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Making the case for an all-subsea development
The new "dry" subsea compression system is designed to boost recovery from the Midgard reservoir on Ăsgard from 67 to 87%, and from 59 to 84% from the Mikkel reservoir, for an additional 306 MMboe of total output. The solution involves the connection of two 11.5-MW centrifugal compressors equipped to handle 21 MMcm/d to existing subsea templates and piping the produced hydrocarbons 40 km from the Asgard B semisubmersible production platform. Qualification of the technology began in 2005 and involved about 50 components/systems. See: e-news-MAN-Worlds-First-Subsea-TurboCompressor.htm
Meanwhile, a separate, stand-alone project on the Gullfaks field will
employ the industry's first subsea "wet" gas compression system. It is
designed to increase recovery from the Gullfaks South Brent reservoir by
22 MMboe. The solution, involving two 5-MW wet gas compressors with
capacity to handle 10 MMcm/d, is connected to existing subsea templates.
The hydrocarbons will be transported via pipeline some 15 km from the
Gullfaks C semisubmersible production platform. Qualification of
helico-axial multiphase compressor technology for the project has been
ongoing since 2008. See:
In general, a well with subsea compression can produce at lower wellhead pressures, thereby accelerating production and/or increasing recovery. The difference relative to topside compression is an improvement in wellhead pressure depending on water depth, by locating the compressor closer to the wells. The subsea approach is also thought to be more energy efficient than the traditional topside solution.
The business case for an all-subsea development is heavily influenced by the production phase of a reservoir, explains DNV GL in a recent position paper: "All Subsea - Creating Value from Subsea Processing."
The firm finds that, for brownfield projects, the various subsea systems
may be used alone or in combination with other technologies. In contrast,
an all-subsea solution for a greenfield project has more limited
applicability. Meanwhile, FLNG technology is emerging as the preferred
development concept for greenfield gas developments as an alternative to a
subsea tieback directly to shore. For more on the
FLNG market and topsides
weight considerations, see page 58 for a report by Nick White with
The business case for wet subsea compression on Ăsgard was strengthened by the lack of available space on the field's platform topsides, which would have required a new compression platform in the absence of an alternative.
For fields that are larger or have long step-out distances, such as Gullfaks, and thus require greater pressure boost, dry gas compression emerges as a feasible solution for the business case. The configuration also allows for the tie-in of future satellite wells.
The successful installation and start-up of subsea compression in the Norwegain Sea illustrates the industry's ongoing commitment to advancing susbsea technology. However, the length of the qualification process raises an interesting question: Is it possible to advance the pace of technology development without compromising quality and integrity? More research and development is needed in the areas of condition monitoring, power transmission and distribution, and storage, to enhance the case for an all-subsea development.
David Paganie, Chief Editor Offshore Magazine