Influence of the internal varying density flow on the vibrations and fatigue damage of a top-tensioned riser undergoing vortex-induced vibrations

•When the internal fluid density fluctuates with a small circular frequency, the VIV riser will be largely affected. The vibrations may beat with time and contain multiple frequencies.•As the mean value or fluctuation amplitude of the internal varying fluid density increases, the fatigue damage of the VIV riser will become larger.•At a small external flow velocity and a large internal flow velocity, the vibrations of the riser may be dominated by the internal varying density flow, which are more irregular.•For a larger diameter or a broader wall thickness, the riser excited by the internal varying density flow and external VIV is more easily to become fatigue failure. The top-tensioned risers have been widely used in the field of ocean engineering. They are usually used to transport fluid flows from seabed wells to surface platforms. Under the action of ocean currents, these risers are often subjected to vortex-induced vibrations (VIV), which could cause serious fatigue damage. When the riser carries oil and natural gas for the same time, the total fluid density will change with time and space and causes some effects on the riser. In this paper, a nonlinear dynamic model of a top-tensioned riser conveying varying density flow and simultaneously undergoing VIV is established based on the Hamilton's principle. The present model is numerically solved. The results are compared with experiments and CFD simulations, which are in reasonable agreement. With the present model, the influence of internal varying density flow on the vibrations and fatigue damage of a VIV riser is analysed in detail. The results demonstrate that when the internal fluid density fluctuates with a small circular frequency, the vibrations of the riser will be strongly affected. As the mean fluid density or the fluctuation amplitude of the internal varying fluid density increases, the fatigue damage of the riser will become larger.