Alignment calibration and correction for offshore wind measurements using scanning lidars

Abstract

Long range scanning lidars offer great potential to measure wind characteristics for a variety of application in offshore wind energy. In order to derive accurate wind measurements it is critical to thoroughly calibrate the beam alignment. In offshore environments, the pitch and roll angle of a scanning lidar can be determined from sea surface levelling (SSL). This paper compares two methods to perform SSL, a method based on PPI scans and a novel method using RHI scans. Moreover, a previously described method to identify the distance at which the laser beam enters the water is extended to be applicable to larger measurement distances. Based on data from an offshore campaign in the North Sea, the differences between the RHI and PPI methods and their sensitivity to the selected elevation angles (PPI) and distances (RHI) are analysed. It is shown that both SSL methods prove to be robust for determining pitch and roll for all examined settings. The average results of the two methods show only minor differences. The standard deviation across all settings and both methods is 0.010° for pitch and 0.024° for roll. Compared to typically observed platform movements introduced by thrust on the wind turbine, the differences between different SSL settings are small. Comparison of the platform movements measured by the high sensitivity inclinometer and SSL shows that even slight differences in pitch and roll can be detected by SSL. This indicates a high level of accuracy for both methods investigated.