Conjectures on the Failure of the OWC Breakwater at Mutriku

In a bold experiment, the Basque Government constructed an innovative oscillating water column (OWC) power station breakwater outside the small fishing port of Mutriku during 2007-2008. While the main structure was a rubble mound, the OWC section of the breakwater consisted of 16 chambered vertical wall units formed by stacked concrete layers. Before the power station could be completed however, storms hit the breakwater in December 2007 and March 2008). A further storm in January 2009 result in severe structural damage to a number of the OWC cells. Previous papers have explained some of the contributions to damage, but have not described detailed flows, loads, and consequent responses. In order to understand the causes of the damage, this paper will describe recent numerical model (CFD) experiments to calculate flows, pressures, and resulting loads at critical positions within the OWC cells, both for the intended operational condition (H_s=8m, T_P=12.8s), and January 2009 storm wave conditions (H_s=13.7m, T_P=14.5s). OpenFOAM CFD modelling tools have been used, supported in part by contemporaneous large scale physical model tests in the GWK in Hannover. The CFD modelling has examined key flow processes and dynamic pressures to explore whether these may explain the onset of damage to the OWC cells. Modelling so far suggests that impulsive pressures were developed within OWC chambers where the air turbine or control valves had not been fitted. Around critical corners of the front face and OWC chamber, flows, and pressures following impulsive processes may have seen the onset of cavitation, potentially leading to localised failures, particularly at joints between the stacked cell units.