Coupled CFD Analysis of Mooring Damping Effects on Vortex-Induced Motions of a Deep Draft Semisubmersible

摘要

The accurate evaluation of vortex-induced motion (VIM) of semi-submersible platforms is gaining increasing importance with the recent development of deep draft semi-submersible platforms. Recent available field measurements of semisubmersibles indicate that the VIM response is typically found to be smaller than what is predicted in model tests. The possible candidates for the reduction in VIM response in the field include Reynolds number, wave effects, mass ratios, and external damping from mooring lines and risers. Previous researches by CFD investigation and model tests have studied the Froude scaling law, effects of waves and mass ratio on VIM response reduction. In this study, mooring-induced damping effect on the VIM response reduction is focused and investigated. The Finite Analytic Navier-Stokes (FANS) code is coupled with an in-house 6 degree-of-freedom (DoF) floater motion solver and an in-house MOORING3D code, for the time-domain simulation of the VIM response of a semi-submersible with semi-taut chain-polyester rope-chain mooring system. The large eddy simulation (LES) turbulence model is used to provide accurate predictions of hydrodynamic forces. Simulations are performed for the 1:70 model of the platform. Mooring systems for the semi-submersible with different water depth (WD) and line numbers are designed with the prototype scale, to represent mooring damping in different levels. The simulated motions of the semi-submersible with different mooring system are compared to the experimental data, and the impact of mooring-induced damping on the resultant motion characteristics is investigated by the comparison. The comparison shows that mooring damping is probably one critical reason of VIM response reduction in the field.