Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Down on the windfarm

Britain's first offshore windfarm came on stream last week. Richard Bennett reports on how underwater foundations have played a key role.

Most people would agree that the UK climate is not the best.

The 'green and pleasant land' is wild, wet and windy for too much of the time. John Prescott's 'wake up call' after the recent flooding has focused minds on global climate change, which is expected to make Britain even wetter and windier in the next 100 years.

This is not only good for travel agents, there are some other advantages to be had from the British weather.

Our weather-beaten maritime climate is the best in Europe for wind power. And with the Government committed to the Kyoto Protocol target of producing 10% of electricity supply from renewable sources by 2010, wind power has a big future in Britain.

Current plans are to erect 2000 wind generators by 2010, half of which will be offshore, where winds are stronger and planning constraints weaker than for shore based turbines.

The first offshore windfarm to be built in the UK, at Blyth in Northumberland, came on stream last week. Sited 800m out to sea near an existing on-shore windfarm, this £4M demonstration project features two 2MW Vesta V66 turbines, said to be the largest ever used offshore.

Measuring 62m high overall and mounting a 66m diameter three bladed rotor, each turbine is supported on a tapered tubular steel mast standing in about 8m of water. The windfarm has been built by Amec Borderwind for Blyth Offshore Wind, a joint venture between Border Wind, Shell Renewables, Powergen Renewables and Dutch utility Nuon, and will provide enough power for 3,000 houses.

Foundations are a key factor in siting offshore windfarms.

'We need the right combination of wind, wave, current, water depth and seabed conditions, ' says David Still, Borderwind's general manager. To be economically viable offshore turbines need to be placed on coastal shelves in water less than 20m deep. Foundation and mast designs must resist cyclic loads from the rotor blades as well as wind, wave and tidal loads, with an additional - if slight - risk of ship impact.

At most sites around the coast of the UK this means either deep piling through soft marine sediment or large underwater concrete rafts. At the Blyth site, however, optimal wind and sea conditions coincided with a sandstone outcrop in the seabed, which allowed rock socketed monopiles to be used.

Each turbine tower is flange bolted to a 3.5m diameter steel monopile grouted into a mating rock socket drilled up to 15m into the sandstone. The piles were installed by specialist contractor Seacore from Amec's Wijslift 6 jack-up platform, through a special 20m long guide tube fitted with a sacrificial drill bit which was lowered onto the seabed.

Drilling operations consisted of sinking an 850mm diameter pilot hole, followed by the 3.7m diameter main bore, using one of the largest coring bits ever to be used underwater. The bit was attached to the guide tube by compressed air inflated rubber bladders inside the circumference of the gripper. The system is similar to the standard compressed air jacking equipment used to lift and lower the tubular legs on jack-up pontoons.

When seabed cores were taken in 1998 Border Wind had initially favoured two 750kW turbines, requiring 2.5m diameter rock socketed monopiles. But a pair of 2MW turbines was eventually specified, requiring much larger 3.5m diameter foundations.

The predrilling sequence was chosen to give the right tolerances. 'Verticality was critical and needed to achieve a tolerance of 1 in 100, ' says Seacore project manager Andy Seager.

'The combination of the pilot hole and coring bit technique has enabled us to achieve a far greater accuracy of 1 in 1,750.'

Progress through the layers of mudstone and sandstone averaged 500mm/hour, using a 60t reverse circulation hydraulic drill with air flushing.

To install the 30m long 150t blank ended steel monopile, water was pumped in to sink it slowly through the casing shoe and into the rock socket so its shaped toe rested on the mating chamfer cut by the drill bit.

High strength, polymer based grout was then pumped into the 100mm wide annulus between the drilled hole and monopile.

Once the monopiles were in place the steel masts were bolted on, using the jack-up's 220t crane.

With Denmark, Sweden, Belgium, the Netherlands and Germany already ahead in offshore windpower schemes, the UK has some catching up to do to capitalise on its enviably windy location.

If the Blyth scheme proves successful, offshore windfarms will sprout up around the British coastline over the next 10 years at a rate of up to 40 turbines per year. Key areas are expected to be other sites along the north east coast of England and in Morecambe Bay on the west coast.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.