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Offshore oil and gas technology aids deep-water wind turbine project

06 October 2010

Statoil’s innovative new offshore floating Hywind wind turbine, now successfully moored in the North Sea off the Norwegian coast, is demonstrating how syntactic foam buoyancy technology is contributing to the future of offshore power generation

“It is subsea technology that has already been extensively proven,” comments Trelleborg Offshore’s Gary Howland, as he describes his company’s distributed buoyancy modules (DBMs), originally developed for deepwater support of umbilicals and risers in the oil and gas industry, and which now are helping to reduce project risk in the area of offshore floating wind turbines.

“People see the tower and turbine, but forget that the expertise in designing the subsea portion is also critical, as ultimately it keeps the whole turbine afloat,” he explains. Statoil’s dynamic floating structure weighs 5,300 tons and is 165m tall; with just 65m of this showing above the sea surface. The 13km of power and communications cabling attached to this impressive structure further adds to its weight.

Like the proverbial iceberg, the mass floating below the surface ensures stability. But unless the weight is supported by properly designed buoyancy the whole structure would be much less able to resist the extremes of the offshore environment and cables connected to it are likely suffer premature damage.

For the Hywind project, Trelleborg Offshore designed and supplied 45 polymer-coated syntactic foam DBMs, which provide buoyancy support for a three-ton, 100m section of cable as it exits the turbine spar and descends to the sea bed at a depth of 220m. The buoyancy modules include an internal clamp, which ensures precise positioning and secure fastening on the cable. The cable is supported mid-water in what is known as a ‘Lazy Wave’ configuration, featuring gentle long radius curves that minimise stress while accommodating natural movement created by wind and waves, as Mr Howland explains:

“For optimum buoyancy under different sea conditions, the precise position of the buoyancy modules on the cable is pre-calculated. The positions must be maintained, despite stresses during launch and in operation. The clamp is crucial; the design, material selection and manufacturing technique are critical to ensuring that the finished clamp maintains the buoyancy module position during cable contraction and expansion over the 20 year lifetime of the project.”

Trelleborg Offshore has in excess of 35,000 DBMs in service around the world. Having faced the technical challenges set by the offshore oil and gas industry, the company is confident that it can quickly and easily adapt its proven technology for use in these far-offshore wind power generation projects. “This will greatly reduce project risk and make development of the industry far quicker and less costly,” Mr Howland concludes.

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