Research on the Fundamental Understanding of the Behavior of a Very Large Floating Structure in Waves

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A concept of Very Large Floating Structure (VLFS) has been offered at the Mega-Float project in Japan that has a mat-like thin configuration while the horizontal scale is very large. They developed several computer codes to estimate the motion of VLFS, however the code is too time consuming and is not useful for the conceptual design. In order to solve this problem, the present study has started form formulating an integral equation which represents the hydro-elastic behavior of a VLFS in waves. Following results of the investigation of the integral equation, an application of the ray theory to hydro-elastic behavior of VLFS has been proposed. The theory itself is based on the classical ray theory, in which we used the fact that the hydro-elastic motion of VLFS can be regarded as wave propagation in the platform, and employed an assumption that the wavelength is very short compared with the horizontal size of VLFS. The classical ray theory insists that the wave field in and around the platform is represented as a summation of wave rays. The ray theory has been improved on two points. The shortcoming of the conventional ray theory is that the wave amplitude is suddenly changed along a ray that passes through a corner. This shortcoming is overcome by applying the parabolic approximation. The second improvement is that they extended the computer code to be applicable for arbitrary bottom, environmental and body geometry. Using this method, we demonstrated the influence of bottom topology and the influence of non-uniform rigidity of the structure and several interesting shapes of the structure have been offered to make motion of VLFS smaller. In order to demonstrate the accuracy of the present approach to the estimation of hydro-elastic behavior of VLFS, experiments in a small wave tank with a micro scale model has been also carried out. The ray method has been validated with experimental results.