Abstract: Traditionally wind profile measurements for offshore wind farms have been obtained by using cup anemometers mounted on wind masts. This is a very expensive method to acquire wind profile data, and the wind data will also be influenced by distortion from the mast and the sensors. A much cheaper way of obtaining offshore wind data is using a buoy mounted lidar. In addition a buoy can also measure waves, current profile and other parameters.
To be able to measure the wind profile from a buoy, a ZephIR 300 lidar from Natural Power was mounted on a Fugro OCEANOR Wavescan buoy. The Wavescan buoy is specially designed for severe environmental conditions, and has been in operation world-wide since 1985.
The buoy system was tested off Titran off the island Frøya on the coast of central Norway. This is an ideal test site as it is in a very tough environment and near to a test centre for wind measurements with 3 instrumented met masts. The wind test centre is a part of the NOWITECH infrastructure programme. A reference lidar supplied by Natural Power was also located at the wind test centre. The distance between the reference lidar and the buoy was approximately 3.5 km. The Wavescan buoy was deployed for a period of one month during March-April 2012. The buoy lidar recorded 10 minutes average wind profile at 10 heights from 11.5 to 218m every third hour, while the reference lidar measured the wind at 53 m height continuously. During the measurement period the significant wave height varied between and 0.5 and 3.6m.
The wind speed from the buoy lidar has been compared with the reference lidar showing that there is practically no bias, while there is some scatter with a correlation coefficient (R2) of 0.93. For higher wind speeds, which are mainly towards the coast, R2 is 0.95 with a slighter larger bias. The scatter can be explained simply by the distance between the lidars, and that the reference lidar is located on land. We are therefore planning to compare the buoy mounted lidar measurements with closer offshore wind mast data.