Stiffness degradation of soft clay around offshore monopile is caused by the long-term effect of lateral complex cyclic loading such as wave and wind. Offshore wind turbine structure is a dynamic sensitive structure. It is urgent that the effect of complex cyclic loading on stiffness degradation of soft clay around pile and natural frequency of offshore wind turbine. A series of variable cyclic dynamic shear tests were conducted. The effect of initial shear stress and cyclic shear stress on soften characteristics of soft clay was investigated. It was found that as the initial shear stress is less than the cyclic shear stress, softening index decreases with the increase of cyclic stress ratio. Based on the test results, a soften model of soften clay with considering the effect of initial stress and cyclic shear stress was then built. By combining dynamic motion equation and this soften model of soft clay, a calculation method of natural frequency for offshore wind turbine structure was established to consider the effect of initial shear stress and cyclic shear stress. This method is verified by combining with the results of practical engineering and numerical data. Some parameters influence analysis were performed to explore the effect of amplitude and number of shear stress on the natural frequency of offshore wind turbine structure. The results showed that natural frequency of offshore wind turbine structure decreases with the increase of initial shear stress. As the amplitude and number is increased, the natural frequency decreases.
