Subsea multiphase boosting and metering represent technologies which have been proven to provide increased recovery factors from subsea fields, and improved field development economics of projects. The experience from successful operation of subsea multiphase boosting system on a global scale has driven the next technology step - subsea multiphase compression. This paper describes the development, qualification and delivery of the world's first subsea multiphase gas compression system.
The development and qualification of the world's first subsea multiphase compressor system with no requirements for an upstream separation facility or an anti- surge system will be described. The compressor is a contra- rotating machine specifically designed for pressure boosting of unprocessed well stream. This unique configuration, developed in collaboration with several oil companies, allows for a compact and robust design that is easy to install by use of light intervention vessels, and is based on the well- proven design developed for subsea booster pumps.
The multiphase compressor is capable of handling high liquid contents without mechanical issues, with gas volume fractions typically in the range of 95%-100%. The application of a multiphase compressor in a subsea environment simplifies a subsea compression system significantly as there is no need for preprocessing. This enables deployment of simplified subsea systems at lower capital expenditure and field development costs. Another feature of the subsea multiphase compression system that will be described is the development of a high performance subsea process cooler. The cooler design is based on free convection to fulfill the design requirements to have a simple and robust system, as the cooler module is installed in a location with limited access for inspection, maintenance and repair.
The main driver in the development of the multiphase compressor was to increase recovery rates in subsea gas fields. The Gullfaks field, operated by Statoil, located in the North Sea, sees the first deployment in the world for subsea multiphase compression. Currently the recovery rate for Gullfaks South Brent is at 62%; however, it is foreseen that the recovery rate will increase to 74%, or by 22 million barrels of oil equivalent, by using the multiphase compression system and combined with low pressure production.