Nonlinear Method For The Computation Of Ship Motions And Wave Loads

The development of structural strength estimation methods requires calculating wave loads more precisely. The aim of this thesis is to provide a practical and efficient method, both to structural estimation and to FEM analysis of partial ship structure or whole ship structure. The nuclear work discussed in the thesis is to modify and improve the strip method which is widely applied among current shipbuilding field. The major work is as follows:Solution of two dimensional radiation or diffraction fluid field. This thesis extends multipole expansion method to adapt to disturbed fluid fields of 2D floating bodies with asymmetric forms, which is applied with symmetric forms originally. And given the solution of potential velocity and hydrodynamic pressure of radiation and diffraction fluid fields, which is incited by a 2D arbitrary form, actual application field of the multipole expansion method has been enlarged.Modification to the linear strip theory. Although the linear strip theory has developed for a long time, based on such a theory there also exist some deficiencies in the software system of ship motions and wave loads. For instance, the software cannot give rational results under the condition when the wave-length is relatively short. The thesis presents some modifications to make the strip theory results meet engineering demands enough even when the oscillating frequency is relatively high. These modifications proposed in this thesis adapt not only to linear methods, but also to nonlinear methods. All modifications serve to improve calculation precision.Prediction of hydrodynamic pressure. The thesis presents a practical engineering method to predict the hydrodynamic pressure on instantaneous wet ship surface. This method applies the extended strip theory to establish the nonlinear ship motion equations in regular head seas, obtains stable time course by resolving in time domain, and then pressure vectors will be available. The pressure includes static pressure, incident waves pressure , diffraction pressure , radiation pressure, and wave impact pressure, and changed with instantaneous draft of each intersection. The characteristic of this method focuses on the matching between the instantaneous pressure distribution and the ship motion in a dynamic sense. On the base of above work, nonlinear characteristics ofhydrodynamic pressure on ship surface are studied by the means that compares the theoretical results with those of model experiments. Applying the method studied in this thesis, theoretical calculation of certain container ship in regular wave has been done on ship motion and hydrodynamic pressure. It also has the results compared with those obtained under multipole conditions in the model tests. As a consequence, we abstract some characteristics of the distribution of instantaneous hydrodynamic pressure on ship wet surface, and the distribution changes with forward speed and wave parameters.Solution of nonlinear ship motion equations in oblique regular seas. In this thesis, a new method is presented about calculating nonlinear ship motion responses in oblique seas based on minimized theory. Because there are no horizontal fluid restoring forces in ship motion, the responses such as sway and yaw in time domain will not converge to a stable state. How to overcome the diverse solution is a critical factor in the nonlinear ship motion solution in oblique seas. The steady solution of nonlinear ship motions in oblique seas can be gained by adopting the new technique presented in this thesis. This work promotes the application of theoretical prediction methods of nonlinear ship motions and wave loads.Solution of nonlinear wave loads in regular oblique seas. Based on strip method, thinking about nonlinear factors such as no straight shipboard on waterline, the instantaneous variation of wet surface shape and slamming effect of ship bottom, the numerical methods of nonlinear fluid force in large amplitude oblique regular seas are discussed. In the thesis, the minimized theory is introduce...