Study On Leakage And Dispersion Mechanisms Of Underwater Gas And Oil Pipelines

Submarine gas-oil pipelines play an important part in offshore gas and oil field development and production system.During the production,fatigue failure,the third-party destruction,internal corrosion may easily trigger the gas and oil leakage thereby generating environmental pollution and injury and loss of life.In terms of gas and oil leakage and diffusion in undersea multiphase pipeline,the development and migration law of gas-oil plume and the interactive mechanisms with water surface are investigated in this study.Theoretical and experimental analysis combined with numerical simulated methods are adopted to study the present problem.Moreover,the dynamic dispersion process of gas-oil plume in large scale shallow sea environment is further explored.(1)Gas-oil two phase leak test loop and underwater leakage experimental system are designed and constructed.The characteristics research of oi,gas leakage and oil-gas two phase leakage under stratified and slug flow and plume development process are investigated respectively.The results indicate that the two phase leakage and dispersion characteristics under stratified flow and slug flow are significantly different from those of single phase leakage.Twophase plume will possess longer rising time,further horizontal migration distance and lower fountain height at water surface.As to horizontal leakage under stratified flow,horizontal migration distance and rising time of gas-oil plume present increasing trend with increasement of gas superficial velocity.The horizontal migration distance and plume rising time decrease with the increase of the gas phase velocity,while the maximum fountain height increases with the increase of the gas phase superficial velocity.(2)Eulerian-Eulerian method and Eulerian-Lagrange method are adopted to describe the migration and diffusion characteristics of oil leakage in water based on the different mode of subsea oil leakage.Oil migration process under different leakage mode in large scale is investigated,and the influence of ocean environment,leakage rate,oil physical property,leakage position and other facters on the diffusion behaviors of underwater oil are analyzed.Additionally,dimensionless analysis of the rise time and horizontal migration distance of spilled oil under jet mode are conducted,and the mathematic relationships between them and influencing factors are established.(3)Based on volume of fluid method coupled with discrete phase model,combining with ideal gas equation and bubble size distribution equation,the prediction model of underwater gas leakage and dispersion is established and this prediction model is verified by laboratory small-scale experiment of gas leakage.The k-ε、SAS、LES turbulent methods are compared to capture the turbulent characteristics of gas plume under bubble jet flow,and the results indicate that the prediction of plume behaviour by multiple size group model combined with LES turbulent method is closest to actural situation and the behavior of necking and bulging can be captured in this way.(4)The gas leakage dispersion process in shallow sea is predicted based on EulerianLagrange method.The results indicate the gas pool that forms at water surface when plume reaches,presents the shape of ellipse under the combined effects of ocean current and wave.The diameter of gas pool keeps increasing with time and then maintains stable.Plume shows a longer rising time and a longer migration distance under condition of current combined with wave.The degree of influence of various influencing factors on rising time is ranked from large to small: water depth,leakage rate,ocean current velocity;the degree of influence of various influencing factors on horizontal migration distance is ranked from large to small: water depth,ocean current velocity,leakage rate;the degree of influence of various influencing factors on gas pool radius is ranked from large to small: water depth,leakage rate,ocean current velocity.(5)Distribution prediction model of two-phase flow getting through the small hole under stratified flow is established,according to the pressure drop calculation formula of two-phase getting through pore plate,the reliability of the model is verified by laboratory distribution experiment.The transient prediction model based on coupled VOF and DPM model is established to study the developmemt process of gas-oil plume under stratified flow,which is verified by laboratory small-scale gas-oil leakage experiment.The results indicate that the prediction errors of plume rise time and horizontal migration distance are-4.97% and-3.84%,respectively.(6)The prediction model of gas-oil leakage and dispersion under slug flow is established based on coupled VOF-DPM model.The average calculation error between the predicted plume rise time and the experimental value is-5.02%,and the average calculation error between the predicted horizontal migration distance and the experimental value is-8.96%.The numerical study of gas-oil leakage dispersion under slug flow in large-scale finds that the gas-oil plume is not separated due to the entrainment effect of the bubble plume in still water.The plume will be curved to the downstream under the effect of wave oscillation and shear effect of current while the rise time is longer and the gas-oil separation will occur in the upper part of the plume.When the leakage inclines to 45° obliquely upward,the plume rise time will be shorter than the case of vertically upward and horizontal leakage.The plume rise time and horizontal migration distance present a decreasing trend with the increase of diameter of leakage hole.