Dynamic performance of suction caisson supported offshore wind turbines under different operational conditions

Suction caissons are promising foundations for offshore wind turbines in shallow coastal regions. To investigate dynamic performance of suction caisson supported offshore wind turbines under various operational conditions, a series of wind tunnel tests were conducted on a scaled model based on the NREL 5 MW wind turbine. The model was designed and fabricated in accordance with similitude principles and installed in saturated sand using suction installation methods. Eleven scenarios were analyzed, ranging from cut-in to cut-out wind speed, with pitch adjustment failure taken into account. The dynamic responses, including nacelle displacement, tower-top acceleration, tower-bottom bending moment, and earth pressure variations, were evaluated and compared under different conditions. The results revealed that increasing wind speed reduces the natural frequency while increasing the damping ratio. When approaching failure, the ground support conditions become notably unstable, as evidenced by significant fluctuations in earth pressure behind the suction caisson and in the tower-bottom bending moment. The unstable ground support conditions amplify variations in both the natural frequency and the damping ratio. Several regression equations were developed to estimate rotation angle of foundation, natural frequency, and global damping ratio enabling better practical design of suction caisson supported offshore wind turbines.