Risks associated with wind-turbine blade failures

The CEGB is investigating the potential of large wind turbines for generating electricity. The paper considers the risk to people and property, in the unlikely event of a serious structural failure of the main rotor. The greatest risks are found very close to the machine, typically within one blade diameter of the tower base. Outside this area, the overall probability of being struck by a blade is around 10¿¿7 per year for a fixed building. For people who are mobile, the risks are substantially less. The range obviously depends on the initial speed, which is shown to be limited by aerodynamic effects to around Mach 0.8 or 0.9. It also depends on the configuration of the fragment during its flight. General reasoning suggests that gliding effects are unlikely to occur; the fragment tumbles and this simplifies the calculation considerably. Blade or fragment throw distances would typically be a few hundred metres or up to 800m for a once-in-fifty-years high-wind runaway. Probabilities of impact for a projectile are typically below 10¿¿5 per square metre within two or three hundred metres (and 10¿¿6/m2 for high-wind runway over a maximum range of 800 m). The possibility of a stable flight configuration, though very remote, does exist. Calculations incorporating the effects of low drag and high lift, which would apply in such circumstances, predict ranges of more than 2km. However, the risk levels are reduced by an order of magnitude, as compared with tumbling fragments, due largely to their inverse dependence on the area of the potential hazard zone.