Natural frequency of offshore wind turbines on monopiles in sand

Abstract

Offshore wind turbines are important for developing sustainable alternative energy sources. Assessing their natural frequencies is a major challenge since resonance can cause damage and even failure, especially for long-term dynamic behavior owing to environmental loads. Several model tests were conducted to understand the long-term dynamic responses of monopile-supported wind turbines buried in sand. An electrodynamic actuator was used to obtain the resonant responses of the structure and a cyclic loading device based on eccentric masses was used to apply long-term cyclic loading. The development of the natural frequency under long-term cyclic loading was investigated for two relative sand densities, three cyclic load amplitudes, and four cyclic numbers. Based on the test results, the frequency increased with increased cyclic number owing to the densification of sand. A larger amplitude results in a larger frequency for medium-dense sand, whereas the opposite tendency exists for dense sand; this is attributed to the reduced embedded depth of the monopile in dense sand accompanied by the subsidence of sand. A non-dimensional framework was used to interpret the change in the natural frequency when subjected to long-term cyclic loads. The empirical equation with the non-dimensional parameter provides an assessment of the long-term natural frequency.