The facility, alpha ventus, was built by EWE, E.ON and Vattenfall in the consortium DOTI (Deutsche Offshore-Testfeld- und Infrastruktur Gesellschaft GmbH) at a cost of €250 million. All 12 wind turbine units were installed in less than 7 months, and 6 of the wind turbines are already being test-run and have generated 13 million kWh of wind power.
The wind farm, sited 45 km north of Borkum island in the North Sea, is the world's first offshore wind park that uses a dozen 5 MW wind turbines. At full operation, alpha ventus will generate sufficient electricity for 50,000 homes.
“With the building of alpha ventus, our three companies have shown that offshore wind energy is technically feasible in Germany, even under considerably difficult conditions,” says project director Wilfried Hube of EWE. “The construction of 12 5 MW wind turbines 45 km from the shore in waters 30 m deep, is a true piece of pioneering achievement and is the first of its kind worldwide.”
The offshore wind farm project relied on the experience of other offshore installations, but the harsh conditions of the North Sea provided a true test for the feasibility of offshore wind power in Germany and is a remarkable logistical achievement, the consortium notes. Initial difficulties with construction vessels in 2008 prompted them to select more suitable equipment this year.
“We learned that the construction process and the employed logistics must be very well attuned with one another in order to be build effectively,” adds Oliver Funk of Vattenfall. “In this regard, we faced a very steep learning curve.”
During peak construction this summer, 350 people and 25 ships were simultaneously at work on the offshore wind farm site. The vessels included three new jack-up barges that arrived at alpha ventus almost directly from their shipyard, as well as Thialf, the largest crane ship in the world that installed the jacket foundations for the REpower wind turbines in 6 days.
The construction of alpha ventus was more complex than other offshore wind farms, which have been built outside Germany. Two types of German-manufactured wind turbines were built on two different types of foundations using various construction concepts and “construction has provided us with valuable information for future offshore projects,” says Ralf Lamsbach of E.ON.
The six Areva Multibrid M5000 wind turbines stand on tripods, and feature a rotor diameter of 116 m and hub height of 90 m. They are 178 m total height above the seabed, and weigh 309 t with rotor and hub. So-called ‘jacket’ foundations are used for the REpower wind turbines, which have a rotor diameter of 126 m, hub height of 92 m, 185 m total height above seabed and weigh 410 t.
The second lot of 6 wind turbines will come online by the end of the year. One hundred technicians passed ’helicopter abseil-training’ in Cuxhaven recently, so they can reach the wind turbines from the air if the sea route becomes inaccessible due to inclement weather or high swells.
An internet webcam has been stationed on the FINO1 construction platform showing the progress on the building site, and DOTI is ready to transmit the power to shore. An underwater cable, installed last year by transpower GmbH (formerly E.ON Netz), connects the offshore transformer station to the German power grid.