The Subsea Electrical Power Distribution System (SEPDIS) is a combination of a high voltage transmission system and an industrial power distribution system Naturally an electrical power distribution system of this type requires a given level of operational control and monitoring when used in a surface application Additional monitoring and control functions and novel design features will be required when the equipment is to be deployed subsea The SEPDIS control system must be designed to meet all the required electrical functions and be fully adapted to subsea use in terms of underwater performance, availability, reliability, installation, retrieval and maintenance requirements It should be remembered that any form of subsea intervention and maintenance will inevitably incur high costs and so the system must provide all the necessary control and monitoring functions to enable safe and reliable operation of SEPDIS in the intended manner
The oil and gas industry has, in recent years, focused more and more on marginal and complex fields These type of fields very often imply long offset distances and/or substantial water depth
The challenge, both for the oil companies and the manufacturing and system supplier companies, is to develop technology facilitating economic attractiveness of these type of fields
A large number of tools have been developed and offered to the market In this paper some of the tools requiring high energy supply for operation will be mentioned
Subsea separation and injection systems, in ABB terminology called SUBSIS, is a very attractive concept improving the economics of marginal, complex and fields approaching the high water cut period The general idea is to separate out the water from the well-stream at the seabed, before it enters into the production pipeline The produced water is then re-injected into a disposal well or possibly back into the reservoir for pressure support Depending on the amounts of water produced from the separation process, one or more subsea water re-injection pumps are required The typical unit rating of these pumps is 0 8–2 0 MW
Subsea multiphase booster pumps feature increased oil production by lowering the flowing wellhead pressure, compensating for increased well back-pressure caused by increasing water cut, overcoming function losses in flowlines (increased utilization of the flowlines), etc The typical unit rating of these pumps is 0 5–1 5 MW
Several other subsea power consumers can be mentioned, such as subsea ESP's, booster pumps, water injection pumps, wet gas compressors, centrifugal process separators, heat tracing, etc
All of the above tools are designed to enhance oil and gas production, still they themselves require energy to operate and this energy has to be controlled, very often by quite sophisbcated techniques Several investigations show that an electrical power supply system will be the most cost efficient solution for driving all the subsea processing pumps and other energy consuming systems at the seabed
There are two possible man concepts, the first based on a topside electrical power distribution system, where each subsea consumer is fed by a separate power umbilical from the host facility, while the other concept has one supply cable and local subsea power distribution, in ABB terminology called SEPDIS (Subsea Electrical Power Distribution System).
