Experimental Evaluation of the Natural Frequency of an Offshore Wind Turbine’s Scaled Model

Offshore Wind Turbines (OWT) are slender structures with sensitive dynamics, strongly influenced by the soil-structure interaction. The structure is subjected to cyclic and dynamic loads with frequencies close to the first natural frequency of the offshore wind turbine. To avoid any resonance phenomenon, a precise evaluation of the initial first natural frequency of the wind turbine is essential. The present work deals with the evaluation of the natural frequency of an OWT’s scaled model with monopile foundation. The main factor influencing the natural frequency is the soil-structure interaction which needs to be assessed precisely. To do so, a simple method presented by Adhikari and Bhattacharya, 2012 assimilates the offshore wind turbine as an Euler-Bernoulli beam on a flexible foundation with lateral and rotational springs. The key factor in the evaluation of the natural frequency is the value of the stiffness of these springs. In this way, this paper presents a method combining experimental measurements and a finite element model on Abaqus which allows a precise evaluation of the stiffness of the springs. The proposed method is compared to the existing methods used to evaluate the soil’s stiffness (such as Eurocode 8, 2003). The suggested method gives a fine evaluation of the response of the structure with a mean deviation below 1%, compared to the average errors obtained for the previous methods ranging from 6.6 to 17.4%.