Following lengthy qualification trials in Norway, MAN Diesel & Turbo has been selected as the supplier of the subsea gas compressors for the Åsgard field. According to field operator Statoil, in comparative trials carried out at the operator's K-Lab testing facility at Kårsto, which began in 2008, a MAN centrifugal compressor was run successfully for over 3000 hours in an endurance trial. Following this, the compressor was fed with gas containing 1-5% by weight of liquids for 2500 hours, although it is believed the machine can handle significantly higher liquid volumes. Other machines considered for Åsgard included compressors from Siemens, GE and Dresser Rand.
‘Two 10MW MAN subsea compressors will be installed in a 70x40x20m high subsea station weighing around 2500t, and will deliver 21 million m3/d of gas, with electric power coming from Åsgard A,' says Torstein Vinterstø, Statoil's project manager for the compression project.
Startup of the compressor station is scheduled for October 2014, which could make Åsgard the first full-scale subsea compression project to come onstream. In December last year, Statoil awarded Aker Solutions a NKr3.4 billion EPC contract to deliver the complete Åsgard compression system.
Subsea compression has been identified as a more economical solution to the Åsgard Minimum Flow Challenge, compared to the installation of a new compression platform (OE December 2010). Åsgard has two subsea satellites, Midgard and Mikkel, located some 40-50km away in 240-300m of water, which produce gas and condensate tied back to the Åsgard B floating platform. Production has declined more rapidly than expected and by 2014 the fields' reservoirs will not have enough gas pressure to produce steadily. Gas pressure needs to be boosted to maintain stable production rates and a minimum gas flow is necessary to avoid the accumulation of MEG (hydrate inhibitor) in the flowlines – MEG flows from Åsgard A to Mikkel and Midgard and then back to Åsgard B. By installing subsea compression, Statoil intends to boost gas output pressure from the satellites to maintain production, and estimates that an additional 28 billion cubic metres of gas and 14 million barrels of condensate will be recovered.
The subsea station will incorporate two compression trains, with a common inlet scrubber and condensate pump, and two anti surge coolers. The compact horizontal MAN compressor has magnetic bearings, the motor and compressor are housed in the same casing, and the motor is cooled by the process gas. According to Vinterstø, Statoil will purchase three compressor modules, with one acting as standby onshore, and is targeting being able to replace a module in less than 10 days in sea states with significant wave heights up to 4.5m.
Two other subsea compression projects could be hot on Åsgard's heels. One is for Statoil's Gullfaks field for which a helico-axial multiphase compressor supplied by Framo has been under test near Bergen. The other is the much larger subsea compression station for Shell's Ormen Lange project, consisting of four 12.5MW trains (OE January 2010). The 12.5MW Ormen Lange Pilot, measuring 36x6.5x13m high and weighing around 1080t, is nearing the end of integration system testing at Aker Solution's Egersund yard where it was built, and, once disassembled into seven main modules, is expected to be on its way to Shell's Nyhamna onshore gas facility in February or March. There, the pilot, based on a vertical GE gas compressor, will be submerged in a 42x28x14m deep test pit and put through its paces on Ormen Lange gas and condensate for up to two years of testing.
In January, Aker Solutions' Egersund yard shipped a newly built 900t process module to Nyhamna which is designed to simulate a range of flow conditions, including slugs, to test the pilot. The Nyhamna test loop has capacity to deliver up to 15 million m3/d of gas, 1800m3/d condensate, 16MW of electrical power and 40-155 bar operating pressure.
As advanced as subsea compression for Ormen Lange is, it is still not a ‘done deal' as the solution for adding gas compression to the deepwater field in a few years' time. According to Bernt Granås, Shell's project manager for Ormen Lange, the alternative of a floating gas compression platform for operation in around 900m of water, is still on the table.
‘A floating platform remains the base case,' says Granås. ‘We have developed a conceptual design for a tension leg platform, which would have a 32,000t topsides consisting of five large modules.'
Whatever the final outcome for Ormen Lange, there is no doubt that the subsea compression ‘genie' is well and truly out of the bottle and here to stay. OE
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