| Flexible riser is one of the important components of the offshore oil and gas production system.It is prone to vortex-induced vibration under the load of its own weight,top tension and ocean current.In practical engineering,marine risers often appear in the form of riser groups.The interaction between risers makes multi-riser vortex-induced vibration more complicated.At present,the research on vortex-induced vibration of flexible multi-risers is mainly focused on two risers,and the numerical research is very limited.In this paper,the viv-FOAM-SJTU solver is extended to solve the problem of vortex-induced vibration of two risers.Based on this,the vortex-induced vibration of two flexible risers with top tension is numerically studied.In this paper,the flow field,structural calculation,fluid-solid coupling interpolation and input-output modules of the viv-FOAM-SJTU solver are firstly modified to realize the expansion of the vortex-induced vibration solution of two risers,and the effectiveness of the solver is verified by numerical simulations of vortex-induced vibration of an isolated riser in uniform flow and two tandem risers in stepped flow.Then,the effect of immersion length on the vortex-induced vibration of two tandem risers is studied.The influence of immersion length on the crossflow displacement responses of two risers is not obvious,the crossflow frequency responses of two risers are very close,and the crossflow dominant vibration modes are all first order.When the immersion length is 0.55 L and 0.7L,the high frequency and low amplitude vibration occurs in the in-line direction of two risers,in which the upstream riser shows pure second order vibration mode,while the downstream riser shows multi-modal vibration characteristics.With the increase of immersion length,the in-line vibration equilibrium positions of two risers shift toward the flow direction.Finally,vortex-induced vibration of two flexible risers with different arrangements is studied in the paper.In the conditions of tandem arrangements,the riser spacing has a greater influence on the vortexinduced vibration response of the downstream riser.The crossflow displacement of the downstream riser increases with the increase of the riser spacing,and the growth rate decreases gradually.The dominant vibration frequency of the downstream riser is lower than that of the isolated riser,which shows obvious first-order broadband vibration.In the conditions of side-by-side arrangements,the displacements of the lower riser are greater than that of the isolated riser,and the residual vibration response characteristics of the two risers are basically not affected by the riser spacing.In the conditions of staggered arrangements,the displacement of the upstream riser increases with the increase of stagger angle,the frequency response is basically not affected.The crossflow frequency of the downstream riser is basically not affected by the stagger angle,while the displacement is greater than that of the isolated riser in the same working condition when the stagger angle is 0 °,20 °,90 °,and is basically the same as that of the isolated riser when the stagger angle is40 °,60 °.The in-line maximum displacement of the downstream riser is greater than that of the isolated riser when the stagger angle is not 0 °,while the first-order frequency proportion in the frequency response decreases with the increase of the stagger angle,and gradually approaches to the isolated riser. |
