Comparative performance of Radar, Laser, and Waverider Buoy measurements of ocean waves – Part 1: Frequency domain analysis

Abstract

Accurate ocean wave measurements are needed for the safe design and operation of offshore facilities, but despite many ocean wave measurements, the accuracy of wave measurement systems remains an ongoing issue. Of paramount importance are measurements during extreme sea states. This paper examines wave measurements made with an Optech Laser, a Rosemount WaveRadar, and a Datawell Waverider buoy at North Rankin A platform (NRA), Australia; Ekofisk, North Sea; and several South China Sea locations. We evaluate the relative performance of these instruments based upon various frequency domain comparisons, including comparisons of their 1-D frequency spectra using spectrograms, spectral moments, high-frequency tail slopes, and significant wave heights derived from their wave spectra. A spectral relationship (transfer function) in terms of mean spectral ratio of the instruments is developed, which can be used for spectral calibration. On average, Laser and Waverider spectral estimates agree well at all sea states. However, at low wind speeds, the higher frequency spectral levels of the Laser are relatively high and noisy compared with the other two instruments. Radar higher frequency spectral estimates are relatively low compared to the other two instruments, particularly at lower sea states. In additionally, the higher frequency tail slopes of all three instruments vary between f‒4 and f‒5. However, at higher sea states, the Waverider tail slopes become steeper than f‒5. The Radar produces the lowest significant wave heights (Hm0) compared to the Laser and Waverider, but its second moment period (Tm02) estimates are longer than the Laser and Waverider.