A Very Large Floating Structures (VLFS), which is moored at the site where ocean waves are protected by such as breakwater, have been studies past several years. In the concept, a huge pontoon type floating structures is moored to dolphins with fenders in calm sea area where wave height is reduced by breakwater constructed near the floating structures. In addition to the studies, new type VLFS have been proposed to widely use floating structures in severe wave conditions. One of the concept is the VLFS moored inside a reef topography where ocean waves decreasing by wave breaking. The site condition inside a reef differs greatly from conventional site conditions due to the wave breaking in very shallow water depth, and long period waves, wave setup and current appears at the site.
　In this study, the hydraulic model tests were carried out on the VLFS moored inside both a ring reef in an isolated island and a coastal reef in southern semi-tropical islands in Japan to understand the response characteristics of VLFS in waves and currents. And numerical computations were also carried out to estimate vertical displacements and strains of VLFS inside the ring reef.