Subsea Compression – Åsgard Subsea Commissioning, Start-Up and Operational Experiences

Abstract

The Åsgard Subsea Compression (ÅSC) project has since project sanction in 2011 engineered, constructed, installed and commissioned the world's first Subsea Compression Station (SCSt). The ÅSC SCSt includes two individual compressor trains, each containing an 11.5 MW centrifugal compressor module, condensate pump module, scrubber module and inlet- and discharge cooler modules. The compressor trains are installed at a water depth of 260 meters, in a shared subsea structure with a total weight of 5100 metric tons, a footprint of more than 3300 m2, and height of 17 meters above the seabed. The SCSt is connected to a subsea manifold station routing all flow from the wells via the compressor trains to the gas processing platform Åsgard B. The installation includes a comprehensive SURF scope with more than 60 km of new flow lines, 11 PLEMs and 50 tie-in points. The compressor station is powered from the Åsgard A FPSO where a separate high voltage power module weighing 850 metric tons has been installed. The subsea station is remotely operated from the Åsgard B platform. The subsea compression station is compressing the gas flow from the Midgard and Mikkel reservoirs, boosting the production and securing stable flow conditions in the subsea flow lines towards Åsgard B. The subsea commissioning has been carried out during summer, autumn and winter 2015/2016. It has involved control system commissioning, dynamic commissioning with N2 and MEG, before final commissioning with hydrocarbons. In order to secure efficient commissioning with involvement of a large number of vendors and support personnel, a remote operation room for the commissioning team was installed at Statoil's Stjørdal office outside Trondheim where the Åsgard Operations land organization is housed, thus avoiding constraints imposed by offshore training, helicopter travel and shortage of offshore cabins. This paper describes the subsea commissioning phase, including experiences and lessons learned. Challenges including cooperation between onshore room, offshore platform, and installation vessel, as well as performing commissioning on one train with the other in operation, are discussed. In addition, it gives an outline of the experiences gained during the start-up and initial operation phases.