Blade cracks signal new stress problem, preventative investment needed on turbines with large LM blades

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The wind industry's major supplier of rotor blades is advising operators of large turbines equipped with its 19 metre blade to ensure their machines are installed with specially developed vibration detectors. The warning, from LM Glasfiber of Denmark, follows several instances of stress cracks appearing on blades which have been subjected to "violent edgewise oscillation." Should such vibrations occur, a detector would trip a wind turbine's safety system, bringing it to a standstill.

Blade damage has been recorded on 18 blades, or 0.5%, of the 3700 LM 19 m blades installed on wind turbines around the world with rated capacities of 600 kW and over. LM supplies blades to all Danish manufacturers of utility scale wind turbines, apart from Vestas, and to manufacturers abroad, including companies in Germany, Spain and India.

In nearly all cases, the damage resulting from edgewise oscillation has amounted to serious longitudinal cracks on the trailing edge of the blade a couple of metres from the root. Only once have the oscillations been so severe as to cause more damage. A blade oscillated so violently on a turbine in Denmark that it hit the tower, causing a piece of the blade to break away and fall to the ground. From the Danish wind turbine test centre at Risø, Carsten Skamris reports that all turbines in Denmark equipped with 19 m blades are currently programmed to stop in winds of 20 m/s, instead of the normal 25 m/s, until a technical solution to the problem has been found.

Reports from the few occasions that the edgewise oscillation phenomenon has been observed by human eye suggest that either the vibration dies away of its own accord, or the wind turbine stops as a result of a signal from its vibration sensor. "We recommend that owners of 500 kW wind turbines or larger machines get them equipped with a specially developed vibration detector," says LM's director Anders D. Christensen. "In some cases the normal vibration sensing function is sufficiently sensitive, but personally I wouldn't wait a moment before getting the vibration detector installed," he adds. As well as helping guard against acute blade damage, the installation would prolong a rotor's operating life since all dynamic loads influence this, says Christensen.

Commenting on the problem, Windpower Monthly's technical consultant David Milborrow explains: "You would expect the blade to move to and fro in the direction of the wind, in and out of the rotor plane, but you would not expect it to twitch in the direction of rotation, or not as much." Christensen likens the effect to that experienced when some of the first supension bridges dramatically self destructed.

The instances of cracks on 18 blades have occurred over the past eight to nine months in Denmark and abroad, but the oscillation problem has been known about for longer. LM blades are, in fact, theoretically designed to withstand the extra stress. A Danish group of technicians from LM, Micon, Nordtank, Bonus, Denmark's technical university and the wind turbine test station at Risø has been studying the problem for the past 18 months. A technical solution has been devised, says Christensen, but he declines to reveal the details.

He points out that theoretically edgewise oscillation can occur on all rotor blades, but the problem increases with blade size and wind strength. It is typically associated with stall regulated rotors in winds of 16-17 m/s and apparently starts during a gust. Whether it starts at all depends on a series of factors: the wind turbine and blade and its physical properties such as weight, stiffness and structural shock absorption as well as the influence of the prevailing weather such as turbulence, temperature and so on, says Christensen.

Edgewise oscillation is not necessarily transferred back to the nacelle, where a wind turbine's normal vibration detector is placed. For this reason Christensen recommends installation of a specifically developed detector -- the TAC 84 from DanControl Engineering, a Danish company. DanControl's Erik Nielsen says the device costs DKK 10,000. Since stoppages caused by it would be few and far between and a wind turbine can start up automatically again, the loss of income would be negligible, say the experts.

No decision has yet been made on who should pay the cost of the problem: the wind plant owner, the blade supplier or the wind turbine supplier. Chairman of the Danish wind industry's association, Ole Bøgelund Nielsen, says he will gather all the involved parties, including Det Norske Veritas and the insurance companies, to discuss a co-ordinated solution to the problem.

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