Through two deployments of subsea gas compression systems at the Norwegian Continental Shelf in 2015, subsea compression is about to be recognized as an upcoming and important technology for future exploitation of offshore gas reserves. Compared to other technologies, unmanned subsea compression in the vicinity of wells is the far most effective method to both accelerate the gas production and increase the total recovery. Although subsea compression in most cases is less costly than a platform or floater solution, cost is still a major obstacle preventing widespread utilization of subsea compression solutions. The major cost elements for a total subsea compression development are SURF (subsea-hardwareumbilicals-risers-flowlines), installation and compression station. The contribution of each cost element will depend on reservoir characteristics, infrastructure, tie-back, water depth, etc. On top of the cost estimates, operators add to their budget a contingency cost related to conceived risk. Large saving potentials are seen for well-to-host integrated design and delivery. Additionally, simplified compression systems with smaller footprint and less requirements for utilities would increase the attractiveness of subsea compression.
Although the potential gain by utilizing subsea compression has been known for a long time, the technology must still be considered as novel with only two systems in operation, both deployed in 2015 by Statoil on the Norwegian Continental Shelf (NCS) and each using a different technology.
The subsea compression system at Åsgard field is based on separating out the liquid from the gas in order to compress the gas under similar conditions as known from topside industry. This system is quite effective for relatively wide operating conditions, but due to the cost and size, only a few fields can be made profitable with this solution.
The subsea compression at Gullfaks is based on direct boosting of the untreated multiphase wellstream by a specifically developed wet gas compressor. In this way, the electric pump is made obsolete. Compared to the Åsgard solution, this system is considerably more compact and less expensive. However, the wet gas compressor cannot deliver comparable performance or operating envelope.