Statoil’s ambitions to create the first “subsea factory” are about to be realized as the Gullfaks and Åsgard subsea gas compression projects offshore Norway reach the end game.

The first subsea wet gas compressor in the world has been installed at the Gullfaks C platform in the Norwegian North Sea, and commissioning is underway.

The structure and module installation campaigns for the project were completed by Subsea 7 in May and June, and the compressors were installed at the end of June. The protective structure and compressor station were installed in early May by the heavy-lift vessel Oleg Strashnov. The subsea infrastructure will be tied back to the Gullfaks C platform in late summer and autumn, and the compressor will be hooked up between the L and M subsea templates and Gullfaks C. Provisions also have been made to tie in other subsea wells to the compressor through existing pipelines.

“This is the first compressor of its kind in the world. It is a milestone, not just toward the compressor startup but also for Statoil’s subsea factory visions,” said Steinar Konradsen, owner representative for the project.

More than 50% of Statoil’s equity production comes from subsea, and the company has 540 subsea wells in operation.

“On Gullfaks, we will increase recovery by 22 million barrels of oil equivalent,” said Margareth Øvrum, Statoil’s executive vice president for technology. “I think the combination of large gas fields and long tiebacks of subsea wells will create large demand going forward for subsea gas compression. We are looking for more implementation opportunities.”

Statoil believes subsea gas compression is an important technology leap for increasing the recovery rate and lifetime of the Gullfaks South Brent reservoir. Recovery levels will be increased from 63% to 73% using the new technology combined with conventional low-pressure production in a later phase.

System design
The compressor system, which comprises two 5-MW wet gas compressors that together can handle a flow rate of 10 MMcm/d (353 MMcf/d), has been connected to existing subsea templates and piping that are 15 km (9 miles) from Gullfaks C. From the compressor station, a power and umbilical cable will be tied back to Gullfaks C. Power and management modules will be integrated on the Gullfaks C platform.

The OneSubsea Multiphase Compressor being used on the project is the world’s first subsea wet gas compressor with no requirements for an upstream separation facility or an anti-surge system, which simplifies the subsea system requirements. The compressor is a contra-rotating machine specifically designed for pressure boosting of an unprocessed wellstream. The multiphase compressor is capable of handling high liquid content without mechanical issues, with gas volume fractions typically in the range of 95% to 100%.

Some of the key advantages of the multiphase compressor, which enables cost efficiencies and improved operational performance, include continuous operation on all liquid fractions (0% to 100%); operation 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, allowing long step-outs without subsea variable speed drives; and a low module weight (about 60 mt), allowing light vessel intervention.

Alongside the subsea operations, much of the preparation work for the startup of the subsea compressor has been made on the Gullfaks C platform. The work, being carried out by Apply Sørco, is in the final stages.

Åsgard subsea installation
As work progresses on Gullfaks, Statoil also is installing the modules for the Åsgard subsea gas compression project.

The project will boost falling gas pressures from the Midgard and Mikkel satellite reservoirs, allowing stable production to continue and 280 MMboe to be recovered. Statoil expects the technology to boost recovery rates on Mikkel to 84% from 59% and on Midgard to 87% from 67%.

Twenty-two modules are being installed and connected in 300-m (984-ft) water depths. These comprise two identical 11.5-MW compressor trains weighing 1,500 mt each. The installation work is being carried out by the North Sea Giant vessel, which was rebuilt for the purpose.

The modules are being put in place in the large subsea frame that was installed on the field in summer 2013. The smallest modules, which weigh up to 70 mt, are being installed via the vessel’s moonpool.

Several larger modules will be installed from the North Sea Giant’s crane system, which has been modified to incorporate a special handling system.

This lifting system is designed to carry a load of up to 420 mt and can operate in up to 9-m-high (29-ft-high) waves. Each module is lowered into the sea via a crane and guided into place with an ROV and cables.

“This crane system makes the installation work safer and more efficient,” said Torstein Vinterstø, project director. “To ensure quality at every stage of the process, all operational crew have practiced the operations in a specially designed simulator. We have also performed a number of sea trials prior to installation.”

The installation sequence has been carefully planned so that startup work can be carried out on the first compressor train while work to install modules on train two continues, Vinterstø said.

A dry-gas compressor system is being used on Åsgard, with gas and liquids separated before boosting. The liquid is boosted by a pump and the gas by a compressor. After boosting, gas and liquids are mixed into the same pipeline before transport to Åsgard B. Statoil started testing the technology at the Kårstø laboratory as early as 2008.

“Åsgard subsea gas compression with its two trains, separators, mixers, pumps, spoolers and compressors is, for all practical matters, our first subsea factory in place,” Ovrum said. “We have qualified 43 different technologies in this project, and that is amazing.

“Subsea gas compression is no doubt a crucial technology and a key delivery to our mission of the subsea factory,” she continued. “With subsea compression we now have all the vital elements for a subsea factory in place.

“We can control the production flow. We can separate oil, gas and water on the seafloor, and we can inject water back. We still need to further develop and refine several elements, for instance, extended-reach power supply and much better oil and water separation. We will take subsea technology longer, deeper and colder,” she added.

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