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Wind of change

Government has high hopes for the future of offshore wind. But before ministers can start counting the megawatts, engineers have to build the infrastructure. Bernadette Redfern visits Scroby Sands.

It is midwinter and the hordes of tourists that flock to Great Yarmouth during the summer months are long gone, put off by choppy seas and bitter winds. Contractors building a new offshore wind farm, 2.5km off the coastline of Scroby Sands, are not so lucky.

Yellow towers are springing up from the North Sea, bringing a splash of colour to the grey horizon. Client Powergen hopes to bring 60MW of clean, green energy to 41,000 homes in East Anglia through the 30 turbines of the Scroby Sands wind farm.

Giant 4.2m diameter, 205t piles will support each of the 60m tall wind turbines, designed and manufactured by Danish company Vestas.

A huge number of surveys have been conducted over the past seven years, including bathymetric surveys to map the seabed.

'The biggest challenge was the dynamic nature of the sand, which is constantly moving, ' says Andy Hilton, project manager for Offshore Design Engineering (ODE), which is managing the project on behalf of Powergen.

Originally turbines were to be built on a regular grid in rows of four. But the team knew that this was likely to change. By October 2003 a new survey had revealed massive changes in the seabed, which in some areas varied in depth by up to 11m.

With the piling rig unable to operate if the water level is very shallow, the new topography meant that the furthest row of piles could not be driven. The bed had formed tall sand banks which rose up almost to sea level. The configuration had to be changed and another two rows of four piles were added to the north of the site, where luckily the seabed was lower. The turbines now sit at 500m centres east to west and 375m centres north to south.

'Before we took over as project manager in 2001, the developers knew that the bed was irregular. Consequently the piles were designed to be driven 8m further than necessary as a safety precaution, ' says Hilton.

Engineers from the Mammoet/Van Oord joint venture have so far successfully installed 23 of 30 monopiles, despite restrictive licence conditions imposed to reduce noise levels. These limit the piling to 7am to 7pm Monday to Friday, 8am to 6pm on Saturdays and none at all on Sundays.

Erection of the turbine masts and hubs will begin in March.

There are four basic operations involved. The 60m tall masts are constructed in two halves, the first of which is bolted to the pile and the second to the first section. The hub can then be lifted on to the top. These will arrive at site with two of the 40m radius turbine blades already attached in a formation known as bunny ears. The final blade will be lifted into a vertical position parallel with the mast.

Who's who

Owner/client: Powergen Renewables

Offshore Developers: Powergen and Vestas

Project manager: Offshore Design Engineering

Turbine design and construction: Vestas Celtic Wind Technology

Pile design: LIC Engineering

Pile construction: Mammoet-Van Oord joint venture

Surveys: Andrews Survey, Coastline, Exploration Associates, Fugro, Gardline, Halcrow, LIC Engineering, Niels Winther, Seacore and Unicomarine Turbine routine

During construction, contractors take five 50m long, tubular steel mono-piles out to their mobile offshore platform, returning to shore only when all have been driven. Along with a crane and facilities for the crew, the mobile platform also provides the support for the piling rig including a hydraulic hammer known as a jumping jack.

'Initially the pile is held in the grip of the rig and allowed to self penetrate into the seabed. The contractors then check its orientation before they place the hammer and start to drive, ' says Hilton.

It takes between 75 and 90 minutes to drive one of the 4.2m diameter, hollow steel piles down 30m. Rather surprisingly time has been saved by using less force.

Reducing the driving impact of the hammer from 750kN to 400kN has shaved over 30 minutes off the time.

Hitting the sand hard forced water out from between individual grains, creating a much harder surface, which resisted the driving force. 'We have definitely learned a lot as we've gone along, ' says Hilton.

Also unusually the hammer has been designed to hit the 325mm wide face of the top flange of the pile, which will later be bolted to the turbine mast. 'Normally the hammer would just hit the edges of the pile for fear of damaging the face, ' says ODE structural engineer Mike Boyles.

Once the pile is driven, men literally climb into the pile to check all of the dimensions for deformation.

Before the tower and turbine can be attached to the foundation, the team must fix a platform for maintenance purposes and attach access ladders and fenders for boats to drive up to. These are connected by two rings, which simply slip over the pile. Sacrificial zinc anodes are distributed across the lower ring at seabed level, to provide cathodic protection.

Given the rough environment of the North Sea, scour protection must also be applied to the base of the pile.

A 1m depth of rock ranging from 50 to 300mm diameter is placed around the base to protect it from erosion.

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