Instability analyses of a top-tensioned riser under combined vortex and multi-frequency parametric excitations

This study investigates the non-linear dynamic responses of a top-tensioned riser (TTR) under combined parametric and external vortex excitations. In real sea conditions, risers simultaneously encounter waves and flow. A general analysis considers only parametric excitation resulting from platform motions or vortex-shedding excitation due to flow bypassing the risers. However, the instability of a TTR under combined excitations can lead to fatigue damage; therefore it is essential to explore the dynamic characteristics in order to maintain the safety and avoid accidents. A derived TTR model subjected to multi-frequency parametric and vortex shedding excitation has been proposed, and different cases of excitation combinations were compared. The effects of key design parameters on the dynamic properties of a TTR under combined excitation are discussed. The surface distribution of instability for a TTR system under multi-frequency parametric and vortex-shedding excitations was analyzed on the basis of various significant wave height and wave peak period combinations, and extreme sea states were considered to determine the maximum strength of a TTR. Several interesting phenomena were observed. The instability of a TTR system excited by combined excitation could be more unstable than that excited only by parametric excitation or vortex-shedding excitation. It can be concluded that the effect of vortex shedding is predominant in the excited instability of a TTR under combined excitation when the outside sea state is mild, but when the sea states is severe, parametric excitation contributes more to the instability, vortex-shedding excitation only strengthens the consequent instability. More modes and a higher frequency of vibration can be excited when extreme states are more severe.