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Kent's sea change

The £1.95bn London Array will soon be powering almost half a million homes in south east England. Declan Lynch took a trip out to sea to watch its construction.

Ships navigating the Thames Estuary will notice the huge wind farms being constructed as part of the UK’s Round Two offshore wind programme.

As the area becomes one of the key power hubs for the UK, none is bigger than the £1.95bn London Array offshore wind farm, 28km off the Kent coast. The wind farm is the biggest in the Round Two offshore wind programme and will be the biggest in the UK once completed.

Precursor to Round Three

Keen interest is being taken in the construction of London Array because it is seen as a precursor to much bigger Round Three programme - central to the UK’s efforts to meet its renewable energy targets.

Work on site has been challenging for the team as offshore wind turbine assembly is a relatively new industry.

“It’s certainly been a learning curve,” says London Array commercial manager Matt Britton.

“Accessibility [to the site] isn’t so great and the equipment is very much more expensive,” adds Britton.
London Array is also the name of the consortium of Danish energy firm Dong Energy (50% stake), German energy giant Eon (30%) and Abu Dhabi renewable energy firm Masdar (20%) which is building the turbines.

Project team

Its project team is made up of staff from Dong, Eon and other engineers brought in to help construction.

The team is constructing the 1GW wind farm in two phases, and the first 650MW stage is well underway. Engineers will construct 175, 3.6MW Siemens turbines, two offshore substations and an onshore substation linked by transmission cables.

“It’s certainly been a learning curve. Accessibility isn’t so great and the equipment is much more expensive”

Matt Britton, London Array commercial manager

Each wind turbine consists of a circular steel monopile up to 5.7m in diameter, weighing up to 650t and measuring up to 68m in height. On top of the monopile sits a steel transition section connecting the wind turbine tower to the monopile foundation. The transition pieces weigh between 245t and 245t and have a vertical height of up to 28m.

“We’ve used our expertise working on other wind farms,” adds Britton. Dong and Eon are major players in Europe’s offshore wind programme.

Suitable site

The London Array project began in 2001, with a series of environmental studies in the outer Thames Estuary confirming the site was suitable for a wind farm.

Two years later, in December 2003, The Crown Estate, which owns this part of the estuary, granted the London Array consortium the option to take a 50 year lease for the site and a cable route to shore.

“We use a process called a ‘soft start’ so fish can move away before full power is applied”

Matt Britton, London Array commercial manager

The government was keen to develop the wind farm because of the high local wind speeds, and high local power demand in south east England.

By 2007 with all the necessary permissions and licences in place, the project team began developing the wind farm and planning how to protect environmental and community interests.

Dong Energy developed the foundation design, supported by consultant Ramboll. They eventually opted for the steel monopile solution.

“We couldn’t use a gravity base foundation because the ground wasn’t flat enough,” says Britton. “Jacket foundations are not suitable at this depth [which is as shallow as 5m] so monopiles were the only option.”

For phase one, 177 monopile foundations are required - one for each of the 175 turbines - and a further two for the two electrical substation.

Conical connection

Fortunately for the team, the monopiles for this project incorporated the conical connection between the pile and transition piece. Turbines at earlier wind farms, such as Gunfleet Sands off the Kent coast and Burbo Banks off the North West coast, had used a straight grouted connection and experienced turbine slippage - a problem revealed by NCE last year (NCE 22 April 2010).

“We were pleased there would be no need to retrofit,” Britton added.

The monopiles are fabricated in Germany, transition pieces in Denmark and most are then floated over to Harwich, and stored until needed on site.

The project office is in Ramsgate but Harwich is better suited to handle the extra ship movements.

Work installing the foundations began in March this year, and by September over 70 monopiles and transition pieces had been installed.

Purpose built vessels

Two purpose-built vessels - the MPI Adventure and A2SEA Sea Worker - transport the monopiles from Harwich into position on the seabed. Both vessels are jack up barges, with extendable legs that lift the deck out of the water, creating a stable platform.

A crane mounted on the jack up barges lifts the monopoles into position ready to be driven into the seabed.

“We use a process called a “soft start” so fish can move away from the noise before full power is applied,” adds Britton.

Once the monopile is secure, the transition is assembled in a similar manner, and engineers insert grout in the connection.

It is delicate work which requires calm weather; as a result the team members are avid listeners to the weather forecast.

“We are looking for weather windows - we need a minimum of a 12 hour window to allow the vessel to get out to sea and begin installation,” adds Britton.

Engineers began construction on the eastern side of the wind farm, in shallow water at depths of about 5m.

“These were harder to work in because there is a smaller time frame to install the foundations, so we wanted to work on them in the better weather,” adds Britton.

Shallow water

For the very shallow water where the depth dropped to just 2.5m engineers had to charter the vessel HLV Svanen which had a shallower draught than the jack up barge.

All the foundations are expected to be completed by the end of year, with attention then turning to installing the 3.6MW Siemens turbines.

Each turbine will have a hub height of 87m above sea level and a 120m diameter rotor. The turbines will be shipped from Denmark.

The turbines are linked by a 33kV transmission cable which is also linked to the substation.
“Laying cables is quite a challenge,” says London Array marine co-ordinator John Bray. “There always seem to beknots or breakages.”

Laying the cables is the offshore support vessel Jan Steen, supported by the tug MCS Ailsa. The first three array cables are now in place, measuring between 720m and 3.1km long. A total of 200km of cable is required for the scheme.

Two substations

All the array cables feed into the two substations, where the electrical current is stepped up to 150kV. Four export cables run from the two substations to the onshore substation at Cleve Hill. This is where the power is stepped up again to 400kV to be fed into the National Grid.

“Eon will buy 30% of the energy produced by the wind farm, with the rest left on the open market,” explains Britton.
Construction is due to be mostly completed by next year, followed by testing and commissioning. The array is due to come on stream by the end of 2013.

London Array hasn’t given timescales for phase two, as it is subject to environmental licences. When operating at full capacity, the wind farm will produce enough electricity to power 750,000 homes in the South East of England.

Project team

Client: London Array
Dong Energy (50%)
Eon (20%)
Masdar (20%)
Wind turbine construction and erection: Siemens Wind Power
Foundations: Per Aarsleff and Bilfinger Berger
Offshore substation:
Future Energy
Onshore substations: Siemens Transmission and Distribution

 

Safety at sea

With difficult conditions offshore, it is clear health and safety is paramount.

“We encourage not only reporting of accidents, but near misses and observations,” adds Bray. “We also want to encourage staff to stop work if they feel in any way unsafe.”

There have been fatal accidents on other offshore wind farms, and the offshore working environment poses unique hazards in addition to those on any land based construction site.

Coastguards are on standby to assist with any operations. London Array has a good safety record with no fatalities, and very few injuries, but the team remains vigilant.

“It’s all the dangers of a construction site but on water,” adds Britton.

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