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

日本Mega-Float项目提出了超大型浮体结构的概念，该浮体结构呈席状薄壁结构，水平尺度很大。他们开发了几个计算机代码来估计超大型浮体的运动，但这些代码太耗时，对概念设计没有用处。为了解决这一问题，本研究从建立描述超大型浮体在波浪中的水弹性行为的积分方程入手。根据积分方程的研究结果，提出了射线理论在超大型浮体水弹性力学中的应用。该理论基于经典的射线理论，利用超大型浮体的水弹性运动可以看作是波浪在平台中的传播，并假定与超大型浮体的水平尺寸相比，其波长很短。经典的射线理论认为，平台内部和周围的波场可以表示为波射线的总和。射线理论在两点上得到了改进。传统射线理论的缺点是波幅沿穿过拐角的射线突然改变。这一缺点可以通过应用抛物线近似来克服。第二个改进是，他们将计算机代码扩展到适用于任意的底部、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不均匀刚度的影响，并提出了几种有趣的结构形状，以减小超大型浮体的运动。为了验证本方法在估算超大型浮体水弹性特性方面的准确性，还进行了小型波浪水槽中的微尺度模型试验。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.