The objective

The giant Ormen Lange Field is located 120 km [75 mi] off the Norwegian coast in 900 m [2,955 ft] of water. Natural gas production started in 2007, and at its peak, the field supplied about 20% of the UK's gas requirements, playing a key role in reducing the amount of coal in the country’s energy mix.

From the beginning, Shell and its license partners were interested in using subsea instead of topside compression to boost production. Compression is a mechanism for transferring energy to the production stream to overcome losses during extraction and transport. Compared with the option of lowering the near-end pressure using topside technology, compressing gas near the start of a long production flowline (where gas pressure is higher) is significantly more energy efficient—with correspondingly lower CO2 emissions—over the productive life of the asset. However, the technology’s high costs and lack of maturity made it unfeasible at the time.

The solution

Interest was rekindled in 2017, when OneSubsea commissioned the world’s first subsea multiphase compression system in Equinor’s Gullfaks South Field in the North Sea. The system has demonstrated a high level of reliability and reached Technical Readiness Level (TRL) 7 in early 2019.

OneSubsea’s award-winning subsea multiphase compressor is the world’s first and only true subsea wet gas compressor. Conventional compressors require perfectly dry gas to operate safely and efficiently. Complex equipment is necessary to eliminate any liquids and pump them separately, which makes the marinization of conventional technology complicated and expensive.

The subsea compression system from OneSubsea, on the other hand, can operate with the unprocessed multiphase well stream—including condensates, produced water, and MEG—with liquid fractions ranging from 0% to 100%. It also tolerates sand and solids. As a result, the size and weight of the solution decreases by about 60%, with a corresponding reduction in cost. Fewer components also mean increased reliability. In addition, core components, such as the motor, bearings, and seals, are part of OneSubsea’s proven boosting portfolio, which has delivered more than 4 million hours of operating time. All these factors led to Shell awarding an integrated EPCI contract to Subsea Integration Alliance; OneSubsea will supply the compression system while its partner, Subsea 7, will perform the installation.

The results

The system includes two compression stations, each containing two compression modules. Each compressor can provide up to 50-bar differential pressure (DP), and they are initially configured in parallel for a large volumetric flow capacity. As production declines, higher DP and compression ratio become beneficial, and the compressors can then be easily configured in series, enabling up to 100-bar DP.

The record-breaking 32-MW subsea multiphase compression system will use onshore VSDs located 120 km away, an industry first. Eliminating subsea VSDs or a topside floating alternative improves project economics. The solution’s technical feasibility was validated using OneSubsea’s state-of-the-art umbilical simulator and multiphase flow loop. Installation is scheduled for completion in 2024.

The recovery factor is expected to increase from 75% to a world-class 85%, with up to 30% (up to 50 billion m3) of additional gas production. An added benefit is the estimated reduction in power consumption of >50% per unit volume of gas produced, compared with a conventional topside solution. Once the system is commissioned, Subsea Live data-driven performance service will use remote monitoring and data analysis to optimize subsea asset performance through AI, automation, and OEM expertise, providing prescriptive health, operational, and production insights.