Siemens will formally respond to OSHA's report this month, which largely confirms what Siemens has already said about the episode: technician and procedural errors during work on the 2.3 MW turbine caused an "overspeed" incident, leading to the eventual collapse of the entire structure (Windpower Monthly, September 2007). OSHA reports that three wind technicians were performing maintenance on the unit. One worker applied the service brake on the turbine in order to work inside the nacelle. He also turned the pitch of all three blades to the maximum wind resistance position and closed all three energy isolation devices on the blades. These computer devices control the blade pitch to ensure safety of operation.
Before leaving the confined space, the worker did not return the energy isolation devices or the blades to their normal operational position. As a result, once the service brake was released, the rotor spun out of control until a blade hit the tower, causing it to buckle. Chadd Mitchell, who was working at the top of the tower, died in the collapse. William Trossen, who was on his way down a ladder in the tower when it collapsed, was injured. The third worker was at ground level outside the tower and unharmed.
Siemens has conducted its own research and implemented changes in a number of key areas "to prevent something like this from ever happening again," says the company's Melanie Forbrick. She suggests the situation might have been prevented if the turbine's software -- which normally implements the ideal and safe blade pitch angles -- had taken precedence over the technician's work. "In this situation what had happened is that the blades became physically locked and the computer couldn't take over and correct what the operator had done," she says.
Siemens has since made an enhancement to its wind turbine software that will make it impossible for an operator to fully override the computer system that normally adjusts pitch control for both optimum wind production and for safety. Now, Forbrick says, "That's not possible, the operator can't act on his own. We have to be able to make sure there is a back-up system to provide correction."
Among the OSHA's findings, however, were also several violations of safety rules: workers were not properly instructed and supervised in the safe operation of machinery, tools, equipment, process, or practice they were authorised to use or apply. The technicians working on the turbine each had less than two months' experience and there was no supervisor on site. The workers were unaware of the potential for catastrophic failure of the turbine that could occur as a result of not restoring energy isolation devices to the operational position, which affects pitch control.
Furthermore, OSHA says the company's procedures for controlling potentially hazardous energy during service and maintenance did not fully comply with Oregon OSHA regulations. This includes using detailed procedures and applying lockout or "tagout" devices to secure hazardous energy in a "safe" or "off" position during service or maintenance.
Forbrick says Siemens has taken action. "We issued a number of supplemental work procedures and communication protocols for certain work," says Forbrick. "We have had a number of safety meetings and employees have gone through safety procedures. For customers, we issued technical advisories for when they are doing their own work that they should follow. That was once again making sure we were protecting the well being of our employees, our customers and others."
Forbrick says no compensatory payments have been made to any of the workers or their families as Siemens is going through "the proper channels," which is allowing the American system of worker's compensation to run its course.