Wind Turbine Intermediate Foundation Performance through Frequency Analysis

The current practice in designing the foundation for the onshore wind turbines (WT) relies on the use of rotational stiffness to size the foundation such that the serviceability requirements are met. A rotational stiffness is considered acceptable if it provides the right boundary (foundation) rigidity to ensure a non-amplified dynamic response of the WT. The common method used to verify acceptable foundation performance is through direct measurements of applied moment due to wind loading and direct measurement of tilt. The accuracy of the tilt measurements is questionable due to the small tilt magnitude, and the required high frequency of tilt measurements. This paper presents an alternative method to evaluate foundation stiffness from the dynamic response of the WT by estimating the WT fundamental frequency (f(n)). Numerical model is then calibrated by changing the boundary rigidity until the f(n) from the model matches the f(n) obtained from the WT dynamic response. The rigidity from the calibrated numerical model is then compared to the design intent for the foundation, which allows for the evaluation of the foundation performance.