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Floating turbines: Taking wind power to uncharted waters

  • Green Economy Society
  • Jul 30, 2020
  • 3 min read


The last decade saw incredible growth in the offshore wind industry, with global installed capacity now over 28 gigawatts – a staggering 13 times more than in 2009 (see figure 1). This has been driven by the need for zero-carbon energy, of which the strong and consistent winds at sea are a huge potential source.

However, the continued expansion of offshore wind faces a major problem in deep waters. At greater depths, it becomes increasingly expensive to construct turbine towers from the ground to sea level. So far, offshore turbines have been mainly constructed in shallow waters, but 80% of potential offshore wind locations are over 60m deep (Musial, 2020). In these waters, fixed-bottom turbines are no longer economically viable. A new approach is needed: floating turbines.

Currently, a variety of different floating platform designs are being pursued, the three main ones being: spar buoy, semi-submersible and tension leg platforms (see figure 2). These designs have all seen use in the oil and gas industry, so have all been shown to be feasible. One of the biggest challenges to implementing floating wind power on a wide scale is finding and optimising the best design for turbines.


The spar buoy uses a tall, buoyant substructure to support the turbine. This is then weighted down at the base for stability and moored to the seabed to prevent it from drifting. As this design sits so deep in the water, it is much less affected by wind and waves at the surface. However, this also means that the design is only viable in very deep water - typically at least 100m (IRENA, 2016).

Equinor, a Norwegian company, is at the forefront of spar buoy wind power development. They have experience of the technology from offshore oil and with total assets of $118 billion, are in a strong position to maintain this lead. Their recent project, Highwind Scotland (see figure 3), is the world’s first full scale floating wind farm. It started electricity production in 2017 and has since been achieving significantly high capacity factors – greater than any other offshore wind farm in the UK (Smith, 2020).


Forecast to become the market leader by 2022 (Hannon et al, 2019), the semi-submersible platform design is both cost-effective and versatile. It has a similar design to the spar, but instead of a single large and deep pontoon, it uses multiple smaller ones. These are arranged around the turbine to provide a wide base, which gives it static stability and means less base-weighting is required. The structure sits high in the water, which makes assembly, transportation and installation all possible in shallower waters. That said, having so much of structure near the surface also means it is greater affected by wind and waves – which becomes more problematic in the rougher waters further out to sea.

The tension leg platform again uses a buoyant substructure to support the turbine, but unlike the other designs, it is stabilised by mooring cables. This uses less material (as no base-weighting is required) and reduces wind and wave induced motion, but poses assembly challenges, as the design is inherently unstable until the cables are attached. Additionally, their feasibility is limited by seabed conditions, as the ground must have sufficient strength to resist the large upwards forces from the cables. These challenges partly explain why only one tension leg turbine has been constructed to date – a demonstration scale turbine, which operated for a year in 2008 (Hannon et al, 2019). However, companies like GICON are looking to overcome the challenges, and are working with universities and research groups across Europe on the design (GICON, 2018).

The next decade will likely see a mixture of all three designs being built in different situations, due to each turbine’s unique set of challenges. While floating wind is currently expensive, the cost is forecast to fall as more projects are completed and investment risk decreases. Industry experts predict floating wind will be price competitive with fixed-bottom offshore and natural gas by 2030, at about $0.05 per kWh (Musial, 2020). More innovation will certainly be occurring over this period too. Projects on multi-turbine floating platforms, platforms that also use wave or solar power generation, and better optimised turbine designs are just a few of the exciting things on the horizon for offshore wind.

By Alex Westwood

References:

Hannon, M. Topham, E. MacMillan, D. Dixon, J. Collu, M. (2019). Offshore wind, ready to float? Global and UK trends in the floating offshore wind market. https://strathprints.strath.ac.uk/69501/13/Hannon_etal_2019_Offshore_wind_ready_to_float_global_and_uk_trends_in_the_floating_offshore_wind_market.pdf

International Renewable Energy Agency (2016). Floating Foundations: A Game Changer For Offshore Wind Power. https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2016/IRENA_Offshore_Wind_Floating_Foundations_2016.pdf

Smith, A. (2020). UK offshore wind capacity factors. https://energynumbers.info/uk-offshore-wind-capacity-factors

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