The Åsgard Field compressor module—the world’s first subsea gas compression system—towers above a worker shortly before a work barge crane lowered the module to the bottom of the Norwegian Sea. The module has twin 13,400-hp units with identical sets of compressors, pumps, scrubbers and coolers fitted together in an 1,800-metric ton steel frame. (Source: Statoil ASA)
The ethos of subsea processing has always been about moving as much of the conventional processing capabilities from the surface to the seafloor—helping to debottleneck topside facilities and subsea pipelines, increasing recovery rates and, in some cases, creating new possibilities for profitable field developments.
Technological development has accelerated in recent years, with core technologies such as boosting, separation, power and compression now routinely considered as part of new development scenarios. They also are seen as an opportunity to improve return on investment on aging brownfield projects. GE Oil & Gas, having worked in the subsea processing space for more than 20 years, has its inventory of knowledge and technology contained in what it calls the “GE Store,” which allows it to tap into different technological areas.
For the subsea sector, this means leveraging its capabilities in rotating equipment, flow assurance, subsea production, water treatment, power systems and advanced monitoring and diagnostics.
Subsea gas compression is one emerging technology that can significantly improve the economics of many mature gas fields. By placing such systems on the seafloor, the producers can accelerate production while eliminating the need for costly topside facilities.
GE’s Blue-C technology is at the heart of both its subsea dry and wet gas compression system. Initially designed for dry gas, a wet gas version of the compressor that can handle up to 5% liquid has now been developed, eliminating the need for a subsea separator and resulting in a more compact and lower-cost system.
Even in challenging environments such as long-distance tiebacks, low-pressure reservoirs or wells with difficult flow assurance conditions, subsea boosting can be used to unlock and enable access to these assets.
GE Oil & Gas has drawn inspiration from its GE Aviation sister business to explore a new approach that has the potential to fundamentally change and simplify subsea boosting. The company is collaborating closely with leading operators to build a demonstrator test unit aimed at confirming its ability to handle a variety of flow rates and fluid compositions.
The system can be configured with either centrifugal or helico-axial stages, with each system again designed for high reliability. Key benefits include life-cycle cost reduction, operational flexibility and reduced topside and subsea footprint compared to conventional subsea boosting.
In addition to the existing market for subsea boosting, this new concept provides the potential to unlock additional barrels in many brownfield applications where there is either not enough space topsides to accommodate the equipment associated with conventional boosting systems or the cost of conventional systems is prohibitive.
One of the remaining subsea processing building blocks still to be fully developed is seawater treatment for injection.
Produced water can be directly reinjected into the reservoir, but seawater requires treatment prior to injection to remove sulfates and other divalent ions that can cause souring and scaling of the reservoir. This is typically done topside on a platform or floating production, storage and offloading vessel using large processing facilities that use membranes. The treated water is then injected into the reservoir through high-pressure risers.
By tapping into another of its sister business’ expertise, focused on the power and water sector, GE Oil & Gas explored the feasibility of a seabed seawater treatment solution using a combination of nanofiltration and ultrafiltration membranes.
This system has been designed for minimal intervention, with five years between replacement of the membranes. Although subsea seawater injection will not make sense in all business cases, it can be very attractive for deepwater assets, long-distance step-outs or where topside footprint or weight is constrained.
Subsea power enabler
Subsea power is still seen by many as the most critical enabler for subsea processing, and there has been intense focus by the industry in recent years to develop the core building blocks and enable subsea alternating current (AC) power supply, transmission and distribution systems—commonly known as “type-3” systems. These systems are key enablers for longer step-outs and for multiple loads such as distributed compression and boosting systems.
GE has qualified a complete type-3 AC power system that includes subsea variable speed drives, uninterruptable power supply and subsea switchgear and high-voltage connectors.
For longer step-outs beyond 93 miles, there might be the need to consider low-frequency AC, while direct current (DC) transmission is seen as the only potential solution for ultralong-distance step-outs of more than 311 miles. However, it is widely accepted that there are still some major technical challenges to low-frequency AC systems, while significant resources and time also would be required to develop a DC system.
GE already has completed significant preliminary R&D on DC topologies including ongoing work on a high-voltage DC connector under a Research Partnership to Secure Energy for America program and will be ready to support the development of DC solutions when the business case becomes clearer.
Continuing the evolution
The offshore industry has come a long way over the last 15 years or so, tackling and solving some of the major challenges presented by subsea processing.
Some of these achievements would not have seemed possible before—from the world’s first subsea separation and injection system in 2001 to the world’s first subsea gas compression system in 2015.
The industry is now at the point where it has the technology at its disposal, has developed the project execution experience and built a global pool of subsea processing experts. To continue this evolution, the industry needs to intensify collaboration, push the boundaries of what is possible and be prepared to do things differently. In this way, it can take the lead in making efficient and integrated subsea processing systems a new standard for the offshore sector.
Alisdair McDonald is with GE Oil & Gas.