ABSTRACT
Sensitivity of the Offshore Wind Turbine (OWT) natural frequency on the monopile length lower and beyond the active length was provided for the sake of safe design purpose. Four types of monopiles having aspect ratios (Depth to the diameter of the foundation) Lp/D of 4, 5, 6 and 8 were studied numerically. The analytical model proposed here for constructing the OWT natural frequency considering soil-pile interaction can be conveniently constructed by monopile head springs characterized by lateral stiffness KL , rotational stiffness KR , cross-coupling stiffness KLR and parabolic soil stiffness variation with depth. However, recent research conclusively showed that if the monopile length lies below the certain depth so called active length, it might cause shifting of the natural frequency to the forcing excitation frequencies range.
