| The dynamic behavior of pipes conveying fluid is of considerable importance in many engineering fields,and oscillations have been observed in oil pipelines,various elements of high-performance launch vehicles,missiles,moving wire and belts,nuclear reactor system components,etc.These vibrations and shocks will produce time varying stresses and consequently lead to accumulation of fatigue damage of the riser system and downstrean equipment.The flow pattern of gas liquid two phase flow can change violently and generate rapid changes of the mass distribution,flow rates and pressure fluctuations along the riser which may lead to large vibrations and shocks on the pipeline and the equipment.In addition,the elongate structural characteristics and constraint methods of offshore riser system make it easy to bend vibration.In this paper,theoretical analysis,numerical simulation and experimental research methods are adopted to establish the FSI model and numerical solution methods for the dynamic response calculation of pipe conveying gas-liquid two-phase flow and investigation on its characteristics were studied.The main research contents and conclusionsare as follows:(1)The experimental facility of riser system was constructed.The characteristics of the two-phase flow were measured.7 flow patterns were observed including typical severe slugging,severe slugging ?,trasition flow,stable bubble flow,pulsant bubble flow,foam flow,hydraulic slug flow.The flow pattern map including 7 types of flow patterns was plotted in the experiment of gas-liquid two-phase flow.The detailed characteristics and the pressure pulsation were analyzed in time-frequency domain.(2)2D and 3D CFD model for the full size experimental facility was developed simulate the gas-liquid two phase flow in the riser system.In 2D simulations,the VOF method was adopted to track the gas-liquid interface,and the long subsea pipeline was equivalent as a tank to improve calculating efficiency.Four flow patterns were numerically simulated including typical severe slugging(SSI),severe slugging ?(SSII),transition to severe slugging(SST)and stable flow.The simulation results of the four flow patterns were compared with the experimental data.The transient characteristics of the typical severe slugging were analyzed thoroughly.The results show that the instantaneous parameters of SSI,SSII is periodic,which fluctuate stronger at the top of the reser.(3)The simulation of severe slugging and transition flow were implemented with the riser vibrating in horizontal direction using dynamic mesh technology.The results of simulation show that horizontal vibration of riser has little effect on the characteristics of the two phase flow,while the flow parameters slightly fluctuate along with the vibration of pipeline.Severe slugging was simulated by 3D numerical model.It shows that the velocity of the gas slightly is larger than the liquid during blowout,and the liquid fall mainly along the wall.3D simulation is in huge expense,so it is difficult to be applied actually.(4)The FSI dynamic model for the pipeline conveying two-phase flow was derived using the balance method.The unified model based on slug flow for gas-liquid two-phase flow was introducted to determine the relevant parameters of the two-phase flow for dynamic response calculation.The numerical solution methods were explained detailedly,and mathematical models and numerical methods were verified.In the simulation verification,the calculated results of the presented model were compared with the results by CFX and Ansys;The laboratory experiments were conducted to measured the piping material performance and damping ratio of two-phase flow pipeline;The experiments of the dynamic response of the span section was carried out.The experimental results were compared with the calculation results by presented model to verify the reliability of the mathematical model and numerical methods.(5)The modal analysis of the riser system conveying tow-phase flow and the experiment of gas-liquid tow-phase flow in the riser system were implemented to predict and evaluate the vibration characteristics of the marine risers caused by two phase flow in different flow regimes.The influence of the variation of fluid density distribution and different elastic foundation coefficients on the vibration modals were considered in the modal analysis.The flow pattern map including 7 types of flow patterns was plotted in the experiment of gas-liquid two-phase flow.The time domain and frequency domain analysis of the pressure fluctuation characteristics of different flow patterns were implemented to analyze the possibility of resonance caused by 7 types of flow patterns in different elastic foundation and predict the coupling vibration characteristics of the riser system conveying gas-liquid two-phase flow.Finally,the dynamics response of the riser system was tested to verify the accuracy of the coupling vibration analysis method.The analysis results show that:gas-liquid tow-phase flow and elastic foundation have significant impact on natural frequency and mode of vibration of the riser system.Severe slugging I,severe slugging III,slug flow and disturbed bubble flow can cause resonance when the riser system has no elastic foundation.When the elasticity coefficient of the foundation is large enough,two-phase flow can't cause resonance of the riser system.The vibration response of the riser system is consistent with the fluctuation of the flow characteristics of the gas-liquid two-phase flow in the riser.(6)A fluid-structure interaction model for a marine riser system on elastic foundation conveying severe slugging flow was developed based on the modified severe slugging transient model and theories of plane frame structure,and solved to simulate the dynamic response of the marine riser system caused by severe slugging flow using numerical methods.Eulerian method was used to solve the equations of serve slug flow.Galerkin's method was adopted to discretize the dynamic equations in space and Newmark-? method was employed for time-domain integration of the discretized equations.Variable time-steps were employed for higher computational efficiency and accuracy in the integration process.The comparisons of simulation results with the experimental data show that the mathematical model and numerical methods are reasonable.Detailed analysis of dynamic response,internal force and reaction force of the hybrid riser reveals that the dynamic response of hybrid riser is closely related to the periodic characteristics of serve slug flow,and the elastic foundation can suppress the vibration amplitude and the internal force,especially bending stress of the downward pipeline significantly.The amplitude of high frequency vibration is large and the axial stress and bending stress oscillate acutely.These analyses are significant to guide the design of marine risers.(7)The two-way coupling simulation of vortex-induced vibration was implemented using co-simulation of CFX and Ansys.The results show that vortex-induced force characteristics of the vibrating cylinder are very different from the still cylinder.Therefore,the fluid-structure coupling effect should be considered in numerical simulations of VIV.Finally,the VIV simulations,under different slug length,transition velocity and cycle periods,were implemented by programming.The results show that the vibration frequency is locked when the frequency of vortex-induced force is close to natural frequencies of the pipe.The mass of internal flows decrease the vibration amplitude.The vibration amplitude increase when the slug velocity is high.The low frequency components appear with the slug velocity increasing. |
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