Numerical Simulation Of Vortex-induced Vibration Of Flexible Riser In Oblique Flow And Shear Flow

As an important offshore facility for drilling and exploitation,marine risers generally connect to the universal joints at lower ends and join to the sliding joints in platform or to the FSOPs directly at upper ends.Besides the fluid transferred inside with both high speed and pressure,risers are also subjected to the wave and current loads.When current passes by circular cylinders,risers for example,vortices shedding from each side of the structures alternatively,which generate periodic hydrodynamic forces acting on the risers and thus lead to vortex-induced vibration(VIV).Moreover,the vibration of the riser will change its wake structure.Most scholars focus on the research for orthogonal uniform flow in VIV.However,the ocean environments is complicated,under the combination of currents and ocean currents,the actual flow field condition is not orthogonal uniform flow.Based on the immersed boundary method(IBM),a parallel code,named as CgLES_IBM,has been developed to be a FSI solver,combining with an implicit dynamic structural solver,X-code,the direct numerical simulation of the vortex-induced vibration of a flexible riser with various inclined angle flows and linear velocity profiles with different shear rates was carried out.The contents and main conclusions achieved in this paper include:1.The vibration displacement of flexible riser under oblique flow shows obvious standing wave characteristics,and the spectrum of vibration shows single spectrum.The phase difference of the bidirectional vibration of the riser is switched in the direction of the in-line vibration node.Correspondingly,the vibration trajectory of the riser alternately changes between counterclockwise “8” and clockwise “8”.With the increase of the inclination angle,the range of the energy excitation region increases.The three-dimensional characteristics is enhanced by the axial flow,and the wake exists the irregular and intermitten region for local region,but most of the regions are still in 2S mode.The independent principle(IP)is more accurate in the prediction for vibration in crossflow and lift coefficients,but there is a large error in prediction for in-line vibration and wake pattern.2.The in-line vibration of flexible riser subjected to shear flow exhibits a mixed traveling and standing wave pattern.With the increase of the shear rate,the vibration spectrum exhibits multi-frequency response and energy transfer to low frequency.The phase difference with small shear rate appears 180 ° shift in the in-line vibration node,and the largest shear rate computation shows irregular phase difference shift,leading to chaotic trajectory.The energy transfer coefficient shows that the high velocity region of the riser is the energy excitation region,and the vortex splitting may occur in the excitation region and the damping region,both of which are due to the shift of vortex shedding frequency.