Coupled Analysis of Deepwater Floating System Including VIV in Time Domain

Deepwater floating systems consist of a vessel, risers, and mooring lines. To accurately simulate the floating systems in current, wind, and waves considering (1) bending and torsional stiffness of riser, (2) elongation of the mooring/riser elements, (3) complex end conditions, (4) internal flow effects, and (5) vortex induced vibration, it is necessary to evaluate the vessel motions and mooring/riser behaviors simultaneously in time domain. However, because the size of the system matrix increases significantly as the number of mooring/riser increases, it is quite time-consuming to solve all equations including both mooring/riser and vessel dynamics simultaneously. The present study was performed in order to develop a program for this problem. The 6DOF vessel dynamics is described by the Cummins equation. And the mooring and riser are modeled with the help of finite-element beam. The Newmark method is used as the time marching scheme of the FEM equations for each mooring/riser and the vessel. The coupled equations of the mooring/riser segments and vessel are solved alternatively at each time step. Mooring/riser and the vessel motion affect to each other in the way that the components of the forces at the segment ends are determined as functions of displacements and slopes of them. This procedure makes it possible to consider the coupling effects between vessel and mooring/riser efficiently. Also no iterations are required to match the vessel motion with the riser dynamics. This new approach allows us to use parallel computations and to deal with as many mooring/riser at the same time as necessary. The hydrodynamic forces induced by current are calculated by using the Morison’s formula. The VIV (Vortex Induced Vibration) effects are included in the way that the frequency and the shape of the riser vibration due to VIV are pre-calculated by iterations in the frequency domain. Then the finite element mooring/riser model is modified to consider the hydrodynamic loads including VIV and integrated in the final equations of the floating system in time domain.