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 b [Adhikari and Bhattacharya, 2012] assimilates the offshore win turbine as an Euler-Bernoulli beam on a flexible foundation wit lateral and rotational springs. The key factor in the evaluation o the natural frequency is the value of the stiffness of these spring In this way, this paper presents a method combinin experimental measurements and a finite element model o Abaqus which allows a precise evaluation of the stiffness of th springs. The proposed method is compared to the existin methods used to evaluate the soil's stiffness (such as [Eurocod 8, 2003]). The suggested method gives a fine evaluation of th response of the structure with a mean deviation below 1% compared to the average errors obtained for the previou methods ranging from 6.6 to 17.4%. today are rated at 5 MW as described by [Jonkman et al., 2009] and submitted to various dynamic loads as shown in Figure 1. In fact, the structure is subjected to wind, waves and currents with an excitation frequency below 0.2 Hz. The rotor frequency needs also to be taken into account and varies between 0.12 and 0.2 Hz. In addition to this, the frequency of the blades passing in front of the mast in the bandwidth from 0.36 to 0.6 Hz is considered. To avoid any resonance phenomenon, the offshore wind turbine is designed with a flexible mast and a rigid foundation so that its first natural frequency is between 0.2 and 0.36 Hz. This structure is called "soft-stiff". In this study, a structure founded on monopile is considered, as around 75% of the current offshore wind turbines have such foundation.