Cheaper foundations

Offshore wind plant foundations of the future will be far cheaper, bringing down the price of salt water kilowatt hours. This is the Danish wind industry's conclusion following a study conducted by three engineering companies, NN&R, LIC Engineering and Rambøll. It came up with three alternatives to the cumbersome concrete caisson concept used in Denmark's two offshore wind plant so far, at Vindeby and Tunø Knob.

All three concepts -- each designed to bear a 1.5 MW turbine -- use steel instead of concrete for water depths of five, eight and 11 metres. The first is a single steel tube rammed into the sea bed to a depth determined by water, wind and weather conditions. The advantages of the method are speed and simplicity, although a rock filled sea bed could make it unusable. The second concept, a gravity foundation, consists of a hollow steel caisson, filled with ballast once on the sea bed. The advantage of steel over concrete is that the weight of a concrete caisson has to be increased greatly for deep water. The third concept is that used for oil platforms. A hollow column of steel, upon which the wind turbine would be mounted, is supported by a three-legged lattice construction hammered into the sea bed. The advantages are its low weight and minimum preparation of the sea bed.

The report concludes that the three concepts cost about the same and that steel is the cheapest material. A fourth solutionbeing studied is a sea based platform. "In everyday language I'm working with a concept where a sort of upside down can is sucked onto the sea bed with the help of a vacuum in the cylinder, says Jeans Ole-Back of Denmark's Geoteknisk Institut. He has been granted DKK 300,000 to research the idea, reports the Danish wind industry's newsletter.