Presentations

Abstract

Scanning lidars are beneficial for offshore applications and large wind farms due to their ability to determine the wind conditions over large areas and at long ranges. Especially in dual-Doppler mode, they can offer measurements with high precision and high spatial resolution. However, precise laser beam alignment is crucial, as even a small deviation can significantly affect the measurement location and impede measurement accuracy. This brings several challenges. While on land hard targets are used to calibrate the alignment, offshore it is often more challenging to find suitable high-precision hard targets. Sea surface levelling (SSL) offers an alternative solution by using the sea surface as a reference (Rott et al. (2017) and Gramitzky et al. (2024)). Besides the beam alignment calibration, the movement of the offshore platform – i.e. a transition piece – introduces additional uncertainties into the alignment of scanning lidars. Finally, if configured in dual-PPI mode, the height difference between the two lidars varies across the scanned planes. This contribution addresses these challenges and presents and extension of the SSL method to derive the elevation offset in addition to pitch and roll. It then systematically quantifies the uncertainties associated with SSL beam alignment calibration. In a next step, the uncertainties in beam alignment calibration are compared and evaluated against the effects of the platform motion of a transition piece of an operational turbine on the beam alignment accuracy. Finally, the effects of height deviations between the two beams of a dual-Doppler system are discussed in a practical setup. This is done using real world measurement data from an offshore measurement campaign including two typical dual-Doppler measurement setups: a dual-PPI and a step-stare configuration. The new SSL method is tested using a scanning lidar on a transition piece of an offshore wind turbine. Results were compared with a high-resolution inclinometer and CNR mappings of hard targets, showing a high degree of robustness and reproducibility. However, the method showed sensitivity to the configuration of the SSL scan, especially for the elevation offset. Moreover, a detailed sensitivity analysis of factors affecting uncertainty revealed  the influence of incorrect assumptions about distance estimation from the CNR signal and the influence of waves. Comparison with platform tilt measured by the co-located inclinometers reveals that platform motion quickly becomes the dominant error source of beam alignment over larger distances. Analysis of the inclinometers also shows that errors of several meters are to be expected in beam alignment of a typical measurement from a transition piece. However, theoretical considerations indicate that the effect on the measured mean wind speed is expected to be small. This is confirmed by an excellent agreement between the dual-Doppler approach and a reference measurement using a co-located nacelle mounted lidar. The comparison between the dual-PPI and the step-stare mode highlights the trade-off between spatial resolution and the precision of spatially distributed dual-Doppler measurements.  In conclusion, this work demonstrates that, despite the challenges with beam alignment accuracy in offshore measurements, highly accurate wind measurements with dual Doppler can be achieved.