Research on the natural frequencies of an integrated offshore wind turbine model considering blades

The offshore wind turbine (OWT) is a high-rise structure that is extremely sensitive to dynamic loads such as wind, waves, and earthquakes. Therefore, in order to avoid resonance, it is extremely important to accurately calculate the natural frequencies of the OWT at the design stage. The integrated finite element model of DTU (Technical University of Denmark) 10 MW OWT is established based on OpenSees finite element software. The correctness of the superstructure simplification method is confirmed by comparing it to the frequencies calculated by the wind turbine dynamic analysis software HAWC2 for the single-blade model and the fixed constraint model at the bottom of the tower. The frequencies and mode shapes calculated by the overall model are compared with those calculated by the fixed model at the mudline and the concentrated mass model at the top of the tower. Then, the parameters of the soil shear modulus G0, void ratio e, pile embedded length L, pile diameter D, water depth, and blade stiffness are analyzed using the lumped mass model and the blade model at the top of the tower, and some regular conclusions about the variation of natural frequency with the parameters are obtained. Finally, based on the blade model, the influence of short-term cyclic loading on the natural frequency for OWT monopiles in dense sand is studied. The results indicate that the pile-soil interaction has a significant impact on the natural frequencies, the blade has a higher influence on the second bending frequency of the OWT, and the front-after direction is more significantly impacted than the side-side direction. The short-term cyclic loading are unlikely to significantly affect natural frequency for OWT monopiles in dense sand.