The world’s largest offshore wind farm is currently under construction in the Thames Estuary

Providing power
Providing power: electricity is taken from the turbines and transferred to the shore

Of all the components that will make up the UK’s new energy landscape in the coming decades, wind is perhaps the most contentious. Supporters and opponents are seemingly entrenched in their positions, with the intermittency of wind being the biggest stumbling block to the acceptance of wind turbines and farms.

For the supporters of wind energy, the potential of offshore wind is the trump card; stronger, more sustained in magnitude and direction, and much less intermittent than onshore wind, the wind out to sea is said to offer real possibilities for the reliable generation of renewable power.

But it’s far more difficult to build off shore than on shore, and, as yet, there are no really large offshore wind farms. The current largest is Walney Island, off the coast of Cumbria, whose 102 turbines have a combined capacity of 367.2MW and power some 320,000 homes in the north west. Even that is a newcomer to the UK’s energy mix, coming on stream less than a fortnight before The Engineer went to press.

But Walney is fairly modest in size. A far larger installation, billed by its developers as the world’s first truly industrial-scale wind farm, is currently under construction. The London Array, sited in the outer Thames Estuary between the Kent and Essex coasts, will have a generating capacity of 1,000MW, making it the first wind farm to have a capacity comparable to a land-based power station; for comparison, the Sizewell B nuclear power station has a capacity of almost 1,200MW.

The site for the London Array is between two bastions of the British seaside resort, Margate and Clacton, around seven miles off the shore and in water up to 25m deep. Covering an area of about 230km2 (90 square miles),  the array will, when complete, consist of 341 separate turbines, each with a capacity of 3.6MW.

The story of the London Array began in 2001, when a survey of the estuary identified the area as being suitable for a large wind farm, having high wind speeds; a range of water depths suitable for turbine installation; nearby ports for construction, operation and maintenance; suitable ground conditions; an accessible high-voltage network connection; and, not least, a ready demand for electricity. When complete and fully online, the array will generate enough electricity for 750,000 homes, its developers claim, which is about a quarter of Greater London’s population.

The developers, themselves called London Array, are a joint venture company 50 per cent owned by DONG Energy, Denmark’s biggest energy company, which is itself 76 per cent owned by the Danish government.  A further 30 per cent is owned by E.On Renewables and 20 per cent by Masdar, the Abu Dhabi state-owned renewable energy company.

The site for the London Array is between two bastions of the British seaside resort, Margate and Clacton, around seven miles off the shore

Originally, the venture was shared equally between DONG, E.On  and Shell WindPower, but the Anglo-Dutch oil giant pulled out in 2008, preferring to boost its investment in onshore wind farms in the US. The remaining two partners bought out equal parts of Shell’s stake, but the project was in doubt for some months until Masdar came on the scene, buying a proportion of E.ON’s share.

Sea worker
Windy work: jack-up barge A2SEA Sea Worker is shown here installing the monopile foundations

The project is proceeding in two phases. In the first, which is currently well under way, 175 turbines are being installed in a 100km2 area, between 650m and 1km apart. These will send power to two offshore substations and a new offshore substation at Cleve Hill, near Graveney in Kent. Connecting the turbines to substations are 200km of array cables, buried in the seabed, while 220km of export cabling takes the power from the offshore stations to Cleve Hill. London Array claims that Phase 1’s 630MW of power will displace more than 925,000 tonnes of CO2 a year, equivalent to taking more than 289,000 cars off the road.

As The Engineer went to press, 133 of the turbine foundations had been installed, with 54 of the turbine installations complete. Both of the offshore substations had been installed and commissioned, with 41 of the array cables connecting the turbines to the export cables laid and two of the export cables in place.

The second phase, consisting of a further 370MW of capacity from 126 turbines, is currently awaiting studies of its effect on bird life, according to project director Richard Rigg.

