World’s first true subsea wet-gas compressor eliminates pre-processing for wellstream fluids

The industry’s first wet-gas compression station will be installed 15 km from the Gullfaks C platform, to pump unprocessed wellstream fluids without pre-processing. Photo courtesy of OneSubea.

OneSubsea has introduced the world’s first true subsea multiphase compressor, which can handle unprocessed wellstream fluids, without the need for pre-processing. This capability will enable deployment of simplified subsea systems, at lower capital expenditures and field development costs. The advanced compression technology can handle high liquid content, without mechanical issues, with gas volume fractions typically in the range of 95% to 100%.

The multiphase compressor is a contra-rotating machine, designed specifically for pressure boosting of an unprocessed wellstream. There are no requirements for an upstream separation facility or an anti-surge system, which greatly simplifies the subsea system requirements. Developed in collaboration with Statoil and Shell, the subsea multiphase compressor allows for a compact, robust design, that is easy to install by use of light intervention vessels, and is based on more than 25 years of subsea boosting experience.

The main driver in the development of the multiphase compressor was to increase recovery rates and to cost-effectively lengthen the tie-back distances of subsea gas fields. Statoil’s Gullfaks field in the North Sea will see the world’s first commercial deployment for subsea wet-gas compression. Statoil foresees that the recovery rate will increase to 22 MMboe, by using the wet-gas compression system.

The wet-gas compression station for Gullfaks will be installed approximately 15 km from the Gullfaks C platform; commissioning is planned for 2015. Key advantages of the multiphase compressor, which enables cost-efficiencies and significantly improved operational performance, include:

• Continuous operation on all liquid fractions (0–100%)
• Operates in all flow regimes
• Standard fluid film bearing technology
• Surge-free operation, so no anti-surge system is required
• Sand- and solid-tolerant design
• Low speed allows for long step-out without subsea VSDs
• Low module weight (approx. 60 t) allows for installation with a light intervention vessel. 

During the third week of March 2015, OneSubsea officially handed over the subsea multiphase compressor to Statoil, preparatory to its installation at Gullfaks field. OneSubsea expects the device to achieve performance and reliability stats similar to the firm’s multiphase pumps, which run 15 years meantime between failures. The compressor is a 2015 OTC Spotlight on New Technology Award recipient (see also p. 162).