On Assessing the Accuracy of Offshore Wind Turbine Reliability-based Design Loads From the Environmental Contour Method

We discuss the use of the environmental contour method to derive design loads for an active stall-regulated offshore wind turbine. Two different Danish offshore environments, Rodsand and Horns Rev, are considered for the locations of the turbine. The accuracy of the derived design loads is assessed by comparing them with exact solutions derived using full integration over an accurate description of the failure domain. The error in estimating design loads is introduced because two key assumptions of the method are violated: (i) the limit state surface especially in the operating range of the turbine is not well approximated by a tangent hyperplane at the design point; and (ii) failure in any of the possible turbine states (e.g., operating or parked states discussed here) needs to be considered in computing accurate failure probabilities. It is recommended that environmental contours and iso-response curves should be plotted and interpreted before establishing design load levels.