Wave loads on U-OWC breakwaters for the wave energy exploitation

The thesis treats the analysis of wave load conditions acting on a U-Oscillating Water Column (U-OWC) modified breakwater. First the study investigates the results obtained by a small-scale experiment conducted on this device, integrated into a breakwater model, deployed at the NOEL laboratory in Reggio Calabria, Italy. The U-OWC, a type of OWC named also Resonant Wave Energy Converter (REWEC), is designed to absorb incoming sea waves and convert them into electrical energy. The study examines wave pressures, forces, acting on the U-OWC under various wave conditions, providing insights into the device's potential to improve breakwater stability. Wave pressure fluctuations in the U-duct determine oscillations in the water column inside the pneumatic chamber, producing hydrodynamics forces in the active parts of the U-OWC. The study presents detailed calculations for these forces and finds that the U-OWC breakwater structure generally exhibits increased stability under all wave conditions. Then the wave pressure distributions along external walls of the U-OWC breakwater and the related wave forces have been calculated. It has been obtained wave loads under quasi-standing waves, and impulsive wave forces produced by breaking waves, which occurs under severe wave conditions. Data post-processing highlights three primary wave load types observed: “Quasi-Standing Waves (QSW)”, “Slightly Breaking Waves (SBW)”, and “Impact Loads (IL)”. Each type presents different wave pressure and force distributions, which are analyzed in terms of peak forces and probability of occurrence. The study identifies specific behaviors for each wave load category, with impact loads showing significantly higher pressures than quasi-static loads. A probabilistic analysis examines various parameters such as water depth, wave height, and seabed topography to understand their impact on wave loads. Results show that wave forces on the U-OWC breakwater increase significantly under lower water depths, higher wave heights, and reduced dimensions of structural elements like the U-duct width. The reliability of results for the three classes of loading conditions is firstly demonstrated by comparison with the existing Goda’s model, one of the most effective approaches used in literature to determine the pressure distribution and force on seawalls or vertical breakwaters, for both breaking and non-breaking incoming waves. Goda's model provides satisfactory estimates for Slightly breaking waves but tends to overestimate experimental results for Quasi standing waves and underestimate impulsive loads. Finally, two different models are developed in the Thesis to estimate the different wave loads acting on a U-OWC breakwater. They are the time-domain Quasi-Determinism (QD) nonlinear model and the Pressure Impulse model, developed for predicting the wave forces acting on a U-OWC (Oscillating Water Column) breakwater under non-breaking and breaking wave conditions, respectively. Finally, the both models are validated against experimental data for wave load prediction on the caisson breakwater embodying the U-OWC. The QD model shows excellent accuracy for quasi-standing wave forces but is limited to non-breaking wave scenarios. Pressure impulse model, developed and applied to the U-OWC, provides satisfactory estimates for slightly breaking waves and impulsive loads.