In the present paper the effects of aerodynamic damping and earthquake loads on the dynamic response of flexible-based wind turbines are studied. A numerical analysis framework (NAF) is developed and applied. NAF is based on a user-compiled module that is developed for the purposes of the present paper and is fully coupled with an open source tool. The accuracy of the developed NAF is validated through comparisons with predictions that are calculated with the use of different numerical analysis methods and tools. The results indicate that the presence of the aerodynamic loads due to the reduction of the maximum displacement of the tower attributed to the dissipation of earthquake excitation energy in fore-aft direction. Emergency shutdown triggered by strong earthquakes results to a rapid change of aerodynamic damping, resulting to short-term instability of the wind turbine. After shutdown of the wind turbine, enhanced dynamic response is observed. For the case where the wind turbine is parked, the maxima displacement and acceleration of tower-top increase linearly with the peak ground acceleration. With the use of the least-square method a dimensionless slope of tower-top displacements is presented representing the seismic response coefficient of tower that can be used to estimate the tower-top acceleration demand. Moreover, on the basis of the seismic response coefficient, an improved model for the evaluation of load design demand is proposed. This model can provide accurate predictions. KEYWORDS aerodynamic damping, dynamic behavior, earthquake intensity, seismic demand, wind turbines | INTRODUCTIONWind energy technology has a leading role in the renewable energy sector. 23.4 GW of new installed capacity of wind turbines that was added in China 1 for 2016; this value corresponds to the 42.8% of the overall added capacity globally for the same year. New installed wind farms are located along the northwest and southeast coasts of China. These wind farms are located in earthquake prone areas, close to the Eurasian and Pacific seismic belts. Wind farms that are located in the aforementioned areas are susceptible to damage on the event of an earthquake.Over the past decades it has been well accepted that earthquakes have a significant effect on the structural dynamic response of wind turbines. 2 Response spectrum method and finite element method (FEM) have been commonly used for the estimation of the seismic load demand of wind turbines. 3 With the use of the response spectrum method the rotor and nacelle are modeled as point lumped masses at tower-top. The seismic load demand of wind turbines can be calculated on the basis of the natural periods, mode shapes, and mass distribution of the tower. 4,5 Usually the aerodynamic damping and higher-order modes are neglected because of the overestimation of the seismic load demand. On the basis of experiments, 6 it is concluded that the seismic excitation increases significantly the lateral displacement of tower-top. On the other hand FEM is applied for the analysis of win...

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This paper presents the simulation of offshore wind turbines (OWTs) suffering from turbulent wind and ice-induced vibrations (IIVs). To ascertain the effectiveness of IIVs, the fully coupled simulation of the NREL 5 MW OWT is implemented. The OWT model, which is processed as a multibody system, takes the aerodynamic load and the IIV simultaneously. Firstly, the Kaimal spectrum is used to simulate the turbulent wind conditions. Then, the aerodynamic load acting on the blades is solved by the blade momentum theory. The IIV can be explained as the forced vibration or the self-excited vibration. The Matlock model and the Määttänen model are employed here to solve the forced ice-induced vibration and the self-excited ice-induced vibration, respectively. Finally, the kinetics and the kinematic computation are coupled with aerodynamic load calculations. The dynamic responses of some crucial parts of the OWT model reveal some important results. The results prove the great effectiveness of IIVs impacting the OWTs, especially for the tower. The frequency-domain responses note that the IIV affects the particular frequencies remarkably.

The offshore wind turbines (OWTs) constructed at the northern sea areas under cold climate are frequently subjected to floating ice loads. It is imperative to reduce the damage owing to the floating ice with some appropriate approaches. The purpose of this paper is to ascertain the effectiveness of the tuned mass damper (TMD) and the ice-breaking cone for reducing floating ice loads on OWTs. The National Renewable Energy Laboratory's (NREL) 5 MW OWT, which is treated as a multibody system with rigid and flexible parts, is adopted as the example model here. The multiple loads taken into consideration in the fully coupled simulation include floating ice and turbulent wind. The aerodynamic load acting on the blades is solved by the blade element momentum method based on a full-field turbulent wind farm generated by the Kaimal spectrum. The Matlock model and the Ralston model are adopted for evaluating the floating ice loads on the cylindrical and conical structures, respectively. The TMD system in the nacelle and the ice-breaking cone on the tower at the mean sea level are the two load reduction approaches of concern in this paper. A weak aeroelastic simulation of the OWT model is conducted. The solution of flexibility effectiveness depends on some accurate mode shapes by the linear modal representation. Finally, Kane's method is used for predicting the motion of the whole OWT. The relevant results reveal some positive effectiveness of the TMD system and the ice-breaking cone for reducing the floating ice load. The displacement of tower top decreases significantly with the utilization of the two approaches. The TMD system has a better performance for the side-side displacement than the fore-aft displacement. By switching the ice failure mode from crushing to bending, the ice-breaking cone reduces the floating load more effectively than the TMD system. It affects equally significantly the fore-aft and side-side displacements.