Offshore gas production involves installations on the seabed which are controlled and powered from a land-based or sea-based terminal or host facility. Well fluid is transported via pipelines from a subsea production system to a receiving terminal to be further processed before the products are supplied to market. In the initial phases of production, the fluid reservoir pressure is usually sufficient for feeding the hydrocarbon fluids through the pipeline. Later in production, or in the case of very long distance between the well fluid reservoir and the receiving terminal, boosting of fluid pressure and flow may be required in one or more compression systems along the feed line in order to maintain flow rate and production level.
Compressors used in subsea compression systems are adapted to process wet gas containing a certain ratio of liquid. Above such a ratio, liquid pumps will be required. In the compression system, well fluid containing gas and liquid enters a separator or scrubber in which liquid is separated from the well stream and fed to the pump, providing predictable operating points for both the compressor and the pump with respect to liquid volume fraction or level. The pump is operated to pump the liquid downstream, typically by injecting the liquid into the compressed gas that is discharged from the compressor, whereby a re-mixed multiphase well fluid leaves the compression system at a raised pressure level and flow. Nevertheless, the subsea compression system may optionally be arranged for discharge of boosted gas and liquid flows via separate export lines.
Conventionally, each compressor and pump is typically driven by a dedicated electrical motor respectively which is supplied operating and control power via an umbilical connecting the compression system with its host facility. Each compressor or pump motor in the compression system requires for its operation an individual setup of power and control gear for a variable speed drive, such as subsea switchgear, wet-mate electrical connectors, high voltage electrical jumpers and electrical control system components, cooling and lubrication circuits including valves and flow and pressure control, etc.