Development Of Computer Code For Vortex-Induced Vibration Of Long Flexible Riser And Its Applications

With the development of offshore oil and gas exploration to the deep sea,the vortex induced vibration has become a key problem to be solved in the design of deep-sea platform risers.After a century of research on vortex induced vibration,it is still an enduring problem,because it is a very complicated nonlinear problem of fluid-structure interaction,and there are many questions about the intrinsic relationship between the phenomenon and mechanism of VIV.Vortex-induced vibration will occur in both in-line and cross-flow directions for offshore risers,and the in-line vibration of the riser will affect the cross-flow vibration,which makes the vortex induced vibration more complicated.Within the country,time-domain numerical prediction of the vortex induced vibrations of slender flexible risers at high Reynolds numbers is not mature,the prediction and analysis of in-line vibration and numerical simulation of VIV of risers with large aspect ratio being especially weak.Therefore in the design of the riser structure,a safety factor of nearly 10 had to bu used to ensure safety.Thus it is more and more important to accurately predict the vortex-induced vibration response of slender flexible riser.Using open source library OpenFOAM as the development platform,based on solving the viscous flow governing equation Navier Stokes equation and equation of structure dynamics,this paper develops a solver viv-FOAM-SJTU for the prediction of the time-domain VIV response.The solver has good versatility,which allows simulation of flow around a stationary cylinder,forced and free vibration of cylinder and VIV of long flexible riser.Prediction of self-excitation response is obtained with the result of the forced motion.The development of solver is based on the idea of the object-oriented programming in C++language.Almost all the governing equations in OpenFOAM can be used for the solover using the unified module interface.In order to simulate the flow-structure interation of big flexible riser,the moving grid technology that comes with OpenFOAM,that is,using Laplace equation to solve the grid displacement,has been unable to meet the need of the large displacement of the riser strip.In this paper,we study and develop the radial basis function（RBF）dynamic grid technique based on the dynamic mesh module of OpenFOAM.In order to validate the developed solver,benchmark cases of rigid cylinders and long flexible risers VIV are performed.1.Large eddy simulation（LES）method is used along with Smagorinsky sub grid model to analyze the flow around a three-dimensional stationary cylinder at high Reynolds numbers,and the influence of different aspect ratios is studied.The instantaneous characteristics of the velocity field and the vortex of the circular cylinder are analyzed.The random fluctuation and the three dimensional characteristics of turbulent flow are demonstrated.On the basis of this,the influence of aspect ratio of the cylinder on the simulation results is studied.Through the analysis of the mean flow field at the characteristic position,it is revealed that the two dimensional simplified model cannot capture the return flow zone and its results is very different from the experimental results.The appropriate aspect ratio suggestions for simultions of flow around a three-dimensional cylinder is given.2.Based on viv-FOAM-SJTU the standard problem of the vortex induced vibration of rigid cylinder is verified.The vortex induced vibration of an elastically supported cylinder with one and two degrees of freedom and low mass ratio(m^（9）（28）2.4)was calculated by using the Reynolds averaged method（RANS）and the SST-kw turbulence model.Numerical simulations reproduce the response branches and the corresponding vortex modes in the experiments,and further gives the lock-in range of vortex induced vibration.Through the contrast between the VIV results of one and two degrees of freedom,it is deemed that the influence of the in-line degree of freedom on the VIV is not only the production of in-line vibration.More important is the coupling effect of in-line degree of freedom on the cross-flow degree of freedom,which leads to the fact that the two degrees of freedom can stimulate higher response than one degree of freedom.3.Based on viv-FOAM-SJTU the standard problem of the vortex induced vibration of long flexible riser is verified.The time domain response of the vortex induced vibration of the top tension riser was studied,and compared to the standard test of F.J.Huera Huarte（2006）.Respectively in-line and cross-flow vibrations of the structure are analyzed in detail.The riser’s dominant vibration modes,envelopes,mean in-line displacements obtained in the numerical simulation agree very well with test results.The results also show multi modal characteristics of the VIV of the riser.From the angle of natural frequency,vibration frequency and vortex shedding frequency,the occurrence mechanism of the dominant modes of vortex induced vibration is analyzed.Based on the study of the flow field（hydrodynamic coefficients and the vortex fields）,the intrinsic relation between the flow field and induced vibration response of the riser is analyzed.After the validation of benchmark cases,the applications of the developed solver are performed.1.First,the mechanism of the inherent relation between the forced vibration and the self-excited vibration is further studied.The phase angle between the displacement and the lift force is solved by the complex demodulation method.When the cylinder is forced to move with certain frequencies and amplitudes,with the changes of the phase angle the wake mode will switch back and forth between 2P and2P_O,which explains the switching phenomenon of the self-excited motion in the upper and lower branches.Based on the energy balance of the self-excitation motion,the method of predicting the self-excitation response with the result of the forced motion is proposed and verified.2.Through the external definition of the dictionary file,the control parameters of the riser and the flow field can be modified,based on which the applied research of parameters analysis of VIV is carried out.The effects of top pretension,inlet flow velocity and mass ratio on vortex induced vibration were investigated,and the response modes and trend analysis under different working conditions were presented.The change of the top tension and mass ratio will affect the natural frequency of the riser,thus changing the vibration mode of the riser.With the increase of the top pretension,the vibration mode of the riser is reduced.The increase of the mass ratio will increase the vibration mode of the riser.The flow velocity also has an effect on the vibration mode,because the change of flow rate leads to the change of vortex frequency,which influences the vibration frequency of the riser.The higher the flow velocity,the higher the vibration mode of the riser.3.Interference effects between tandem vertical risers is also one of the research contents.The influence of the spacing between the risers on the interference is discussed.The numerical results is shown that the distance between the vertical risers has little effect on the vibration mode of the upstream riser,but it will affect the mode of the downstream riser.Under the same conditions with the benchmark case,the dominant mode of the upstream riser is 4th order,consistent with the vibration mode of a single riser.The dominant mode of the downstream riser is 3rd order,slightly lower than the upstream riser.With the increase of riser spacing,the weight of the 3rd mode of the downstream riser is gradually reduced,and the weight of the 4th mode is gradually increased.Thus as the spacing increases,vibration mode of the downstream riser moves toward the mode of a single vertical riser.4.The vortex-induced vibration responses of the vertical riser with aspect ratio of500,750 and 1000 in the uniform flow and shear flow are investigated.The increase of the aspect ratio will increase the equilibrium position of the in-line direction and the vibration mode of the riser.When the aspect ratio is 500,the cross-flow vibration is shown as a standing wave with a 3rd order single mode.As the aspect ratio increases,the cross-flow vibration gradurally turns into multi mode.When the aspect ratio is 1000,the vibration become more complex.There are two dominant mode and several vibration mode.The cross section of flow has an influence on every mode amplitude.At some conditions,the shear flow can excite modes that are slightly higher than that of the uniform flow.In this paper,the solver for the prediction of the vortex-induced vibration of a rigid cylinder and a flexible riser is developed.By the numerical simulations of the vortex induced vibration under various complex conditions,the reliability of the solution is proved effectively.Through the analysis of vortex induced vibration,the understanding of its internal mechanism is deepened,which provides a valuable reference for further improvement of the prediction model of vortex induced vibration.