COUPLED CFD-FEM SIMULATION OF FLOATERS WITH MOORING SYSTEMS

Abstract

The Finite-Analytic Navier-Stokes (FANS) computational fluid dynamics (CFD) code is coupled with an in-house nonlinear finite element (FEM) mooring analysis program, MOORING3D, to study the dynamic responses of a floating body with a mooring system in complicated environmental conditions. A six degree-of-freedoms (DoFs) motion solver based on the 4th order Runge-Kutta scheme is used as the interface between CFD and the MOORING3D. A specially designed coupling methodology to adapt to the Runge-Kutta scheme is developed. The hydrodynamic loads on the moored floating system is estimated by the CFD using the large eddy simulation (LES) model. The FANS code is solved in conjunction with the level-set (LS) formulation to model free surface effects. The 3rd order TVD (total variation diminishing) Runge-Kutta scheme and the 3rd order ENO (essentially non-oscillatory) scheme are used to numerically solve level-set values. Besides, the methodology is developed for an overset grid system of embedding, overlapping, and moving structured grids. The dynamic responses of a catenary anchor leg mooring (CALM) buoy system in waves and currents are simulated with the coupled method to demonstrate its feasibility. The simulation results are compared against numerical simulations based on potential flow theories and available experimental measurements. The free-decay motions, wave-induced motions, and wave-current-body interaction are investigated. The agreements between the simulated response amplitude operators (RAOs) and the experimental data provide validations for the coupled method. The results also show the potential capability of the coupled method in addressing much more complicated free surface flow simulations. The coupled method is additionally applied to investigate the vortex-induced motion (VIM) of a deep draft semi-submersible platform. The VIM of semi-submersible is gaining increasing attention from both industry and academia with the recent development of semi-submersible platforms. The mooring-induced damping effects on VIM are investigated through the coupled CFD-FEM analysis. The LES turbulence model is used to provide accurate estimation of hydrodynamic loading. Varying reduced velocities are considered. The simulated VIM responses are compared with experiments and previous numerical simulations. The results reveal the mooring-induced damping to be a critical factor contributing to the VIM response reduction in the field. The reduction ratio matches well with the previous model tests and field measurements. The results demonstrate that the coupled FANS-MOORING3D code proves to be a powerful tool for the study of complex fluid-body-mooring interaction.