Offshore investment starts to pay off

22 January 2001

The UK’s first offshore wind farm at Blyth, Northumberland, has started operation, feeding 4 MW of power generated by the largest operating offshore wind turbines in the world into the local grid. The UK is one of the windiest countries in Europe and has enough offshore wind energy available to supply three times the country’s present electricity requirements.

The Blyth Wind Farm is the first offshore wind farm to be built in UK waters; at its heart are two Vestas V66 2 MW turbines, among the most powerful wind turbines currently available. They are the largest operational turbines erected offshore anywhere in the world, although that record will be shortlived – the Middlegrunden farm in the Øresund strait (between Denmark and Sweden) currently under construction will consist of twenty Bonus 2MW turbines, and there are other large offshore sites, notably the recently completed Utgrunden in the Baltic sea 12 km off Sweden which consists of seven 1.5 MW units.

The UK is one of the windiest countries in Europe. The pro lobby has been saying for some time that there is enough offshore wind energy available to supply three times the country’s present electricity requirements, but even viewed in practical terms the potential is still impressive. In a recent consultation document published by the Department of Trade and Industry, a figure of £6 billion in possible offshore wind farm contracts was suggested. And wind power development now has serious financial backing from national government and the European Union.

The Blyth project has been developed by Blyth Offshore Wind Limited, a consortium comprising Powergen Renewables, Shell Renewables, the Dutch utility Nuon and AMEC Border Wind. Construction started in July 2000. Vestas, AMEC Marine, Seacore and Global Marine Systems were the main contractors on the project. AMEC supplied the foundation piles for the turbines and installed them under the guidance of Vestas. Seacore drilled the rock sockets for the foundations, and Global Marine provided the cable installation from the turbines to the shore.

The two wind turbines have been erected approximately one kilometre off the coast of Northumberland, close to the existing Blyth Harbour wind farm in an average water depth of eight metres. The £4 million project received financial support from the European Commission Thermie Programme, and will be monitored and evaluated as a part of the DTI's Wind Energy Programme, which exists to develop and support the UK wind farm industry. The project had originally been awarded an NFFO-4 power purchase contract based on a design incorporating two smaller machines. (The Non-Fossil Fuel Obligation is a government policy intended to promote the wind farm industry). As a result of upgrading to 2MW machines, a new contract was required for the balance of the increased output.

Blyth harbour was the site for the UK's first semi-offshore wind farm in 1992. Nine 300kW machines were installed on the harbour wall for that project, which led directly to the concept for the UK's first offshore wind farm. The following year Powergen Renewables joined with Border Wind in an application to the European Commission for support under the EC Thermie Programme. With applications for regulatory permissions well under way, Shell Renewables and Nuon UK became involved in the project. Drilling started in August 2000, the two 66m rotors were installed in early October, commissioning began in late November, and the site was officially opened on 7 December one day before the start of commercial operation.

The two 2 MW turbines, spaced 300m apart, are each supported on a tapered tubular steel mast standing in about 8 m of water. Each turbine tower, with its 66m diameter three bladed rotor, is flange bolted to a 3.5m diameter steel monopile grouted into a mating rock socket drilled 15m into the seabed. The windmills have a hub height of 62m above sea level.

The site

The site of the wind farm lies off the Northeast coast of England, the turbines being located on the North Spit, an area of permanently submerged rock bed. This is the first such project to be exposed to the full force of the North Sea; the minimum water depth is 6m, the tidal range is 5m and waves of up to 8m are expected, a strong contrast with other European offshore wind farms in the Baltic and elsewhere which have been exposed only to a 0.5m tidal range and 2m waves at most.

An Environmental Impact Assessment and numerous site surveys were carried out to confirm the sea bed conditions. Boreholes were drilled at the proposed locations of both turbines. This confirmed that there were different ground conditions at each location, even though the turbines are only 300m apart. A 'Front End' engineering study was produced by the project partners to allow contractors to be supplied with base line data for their design work. The turbine manufacturer then took responsibility for the design of the tower and foundation.

Drilling contractors Seacore, whose experience includes a similar 5 by 0.5 MW wind farm in the Baltic Sea just off the coast of the Swedish island of Gotland, completed the seabed survey in 1998. The company’s marine geotechnical division initially sank two 20m deep boreholes to determine the sub-sea ground conditions to enable Border Wind to design the favoured monopile foundation concept for the Blyth turbines.

Deep sockets

In summer last year Seacore returned to the site, where, operating 24h a day in bad weather, they used special large diameter sacrificial coring bits to assist with drilling deep sockets in the seabed rock to receive the turbines’ steel monopile foundations.

For the Blyth project Seacore adopted a similar approach to Gotland, working closely with Border Wind on the design and installation of the wind turbine monopiles. Border Wind had initially favoured two 750 kW turbines, which required 2.5m diameter rock socketed monopiles, but the project subsequently developed to the specification of a pair of 2MW turbines, which required the much larger 3.5m diameter foundations.

Seacore adapted its own proven reverse circulation drill rig and rock socketing technique to suit the Wijslift 6 jack-up platform and seabed rock. With the hole drilled to required depth the drill rig was moved back from its drilling position and the pile gates centralised over the drilled socket. A monopile approximately 30m long and weighing 150t was towed out on a barge from the nearby Blyth dock, then raised by the jack-up’s crane and lowered through the pile gate.

Water was pumped into the 3.5m diameter blank ended steel monopile pile to slowly sink and lower it through the casing shoe and into the rock socket so its shaped toe rested on the mating chamfer cut by the drill bit. To anchor it in place the pile gate held the monopile vertically while high strength, polymer based grout was pumpd into the 100mm wide annulus between the drilled hole and monopile.

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