Power Engineering - Siemens looks at excessive wind turbine blade speed in tower fall

31 August 2007 - Rapidly turning turbine blades might have contributed to the buckling of the massive wind tower that killed one worker and injured another at the Klondike III wind farm near Portland, Oregon.

"An overspeed operation may have occurred while performing a sequence of procedures," said Melanie Forbrick, a spokeswoman for the German turbine maker.

Siemens declined to provide further details about how the blades might have begun turning at excessive speeds or whether inspection procedures may have stressed the system enough to collapse the steel tower. If the preliminary findings hold, they would move attention away from possible flaws in the tower itself.

"It doesn't appear there was a structural design issue" with the tower, Frobrick said.
The company issued additional safety protocols to the workers in its wind division on Tuesday and resumed inspections of its turbines. Siemens suspended the procedures worldwide after the tower collapsed.

Chadd Mitchell, 35, of Goldendale, Wash., a Siemens employee, was working at the top of the 231-foot tower when it buckled. The rotor and the nacelle, which houses the drive train and generator, crashed to the ground. Mitchell died in the fall.

Bill Trossen of Minnesota, who was midway up the inside of the tower, was hospitalized for a broken thumb.

It marks the first time someone has died in a tower collapse, according to the American Wind Energy Association, an industry trade group that tracks wind farm development.

Siemens' Frobrick said the collapse was a first for the company. "Nothing like this has happened before."

Jim Johnson, a mechanical engineer with the federal National Renewable Energy Lab's wind technology center, all but ruled out the possibility of a structurally deficient tower. "It's not impossible, but it's extremely unlikely for a tower to be at fault on its own and to buckle and fall," he said Tuesday.

Wind turbine towers are made of cold-rolled carbon steel. Several pieces are butted together and welded to form the entire tubular structure.

Johnson also found it difficult to come up with a scenario under which the blades turned quickly enough and the braking systems came on suddenly enough to torque or vibrate and buckle the tower.

The winds were blowing about 25 mph at the time of the incident, nearly optimum conditions for wind-power generation. The blades are designed to feather out and stop at about 55 mph.

"Maybe I can see a situation where the rotor or the brake might burn," he said, "but not a catastrophic case like this."
Siemens' Frobrick declined to address whether the workers' actions could have contributed to the collapse, other than to say "there's some indication, based on preliminary findings, that this excess speed occurred after a sequence of procedures were performed."

Portland-based PPM Energy, which is the project developer and owns wind farms across the country, declined to comment and referred all questions to Siemens.
The Klondike III wind farm is in its final stages of construction.

It consists of 44 Siemens 2.3 MW turbines and 80 GE Energy 1.5 MW turbines. The project remains on track to begin operating this fall, PPM Energy said.

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