The return of the Clipper Liberty wind turbine

The Steel Winds project in Lackawanna, New York, was selected as a POWER 2007 Top Plant because of its unusual location (a former steel mill and Superfund site) and because it was the first commercial deployment of the Clipper Windpower 2.5-MW turbine. That report was written just as the project entered commercial service but before a major gearbox problem was identified. For many new designs, it isn’t a question of if problems will occur but of how the manufacturer responds when problems inevitably do occur. For its handling of Liberty’s problem, Clipper Windpower gets an "A."

What would you expect to find in the first year of any new high-technology, commercial-scale product release? From a new model car to a computer operating system, or perhaps a gas or steam turbine, history has taught us to expect at least a few teething troubles. And so it goes that most every class of industrial or commercial product seems to require an initial phase of debugging before it achieves dependable levels of performance and efficiency.

Original equipment manufacturers can invest thousands of hours in design and prototyping in an attempt to eliminate bugs, but perfection is an elusive goal. It is not usual in this business to spend a year or two — sometimes longer — of field experience refining the operation of new machines. The early experiences of F-class gas turbines is one example.

Growing pains

The same design cycle applies to the wind power industry. In late 2006, for example, Clipper Windpower’s first eight commercial 2.5-MW Liberty turbines rolled off the assembly line at the company’s new Cedar Rapids, Iowa, manufacturing facility. Installed during one of the coldest winters on record in New York, the turbines were placed into service in the first quarter of 2007 on the U.S. banks of Lake Erie in Lackawanna, near Buffalo. The project was known as Steel Winds and was recognized as a POWER Top Plant last year (Figure 1). (See the December 2007 issue for a complete description of the project.)

1. Rust Belt goes green. The Steel Winds Project, the first commercial deployment of theClipper Windpower 2.5-MW turbine, is located on the site of a former Bethlehem Steel mill and Superfund site along Lake Erie in Lackawanna, south of Buffalo. The 1,600-acre site is under redevelopment for public use. Courtesy: Clipper Windpower

Erected on a former Bethlehem Steel industrial site, the eight 2.5-MW Clipper Liberty wind turbines are owned by First Wind (formerly known as UPC Wind), a company that operates 92 MW of energy capacity through three wind farms.

For the first several months, all ran smoothly. Then an on-site engineer at Steel Winds heard an unfamiliar noise in the drivetrain. An inspection and analysis revealed a supplier-related drivetrain timing issue that had caused some gear teeth in one turbine’s secondary stage to break.

After a quick assessment of Clipper’s small but growing fleet, the company decided to test all drivetrains fleetwide. While it was addressing this issue, it looked more closely at its turbines to verify that everything in the field was running as expected. That uncovered a minor problem with an adhesive joint on the blade’s aft shear web, which Clipper elected to address on all its turbines.

Work had to be conducted during the depth of winter with temperatures well below zero.

"We decided to conduct blade repair in winter in order to get the turbines up and running rapidly for the customer," said Jeff Maurer, vice president of Fleet Services at Clipper Windpower, a wind industry veteran who has commissioned more than 1,000 turbines and overseen fleet operations at more than 3,000 wind turbines for GE Wind Energy. "Although it was extremely difficult, we pulled it off and now have the Steel Winds site fully operational."

Result: All eight of the turbines at Steel Winds were quickly returned to service.

Because First Wind has filed a prospectus with the Securities and Exchange Commission (SEC), its representatives could not be quoted in this story. "We address the issue of Clipper’s turbines in our prospectus with the SEC," said John Lamontagne, a spokesperson for First Wind.

According to the prospectus, between May 17, 2008, and August 31, 2008, the cumulative performance of the turbines at Steel Winds met the availability warranty rates agreed to by Clipper. "We anticipate this project will have an approximate 31% to 33% net capacity factor over the next 24 years," says the prospectus. "Assuming a 32% net capacity factor, the average annual electrical production is projected to be 56 GWH."

Strong wind rising

If anything, the situation facing wind turbine builders may be more severe than what traditional power industry manufacturers have to deal with. After all, how often does an industrial turbine supplier release a new line of larger machines? The wind industry, on the other hand, ramps up its power output every couple of years. At the end of the nineties, turbines of 750 kW were the norm. They were replaced by 1.5-MW models, which became standard a few years ago. Now, models in the 2- to 3-MW range are beginning to dominate. Yet 3.5-MW and 5-MW gear is already running in the field, and companies have announced 6- and 7.5-MW models as being in the pipeline.

