Powering up for Subsea Compression
Monday 19 September 2016
Many years in the works, subsea gas compression technology made its commercial debut off Norway last year, first at Statoil’s Aasgard field and, just months later, in a system for wet gas compression at the Gullfaks development.
By placing the boosting equipment on the seabed, closer to the subsea wells, Statoil says it will be able to recover significantly more gas from its reservoirs. The technology also eliminates the need for major platform modifications to accommodate topsides compression modules.
The Shell-operated Ormen Lange field, also off Norway, was a strong contender to be the first subsea compression site. In 2006, the field partners, led at the time by Hydro, asked GE to supply subsea compression for the dry gas development, which ties back to an onshore processing plant at Nyhamna. In 2009, GE introduced the Blue-C subsea compression unit, a marinised, vertically oriented version of its gas compression technology. The unit, along with a subsea power distribution system, was installed in a test pit at Nyhamna in 2011.
In 2014, halfway into the four-year pilot programme, Shell and the Ormen Lange field partners, citing economic constraints, announced that a concept selection for offshore compression, either subsea or platform-based, would be postponed and other options considered.
“The Ormen Lange licence group believes in the subsea compression technology, and still regards the qualification of this technology to be an important stepping stone for the Ormen Lange future development alternatives,” the chairman of the project’s management committee, Odin Estensen, said at the time, adding that subsea compression technology “is a key contributor for ongoing and future field developments on the Norwegian continental shelf.”
Test success
Ormen Lange lies 120 kilometres north-west of Kristiansund in water depths of 850 to 1100 metres (2800 to 3600 feet). Aasgard, by contrast, is in depths of 300 metres (990 feet) and lies 200 kilometres off the coast. The subsea compression system receives power from the Aasgard A platform 40 kilometres away.
“What was unique about Ormen Lange versus Aasgard was the step-out distance,” says Alisdair McDonald, head of subsea power and processing at GE Oil & Gas. “The gas compression station was to be about 120 kilometres offshore from the Nyhamna facility. Due to that step out distance, it wasn’t possible to have topsides variable speed drives, so the decision was made to investigate the potential for subsea power electronics.
“That’s where GE and the power conversion team collaborated with Shell and the licence to develop that technology,” he explains. “That gave us the possibility to put the compressor 120 kilometres from shore.”
The Ormen Lange pilot programme “had about 90%-plus GE technology”, McDonald says: the Blue-C, which he describes as “the world’s first specifically designed subsea centrifugal gas compressor”, along with a subsea power system that included variable speed drives (VSDs) and subsea switch gear.
“This was to be the world’s first subsea gas compression system with a subsea power supply, transmission and distribution system,” he says.
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