Full Time-Domain Nonlinear Coupled Dynamic Analysis Of Deepwater Floating Structures And Mooring/Riser Systems

Floating platforms, such as Spar, TLP and FPSO, have been widely used for oil and gas production in deep water. They are often maintained in position by a mooring system which resists the influences of all kinds of harsh marine environment. The coupled analysis of platforms with their mooring/riser systems is of primary importance in studying the dynamic behavior of offshore structures. With the increase of water depth, the mass and damping of mooring lines and risers become nontrivial and the surface-platform motions can be appreciably affected by them. Therefore, it is important to include dynamic interactions between surface platform and its mooring system. In this case, an integrated nonlinear coupled-dynamic analysis approach is presented so that all the interactions among platforms, mooring lines/tendons, and risers, can be fully evaluated.By taking the3D Laplace equation as the basic governing equation, a mathematical model about second-order time-domain wave interaction with structures is founded. The Taylor series expansion is used to make the instantaneous body-surface boundary condition and the instantaneous free-surface boundary condition satisfied on the mean body surface and the still water surface, respectively. With the use of perturbation expansion and the separation of series, the corresponding first-order and second-order boundary value problems are respectively established. By choosing the Rankine source and its images about the seabed as the Green's function, a boundary integral equation about the velocity potential of an arbitrary point in the wave field is obtained based on the Green's second theorem. The computational formulations of wave forces are developed in detail. The motion equations of a rigid body are established and the process of numerical implementation using the fourth-order Runge-Kutta method is introduced.The higher-order boundary element method is then used to establish a set of linear equations, with the nodal velocity potential of body-surface elements and the nodal normal derivative of velocity potential of free-surface elements as the unknowns. The triangular polar coordinate transformation is used to deal with the singular integration, and the singularity of the Green's function is then removed; The direct method is applied to solve the solid angle coefficient, which guarantees the calculational precision in different conditions; By use of the symmetry of the structures, the matrix equations are simplified, and the amount and time of calculation are reduced greatly. For the static and dynamic analysis of mooring-lines/risers, a three-dimensional elastic rod theory is chosen to modeling their motion in a global coordinate system, and the corresponding governing equations can be established. The finite element method is used to implement the developed theory into a numerical model with different boundary conditions. Newmark method combined with Newton-Raphson iterative schemes are used in solving the line dynamics.The mooring dynamics program is then coupled to the hull dynamics program through the matching conditions at the fairleads. The first order and second order motion equations for hull are then established respectively. In every time step, the fourth-order Adams-Bashforth-Moulton method is adopted to update the velocity potential and the instantaneous wave elevation on the free surface, and the dynamic equations for the mooring system and hull are solved simultaneously using Newmark method including N-R iteration.For the nonlinear interaction between waves and structures, the second-order wave-diffraction problems for a vertical cylinder and a truncated cylinder are first calculated. The first-order and second-order wave forces are presented, and the results are compared with others and they coincide very well. The problem of wave-interaction with a floating hemisphere confined by linear springs is then studied. The first-order and second-order displacements of the hemisphere are compared with those in frequency domain respectively, and they coincide very well.For the static and dynamic analysis of mooring-lines/risers, a systematic study has been conducted. The static problems of mooring lines with/without current are firstly calculated, and the static configuration and the tension distribution are presented. Then, the static and dynamic analysis of a vertical mooring line attached to a buoy is conducted. Both the quasi-static and dynamic analysis results are presented, the effects of the line dynamics can be observed. The mooring-line dynamics problems are further studied. The dynamic configuration and the tension at the top point are presented. The maximum tension in the line and the mooring-induced damping are also studied in the paper. Lastly, the static analysis of a riser with different boundary conditions is executed. The static configurations of the riser are compared with the published results, which validate the numerical model.The coupled dynamic analysis for nonlinear interaction between waves and deepwater floating structures with their mooring/riser systems is conducted in emphasis. The computational results include the platform motion, the mooring/riser tension, the time histories of wave elevation and dynamic pressure at the waterline points, and the3D contour map of wave elevation and dynamic pressure. A moored classical Spar in two different water depths is first studied. The results of coupled quasi-static approach are compared with that of coupled dynamic approach. From the results, we can see that the coupled dynamic approach must be used to predict the platform motion and mooring tension exactly in deep water. A TLP in regular waves is then calculated. It can be seen that the platform mainly goes through translational motion in the horizontal plane, and the heave and angular motion are relatively small. The surge-induced set-down of the TLP can also be observed. Lastly, a Truss Spar in irregular waves is studied. The numerical results are compared with the model test results. From the comparison, we can conclude that the numerical model can predict the motion responses of ocean platforms and the mooring tensions accurately. As we can see from the motion response of the Truss Spar, the platform goes with slow-drift motion and wave-frequency motion in a mean offset position, and the second-order analysis is very important.