Making the case for an all-subsea development

Statoil and its partners are moving the industry one step closer to an all-subsea field development solution.

David Paganie • Houston

Statoil and its partners are moving the industry one step closer to an all-subsea field development solution. Last month, following a lengthy qualification and testing period, the industry's first subsea gas compression facility became operational in the Norwegian Sea. It also marks a milestone in Statoil's quest to qualify and prove all of the elements of a full-scale "Subsea Factory."

The new "dry" subsea compression system is designed to boost recovery from the Midgard reservoir on Åsgard 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.

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.

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.

Business case

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 Granherne.

The DNV GL paper suggests that the Åsgard and Gullfaks projects, together, illustrate that the strongest candidates for brownfield subsea compression are likely projects in mature areas such as Northwest Europe. The infrastructure in the area is sufficient for pipeline transport, onshore processing is developed, and power is available from a stable grid onshore. For more on the market in Northwest Europe, see page 32 for a report by Markus Nævestad and Jo Husebye with Rystad Energy.

The business case for wet subsea compression on Åsgard 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

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