A sea change for Hywind

2 July 2014

Interest in harvesting the stronger winds further out at sea has been slow to develop because fixed towers at depths of 100 to 500 metres are not a practical proposition. But interest in floating towers is growing. The leader in the field is Statoil’s Hywind, which is now to be trialled at wind farm scale off the coast of the UK.

The world's first operational deep-water floating wind turbine on a commercial scale was Statoil's Hywind, located in the North Sea off the Norwegian coast. The entire rig was towed out to sea and anchored in June 2009. The 2.3 MW turbine was constructed by Siemens and mounted on a floating tower with a 100 metre deep draft.

Although Statoil has repeatedly said that floating WTG technology is still immature and commercialisation is distant, its enthusiasm for the potential offered by the technology has grown as data from the Hywind trial has grown. In less than two years the turbine generated 15 MWh of electricity. Statoil reported that during 2011 it achieved a capacity factor of more than 46% - and that in the months before the windy season started.

During the following couple of years commercial interest seemed to dry up and Statoil's reported €50 m investment was looking in danger, especially when in July 2013 it was obliged to pull out of its commitment, supported by $4m of US DoE cash, to install a 12 MW wind farm off the coast of Maine, USA, over what it called 'political uncertainly' created by the state government's policies.

But interest has revived with the news that Hywind has been selectd for the UK's first floating wind farm, to be located off the Scottish coast. The UK's Crown Estate has granted an agreement for lease to Statoil for the next phase of the project, which consists of five 6 MW floating turbines operating in waters exceeding 100 m in depth at a site in Buchan Deep, approximately 20-30 km off the coast of Peterhead, Aberdeenshire. At a total capacity of 30 MW, the scheme, subject to the necessary consents and Statoil's final investment decision, is set to be the largest floating wind project to date in Europe and one of the largest announced worldwide.

Working closely with the Crown

"The UK has one of the best offshore wind resources in the world and floating technology can play an important part in realising this potential"

The Crown Estate and Statoil have been working together over the past two years to progress this project. Statoil will now look to secure the necessary consents from Scottish government.

The UK has one of the best offshore wind resources in the world and floating technology can play an important part in realising this potential, allowing developers to access new sites in deeper water, and helping to substantially lower the cost of energy generated from offshore wind. The site at Buchan Deep will also play an important role in demonstrating that the technology can operate as part of an array as well as showing how knowledge, gleaned from Hywind's first stage, has been incorporated into the latest designs.

Statoil's senior vice president for Renewable Energy, Siri Espedal Kindem, said: "This is a significant milestone for the Hywind Scotland Pilot Park. It represents a new step in the development towards a future floating commercial scale park. 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."

Scottish government secretary for Finance, Employment and Sustainable Growth, John Swinney, said: 'Scotland has a huge offshore wind resource but to maximise this opportunity we need to move into deeper water. The agreement ... offers the first step towards harnessing this resource.'

The Crown Estate, which manages almost the entire UK seabed as well as renewable energy generation rights on the UK continental shelf, plays an active role in supporting growth in the offshore renewable energy industry. It is currently running a test and demonstration programme which includes a leasing round for offshore floating wind projects, designed to highlight the commercial viability of the technology.

First Hywind

In June 2009 StatoilHydro and Siemens installed the world's first large-scale floating wind turbine approximately 12 km from Karmøy on the west coast of southern Norway at a water depth of about 220 metres.

"The Hywind project was developed by StatoilHydro, Siemens supplied the wind turbine and the float tower was constructed by Finnish engineering company Technip."

The Hywind project was developed by StatoilHydro, Siemens supplied the wind turbine and the float tower was constructed by Finnish engineering company Technip. It is a 117m long steel cylinder, displacing 5300 cubic metres and weighing 3000 tonnes when filled with its ballast of water and rock. Its anchoring system enables the technology to be used at depths from about 120 to 700 m or possibly even more. The whole is based on methods already used in the offshore drilling industry. The unit has been undergoing testing since 2009, although the initial intention was to run the programme for two years.

Hywind is designed to be suitable for installation in such water depths in order to open up new possibilities for offshore wind technology. Foundations become very expensive at water depths of more than
30-50 m, which might limit the large scale exploitation of offshore wind power, particularly in countries with little or no shallow water areas near the coast line.

