Estimation of long-term structural loads on a wind turbine
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With the introduction of the third edition of the International Electrotechnical Commission (IEC) Standard 61400-1, designers of wind turbines are now explicitly required to use statistical extrapolation techniques to determine nominal design loads. In this study, we use field data from a utility-scale 1.5 megawatt turbine sited in Lamar, Colorado to compare the performance of several alternative techniques for statistical extrapolation of extreme rotor and tower loads, with the goal of recommending which procedure might best suit the problem. Estimation of long-term fatigue damage is also studied. Such estimates are obtained using both the conventional rainflow cycle-counting method in the time domain, as well as an alternative spectral approach, and the advantages and limitations of the spectral method are discussed. In addition, the importance of using statistical extrapolation of fatigue cycles within the conventional approach is explored. While we are primarily interested in the prediction of long-term extreme and fatigue loads, we are also interested in assessing the uncertainty in our predictions as a function of the amount of data used. Towards this end, we first obtain estimates of long-term loads by making use of all of the data available, and then we obtain similar estimates using only subsets of the data. From these separate estimates, conclusions are made regarding what constitutes a sufficient amount of data upon which to base a statistical extrapolation. This study makes use of field data in addressing statistical load extrapolation issues, but the findings should also be applicable to simulation-based attempts at deriving wind turbine design load levels, where similar questions regarding extrapolation techniques, distribution choices, and amount of data needed are just as relevant.