Performance Levels and Fragility for Offshore Wind Turbine Support Structures during Extreme Events

Successful development of the offshore wind energy resource depends upon the reliability of the structures that support the electricity-generating rotor-nacelle assemblies that form the core of a wind turbine system. The risk posed to offshore wind turbines (OWTs) by extreme environmental events such as Atlantic hurricanes requires a more detailed examination of post-elastic structural performance during events occurring at greater return periods than the design conditions. This paper focuses on the assessment of the fragility of OWTs with jacket-type support structures under extreme environmental loadings. Performance levels ranging from undamaged to near collapse of the jacket supported OWT are defined and are assessed using static pushover analysis of the support structure for given wind and wave conditions. A numerical example is presented based on an environmental hazard model for a site off the coast of the state of Massachusetts along the Atlantic coast of the U.S. Using probabilistic models for the structural demands and capacities, fragility curves are developed for two damage states. It is shown how such fragility curves can be convolved with estimates of the economic consequences associated with the various performance levels to develop a complete risk profile for the support structure.