Investigation of mooring damping effects on vortex-induced motion of a deep draft semi-submersible by coupled CFD-FEM analysis

The vortex-induced motion (VIM) of semi-submersible is gaining increasing attention from both industry and academia with the recent development of deep draft semi-submersible platforms. Field measurements of semisubmersibles reveal that the VIM responses are typically found to be smaller than those observed in model tests. The possible factors causing the VIM response reduction in the field measurements include Reynolds number, wave effects, mass ratios, and external damping from mooring lines. The objective of this study is to investigate the mooring-induced damping effects on VIM. The Finite Analytic Navier-Stokes (FANS) code is integrated with an in-house six degree-of-freedom (DoF) motion solver and an in-house nonlinear finite element (FEM) code MOORING3D, for the coupled CFD-FEM analysis of the VIM of a moored semi-submersible. The large eddy simulation (LES) turbulence model is used to provide accurate estimation of hydrodynamic loading. Simulations are performed for the 1:70 model of the platform under varying reduced velocities. Full-scale mooring systems with different water depths and mooring line numbers are designed to represent different levels of mooring damping effects. The simulated VIM response is compared with the experiments. The study reveals that mooring-induced damping is a critical factor contributing to the VIM response reduction in the field.