At around 4:00 a.m., the three blades of a wind turbine fell to the ground in a windstorm. The operator picked up the pieces of carbon fibre strewn for 40 m around the turbine’s tower. Impact marks were visible on the tower. The operator set up barriers and brought in security guards to keep watch.

Commissioned in 2002, the 600 kW wind turbine was shut down for maintenance after its low-speed shaft had completely broken a few days earlier. The breakage caused the rotor to become disconnected from the gearbox, rendering the mechanical brake inoperative. Although the blades were pitched to a safe position (parallel to the wind and with the blades’ air brake engaged), the 25 m/s winds caused the blades to break due to excessive rotational speed.

After assessing the damage, the operator concluded that damage to a bearing supporting the shaft was the most likely cause of the low-speed shaft’s breakage. This may have placed high bending stress on the rear portion, at the gearbox’s inlet, causing it to break. No weaknesses were found in the shaft’s material structure. Inspections performed on the other turbines in the operator’s farm did not reveal any problems.

To prevent this incident recurring, the operator plans to equip all its wind turbines with inductive presence sensors. Connected to the turbine’s control system, it would bring the turbine to a safe condition as soon as the front bearing sags by more than 1 mm. A visual and operational inspection of the bearing/low-speed shaft assembly would then be carried out. In addition, vibration checks of the drivetrain are scheduled at regular intervals to look for any misalignment or unusual stresses.

The turbine was put back in service after its tower was repaired and its damaged parts (blades, gearbox, low-speed shaft) were replaced.