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Two Danish blade manufacturers, Vestas, which produces blades for its own wind turbines, and LM Glasfiber, the world's largest supplier, have reported replacement of a number of defective blades. Both companies also say they have improved manufacturing of their 19.2 and 19.1 blades, respectively.

Vestas, having discovered a bonding problem on a handful of its 19.1 metre blades, is retrofitting all 289 of the blades made before March 1994, now installed on 114, 500 kW turbines. Ninety of the affected blades are on turbines in the US and 199 on turbines in Europe. The problem came to light following inspection of a 500 kW unit in Denmark, which lost a blade after being struck by lightning in January last year. Faults in the bonding between the blade root and aluminium section were spotted. "It became clear that the degreasing process, which takes place before bonding, was insufficient," says Vestas. As a result an improved bonding process was introduced last March and a method developed, using compressed air, to locate the fault on other blades. All faulty blades found were replaced.

Despite the testing programme, two blades failed this year, in January and February, revealing that the bonding fault was exacerbated over time. Following another inspection of all the 500 kW blades made before March 1994, the company says 12 blades were discovered to have two or more bonding faults, 34 had one fault, while 243 were fault free. A decision was taken to retrofit all 289 blades by re-bonding the affected parts. The worst affected blades will be replaced immediately and those with just one defect will be replaced within the next three months, says Vestas, followed by the remainder over the next 18 months.

LM Glasfiber reports that 13 blades, supplied to a single customer, have been replaced or repaired after cracks appeared. The 13 are among 225, 19.1 metre blades supplied so far. The cracks all appeared around the point where the blades are lifted for transport and installation. LM has now given new instructions for handling of blades and, as an extra precaution, strengthened the area where the cracks appeared on all subsequent production runs. The company says there is no reason to replace other 19.1 blades. "But of course we won't let our customers down if problems do show up," says LM's Anders Christensen.

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