Numerical study of coupling between multiphase flow-induced vibrations (mfiv) and vortex-induced vibrations (viv) of slender flexible pipes.

The dynamic response of a riser subjected to external and internal flow-induced vibration is studied numerically through the coupling of a computational fluid dynamics (CFD) solver and a finite element (FE) solver. This thesis describes the vortex-induced vibration (VIV) of elastically mounted rigid cylinders, empty flexible risers and fluid conveying risers using three-dimensional (3D) numerical simulations. The dynamic characteristics of the riser, such as inline (IL) and crossflow (CF) displacements and vibrating frequencies, are provided and discussed. The effects of fluid flow, including velocity magnitude and profile, on the dynamic response are also investigated. A 1 degree of freedom (1DOF) VIV of an elastic cylinder is studied first using 2D and 3D numerical models. The effects of mass and damping ratio on the amplitude of vibration of the cylinder over a reduced velocity range are analysed. Results show a good agreement with published experimental studies. It is demonstrated that increasing the mass and damping ratio reduces the maximum vibration amplitude of the cylinder. A 3D numerical study is then carried out to investigate the effects of velocity magnitude, velocity profile, the elastic modulus of the riser, orientation, and aspect ratio. The study establishes that the IL displacement, CF vibration mode, and frequencies increase with the flow velocity, aspect ratio and reduction of elastic modulus. In addition, the VIV of the riser transporting single-phase internal flow is presented. The numerical study describes the effects of internal flow velocity magnitude on the dynamic response of the riser. It is shown that the internal flow velocity magnitude affects the vibration mode of the riser. Finally, the effects of internal air-water volume fractions on the dynamic response of a riser transporting a two-phase flow are examined in detail. The influence of gas fraction on a pipe with no external flow, on a jumper pipe with no external flow and on a fluid conveying pipe exposed to external flow is investigated. The numerical results show that a variation of the air volume fraction slightly affects the dynamic characteristics of the riser.