Numerical Simulation Of The Natural Venting Process Of Large-drop Oil Pipeline Shutdown

It is inevitable for the crude oil and product oil pipepline with steep drop sections to encounter a planned or accidential shutdown construction opearation during the normal operations.In order to reduce the risk of the construction by decreasing the static pressure or meet the requirements of construction,the oil in the steep drop sections should be discharged or partially discharged.When using natural emptying method to discharge the oil,it is important to calculate flow parameters and parameters assosicated with operating under the different operation conditions,such as emptying time,pressure along with pipeline,volume flow changing with time,which povides guidance for making a proper plan.Due to the difference of terrain condition,pipeline operating conditions and oil properties,it may appear two types of flow state in the big drop section during the emptying process,namly full pipe flow and slack line flow.Since the difference of the nature model between the two types of flow state,what kind of flow state will appear under different operating conditions should be firstly determined,and then the built model of each type can be used to calculatet flow parameters.Thus,this paper focus on calculating the parameters accosiated with making a proper pipeline emptying plan,the main contents and conclusions of this peper are as following:(1)Since the properties of oil have important impact on its flow state,the calculation formula of oil properties of both the black-oil model and the component model were summarized based on the oil phase behavior theory.Subsequently,after analyzing the existing methods of the characteriazation of the oil pesudo-components,three parameters gamma distribution was selcected to spilt the plus fraction into single carbon number via using the optimization method to determind distribution parameters,which was proved to be effctive compared with the experimental data.Considering the phenomenon of gas escape as the pressure reducing below the saturated vapor pressure during the emptying process,oil component model calculation program was writed by combining the vapor-liqiud equilibrium model and the above thoeries,which provide the mass transfer model for the flow simulation.(2)Because of the difference of the nature model between the full pipe flow and slack line flow,two types of flow state were quanlitatively analyzed and according to the basic principles of fluid meachanics,a quantitative model was established to determind the potential flow state under the different emptying condtion of big drop section.(3)For the simulation of full pipe flow state,based on the liqiud transient flow theory,a numerical model was established by using the coordinate transformation method,the method of characteristic and finite difference method.The validity of this model was verified by an example;For the simulation of salck line flow,considering stratified flow pattern is the main flow pattern in the downward inclined pipeline,a gas-liqiud two phase transient flow model was established according to the simplified two-fluid model,which was suitable for the flow condition of big drop pipeline emptying process.The finite difference method was adopted to discretize this model and the vadility of this mode was verified by an example.(4)Based on the above theories,a software was developed by using the C#progarming language,and take the emptying process of the big drop section in the ku-shan line for the example,numerical simulation was carried out to calculate the relevant parameters of oil flow state under the different operating conditions and the emptying time.The simulation results show that,with the downstream outlet discharge area increasing during the venting process,the oil inside the pipeline is more inclined to flow as the state of slack line flow.Meanwhile,the larger the discharge outlet area is,the more prominent the slack line flow state in pipeline is.What's more,when the downstream drainage area reaches a certain level,the method of increasing the discharge area of the vent to shorten the time is not effective any more.And with the slack line flow in the pipeline becoming more prominent,flow parameters of the oil fluctuate more strongly,which is easy to cause pipe vibration and is bad for the security of operation.Thus,downstream discharging outlet area is better not to be set too large so as to reduce risk.