The Prediction Of Ship Large Amplitude Motions And Wave Loads In Time Domain

Presently, the strip theory has become an ordinary and utility method for the calculation and prediction of ship motions and loads in waves , because of its virtues that other methods are beyond compare . Based on the commonly used strip theory with the extension of taking into account the instantaneous variation of the wetted hull surface during the motion and its effects on the ship hydrodynamic characteristics, this dissertation mainly calculates the ship motions and loads in waves in time domain and elementarily investigates some nonlinear characteristic of ship motion and hydrodynamic coefficient. It is hoped that the research had an engineering value for the predication and evaluation of seakeeping performance in the concept ship design stage . The research includes two parts: Firstly , following the Frank close fit method, it calculates the ship hydrodynamic coefficient , and includes the calculation of added mass and damping coefficients of different drafts. The calculated drafts covers the whole possible ship draft in the large amplitude motion; Secondly, the research creates and solves the ship large amplitude motion equations .On the one hand , the actual calculation takes into account the coupling between the different motion modes ,on the other hand ,in the process of calculation of the ship hydrodynamic coefficient ,it considers the influence of the nonlinear factor .The hydrodynamic coefficient used in the calculation of Froude-Krylov force, diffraction force and radiation force changes instantaneously following the draft, and the hydrostatic buoyancy is computed very accurately for the instantaneous immerse hull. At last ,the ship motion equations are performed numerically with the fourth-order Runge-Kutta scheme in time domain,and sea loads are computed at the same time. The research reveals: Within the range of some frequencies , the associated hydrodynamic coefficient doesn't change following the ship draft basally, and appears some difference only in excess of an frequency; For the rolling mode, the hydrodynamic coefficient doesn't varyregularly along with the draft change, but appears the fluctuating phenomenon ;For the sections close-by stem, its hydrodynamic coefficients fluctuate sharp, and are far from the value of the average place ,what's more, there is a difference in the rate of value change. In general, the upward amplitude curve' curvature is larger, and it changes quicker. On the contrary, the downward is smaller and slower; Because the flare force contains the square of the wave elevation velocity, it results in an difference between the flare force frequence and the associated moving frequence; The flare force decreases the downward amplitude of heaving oscillations; The speed of ship has an ignorant effect on the peak values of three calculated values, but there is an obvious effect on the oscillating frequence; The wave amplitude has an effect on the ship motion, and there is an linear relation between the oscillating amplitude and the wave amplitude on the whole .By the way ,there are some experiences and lessons according to the progress calculation: Before the calculation of the ship motion, we should checkout the balance in a calm water firstly, namely, the whole displace of ship and the longitudinal position of the center of gravity must be consistent with the draft in calm water, or else ,the calculation may show the "floating" phenomenon; The mixed language programming has a lot of virtues, but it exists a fatal limitation-the debug of DLL. It brings great inconvenience because we can't see the array value in the frame of DEBUG.