Subsea Compression - Åsgard and Beyond

Abstract The comprehensive R&D effort that was initiated in the beginning of the eighties resulted in a highly successful and profitable development and implementation on the Norwegian continental shelf, namely the subsea industry. A series of successful tie-backs and long distance multiphase flow lines is today in operation and subsea processing in general, and subsea compression in particular, represented by Gullfaks Subsea Compression (GSC) and Åsgard Subsea Compression (ÅSC), is becoming a more attractive feature as many fields move into the tail-end. This article gives the status and experiences so far with focus on ÅSC and how the technology may be further enhanced and matured in future projects. Long-term technology development and qualification work together with the vendor industry has opened for low-pressure production by implementation of subsea compression systems close to the flowline inlet, i.e. close to the wellheads. This represents a significantly more efficient Increased Gas Recovery (IGR) tool compared to downstream compression, by the fact that it gives a packing of the downstream system in terms of an increased gas density, reduced gas velocity and hence reduced frictional losses. ÅSC, that was started up in 2015, is by far the larger and more advanced subsea processing installation. The article reviews the experiences after 9 years in operation with close to 100% availability of the subsea station and a retrospect on important activities during project execution. ÅSC Phase II was initiated in 2016 with the objective to develop a tandem compressor variant that could significantly increase the pressure ratio and replace the existing ones at the subsea station to prolong the field life time and sustain the gas production further into the low-pressure region. The qualification program that led up to the new compressor technology is presented along with experiences after one year in operation. Future implementations of the subsea compressor technology target to reduce complexity and cost. During ÅSC II the centrifugal gas compressors were qualified for continuous liquid injections to enhance pressure ratios and further R&D is undertaken to allow them for full well stream operation. Details are given for a selection of these technology elements. Equinor has identified several potential business cases that can benefit from subsea compression. To optimize these business cases, mature the concepts, and accelerate project development, an early phase project cluster that focuses on subsea compression has been established. The cluster has a strengthened mandate to maximize synergies and standardize solutions, where a key challenge is to strike the right balance between standardization and project-specific solutions.