Esa Eranti, a coastal and offshore engineering specialist, has developed what he claims to be an innovative and cheap foundation that can cope with moving ice -- seen as one of the biggest problems for offshore wind potential in the north Baltic Sea. The foundation has been designed for water depths up to 20 metres with firm to hard seabed conditions. It consists of a thin walled, cylindrical steel shell with a conical upper part and a ring footing. The shell is filled with granular material for stability. Compared to a traditional caisson foundation the shell is much lighter, says the company. It weighs 200-400 tonnes, has no inner compartments and has an open bottom. This can be compared to the 1800 tonne concrete conical foundations used for the 165 MW Nysted wind plant in Danish waters. The shells are floated out to location and then sunk onto the prepared sea bed.
The Eranti foundation type features in a feasibility study for a 100 MW offshore wind farm developed by Hyötytuuli Oy outside of Pori on Finland's west coast. Hyötytuuli Oy is owned by nine major municipal utilities.
Meantime, Hollming Works is busy marketing its Windarc concept. The advantage of this foundation, says Esa Holttinnen of Windarc, is that the wind turbine, complete with rotor, can be erected on the Windarc foundation before being towed out to sea, allowing for full scale testing at the place of assembly. The foundation is a huge six-sided, hollow steel box -- 25-30 metres in diameter for a 2 MW turbine -- with a central steel column on which the turbine tower is mounted. It is sunk into place by filling it with water. The concept, weighing around 1300 tonnes, is suitable for 2-5 MW turbines and water depths of 10-35 metres. Holttinnen says is represents significant cost savings on foundations used to date and that several complete turbines can be floated into place at one time using a specially constructed barge. The only limitation is the need for good weather and wave heights of one metre or less. It can be used on any seabed surface and is resistant to seabed erosion and ice movement.
Hollming is a machinery and equipment manufacturer for the offshore and shipbuilding industries with four plant in Finland.