Having successfully completed testing of the world’s first subsea gas compression system on the Ormen Lange Pilot project offshore Norway, Anglo Dutch major Shell and its partners are now gearing up to kick start the process of simplifying the total system that is seen as the next step for the technology, GE Oil & Gas said.

The Ormen Lange Pilot has been undertaken by Shell and its partners Petoro, Statoil, Dong and Exxon Mobil since 2011 at Shell’s test facility at Nyhamna in Norway where gas from the Ormen Lange Field reaches shore.

Alisdair McDonald, head of GE Oil & Gas’ Subsea Power & Processing business, talked to SEN about the next step for the key technology following the testing stage.

“Subsea compression technology is ready for subsea use. The drive in the industry is now to simplify the total subsea compression system, and making the compressor more robust and tolerant to liquids is a key part of this,” McDonald said. “GE is currently qualifying a liquid tolerant compressor, which will be an important step in this direction.”

In terms of how much the system costs and how applicable it is for use on development projects on the Norwegian Continental Shelf (NCS) and across the world, McDonald said, “The cost will be very dependent on field size and requirements (number of compressors, step-out, etc.). Subsea compression may also be economic for smaller developments given that the power system infrastructure around is beneficial and the reservoir properties are suitable.

“The main benefit of subsea compression systems is increased recovery. For many fields the challenge is that the wellstream does not have enough pressure to push through the pipeline, especially when reservoir pressure declines,” he said. “The compressor basically helps the gas stream flow, allowing the well to produce more easily.”

When asked about the future for subsea compression systems, McDonald noted, “Subsea compression is definitely an important part of the subsea puzzle in the future, but not for all fields. It will depend on the reservoir and on the infrastructure in the area. As the systems become less complex and the technology is adapted in the market, we will see more subsea compression developments in the future.”

GE Oil & Gas was heavily involved in the program and said the test produced “a full subsea power supply, transmission and distribution system that further advances the development of hydrocarbon processing on the seabed.”

“The Ormen Lange Pilot was a first of its kind and was designed to test a full-scale integrated subsea compression system in submerged conditions with real hydrocarbons,” GE said.

The system allows operators to conduct gas compression on the seabed, reducing the need to introduce additional power generation on nearby offshore facilities.

Statoil Startups

Prior to the end of the pilot, Norway’s Statoil saw the potential of the technology and became the first company to apply subsea gas compression with two project startups in late 2015.

In October 2015, Statoil and partners Petoro and OMV started the world’s first wet gas compression on the seabed of the Gullfaks South Field off Norway.

The use of this technology will increase recovery by 22 MMboe and extend plateau production by about two years from the Gullfaks South Brent reservoir, Statoil said.

“Subsea processing and gas compression represent the next generation of oil and gas recovery, taking us a big step forward,” said Margareth Øvrum, executive vice president for technology, projects and drilling at Statoil.

Gullfaks South is a satellite field linked to the Gullfaks C platform. Gas is compressed in 135 m (443 ft) of water, raising pressure in the pipelines and accelerating gas flow to the platform.

The Gullfaks system provides a multiphase compression model for smaller fields, where simplicity and relatively low-boost pressure drive investment decisions.

In field-life extension efforts on Gullfaks, Statoil worked with OneSubsea and other suppliers to qualify a helico-axial multiphase compressor technology. Following successful onshore compressor testing in 2011, Statoil and its partners selected subsea compression as the preferred concept—a giant leap forward for the technology.

The main components include inlet coolers, a multiphase mixer unit and two multiphase compressors. Three additional flowline hubs are available to tie-in future Gullfaks satellite fields.

In September 2015, Statoil also started the Åsgard subsea gas compression.

During 2013, the Åsgard system involved one subsea template containing two parallel compressor trains installed in 300 m (984 ft) of water. As of September 2016, Train 1 had been in operation for one year and Train 2 for nearly eight months.

The more sophisticated Åsgard model is best suited for large fields or wells with a great step-out distance requiring a larger pressure boost. Åsgard’s subsea system involves gas-scrubbing equipment upstream of centrifugal dry gas compressors.

At Åsgard, gas needs to be boosted from the Mikkel and Midgard satellite fields to the Åsgard B platform to maintain stable production rates and avoid accumulation of mono-ethylene glycol in the flowline.

Gas production from the Midgard and Mikkel reservoirs is about 40% higher than before subsea compression started. Field life was extended by many years, Statoil added.

Placing a compressor closer to the well helps maximize production rates and energy efficiency. Subsea compression also requires relatively little supporting steel and operating personnel, giving it a low physical footprint compared with compression done on a platform. Statoil is considering the use of this technology on its other properties, both in Norway and elsewhere.

“The two projects are the first of their kind worldwide, and represent two different technologies for maintaining production when the reservoir pressure drops after a certain time,” Statoil noted.

“Subsea compression has a stronger impact than conventional platform-based compression. It is furthermore an advantage that the platform avoids increased weight and the extra space needed on the platform for a compression module.”

—Steve Hamlen

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