Stability and dynamic responses of a top-tensioned riser under two different parametric excitations

Top-tensioned risers are commonly used to transport fluid from seabed wells to floating platforms at the sea surface. Under wave action, the floating platform may heave. The tension at the top of the riser may change, potentially causing parametric excitations on the riser. The riser may transport oil and natural gas at the same time. The internal total fluid density fluctuations may induce parametric excitations on the riser. In this study, a top-tensioned riser under parametric excitations from periodically fluctuating top tension and varying internal fluid density is simulated and analyzed extensively. The variations in the top tension are modeled using a sinusoidal function over time. Meanwhile, the varying internal fluid density is described with a function of time and space. The governing equations of the riser are derived based on Hamilton's principle. The present model is solved numerically and validated. The analysis shows that if the top tension and internal fluid density fluctuate outside parametric resonances, the riser may become unstable. In different parametric resonances, different riser modes are excited, leading to irregular vibrations with multiple frequencies.