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London Array (wind farm)
28km north-east of the Kent coast, outer Thames Estuary, UK
associated engineer
Dong Energy
date  July 2009 - 4th July 2013
UK era  Modern  |  category  Power Generation  |  reference  TR418977
London Array in the Thames Estuary is the world’s largest offshore wind farm (2016). Its 175 turbines and three substations are controlled from an operations centre at Ramsgate in Kent. The biggest element in the country's Round Two offshore wind programme, and a herald of the much bigger Round Three programme, it is central to meeting the UK’s renewable energy targets.
The project was developed by a consortium — London Array Ltd — consisting of Danish energy company Dong Energy (50% shareholding), German energy corporation E.ON (30%) and Abu Dhabi renewable energy company Masdar (20%), which replaced original partner Shell Wind Energy in July 2008.
The project got underway in 2001 with a series of comprehensive environmental studies in the outer Thames Estuary confirming that the location as suitable. The site has both high local wind speeds and high local power demand from south east England. It shares the estuary with the much smaller Kentish Flats Offshore Wind Farm (completed 2005), which is closer inshore.
The 245 sq km site lies some 28km north east of the Kent coast, between Margate and Harwich, beyond the estuary’s main shipping lanes. It covers the intertidal sandbanks of Kentish Knock and Long Sand as well as the Black Deep Channel, with water depths ranging from zero to 25m. Two construction phases were planned. The first was completed in 2013, delivering the 175 wind turbines now onstream, and covering 100 sq km of the site. Plans for a second phase were dropped in 2014.
The 175 turbines of the first phase have a combined capacity of 630MW. The array is served by two offshore substations and one onshore, all linked by networks of cables. Its use is designed to reduce UK carbon dioxide emissions by more than 925,000 tonnes annually. The second phase would have added up to 166 more turbines, potentially increasing total capacity to 1GW (1,000MW).
In December 2003, seabed owners Crown Estate granted the consortium the option to take 50-year leases on the site and a cable route to shore. London Array Ltd applied for planning permission in June 2005. In 2006, planning consent was granted for the offshore works, and in 2007 the onshore works were approved after a public enquiry. The project team began refining the design, with suitable protection of environmental and community interests.
Onshore construction began in July 2009, with the Cleve Hill substation (TR050639) near Graveney in Kent. Works for its connection to the National Grid commenced in February 2010, with connection enabled for 125MW in October 2011 and for the entire 630MW in April 2012.
In May 2010, the port of Ramsgate in Kent was designated the London Array’s construction, operations and maintenance base. The port of Harwich in Essex handled extra ship traffic during construction.
The Ramsgate base occupied a temporary structure until a new building opening in April 2012. This was used by up to 120 employees during construction and is now a long-term headquarters for the Array. The base includes a pontoon and fuel pumping facility, funded and constructed by London Array Ltd in partnership with Thanet District Council. The pontoon can berth up to six service vessels, which can reach the wind farm in around an hour.
In March 2011, the first of 177 foundations (175 turbines and two sub-stations) was constructed by joint venture contractors Per Aarsleff A/S and Bilfinger Berger Ingeniuerbeau GmBH from the jack-up barge A2SEA Sea Worker. Installation began on the eastern side of the wind farm, in waters around 5m deep. Where water depth reduced to just 2.5m, work was carried out from the shallow draught charter vessel HLV Svanen. At its peak, the first phase had 1,000 people and 60 vessels working offshore.
Dong Energy developed the foundation design with Ramboll UK. Each foundation — and there are 175 different designs — consists of a tubular steel monopile 4.7-5.7m in diameter, driven 20m to 50m into the seabed. Piles are between 33m and 67m long and weigh 190-650 tonnes (the first one was 268 tonnes). They are set out in a grid, spaced at 650m x 1-1.2km.
Each monopile is topped by a large yellow-painted transition piece (half under water) fitted with access ladders and platforms. The join is made using a grouted conical connection. There are six types of transition piece, each 20-28m high and weighing 245-345 tonnes. The monopiles were fabricated in Germany and the transition pieces in Denmark. Both were towed to Harwich and collected by barge for use on site.
In summer 2011, Sea Worker was joined by the purpose-built self-propelled jack-up vessel MPI Adventure, which worked in water up to 25m deep. The barges were fitted with 60m cranes to lift the monopiles, driving them into the seabed with 225 tonne anvils and hydraulic rams. Piling had a 'soft start' increase in noise level so fish could swim away from the area before full power was applied. Monopile installation required a minimum 12-hour weather window, and each foundation took an average of two days to complete.
In July 2011, the two offshore electrical substations were completed by Future Energy, a joint venture between Fabricom, Iemants and Geosea, in a design, build and install package. The three-level structures were lifted into place onto the transition pieces by a 3,300 tonne capacity floating crane. Each weighs over 1,260 tonnes and is 25m long, 23m wide and 22m high, with two electrical transformers and three winding transformers.
