Denmark's Dong Energy achieved a milestone at its 209 MW Horns Rev II project in the North Sea off Denmark's west coast when on March 19 it installed the first of 91 Siemens 2.3 MW turbines. When operational later this year, Horns Rev II will be the world's biggest offshore wind farm, until next year. It is located alongside the 160 MW Horns Rev I wind farm of troubled Vestas 2 MW turbines, built in 2002. The turbines have already undergone one major retrofit following gearbox trouble. From project owner Vattenfall, Jan Grønbech, who heads Horns Rev I operations, does not refute that more trouble has arisen. "We have a round we have to get through, but we have a confidential relationship with Vestas and this is sensitive stuff," he says.