Thesis - Offshore Turbulence Intensity

The turbulence intensity is a measure of the degree of fluctuation in the horizontal wind speed. This is an important parameter in the design of wind turbines, since the turbulence is responsible for fatigue loading of the turbine. Traditionally turbulence intensity is measured with anemometers mounted on meteorological masts at the site of a prospective wind farm. However, as offshore wind farms are placed further from land and in areas with deeper water, the price of offshore meteorological masts increases rapidly.

 The goal of the project is to investigate an alternative solution to obtaining reliable ambient offshore turbulence intensity data, by avoiding the high cost of a tall meteorological mast on-site. For this purpose, it is necessary to have a full description of the offshore turbulence intensity and the different variables driving its behaviour. Based on that description, a vertical extrapolating model will be proposed. In addition, spatial distribution patterns between different sites will be investigated.

At this moment, LiDAR technology, Light Detection and Ranging, has been established as an alternative solution to the use of meteorological masts for Wind Resource analysis. These met masts are not able to describe the wind conditions at hub height of the new generation of WTs as their measurements height are relatively low compared with those hub heights.

With the increasing relevance and use of LIDARs within the industry, it is important to assess the accuracy of LiDAR turbulence measurements and its relationship and dependency with certain variables as wind speed, wind direction and turbulence level itself. LiDARs measure the mean wind speed with high accuracy. However, with the present lidar technology it is not possible to obtain the fluctuation of the wind speed, and hence the turbulence intensity, accurately. For this reason, alternative solutions for having reliable offshore ambient turbulence intensity measurements should be explored.

PURPOSE

The purpose of this project is to create alternative approaches in order to provide a complete description of offshore ambient Turbulence intensity to be used as reliable input for both calculation of the annual energy production of a wind farm and in loads analysis.

It is the intention of this project to carry out the following analysis:

1. Offshore ambient turbulence intensity versus different parameters will be analysed

Turbulence intensity versus wind speed

Turbulence intensity versus wave height

Turbulence intensity versus wind direction

Turbulence intensity versus vertical profile

Turbulence intensity versus Weibull k values

Turbulence intensity versus distance to shore

1. Spatial distribution of Offshore ambient turbulence intensity between different offshore sites will be investigated in order to identify possible existing patterns

1. Vertical variation of Offshore ambient turbulence intensity will be also analysed in order to confirm if the proposed methodology in the IEC 61400-1 Ed3 standard is applicable or not

1. Vertical offshore turbulence Intensity model definition

1. Investigation of a potential mixed solution (Lidar and a smaller met mast) will be explored

1. Extreme turbulence intensity distribution

It is not the intention of this project to investigate the wind energy content through spectral analysis as no high frequency data will be available.