As with any project of this size, the London Array is a collaboration between a number of suppliers from different nations. The main contractors for Phase 1 are Siemens Wind Power, supplying the turbines, each of which is fitted with a 120m rotor; ABJV, a joint venture between Par Arrsleff and Bilfinger Berger, specialist Danish companies, is supplying the foundations and the transition pieces that connect them  to the turbine assemblies and is also handling their installation; four vessels, the jack-up barges MPI Adventure, A2SEA Sea Worker and MPI Discovery and Ballast Nedam’s heavy lift vessel (HLV) Svanen, which is designed specifically to transport heavy, pre-cast concrete structures from shore to sea, are working on the installation procedure.

Floating crane

The offshore substations have been built by Future Energy, a joint venture of three companies — Fabricom, Iemants and Geosea — with electrical equipment coming from Siemens Transmission and Distribution (T&D), which is the main contractor for the Cleve Hill onshore substation.

The vital cable links, using 150kV export cable, come from Norwegian specialist Nexans; the 33kV array cables are supplied by JDR Cable System, an Anglo-Dutch company based in Hartlepool. Both sets of cables are installed by Visser and Smit Marine Contracting, along with Global Marine Systems.

The Operations and Maintenance base, at the Port of Ramsgate, which opened in March, was designed by BBLB Architects. Housing 70 technicians and 20 support staff and acting as a base for five maintenance vessels, the building was itself designed along environmental lines. The Cleve Hill substation, work on which began in 2009, was designed by the London-based RMJM architecture practice and, in keeping with its coastal location, is designed to look like a row of beach huts (although the resemblance is somewhat impressionistic).

The foundations for the turbines are monopiles — long, cylindrical tubes weighing 650 tonnes and measuring up to 68m tall and 5.7m wide. These are pounded into the seabed using hydraulic hammers mounted on the jack-up barges using a ‘soft-start’ process, designed to allow marine mammals to move away from the process before the noise and vibration  build up to full power.

The offshore substations are, lke the turines, mounted on monopiles and transition pieces, but are far larger structures than the turbines

The transition pieces, which provide the platform for the wind turbine itself, also vary in size because of the different seabed conditions and water depths. Up to  28m in height, they weigh 245–345 tonnes and are grouted in place  onto the monopiles.

Designing these structures was a somewhat fraught process. The original design chosen for the array was found to be flawed in 2009; consisting of a parallel-sided transition piece fitted on top of the steel tube of the monopile, with the gap between the two filled with cement grouting, concerns arose when almost two thirds of the 948 turbines installed so far were found to have shifted within their foundations. The problem turned out to be that the turbines were constantly bending, which crushed the grout at the top and the bottom, leading to them sliding around. The Norwegian classification organisation that had originally recommended the design, Det Norske Veritas (DNV), carried out additional studies and proposed an alternative design with conical connections within the grouted section that compensate for the forces that act upon the structure while in operation.

Although DNV has approved the new design, London Array is not taking chances; Rigg said that monitoring systems are being installed, including rulers to make any slippage apparent and accelerometers to pick up unexpected turbine movement.

Floating crane
Floating crane

The offshore substations are, like the turbines, mounted on monopiles and transition pieces, but are far larger structures than the turbines. With three stories measuring 22m in height and weighing 1,260 tonnes each, the substations measure 25 x 23m and contain two 180 MVA transformers, which step up the voltage from the turbines from 33kV to 150kV before exporting it, via Nexans cables, to Cleve Hill. There, further generators will boost it to 400kV, the voltage at which it can be fed into the National Grid.

This was where the HLV came in. Another HLV, Rambiz,took the topside structures from Belgium, where they were built, and lifted  them into place atop the oversized transition pieces. ‘We should never underplay the difficulties of working off shore, especially when we’re using a 3,300-tonne-lifting-capacity crane,’ said Rigg. ‘But it’s fair to say that everything went swimmingly.’

