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Back to Overview Why wind turbines are learning how to swim
Less material, easy assembly, suitable for deep sea waters – the technology boasts many advantages

The news sparked uproar across the globe: In mid-June, the regional government of the Canary Islands authorised the Norwegian energy company Equinor to erect wind turbines on floating platforms off the coast of Gran Canaria. The 200 megawatt (MW) project would not only be the world’s largest floating offshore wind farm but it would also be the first time that such a farm had been built and operated on such a large commercial scale. Hywind Scotland, currently the largest wind farm in the world and owned by Equinor, has a much lower total output of 30 MW. The Norwegian company plans to invest around 860 million euros in the project off the Spanish islands, which is scheduled to become fully operational in 2024.

“Floating technology can make new regions accessible”

Yet Equinor isn’t the only company relying on floating technology. Experts believe that wind turbines on floating platforms hold enormous potential. “Floating technology can make new regions accessible”, says Stephan Barth, Managing Director of ForWind, the Centre for Wind Energy Research at the Universities of Oldenburg, Hanover and Bremen. Wind turbines built on a fixed foundation can only be operated economically at depths of up to 60 metres. “But water depths near coastlines often increase rapidly – especially in Asia and the US,” explains Barth. The Japanese coastline, for example, has a steep drop, with the result that wind power plants with fixed foundations are not compatible.

However, according to WindEurope, formerly known as the European Wind Energy Association, in Europe around 80 percent of the area suitable for offshore wind power is in fact located above ocean depths of more than 60 meters. This is the case, for example, with the Atlantic coasts of France, Portugal and Spain. Not to mention the fact that wind speeds are higher and more regular the further the location is from the coast. As such, floating wind farms generate more electricity throughout the year and have a higher capacity utilisation than those with fixed foundations. According to an article by Power Technologie, an information platform for the energy industry, Hywind Scotland’s five wind turbines, which have been in operation since autumn 2017, have achieved a capacity utilisation of 65 percent and have thus significantly exceeded expectations.

Pilot plants in Asia and Europe

Until now, apart from the wind farm off the Scottish coast there has only been a modest number of floating plants, which are mostly pilot plants in Asia and Europe being used to test the technology with outputs in the single-digit megawatt range (this interactive map from WindEurope provides an overview). But this is set to change in the coming years. WindEurope lists more than 50 FOW projects (Floating Offshore Wind) worldwide, which are currently in various stages of development. In Europe, six wind farms and individual turbines are expected to go into operation in the next two years – including some off the coast of Great Britain, Spain and Portugal. These are pilot projects with a respective output of between 1 and 49 MW, which are to develop the technology further. But they are by no means market-ready.

The issue is that it simply costs an arm and a leg to get windmills to float. The rotor is usually more than 100 meters in diameter and weighs hundreds of tons. The beasts maintain buoyancy with the help of steel and concrete floats that can extend 80 metres into the depths of the ocean. Or alternatively: They stay afloat with the help of elaborate ballasting systems into which water is pumped back and forth.

“Until now, floating plants have been significantly more expensive than stationary ones,” explains wind power expert Barth. Although the floating farms use conventional turbines, their platforms are fabricated from custom parts which have to be specially made. These custom creations are manufactured in shipyards and can usually only be erected in particularly deep ports, as FAZ reports in an extensive article (in German). As such, the five Hywind Scotland turbines, for example, were in fact built and erected in Norway and then towed to their location off the east coast of Scotland, as the country doesn’t have a suitable port itself.

The costs are still too high

This is why the costs are much higher than for mass produced wind turbines, which are also becoming more and more common in the offshore sector. “Floating technologies are at the beginning of the learning curve,” says ForWind employee Barth, but the growing number of investments in this technology and the higher unit numbers mean the costs are likely to fall significantly over time, assures the expert.

One project has explicitly set its sights on reducing the costs of floating platforms and Essen-based company innogy is involved in the venture. The energy provider is planning on testing what is known as the TetraSpar Concept together with petroleum company Shell and Danish company Stiesdal Offshore Technologies A/S (SOT). The project focusses on a tubular steel support structure with a keel extending below, developed by Danish company SOT. The keel gives the structure the necessary stability so it doesn’t capsize in case of high wind speeds and strong waves. Similar concepts are already being applied to oil rigs, for example.

The Danish company "Stiesdal Offshore Technologies A/S" (SOT) installs offshore foundations for any water depth

Notably less material is used in comparison to other floating platforms, thanks to the tubular steel structure. Another major advantage is that the wind turbine can be assembled in the shallow waters directly at the edge of the quay, explains an innogy spokeswoman. The components of the platform are assembled in the Danish port of Greena and then lowered into the water. The turbine (a 3.6 MW offshore wind turbine from Siemens Gamesa) is then attached to the floating platform with a crane.

A tugboat then drags the floating wind turbine from Greena to the test site located about ten kilometres off the Norwegian coast near Stavanger. Once on-site, the keel is then lowered into its suspended position with the help of ballasts. At a depth of 200 metres, the structure is anchored to the seabed with three anchor chains and then connected to the power grid via a cable.

Simplified installation

The components are being manufactured this year and the plant is scheduled to go into operation in 2020. Around 18 million euros have been earmarked for the pilot project. “We hope that the design will give us a clear competitive advantage over other concepts, due in part to more efficient manufacturing, assembly and installation processes as well as lower material costs,” says the innogy spokeswoman.

Floating systems of this type also have the added advantage that the installation can be simplified, explains wind power expert Barth. When it comes to TetraSpar, the entire plant can be assembled in the port, whereas special ships are needed for wind turbines with fixed foundations. After the seabed has been extensively examined, these ships ram steel pipes deep into the subsoil. The tower, turbine and blades are then installed with the help of large crane ships. Operating these stationary turbines is very costly and is jeopardised by bad weather and high waves.

One thing is clear: Floating wind turbines could open up new regions for wind power and notably simplify logistics. However, in order to do so, they must become more affordable. Mass production of standardised components could make this feasible. Then floating wind turbines would soon be yesterday’s news.

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