Experimental Study And Numerical Simulation On The Vortex-Induced Vibration Of Marine Risers Conveying Fluid

Marine risers are the main connecter between a platform and the mouth of a well in the seabed and can be applied to floating platform, fixed platform and drilling shipping systems. The risers generally contain flowing oil or high pressure gas and bear the action of wave and current outside. When current or wave flows across the risers at certain speed, there will appear vortex shedding, and if the frequency of the risers is close to that of the vortex shedding, oscillation will make the vortex shedding frequency fixed nearby to the structural natural frequency and cause large amplitude vibration which is called'lock-in'state. So the vortex-induced vibration is an important factor in the design of all riser system, including drilling, production and export riser.When internal fluid travels along the curved path inside the deflected risers, it experiences centrifugal and coriolis accelerations because of the curvature of the risers and the relative motion of fluid to the time dependent risers motion, respectively. Those accelerations exert against the risers and, in turn, affect the dynamic behavior of the risers and cause additional vibrations. Risers subjected to the vortex-induced vibration and the internal fluid may experience large response which gives rise to oscillatory stresses and cause fatigue damage. Only few numerical studies were focused on the VIV considering internal flowing. For the experimental study, has not been carried out yet. So the experimental study and numerical simulation on VIV considering internal flow flowing has both scientific and practical value.In this paper, considering internal flowing fluid and external marine environment, the scale experiment on the rigid riser in the shallow water and no-scale experiment on the flexible riser in the deep water were both conducted. For the numerical simulation, the equation of motion was derived based on work-energy principles and vortex strength was expressed using wake oscillate model. The program in Matlab was complied to get the dynamic response. Comparisons were made between the experimental and numerical results. The results indicate that: for the rigid riser in shallow water, the effect of internal flow on its dynamic characteristic and dynamic response is not obvious because of its high stiffness and low internal flow speed. For the flexible riser in the deep water, with the increasing of the internal fluid speed, the natural frequency from the lower one to the higher one all will decrease, the response amplitude increases while the vibration frequency decreases. And the internal fluid flowing lessens the correlation of the vibration between different sections. In addition to that, by plotting both in-line strain and cross-flow strain simultaneously, the figure-of-eight of bending strain is also observed, and the trajectories in different cycle are more accordant with the increase of internal flow speed.