Offshore Foundations: Traditional Concrete
Foundation being floated out to Tunoe Knob Photograph © 1996 by Flemming Hagensen	The first offshore pilot projects in Denmark (and the world) used concrete gravity caisson foundations. As the name indicates, the gravity foundation relies on gravity to keep the turbine in an upright position. Vindeby and Tunoe Knob Offshore Wind Farms Vindeby Offshore Wind Farm and Tunoe Knob Wind Farm are examples of this traditional foundation technique. The caisson foundations were built in dry dock near the sites using armed concrete and were floated to their final destination before being filled with sand and gravel to achieve the necessary weight. The principle is thus much like that of traditional bridge building. The foundations used at these two sites are conical to act as breakers for pack ice. This is necessary because solid ice is regularly observed in the Baltic Sea and the Kattegat during cold winters. Disadvantage of Concrete Using traditional concrete foundation techniques the cost of the completed foundation is approximately proportional with the water depth squared - the quadratic rule. The water depths at Vindeby and Tunoe Knob vary from 2.5 m to 7.5 m. This implies that each concrete foundation has an average weight of some 1050 metric tonnes. According to the quadratic rule the concrete platforms tend to become prohibitively heavy and expensive to install at water depths above 10 metres. Therefore, alternative techniques had to be developed in order to break through the cost barrier, as we shall see on the next pages.
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