Effect of Coupled Non linear Wave Kinematics and Soil Flexibility on the Design Loads of Offshore Wind Turbines

The design driving loads on offshore wind turbine monopile support structures at water depths of 35m, which are beyond current monopile installation depths, are derived based on fully coupled aerohydroelastic simulations of the wind turbine in normal operation and in storm conditions in the presence of flexible soil conditions. The impact of moving to 35m water depths on monopile sub structure loads is quantified using irregular non linear wave kinematics interactions with the reduced natural frequencies of the sub structure resulting from soil flexibility. The wave kinematics is modeled using a second order nonlinear irregular wave formulation and further corrected to satisfy the free surface conditions using a Taylor series expansion about the mean water level. Such a wave kinematic representation allows direct representation of the wave crest kinematics above the mean sea level without the need for geometric stretching methods. The effect of the nonlinear wave interaction sum
frequencies on the support structure is investigated when the structural natural frequencies are reduced due to soil flexibility. The impact of the wave sum frequencies during the occurrence of extreme waves under storm conditions and on normal operation fatigue loads is brought out. The fatigue damage equivalent loads are evaluated also with wind/wave misalignment, which result in highly increased loads in the tower side to side direction. The results underscore the need for reduced uncertainty in soil properties and adequate damping in the support structure during wind/wave misalignment, without which monopile sub structural loading is highly amplified at 35m water depths compared to the design conditions at 20 m depths.