Experimental study of the effect of wind above irregular waves on the wave-induced load statistics

The design load from waves on offshore structures are often estimated by aid of experimental studies in wave flumes and basins. When going from open sea to laboratorial conditions a number of factors are either added or omitted such as wind above waves. The paddle-generated waves are based on a spectrum taking the indirect effect of wind into account, whereas the direct effect of wind is left out. Previous studies have focused on the direct effect of wind on waves themselves, but no investigations on the load have been made. The question is whether or not airflow separation, vortexes etc. in the wind field alter physical properties such as steepness of waves, the number of breaking waves and hereby the force; especially the in-line force. To investigate the matter, an experimental study of depth-integrated force on a circular cylinder from irregular waves both with and without wind above, was conducted. An objective, when conducting the tests, was to achieve the same significant wave height whether wind was present or not. Exceedance probability curves for wave crest height, depth-integrated force and pressure were obtained. Moreover, a more descriptive assessment of the phenomena was done by studying the average force shape of the hundred largest force events. In addition, there was applied a phase-based harmonic separation method to explore the wind's effect on the harmonic force components of higher order. The front steepness of the waves was clearly increased with the introduction of wind and further increased, when increasing the wind velocity. The introduction of wind consistently increased the number of breaking waves detected with a breaking criterion. The wave-induced load in the tail of the exceedance probability curve was only increased for some of the sea states, when wind was present. This was very dependent on the size of the crest height of the largest breaking waves. The maximum wave-induced pressure in the tail of the exceedance probability curve was on the contrary increased for all sea states, when wind was present, and it kept increasing with increasing wind velocity. For one of the sea states with increased force extrema, the averaged force shape for the hundred largest events of a case with wind revealed a shape with a spiky peak consistent with that generated from breaking waves. This was not present for the case without wind. The separated harmonic contents of the force up to fourth order also showed a tendency towards more energy at harmonic components of higher order for the case with wind.