Wave Buoy Performance in Short and Long Waves, Evaluated Using Tests on a Hexapod

Abstract

Offshore wave conditions can be measured using wave buoys, which are generally designed for wind waves. Longer waves (swell or bound second-order waves) are very relevant for certain maritime structures. The accuracy of the instrumentation in a typical wave buoy in long and short waves was therefore studied, and it was investigated if the buoy can be applied in longer waves. A Waverider buoy was placed on a hexapod, which applied regular and irregular prescribed motions in multiple degrees of freedom. The hydrodynamic response of the buoy in waves and the effect of its mooring system were not evaluated; the buoy was assumed to follow the orbital motions of a wave and to rotate with its slope. The tests showed that the buoy sensors measure accelerations and rotations with periods between 1.5 and 35 s very well. Vertical displacements derived from the accelerations by the buoy are accurate for the period range of 2 to 20 s. In longer waves, the motions are significantly underestimated, even though the accelerations are accurately measured. This will not lead to large errors in normal operation, as the energy of such long waves is generally low. This explains why the buoy also performs well when it is subjected to irregular motions (less than 2% error in the significant wave height of a half-hour measurement in realistic irregular sea states with peak periods between 5 and 20 s can be expected). It can be concluded that the buoy accurately measures accelerations. The accuracy of the derived displacements decreases when very long swell wave energy (> 20 s) is present. Review of the internal integration procedure may be considered when there is a specific interest in measuring longer waves.

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

“…However, if either of these two effects were the cause of preferential avoidance of wave crests, one may expect that the measured (crest-to-trough) wave height would also be affected, which we do not observe. In previous work, we have shown the latter of these two effects (the avoidance of An alternative explanation offered for the systematic underestimation of crest heights, which is consistent with in situ observations, is that the Lagrangian motion of a wave-following buoy cancels out second-order nonlinearity and essentially linearizes the resulting measurements (van Essen et al 2018;Casas-Prat and Holthuijsen 2010). This is partially correct.…”

“…These displacements are projected onto the coordinate system of heave, north, and east displacements. In our experiments, we effectively assume that there is no error in this process [see van Essen et al (2018) for a discussion of this assumption], and consider the position of our model buoy to be what a wave buoy in the ocean would measure.…”

Lagrangian Measurement of Steep Directionally Spread Ocean Waves: Second-Order Motion of a Wave-Following Measurement Buoy

“…For steep wave groups in unidirectional and weakly spread conditions, bound second-order subharmonic or ''difference'' waves form a depression in the wave-averaged surface elevation often referred to as a setdown (Longuet-Higgins and Stewart 1962). In the case of crossing and strongly directionally spread wave systems, the setdown of subharmonic bound waves underneath a wave group can turn into a setup, enhancing the maximum crest amplitude, as theoretically predicted (Okihiro et al 1992;Herbers et al 1994;Toffoli et al 2006;Christou et al 2009) based on second-order interaction kernels (Hasselmann 1962;Sharma and Dean 1981;Dalzell 1999;Forristall 2000), observed in field data (Santo et al 2013;Walker et al 2004;Toffoli et al 2007) and recently in detailed laboratory experiments (McAllister et al 2018).…”

A multivariate pseudo-likelihood approach to estimating directional ocean wave models

“…Selecting these frequencies is difficult, but there are principled ways to choose them. In particular, the buoy data does not accurately represent the data which we are interested in modelling at the lowest and highest frequencies (van Essen et al, 2018). As such, we select a low-and high-frequency threshold and use only the frequency interval between the thresholds in our analysis, as we shall now detail in Section 4.1.…”