Swedish officials have pointed to incorrect bolt installation as the root cause of a Vestas turbine collapse at the Lemnhult wind farm in late 2015.
Government accident investigation authority SHK said the three-year-old V112 which failed on 24 December showed clear signs of fatigue and corrosion at the join between the bottom and second section of the tower.
“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 flange surfaces in the main wind direction showed signs of corrosion which most likely originated from the bolts.”
SHK said the cause of the fatigue was “the pre-tension force in the joint was too low”.
It said the reason for not achieving the required standard “was due to the bolts, tower sections and tools not being protected from rain and snow during installation”.
SHK added that the tools were not maintained properly and that the assembler who performed final torqueing of the bolts “had no previous experience” and “had not received the manufacturer’s internal training”.
Lemnhult also experienced problems with “loose and broken bolts” prior to the accident. “These were not reported by the operator,” said SHK.
Officials recommended that Vestas “follow up on the compliance of instructions and manuals on installation sites”.
Vestas head of communications Anders Riis said the company is “satisfied” with the findings, which confirm the root cause analysis carried out by the manufacturer.
“As a result of the incident and for precautionary purposes Vestas has issued global instructions underlining the correct procedure for both bolt handling and pre-tensioning process to avoid any recurrence,” he added.
Siemens last year suffered a similar bolt failure at a wind farm in Hawaii, albeit at the top of the tower. Incorrect tensioning was again the main suspect.
Image: the collapsed turbine (SHK)
