Offshore wind power: The Danish experience

The North Sea is considered to be a difficult stretch of water for shipping. And now wind farms too have to cope with the rough weather – wind, heavy seas, salt water, spray, and ice all attack the materials. The biggest wind farm in the world, Horns Rev, is designed to prove that offshore technology is up to coping with these demands.

With its eighty 2-MW Vesta wind turbines, the Horns Rev wind farm has been standing since 2002 amid the North Sea waves at a distance of between 14 and 20 km from the sandy beaches of Blåvandhuk. This is the most westerly point of Jutland, the Danish mainland. The wind farm’s total output of 160 MW is enough to supply 150,000 Danish homes with electricity. The annual power harvested is around 600 million kWh.

Even just building the wind farm was a venture with many challenges. With rotors 80 m in diameter, the wind turbines had to be installed by special vessels. First of all, the monopile foundations consisting of steel tubes about 4 m thick were rammed into the sea floor to a depth of approx. 25 m. At this point, the water is between 6 and 14 m deep, and the individual installations each lie 560 m apart. Transition pieces with ready-mounted working platforms, boat moorings, and cable protection were placed on the foundations.

The wind turbines were then mounted by assembly crews using special jack-up vessels. These jack-ups stand firmly on the seabed thanks to their retractable feet. They carry two complete wind turbines at a time and use their own cranes to place them on the foundations. The towers on which the nacelles containing the generators and rotors are mounted are 70 m high. A single tower weighs 160 t, and each nascelle 79 t.

Cables link the installations to a transformer substation, from where the electricity flows into the Danish grid via a cable buried in the sea floor. The large Danish power company, Elsam, monitors the installations from onshore. New maintenance concepts were needed because the farm lies well out to sea. The nascelles are thus provided with platforms onto which maintenance personnel can be lowered from a helicopter when the sea is too rough for boats to berth.

In the long term, the German authorities expect wind farms with a combined output of up to 25,000 MW to be installed in the German North Sea and Baltic Sea. At the end of 2003, applications had been made for around 30 projects by various initiators. Designed for outputs of several hundred megawatts, such offshore wind-power projects involve investment volumes running into the hundreds of millions – a new dimension for the wind power sector. The major construction and operating risks also present new challenges to the wind power sector, which is characterised by small and medium-sized enterprises. In order to be able to finance and insure the projects, many questions still have to be answered – from licensing law and infrastructure to plant technology and economic efficiency.

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