Time Domain Simulation Of Wave Action With Sea Platform In Deep Water

Based on the high-order boundary element method, the wave force on small-scale members of a composite structure was firstly investigated in diffraction field in time domain. The study is mainly focused on the hydrodynamic characteristics of a Truss Spar and a semi-submersible platform in the South China Sea with 100-year marine environmental disaster.By taking the 3D Laplace equation as the basic governing equation, a mathematical model with respect to the interaction between linear waves and arbitrary 3D structures was founded. The Taylor series expansion was used to satisfy the free surface boundary condition on the mean static water and the solid-wall boundary conditions on the average body surfaces, respectively. Therefore, the computational domain can be restricted to a fixed time-invariant scope. With the use of perturbation expansion and the seperation of the incident and diffraction (scattering) waves, a linear boundary value problem was set up. By introducing the Rankine source and its image on the sea bottom as the Green function, a boundary integral equation for the velocity potential of an arbitrary point in the wave field was obtained based on the second Green identity.Through a series of complicated transformation, the integral equation was discretized into matrix equations, with the nodal potential of body elements and the nodal normal derivative of velocity potential of the water surface elements as the unknowns. The singular integral problems emerged in calculations were solved through triangular polar coordinate transformation; the solid angle coefficient was acquired through direct calculation, which guaranteed the calculation precision in different conditions. With the symmetry applied to the solution process, the matrix equations were simplified, and the amount of calculation was reduced greatly.In the time-domain calculation, the fourth-order Runge-Kutta method was adopted to update the velocity potential and the instantaneous wave elevation on the free surface. As to the motion equations for objects, the fourth-order Runge-Kutta method was employed again for iterative solution. A ramp function was added at the beginning of calculation. A damping layer was arranged at the boundaries of the computational domain to ensure that steady calculation results can be obtained within a finite time and space domain. Formula for calculating the wave forces on the small-scale members in the diffraction field were firstly derived. The water-particle velocities produced by the diffracting waves were solved using integral equations, and the acceleration was obtained based on the time difference. For some specific examples, the accuracy of the velocity and the acceleration obtained with this method was verified by comparison with the analytical solutions. For a given structure, the characteristics of the wave load on the small-scale members under the action of regular waves and freak waves were analyzed. Then the influence of the size of an upper large-scale structure on the wave load on a small-scale structure was investigated. Finally, the wave load on the medium small-scale member bars at different positions in the diffraction field was analyzed. For oblique members on moving body, the solution procedure was deduced in detail.The hydrodynamic characteristics of a Truss Spar and a semi-submersible platform in South China Sea with 100-years marine environmental disasters were investigated in emphasis. The viscosity damping effect on first order motion respose was studied by calculating the damping matrix, and the different motion nature period of each platform was obtained by analzing the displacement-time curve. Time domain simulation was carried out for each platform reponse under four kinds of loading combinations, and the time history of wave loadings, replacement, and mooring tension forces were get and analyzed, respectively. The wind and current loads on each platform were calculated and then the equilibrium positions under these loads were obtained. The wave run-up for Truss Spar and minimum air-gap for semi-submersible platform were given under waves. The changing of the wave surface and the wave height distribution for regular waves and irregular waves were described, respectively.