The Dynamic Instability And Pushover Analysis Of Offshore Monopile Wind Turbines

With more and more offshore wind farms constructed in regions of active seismicity,the dynamic response and structural safety of offshore monopile wind turbines subjected to earthquake,wave and aerodynamic loads are attracting more and more attention.In recent two joint earthquake-wave shake table model tests conducted in Dalian University of Technology,subjected to unidirectional excitations,the scaled models of offshore wind turbines exhibited significant bi-directional whirling motions with elliptical nacelle response tracks,indicating the existence of dynamic instability.Such dynamic instability of offshore monopile wind turbines may lead to structural responses larger than the responses predicted by linear structural dynamic theories,thus significantly undermining the structural safety.Therefore,it is of great importance to investigate the triggering mechanism and vibration pattern of the dynamic instability of offshore monopile wind turbines.Using a 64:1 scaled simplified model of the NREL-5MW offshore monopile wind turbine in 25m-depth water as the underlying structure,this paper studied the dynamic instability of offshore monopile wind turbines both theoretically and experimentally.For the theoretical part of this study,two nonlinear coupled integro-differential equations of motion containing cubic nonlinearities due to curvature and inertia are used and solved by both a multiple-scale method and a numerical Galerkin method for the simplified model of vertical cantilever beam.Different from literature,the top mass on the beam is considered in the present study to reflect the effect of heavy nacelle.The results from the multiple-scale method show that,subjected to unidirectional harmonic base excitations with a large amplitude and a frequency close to the natural frequency of the structure,the offshore monopile wind turbines may experience dynamic instability,showing bi-directional whirling motions instead of planar motions.For harmonic excitations,the results from the numerical Galerkin method agree well with the results from the multiple-scale method.For narrow-band random excitations and scaled seismic excitations,the results from the numerical Galerkin method show that,it is easier to trigger the dynamic instability with intense harmonic or narrow-band random excitations with structural resonant frequency,while it is harder to trigger whirling motions with seismic excitations.For the experimental part of this study,extensive shake table tests are carried out for the scaled shake table model,including various harmonic and scaled seismic excitations loaded in one or two directions.Eight load cases with unidirectional harmonic excitations triggered the dynamic instability of the scaled model,while all load cases with unidirectional scaled seismic excitations failed to trigger dynamic instablility.The experimental results show good qualitative agreement with the theory,indicating that the dynamic instability of offshore monopile wind turbines tends to be induced by large harmonic excitations with structural resonant frequency.The experimental results also show that it is easier to trigger dynamic instability with bi-directional excitations.It is worth mentioning that before this study the whirling motion of a flexural cantilever beam has been never observed in laboratories.Our research thus has filled up the validation gap between analytical analysis and experiments.Besides,in order to investigate the elasto-plastic behavior of offshore monopile wind turbines,this paper also conducted a number of static Pushover analysis on the original NREL-5MW offshore monopile wind turbine operating in 25m-depth water,using eight lateral load patterns in which four new patterns take the effect of the combination of seismic and wave loads into account.