Offshore wind farms are generally built in shallow areas along the coast. In order to be able to build wind turbines, a supportive and durable foundation is necessary. The industry has predominately relied on monopiles; these pipes with different lengths and diameters are rammed into the seabed, and the wind turbines are mounted on top of them. This concept does not work in deeper water depths or with heavy turbines, because the required pipe lengths and diameters currently cannot be manufactured at a reasonable cost. However, new monopiles are currently being developed that could change this.
Grafic: German Offshore Wind Energy Foundation - Blickfang
Two different foundations were used in the alpha ventus test field. One variant comprises tripods that serve as bases for six AREVA Wind turbines. The three-legged steel structure provides a stable hold on the seabed and converges upwards to form a tube on which the tower of the turbine is mounted. Each tripod weighs 700 tons and is 45 metres tall, of which 30 metres are under the water's surface. At all three corners, the tripods are firmly connected to the seabed with rammed pipes.
Foto: German Offshore Wind Energy Foundation - AREVA Wind - Jan Oelker, 2008
The other foundation variant used consists of "jackets", on which six REpower turbines were built. These square steel structures come from the oil and gas sector and are significantly lighter than the tripods. At a height of 57 metres and a base area of 17 x 17 metres, they weigh about 320 tons. Four rammed pipes anchor the structures to the seabed. In contrast to the usual method, the rammed piles were installed in the test field first; then, the jackets were placed on them with the world's largest crane vessel “Thialf”.
Foto: German Offshore Windenergy Foundation - DOTI - Matthias Ibeler, 2009