NUMERICAL SIMULATIONS OF VORTEX-INDUCED MOTIONS OF OFFSHORE PLATFORMS IN DEEP SEA

Offshore platforms with slender vertical hull are subject to vortex-induced motions (VIM) when exposed to currents. VIM involves flow separation, vortex shedding and/or reattachment. It forms free shear flow and becomes completely unstable and sophisticated after separation. Predicting VIM response for offshore platforms with complex geometries involves mainly three key aspects: an efficient and robust rigid body motion solver, a extensible model for various kinds of mooring system and a high accuracy model for massively separated turbulent flows. Based on an existing solver naoe-FOAM-SJTU developed on top of the OpenFOAM framework [1], we extended the current solver to have full capability of simulating VIM for offshore platforms. An Euler angle described rather than the OpenFOAM built-in quaternion described six-degrees-of-freedom rigid body motion solver was implemented to handle dynamic motions of platform. In addition, the platform is constrained by mooring system, which is modelled as several distributed linear springs in simulations. The massively separated flow at high Reynolds numbers is addressed by Shear Stress Transport based Detached-Eddy Simulation, a hybrid RANS-LES method which functions like RANS in the boundary layer and transits from RANS to LES after flow separation.