The idea, from Clipper's project managers in the field, eliminates the need for $200,000 cranes by using a pair of ordinary industrial trucks - and could forever solve one very expensive wind industry headache.
A hydraulic bucket truck, a flatbed with a winch and a specialised toolkit worth $15,000 is all it takes, says Clipper vice-president Craig Christenson, an engineer with 15 years' wind experience. "You can go down to your local industrial equipment rental place and rent the trucks," Christenson says. "This took very little modification to meet our needs."
Christenson traces the idea back to 2008, when Clipper faced a fleet-wide remediation programme to correct blade problems on hundreds of its 2.5MW turbines and began calling out traditional construction cranes. "Some of these cranes in the US, the lattice-style cranes, come in on 25 or 30 trucks," Christenson says. "To bring in a large crane just for one blade, you add $200,000 for that replacement to what is typically about $100,000 a blade. You've now tripled the cost."
Last year, partway through remediation, the Clipper field technicians proposed the idea. "My initial gut reaction was, it's impossible. The machine wasn't designed for that," says Christenson. "But they came back and showed us; we just couldn't take no for an answer."
Winch, bucket truck and more
A group of Clipper engineers worked with a crane company to perfect the method: winch on one side, bucket truck on the other, point the blade downward, connect the cables, unbolt and lower.
"The basic instruction is very specific: you can't drop it," Christenson says. "But we had to put very detailed procedures in place, and safety conditions, so that, number one, no blade was dropped - and, number two, no one was hurt. Now it's standard procedure for us."
Clipper used the new method to finish off the final third of the system-wide blade retrofit project, completed in January. And the company sees ongoing savings because of the small-scale trucks involved. Many of Clipper's turbines, planted in Midwestern farmland, are built with nine-metre-wide construction roads that are subsequently narrowed to three metres for the 20-year operation cycle.
"It's a huge deal to bring in these large cranes afterwards on some of these projects," Christenson says. "You have to pay the farmer for the damaged crops, you have to pay for the road to be rebuilt. In some cases, rebuilding the road can be as expensive as the crane costs."
Clipper's toolkits can be shipped to project owners. "These machines do get hit by lightning," Christenson says. "Typically, there are reasons that you'd want to take a blade off over a 20-year period, so it might be justified by an owner to have their own set of the tools."
Meanwhile, Christenson realises that other companies are working on related methods and a lucrative new market is up for grabs. GE Energy, he believes, is working on a system that uses a giant sock to lower blades. Vestas has a system where a small crane is winched up to the hub and bolted to the top of the tower (Windpower Monthly, September 2009). While Clipper knows its method will be hard to beat in simplicity or cost, Christenson says the company is hesitant to let others use it before a patent is secured - a process that could take up to two years.
"We want to own the intellectual property, the exact tooling that's used up in the turbine to execute the blade change-out safely," he says. "So what we have is a solution and a patent application in place on it. But I wouldn't be surprised if GE has this alternative and they're trying to protect it as well. We're all racing to find ways to change the game."