Wind Gust Factors in a Coastal Wind Climate

Abstract

Gust wind speeds in high wind speed conditions are important for load estimation on large structures in exposed locations, like a multi-megawatt wind turbine. Modern wind turbines operate at wind speeds up to 25 m/s so it is important to know the expected amplitude, acceleration and relevant scale of typical wind gusts under operating conditions.The wind measurement station at Frøya is located near the village of Titran on the western tip of the island. The site (Fig. 1) is well suited for measuring coastal and marine wind conditions relevant for offshore and coastal wind farms. The database contains several hundred hours of wind data from high wind speed conditions (>15 m/s). A 100 meter high meteorological mast with 2D ultrasonic anemometers at 6 levels have been used to collect wind data over a time period of 5 years from 2009 to 2014.Gust factors (G) measured at Frøya have been studied and correlated with height, wind speed, turbulence intensity (TI) and atmospheric stability, and compared to existing models.The gust factor mainly depends on turbulence intensity and gust averaging time. This indicates that the “peak factor” (kP) is a better measure for gustiness when turbulence values are available. kP decreases with increasing turbulence intensity and averaging time, but is less dependent on atmospheric stability. Its pdf follows a Gumbel distribution. An asymmetry of the measured gusts could be observed, showing a higher fall time compared to rise time.