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Title : ON THE MOST LIKELY EOG AMPLITUDES
Authors : Gunner Chr. Larsen and Kurt S. Hansen

Summary :
For most structures exposed to wind loading, extreme response is associated with extreme wind speeds. However for wind turbines - especially for pitch regulated wind turbines - the design driving thrust loading is often associated with severe wind gust situations occurring during normal operation, and these are thus related to the mean wind speed regime [5m/s; 25m/s]. Based on knowledge on the load response characteristic of the wind turbine (in terms of aerodynamics and pitch action) it is possible to identify a general critical mean gust shape that may potentially result in ultimate thrust loading. In the IEC 61400-1 design code, this particular load case is defined as Extreme Operational Gust (EOG). The code prescribes the gust shape, extend and magnitude for recurrence periods 1 year and 50 years, respectively. In a recent cogent study, based on wavelet filtering combined with extreme statistics, the most likely extreme amplitudes of wind speed gusts (with shapes and time scales identical to those specified for the EOG load case in the IEC-61400 code) has been assessed. The estimated extreme gust amplitudes from the analysis shows substantial qualitative and quantitative differences compared to the specifications in the most recent IEC-61400 code revision. Assuming the mean wind speed to be Rayleigh distributed (as specified in the code) and adopting the conjecture that wind speed gust amplitudes scale directly with the turbulence standard deviation, the EOG amplitudes have been estimated for the 3 WTGS classes combined with the 3 turbulence classes prescribed in the IEC-61400 code. The estimation is performed for recurrence periods 1 year and 50 years, respectively, and the estimated amplitudes are finally synthesised into a simple algebraic expression.

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