Numerical Sutdy On Vortex-induced Vibration Of Marine Riser

As the main channel connecting offshore platforms and subsea equipment,marine riser is an indispensable component in the offshore oil and gas resourcedevelopment. When the ocean current flow past the riser with a certain velocity, thevortex will generate, and the alternate vortex shedding will cause the vortex-inducedvibration (VIV) of riser. When the current velocity is in a certain range, thefrequency of vortex shedding will be close to the natural frequency of marine riser,which will cause the lock-in phenomenon. At this point, the vortex sheddingfrequency will remain in the vicinity of the natural frequency of riser, causing theresonance and aggravating the fatigue damage of the riser.The VIV characteristic of the marine riser is not only influenced by the size andmaterial parameter of the riser, but also has a relationship with the flow velocity.And generally, the reduced velocity is used to describe the relationship between thematerial parameters of the riser and the flow velocity. Research of the variation ofthe key parameters such as lift and drag coefficient, vortex shedding frequency andcentral displacement in a series of reduced velocities will find out the range ofreduced velocity in which the lock-in phenomenon will occur. Meanwhile, theresearch on VIV suppression device can improve the service life of the riser, whichhas a high value in engineering application.In this paper, the vortex shedding characteristic of the rigid riser, the VIVcharacteristic of the flexible riser and the vibration suppression effect of fairingwere researched by using the computational fluid dynamics software ANSYS CFX.And the data were collected and contracted. The concrete research contents are asfollows:1. The influence on the simulation results of different turbulence models werestudied when the Reynolds number is200. Compared with the results in references,the results showed that the LES turbulence model, which has relatively highcomputer accuracy and fast convergence speed, is suitable for numerical simulationof the VIV of riser.2. The vortex shedding characteristic of rigid riser was studied in a series ofReynolds numbers. With the analysis of data, the variation of flow field, lift anddrag coefficient and the vortex shedding frequency were obtained.3. Considering fluid-structure interaction (FSI), the VIV characteristic offlexible riser was studied. The results showed that the lock-in phenomenon willoccur when the reduced velocity is in the range from4to8.4. The suppression effect of fairing was studied. The results showed that fairing can reduce the drag force and the frequency of lift force effectively. Meanwhile,fairing can change the vortex shedding mode.