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

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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.