Prediction Of Vertical Ship Motions And Wave Loads Based On 2-1/2D Theory

In marine hydrodynamics field, potential flow theories are widely used to predict the ship motions and wave loads in waves . Among various potential theories .strip theory is the most popular one ,because strip theory can give the predicted ship motions and wave loads with enough engineering accuracy that are useful for design .besides that ,strip theory cost much less computer time than other potential theories such as three dimensional theory. In ship structure design .direct calculations need strip theory to provide statistical values of wave loads. Strip theory assumes the ship hull is a slender body, two dimensional governing equation is used to describe the nonsteady disturbed flow near the ship. At the same time .strip theory assumes the forward ship speed is low and the oscillating frequency is high. So that the linear free-surface boundary condition becomes the zero speed two-dimensional free-surface boundary conditions. When ship speed is very high, three-dimensional effect of free surface becomes dominant .the ship motions responses given by strip theory do not agree well with ship model experiments. A high speed slender body theory is proposed to solve the seakeeping problems of ship motions which involes the forward speed effects of free surface .This theory adopts two dimensional governing equation according to the fact that most high speed ship have a slender body .Different from strip theory .high speed slender body theory keeps the three dimensional free surface conditions and assumes that there are no waves in front of the ship .the high speed slender body theory adapts to the ship motions and wave loads prediction of high speed ships very well. This paper carries out the following research work to investigate the high speed slender body theory.First .this paper calculates the radiation added mass and damping in heaveand pitch and diffraction forces according to high speed slender body theory by two dimensional nonlinear time domain source method .The numerical results are compared with strip theory.Second ,Vertical ship motions and wave loads in regular waves are solved by high speed slender body theory, The numerical results are compared with strip theory and experiments to verify and validate this theory.Third .The fluid loads composition calculated by the two theories are compared in order to analyse the hydrodynamic coefficients effects on ship motions and the differences between the two methods.Last,the ship cross section numbers are refined in order to investigate how the relative error of fluid solution , pressure motions responses changes.