Senior Research Engineer FDH Infrastructure Services, LLC Raleigh, North Carolina
Presentation Description: Anchor bolts secure wind turbines to foundations. Insufficient tension in anchor bolts causes excessive fatigue stress and uneven pressure on the turbine foundation. It is important to establish records of bolt tension to be able to identify non-uniform foundation stress, defective anchor bolts, and anchoring system failures. Current practice requires annual tensioning of 10% of the anchor bolts in a wind turbine. This process does not always evaluate the existing tension in the bolts and is labor, time, and cost intensive, involving mechanical tools and hydraulic jacks. A need exists for a nondestructive method that can identify tension in anchor bolts without applying the current tensioning process. Such a nondestructive method would reduce the cost of wind farm maintenance and increase the safety of the maintenance process. This study proposes a nondestructive method to determine residual tension on wind turbine anchor bolts by analyzing the vibration response of the anchor bolts. The core idea of the method is that the natural frequencies of vibration in a cantilever beam is a function of the stiffness of support. In this method, the rotational stiffness of the support at the grip nut is a function of the existing tension in the anchor bolt. Therefore, the natural frequencies of the cantilever part of the anchor bolt is an indication of the tension in the tensioned part of the bolt. A proprietary dispersive wave (DW) testing method is used to obtain the vibration behavior of anchor bolts. With customized software, the vibration response of the anchor bolt, as measured by the accelerometer, is then analyzed in the frequency domain to identify modal frequencies, which have a relationship to the post-tensioned force in the anchor bolt.
Methodology: An empirical model relating the existing tension to the frequency response of the anchor bolts is developed for the wind farm.The tension can then be predicted by using the model corresponding to known parameters of anchor bolt.
Identify the existing tension in wind turbine anchor bolts using the developed model that corresponds to known parameters (length, diameter, different modes of frequency, etc.) for the anchor bolts being tested.