Modelling the Vertical Wind Speed and Turbulence Intensity Profiles at Prospective Offshore Wind Farm Sites

Monin-Obukhov theory predicts the well-known log-linear form of the vertical wind speed profile. A turbulence intensity profile can be estimated from this by assuming that the standard deviation of the wind speed is proportional to the friction velocity. Two parameters, namely the aerodynamic surface roughness length and the MoninObukhov length, are than needed to predict the vertical wind speed and turbulence intensity profiles from a measurement at one height. Different models to estimate these parameters for conditions important for offshore wind energy utilisation are compared and tested: Four models for the surface roughness and three methods to derive the Monin-Obukov-length from measurements. They have been tested with data from the offshore field measurement Rødsand by extrapolating the measured 10 m wind speed to 50 m height and comparing it with the measured 50 m wind speed. The mean wind speed at 50 m height is under-predicted by 2% to 5% when extrapolated from a 10 m measurement. For the sea surface roughness it has been found that the simplest approach, a constant roughness, gave the best result. From the models to derive the Monin-Obukhov length the bulk method performed best. It has been shown that this is due to shortcomings of the description of the stability influence in the stable regime. The measured turbulence intensities show a large spreading, which is not captured in the modelling by any of the sea surface roughness models.