Impact of mooring configurations on wave attenuation of porous floating breakwater: A comparative experimental study

Abstract

Floating breakwaters are increasingly used for coastal protection, especially in deep-water environments where traditional fixed breakwaters are impractical. The performance of porous floating breakwaters (PFBs) is highly dependent on their mooring systems, which has not been thoroughly investigated to date. This study presents a comparative experimental investigation of four mooring systems, i.e., fixed, pile-restrained, catenary, and taut, for PFBs, focusing on their wave transmission coefficients ($K_t$) and motion response (heave, surge and pitch). The PFB, constructed from cubically packed stainless-steel spheres, was subjected to periodic waves with varying steepness and ratio of width ($B$) to wave length ($L$). Fixed mooring, with no dynamic response, provided good wave attenuation, especially when fully submerged ($K_t\approx 0.5$ for $B/L\ge 0.18$). Pile-restrained mooring, allowing vertical motion, performed comparably to emerged fixed mooring, with minimal impact from heave motion. Catenary mooring, characterized by high compliance, exhibited poor performance for long waves ($K_t>0.8$ for $B/L<0.3$) due to large surge and heave motions. Taut mooring, with high stiffness due to pre-tensioned mooring chains, demonstrated superior attenuation when fully submerged, outperforming fixed mooring in some cases. However, its performance degraded when the PFB was not fully submerged and without the pre-tension. The study highlights the critical role of restricting translational motions (surge and heave) in enhancing wave dissipation. Submergence was also found to be a key factor, with fully submerged PFBs dissipating more energy. These findings provide valuable insights for optimizing mooring systems in practical applications.