Dynamic analysis of deepwater risers conveying two-phase flow under vortex-induced vibration

The vortex-induced vibration of the riser transporting flow was analyzed in this paper. The dynamic model which couples the effect of internal flow and external flow has been proposed based on Euler-Bernoulli beam model, slip-ratio factor model and wake oscillator model. The finite difference scheme and generalized integral transform techniques have been adopted to solve the partial differential equations. The influence of the internal flow velocity, void fraction, aspect ratio and external flow velocity was studied on the vibration frequency, amplitude and modes of the riser. The results show that internal flow will lead to the drop of natural frequencies of the riser. For deepwater rises, the internal flow should be taken into consideration and the two-phase flow has significant influence on the dynamic response when the riser is long. The increase in external flow velocity and aspect ratio may lead to the high mode vibration. When the riser is under the combination action of internal flow and cross-flow, the frequencies decrease and the vibration amplitudes increase with the increase in gas fraction. Under the same external flow velocity, the two-phase flow in the riser will induce a higher-order modal vibration.