Whirling Motion of Monopile Offshore Wind Turbines Subjected to Harmonic and Random Base Excitation

Abstract

In April 2014 a series of 30:1 scaled shake table test of the NREL-5MW monopile offshore wind turbine were conducted. Significant whirling motions were observed during the test [1], which were seldom reported for civil engineering and offshore engineering structures. This study is thereby devoted to revealing the mechanism of the whirling motion of a typical tall monoplie offshore wind turbine. The original shake table model is simplified into a uniform cantilever beam with a large lumped mass on the top. Two nonlinear coupled integro-differential equations of motion containing cubic nonlinearities due to curvature and inertia are used and solved by both analytical and numerical approaches. The ElCentroExp random base excitation and lateral harmonic base excitations with different amplitudes and frequencies are considered in the analysis to fathom the instability mechanism. The analysis results show that, for harmonic base excitations with frequency within a specific range, when the amplitude of base excitation and the initial perturbation exceed a certain threshold, stable whirling motion response will be triggered and the motion of shake table model will be controlled by whirling. However, the analysis results also demonstrate that, subjected to the ElCentroExp base excitation or its equivalent lateral harmonic base excitations, neither analytical nor numerical approach can produce whirling motion response, regardless of the damping ratio. The LM model will always show planar motion response. This differs significantly from the observation of shake table model test.