Locked turbine blades and an unplugged circuit board may have been behind the sequence of events that buckled a wind turbine tower and sent a technician plunging to his death.
Chadd Mitchell, 35, a technician for turbine manufacturer Siemens Power Generation, died Aug. 25 when a tower at the Klondike III wind farm in Sherman County collapsed. Mitchell was in the generator box, or nacelle, 231 feet from the ground when the incident occurred.
Portland-based PPM Energy, developer of the Klondike project, presented details about the accident to state regulators Friday.
Mitchell and two other technicians were performing routine maintenance before the project’s commercial operation, PPM Energy and Siemens said.
Scott Winneguth, PPM’s director of wind plant energy, said two workers were inside the turbine checking the bolts that held the sections of the tower together. The third, Mitchell, climbed up to the nacelle to pick up tools and perform other “housekeeping chores,” Winneguth said.
About 2 p.m. the blades were set flat to the wind “in the full-power position,” Winneguth said, which ran counter to safety procedures and proved fatal.
Mitchell then entered the nose, or hub, “rumor has it,” Winneguth said, “to get his cell phone.” Mitchell followed hub-entry procedures, securing the blades so they wouldn’t turn while he was inside, according to computer readings. He returned to the nacelle and performed one last task, which required replacement of a circuit board.
The unplugged circuit board put several components in default mode, which released the hydraulic brake on the blades. Because the blades were flat to the wind, they began turning too quickly and, bending from the force, hit the tower. The force from the hit, the flying debris and 25 mph winds buckled the steel tower.
A second worker, Bill Trossen, was in the turbine tower, but saved by his safety harness. He was knocked unconscious and broke his thumb. The third worker had left the tower.
By Gail Hill
17 November 2007
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