Study On Calculation Of Motions And Loads On Floating Bodys In Large Amplitude Waves

Numerical simulation and analysis of motions and loads of ship and ocean structure under waves with large amplitude are very important in theory and engineering application. With the development of Chinese economy, more and more new types of ship and ocean structure are developed. The new design method is required for achieving safe ships and ocean structures. With the wide use of the three-dimensional finite element method, the calculation of wave loads on floating body in the waves is in need more and more. In this paper, the interaction between non-linear waves and surface-piercing body with arbitrary shape in three dimensions is investigated on the basis of the potential theory. The Green-Function method both in frequency-domain and in time-domain with considering of fully non-linear effect is used in the calculation.As the wave simulation in the time-domain requires more memory and CPU-time than in the frequency-domain, an efficient NURBS approach based boundary element program is developed firstly in this paper. NURBS (Non-Uniform Rational B-Spline) is a new spline method that can express the complex three-dimensional surface exactly. By using the NURBS method, the continuity of potential function and its derivative between the grids can be affirmitived. And the fluid speed on boundary surface can be accurately calculated. Comparing with the traditional constant panel method, the NURBS based BEM is more accurate and can reduce the CPU time consuming and memory requirement. Now the NURBS method is widely used in the CAD software. The application of the NURBS based BEM can help the integration of CAD and CFD system. In this paper, the detail of NURBS based BEM is anylased and a computer program of NURBS based BEM is developed. The diffraction problem of body in infinity fluid is calculated. The computation result is compared with the analytic result satisfactorily.In this paper, the NURBS based BEM is used to calculate the wave-body interaction problem in the frequency-domain. By using the self-adaptive step size Simpson integral method and use a certain integral transform method, a numerical method for the calculation of the Green function of three dimensional translating and pulsing source is developed and used in the computation of wave loads. Numerical examinations on first-order wave forces, first-order body responses, velocities of fluid particle and second-order mean drift forces in the frequency-domain show that the NURBS based BEM is more accuracy and more computational efficient than conventional const panel BEM. The relationship between wave parameters and wave loads is investigated. The exceeding probability of design waves with different wavelength is calculated based on the wave data of long-term distribution.In this paper, the fully non-linear problem in the time-domain is analysed. By using the mix Eurian-Lagrange method, the free surface boundary condition is satisfied on the instance free surface and on body surface. The standard fourth order Runge-Kutta method and the fourth order Adams-Bashforth-Moulton method are used as the time step method. TheNURBS based BEM method is used to calculate the velocity potential at each time step. An artificial damping zone is adopted as the far field radiation condition on the free surface to avoid the wave reflection from the outer boundary, and the present method is found to be numerically stable and accurate. The pressure on body can be obtained by solving a boundary integral equation of time derivative of velocity potential. The wave generation and attenuation in a numerical wave tank and the wave load on a cylinder are calculated. The time-domain wave-body interactive is also calculated in this paper. An iteration method is used in the calculation of time derivative of speed potential. The free motion of floating truncated cylinder and sphere is calculated and the results of wave forces and body motion are presented. All of the numerical results in this paper, including the time-domain results and the frequency-domain results, agree well with published e...