Uncertainty in the Natural Frequency Of wind Turbines Supported on Monopiles in Spatially-Variable Clays

The contribution of wind energy in the global energy mix has reached 1TW milestone in the past year with half of this power achieved in that year. It is planned to triple this capacity by 2030 (Global Wind Report). The major loads experienced by wind turbines are cyclic in nature (wind, wave, and operational loads); therefore, the dynamic analysis of these structures could be a governing design step. In this paper, the uncertainty in the natural frequency of a wind turbine supported on monopiles in clay is quantified using a random finite element model that considers soil variability. 1-Dimensional idealization of the monopile, tower and nacelle with p-y and M-springs representing the surrounding clayey soil is adopted. The recently developed PISA model (Byrne et al. 2020) that is calibrated specifically for monopiles was used to develop the soil reaction curves. The effects of overconsolidation, correlation length, undrained shear strength, soil modulus and amplitude of vibration were investigated. Results indicated that the uncertainty in the natural frequency of the turbine was found to be low even for high soil variability because of variance reduction due to averaging along the monopile depth to fixity and relative soil to pile stiffness.

Keywords: Wind turbines, Natural frequency, Soil variability, Random finite element, Resonance.