Experimental Research And Numerical Simulation Of Vortex-Induced-Vibration Under Flow Interference

In recent years,with the continuous increase in global energy consumption,oil and gas exploration and development have gradually shifted from land to ocean.Riser plays an important role in deep-sea oil and gas development.When the current flows through the riser at a certain speed,it is easy to cause vortex induced vibration,resulting in fatigue damage and even major production accidents.The riser groups close to each other will cause flow interference,which makes the excitation response and vibration mechanism of the riser more complicated than that of the isolated riser,and the possibility of fatigue failure increases.The traditional vortex-induced vibration theory and analysis model are mainly established based on the study of a single cylinder,the analysis of multi-cylinder vortex-induced vibration under the action of flow interference has important research value and engineering significance.In this paper,the method of combining model test,numerical simulation and empirical model is used to conduct a systematic and in-depth study on the vortex-induced vibration problem under the action of ocean riser flow interference.Firstly,vortex induced vibration model tests were carried out in a wave flume with data acquisition instruments based on different test principles,and the relationship between the response characteristics of the riser model under different working conditions and the incoming flow velocity was obtained;Secondly,a two-dimensional fluid structure coupling numerical model is established by using UDF function and dynamic grid technology in Fluent software,and the results are compared with the previous experimental results,mainly study the effect of changes in spacing ratio,mass ratio,incoming flow angle and other conditions on flow interference.Finally,numerical simulation research is carried out based on Orca Flex and SHEAR7 software,and the vortex-induced vibration response characteristics before and after the interference of the riser model are predicted and analyzed through superposition calculation.The results show that the riser model will have a strong nonlinear dynamic interaction with the fluid in the “lock-in” region,and the probability of continuous large amplitude vibration and fatigue failure will be increased accordingly.The vibration amplitude along the rectangular direction of the riser model is characterized by small ends and large middle;The change of the incoming flow angle will have a direct impact on the flow interference between the riser models.With the continuous increase of the angle,the interference on the downstream riser will gradually weaken;With the increase of mass ratio,the range of “lock-in”zone is greatly shortened and the vibration amplitude is obviously decreased;The shielding effect of the upstream cylinder leads to the lag between the “lock-in” zone and the maximum amplitude of the downstream cylinder,and the increase of the spacing ratio makes the interference between the two cylinders continuously weaken;The Blevins wake model takes into account the transverse lift coefficient,so that the velocity can be reduced to the maximum extent,resulting in the response characteristics of local velocity,motion amplitude and resistance coefficient are less than those of Huse model;With the increase of spacing ratio,the probability of fatigue damage of downstream riser gradually decreases,and the response characteristics are closer to that of isolated riser.