As the monopile supported offshore wind turbine (OWT) is a dynamic sensitive structure, one of the major challenges in its design is the assessment of the natural frequency to avoid resonance during the lifetime. Since the characteristics of OWTs under dynamic loading and their long-term behavior are not fully understood, to study their natural frequency considering soil-monopile interaction, a series of scaled model tests in sand were performed. The first part was about the initial resonant frequency subjected to different forcing amplitudes and the second part was about the change of the natural frequency under long-term horizontal cyclic loadings. Based on the test results, the effects of pile-soil interaction, related to the loading amplitude, embedment depth, soil density, and cyclic numbers, on the natural frequency of OWTs are presented by a non-dimensional group based on the explanation of the governing mechanism. As the soil nonlinearity leads to a degradation in the natural frequency of monopile supported OWTs in the sand and the cyclic loading results in an increase, the choice of the natural frequency closer to the upper limit of the 1P band is suggested in practice based on the tradeoff of the two above effects.
