A New Identification Method for Bolt Looseness in Wind Turbine Towers

Table 6

Vibration characteristics before and after bolt tightening of the six wind turbine towers.

Tower ID	Comparison of first-order natural frequency before and after the bolt retightening	Comparison of first-order damping ratio before and after the bolt retightening	Comparison of first-order vibration mode before and after the bolt retightening	Comparison of first-order phase difference before and after the bolt retightening
1	No change	Difference is less than 2%	Similar	There is a sudden change in the phase difference between the upper and lower plates before tightening, and the sudden change disappears after tightening
2	No change	Difference is less than 2%	Similar	There is a sudden change in the phase difference between the upper and lower plates before tightening, and the sudden change disappears after tightening
3	No change	Difference is less than 2%	Similar	There is a sudden change in the phase difference between the upper and lower plates before tightening, and the sudden change disappears after tightening
4	No change	Difference is less than 2%	Similar	There is a sudden change in the phase difference between the upper and lower plates before tightening, and the sudden change disappears after tightening
5	No change	Difference is near 10%	Similar	There is a sudden change in the phase difference between the upper and lower plates before tightening, and the sudden change disappears after tightening
6	No change	Difference is less than 2%	Similar	There is a sudden change in the phase difference between the upper and lower plates before tightening, and the sudden change disappears after tightening