December 1, 2008

The return of the Clipper Liberty wind turbine

Drew Robb

Inside the drivetrain

During root cause analysis (RCA), Clipper used the Six Sigma quality process. It spent three months determining the exact cause of the gear failure, identifying a corrective action, and validating it. As part of this effort, it brought two drivetrains back to its Cedar Rapids factory for teardown and component analysis.

"Improper timing in the drivetrain led to uneven stress between the gears," said Christenson. "In some cases, this had led to premature failure of the gear teeth."

The RCA revealed that gear sets from both of Clipper’s gearing suppliers had timing deficiencies due to supplier-related gear tolerance discrepancies. In response, suppliers improved their manufacturing processes, and Clipper developed a timing measurement fixture as well as a drivetrain test stand and qualification testing process in order to confirm proper timing in each unit by measuring load distribution in the gear mesh before approving the gearbox for shipment. Tooling and process improvements also were implemented at the gear vendors’ plants to verify gear set and timing quality before gears were shipped to Iowa (Figure 2).

2. Switching gears. Early gearboxes used by Clipper Windpower were victims of supplier-related gear tolerance discrepancies that caused gearbox failures. Clipper Windpower used root cause analysis techniques to develop action plans to resolve the problems, upgraded manufacturing techniques and quality control inspections, and replaced all the affected gearboxes in the field. Courtesy: Clipper Windpower

The RCA results were subjected to independent evaluation by engineers and consultants hired by First Wind. The owner’s representatives confirmed Clipper’s findings and supported the remediation plan, which included changing out the drivetrains at each of the eight Steel Winds turbines.

Blade inspections next

Under detailed inspection at Steel Winds, someone observed one blade that had a loose internal structural reinforcement panel, also known as the aft shear web. This is a longitudinal spar in the root area (widest part) of the blade that connects the high-pressure and low-pressure skins to each other. The RCA discovered that the connection of the spar to the blade skins lacked the required strength to withstand the loading experienced during turbine operation.

"Although a fleetwide inspection revealed this defect to affect less than 10% of our blades, we decided to conduct a rotor blade reinforcement of all blades at Steel Winds by adding a shear clip, which strengthens the connection of the spar to the blade skins," said Maurer.

Clipper also decided not to wait for the end of winter to implement the fix. To return turbines to service as quickly as possible, during extreme conditions, it developed a process to carry out the field repairs. The workflow steps consisted of preparation, grinding, lamination, post-curing, and cleanup.

During blade preparation, for example, heating elements and blankets were used around the blade to produce the required temperature for curing: a constant 160F, despite biting winter temperatures. A mobile generator was used to power the lights as well as hot air units to warm the inside of the blade. The company also developed a control system to get the blades up to a certain constant temperature to be able to cure them properly. Once prepared, blades went through grinding, lamination, and post-curing, which verifies the blades have cured properly and are ready for installation. Between each phase, independent quality control (QC) specialists verified that the blade met requirements before it moved on to the next step.

"Temperatures as low as 20 to 30 degrees below zero were experienced at Steel Winds, so it was a real effort to develop an effective and consistent post-curing process," said Maurer. "Our work in this regard will prove invaluable for other wind turbine manufacturers who are forced to do blade remediation in cold climates in the future."