3-D Computational Method Of Wave Loads On FPSO Tankers And Semi-submersible Platforms

The wave load computation is one of the most vital research fields in offshore engineering. The major purpose of this thesis is to present a full solution scheme of auto-meshing of body wet surface, linear wave load computation, nonlinear wave load computation and direct strength analysis for structures of FPSO tankers and semi-submersible platforms. For this reason, the following problems are emphatically investigated.Firstly, the method of accumulative chord length cubic parameter spline function combined with analytic expression is adopted to generate the mesh of body wet surface, which can be widely applied to ships, FPSO tankers, and semi-submersible platforms. On the basis of above, a further treatment for mesh is proposed in order to meet the needs of nonlinear hydrodynamic computation and the removal of irregular frequency effects with extended boundary integral equation method.Secondly, based on the three dimensional potential flow theory, the model for the solution of motion and wave load responses of lowly sailing or moored floating body is established in frequency domain, in which irregular frequency effects are removed with the extended boundary integral equation method being used. The linearized restoring forces acting on the floating body by the mooring system are calculated according to the catenary theory, which are expressed as the function of linear stiffness coefficients and the displacements of the upper ends of mooring chains. Large amount of computations are carried out for ships and semi-submersible platforms, and the comparisons between calculation results and model testing results are conducted. For FPSO tankers, the effects of mooring system on motion and wave load responses are discussed.Thirdly, a kind of three-dimensional nonlinear time history simulation method of floating body motions and wave loads is presented. With the nonlinear factors due to the instantaneous position variation of body surface being considered, the nonlinear motion equations are established and solved in time domain, in which the incident wave forces and hydrostatic restoring forces are calculated according to the instantaneous body surface condition and thediffraction forces and radiation forces are obtained on the assumption that the body is oscillating in harmonic mode at the instantaneous mean wet surface. For improving the calculation efficiency, only the dominant nonlinear factors of incident wave forces and hydrostatic restoring forces are considered, and the diffraction and radiation problems are solved according to the still balanced' position, by which a kind of practical method with enough accuracy and efficiency is presented. Two programs have been developed, and the nonlinear fluid forces, nonlinear motions and nonlinear wave loads of container ship and FPSO tankers are computed, and the contribution of each nonlinear factor is evaluated.Furthermore, the above-mentioned nonlinear method is adopted to predict the relative motion and velocity of the bow, which are used in the judgment of the happening of slamming and green water. The bottom slamming forces and bow flare slamming forces are computed by matching the two-dimensional section and the three-dimensional hydrodynamic computation mesh. For green water loads, the differences of green water pressure and water height distribution between dam breaking model and flood wave model are analyzed, and an improved model is presented, which can include the effects of the change of relative water height over freeboard. Then, the slamming loads and green water loads are considered in the equations of nonlinear body motions.Based on the above work, the study on design wave loads of FPSO tankers and semi-submersible platforms is carried out. The design wave loads of FPSO tankers are investigated with the spectrum analysis method, and compared with those regulated in oil tanker rules. The three dimensional nonlinear method is introduced in the design wave analysis of semi-submersible platforms and the differences between the nonlinear method and linear method...