Statoil is pioneering the world’s first subsea gas compression project to eventually boost production when needed on its flagship Asgard field in the North Sea.

The company is in the vanguard of a growing industry movement toward the general acceptance and implementation of various subsea processing solutions to improve recovery of oil and gas reserves from brownfield developments across the globe. These solutions include seabed separation, single and multiphase hydrocarbon boosting, raw seawater injection, sand handling, and produced water reinjection.

All of these solutions will play a key part in enabling the industry’s inexorable advance into deeper waters and more remote environments.

Gas compression is arguably the most important piece of equipment in the subsea toolbox as the industry positions itself to meet these future challenges.

A field schematic shows the planned layout of Statoil’s Esgard field

A field schematic shows the planned layout of Statoil’s Esgard field offshore Norway, which will feature a huge subsea compression station weighing 4,800 tonnes. (Images courtesy of Statoil ASA)

According to Statoil Senior Vice President for Subsea and Marine Technology Bj?rn K?re Viken, “Subsea compression is the most important single part of the subsea factory. My opinion is that we are not that far from realizing a subsea factory – Statoil is ahead on this. All the other operators are thinking along the same lines, but someone has to go in front. And when it comes to subsea compressors and subsea technology in general, we are in the lead. This is an important technology to have in our toolbox.

“But also importantly, this is being business-driven. We are not doing this from a technology point of view itself; it is connected strongly with the business case. It has to compete with other concepts.”

Statoil first officially confirmed it was opting for gas compression on the seabed for ?sgard in October 2010 before making the final investment decision last year.

So why opt for subsea compression? As any field ages, its natural pressure declines, meaning that compression is needed to extract more gas and get it to a platform. Until now, the standard solution has been for that compression to be done on a platform or from shore. However, the closer the compressor is placed to the wellhead, the more gas can be extracted because the compressor’s suction pressure will be lower than if it is installed on a platform. To achieve this, Statoil will place two massive compressors near the wells on Asgard that will provide enough pressure to transport the gas to the platform.

The Gullfaks 2030 and Ormen Lange pilot projects will eventually benefit from the technology; technology qualification and testing are under way at both developments. Asgard was not the first field to opt for subsea compression – another opportunity was assessed in 2008 by Statoil for its Tune field offshore Norway. Although there was a clear production acceleration and increased recovery effect, it was not possible to develop a sufficiently robust business case at the time. The knowledge gained from studying this opportunity for what was a relatively small field where a simple multiphase compressor system could be applied did not go to waste and is being taken further in the work being done on the Gullfaks subsea compression project.

According to Statoil, the Gullfaks South recovery rate already is 62%, and the combination of subsea compression and conventional low-pressure production in later phases could lift the total recovery rate to 74%. This would increase production by 3 Bcm of gas, or an additional 6%.

Statoil also has identified several other small- and medium-sized fields as potential candidates for the technology. But it is Asgard that leads the way, with the subsea compression system aimed at increasing pressure from the field’s Midgard and Mikkel reservoirs in Blocks 6507/11 and 6407/12, and 6407/7, respectively. This could enhance recovery by up to 28 Bcm and 14 MMbbl of condensate (around 278 MMboe), the company has estimated.

Two conventional compressor trains, each with an inlet/anti-surge cooler, will be used along with a common scrubber and condensate pump. The moisture will be removed from the gas before it is compressed on the seabed.

Subsea no longer necessarily means small, as this scale image from Statoil demonstrates.

Subsea no longer necessarily means small, as this scale image from Statoil demonstrates. The Esgard compression station will feature two 11.5 MW compressors, with the substantial power requirement to be supplied by a high-voltage electricity system from the Esgard A floating production platform.

The Midgard and Mikkel gas reservoirs were developed as subsea installations in the Halten Bank area of the Norwegian Sea, around 200 km (124 miles) offshore. The wellstream from the two fields, located 50 km (30 miles) and 70 km (43 miles) away, respectively, is sent in the same pipeline to the Asgard B platform. Current analysis shows that by year-end 2015, the fields’ pressure will become too low to avoid unstable flow and maintain a high production profile to the platform, with the low pressure unable to transport the gas.

For Asgard, qualification of components began in 2005, with work including the development of a suitable scrubber capable of handling solids as well as liquids.

Statoil said the application of the two massive 11.5 MW compressors, plus coolers, separators, and pumps in one subsea template, will involve an investment of almost US $2.6 billion. Much has gone into developing a compact compressor design, with motor and compressor housed in the same casing and motor cooling using the process gas.

A high-voltage electrical power distribution system will be provided via submarine cables from the Asgard A FPSO.

The $590 million contract for the design and construction of the gas compression system is being carried out by Aker Solutions, which is also doing the modifications on Asgard to provide electricity to the system.

The compression station (4,800 tonnes) and manifold station (900 tonnes) will be installed in 250 m to 325 m (820 ft to 1,066 ft) water depth by Italy’s Saipem, which also will carry out the lifting of a new module onboard the Asgard A FPSO.