"The wind industry is continually pushing the envelope and extending the capabilities of the entire supply chain as it expands," said Maurer. "At each stage, gearbox and blade manufacturers are forced to provide parts that go beyond the bounds of current technology. In many cases they are also forced to build new facilities and new equipment to manufacture parts that are larger than those demanded by any other industry."

Every new generation of turbine requires a complete redesign, as you are moving into a whole different level of stress and strain within the turbine. The market demands more capacity while at the same time it insists upon a lower cost for wind energy. So if you have a blade that produces 10% more energy, then that larger blade has to cost less than the gain it provides.

"If it’s proportionate, then there is no reason to build the larger machine," said Maurer. "Every step of the way, therefore, you have to push the technology forward while keeping it cost-effective. If you don’t experience any issues at all, it is probably the case that the machine has been overdesigned and therefore is too expensive. Product design is a delicate balance of robustness and cost-competitiveness."

For Clipper, though, this not only meant bringing a new turbine onto the market but also establishing a manufacturing plant in Iowa from scratch. The 2.5-MW Clipper Liberty, the largest wind turbine made in the U.S., has rotor diameters ranging from 89 meters (292 ft) to 100 meters (328 ft). It includes features such as variable-speed technology, low-voltage ride-through, and four MegaFlux permanent magnet generators — a departure from previous models deployed since the modern wind energy industry began in the early 1980s.

In early 2007, with growing sales orders and more on the horizon, Clipper ramped up operations significantly, increasing its plant floor space from 215,000 to 330,000 square feet and its plant workforce from 154 to 297. The ISO 9001:2000 Quality Management System was used to maintain a high standard in the turbines produced. Despite these processes, its first production machines experienced the above-noted teething pains.

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."

Crane convention

To remove the drivetrain on the Clipper turbine, the blade rotor must first be removed, so it made sense to make the gearbox and blade repairs simultaneously. The plan was to ship a new and final-inspected drivetrain to the site, remove the rotor, and begin reinforcing the blades on the ground while mounting the replacement drivetrain on the tower. Once the rotor blades were reinforced, the rotor would be reattached and the turbine recommissioned. The defective high-speed pinions were replaced as part of the drivetrain remediation process.

Bringing down eight sets of rotors and gearboxes, and replacing them on-site also proved to be quite an undertaking. Four cranes were required at Steel Winds: one crane dropped the rotors, one replaced the gearboxes, a third reinstalled the rotors, and the fourth performed other various activities.

"The crane usage is one of the more costly elements," said Maurer. "We organized shifts so we could have staff installing rotors 24 hours a day in order to obtain the best efficiencies with our crane operations."

As well as Clipper technicians, QC experts from Europe, Occupational Safety and Health Administration safety staff, independent engineers representing customers, crane operators, training staff, and many others were present at the Lackawanna facility.

By all indications, this wind turbine remediation project will be no more than a hiccup in Clipper’s long-term goal to compete with the established wind turbine manufacturers. Clipper finished work at Steel Winds in May. From June 1 through the end of September, the plant’s availability rating was 96%.

Moving on

Having produced some 400 turbines to date, Clipper is targeting another 300-plus wind turbines by year-end 2009 and will expand production further the following year. According to Maurer, the company currently has sufficient plant capacity and equipment, a trained workforce, and processes in place to assemble more than 500 Liberty turbines annually, with potential for further capacity extensions (Figure 3).

3. Gearboxes good to go. Upgraded gearboxes under assembly at the Clipper Windpower Cedar Rapids wind turbine manufacturing facility. Courtesy: Clipper Windpower

Sales of the Liberty hardly skipped a beat. The company sold 370 2.5-MW turbines in 2006 and 825 units in 2007. Most of last year’s orders were from existing customers, such as First Wind, and were confirmed once the RCA was completed. Furthermore, the company has announced an additional 1,500 units (3,750 MW) in joint development and contingent sale agreements.

Encountering problems with a new design, produced in a new manufacturing plant while key vendors are ramping up their production, is not unusual. What is unusual is that Clipper didn’t immediately jump to conclusions about the cause of the failure. Instead, it took time to identify the root cause and then modified its manufacturing processes, and those of its key suppliers, to ensure that the problem never happens again. That is the real definition of quality.

— Drew Robb is a Los Angeles-based writer specializing in engineering and technology issues.

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