Development of Novel Pressure Control for Subsea Pumps and Compressors
Authors: Kjellnes Hans Fredrik Lindøen , Jenssen Trond William , Almqvist Joakim , Solberg Petter , Russenes Carsten Falck 
Published: April 2019

Abstract
Subsea boosting has been building a track record at increasing depths and higher pressures. This has introduced certain new challenges. Continuous development of the technology has been required to maintain the historical high reliability and operability. This paper identifies operational challenges associated with a specific deepwater field and how they were resolved. The close collaboration between the operator's and the pump supplier's teams is emphasized as a success-factor. Insight is given into the development team's problem-solving strategy, as well as the applied technology itself. Extensive use of digital tools such as advanced dynamic modelling and virtual prototyping has been applied to debug concepts ahead of physical prototyping. This resulted in a fast track project with only very few time-consuming and expensive re-iterations.
In 2014 the world's deepest seabed boosting pump system was successfully installed and commissioned. The permanent real-time condition monitoring system allowed the pump manufacturer to remotely monitor the pump performance. During the first few months of operation, it was determined that the shut-in pressure gradient was significantly steeper than specified. The production pressure build-up following a pump stop was more abrupt than the pump's barrier fluid pressure control system was designed to deal with. Because the gradient of the pressure increase couldn't be altered, a limitation on the pump's maximum pressure drawdown was immediately put in place. This was done to minimize the amplitude of the pressure increase on shut-in, and to prevent the production pressure from exceeding the pump's barrier fluid pressure. Without such a limitation, this condition could result in a pump breakdown. Continuous operation with this constraint in place would lead to significant curtailment once additional pressure drawdown was required to maintain the nameplate production.
Seabed pumps are equipped with a barrier fluid system, which is regarded among the main success factors leading to the high meantime to failure. The barrier fluid system provides the pump with clean fluid at a correct pressure. The barrier fluid is used for lubrication of bearings and seals, heat transfer, and electric insulation. It also constitutes a barrier, hence its name, for any production fluid ingress into the electric motor through pressure control. The pressure is being closed-loop regulated to stay within a certain band above the production pressure.
Barrier fluid is conveyed between host facility and the subsea pump through small-bore tubing in the umbilical. Thus, quick volume exchanges between topside and subsea is limited. As the umbilical length increase, the response time, as given by speed of sound, also becomes a limiting factor. A subsea pressure control system is the most common solution in the industry for larger depths and long tie-backs.
As the well pressures were depleting for the described deepwater field, the drawdown limit posed a risk for curtailed production. To avoid falling below the nameplate production of 170 kbbls/day, the full differential pressure capability of the pumps was soon required. The novel pressure control technology was developed, qualified and successfully implemented on the pumps. It allows for safe operation through ultra-quick production pressure changes without the need for upgrades to the umbilical. In fact, the technology allows for longer step-out and further cost savings on future umbilical and seabed boosting deployments as even smaller-sized umbilical tubing can be utilized. The successful development of the novel pressure control system prevented production curtailment altogether. The system is now successfully operating subsea, and the pumps are helping the operator to utilize the full production potential of the field.