# Recent Advancement in Assessment and Control of Structures under Multi-Hazard
## 6 Multi-Hazard Assessment and Control of Wind Turbines
### Multi-Hazard Assessment of Wind Turbines  
[PERSON] [155], as well as [PERSON] and [PERSON] [156] highlight the importance of multi-hazard consideration in site-selection and design of wind turbines. They present a multi-hazard probabilistic framework to evaluate the structural reliability of offshore wind turbines. Considering wind and seismic action, their results show that annual probabilities of failure are higher when seismic action is considered. Comparing two identical wind turbines, one in the Gulf of Mexico and the other off the coast of California, they conclude that, although the latter location is more favorable in terms of power production, annual probabilities of failure are higher due to higher seismicity. [PERSON] et al. [157] present a Monte Carlo simulation-based framework for the estimation of multi-hazard fragility curves of wind turbine structures. They provide an example application of the framework, to derive failure probabilities of a prototype wind turbine structure, conditioned on wind velocity and peak ground acceleration for different operational states of the turbine. Their results show that aerodynamic damping plays an important role in the seismic fragility. Fragility in an operational state, for seismic action in the fore-aft direction, increases with wind speed up to the rated wind speed, after which it starts to decrease. When the rotor is operating at the rated condition or is parked, the probability of failure is larger than 50% and the peak ground acceleration exceeds about 70% of the acceleration of gravity. [PERSON] and [PERSON] [158] present similar conclusions regarding the importance of aerodynamic damping, stating that, while wind action is more damaging at the operational state, seismic action can be more threatening when the rotor is parked. [PERSON] et al. [159] report, for a 5 MW offshore wind turbine, that seismic action contributes more than wind and wave action to structural demands such as base moment and tower-top displacement. They also report on the fragility of sensitive equipment located in the nacelle, which are found to be prone to severe damage at moderate ground shaking intensity. [PERSON] et al. [160] investigated the fragility of a prototype 5 MW offshore wind turbine structure subjected to aerodynamic forces and wave loading. They considered different operational states of the rotor and derived fragility curves for both the supporting tower and the rotor blades. Their results show that, when the wind speed is between the cut-in and cut-out range, exceedance probability of the blade failure is much higher than that of the tower failure. They also highlight the impact of aerodynamic damping in reducing wind-induced vibrations of the tower. [PERSON] et al. [161] studied the effect of soil structure interaction (SSI) on the 5 MW offshore prototype model. Their results show that the fore-aft displacement demand on the tower is significantly affected by SSI. [PERSON] et al. [162] investigated the fragility of a 5 MW wind turbine prototype subjected to wind and seismic action. Their results show that failure due to exceedance of tower-tip displacement and rotation is more likely than yielding or buckling of the tower.