Freeze soil area product oil pipeline commissioning simulation method based on discrete cavitation model

Abstract

The invention discloses a permafrost region product oil pipeline commissioning simulation method based on a discrete cavitation model. The permafrost region product oil pipeline commissioning simulation method comprises the steps that an equation set of a water head H is obtained based on deformation of a continuity equation and a momentum equation of pipeline transient flow simulation; on the basis of the continuity equation and the momentum equation, a pipeline along-line temperature drop calculation formula is introduced in combination with temperature conditions; dividing computational grids according to a finite volume method, solving a water head H equation set by adopting a characteristic line method to obtain a compatibility relational expression, and obtaining uniform time step length and space step length; the gas phase volume is described based on pressure boundary conditions and a compatibility relational expression, multiphase transient flow is calculated by combining a discretization equation, a single-temperature hypothesis and a single-speed hypothesis, a pressure time-varying curve along a grid is output, and a temperature time-varying curve is output based on a temperature drop calculation expression. According to the method, simulation calculation is carried out on typical working conditions, and production scheme simulation and risk assessment in an accident state of the oil pipeline in the permafrost region with very complex conditions are realized.

Description

Technical Field

[0001] The invention relates to the technical field of oil pipeline production simulation calculation, and in particular to a method for simulating the production of finished oil pipelines in frozen soil areas based on a discrete cavitation model. Background Art

[0002] my country's permafrost areas are widely distributed. As buried pipelines have the advantages of relatively low cost, large transportation volume, small land occupation and short construction period, laying buried oil pipelines in permafrost areas is the main way to transport oil and gas resources over long distances at present and in the future, and will inevitably pass through these areas. However, there is no mature experience and technology for reference in China for laying, commissioning and operating oil pipelines in permafrost areas, especially in permafrost areas with high average altitude, widespread permafrost area, thick permafrost layer, extremely harsh natural conditions and extremely complex geological conditions, such as the Qinghai-Tibet Plateau.

[0003] The commissioning process of a refined oil pipeline is a complex multiphase flow process, and a scientific and systematic assessment of the hydrothermal risks involved is required before commissioning.

[0004] Therefore, the prior art requires a method for simulating the commissioning of a finished oil pipeline in a frozen soil region. Summary of the invention

[0005] In view of this, the purpose of an embodiment of the present invention is to propose a method for simulating the commissioning of a refined oil pipeline in a frozen soil area based on a discrete cavitation model. A method for simulating the commissioning of a refined oil pipeline is proposed based on a discrete cavitation model (DGCM) and a finite volume solution method of a Godunov format, and simulation calculations are carried out on typical working conditions, thereby realizing the commissioning scheme simulation and risk assessment of the oil pipeline in the frozen soil area with very complex conditions under accident conditions, thereby ensuring the safety and integrity of the pipeline system design, commissioning, operation and maintenance.

[0006] Based on the above purpose, an embodiment of the present invention provides a simulation method for the commissioning of a finished oil pipeline in a frozen soil area based on a discrete cavitation model, comprising the following steps:

[0007] The continuity equation and momentum equation based on pipeline transient flow simulation are transformed to obtain the equation group of water head H;

[0008] On the basis of continuity equation and momentum equation, the temperature drop calculation formula along the pipeline is introduced in combination with temperature conditions;

[0009] The computational grid is divided according to the finite volume method, and the characteristic line method is used to solve the head H equation group to obtain the compatible relationship, and obtain the uniform time step and space step;

[0010] The gas phase volume is described based on the pressure boundary conditions and the compatibility relationship. The multiphase transient flow is calculated by combining the discretized equation, the single temperature assumption, and the single velocity assumption. The pressure variation curve along the grid is output, and the temperature variation curve is output based on the temperature drop calculation formula.

[0011] In some embodiments, the equation group for the water head H obtained by deforming the continuity equation and momentum equation based on the pipeline transient flow simulation includes:

[0012] According to the characteristics and assumptions of gas-liquid two-phase flow, the control equation is established to describe the behavior of gas-liquid two-phase flow. The control equation includes the continuity equation and the momentum equation. Based on the deformation of the continuity equation and the momentum equation, the equation group of the water head H is obtained:

[0013]

[0014]

[0015] Among them, x and t are space and time coordinates respectively, u is the average flow velocity in the pipe cross section, D is the inner diameter of the pipe, f is the friction coefficient, and a is the wave speed.

[0016] In some embodiments, the continuity equation and momentum equation are:

[0017]

[0018]

[0019]

[0020] Among them, x and t are space and time coordinates respectively, p is pressure, ρ is density, u is the average flow velocity in the pipe cross section, D is the inner diameter of the pipe, f is the friction coefficient, and a is the wave speed.

[0021] In some embodiments, the method further includes: obtaining a volume fraction calculation formula based on the volume of the gas phase and the liquid phase to quantitatively describe the two-phase flow in the pipeline, and the volume fraction calculation formula is:

[0022]

[0023] Among them, V g is the gas phase volume, V l is the volume of liquid phase, V m is the volume of the mixture.

[0024] In some embodiments, the temperature drop calculation formula along the pipeline introduced in combination with the temperature conditions includes:

[0025] The temperature drop calculation formula along the pipeline is introduced by combining the starting point temperature of the pipeline and the soil temperature around the pipeline. The temperature drop calculation formula along the pipeline is:

[0026] T L =T0+(T R -T0)e -aL

[0027]

[0028] Where K is the total heat transfer coefficient, W / (m 2 ℃); T R is the starting temperature of the pipeline, ℃; T0 is the soil temperature around the pipeline, ℃; T L is the temperature at a distance L from the starting point, °C; D w is the outer diameter of the pipe, m; G is the mass flow rate of the oil-water mixture, kg / s; c is the specific heat of the oil, J / (kg℃).

