Modeling and mitigation of excessive dynamic responses of wind turbines founded in warm permafrost

Field monitoring data provide evidence of excessive vibration on the wind turbines founded in warm permafrost (referring to soils frozen for more than two consecutive years with a temperature close to its melting point), which was attributed to resonance between the rotor and the tower-foundation system. This study presents a finite element (FE) model of the turbine-tower-foundation-soil system for analyzing causes of resonance and assessing effectiveness of mitigation measures. This model decouples the aerodynamic effects of the rotor from the turbine system. Aerodynamic simulation of the rotor was performed to provide wind thrust time series on the tower top. The lateral interaction between pile foundation and warm permafrost was modeled by distributed p-y springs and details on how to evaluate the permafrost p-y curves were provided. The FE model was used to investigate the sensitivity of wind turbine structural fundamental frequency to the permafrost temperature. It is found that, in addition to 9% seasonal change, the turbine-tower-foundation-soil fundamental frequency can decrease by 7% when the permafrost temperature increases from −2 °C to 0 °C. Analyses by using this model show that resonance can occur when the rotor speed is in the vicinity of the structural fundamental frequency, and increase the fatigue load on both the tower and foundation. The resonance is likely caused by the seasonal and long-term change in fundamental frequency of the turbine-tower-foundation-soil system and the inability of the controller to adjust its control parameters with time. Results show one tuned-mass-damper of 3% mass ratio is capable of reducing the tower peak acceleration and displacement during resonance by 50%, and reducing the peak shear and bending moment in the foundation by 40%. Adopting a controller with parameter updating capability together with using passive structural control techniques such as tuned-mass-dampers may be an effective control design strategy for wind turbines founded in warm permafrost.