Experimental Study of the Statistical Properties of Directionally Spread Ocean Waves Measured by Buoys

Abstract

Wave-following buoys are used to provide measurements of free surface elevation across the oceans. The measurements they produce are widely used to derive wave-averaged parameters such as significant wave height and peak period, alongside wave-by-wave statistics such as crest height distributions. Particularly concerning the measurement of extreme wave crests, these measurements are often perceived to be less accurate. We directly assess this through a side-by-side laboratory comparison of measurements made using Eulerian wave gauges and model wave-following buoys for randomly generated directionally spread irregular waves representative of extreme conditions on deep water. This study builds on the recent work of McAllister and van den Bremer (2019, https://doi.org/10.1175/JPO-D-19-0170.1), in which buoy measurements of steep directionally spread focused waves groups were considered. Our experiments confirm that the motion of a wave-following buoy should not significantly affect the measured wave crest statistics or spectral parameters and that the discrepancies observed for in situ buoy data are most likely a result of filtering. This filtering occurs when accelerations that are measured by the sensors within a buoy are converted to displacements. We present an approximate means of correcting the resulting measured crest height distributions, which is shown to be effective using our experimental data.

Real-world rogue wave probabilities

“… Parameter Physical meaning References Crest-trough correlation Correlation coefficient between wave crest heights and trough depths 18 , 21 , 22 Spectral bandwidth Spectral peak width, controls wave group dynamics 11 Mean period Mean wave period 2 Rel. low-frequency energy Relative low-frequency (swell) energy content 2 , 4 , 23 , 24 Directional spread Short-crestedness of waves 6 Ursell number ( ) Non-linear shallow water effects 25 Benjamin–Feir index Degree of non-linearity, modulational instability 26 – 28 Excess kurtosis Proneness to outliers of sea surface elevation 13 , 26 , 29 Steepness Weakly nonlinear corrections, wave breaking 15 , 17 Significant wave height Reference wave height, total energy 14 Skewness Shape asymmetry between wave crests and troughs 13 , 30 Relative depth ( ...…”

Estimating ocean wave directional spreading using wave following buoys: a comparison of experimental buoy and gauge data

“…Full details of the experimental methods behind the data are given in McAllister and van den Bremer (2020). The experiments were carried out in the FloWave Ocean Energy Research Facility at the University of Edinburgh.…”

Relation between occurrence probability of freak waves and kurtosis/skewness in unidirectional wave trains under single-peak spectra

“…Freak waves [1][2][3] have been reported as a major cause of disasters in the increasing marine development activities [4][5][6][7]. It is important to predict the occurrence of freak waves and avoid the latent threat to the safety of offshore structures, ships, and personnel [2,8,9].…”