An Assessment of Subsea Production Systems

Abstract

Abstract The rapidly accelerating shift to subsea production systems represents a significant departure from conventional operations. Historically, subsea wells have had a good track record. However, complex subsea systems are now being deployed in ways rarely encountered in previous development schemes. These increasingly complex systems present a number of technical challenges. This paper presents an assessment of subsea production systems, considering the technical, operations and safety issues associated with this development modality. This assessment considers the following general areas:subsea processing;flow assurance;long-term well monitoring and,safety & environmental concerns. A review of the state-of-the-art in each of these areas is presented and several technical and operational gaps are identified. One of the most striking findings is the low ultimate recovery anticipated from many subsea wells. Two new technologies that can have a positive impact on recoveries are subsea multiphase pumping and subsea separation/processing. This assessment found, however, that the safety systems seem to be absent from current subsea processing plans. Other concerns include the inability to monitor subsea wells for sustained casing pressure, a widespread problem in the GOM. With regard to flow assurance, much work has been done to develop design tools for flow assurance, little has been done in the area of monitoring subsea production systems to detect and locate these materials. As more subsea systems are placed in operation, the monitoring and operation needs (rather than the design needs) will likely emerge as the top priority. Introduction The subsea environment is perhaps the most remote and unexplored on earth. The remoteness of subsea wells, coupled with a number of complex interactions between subsea wells/flowlines and the ocean environment make monitoring, intervention and routine operation much more difficult. These systems are now being deployed in ways rarely encountered in previous development schemes.1,2,3 One of the forces driving increased use of subsea production systems is the dramatic reduction in development costs when compared with conventional methods.4 In many cases, the use of a subsea tieback is the only viable option to develop these resources. In recent years, we have seen a rapid maturing of the technology being developed for subsea use.5 However, a number of technical issues are associated with subsea production.6,7,8,9 Industry and regulators are increasingly becoming aware that, while reducing up-front capital outlays, long, multiphase flowlines add additional backpressure, reducing flow rates and ultimate recoveries. For example, conventional production operations routinely drawdown wellhead pressures to 100–200 psig. A subsea completed well, however, may have abandonment wellhead pressures of 1,000–2,000 psig due to the backpressure added by the long multiphase flowline. Consequently, there is a growing interest in processing the produced fluids subsea.5,10,11 Strategic technologies that are believed to be essential for the successful implementation of subsea production include multiphase pumping10,11,12,13, multiphase metering14,15,16 and compact separation. One of the challenges posed by subsea production is how to reduce wellhead pressure to allow effective recovery of hydrocarbon resources. Multiphase pumping is one technology being considered to help remedy this situation, as well as pressure boosting deployed in advanced subsea well systems. Other challenges in the subsea arena are in the areas of flow assurance and well monitoring and intervention. Sustained casing pressure has been identified as one of the key areas requiring inexpensive and effective intervention options.9,17 Another key area is the area of blockage monitoring. For the past decade research has focused on developing design methodology, while relatively little attention has been paid to the long-term problem of monitoring subsea flowlines for the buildup of wax, scale, hydrates, etc. There is a need for analysis techniques to help identify and locate partial pipeline blockages and new development of sensors to monitor the flow. This paper discusses some of the few fundamental issues associated with subsea processing. The various options are discussed and the advantages and disadvantages of each type of technology are highlighted. Most importantly, technology gaps are identified that, if not properly address, may limit the application of subsea technology.