A construction fault came to light in the first LM 26.1 metre blades after failures in 1997 and 1998, says LM managing director Anders Christensen. The fault's effect was a tendency for the blade to bulge on the suction side. A separate fault discovered on early LM 29.0 and LM 29.1 blades could lead to cracks appearing in the glue join along the trailing edge around the root and the broadest chord. Lastly, the first 29.1 P-blades lacked tip strength.
Following discovery of the faults, all three blade types have been redesigned after further calculations and dynamic tests. All the blade redesigns have been approved by the Danish national laboratory at Risø and classification societies Det Norske Veritas and Germanischer Lloyd. A large number of the blades have already been supplied.
It has only been necessary to replace a few of the older blades from before the redesign, says Christensen. The remaining old blades are being closely watched by operations and maintenance staff at individual wind stations, mainly in Denmark and Germany, until they can be replaced, he adds. Owners of wind plant are not required to bear the costs of the retrofit work and their blade guarantees will be extended.
About one quarter of all the affected blades can be repaired while still installed. The remainder will be taken to LM service stations in Vojens, Denmark, and Sebexen in Germany. To avoid as much downtime as possible, the blades will be retrofitted at the same time as the turbines are stopped for other repairs.
Christensen says that dealing with the construction faults has strengthened critical elements of LM's blade development program around calculations, control and documentation. "In addition, series production of LM blades now only starts after statistical and edgewise dynamic testing is complete and flapwise dynamic testing is 25% complete," he says. LM has expanded its internal testing capacity with three test stands with the capacity to test blades up to 55 metres. "LM now has the largest in-house capacity in the world for testing rotor blades in the megawatt class," says Christensen.