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

日本的Mega-Float项目提出了超大型浮动结构（VLFS）的概念，它具有类似垫子的薄结构，而水平规模非常大。他们开发了一些计算机代码来估计VLFS的运动，但是这些代码太耗时，对概念设计没有用处。为了解决这一问题，本研究从建立一个表示VLFS在波浪中的水弹性行为的积分方程开始。根据积分方程的研究结果，提出了射线理论在VLFS水弹性行为中的应用。该理论本身基于经典的射线理论，其中我们利用了VLFS的水弹性运动可以看作是波在平台中的传播这一事实，并假设波长与VLFS的水平尺寸相比非常短。经典射线理论认为，平台内和平台周围的波场可以用射线的总和来表示。射线理论在两点上得到了改进。传统的射线理论的缺点是，波的振幅是沿着穿过一个角的射线突然改变的。应用抛物线近似可以克服这个缺点。第二个改进是他们扩展了计算机代码，使其适用于任意的底部、环境和身体几何形状。利用这种方法，我们论证了底部拓扑结构和结构非均匀刚度的影响，并提出了几种有趣的结构形状来减小VLFS的运动。为了验证该方法对VLFS水弹性特性估计的准确性，还在一个小波槽中进行了微尺度模型实验。实验结果验证了射线法的有效性。

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.