Fatal accident investigation

A wind turbine collapse in which a Siemens employee died at PPM Energy's Klondike III wind plant in Oregon last month was the result of a typical runaway rotor occurrence, says Henrik Stiesdal, head of technology at Siemens wind turbine division in Denmark.

In the event of a runaway in winds of 13-14 m/s, the rotor, decoupled from the generator, over-speeds until technology failure occurs at the weakest point. Normally in that situation one of the turbine's safety systems, in this case the airbrakes, would slow the rotor down by turning the blades out of the wind, but under the routine commissioning work being undertaken on the Siemens 2.3 MW turbine at the time of the accident the air brakes were locked in place and the turbine's mechanical brakes taken out of action. For reasons unknown as yet the locking mechanism was not released before the rotor was set in motion.

Siemens spokesperson Melanie Forbrick, says: "There was nothing to indicate that there is a structural design issue with the tower, which is where the immediate suspicion was placed." Stiesdal says that no fault has been found on either the tower or the turbine. The deceased was Chadd Mitchell, 35, of Goldendale, Washington, employed since July. Siemens has reinforced its procedures for service -- including supplementing its "man to man" communication protocols, says Forbrick.

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