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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25367, "Qualification of a Subsea Separator With Online Desanding Capability for Shallow-Water Applications," by M.D. Olson, E.J. Grave, and J.C. Juarez, ExxonMobil Upstream Research, and M.R. Anderson, ExxonMobil Development, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed.
In recent years, companies have executed project-specific qualification programs for subsea-processing technologies. This paper summarizes the results of a qualification program that included a multiphase, subsea-separation system for shallow-water applications. The intent of this qualification program was to develop subsea-separation technologies for the global subsea portfolio, rather than for a specific project. To meet this goal, a separator design was chosen that would meet performance targets over a wide range of operating conditions.
Subsea processing is not a new concept; however, recent economic considerations have led to more applications, ranging from simple single-phase or multiphase boosting to separation/boosting to future compression projects. There has been a modest number of subsea-separation applications in the Norwegian North Sea, in the Gulf of Mexico, off the west coast of Angola, and most recently in the Campos basin of Brazil. Future subsea projects that have been announced include two compression and liquid-boosting units that will be installed in the Norwegian North Sea. These two projects, and a few of the installed units, use the simplest form of subsea separation: two-phase gas/liquid separation.
The most notable projects that have installed three-phase subsea separators, which remove a produced-water stream, include the Troll C pilot unit, Tordis in the North Sea, and, most recently, Marlim offshore Brazil.
Shallow-Water, Three-Phase Separator Design
In the preliminary separator design, bulk separation was provided by two inlet vane diffusers (IVDs) installed on the two inlet nozzles. In this design, the IVDs diffuse the momentum of the inlet in a gradual manner such that the liquid phases are not sheared into smaller droplets, which can lead to liquid droplets entrained in the gas or the formation of stable oil/ water emulsions. Downstream of the inlet section, a series of perforated baffles was provided to straighten the flow paths in the oil/water phases in an attempt to maximize the separation length and minimize recirculation or stagnant zones. In the preliminary separator design, there were no separation internals downstream of the perforated baffles in the settling section. High-efficiency oil/ water-separation internals, such as platepack coalescers or vessel-based electrostatic grids/coalescers, were avoided because of reliability concerns.
A water-retaining weir was included in the design to separate the oil/water outlet compartments. With this design, a single level detector, such as a nucleonic device, can be installed upstream of the weir and can be used to measure both the gas/liquid- and the oil/water-interface levels.
Olson, M.D. (ExxonMobil Upstream Research Company) | Grave, E.J. (ExxonMobil Upstream Research Company) | Juarez, J.C. (ExxonMobil Upstream Research Company) | Anderson, M.R. (ExxonMobil Development Company)
Abstract ExxonMobil Development Company (EMDC) recently completed a qualification program which included a multiphase, subsea separation system for shallow-water applications. This paper summarizes the results from this qualification program. Developments and advances in subsea processing technologies and the application of such technologies have been fueled by recent economic considerations in the oil and gas industry, as fields are maturing and operating companies strive to maintain oil and gas production. To-date, companies have executed project-specific qualification programs which take years of upfront engineering. The intent of this qualification program was to develop subsea separation technologies for the "global" subsea portfolio, rather than a specific project. To cover the "global" subsea portfolio, a separator design was chosen that would meet separation performance targets over a wide range of operating conditions. For subsea applications, availability and reliability is critical, as unplanned intervention costs are extremely high; therefore, only certain internals were recommended to avoid plugging and fouling issues. Also, the separator was equipped with sand handling internals, including sand jetting headers and sand removal cyclones, to allow online fluidization and removal of accumulated sand. Qualification activities included the development of a subsea processing template, design validation of a multiphase separator design using Computational Fluid Dynamics (CFD) and performance tests with model fluids and "live" fluids, and performance tests on the sand handling equipment. High-level results from these tests are shared. Prior to the qualification programs, experience with subsea separation technologies, and subsea processing as a whole, within ExxonMobil was limited to being an active partner with subsea processing projects such as Tordis and Pazflor. Following the execution of the aforementioned qualification programs, technical risks have been mitigated, such that it is now possible to be confident in applying these technologies in the "global" subsea portfolio. This will enable ExxonMobil to reduce the cost and schedule impact of upfront engineering on future subsea processing projects. In the ever-changing business environment of the oil and gas industry, this may become a preferred approach to bring unproven technologies to maturity when the business need is well-established.
Olson, M.D. (ExxonMobil Upstream Research Company) | Grave, E.J. (ExxonMobil Upstream Research Company) | Juarez, J.C. (ExxonMobil Upstream Research Company) | Gul, K. (ExxonMobil Upstream Research Company)
Abstract 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.
While subsea processing is not a new concept, recent project economics have led to more applications, ranging from single-phase or multiphase boosting and subsea separation and boosting to future gas compression projects. Subsea equipment suppliers are working to establish technologies that can meet the unique challenges of subsea processing, and operators are trying to capitalize on these technologies by qualifying them in a timely fashion that satisfies the overall project schedule. Certain technologies, such as subsea compact separation, are considered enabling technologies for subsea processing, especially in ultradeepwater applications.ExxonMobil Upstream Research Company recently completed a technology development and qualification program, which included performance tests on an integrated subsea compact separation system for ultradeepwater applications. Because of extremely high internal and external pressures, unconventional separator designs must be used in ultradeepwater applications. A key challenge is the design of separators that can achieve acceptable gas/liquid and oil/water separation performance while still meeting module size and weight restrictions.
Gas volume fraction, % in this article, electrostatic coalescence improved the With this qualification effort, ExxonMobil has added performance significantly.