Coupled dynamic analysis of floating structures in waves and currents

A computer program, WINPOST, is developed for coupled dynamic analysis of the floating structures. In the program, the floating platform is modeled as a rigid body with six degrees of freedom. Hydrodynamics of the structure, which include the linear and second-order wave forces, added mass, radiation damping and wave drift damping, are calculated from programs WINTCOL and HOBEM. The wave force time series are then generated in time domain based on the two-term Volterra series model. Drag force on the platform is calculated using Morison's formula assuming the wave field is undisturbed. The mooring line dynamics is modeled using rod theory and finite element method, with the governing equation described in a single global coordinate system. The connections between mooring lines and platform are modeled as linear and rotational springs and dampers. Various types of connection can be modeled using proper spring and damping values. An efficient time domain integration scheme is developed based on the second-order Adams-Moulton method. In frequency domain, the nonlinear drag force is stochastically linearized and solutions are obtained by an iterative procedure. Finally, the nonlinear coupled responses of four types of floating structure in waves and currents are investigated with the focus on finding the critical parameters in the dynamics of the floating structures. These four types of structure are tethered spar, classic spar, truss spar and tension leg platform. In the study, the coupled time-domain analyses are systematically compared with model test results, uncoupled analyses and frequency domain analyses.