North Hoyle wind farm off Wales: Europe saw 1567 MW of new offshore capacity in 2013
Credit: RWE

As the wind industry expands, players in the sector are looking to build new projects miles offshore, where the challenges multiply. Penny Hitchin finds out why building deepwater offshore wind farms is a step up for developers and the supply chain alike.

Twenty-five years ago there were no offshore wind farms (OWFs). Today, thousands of offshore turbines belonging to 15 countries can generate around 7 GW of electricity.

Most of the capacity (6 GW) is in maritime north-west Europe. The UK leads the way with over 3.6 GW, while Denmark (1.27 GW), Germany (.52 GW), and Belgium are the other leading players.

Offshore wind generation started in 1991 with 11 turbines installed 2 km off the coast at Vindeby, Denmark. The sector grew slowly over the next decade with small near-shore projects in Danish and Dutch waters. In 2001, Denmark’s Middelgrunden became the first utility-scale OWF, with 20 turbines and a total capacity of 40 MW.

The industry has boomed in the twenty-first century, with new offshore wind capacity coming online most years. According to the European Wind Energy Association (EWEA), Europe saw 1567 MW of new grid-connected offshore capacity in 2013, a one-third increase compared to 2012.

The new capacity included 733 MW in the UK, 350 MW in Denmark, 240 MW in Germany and 192 MW in Belgium. However, EWEA warns that the European offshore market looks likely to plateau until 2015 and then to decline from 2016.

The logistics of installing and operating in the harsh offshore environment pose considerable challenges. As the industry expands, developers must look further afield for their sites. This means going into deeper water and/or going further offshore where the winds are stronger and more consistent.

The challenges multiply in such hostile surroundings. Building industrial scale wind farms with bigger turbines on bigger foundations is going to be a step up for developers and the supply chain alike.

European supply chain

Binding renewable energy targets set by the European Union lie behind the incentive regimes which have attracted developers and investors to create north-western Europe’s offshore wind industry. History and geography have combined to make the North Sea the centre of offshore wind development, with the UK now providing the largest market for deployment. The big ticket capital items – turbines, foundations, blades, nacelles – come mainly from Denmark and Germany. Offshore turbine manufacture is dominated by a handful of companies. Siemens Wind Power turbines are responsible for over 3.8 GW of generation; Vestas Offshore has nearly 1.5 GW while REpower (a German wind company acquired in 2009 by India’s Suzlon Energy Group and now rebadged as Senvion) has over half a gigawatt. Currently the biggest machine installed in fleets is REpower’s 6 MW turbine.

Bigger turbines are needed as OWFs go further from shore and into deeper water. This is attracting new players from Europe and Asia into the market. Two 6 MW Siemens machines were installed at the UK’s Gunfleet Sands demonstrator site in 2013. Vestas has a prototype 8 MW offshore wind turbine installed at the Danish National Test Centre for Large Wind Turbines. Korea’s Samsung Heavy Industries is testing its prototype 7 MW turbine at Fife Energy Park on Scotland’s east coast.

French multinational power conglomerate Alstom is relatively new to the offshore wind market. The company is in at the deep end with its 6 MW direct drive turbine, the Haliade-150, which is currently being demonstrated at Belwind OWF in Belgium. Frederic Hendrick, Alstom’s vice-president of offshore wind, talked to Power Engineering International about designing a large turbine for deployment in deeper water.

“When we decided to go into the market we went straight to the big machines, the next generation. It was a way to close the gap on the competition.” He says the new turbine was designed to be robust and simple, and to be suitable for deployment in water depths of over 30 metres.

To date, wind turbine design has been largely based on conventional gearbox architecture. Gearboxes are one of the most expensive components in a wind turbine. A 5 MW gearbox can cost around $1.6 million and weigh as much as 65 tonnes. Gearbox malfunctions disrupt turbine availability. High replacement costs, significant lead time on replacement components such as large bearings or gears, and delays in getting access to the turbine mean that gearbox failures can lead to significant loss of output.

Alstom has opted for a direct drive machine with built-in ‘fault tolerance’. Alstom’s technology uses a neat feature to ensure that torque transmission is independent of rotor support. This improves the reliability of turbine components. Alstom says its Pure Torque technique protects the generator and gives results that are better than the industry average failure rate.

“From a capex point of view we know direct drive is more expensive but in terms of opex it makes sense. What counts is the total price of electricity,” Hendrick explains.

“Most incidents on the drive chain will not stop electricity generation, but will de-rate the machine. Maybe the maximum power will be 4 MW instead of 6 MW but it will still be producing electricity. If you have a spell when the machine will not produce more than 4 MW, it is completely transparent. We have put fault tolerance into the system, but it doesn’t mean you can leave it like that forever.”

