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Energy
| 08/02/17

Floating offshore wind: more power, better reliability

TCE

The world’s first floating wind farm is being assembled off the northeast coast of Scotland, putting in place a technology that could significantly boost wind energy’s attractiveness by extending its reach to deeper waters, where wind is more powerful and more reliable.

The $332-million CAD (£190 million) wind farm developed by Norway’s majority state-owned oil and gas giant, Statoil, is called ‘Hywind’ and is sited some 25 kilometres off Scotland’s northeast coast, a location where the sea floor is far too deep for conventional bottom-anchored offshore wind turbines.

The further out one goes, the higher the speed and less variable the wind becomes.

When completed, the 30-megawatt project, capable of serving 20,000 homes at peak operating conditions, will comprise five vast wind turbines. Each will stretch 175 metres above the sea and deploy a 78-metre ballast below the water along with three mooring lines attached to the seabed.

The turbines will be spread out across four square kilometres at locations where the seabed lies at a depth of between 95 and 120 metres. Statoil says the technology can site turbines at locations up to depths of 700 metres, and is bullish that still greater depths are achievable. For comparison, conventional offshore turbines can only be sited at depths up to 40 metres.

Much effort has been put into software that help the tower remain upright by twisting the blades to counteract motion from waves, currents and the wind itself.

There are three main design options for floating offshore wind turbines (FOWTs): ballast-stabilised or spar-buoy, tension-leg platform (TLPs), and semi-submersible (see image for details). Spar-buoys, the design favoured by Statoil, are steadied by heavy ballast tanks low in the water. Tension-leg platforms decrease the motion of the turbine in response to sea conditions by tensioned mooring lines. And semi-submersible turbines are stabilised by three to five buoyant cylinders that are partially submerged and again moored to the sea bed, but without tension.

Study of which design is best for a particular location has increased in recent years, and Canadian researchers are among those investigating the options, balancing designs’ optimum energy harvesting with cost. Little research however has been performed that explores the full range of platform design classes. PICS is currently supporting such work, headed up by engineering researcher Meysam Karimi. His paper published last December in the Journal of Ocean Engineering and Marine Energy aimed to discover which platform provided the turbine with maximum stability at minimal cost.

TLPs and semi-submersibles with three outer cylinders turn out to be the best options up to a cost of $4.5 million, not the spar-buoy design favoured by Statoil and a number of pilot projects. Above this price point, TLPs are the optimal platforms, but they achieve only modest performance improvements with exponentially increasing costs. The next steps in Karimi’s research will be to develop a new dynamic model to evaluate an entire farm of FOWTs, and then to more robustly compare the costs of to other clean energy technologies.

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Floating offshore turbines could make wind energy more attractive via locations where wind is less variable

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