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This paper addresses the statistical uncertainty of loads prediction using structural dynamics simulation codes and the requirements for the number and duration of simulations for obtaining robust load estimates. Substantial statistical variation is observed in loads data and a statistical model that enables extrapolation and determination of quantiles is presented. Further reduction in the numerical work necessary to determine extreme loads with an acceptable uncertainty is possible using a stochastic process model for the dynamic responses. The concept of ultimate loads from extreme wind is very similar to the traditional approach to wind loading adopted in codes for wind action on structures, e.g., ISO 4354.1 This approach was established by Davenport2 and was applied to wind turbines in Madsen.3 This statistical approach forms the basis for the Danish wind turbine design standard.4 However, in recent years aero-elastic time-domain load prediction codes have been developed and verified, and are now widely used by the industry. These codes are used not only for calculation of operational loads but also for extreme wind loads. For this study a model of the NREL/NWTC Unsteady Aerodynamics Experiment Phase III turbine was used, see Figure 1. The turbine has a rotor diameter of 10 m and nominal rotor speed of 72 rpm. It is a three bladed turbine with a hub height of 17 m and operates in free yaw downwind of the tower. Loads calculation : In this study two design conditions of the IEC 61400-1 have been considered: · Power Production · Parked (standing still or idling). During the Power Production design condition the wind turbine was modeled as operating in its natural mode, i.e., free yaw and a nearly constant rotation speed of 72 rpm (with small variations due to the generator slip). If we assume that the load case under consideration properly reflects a design driving condition two questions immediately come to mind: · How to determine the characteristic/representative ultimate load for design verification, and · How many simulations are required for a sufficiently accurate estimate of the ultimate load?
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