Purpose built ships

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Installation vessels for offshore turbines must contend with water depths of five to ten metres, tidal currents of up to two knots, tidal ranges up to eight metres, and wave heights up to six metres -- all depending on water depth. They can be required to drive or drill piles weighing up to 150 tons, to a depth of up to 20 metres; to lift turbine components or assemblies weighing up to 175 tonnes to heights of around 55 metres above sea level and to be capable of remaining on location during periods of down-time during adverse weather.

The vessels currently available fall into two categories, large floating craft and jack-up construction vessels. Jack-up vessels have legs, which extend to rest on the seabed and push the body of the vessel clear of the water to provide a stable platform. None of the ships used by the offshore oil and gas sector have proved to be ideal for wind plant installation. For this reason the UK's Mayflower Energy is building a special-purpose vessel for the wind industry.

The vessel, costing EUR 45 million, will be used for the construction of National Wind Power's North Hoyle wind farm off the coast of North Wales. It can accommodate up to ten 3.5 MW turbines, or a maximum load of 7200 tonnes. It is 131 metres long, 38 metres wide and includes a crane for lifting 300 tonnes over a 25 metre radius. It is equipped to stay at sea for between 25 and 60 days, depending on the number of crew. Mayflower has an option on a second vessel, with a 700 tonne crane, and hopes to build more as needed by the industry.

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