Getting technicians onto turbines for maintenance can be expensive, and access is dependent on calm weather conditions. Alstom has designed its offshore turbine to have preventative maintenance visits only once per year, which is less frequent than onshore machines.

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As wind turbines increase in size, blade manufacturers must find ways to engineer blades that are bigger, stiffer and lighter, as well as able to withstand the stresses they experience without succumbing to fatigue. Turbine blades are made from highly engineered, state-of-the-art composite materials. The leading edge of the outer part of a blade is designed to cut through the air at over 150 miles per hour during its 20-25 year working life. As turbines get bigger, speeds will increase.

Hendrick says: “The blades are a very important component. We opted for LM, the blade supplier which has the most extensive offshore experience. We said to them we wanted to go for step change compared to previous machines, but manageable. There is always a risk going for step change, but it was a limited risk, a manageable risk. That is why we went for 150 metre blades rather than 180 metres or larger.”

The first Haliade (built onshore on the west coast of France) has been generating power since March 2012. Hendrick says: “We have certified the power curve on that machine and we are going to close the final certification this year.” The Belwind offshore machine was installed in November 2013 in water depth of 34 metres, “conditions which will be similar for UK Round 3 or Far Offshore Germany,” he explains.

“We faced some difficulties in installing the Belwind machine – not so much in the installation itself – the difficulties we were facing was the very bad weather in the North Sea at the time. Everybody else faced the same difficulty.” The machine is in the process of commissioning and cable is being installed. First power production is expected this month.

France currently has no offshore wind capacity, but there is a development pipeline. The first tendering round in 2012 resulted in a consortium of EDF, Alstom and Dong Energy being awarded three sites with total capacity of 1428 MW. Iberdrola was awarded a fourth site, bringing planned development to almost 2 GW. An estimated €7 billion ($9.6 billion) total investment will be needed for the four sites.

In a second tender round in 2013, bids were invited to develop two 500 MW wind projects off northwestern France.

Two consortia are bidding: GDF Suez is partnering with EDP Renovaveis SA, Neoen Marine and Areva, and proposing to use Areva’s new 8 MW turbine. EDF and partners Alstom and Wpd Offshore will use Alstom’s 6 MW Haliade-150 turbines manufactured at four planned factories at Saint-Nazaire and Cherbourg.

Blade runner: Alstom’s 6 MW Haliade offshore wind turbine
Credit: Alstom

Building on foundations

Big turbines need big foundations. At present, most of the world’s offshore wind turbines use monopile foundations. These are substantial steel tubes which are driven into the seabed, and emerge above the water surface to support the transition piece and turbine. They can be 40 or more metres in length, with a diameter of up to 6 metres and weighing up to 700 tonnes.

Monopile technology is suitable for turbines installed in a sandy seabed. If the surface is too hard it can be problematic to pile-drive. If it is too soft then underwater erosion (scour) around the foundation might make it unstable. There are also restrictions on water depth.

Chris Willow of BVG Associates (BVGA) explains: “Two years ago the industry expected to move away from monopiles because we thought they were not suitable for depths over 25 metres with next generation turbines.

“What we are actually seeing is the development of the so-called “XL” monopile with diameters of 7.5 metres or more, which should extend the operational envelope. That can keep costs down: companies have invested in equipment and know how to produce these structures on a fairly automated basis and the installation contractors know how to install them.”

The more expensive jacket foundations can be used in deeper waters and support a greater weight load. Jackets are based on a lattice steel tower design supported by three or four legs resting on the seabed, secured by piling.

Gravity-based foundations use heavy weights to anchor structures to the seabed. A concrete structure is towed offshore before being sunk and filled with ballast. Concrete gravity-based foundations do not shift once installed and, unlike steel, do not corrode. This technology is in use in OWFs in Belgium, China, Denmark, Finland and Sweden.

Suction foundations, or suction buckets, work by lowering the device to the seabed and creating a vacuum pressure difference by pumping water out of the support structure. The technology has already been used in offshore oil and gas construction.

Floating turbines are a concept that could enable future OWFs to be installed in deeper water. The idea is for the support structure to transfer loads and forces to the water, not the soil. The turbine would be mounted on a large floating vessel attached to the seabed by cables. The first two floating structures, Hywind (Norway) and WindFloat (Portugal), are at the demonstrator stage. They are not expected to be in large-scale commercial use before the 2020s.

The weather factor

The logistics of installing OWFs are complex and depend upon availability of a fleet of specialized vessels. The weather is a key factor, capable of throwing a spanner in the works.