By 6th October the foundations and transition pieces for 70 of the 175 turbines were in place. At this point, each assembly was 68m tall. In situ turbine assembly began in January 2012, when the first turbine tower was bolted on, followed by the fixing of its hub, nacelle and blades. The nacelle is located behind the hub and houses the turbine’s gearbox, generator and controls. For a detailed explanantion, see our video How a Wind Turbine Works.
On 19th October 2012, the wind farm produced its first power and in December the same year installation of all the turbines was completed.
The 3.6MW Siemens turbine generators have a design life of around 24 years and were shipped to the UK by barge from Esbjerg in Denmark. Each has a hub height of 87m above sea level and a 120m diameter three-blade rotor. The total turbine height does not exceed 175m, and the distance between maximum sea level and rotor blade tip is at least 22m to minimise damage risk to smaller vessels. Fully assembled turbines weigh around 424 tonnes (tower 180 tonnes, nacelle 190 tonnes, blades 18 tonnes).
The turbines are aligned to the prevailing south west wind and typically begin generating electricity at a minimum wind speed of 3m/s. They operate at full power from 13m/s but will automatically shut down at wind speeds over 25m/s (Force 9 gale). Some are fitted with a helicopter access platform and navigation lights to warn passing ships and aircraft.
The turbines are connected to each other and to the two offshore electricity substations by a series of array cables, between 650m and 3.2km long, that transmit power at 33kV. About 210km of cabling was laid from June 2011 to June 2012, by the offshore support vessel Jan Steen, the tug MCS Ailsa and underwater remotely operated vehicles. The 150 sq mm array cables are grouped into 500 sq mm three-core submarine cables, laid on the seabed and buried to a depth of around 1.5m by jetting.
In the offshore substations, the voltage is stepped up from 33kV to 150kV. Four export cables, totalling 220km, then transmit the power to the onshore substation where the current is increased to 400kV and fed into the National Grid.
The export cables were installed between November 2011 and July 2012, each in one continuous length of more than 50km. Cable laying began at the south end, in the intertidal zone of The Swale. The tracks were cleared of debris by dragging the seabed, then the cables were laid in trenches using the barge and plough vessel Stemat Spirit. Export cables offshore are buried 1.5-2m below the seabed, while onshore they are 1-1.5m below the surface and connected to the substation via holes drilled through the seawall at Cleve Hill.
On 6th April 2013, engineers completed the commissioning of the wind farm — on schedule. Control passed to the London Array Management Building (TR380643) in Military Road, Ramsgate, which has an operations and maintenance team of around 70 people from Dong Energy and Siemens.
On 4th July 2013, the London Array was opened by UK Prime Minister David Cameron. It had cost £1.95 billion.
The competitive offshore transmission regulatory regime, established in 2009 by Ofgem (Office of Gas and Electricity Markets) and the UK government, requires an independent 'offshore transmission owner' (OFTO) to own and manage a wind farm’s assets between the offshore generator and the onshore grid. London Array’s transmission assets were sold to Blue Transmission Investments Ltd, a joint venture between 3i Group Plc and Diamond Transmission Corporation Ltd (a UK subsidiary of Mitsubishi Corporation), for a transfer value of £459m. The licence for Blue Transmission London Array Ltd, its operating name, was granted on 10th September 2013.
London Array Phase Two was subject to various environmental licences conditional upon its impact on bird life, especially the Red Throated Diver. In February 2014, the project was abandoned owing to concerns over the length of time required for environmental impact assessments to be completed. The consortium’s shareholders surrendered the Crown Estate lease and terminated the grid connection option.
In January 2014, Dong Energy signed an agreement to sell half of its shares in London Array Ltd to the pension and finance organisation La Caisse de dépöt et placement du Québec (La Caisse) for £644m. La Caisse becomes a partner in the joint venture, holding 25% of shares. Dong Energy’s holding decreases from 50 to 25%, though it remains the operation and maintenance services provider for the project.
In December 2015, London Array generated 369GWh of electricity — a record amount produced by an offshore wind farm in one calendar month — and reached a capacity factor of 78.9%. Its previous monthly best had been 317GWh, in November that year. Total output for 2015 was 2.5TWh, or enough to meet the energy needs of more than 600,000 British households, saving 1.075 million tonnes of carbon dioxide emissions.
Architect (Cleve Hill substation): RMJM
Architect (Ramsgate operation/maintenance centre): BBLB Architects
Project director: Richard Rigg
Archaeo consultant: Coastal & Offshore Archaeological Research Services
Foundations: Per Aarsleff A/S with Bilfinger Berger Ingeniuerbeau GmBH
Wind turbines: Siemens Wind Power
Offshore substations: Future Energy
Electrical systems (offshore substations): Siemens Transmission & Distrib Ltd
Array cabling supply: JDR Cable Systems
Export cable supply: Nexans, Norway
Cabling installation: Visser & Smit Marine Contracting
Cabling installation: Global Marine Systems Ltd
Contractor (Ramsgate centre): Mansell, Maidstone
Research: ECPK

London Array (wind farm)