Experimental study of the effect of wind above focused waves and the wave-induced loading

The statistical design load from waves on offshore structures is often estimated by aid of experimental studies of irregular waves in wave flumes and basins. When going to the laboratory things are either added or omitted. One of the latter being the direct effect of an instantaneously wind field above the waves, and there is a concern that it can alternate the steepness of the waves. More recently experimental investigations of wave load statistics generated by irregular waves, points at an increase in wave-induced pressure and dynamic loading, when the direct effect of a wind field above the waves was taken into account. The investigations moreover, shows an increase in the number of breaking waves, which leads to the question if the extreme wave load events were increased due to more breaking waves or if the load from the individual breaking wave was increased, when wind was introduced in the tank. An attempt is carried out to answer if the direct effect of wind increases the wave load of an individual breaking wave event. Therefore, breaking waves both with and without wind, nevertheless with a quite similar wave profile, are investigated experimentally. Deterministic, focused waves are then generated with different distances from breaking point to a model. The point at which the wave is focusing is pushed further down tank, when the wind is introduced. Therefore, the input focus point and thereby the resulting breaking point is adjusted to achieve the same distance from breaking point to model, whether wind is present or not. The dynamical force and pressure are in this study highly dependent on the distance from the breaking point to the model, where the optimal breaking point here is detected around 0.15 meters before or after the model. The front steepness of the waves is increased, when wind is introduced. The maximum generated wave load and dynamic force are occurring in a case with wind. The picture for pressure is less clear, and is in one case largest for a case with wind, and in another largest for a case without wind. Due to the low number of repetition tests, the small differences observed between the wave-induced load in the case with and without wind and the intrinsically, stochastic nature of breaking waves, there is no basis for neither confirming nor disconfirming that the direct effect of wind above a breaking wave increases the load or the pressure.