Transporting and installing the massive turbines and foundations requires huge jack-up vessels. Offshore wind turbines are connected by an intricate arrangement of array cables, connecting each turbine to offshore substations where transformers boost the power to a higher voltage before it is sent to an onshore substation via export cable(s). The array cables and the export cables are laid on the sea bed and then covered to minimize damage to them. Laying cables requires specialist vessels.

Other vessels involved in wind farm construction include survey vessels, guard vessels, accommodation vessels, tugs, diving vessels, marine mammal observation vessels and crew transfer boats. Offshore support vessels are need throughout the operating life of the OWF.

In deep water: Getting technicians onto offshore wind farms can be expensive and hazardous
Credit: Siemens

Developer’s viewpoint

With two OWFs operating and another under construction, RWE dominates offshore wind in Wales. The UK’s first commercial OWF at North Hoyle (60 MW) has been generating for ten years while the 90 MW Rhyl Flats was built four years ago. RWE’s flagship Gwynt y Môr OWF is currently the biggest in construction in Europe. The €2 billion project consists of 160 turbines with an installed capacity of 576 MW. It is due to be fully operational by the end of this year.

In the North Sea, RWE has operational OWFs at Greater Gabbard (504 MW) off England’s east coast, a joint venture with SSE, and RWE is the largest partner at Thornton Bank (300 MW) in Belgian waters. The completion of the German Nordsee Ost (295 MW) project is also planned for this year.

Richard Sandford, head of offshore projects for RWE, says: “Our core offshore market is the UK and Germany, and we have a portfolio of around 5 GW of projects in the development pipeline.”

Next in the pipeline are UK east coast projects at Triton Knoll and Galloper (an extension to Greater Gabbard) and Nordsee One and Kaskasi in Germany. Although the water is not deep, the two German projects are likely to see 6 MW turbines deployed.

Sandford explains: “What we tend to do each time we are developing a project is to look at available technology. We look for turbines which have a track record, and we look at reliability. Lots of new turbines are being developed which is really encouraging but at the moment we are looking at 5-6 MW machines.”

The rationale for offshore wind farms is to generate electricity which can be brought ashore and fed into the national grid. Transmission can be a challenge. In the UK, developers build offshore substations as part of the wind farm and install an export cable to take the power to an onshore substation where it joins the grid. Once construction is completed the developer must sell the transmission assets to an OFTO (Offshore Transmission Owner) via a process run by the regulator.

This ruling is part of the UK regime, introduced as a way of getting capital investment into OWFs. Sandford says: “We like the generator-build option where the generator builds the transmission assets and sells them to an OFTO.”

In Germany, the transmission system operator TenneT is responsible for supplying the grid connection to the offshore substation. Sandford reflects: “At Nordsee Ost the grid connection has been late, which has been quite painful for us.”

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Going further offshore

In 2010, the UK seabed owner, The Crown Estate, awarded leases totaling 25 GW for nine big Round 3 project zones.

RWE handed back the lease for its Round 3 Atlantic Array project in November 2013. Sandford explains: “As you start developing a project you understand more about the sea bed conditions and can start looking for the right solutions.

“The Bristol Channel is a challenging environment, with difficult ground conditions. We found that the type of foundations and substructure needed to make that project work would be expensive. We decided instead to focus on more attractive projects in our portfolio.”

RWE’s Round 3 focus has switched to the North Sea where it is involved in a joint venture with SSE, Statoil and Statkraft at Dogger Bank, a huge area located a long way from shore.

Dogger Bank is the largest of the Round 3 zones but with water depths ranging from 18 to 63 metres, it is also one of the shallowest. The developers say it has the potential for approximately 4 GW of development capacity in less than 30 metre water depths and 8 GW in less than 35 metre water depths

Dogger Bank is 125 to 290 km from land. This means excellent wind resources, but the distance from shore will necessitate a change in working patterns. Sandford says: “We will probably start looking at ‘flotels’ so that we can have teams of people living out there during construction and operations.

“It definitely takes offshore wind to a different level and the industry is working together to find the right solutions. It’s quite a long way off, but it’s all going to be about logistics.”

Future prospects

BVGA’s Chris Willow reflects: “You could say that the challenge of the industry is to first get costs down to £100 ($166)/MWh and then €100 then $100. We are seeing a lot of technology, a lot of ideas coming forward, but the investment needs to happen in a timely manner.”

Currently offshore wind needs government support. Willow says: “The long-term aim is to focus on ensuring the industry is not dependent on public funding by 2030. Companies have the capability but they need the market. It’s a scary roller coaster, with each country trying to get the best deal on cost of energy, but developments are taking us in the right direction.”

Penny Hitchin, is a UK-based freelance writer, specialising in energy-related matters.

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