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，需要更精确和可靠性为基础的方法，和船舶响应的非线性模拟的应用，预计这一目的的设计。本研究主要探讨如何将这些数值模拟应用于船舶的设计阶段。首先，研究了船舶在波浪中的非线性响应的时域数值模拟方法。在斜波中，由于横摇和横摇模态中没有恢复力，在时间积分法中，船舶的横摇和横摇运动在数值上是漂移的，这导致了数值不稳定。这意味着目前的非线性模拟不能适用于设计阶段。本文利用时间积分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舶进行了波浪响应的非线性仿真，以明确各种船舶波浪载荷的非线性。本文还对宽梁浅吃水船进行了一系列的模型试验。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载荷概率计算方法;使用非线性模拟确定性地估计波浪谱中的船舶响应，这个响应的概率由这个波谱的概率得到。在该方法中，设计波的引入，以确定在海浪谱中的最大响应。该方法还可以考虑海洋面积或船舶航行根的差异。因为在海洋的每个区域的每个波谱中估计最大响应。

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.