Phase 1 of the London Array, costed and currently on budget at €2.25bn (£1.95bn), is on schedule, with construction due to be completed by the end of this year and commissioning set to take place immediately afterwards. According to Rigg, the first turbines could be energised by the end of next January.

Habitat impact

A large population of small seabirds is holding  up the second phase of the wind farm project

Naturally, any project this size  out to sea has to contend with environmental problems. But in the case of the London Array, one of the biggest turned out to be small, feathery and fond of fish.

The red-throated diver is a small seabird, which, before a set of surveys that started in 1996, was thought to be confined to the Scottish coast. However, one of the largest concentrations of the birds in Europe — more than 10,000 — was found to be over- wintering in the Thames Estuary.

‘I don’t think anyone expected the number of red-throated divers that we found,’ Rigg said. But the presence of the birds is currently holding up the second phase of the London Array project.

Rigg has a team in place monitoring the divers as part of the consent conditions for Phase 2; this depends on how the birds cope over winter during and after construction of the farm. The team is also doing research into the behaviour of the divers and suitable habitats for them, all of which will be submitted to the Royal Society for the Protection  of Birds and Natural England.

The discovery of the divers  led to the government deciding to exclude any offshore wind farm developments in water  less than 10m deep. And, for  the moment, the humble red- throated diver remains the main threat to the world’s largest offshore wind farm.

Supplier focus

The array’s wind turbines, supplied by Siemens, sweep out an area the size of two football pitches

The London Array’s wind turbines are Siemens SWT-3.6-120 units, developed specifically for offshore applications. Based on an earlier model but with  a larger rotor, the turbine  sweeps out an area of 11,300m3  — equivalent to two football pitches. This, according to the company, indicates that the turbines will generate 10 per cent more electricity than similarly rated machines.

The rotor blades — each  one 58.5m long — are cast from fibreglass-reinforced epoxy in a single piece and are equipped with independent pitching mechanisms to make the best use of prevailing wind conditions. These are attached to a cast  iron hub, which is itself mounted on a hollow alloy-steel shaft. A three-stage, planetary-helical gearbox transfers torque to the generator, which is designed for high efficiency at partial loads.

London Array is one of six offshore wind projects that Siemens is working on with DONG, and the company has in excess of 3GW of installed wind power capacity within the UK.

‘The potential of the industry to create jobs, stimulate the supply chain and regenerate the economic landscape is huge,’ said Matthew Chinn, Siemens Energy UK’s chief executive.

Reaching the 3GW milestone during a recession ‘is a sign of the potential of the industry’,  he added.

The 3.6MW turbine is not the limit of Siemens’ ambitions. Like many wind energy companies, Siemens is working on higher-capacity offshore units and has recently unveiled a 6MW turbine.

With rotors measuring up to 154m in diameter (although it  can also use the 120m rotor),  the turbine replaces the main shaft, gearbox and high-speed generator with a direct-drive low-speed generator, which is claimed to reduce the need for maintenance and to make the towerhead unit light — less than 60 tonnes/MW of capacity.

Moreover, enclosing the entire electrical system inside the nacelle means pre-commissioning testing can be carried out at the quayside, reducing the need for offshore work.

Supplier focus

Nexans’ contract to supply London Array with export cables is worth some €100m (£80m) to the Norwegian cable specialist.

The company is currently designing, manufacturing and supplying the cables, which  are being made at its plant  in Halden.

Each of the four cables is 53–54m long, containing three copper core conductors along with optical fibres for data transmission, so that the performance of the turbines can be monitored from the operations centre in Ramsgate. These will also allow some maintenance to be carried out remotely.

Installing the cables has been no mean feat. Some of the seabed is London clay, a difficult material in which to bury cables, while elsewhere there is soft sand and uneven surfaces.

Moreover, the relatively shallow waters mean that the weather is a big issue; the laying of the second of the export cables was delayed in January of this year owing to bad weather. Extra cable-laying vessels, capable of operating in shallow water, had to be called in.