"The first sign [that something was wrong] was when we started getting noises from the gearbox," he says. "By the time that a handful had failed we realised it was more significant." The culprit was an "intermediate bearing" in the gearbox. "Our priority was to keep it as a bearing issue, so that it did not become a gearbox issue," says Smith.
E.ON instigated a "proactive maintenance plan" with Vestas which is retrofitting bearings in all 30 gearboxes. Vestas' co-operation has been good, says Smith. "The important thing for us is that we maximise the output of the wind farm across the winter period. Their response was very good and they have given us quite a strong commitment." Vestas is conducting an in-depth study into the cause of the failures, he says.
By mid-December, 18 of the 30 turbines had been fitted with new bearings of the same type as those that failed, says Vestas' north European manager, Tom Pedersen. The cause of the failure has yet to be established, but he believes it to be a combination of factors, including an under dimensioned bearing. A modified and strengthened bearing is to be installed on the 12 machines still to be retrofitted.
Vestas has not experienced bearing failure on other 2 MW machines in Europe, but some machines in the US have suffered a similar problem, says Pedersen. He declines to name the gearbox supplier while negotiations continue about who should pay for the Scroby Sands retrofit. Whether the bearing supplier should pay is a matter between it and the gearbox supplier, says Pedersen. He stresses the retrofit is included in the sum set aside by Vestas for dealing with warranty issues (Windpower Monthly, December 2005).
Vestas also supplied turbines for another offshore wind plant off the east coast of England at Kentish Flats. But the 3 MW machines have a totally different type of two-stage planetary gearbox, says Pedersen. He does not expect to see the Scroby Sands problem repeated at Kentish Flats.