Investigations of a Doppler Wind LIDAR on a Moving Platform

The development of offshore wind energy along the Eastern Seaboard is expected to increase over the next decade. As a result, an accurate assessment of wind resource, as well as reliable forecasts over WEAs (Wind Energy Areas), becomes important for cutting maintenance and operational costs. In the past, meteorological towers have been used for acquiring vertical information of wind across a turbine; however, due to the construction of larger turbines and the complexity of the offshore wind profile, alternative measurement techniques, such as remote sensing technology, have been implemented in order to overcome height limitations. Among the most favorable of techniques is the “floating lidar” which measures winds using a Doppler wind LIDAR (LIght Detection And Ranging) on a marine platform.

The deployment of a Doppler LIDAR offshore is challenging because LIDAR measurements include additional Doppler shifts due to the platform motion. To some extent, this can be taken into account by processing the data with a motion compensation algorithm; other errors remain under conditions when there are spatial inhomogeneities in the wind field such as strong wind shear, which is especially important to measure accurately, or turbulence, which if not measured accurately, can effect calculations of structural loading on a wind turbine.

In this presentation, data from a WRF (Weather Research and Forecasting) simulation, along with data collected during the XPIA experiment will be used to assess the impact that a moving LIDAR has on wind measurements. Results from this study aim to determine the effect that strong wind shear and turbulence has on the wind field after platform motion removal. The advantage of conducting these investigations is evident through numerical experiments, which compare cases when the LIDAR is stationary against a moving LIDAR under the same prescribed wind field. In addition, through calculations of the actual measurement height due to platform tilt, a method was found to mitigate the effect that a moving platform has on a measurement during cases involving large vertical shear and turbulence. Other investigations carried out involve horizontal repositioning of the LIDAR beam due to large cases of yaw motion. Results show that a correction, which is considered purely geometrical, can be implemented if a change in the horizontal angle is correctly calculated, and if a sinusoidal fitting function is applied such that the reference angles can be used to calculate the radial winds in the stationary frame of reference. The observational data analyzed from the XPIA experiment are through Leosphere's WindCube. It was found that certain motions had a tendency to exacerbate the motion compensated wind field, while others resembled conditions both before and after the initiation of the platform. Investigations applying methods found during the numerical study show that some of the error, and consequently the artificial fluctuations present in the LIDAR data, are reduced, which is evident through calculations of the structure function, the turbulence dissipation rate, and the turbulent kinetic energy.