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In 2015, the world’s first subsea multiphase gas compression system was installed offshore Norway. The system comprises two-off 5-MW machines configurable for serial or parallel compression. This system has now gained considerable and valuable operational experience. The multiphase compressor not only ensures efficient power system compatibility but also can contribute to stepout topologies because of the low transmission frequency required for the power supply. Minimizing the complexity of both process and power architecture is crucial in terms of cost, robustness, and system reliability.
Gullfaks Subsea Compression
The realization of the subsea multiphase gas compression system was made possible through major technology investments during 2007–2012, including facilities, design models, and engineering as well as component and full-assembly qualification testing. This extensive technology-qualification program made the technology robust and reliable and enabled the engineering, procurement, and construction delivery of the subsea multiphase compression system to the Gullfaks field in March 2015 per schedule (Fig. 1).
The dynamic commissioning of the Gullfaks subsea wet-gas-compression (WGC) system was begun in July 2017. The work was first performed on both compressors in single mode to verify control-system functionality. Different wells and production lines were used to reach desired operation points. After initial commissioning, the compressors were run first in single mode and later in parallel at different suction-pressure setpoints.
The system has been operating successfully since its commissioning in 2017. The subsea compressor system is boosting multiphase gas, increasing the production from several wells, and has recently achieved Technology Readiness Level 7. The system flexibility is exploited in extended ways compared with the initial project phase approach and allows the operator to take advantage of many opportunities, including increased oil recovery. Some key achievements of the system include increased oil production by kicking off dead wells and enabling stable well backpressures.
The development of the multiphase compressor began in the late 1980s in cooperation with a major operator. Initially envisaged was a topside market where compression of single low-pressure wells could be beneficial. Consequently, the target throughput and power rating were low. On the other hand, it was realized that, for such low-pressure conditions, the relative liquid loading and impact from various multiphase flow regimes were immense, resulting in challenges that had never been addressed by the industry. From previous experience, an axial contrarotating impeller arrangement was proposed.
During the following decade, several full-scale prototype compressor units were designed, built, and tested under realistic conditions. This period cumulated with a successful test of WGC2000 in 2000. The specification of this compressor now had been tailored for subsea applications, taking advantage of the company’s development and substantial experience with subsea pumps.
During the subsequent decade, general interest in subsea compression of larger gas fields emerged, and, as that market gradually matured, multiphase compression development regained momentum. Focus had now shifted to a complete subsea compression system aiming for high overall efficiency and ultimate reliability. This development phase cumulated with the technical qualification program for the WGC4000 compressor in 2010 performed with the Gullfaks operator. The successful qualification of this compressor led to a commercial delivery of the Gullfaks subsea compression system in 2015.
The WGC6000 Multiphase Compressor
A Low-Risk Enhancement Program. Following the success of the WGC4000 commercialization, the compressor envelope and power rating were extended by a low-risk enhancement in a technology-qualification program. The aim was to qualify a multiphase compressor capable of meeting the compression requirements for major subsea gas fields.
During 2014–2018, considerable resources were spent qualifying enhancements of the existing compressor, including component testing of impellers, thrust bearings, and electric motors. The first-article next-generation multiphase compressor, the WGC6000, has now been built and successfully tested at full load conditions. Differential pressure (DP) capacity and power rating have been increased by 60% compared with the WGC4000. The volumetric throughput has been increased by 40%. Impeller tests performed on a test compressor in parallel have shown an increased throughput of up to 110% compared with the WGC4000.
In addition to performance-mapping studies, developments to further increase the compressor volumetric capacity, DP, and power rating to meet specific field requirements are ongoing in accordance with established timelines and needs of various projects.
Increased Volumetric Capacity. The key to increasing the compressor volumetric capacity while maintaining its many unique features has been the development of new impeller blades. Analytic analysis and computational fluid dynamics have been iteratively performed with wind-tunnel testing of impeller blade profiles. New model manufacturing techniques using 3D printing technology have made this approach very efficient.
Single impellers were performance-mapped in a dedicated test compressor equipped with advanced high- pressure pitot probes for detailed internal flow measurements. All physical tests were modeled and reproduced with computational fluid dynamics that, combined with the measurements, provided insight to important flow phenomena and enabled the development of new high-performance impellers with good multiphase predictability and efficiency.
Increased DP (Thrust Balancing). The DP generated by the compressor renders thrust loads on the shafts that are supported by fluid-film-lubricated axial bearings. With more available shaft power in the WGC6000, bearing size has been increased to improve the mechanical DP capacity of the compressor. The DP capacity of the WGC6000 first article is 60% greater than that of the WGC4000.
The compressor’s motors and internals are protected by the same barrier fluid as in the single- and multiphase pumps. This allows for use of standard oil fluid-film-lubricated bearings and mechanical seals, one of the key factors for the technology’s high reliability. During operation, the barrier fluid pressure always must be higher than process pressure to avoid ingress into compressor internals. With implementation of the thrust-balancing design, operation with the same barrier fluid pressure on both motors of the compressor is possible, and thus the complexity and cost of the pressure-regulation system is reduced substantially.
Competitive System Solution: WGC6000 and Long-Stepout Power Systems. Compression for major subsea gas fields will require significant amounts of power independent of selected technology. In this context, stepout distance has been perceived as a limiting factor for onshore power supply as an alternative to conventional offshore facilities. However, recent analysis and testing verifies that the stepout distance can be increased by enhancing field-proven power system topologies while taking advantage of fit-for-purpose compressor technology.
While the two-motor contrarotating design enables several process-specific advantages in terms of excellent liquid-handling abilities and surge-free operation, the two-motor solution is also an enabler for long stepouts. This is predominantly because the two-motor contrarotating solution requires a relatively low speed for each shaft. From a power-system perspective, the compressor is operating as two motors in parallel, fed from a common source. The required electrical input frequency is thus correspondingly low.
Both the capacitive stray currents, also referred to as leakage currents, and inductive voltage drop in a cable are directly proportional to the applied frequency. The stepout feasibility for the multiphase compressor is thus greater than the achievable length for a conventional high-speed single-motor machine because of the low supply frequency. Minimal operation frequency is also an advantage in terms of the electrical-system-frequency response.
In 2018 and 2019, several application analyses and verification tests were performed for long stepouts. The most important of these tests is the long-stepout test for the WGC6000. A 120-km high-voltage cable simulator has been built, replicating the actual impedance and frequency response, including skin effect, of an intended supply cable for the WGC6000.
In addition to the cable simulator and adjustable speed drive (ASD) acting as the power source of this setup, a cable sending-end step-up transformer and a receiving-end step-down transformer are included to accurately represent the design topology of the intended power system.
Topologies using topside or onshore ASDs hold significant advantages in terms of unit and sparing costs in addition to mean time to repair. Nevertheless, for applications typically requiring more than 40 MW of subsea compression power combined with long stepouts, subsea ASDs may be more attractive from a system-cost point of view because of the reduced number of required cables and their installation. Consequently, as part of the long-stepout program, an additional system-testing program is being established that will verify compatibility between the WGC6000 and a subsea ASD currently in qualification by another industry leader.
Eliminating offshore processing and facilities such as platforms and local power generation will reduce the overall cost and risk of a gas-field development significantly. An approach that combines these advantages with the opportunities of a simple, yet robust, multiphase compression system to increase the recovery of both gas and condensate is believed to have an extremely competitive advantage.