Abstract:In recent years, harvesting offshore wind energy has become a promising option for overcoming the problem of energy supply while still protecting the natural environment. China is rich in offshore wind energy along the east and southeast coastline, so it makes sense to invest in offshore wind farms. With wind turbine construction moving from onshore to offshore and into deeper and deeper waters, high construction standards and maintenance of the foundations and turbine structures are necessary to meet the adverse offshore environment. Therefore, the technological difficulties and costs of installing offshore wind turbines (OWTs) are increasing markedly and are becoming obstacles to the development of offshore wind energy. OWTs are relatively new structures and are meant to produce energy for 25 to 30 years, but we have no records on their long-term performance. The offshore wind turbine is a slender and flexible structure, and its dynamic behavior is strongly affected by environmental loading and the stiffness of its foundation. The first-order natural frequency of an OWT is generally close to the forcing frequencies imposed by environmental loading (wind, waves, and currents) and the onboard machinery. A change in foundation stiffness results in a change in the natural frequency of OWTs; therefore, rational choices regarding OWT foundations and accurate prediction of their long-term performances are very challenging. This paper establishes a simplified wind turbine model comprised of foundation, tower, and top mass. Based on the elastic restrained bottom condition, the coupling effect between the lateral and rotational stiffness is considered. Based on the improved model, differential equations, and boundary conditions, the effect of foundation stiffness and top mass on the structure's first dynamic response, i.e., its four orders of natural frequency, is studied. The conclusions in this paper can be used to predict the development trend of OWT dynamic behavior based on foundation stiffness variation, and can also guide the design of foundations and support structures in the future. However, rational choices of OWT foundations and accurate prediction of their long-term performances are still very challenging.
