At present, most researches on the vortex-induced vibration of submarine free spanning pipelines ignore the effect of internal flowing fluid; furthermore, there are no research reports considering the coupling effect of internal and external fluid with the free span. In this paper, combining Iwan's wake oscillator model with the differential equation derived for the dynamic response of submarine free spanning pipelines with inclusion of internal flow, the pipe-fluid coupling equations are developed to investigate the effect of internal flow on the vortex-induced vibration of the free spans. The finite element approximation is implemented to derive the matrix equations of equilibrium. The Newmark method combined with simple iteration is used to solve the system of equations. The results indicate that the internal fluid flow may cause the shift of resonance band to the lower frequency and a slight decrease in the peak value; the effect will be more pronounced with the increase of the span length and can be weakened in the presence of the axial tension.
As a project supported by the National Natural Science Fotmdation of China, a model experiment on the vortex-induced vibration of practical risers transporting tluid in currents was conducted in the Physical Oceanography laboratory of ocean University of China in 2005. Because most of the offshore oil fields in China are in shallow water, the experiment was focused on the risers in shallow water. The similarity theory was used in the experiment to derive the experimental model from the practical model. Considering the internal flowing fluid and external marine environment, the dynamic response of the marine riser was measured. Corresponding numerical simulation was performed with the finite element method. Ccnaparisons were made between the results from the experiment and numerical simulation.
