Experimental Study on the Probability of Different Wave Impact Types on a Vertical Wall with Horizontal Slab by Separation of Quasi-static Wave Impacts

Abstract

When the fundamental natural frequency of marine structures is comparable to the dominant frequency of incident waves, the response of the load on the structure will be amplified. Accurately quantifying how wave loads can be amplified by incident wave conditions must thus be considered in any structural analysis, given how sensitive these characteristics are to different wave impact types. Systematic physical model tests of wave impacts on the simple horizontal plate and the vertical wall with a horizontal overhanging cantilever slab were performed. By first comparing quasi-static wave load estimates along a simple horizontal plate (obtained by low-pass filtering the pressure time series at different cut-off frequencies) with quasi-static uplift pressures from established predictive formulations, a cut-off frequency of 7 Hz was found to accurately separate the quasi-static component from impulsive wave impacts. By applying the low-pass filtering approach with the selected cut-off frequency to the pressure measurements for the vertical wall with a horizontal cantilever slab case, the impulsive and quasi-static peaks were attained, which were then used to quantify the probabilities of individual impulsive, dynamic, and quasi-static wave impacts. Incoming wave conditions and structural clearance had a significant effect on the probabilities of different wave impacts. With the increasing wave height and wave steepness, wave impacts on the horizontal slab and vertical wall were increasingly of the impulsive type and less frequently of the quasi-static type, while the probability of dynamic impact types were relatively stable. As the overhanging slab was shifted from elevated to submerged, the dominant type of wave impact on the structure was variable, ranging from impulsive to dynamic to quasi-static as its elevation was lowered. The results indicated that up to 90% of the impacts were of the impulsive type when the overhanging slab was on or slightly over the still water level. Moreover, the presence of the vertical wall increased the magnitude of wave loads and the occurring frequency of impulsive wave impacts for the horizontal slab.

Keywords

• wave impact type
• cut-off frequency
• occurring probability
• structural clearance