Float on – stabilising offshore wind turbines

Following the installation of the world's first offshore wind turbine in 1993, the past two decades have seen the unchecked rise of the monopole as the foundation of choice for wind farm operators.

Today, the vast majority of turbines sited in water less than 25m deep use these giant steel pipes, up to 6m in diameter, to support the nacelle and blades. Anything in deeper waters, between 25 and 60m, has seen a variety of solutions, such as lattice steel jackets or the new generation of extra-large monopiles - up to 10m wide and used in waters more than 40m deep.

Fixed foundations are only capable of so much, however; any larger, and monopiles can start to warp under their own weight during transportation and installation, and their sheer size presents new challenges for ports, ships and lifting infrastructure.

The eventual move to even deeper waters will see the abandonment of these tried-and-tested designs and a move towards floating substructures.

The field is still in its infancy: there are two full-scale designs operating in European waters, Hywind and Windfloat, plus a host of other designs in the planning phase and a number of other smaller experimental designs.

On the 'Hy' seas

Hywind, the first floating foundation in the world, was designed and built by Statoil using a 2.3MW Siemens wind turbine, and has been operating off the coast of Norway since 2009. It employs a slender, cylindrical subsurface design, anchored to the seabed using three mooring lines.

When first covered in the inaugural issue of World Wind Technology in 2010, the design was one year into its planned two-year initial test phase. Statoil's then-vice-president for wind power, Anne Strommen Lycke, told me at the time: "The Hywind design is fully adaptable to any turbine, and we welcome the use of a larger turbine [in the future]." It's a plan that Statoil has pursued in the intervening years, with the announcement of Hywind 2 in November 2013.

Planned to be operational in 2016 off the north-east coast of Scotland, Statoil will install five 6MW turbines at the Buchan Deep wind farm in 100m-deep waters. No longer aimed at proving the concept, this next phase of development plans to see how Hywind performs as part of an array.

Siri Espedal Kindem, Statoil's current senior vice-president for renewable energy, said at the time of the announcement: "We will continue to mature the Hywind Scotland Pilot Park towards a final investment decision by conducting marine surveys and concept studies in order to demonstrate technical and commercial feasibility for future offshore floating wind projects."

Success here could mark the beginning of commercial floating offshore wind farms and pave the way for a number of other competing designs. One of these is Windfloat, the second commercial-scale floating wind turbine to be installed after Hywind, which has been operating in the Atlantic off the coast of Portugal since 2012.

Using a 2MW Vestas wind turbine, the project was developed by Principle Power and EDP using a slightly different design from Hywind: it uses four mooring lines instead of three; a semi-submersible base; and, crucially, is assembled onshore and then towed into position out at sea. This removes the need for offshore heavy-lifting operations, which could become increasingly difficult the further wind farms are sited from the coast.

The success of Windfloat, as with Hywind, has also led to further refinement of the design and could aid the development of some of the first US offshore wind energy capacity.

Principle Power and Deepwater Wind will install five 6MW direct-drive wind turbines approximately 18 miles off the coast of Coos Bay, Oregon, following the US Department of Energy's decision to award a £47-million grant to develop the project.

Hot on the heels of this development, Hexicon has also signed an agreement with SSAB to develop their 'far offshore' platforms, with a first demonstration unit operating in Swedish waters by 2017. Using a floating platform that can automatically turn into the wind, Hexicon's design could be the third full-size solution to generate electricity, offering further choice for wind farm developers.

Speaking at the time of the announcement, Hexicon CEO Henrik Baltscheffsky, said: "This agreement is an important milestone in the further development of our patented solutions. 'Far offshore' wind power is more easily scalable and a prime source of renewable energy. Hexicon's technology offers competitive advantages for the long-term renewable energy market."

Global interest

There is no denying that Europe is currently the world leader in offshore floating turbine technology, with each of these three designs making their debut around the continent's coasts. In fact, several other designs have seen smaller experimental pilots, such as SeaTwirl, which was also installed and tested in Sweden.

SWAY, another concept, was installed in Norway at 1:6 scale. Blue H was installed in 2008 off the coast of southern Italy and operated for six months. Poseidon 37 was also installed and tested in 2008, this time at the Vindeby wind farm off the coast of Denmark.

It's not only Europe getting in on the act, despite the rest of the world's slower uptake of fixed-foundation offshore wind. Japan has already installed two small-scale pilot projects, the Kabashima Island concept and WindLens.

The first of these saw a 100kW wind turbine installed off the coast of one of the country's many islands. WindLens used a scale model with two small, grid-connected 3kW turbines in Hakata Bay.

The US too, installed the DeepCwind floating turbine in the Gulf of Maine in June 2013, ahead of the aforementioned WindFloat Pacific development in Coos Bay.

This global interest in floating turbines could yet see a number of differing designs pursued for full-scale commercial wind farms, mirroring the previous development of fixed-foundation turbines. Even today, several competing designs, including monopoles and steel jackets, are still seen as attractive options for offshore wind farms in shallower waters. The future of deep-water wind energy could certainly lead to several of these designs coming to the fore.