Dynamic responses of a flexible pipe conveying variable-density fluid and experiencing cross-flow and in-line coupled vortex-induced vibrations

The flexible pipes such as marine risers have been widely used in the ocean engineering to transport fluid flows. The fluid flows inside the pipes may contain oil, natural gas and sea water at the same time. As a result, the total fluid density will change with time and space. In addition, these flexible pipes are usually subjected to ocean currents and experiencing cross-flow (CF) and in-line (IL) coupled vortex-induced vibrations (VIV). In this paper, a nonlinear dynamic model of a flexible pipe conveying variable-density fluid and simultaneously undergoing CF-IL coupled VIV is developed based on the Hamilton's principle. The present model is numerically solved and validated. Then, the influence of internal varying fluid density on a straight pipe, IL curved pipe and CF-IL coupled VIV pipe are successively analysed. The results demonstrate that the internal varying fluid density can cause parametric resonances on the flexible pipe. In different parametric resonances, different modes of the pipe will be excited and the CF-IL coupled VIV of the pipe will be affected differently. • A nonlinear dynamic model of a flexible pipe conveying variable-density fluid and undergoing CF-IL coupled VIV is established. • When the internal fluid density varies with time, the pipe will be parametrically excited and different modes will be triggered. • In different parametric resonances, the CF-IL coupled VIV of the pipe will be affected differently. • As the flexible pipe deflects in the IL direction, the IL VIV will be more strongly affected by the internal varying fluid density.