In the first of the two failures, a 600 kW wind turbine maintained by Vestas was filmed by a nearby resident while it dramatically ran out of control and flew apart (Windpower Monthly, March 2008). The video clip was circulated worldwide on the internet and transmitted on television news channels. Public reaction ranged from political demands for better safety controls to calls from Denmark's main national newspaper, Jyllands Posten, for an immediate stop to the operation of "5000 potentially life threatening power plants" that are "a threat to the population."
The failure occurred as the wind turbine was being restarted in strong winds by Vestas' technicians after service work because of worn brakes. The machine was installed by Nordtank Energy Group in 1996 before Nordtank merged with Micon to become NEG Micon prior to that entity's subsequent merger with Vestas. On this particular make of turbine, the blade tips are extended during start up to regulate rotor speed.
During the start-up and just before the clutch engaged, a noise emanating from the nacelle prompted the service team to hit the stop button. At that point there is a loud bang, which according to a failure report by Denmark's Risø national laboratory is presumed to come from the gearbox, which fails.
The turbine noticeably vibrates and the rotor stops dead, instead of coming to a controlled halt as it should. At this point the employed tip brakes snap. With no mechanical resistance and no tip brakes, the rotor accelerates rapidly, the blade tips are thrown off and the turbine runs out of control. The lack of effect of the mechanical brake on the main axle is put down to the gearbox failure. After three hours out of control, a blade hits the tower and the turbine falls apart in seconds.
In the second Vestas wind turbine failure later in the same gale, but on the other side of the country, a blade was discovered on the ground 60 metres from the tower. According to the failure report, Vestas' own examination of the reason for the breakage reveals that some of the blade bolts were not correctly tightened, which caused a stress break and a domino effect on the remaining bolts. The company's assessment is that the incorrect bolt tension could stem from the turbine's installation in 2000. It regrets that the mistake was not detected during routine annual service checks.
Quality control of wind turbine service companies is not a new idea. It was proposed by the Danish Wind Turbine Owners Association back in 1992, but at the time the suggestion was rejected by the energy ministry, which felt that market forces should provide sufficient incentive for companies to carry out quality work. But paying for quality certification has not been a priority among service companies, says the association's Asbjørn Bjerre. His suggestion to Connie Hedegaard last month that all wind turbine owners be obliged to subject their wind turbines to a twice yearly service control was acted upon within days.
A focus on wind turbine failure after the two incidents in February has revealed that during the past three years more than ten such accidents have occurred -- and three to four failures a year is a distinct increase on previous years, says Peter Hjuler Jensen at Risø's wind turbine test centre. While agreeing that such an increase might be expected as wind turbine fleets grow larger and older, he points out that it is not the really old machines that are failing and that the most common form of major failure is a thrown blade.
Routine service is a requirement of insurance companies, says Hjuler, but as turbines get older it is not always worth paying the premiums. "Whether owners want to insure their turbines is up them, but when it is a matter of safety it should be obligatory," he says.
A general problem in connection with bolt tension on blades is that galvanised bolts have a tendency to stick which makes adjustment of them difficult, says Strange Skriver, technical consultant for the turbine owners' association. The best approach is to release them first before applying the right tension, he says. But it is a subject on which experts do not yet agree.
"Generally I'm a bit nervous about blades on the new big turbines. They are getting lighter and lighter. It is particularly in blade development that the tendency has been to use fewer and fewer kilos of material for each kilowatt of capacity," says Skriver. "As a result we are getting closer and closer to the margin for what blades can withstand -- and if they are exposed to extra loads there is a risk that things will go wrong."
Extra focus is also needed on blades because their manufacture still involves a deal of manual labour, he adds. Quality can be a problem as new blade production lines are started up around the world, warns Skriver. Tip brakes that are employed to stop a rotor from over speeding are still a weak link. Experience has revealed that if a rotor is stopped manually while the tip brakes are still being extended they can break off.