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Modelling of extreme gusts for design calculations


Verification of the structural integrity of a wind turbine structure involves analyses of fatigue loading as well as extreme loading arising from the environmental wind climate. With the trend of persistently growing turbines, the extreme loading seems to become relatively more important. The extreme loading to be assessed in an ultimate limit state analyses may result from a number of extreme load events including transient operation (start/stop sequences), faults, and extreme wind events. Examples of extreme wind events are extreme mean wind speeds with a recurrence period of 50 years, extreme wind shear, extreme wind speed gusts and extreme wind direction gusts. The present study addresses extreme wind turbine loading arising only from extreme wind speed gust events. The extreme wind events explicitly accentuated above are included in the currently available draft of the IEC-standard [IEC 61400-1, 1998] as extreme load conditions that must be considered as ultimate load cases when designing a wind turbine. Within the framework of the IEC-standard, these load situations are defined in terms of two independent site variables - a reference mean wind speed and characteristic turbulence intensity. However, the gust events described in the IEC-standard are formulated as coherent gusts of an inherent deterministic character, whereas the gusts experienced in real situation are of a stochastic nature with a limited spatial extension. This conceptual difference may cause substantial differences in the load patterns of a wind turbine when a gust event is imposed. In order to introduce more realistic load situations of a stochastic nature, the NewGust project has been launched. The aim of this project is to obtain experimentally verified theoretical methods that allow for embedding (qualified) predefined gusts (with a well described probability of occurrence) in the stochastic of the turbulence atmospheric wind field. This is in line with what is the common practice when dealing with the fatigue loading.

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