ANSYS multi-physics software has been applied to solve a series of meaningful pre-test Fluid-Structure Interaction (FSI) tasks which was related to dual-pipe and quad-pipe systems subject to vortex-induced vibration (VIV) resulting from the coupling between outer and inner flow. As an instructive supplementary to model test, those numerical tasks were going to treat more complex problems which are difficult or even disable to carry out in engineering basins and provide model tests some pre-test verification and some supplementary wake visualizations. As conclusions, the coupled interaction between outer and inner flow mainly depends on reduced velocity and the central axis distance between pipes. In dual-pipe systems, the drag coefficient of the pipe in the wake is weakened, less than that of single pipe system. However, the in-phase movement between adjacent pipes will enhance the oscillation in the relative direction in quad-pipe systems.
With the development of deep and ultra-deep water petroleum exploitation, multi-assembled risers/pipes systems connecting subsea wells to FPSO, vessel or oil platform become popular in engineering application recently. This kind of joint form produces a congested configuration where interaction or even collision between risers and umbilical mooring lines maybe occur. In case that the central spacing between riser/pipes and riser/mooring is limited, the interaction between them becomes serious and is of a typically prominent phenomenon. Generally speaking, except for turbulence buffeting, hydrodynamic interference between risers can be classified in two categories: wake-induced oscillation (WIO) and vortex-induced vibration (VIV) (Blevins, 2004; Fontaine and Morel, 2006). WIO, also referred to as "galloping", which may rise up to several tens of diameters, is a typical kind of instability. Since the oscillation principles of multi-assembled pipe systems are distinctly different from that belonging to single-pipe systems, much attention should be paid to the hydrodynamic load imposed on the risers specially. In fact, the riser deflection increases with the square of the riser length for a tensioned riser, as well as for the upstream riser in multi-assembled pipe systems approximately.
