Investigation On Gas-liquid Flow And Flow-induced Vibration In Submarine Multiphase Pipelines

The flow safety of submarine mixed pipeline has gradually become a key topic in the field of oil and gas development.The serious slug flow and vibration fatigue loss of marine riser bring great challenges to safe flow.In order to improve the safety design and safe operation level of marine mixed riser system,this paper studies the vibration caused by gas-liquid two-phase flow and pipeline flow in offshore riser pipeline.According to the literature investigation,this paper first builds the large multiphase flow combined riser experimental loop,considering the influence of the underground pipeline fluctuation,studies the gas-liquid flow pattern in the ups and downs-down-rise riser pipeline system under different gas-liquid inlet conditions.According to the observation of the gas-liquid flow characteristics and the pressure fluctuation at the bottom of the riser,four flow patterns are divided:severe slugging flow,dual-peak slug flow,oscillating flow and stable flow.A special flow pattern is found:dual-peak slug flow.By comparing with the experimental data,it is found that this criterion can accurately predict the occurrence range of severe slugging flow in the combined riser system with undulation.By means of experiment and theoretical analysis,the influence of the existence of the undulating tube on the gas-liquid two-phase flow is studied.It is found that a certain liquid slug will accumulate in the undulating tube to block the gas transport downstream.Severe slugging flow can still occur at relatively high gas-liquid flow rate.Based on the experimental observation and liquid holding rate measurement data,this paper divides the vertical flow pattern into severe slugging flow（SSⅠ、SSⅡ、SSⅢ）,unstable slug flow,bubble flow,churn flow and annular flow,and modifies the local flow boundary of the tube.The main frequency f₀ of convection force fluctuation is calibrated and the f₀calculation formula is derived.The statistical analysis of different flow f₀ shows that the area of occurrence caused by severe slugging flow is small and the vibration frequency range is small.Therefore,pipeline resonance is less dangerous.The results show that with the increase of mixing velocity,the F^rms value of different flow patterns varies,the F^rms value in churn flow decreases linearly,while the other flow patterns increase gradually.In addition,with the increase ofβ,the F^rms basically presents the law of first increasing and then decreasing.Of all flow patterns,the maximum F^rms value appears at the slug flow/churn flow boundary,but for high gas content,at the same gas content,the maximum F^rms value appears at the SSⅡ/unstable slug flow boundary.In engineering practice,attention should be paid to the SSⅡflow pattern.By combining theoretical analysis with experimental data,a semi-theoretical semi-empirical model for calculating flow force is obtained.The model can accurately predict the RMS value of flow force and the maximum flow force value that may appear in the flow process.The vibration experiment of riser pipe system in this paper analyzes the frequency domain signal of vibration.The results show that when the gas-liquid flow rate is small or the gas-liquid flow rate is different,the flow is relatively stable,or the periodicity is strong.The power spectral density（PSD）of vibration signal is analyzed.It is found that the change of vibration energy is consistent with the change of F^rmsvalue,but the resonance phenomenon occurs in the pipeline under the working condition point,which greatly improves the vibration amplitude of the pipeline and produces large vibration energy.On the basis of two-dimensional beam vibration model,the pipeline vibration model is established,which can be used to calculate the natural frequency of the pipeline system.The third order natural frequency of the pipeline system is7.8 Hz,19.7 Hz,29.2 Hz.The third order inverse resonance frequency is 5.9 Hz,16.1 Hz,and28.1 Hz,by using the mathematical model and frequency response function method in this paper.Based on the experimental data,the change of pipeline internal flow is studied.The experimental results show that when the liquid quantity is small and the pipeline is vibrated,the gas content of gas-liquid two-phase flow increases,but decreases with the increase of liquid volume.In different flow patterns,the variation characteristics of gas content are different,among which the change of bubble flow and severe slugging flow is the most obvious,and the change of gas content more than 5%is basically under the severe slugging flow pattern.A combined riser system is established on the ANSYS Workbench platform to simulate the pipeline displacement caused by internal flow and outflow respectively.Based on the calculation results,the mechanism of internal and external flow vibration is discussed.The results show that the vibration frequency of pipeline is lower than that of internal flow vibration.The mechanism of pipeline vibration caused by internal flow and outflow is different,and the vibration caused by outflow is mainly caused by the generation and shedding of spoiler vortex.