According to Framo Engineering, processing oil, gas and produced water in a subsea plant yields considerable cost reductions. A number of development projects have been carried out in recent decades. This evolution took off when Norsk Hydro placed the first order for such a plant in 1998; the plant is being installed in the Troll field.
Subsea process plants have several advantages: They perform a separation of the well stream and reinject the produced water, increasing well production; subsea removal of produced water reduces or eliminates the need for water processing up on a platform/production vessel or onshore and new marginal fields can be developed and started up with a minimum of topside equipment. If flaring is not permitted and the gas cannot be economically exported, the SWAG (Simultaneous Water and Gas Injection) concept will take care of the gas on the sea floor. In addition, reinjecting unprocessed produced water into the formation eliminates the need for topside pumps and other equipment for topside processing of produced water.
Two Framo projects involving such processing plants are: Testing a contra-rotating axial (CRA) wet gas compressor and a pilot installation of a subsea centrifugal separator (CeSep).
CRA: Framo's contra-rotating axial (CRA) wet gas compressor has been developed to reduce the pressure of export gas produced in subsea installations. Highly compact, the unit has two electrical motors that each drives a short shaft with impellers. The compressor has low weight and is designed to handle wet gas. The pressure can pump the gas to a remote treatment facility. The reduction in gas volume due to the compression makes it possible to use pipelines of smaller inner dimensions. Rig tests, where pressure build-up and the well stream have been 100 percent recreated, have so far shown that the compressor does what it promises and then some.
CeSep: The company's subsea centrifugal separator CeSep has been developed for continuous separation of oil, gas and water. After separation the water content is down to 0.5 percent, which sharply reduces the problem of hydrate formation. Hydrates - almost a kind of slush - create major mechanical problems as well as corrosion. The separator treats the water to 1,000 ppm purity, making possible direct reinjection. Driven by an electric motor, the centrifuge can be installed or removed using the standard equipment developed for subsea pumps. The design is based on a system that has long been approved for topside installation. CeSep has been installed for testing, and results have been positive.