Structural Robustness Evaluation of Offshore Wind Turbines

Wind turbines are complex structures that should deal with adverse weather conditions, are exposed to impacts or ship collisions and, due to the strategic roles in the energetic supplying, can be the goal of military or malevolent attacks. Even if a structure cannot be design to resist any unforeseeable critical event or arbitrarily high accidental action, this kind of systems should be able to maintain integrity and a certain level of functionality also under accidental circumstances, which are not contemplated or cannot be considered in the usual design verification. According to a performance-based design view, the entity of actions to be resisted and the services levels to be maintained are the design objectives, which should be defined by the stakeholders and by the designer in respect of the regulation in force. For what said above, the structural integrity of wind turbines is a central issue in the framework of a safe design: it depends on different factors, like exposure, vulnerability and robustness. Particularly, the requirement of structural vulnerability and robustness are discussed in this paper and a numerical application is presented, in order to evaluate the effects of a ship collision on the structural system of an offshore wind turbine. The investigation resorts nonlinear dynamic analyses performed on the finite element model of the turbine and considers three different scenarios for the ship collision. The review of the investigation results allows for an evaluation of the turbine structural integrity after the impact and permits to identify some characteristics of the system, which are intrinsic to the chosen organization of the elements within the structure.