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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Largest Offshore Wind Farm Powers Up near London

By Catherine A. Cardno, Ph.D.

The world's largest offshore wind farm, located in the Thames estuary off the coast of the United Kingdom, became fully operational this month.

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The 630 MW London Array became fully operational earlier this month. Located in the United Kingdom’s Thames Estuary, the offshore wind array contains 175 turbines, each with a capacity of producing 3.6 MW. In total, the field is expected to power nearly half a million homes. London Array Limited

April 30, 2013—Earlier this month, the last switch was flipped to enable the largest offshore wind farm on the globe to reach its full capacity. Located in the Thames estuary 20 km off the southeastern coast of the United Kingdom, the 630 MW array contains 175 turbines, each with a capacity of 3.6 MW. The 90 km 2 field is expected to power nearly half a million homes, a number equal to nearly two-thirds of the homes in the nearest county, Kent.

"The United Kingdom is the undisputed leader in offshore wind, with over 3 GW [of] installed capacity, representing more than the rest of the world combined, and we are on track to hit 18 GW by 2020," wrote John Lang, a spokesman for RenewableUK, a nonprofit trade association seeking to expand the use of renewable energy in the United Kingdom, in response to questions posed by Civil Engineering online.

According to Lang, wind power represents approximately half of the United Kingdom's total energy output from renewable sources, that is, energy produced from wind, waves, geothermal heat, or the sun. "This is partly due to the superb natural wind resource that the U.K. has," he said. And the economic advantages of wind are great. More than 12,000 people are currently employed in this sector, and that number is projected to grow to 76,000 within the next eight years, he said. 

The London Array is located in water depths that range from intertidal to 25 m, according to Joanne Haddon, the spokesperson for the array, who also wrote in response to questions posed by Civil Engineering online. In addition to the turbines, two offshore substations and one onshore substation were constructed as part of the project. A network of cables also was necessary to connect the turbines to one another and bring the power ashore.

Each of the turbines was founded on a steel cylinder monopile that was embedded into the seabed using special vessels equipped with four or six extendable legs. Extending the legs raised the vessel and created a stable platform for hammering the pile. Once each pile was placed, a crane mounted on the vessel attached a bright yellow transition piece-containing ladders and platforms for maintenance access-to the top of the pile. The wind turbine was then attached to the transition piece.

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The foundations, transition elements, and turbines are placed using vessels with either four or six extendable legs that lift the vessel’s deck out of the water to create a stable platform from which to work. The water depths range up to 25 m. London Array Limited

Because of the variable depths of water and the complexity of creating the cable, turbine, and substation network, "as many as 60 vessels were on-site at any one time during the peak of construction, which required meticulous planning and coordination," Haddon explained.

Manufactured by the German firm Siemens Energy, the gray, three-blade turbines each have a 120 m rotor and a hub height of 87 m above sea level. They begin generating power when wind speeds reach 3 m/s and achieve full power at 13 m/s, according to the array's website. At a force 9 gale, which is a wind speed of 25 m/s or greater, the turbines shut down for safety reasons. 

Safety also is a concern for aircraft and ships; the bright yellow color of the transition piece was chosen for its visibility. To protect smaller boats that might approach the field, the lowest point of each turbine is always at least 22 m above the maximum high tide. 

The two offshore substations are 20 by 20 m in plan and three stories tall and contain transformers to boost voltage so that less electricity is lost during transmission. These substations also house switchgear and emergency equipment, as well as backup generators and batteries.

The array is connected to the offshore substations by a network of cables that are 650 to 1,200 m long and have a total length of 200 km. The cables also contain fiber-optic wires to facilitate digital communication with the turbines. A total of four continuous lengths of export cables, each more than 50 km long and also containing fiber-optic communications wires, extend from the offshore substations to the onshore substation, which is located at Cleve Hill. These cables were placed in trenches dug into the seafloor mud and then backfilled. The export cables feed into the onshore substation through conduits drilled into the Cleve Hill seawall.

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The array contains two three-story offshore substations that contain transformers, switchgear and emergency equipment, and back-up electrical generators and batteries. Four buried export cables each measuring 50 km in length transmit the power collected offshore to an onshore substation. London Array Limited

While onshore wind is a mature industry by renewables standards and, as a result, "the cheapest form of renewable energy that we have," according to Lang, "offshore wind is at an exciting stage in its development." Within the last year, such major projects as the London Array and the 270 MW Lincs Offshore Wind Farm, located 8 km off the east coast of the country, have come online, he noted. 

But in Lang's opinion, the importance of such projects for Britain goes beyond the number of jobs they create. As he explained, "Offshore wind has the potential to establish a burgeoning supply chain of British companies that will help to ensure not only that the U.K. maintains its place as a world leader of installed offshore wind, but also [that it will be] an exporter of expertise in the technology."

Haddon agreed, noting that "offshore wind is an important part of the U.K.'s energy mix, alongside other forms of renewable energy and conventional energy generation." Not only does offshore wind ensure the security of supply and meet the nation's climate reduction targets; it also keeps costs affordable for consumers, she said. 

The London Array was completed by a consortium of three global companies: DONG Energy, which is based in Fredericia, Denmark, and holds a 50 percent share in the offshore wind farm; the E.ON Group, which is based in Düsseldorf, Germany, and holds a 30 percent share; and Masdar, which is based in Abu Dhabi, in the United Arab Emirates, and holds a 20 percent share. A joint venture known as Future Energy—comprising the Norwegian firm Fabricom AS and the Belgian firms lemants N.V. and GeoSea—designed, built, and installed the offshore substations. 

The announcement at 4:09 PM GMT on April 6, 2013, that the array had reached its full operational capacity capped a fairly speedy design and construction cycle in view of the amount of work involved in creating new infrastructure. Construction began on the project in 2009, although planning began in 2001 when environmental studies established that the site was suitable for an offshore wind farm. 

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Each turbine is founded upon a steel cylinder monopile. A transition piece rendered in bright yellow for visibility from sea and air is placed atop the pile, and the turbine is attached to the top of this element. London Array Limited

According to a press release issued by the London Array, the potential already exists for the field to be expanded, which would boost its total capacity by 38 percent, to 870 MW. Last October the London Array submitted a proposal for expansion to the United Kingdom's Department of Energy and Climate Change.

 

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