Coupled dynamic analysis of deepwater drilling riser under combined forcing and parametric excitation

This paper presents a coupled dynamic analysis of a marine riser under combined forcing excitation and parametric excitation. The forcing and parametric excitation are induced by lateral sea wave and current, and time-varying heave motion of a floating vessel respectively. Firstly, the mechanical model and control equations, which can be solved by Runge-Kutta method, have been established. Then, the Mathieu instability region has been figured out by perturbation method through transforming the control equation into Mathieu equation. On this basis, riser vibration response, bending moment and shear force under the two kinds of actions, sea current (Case I) and combination action of sea current and sea wave (Case II), have been discussed respectively. Results show that, under these two cases, riser vibration responses are both given priority to the first order of vibration mode. The vibration amplitude of riser bending moment under Case II is bigger than that under Case I. Moreover, riser total shear force is mainly determined by the first order mode under Case I, while under Case II, the influences of the first several order vibration modes on the total shear force are non-ignorable.