This paper summarizes key results of the development and verification for a subsea compact separation control system based upon a literature review, control system design, dynamic simulations, and an integrated system test. The compact separation system being considered is designed for applications in water depths up to 3,000m. The control system is a key technology element of the compact separation system, and its satisfactory performance is critical to ensure stable operation of the overall system. A thorough literature review was conducted to evaluate the control system designs for comparable subsea separation systems in industry, to understand the technical challenges, and to incorporate the key learnings into the development of this control system design. From this work, best practices were identified and were used as guidelines. A preliminary control system was developed, and the robustness of its design was assessed through dynamic simulations. Later, an integrated system test was performed with real crude and methane at realistic operating conditions to evaluate the performance of the compact separation equipment and to validate the model of the control system. The dynamic simulations and integrated system test results demonstrated the control system response following transient events, such as slugging, and provided insight into the system dynamics that then led to further modifications of the control system design and enhanced the overall system performance. This paper also delineates challenges associated with the design of a control system for a subsea compact separation system consisting of multiple, closely integrated gas-liquid and oil-water separation units. For example, the relatively small control volumes in compact separation equipment and the interaction between the different components can both add complexity to the control system design. Lessons learned from the modeling and implementation of such a sophisticated control system are discussed with the intent to serve as a reference for future subsea separation projects in the oil and gas industry.

ExxonMobil Upstream Research Company (EMURC) recently completed a subsea technology development and qualification program which included performance testing of an integrated, subsea compact separation system with electrocoalescence for ultra-deepwater applications. To the authors' knowledge, this was the first time that an electrocoalescer had been tested with a gravity-based, compact separator (e.g., a Pipe Separator).One challenge often seen with conventional gravity separators is the formation and build-up of stable emulsion layers, mainly associated with the processing of medium and heavy oils. In an earlier test program, the Pipe Separator, the primary oil-water separator in the subsea compact separation system, performed well; however, emulsions that were not separated in the main pipes of the Pipe Separator tended to accumulate in the outlet section. There, the emulsion layer then had to either flow out of the oil outlet (penalty on the oil quality) or the water outlet (penalty on the water quality).The medium and heavy oil trials were particularly challenging, especially at lower water cuts. In order to achieve the desired oil and water qualities, the total liquid flow rate through the system had to be reduced. To overcome this limitation, additional trials were performed later with a compact electrocoalescer, the Compact Electrostatic Coalescer (CEC™) supplied by Fjords Processing (formerly Aker Process Systems), which effectively coalesced dispersed water droplets in the oil-continuous feed into larger droplets such that they could be separated easier in the downstream Pipe Separator. The results from the additional trials demonstrated that electrocoalescence enhanced the oil-water separation performance of the integrated system at flowing and operating conditions that would have otherwise been very challenging. This paper presents a summary of the results from these additional trials.The cost of developing and deploying a subsea separation system is significant; and therefore, it may not be economical if the system is unable to achieve a sufficient capacity. The design of subsea processing systems is often a balance between what is practically achievable under the module size/weight constraints, and what production rate is required for project economics. By understanding, with confidence, the maximum liquid handling capacity of the integrated system with electrocoalescence, technical risks could be minimized, and a future subsea separation project could become more attractive. As such, the results from this test program may be of interest to operating companies considering similar technologies or future subsea separation projects.Industry activity in the area of subsea processing, and subsea separation in particular, has increased over the past two or three decades, leading to a handful of applications ranging from single-phase or multiphase boosting, separation and boosting, and compression projects. While the benefits of subsea separation and boosting are generally recognized by industry, the design ...

ExxonMobil Upstream Research Company (EMURC) conducted a comprehensive laboratory testing program of a produced water de-oiling system primarily targeted for subsea applications. The test program included performance evaluation as well as durability testing of a two-stage mixed-flow de-oiling hydrocyclone technology. The objectives of the test program were to evaluate the separation performance of two system configurations i.e. decoupled and integrated hydrocyclones stages, to understand the overall system integration effects on the performance, and to determine system sustainability to sand loading. Subsea produced water treatment in deeper water applications requires robust, reliable and compact separation equipment. The performance testing confirmed the feasibility of a multi-stage mixed-flow hydrocyclone system for subsea applications and demonstrated that, within a given operating envelope, the system can treat challenging produced water streams with high oil-in-water (OIW) content of up to 5%, reducing it to a few hundred ppm level. In addition, the issue of turndown can be addressed by designing the de-oiling system with multiple parallel banks of liners. The de-oiling system testing was conducted with light and heavy crude oils, of gravities of 36 °API and 19 °API respectively, to map performance characteristics of a two-stage system with the first hydrocyclone stage of bulk oil removal and the second stage of water polishing. Variations in test conditions such as the flow rate, temperature, inlet oil concentration and the reject ratios were introduced to establish optimal operating ranges for each stage as well as for the system. The performance of the decoupled and the integrated system was evaluated by determining separation efficiency break-points at different operating conditions. Accelerated sand erosion tests were performed on a dedicated sand erosion test loop to evaluate different erosion resistant coatings for hydrocyclone liners. The erosion tests helped identify a promising coating solution that can withstand continuous sand loads for long-term operations. The test program demonstrated that a two-stage mixed-flow hydrocyclone de-oiling system can meet challenging subsea produced water treatment and reinjection requirements over a wide range of operating conditions. The paper presents the key results of the overall performance tests as well as the results of sand erosion testing of the hydrocylone liners with different coatings.

Re-injection of produced water can be a cost-effective way of disposing produced water and improving hydrocarbon recovery for subsea production and processing systems in deeper water or with long tieback distances. Injecting produced water with high oil-in-water content and suspended solids into a well can be detrimental to relative permeability in the formation near the wellbore region causing loss of well injectivity over time. Monitoring re-injection water quality trends can provide an early indication of injectivity decline and potentially avoid loss of an injection well. As the industry moves closer to having a fully-functioning subsea production and processing system, there has been an increased effort in developing reliable in-line subsea Produced Water Quality Monitoring (PWQM) technologies. Such monitoring sensors can measure the quality of re-injection water in terms of oil and solids content (size and concentration) thereby facilitating performance monitoring of subsea separation and produced water treating systems by identifying upset conditions. Development efforts to meet subsea PWQM requirements for produced water re-injection application included: (1) development of subsea sensor concepts with active cleaning systems to enable long term operation without compromising performance, and (2) extension of the Oil-in-Water (OIW) measurement range significantly to support subsea production/processing and produced water reinjection applications. The new sensor designs that were developed primarily for subsea application for improved measurement performance and with self-cleaning functionality can also be used topsides. Two different PWQM sensor prototypes were developed to meet the subsea PWQM requirements. The measurement performance of both prototypes was verified in a flow-loop by comparing the measurement readings with known reference. The impact of variations introduced in test conditions such as changes in pressure, temperature, salinity, and flow velocity on the performance of the prototypes was also investigated. The active cleaning systems were also tested to examine their effectiveness under normal and simulated fouling conditions. The paper discusses the technical gaps in subsea PWQM technologies, flow-loop testing of the PWQM sensors prototypes and the key test results.