Experimental and Numerical Studies on the Dynamic and Long-Term Behavior of Offshore Wind Turbines in Clay

Monopile foundations for offshore wind turbines (OWTs) are exposed to long-term dynamic loads from wind and waves. Hence, the long-term dynamic behavior of a monopile supported OWT is necessary for the sake of stability during the serviceable period. To assess the safety of the system, the serviceability limit state regarding the allowable tilt of the monopile at mudline is satisfied, and the fundamental frequency of the system is kept away from the rotor and blade passing frequencies. The present study investigates the long-term performance of monopile supported OWT in clay using a series of scaled model tests. The fundamental frequency and damping of an OWT system and the rotation and lateral deflection of a monopile head are examined for various load cycles with different amplitudes. The effect of long-term loading cycles and amplitude on the soil-pile stiffness is evaluated. It is observed that the fundamental frequency of the system decreases with the number of load cycles, whereas damping of the whole system is increased. The responses regarding accumulated rotation and deflection at monopile head are found to be increasing with the number of load cycles. For the numerical simulation, soil is modeled as viscoelastic material with a stiffness degradation function in three-dimensional finite element analysis. Finally, the fundamental frequency and response of the OWT obtained from the model test are validated with numerical results. A good agreement is observed between experimental and numerical results.