Numerical study of vibrations of a vertical tension riser excited at the top end

This paper presents numerical simulations of vortex-induced vibrations of a vertical riser which is sinusoidally excited at its top end in both one and two directions in still water. A computational fluid dynamics method based on the strip theory is used. The riser's responses to both top-end and two-end excitations are carefully examined. In low reduced velocity cases, the in-line vibrations consist of three components, the low-frequency oscillation, the first-natural-frequency vibration during the riser reversal, and the second-natural-frequency vibration due to vortex shedding. The sheared oscillatory flow along the span causes low-frequency oscillations in higher modes in the in-line direction, thus forming 'X' shaped, 'II' shaped, and 'O' shaped trajectories at various positions along the span when the riser is excited at its top end in one direction. In the presence of excitations in the other direction, more complex trajectories appear.