Vibrations fix looks solid

The wind industry's major supplier of rotor blades, LM Glasfiber of Denmark, is so convinced it has solved the problem of edgewise vibrations that it is now offering customers a ten year guarantee on new blades as well as existing blades that are up to five years old. The work on the problem-which caused severe cracks in blades-was conducted by LM in co-operation with the Danish wind test centre at Risø and turbine maker Bonus Energy A/S.

LM's Anders Christensen says the solution has been found by installing a damper function in the blades, which "kills" edgewise vibrations. In existing blades, the problem can be solved by applying stall strips to the edges, along with vibration detectors.

Stress cracks were first discovered on 19-metre LM blades in 1996 as a result of "violent edgewise oscillation." Edgewise vibration has haunted companies like Danish NEG Micon, which conducted a major retrofit of wind turbines with LM blades in 1997 and early 1998.

Christensen says the problem has been difficult to solve because it covers both large and small blades and because it is not just an isolated blade problem, but a phenomenon which happens under certain conditions between the wind turbine and its blades. "This problem has not been limited to 19.1 metre blades-in principle, edgewise vibrations can occur in all turbines with stall regulated blades," Christensen says. "This has been a stone on the road to development of rotor blades in the same way that fatigue was a problem earlier. Now it is unlikely that owners of new turbines will notice any effect of edgewise vibrations at all."