Sweden’s Accident Investigation Authority (SHK) has blamed fatigue in bolts on the tower of a Vestas V112-3.0MW turbine that collapsed in late 2015, highlighting inadequate assembly.
The turbine collapse at the 98MW Lemnhult wind farm south of the town of Vetlanda in Jönköpig county on Christmas Eve 2015 was the first such incident involving one of Vestas’ MW-class machines.
The SHK in a summary of its investigation said bolts that had held together the flange between the first and second of six sections in the tower structure failed, and the upper part of the tower fell.
“The bolts that had held together the joint had suffered from a fatigue process and the bolts could no longer withstand the loads of normal operation,” the SHK said. It added that the cause of the bolts’ fatigue was that the pre-tension force in the joint was too low as bolts, tower sections and tools had not been protected from rain and snow during installation.
That affected the lubrication of the nuts, which led to increased friction in the contact surfaces of the joint.
“The assembler who performed the final torqueing of the bolts had no previous experience from such work, and had not received the manufacturer’s internal training course to that end,” the SHK said.
As a result, the SHK report recommended Vestas should follow up on compliance with instructions and manuals on installation sites.
The company’s own initial internal investigation into the incident made public last year also pointed to a failure in the bolted flange between tower sections one and two.
Vestas head of external communications Anders Riis said: “We welcome the Swedish Accident Investigation Authority’s engagement and are satisfied that the findings in their report confirm the conclusions of Vestas’ root cause analysis.
“It is correct that the employee who gave final torque to the failed flange hadn’t received the prescribed training. Today, we control all site workers’ training certificates and have Vestas employees leading and controlling subcontractors’ work at site.
“Additionally and for precautionary purposes, Vestas has also issued global instructions underlining the correct procedure for both bolt handling and pre-tensioning process to avoid any recurrence.”
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