The problem first surfaced on the morning of 5 April at MidAmerican Energy's recently completed 200MW Eclipse wind farm in Iowa, when a company service technician discovered a blade from one of the 87 turbines lying on the ground.
Siemens convened a team of experts to conduct a root-cause analysis, but six weeks later a second overnight blade failure occurred, this time at Pattern Energy's 265MW Ocotillo project in California. The turbine maker immediately curtailed all turbines using the B53 blades, the type involved in the two incidents. About 700 turbines were affected, most located in the US. Perhaps more importantly, the failures dented Siemens reputation for reliability.
Possibly with that in mind, the company moved quickly to find answers and announced the results of its investigation in early July.
"The analysis revealed that the root cause of the two incidents is not related to the design of the blade," said a Siemens spokesperson.
The fault that caused two B53 blades to break off Siemens' SWT-2.3-108 wind turbines in the US has been traced back to an adhesive bonding failure after an intensive investigation by the manufacturer.
The fractures occurred at the base of the blade, where pre-cast metal inserts are used to attach it to the nacelle. The adhesive used to bond those inserts to the fibreglass laminate of the blade failed, causing it to separate from the hub. The reason, the company said, was that the surface preparation of the inserts was not sufficient to allow the adhesive to stick.
The inserts, or root segments, are manufactured for Siemens by a number of third-party suppliers.
The attachment of blades provides little margin for error, said Aris Karcanias, a managing consultant at Navigant's BTM Consult: "It's where a lot of loading occurs and, therefore, it is a delicate procedure around how you bond the metal to the composite. But it's a well-drilled procedure in the wind industry and is not really a common source of failure."
Most of the turbines using B53 blades were back in service by early July. But the inspections turned up an unspecified number of blades that showed signs of delamination and would have to be replaced, said Siemens.
The company added that as a precautionary measure, it would apply a minor field modification to all existing B53 blades to enhance the bond between the root segments, an upgrade Siemens says is in compliance with DNV's certification for that blade type. The minor field modifications to existing B53 blades could be seen to signify that Siemens is confident in its solution.
However, Pattern Energy said seven blades at the Ocotillo wind farm needed to be replaced. The company shut down all 94 turbines at the project after the blade break and is now bringing them back online one at a time, work that is expected to continue at least through to the end of July.
Siemens did not respond to enquiries about the cost of curtailing the turbines or the name of the third party supplier.
MidAmerican declined to comment on the incident or the status of the Eclipse project, referring all questions to Siemens. There were no injuries at either project as a result of the blade breaks.
One impact of the blade failures, he says, could be that already rigorous quality control standards for supplied components are tightened. In addition, Karcanias added, there is already a movement in the industry towards pre-emptive maintenance and condition monitoring to head off potentially expensive problems before they can occur. "Nothing is fail-safe, but the industry is already proactively seeking solutions to failure before they happen."