Small-scale field experiment on wave forces on a U-OWC breakwater

A small-scale field experiment is conducted on a U-Oscillating Water Column (U-OWC) incorporated into a model of caisson breakwater at the Natural Ocean Engineering Laboratory (NOEL) laboratory of Reggio Calabria (Italy). The U-OWC or REWEC (REsonant Wave Energy Converter) is a device belonging to the family of Oscillating Water Columns (OWCs), characterized by the introduction of a U-duct. Such a device is innovative, absorbing a high percentage of incoming sea energy and, then, to produce electricity via a Power-Take-Off (PTO). The aim of the present study is the investigation of the wave pressures and forces acting on the U-OWC monolithic coastal defense structure. Moreover, the structural response of the U-OWC structure is also investigated under the occurrence of wave crests, by calculating the hydrodynamic forces realized in the active parts of the U-OWC. It is shown as a global additional force is realized, which contributes to increase the overall stability of the structure. Considering the wave loads acting on the U-OWC caisson, the post-process of the overall dataset recorded during the real field experiment at NOEL laboratory reveals the occurrence of both quasi-static loading (non-breaking), and of impact loading (impulsive) due to wave breaking. Thus, a systematic analysis is pursued in order to identify the loading cases acting on the U-OWC breakwater. When high non-breaking sea waves interact with the breakwater, “Quasi-Standing (QS)” wave pressures and forces occur at modified structure with two peaks of equal intensity. Instead, the impact of extreme breaking waves against the U-OWC structure generates asym- metrical wave pressure records beneath the wave crests, with two peaks with different intensity: the first one greater than the second one. In these cases, “Slightly Breaking (SB)” wave forces and “Impact Loads (IL)” are identified. Wave pressure distributions and forces acting on the U-OWC model are, then, investigated for the three classes of wave loads. Moreover, the probability of occurrence of maximum positive peak forces is determined from the experimental dataset, given a number of parameters. In particular, when QSWs occur, the maxima peak forces are recorded for deeper water depths dn , d’ and a, at given significant wave height HSi . Instead, for SBWs and ILs, the extreme wave loads are realized, when the most severe sea states occur with higher HSi , and for decreasing water depth dn . d’ and a. Then, the influence both of the length of the berm and of the width of the U-duct in the front structure is considered. When the dimensions of both structural elements is reduced, the maxima wave forces occur in a fixed recorded sea state. Then, the Quasi-Determinism (QD) nonlinear model at the U-OWC breakwater is introduced for evaluation of Quasi-Standing wave loads, showing a good agreement among analytical and experimental results. Finally, the Goda’s model is applied to calculate the wave pressure distributions acting on the external walls of the U-OWC model. The theoretical results given by the Goda’s formulae are compared with experimental data, overestimating experimental results for both QSWs and SBWs and, significantly, underestimating IL conditions.