A week ago, Statoil AS, Norway’s largest energy company, revealed plans to build a demonstration site testing floating offshore wind turbines off the coast of Scotland. It’s a move that rubber-stamps the industry’s gradual shift to floating platforms, but is the technology up to the task. PES takes a look at the various engineering challenges.
Statoil is also considering Norway and the US state of Maine to test the commercial potential of its “Hywind” project, and it aims to build three to five Hywind machines at the site, when selected. A 2.3-megawatt prototype 10 kilometres offshore at Karmoy in Norway has been working “beyond expectations” at waters 200 metres deep, using a Siemens AG turbine and floating technology provided by French company, Technip SA.
The vision for large-scale offshore floating wind turbines was introduced by Professor William E. Heronemus at the University of Massachusetts in 1972, but it was not until the mid 1990s, after the commercial wind industry was well established, that the topic was taken up again by the mainstream research community. Current fixed-bottom technology has seen limited deployment to water depths of 20 m, but as the technology has advanced into deeper water, floating wind turbine platforms may be the most economical means for deploying offshore wind turbines at some sites.
Technically, the long-term survivability of floating structures has already been successfully demonstrated by the marine and offshore oil industries over many decades. However, the economics that allowed the deployment of thousands of offshore oilrigs have yet to be demonstrated for floating wind turbine platforms.
For deepwater wind turbines, a floating structure may replace driven monopoles or conventional concrete gravity bases that are commonly used as foundations for shallow water turbines. A floating structure must provide enough buoyancy to support the weight of the turbine and to restrain pitch, roll and heave motions within acceptable limits. The turbine design philosophy for floating may be impacted if platform dynamics require a more dynamically compliant machine but the platform costs are likely to dominate the cost tradeoffs. Therefore, it is assumed that the economics of deepwater wind turbines will be determined primarily by the additional costs of the floating structure and power distribution system, which are offset by higher offshore winds, close proximity to large load centres (e.g. shorter transmission runs), and greater public acceptance due to lower visual and environmental impacts.