Subsea gas compression rises up as a viable and attractive solution for any case where the natural pressure of the reservoir constrains the gas recovery and/or the transport capabilities of the pipelines. One typical application is where the weight of the gas column (particularly in extreme depths) prevents maintenance of production by natural pressure, forcing the operator to leave behind considerable amounts of gas in the reservoirs. Another application is where relatively low flow causes liquid to accumulate in the flowlines, creating instability in the production rate and eventually leading to shutdown. Yet another attractive application is to enable longer step-out distances, including subsea-to-shore fields.
The main conclusions are that the subsea gas compression technology has after 25 years of development reached a readiness level that makes it not just a more energy-efficient option than topsides, but also a more economically attractive one by reducing both CAPEX and OPEX and increasing the total recovery.
The 25-year journey that made the subsea compression technology a viable and attractive option has provided a number of contributions to the technical knowledge base of the oil & gas industry. This paper gives an overview of the two pioneer projects on subsea compression, a brief history of the technology from inception to date, four key achievements which contributed to make it possible and naturally and three key features of the next generation of subsea compression, currently under development.
For many different reasons, sustaining production in gas fields may be or may become technically or economically unfeasible. The challenges vary from liquid accumulations in the pipelines, too long step-out distances, or simply too low production rate by natural well pressure making production less economically attractive. One traditional solution in such cases is topside compression, which has proven a good option.
Subsea compression, which was not more than a promising concept until not long ago, addresses some of the topside solution limitations and weak points. The first major advantage is the increased recovery. Since the subsea compressors can be closer to the wells and upstream of a riser or tie-back pipeline, the suction pressure can be drawn further down than if they were deployed topside or onshore. Therefore with the same power consumption, subsea compressors can sustain higher production rates and for longer than the topside alternative, resulting in an increased total recovery. Moreover, being unmanned and installed at the bottom of the sea, the total amount of raw material is substantially lower and the operation costs are reduced to a minimum. Furthermore, the reduced amount of raw material utilised, the elimination of human activities (during normal operation) and the optimisation of process energy efficiency makes this technology also more attractive from the HSE standpoint.
The reason why subsea compression did not come true earlier is mainly because the enabling technologies were not yet qualified. After a number of studies and technology qualification programmes over more than 2 decades, these enabling technologies finally reached the necessary readiness level and made subsea compression commercially available. A number of related works are mentioned in this paper and two in particular are presented in more detail, namely Ormen Lange Subsea Compression Station Pilot and Åsgard Subsea Compression.