Subsea Compression Technology Surfacing

photo of subsea station
A conceptual design of GE’s subsea compression system. According to GE, this technology is more easily applied to larger gas fields, but the potential is there for smaller fields as well, thanks to the ongoing evolution of the technology and its increasing competitiveness.

This article was originally published in the April issue of COMPRESSORtech2. Get every issue in your inbox/mailbox and access to our digital archives with a free subscription.

Editor’s Note: COMPRESSORtech2 talked with Alisdair McDonald, Subsea Power & Processing business leader at GE Oil & Gas in Aberdeen, Scotland. McDonald shared some insight in GE’s technology for subsea compression and the related market going forward.

COMPRESSORtech2: How long has GE been pursuing subsea compression technology?

McDonald: GE began working on subsea compression in 2005 with Norsk Hydro, and latterly with Norske Shell AS, as the main technology provider for the Ormen Lange Pilot. This was to be the world’s first subsea compression system tested with a full subsea power supply, transmission and distribution system. The compression and power system was successfully tested for more than 4000 hours over a period from 2012 to 2015 in a submerged environment at Shell’s Nyhamna facility in Norway. The compressor, GE’s Blue-C, was the first of its kind and was delivered to the Pilot in 2010.

For the Ormen Lange Pilot, the Blue-C was configured to deliver 16,762 hp (12.5 MW) of power (tested up to 119,310 hp [14.4 MW] during the Pilot operation) but other motor sizes are available that enable the Blue-C to be configured from 5000 to more than 27,000 hp (4 to 20 MW).

What makes the Blue-C compressor suitable for underwater operation?

The Blue-C is a high-speed centrifugal compressor with a high power density. It is specifically designed for subsea operation with a vertical orientation to minimize subsea footprint and ease the draining of liquids. The compressor uses a high-speed induction motor supplied by GE’s Power Conversion business, operating at more than 10,000 rpm. The motor by Power Conversion comes from the same motor family used in GE’s topside Integrated Compressor Line (ICLs).

The Blue-C is a truly integrated compressor and uses active magnetic bearings to levitate the rotor to avoid friction. Process gas is used to cool the motor and avoid the need for external cooling loops subsea.

BlueC Compressor
The Blue-C compressor by GE Oil & Gas in a section view. It is a high-speed centrifugal compressor specifically designed with a vertical layout for subsea operation. It utilizes active magnetic bearings and process-gas motor cooling.

Did you encounter any surprises when GE was in the testing process for Blue-C ?

GE completed the testing of the Blue-C compressor system at its Nyhamna facility in Norway at the end of 2015. The original intention of the pilot was to test with the same dry gas conditions as anticipated in the Ormen Lange field. However, during the test program, the compressor experienced wet gas conditions up to 5% liquid mass fraction (LMF). These unexpected conditions confirmed the inherent robustness of the Blue-C’s design to wet gas.

Creating a compressor capable of handling wet gas has other benefits; it removes the need for an upstream separator or scrubber and eliminates other ancillary equipment such as condensate pump and its adjustable speed drive. In addition, the technology developed for wet gas adds robustness and reliability to dry gas compressors.

GE is currently completing qualification of a topside wet gas compressor at Statoil’s K-Lab in Karstø, Norway as part of a joint industry program between GE, Statoil and Chevron. The new wet gas compressor includes a novel impeller design to tolerate wet gas conditions. The design includes changes in geometry and material and uses coatings technology from GE’s Aviation business — a coatings technology normally used to protect the engine blades of aircraft used in extremely harsh desert conditions. This sharing of technology across different industries is a strong example of the “GE Store” — the global exchange of technology and knowledge across GE’s businesses that helps drive innovation. At the end of the qualification program, the new wet tolerant design will be available for both topside and subsea operation.

At what point in the extraction process does subsea compression become a good option?

Gas reservoir pressures typically plateau and decline over a number of years to a point where there is insufficient pressure or flow rate to produce back to the host. Subsea compression is typically applied just after plateau decline. The advantage of subsea compression is that it can be placed much closer to the wellhead, increasing efficiency by reducing the impact of pipeline pressure drop. In general, we see subsea compression being applied mainly to larger gas fields where the economic benefits are more easily realized. But there is potential to extend this to smaller fields, as the technology continues to evolve and becomes more competitive.

Are you seeing interest from customers? What are their concerns?

More recently, we have seen an increase in activity related to several large compression opportunities and we are actively engaged in supporting with GE’s compression solutions.

The most common concern voiced by operators is that the technology has a limited track record. The Ormen Lange Pilot was the first subsea gas compression project and now Åsgard subsea compression has been installed. However, confidence among operators is increasing based on the success of these projects, and operators are prepared to seriously evaluate the potential offered by subsea compression compared with conventional compression from onshore or floating structures.

What are the best arguments for more subsea compression?

Subsea compression technology has evolved considerably since the Ormen Lange Pilot and Åsgard to the extent that system designs are now 50% smaller and lighter, improving the economic and technical viability of many large compression projects. GE is playing a key role in that evolution process as both a systems integrator and as a complete systems technology provider. At GE, we have the advantage of having almost all the key technology in-house — another aspect of the GE Store. Our scope of supply not only includes the Blue-C compressor, but it extends to the power system, system design, high voltage connectors, process connectors, electric actuators, controls systems and monitoring and diagnostics. Being able to design a system predominantly based on GE Store technology gives us the freedom to collaborate across the businesses without some of the traditional commercial or technical interface challenges that can be faced by conventional systems integrators.

What is GE’s investment forecast for subsea compression?

GE has invested significantly in subsea compression technology and system design over many years and will continue to push the boundaries of this exciting technology. GE’s wet gas compression program with its partners, Statoil and Chevron, is an example of how GE is partnering with customers to develop solutions that meet their specific needs and of the continued investment we are making to develop systems that bring more value.

GE expects the uptake of subsea compression to increase as the industry becomes more familiar with the technology, its benefits and the level of perceived risk.

What is clear is that technology and systems thinking is driving a step change in the economics to make subsea compression more attractive than ever. In the end, the path taken may be similar to that experienced by subsea boosting, which took several years before becoming embedded in the operators’ toolbox.