The wind turbine supplied by Siemens was a SWT-2.3-82 with a 65 m hub height and a rotor diameter of 82 m. Statoil was responsible for installing the floating structure, which extended 100 m beneath the surface and was fastened to the seabed by three anchor wires.

Statoil and Siemens jointly developed a control system for the Hywind turbine to address the special operating conditions of a floating structure. In particular, the system takes advantage of the turbine's ability to damp out part of the wave-induced motions of the floating system.

Other offshore developments

"Interest in deepwater turbines is growing in the USA"

Interest in deepwater turbines is growing in the USA. Principle Power, a wind farm developer based in Seattle, received initial regulatory approval in February to build an array of five 6 MW wind turbines floating 16 miles off the Oregon coast.

The pilot project off Coos Bay would be the first offshore wind facility on the West Coast. It also would be the biggest demonstration to date of technology that places floating turbines on platforms in deep water.

The turbines - no supplier named yet -would be much larger than typical turbines on land-based wind farms, able to tap the strong ocean winds that blow consistently in southern Oregon and suggesting capacity factors around 40%, compared to the 20-30% typical of turbines on land. Each WTG would be supported on three floating platforms moored by cables anchored to the sea floor at depths of up to 1400 feet. The venture is estimated to cost about $200 million and is expected to be operational by 2017. It has received $4 million in Energy Department funding as an advanced demonstration project.

The UK's Energy Technologies Institute has commissioned, and is funding, naval architect Glosten's PelaStar tension leg platform floating system. The company has applied for consent to install it at the deep water Wave Hub in Cornwall.

The tension-leg platform development utilises 'tendons' manufactured from high-performance materials. This tendon material, combined with innovative end connections, is said to address the technical challenges faced by tension-leg mooring systems - namely eliminating the need for in-field pre-tension adjustment mechanisms, providing system simplicity and low weight, reducing system construction and installation cost, allowing tendon installation from the support barge at the same time the platform is installed, reducing system fatigue loading with small-diameter tendons having high structural damping and reducing the quantity of mooring line when compared to spread-mooring systems. Pitch, roll, and heave motions and accelerations are eliminated.

The hull is an optimised steel structure that is said to be simple and easy to build. There are no mechanical systems operating when deployed, and it avoids fatigue concerns with the welded joints of tubular members found on semi-submersible platforms.

RES Offshore has been contracted to provide construction advice, strategy development, engineering design reviews and turbine procurement services.

Glosten Associates has selected the 6 MW Haliade 150 for the floating platform, currently under construction. RES is working with project partner Alstom which is supplying the turbine. The aim is to install it in 2015. Chris Morgan, RES Offshore CEO, commented: 'Cost effective deployment of floating turbines will significantly increase the potential for renewable energy generation from deepwater sites.'

The floating Haliade arrangement is one of the concepts to be pursued at Alstom's projected research and engineering centre for offshore technology to be built at its marine renewable energies R&D site at Nantes, on France's Atlantic coast. The site is near the Jules Verne technology research institute, which is already active in offshore wind as well as other marine renewables.

The centre is intended to develop the technology and improve the manufacturing process. Alstom expects to employ around 60 employees in 2015 and up to 200 in 2020 and is currently building two factories at nearby St Nazaire, producing nacelles and generators for the direct-drive permanent magnet generator for Haliade 150, which should come on stream in September.

The first five units are destined for the Deepwater Wind project in the United States for which Alstom signed the contract in February. The units will be installed in the pilot wind farm located off the coast of Rhode Island, USA. Contract scope includes supply of the turbines and 15 years of operation and maintenance support for the Block Island Wind Farm owned and operated by Deepwater Wind. Alstom also has contracts to supply 238 turbines to three offshore projects under development in France.

First Hywind
For the Hywind prototype Statoil Hydro relied on the ‘spar buoy’ concept, an underwater floating structure of steel and concrete with ballast tanks – a method that has been used for years with floating offshore rigs. (Image courtesy of Siemens)
Hywind on location 
(Photo Trude Refsahl - Statoil) Hywind on location. (Photo Trude Refsahl - Statoil)
Artist’s impression of the Pelastar platform Artist’s impression of the Pelastar platform

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