Britain's weather-beaten maritime climate offers the best potential in Europe for wind power, but it has lagged behind its continental neighbours in exploiting this form of energy.
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.
There are plans to erect 2000 wind generators by 2010, half of which will be offshore - where the winds are stronger and planning constraints weaker than for shorebased turbines. Foundations are a key factor in choice of site.
'We need the right combination of wind, wave, current, water depth and seabed conditions, ' says David Still, general manager of AMEC Borderwind which recently completed the UK's first offshore windfarm, at Blyth in Northumberland.
The project, which came on stream in December, has been built for Blyth Offshore Wind, a joint venture between Borderwind, Shell Renewables, Powergen Renewables and Dutch utility Nuon. It will provide enough power for 3000 houses.
Sited 800m out to sea, adjacent to an on-shore windfarm, this £4M demonstration scheme 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.
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 UK coast this means either deep piling through soft marine sediments 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 on to the seabed.
Drilling operations consisted of sinking an 850mm diameter pilot hole, followed by the 3.7m diameter main bore, employing one of the largest coring bits ever to be used underwater. This 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.
Following coring of the seabed in 1998, Borderwind initially favoured two 750kW turbines, which required 2.5m diameter rock socketed monopiles, but the project subsequently developed to the specification of the pair of 2MW turbines, which required the 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 1750.' Progress rates, 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 slowly sink it 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 rapidly 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.