, which has also been supported by field experience. The pump station, including the new liquid extractor, was installed subsea in the Otter field in October 2018. The Otter field, with the MPP and the liquid extractor, has been in operation for 1,057 days at the time of writing with 100 % reliability and will significantly increase the total recovery of the Otter field compared to natural production.