Application of Nonlinear Simulations of Wave Loads to Design of Ship Structures

近年来，在船舶结构的设计阶段，需要更精确、更可靠的方法，为此，需要将船舶响应的非线性模拟应用于设计中。在这个研究项目中，如何将这些模拟应用到设计阶段进行了研究。首先，研究了船舶在波浪中非线性响应的时域数值模拟格式。在斜波浪中，由于船舶的横摇和偏航模式没有恢复力，采用时间积分法对船舶的横摇和偏航运动进行数值漂移，导致数值不稳定。这意味着目前的非线性仿真不能适用于设计阶段。在本研究中，利用时间积分方法中的数字滤波技术克服了这一困难。其次，对一艘油轮、一艘集装箱船和一艘宽梁浅吃水船进行了波浪作用下船舶响应的非线性模拟，以阐明各种船舶波浪荷载的非线性特性。并对一艘宽梁浅吃水船进行了一系列模型试验。在此基础上，提出了一种将非线性仿真应用于设计阶段的方法。第三，提出了一种考虑船舶响应非线性的船舶最大波浪荷载概率计算方法；采用非线性模拟方法对某一波谱中的船舶响应进行了确定性估计，并由该波谱的概率得到该波谱中船舶响应的概率。在该方法中，引入设计波来确定波谱中的最大响应。该方法还可以考虑到海洋面积或船舶导航根的差异。因为最大响应是在海洋的每个区域的每个波谱中估计的。

Recently more accurate and reliability-based methods are required in the design stage of ship structures, and the application of nonlinear simulations of ship responses to the design is expected for this purpose. In this research project, how to apply these simulations to the design stage is investigated.Firstly, a numerical scheme in time domain is studied to simulate nonlinear responses of a ship in waves. In oblique waves, sway and yaw motions of a ship are drifting numerically in time integral method, because of the absence of restoring forces in sway- and yaw-modes, and this results in numerical instability. This means that the present nonlinear simulation cannot be applicable to the design stage. In this research, this difficulty is overcome making use of the digital filter technique in time integral method.Secondly, nonlinear simulations of ship responses in waves are performed for a tanker, a container ship and a wide-beam, shallow-draft ship, in order to clarify the nonlinearity of wave loads in various ships. A series of model tests are also conducted for a wide-beam, shallow-draft ship. On the basis of this results, a method to apply the nonlinear simulation to the design stage is considered.Thirdly, a method to obtain the probability of maximum wave loads for a ship is proposed, in which the nonlinearity of ship responses are taken into account; the ship responses in a wave spectrum are estimated deterministicly using nonlinear simulations, and the probability of this response is obtained by the probability of this wave spectrum. In this method, a design wave is introduced to determine the maximum responses in a wave spectrum. The differences of the area of the ocean or the navigation root of a ship can also be taken into considerations in this method. because the maximum responses are estimated in each wave spectrum of the each area of the ocean.