Offshore Wind – The London Array: An offshore wind farm of Olympic ambition

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Last December, London Array Limited signed contracts worth almost €2 billion with seven European suppliers for the supply and installation of the first, 630 MW phase of what will be the world’s biggest wind farm. The Olympic Games will not be the only thing that Londoners will be celebrating when the wind farm becomes operational in 2012.

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Tim Probert, Deputy Editor

The Olympic Games will not be the only show in London town come 2012. Soon after the world’s biggest sporting event comes to the UK’s shores, the world’s biggest marine wind farm will be commissioned some 11 km offshore.

With a capacity of 1 GW when fully operational, the €2.2 billion ($3.1 billion) London Array will be by quite some distance the world’s largest offshore wind farm. When fully complete the project would generate enough electricity to power 750 000 homes ” around a quarter of Greater London. The site is eleven km off the North Foreland on the Kent coast in the area of Long Sand and Kentish Knock, and will cover 230 km2 between Margate in Kent and Clacton in Essex.

The wind farm was initially planned to be built by a 50:50 joint venture ” London Array Limited ” between Shell WindEnergy and DONG Energy. In May 2008, however, Shell announced that it was pulling out of the project, citing concerns over the project’s profitability.

In the following July, E.ON UK and DONG Energy announced that they would buy Shell’s stake and subsequently, in October 2008, London Array announced Abu Dhabi-based Masdar would join E.ON as a joint venture party in the scheme.

Under the agreement, Masdar purchased 40 per cent of E.ON’s half-share of the scheme, giving Masdar a 20 per cent stake in the project overall. Therefore, London Array Limited is 50 per cent owned by DONG Energy, 30 per cent by E.ON UK and the final 20 per cent by Masdar.

The economic downturn caused many in the power industry to fear for the viability of offshore wind farms like London Array, which are among the more expensive forms of power generation, renewable or otherwise. Had it not been for the UK government’s decision to increase the number from 1.5 to two Renewable Obligation Certificates (ROCs) for which offshore wind farms are eligible, it is widely viewed that the London Array project would have been scrapped.

The completed wind farm will consist of 271 wind turbines. The first, 630 MW Phase I will consist of 175, 3.6 MW turbines and two offshore substations, and is expected to be online in late 2012/early 2013. Phase II is expected to generate 370 MW of power, but details of the timeline and cost have yet to be finalized.

The turbines for Phase I are to be assembled at the port of Ramsgate in eastern Kent. All turbines, as well as two offshore substations will be erected on monopiles. In addition, the first stage includes 220 km of 170 kV subsea export cable connecting the offshore substations to the shore and 210 km of 33 kV array cables to link the turbines to each other and to the offshore substations.

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Project participants

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Siemens Energy has been appointed to supply the turbines for the wind farm and to connect the London Array to the power supply network for Phase I. In a €1 billion deal, the German conglomerate will deliver 175, 3.6 MW turbines similar to those in operation at DONG Energy’s 90 MW Burbo Banks offshore wind farm and to those being installed at DONG Energy’s Gunfleet Sands project.

The 1 GW London Array offshore wind farm will cover an area of 230 km2 and provide almost half of the UK government’s 2015 target of 15 per cent of electricity supplied by renewables Source: London Array Limited

The wind turbines, which will be fitted with a 120-metre diameter rotor and have a hub height of 87 metres above sea level, will be installed by a joint venture between Denmark’s Per Aarsleff A/S and Bilfinger Berger Ingenieurbau GmbH of Germany. The same company will also supply and install the 177 monopiles, including those needed for the two offshore substations.

These offshore substations will be designed, fabricated and installed by Future Energy, a joint venture between Fabricom, Lemants and Geosea, while Siemens Energy will undertake the electrical systems and onshore substation work.

Siemens Energy also signed a €128 million contract to supply electrical equipment to connect the wind farm to the grid. This includes two offshore substation platforms, which will be installed at the wind farm. The substations bundle the power generated by the 175 Siemens SWT-3.6 wind turbines, before it is transported via high-voltage subsea cable to the coast.

On each of the substation platforms there will be two 180 MVA transformers and medium-voltage switchgear at the grid access point in Cleve Hill, Kent. The requisite protection and instrumentation and control equipment will also be installed on the platforms. Distribution over two platforms has the advantage that the cable routes within the wind farm can be shorter, and power transmission losses are kept to a minimum to enhance the wind farm’s energy efficiency.

