Vortex-induced vibration dynamics of a flexible fluid-conveying marine riser subjected to axial harmonic tension

This study investigates vortex-induced vibration dynamic responses of a marine riser transporting internal flow under the action of axial harmonic tension by means of finite element method. A van der Pol wake oscillator is adopted to represent the fluctuating lift forces, and nonlinear hydrodynamic force is also introduced. The goal of the present work is to discuss VIV dynamic responses of a fluid-conveying riser to guide the design and usage of riser in offshore oil field. The constant tension analyses are first obtained to determine the lock-in region of cross-flow velocity. Then, the influence of excitation frequency and harmonic tension amplitude on the resonance regions, the displacement amplitudes, the maximum stresses for various internal flow velocity, has been investigated. The results show that the dominated resonance region changed from twice the fundamental frequency to the fundamental frequency when harmonic tension amplitudes and cross-flow velocities are simultaneously increased. It is also revealed that the foremost two lock-in regions, respectively, appeared near the fundamental frequency and twice the fundamental frequency for varying cross-flow velocities.