ESTIMATION OF NATURAL FREQUENCY OF OFFSHORE MONOPILE WIND TURBINES CONSIDERING FOUNDATION FLEXIBILITY AND TOWER MASS

With world effort focused on the reducing impact of greenhouse gases, wind (offshore or onshore) has been a serious contender for large-scale, cost-effective and cleansource of energy. In offshore wind energy technology, monopiles are the most frequently used substructures to resist lateral cyclic loads produced by wind and waves [1].Monopiles are the most common sub structure design, especially where turbines of rated capacity less than 5 MW are used in water depths of less than 30m [2]. In offshorewind turbines, support structures are highly sensitive to dynamic loading [3]. Thus, estimation of the structure natural frequency is an important design stage [4]. Inoffshore wind turbines, dynamic properties of the structural system are strongly influenced by the foundation stiffness, especially in monopile support structures [5]. Theaim of this work is to provide an analytical estimation for the natural frequency of monopile supported offshore wind turbines using Rayleigh method. Foundation stiffnesseffect is simply considered as a rotational spring. Simple equations relating natural frequency to the structural parameters including mass and foundation stiffness arepresented which are beneficial in the preliminary stages of the design.