Nexans will supply four high-voltage export cables for the London Array ” two in 2011 and two in 2012 ” all of which will be manufactured at its Halden facility in Norway Source: Nexans

The transformers on the substation platforms will step-up the 33 kV generated by the wind turbines to a transmission voltage of 150 kV. In addition to a substation with four 180 MVA power transformers (400/150 kV), Siemens will also install four 50 MVAr reactive-power compensators at this access point to fulfill the British grid’s requirements ” Grid Code ” on the quality of the electricity fed into the grid. Siemens will also prepare the requisite design studies for the grid access of all of the wind farm’s electrical components and prepare the studies to demonstrate fulfillment of grid access requirements.

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Bringing the power to shore

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Three types of cables are to be installed: inter-array cables connecting groups of turbines to the offshore substations; export cables for transmitting electricity from the offshore substations to shore; and onshore cables to connect to the onshore substation.

The wind turbines will be connected by buried inter-array cables that carry the electricity to offshore substations where the voltage is increased. Four export cables would then carry the power from the offshore substations to shore, a distance of around 50 km, and connect to the onshore substation at Cleve Hill, near Graveney in Kent.

JDR Cable Systems of Hartlepool in the UK has been awarded the contract to supply the 209 km of various lengths of inter-array cables needed to connect the wind turbines to the offshore substations. The four 170 kV subsea export cables will be supplied, but not delivered, by Nexans.

Nexans will supply four export cables: two in 2011 and two in 2012, all being delivered in single continuous lengths from its factory in Halden, Norway. The cables will also incorporate fibre optic elements manufactured at the Nexans’ factory in Rognan, Norway. In addition, the contract also includes the cable accessories such as repair joints and terminations.

While other offshore wind farms have elected to allow 100 per cent redundancy, this obviously costs more, so London Array Limited has selected four 170 kV DC cables in a €100 million deal. Olivier Angoulevant, export sales manager for Nexans’ high-voltage cables division, said: “So if something happens to one cable, like damage from a fishing vessel, the wind farm can still transmit 75 per cent of the power.” Angoulevant expects that the cables will take approximately four months to manufacture in Halden.

The export cables, designed to be operate for 40 years, will typically be buried at a depth of between 0.5 metres and 3 metres, subject to local conditions. It is expected that each export cable will take around six weeks to lay and bury. Nexans offers a turnkey supply and installation, but on this occasion it was decided by London Array Limited to use a third party ” Britain’s Global Marine Systems ” to install the export cables due to insurance purposes.

Installing export cables for offshore wind farms can be a tricky business as E.ON UK found to its cost when the Blyth offshore wind farm ground to a halt in 2006 after an export cable had been damaged by the rocky conditions on the seabed. It was not until January 2009, after the cable was replaced by a special steel-clad system, that the wind farm was fully operational again.

Nexans had a good recession, claims Angoulevant, and order books are full. The French firm has signed a deal with Norway’s StatoilHydro and Statkraft to supply two 21 km export cables for the 315 MW Sheringham Shoal offshore wind farm off the Norfolk coast; and with Centrica for the 250 MW Links project for two 50 km lengths of export cables. Both projects are currently under construction. Last September, it also won a €39 million contract to supply a 52 km, 170 kV subsea power export cables for Phase I of the 165 MW Belwind project, located on the Bligh Bank, 46 km off the Belgian coast of Zeebrugge. Nexans first offshore project was the Horns Rev facility, located 16 km off the coast of Denmark, and was the first installation of its 170 kV XLPE subsea cable.

With an annual consumption of 700 000 tonnes to 800 000 tonnes a year, Nexans is the world’s largest private consumer of copper and, despite the downturn, it may be forced to examine expanding its Halden manufacturing centre on Norway’s southern coast.

“Following the London Array contract, the decision to expand Halden was brought closer,” hinted Ragnvald Graff, Nexans’ offshore subsea manager, “if we get another big project like that we will have to consider it further. The critical factor now is the exclusion capacity of the tower [which contains the vertical production line used for XLPE cables, which at over 100 metres, is one of the tallest structures in Norway]. We are considering adding another production line in the same tower and the decision could be made before the end of the first quarter of this year.”

The London Array is expected to make a substantial contribution to the UK government’s target of providing 15.4 per cent of all electricity supply from renewable sources by 2015. Based on the current schedule, it is anticipated that it will account for nearly seven per cent of this challenging target.

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