FEATURE: A floating future for wind

Mark Venables

(Credit: Statoil)
(Credit: Statoil)

When installed, the wind turbine will tower 178m above the North Sea, rising twice as high as the Statue of Liberty.

But it is not its height that makes this turbine special. It’s one of five emerging from the sea as part of Hywind Scotland – the world’s first floating windfarm.

“With floating wind, we’re trying to access deeper waters, particularly where there’s quite high wind speeds as wind is the fuel of the industry,” explains Rhodri James, a manager at the Carbon Trust.

According to the Energy Technologies Institute (ETI), floating turbines could account for almost half of the 55GW of power predicted to be generated by offshore turbines by 2050. “If offshore wind deployment reaches 40GW, the ETI expects that 8-16GW of this could be provided by floating wind technology, the majority of which would be in Scottish waters,” says James. There is also, he says, enormous potential for floating wind power in Japan and on Mediterranean and Atlantic coastlines.

Despite its promise, floating wind technology is still in its infancy, and is yet to be demonstrated at large scale. Several projections suggest that floating wind power can reach cost parity with fixed-bottom turbines during the 2020s if adequate support is provided by government. But the challenge for the industry is to optimise designs and build a commercial model that will deliver the economies of scale needed to drive down costs.

In search of deeper waters and stronger winds (Credit: Statoil)

In search of deeper waters and stronger winds (Credit: Statoil)

Anchored to the seabed

Hywind technology consists of a wind turbine placed on top of a ballasted cylinder. The turbines are anchored to the seabed by three mooring chains, connected to a suction anchor. The chains are close to 900m long and weigh 400 tonnes. The suction anchors themselves are 16m tall and 5m in diameter.

The Hywind Scotland windfarm, floating 25km off Peterhead on the east coast, will consist of five turbines, each rated at 6MW, which will provide renewable energy to 20,000 households. The objective of the pilot project is to demonstrate the cost efficiency and feasibility of multiple floating turbines in a region that has optimal wind conditions.

Floating wind power is not a new concept. The technology was first conceptualised by Statoil in 2001, a scale model was used to test the concept in 2005 in Trondheim, and Hywind Demo, the world’s first full-scale floating wind turbine, with 2.3MW output, was installed off the island of Karmoy, north of Stavanger in Norway in 2009.

“Through six years of testing in the North Sea, the concept has been verified, and performance exceeds expectations,” says Elin Isaksen of Statoil. The company has used the data from the demonstration to create simulation tools, which have been used to optimise the concept for future projects.

The experience has also been useful in optimising and controlling the motion of the floating turbines which, says Isaksen, will be increasingly important as their size increases. “Development work is also ongoing to improve and optimise the fabrication process, the mooring system and the installation methods, all with the aim of reducing the overall cost of energy,” she adds.

Since Statoil installed its single turbine off the coast of Norway in 2009, there has been another installed off Portugal, along with a few off Japan. “We’re now moving to the stage of the first arrays,” says James. “But it is not really new technology. It’s bringing together a lot of existing technology from other sectors, from fixed offshore wind turbines, offshore oil and gas and marine energy. Most of the floating platforms will be scaled-down versions of the oil and gas platforms we have seen over the past 20 years.”

It is expected that the windfarm in Scotland will be operational by the end of this year, with Statoil reporting that the project is on schedule. “Right now we are in the assembly phase,” said Isaksen in May. Since then, the turbines have been assembled onshore at Statoil’s Stord base on the west coast of Norway.

They were then lifted by Saipem 7000, the world’s second-largest crane vessel, onto the ballasted, floating sub-structure, leaving them standing in the water. In the summer, the five floating turbines were towed across the sea to Scotland for final installation with suction piles and mooring.

Scotland is particularly well placed to take advantage of the technology, says James, thanks to its rich experience in the oil and gas sector.  While countries such as Japan, the US and France are keen to develop floating wind power, the lack of either an established oil and gas sector or offshore wind industry may impede the rate of technology development and limit the cost reduction that can be achieved. By tapping into an existing knowledge base and benefiting from an established supply chain, Scotland can play a key role in supporting the global floating wind-power market.

Windfarms can unlock more power generation (Credit: Statoil)

Windfarms can unlock more power generation (Credit: Statoil)

Growing interest

Hywind is not the only floating wind project in the offing. Pilot Offshore Renewables is in the early stages of developing a windfarm to the south-east of Aberdeen off Scotland’s east coast.

Meanwhile, the Floatgen project, a consortium of seven European partners, is aiming to extend the development potential of offshore windfarms to deeper waters with more optimal average wind speeds. This will involve the construction and installation of a 2MW demonstration structure off the west coast of France at Le Croisic, at the Ecole Centrale de Nantes’ SEM-REV test site, by the end of this year.

This project is using a new approach called a ‘damping pool foundation,’ based on technology from one of the partners, Ideol. It is under construction in Saint-Nazaire harbour.

“The piston movement inside of the pool acts in opposition with the surrounding sea movements and stabilises the entire structure,” explains Thomas Choisnet, chief technology officer of Ideol. “The advantage of this is that it can be used in both shallow and deep waters; this is particularly interesting where excellent wind resources are coupled with challenging sea-bottom conditions.”

Fixed-bottom offshore turbines have seen increased deployment and dramatic cost reductions, and floating wind power is riding off that success. “Floating turbines can play quite a key role in accessing fairly strong wind speeds, particularly where they might be fairly close to shore,” says James.

Choisnet points to the limitless potential to produce renewable electricity with floating wind technology as a reason for optimism. “The market opportunities are huge: floating wind turbines can be installed on oceans worldwide, offering a solution to the problematic issues faced by countries where offshore bottom-fixed turbines are incompatible with the great depths off their coasts,” he says.

Content published by Professional Engineering does not necessarily reflect the views of the Institution of Mechanical Engineers

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