[0029] In some embodiments, the compatible relationship obtained by solving the head H equations using the characteristic line method includes:

[0030] For each grid or unit, the characteristic line method is used to ignore the nonlinear terms in the head H equation group and solve them. The slope and characteristic velocity of the characteristic line are calculated, and the integration is performed along the characteristic line to obtain the compatibility relationship on the characteristic line:

[0031] H P =C pc -B pc Q Pu +R pc Q Pu |Q A |

[0032] H P =C mc +B mc Q P +R mc Q P |Q B |

[0033] Among them, C pc , B pc , R pc is a constant on the positive characteristic line, C mc , B mc , R mc is a constant on the negative characteristic line, Q pu is the upstream flow, Q p is the downstream flow, Q A , Q B is the flow on the positive and negative characteristic lines, and then, the following calculation format can be obtained:

[0034]

[0035]

[0036] Among them, Δt is the time step and Δx is the space step.

[0037] In some embodiments, the pressure boundary condition is: when the pressure of any node on the computational grid in the pipeline is lower than the saturation pressure, the pressure is taken as the saturation pressure.

[0038] In some embodiments, describing the gas phase volume based on the pressure boundary condition and the compatibility relation includes:

[0039] The gas phase volume is described based on the following formula:

[0040]

[0041]

[0042] in, is the weighting factor, V' is the gas phase volume at the current time step, V t-2Δt is the gas phase volume in the first two time steps, Q pu is the upstream flow, Q p is the downstream flow.

[0043] In some embodiments, outputting a pressure versus time curve along a grid includes:

[0044] The pressure variation curve along the line with time is obtained by the pressure drop iteration formula. The pressure drop iteration formula is:

[0045] P (i+1) =P (i) +(150×(1-ε) 2 μ g ×ΔxV g ) / (ε 3 ×dp 2 ρg)+(1.75×(1-ε)×u l ×Δx) / (dp×ε 3 ×ρl)

[0046] Where P(i) represents the pressure in the i-th grid unit, P(i+1) represents the pressure value of the i-th grid unit in the next time step; ε is the volume proportion of the solid phase or liquid phase in the multiphase flow; μ g is the dynamic viscosity of the gas; μ l is the dynamic viscosity of the liquid; Δx is the length of the grid unit; V g is the flow rate of the gas; V l is the flow rate of the liquid; dp is the diameter of the particle or droplet; ρ g is the density of the gas; ρ l is the density of the liquid.

[0047] In some embodiments, outputting a temperature variation curve over time based on a temperature drop calculation formula includes:

[0048] Based on the temperature drop calculation formula combined with the discretization equation, the temperature iteration formula is obtained to output the temperature change over time. The temperature iteration formula is:

[0049] T (i+1,j) =T0+(T (i,j) -T0)e -al

[0050] Among them, T0 is the initial temperature, l is the length of the pipeline, i is the horizontal coordinate of the grid unit, and j is the vertical coordinate of the grid unit.

[0051] The present invention has at least the following beneficial technical effects:

[0052] (1) The method of the present invention proposes a physical process splitting solution method for the control equation based on the discrete cavitation model (DGCM) and the Godunov format;

[0053] (2) The oil pipeline in the permafrost region with very complex conditions was simulated for commissioning and risk assessment under accident conditions;

[0054] (3) Compared with traditional numerical calculation methods, this method has clear physical meaning and better robustness. It has certain guiding significance for the safety and integrity of pipeline system design, commissioning, operation and maintenance, and can be extended to other pipeline commissioning problems. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For ordinary technicians in this field, other embodiments can be obtained based on these drawings without paying creative work.

[0056] Figure 1 A schematic diagram of an embodiment of a method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model provided by the present invention;

[0057] Figure 2 A schematic diagram of the characteristic line method calculation provided by the present invention;

[0058] Figure 3 This is a schematic diagram of the temperature field calculation of the buried pipeline provided by the present invention. DETAILED DESCRIPTION

[0059] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0060] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are for distinguishing two non-identical entities with the same name or non-identical parameters. It can be seen that "first" and "second" are only for the convenience of expression and should not be understood as limitations on the embodiments of the present invention. The subsequent embodiments will not explain this one by one.

[0061] The present invention proposes a method for simulating the commissioning of a refined oil pipeline based on a discrete cavitation model (DGCM) and a finite volume solution method of a Godunov format, and performs simulation calculations on typical working conditions. The method of the present invention comprises the following steps:

[0062] S1. The continuity equation and momentum equation based on pipeline transient flow simulation are transformed to obtain the equation group of water head H;

[0063] S2. Based on the continuity equation and momentum equation, the temperature drop calculation formula along the pipeline is introduced in combination with the temperature conditions;

[0064] S3. Divide the computational grid according to the finite volume method, use the characteristic line method to solve the water head H equation group to obtain the compatible relationship, and obtain a uniform time step and space step;

[0065] S4. Describe the gas phase volume based on pressure boundary conditions and compatibility relations, calculate multiphase transient flow by combining discretized equations, single temperature assumption, and single velocity assumption, output the pressure variation curve along the grid over time, and output the temperature variation curve over time based on the temperature drop calculation formula.

[0066] Furthermore, in S1, the equations for the water head H obtained by deforming the continuity equation and momentum equation based on the simulation of transient flow in the pipeline include:

[0067] According to the characteristics and assumptions of gas-liquid two-phase flow, the control equation is established to describe the behavior of gas-liquid two-phase flow. The control equation includes the continuity equation and the momentum equation, which are specifically expressed as:

[0068]

[0069]

[0070]

[0071] Among them, x and t are space and time coordinates respectively, p is pressure, ρ is density, u is the average flow velocity in the pipe cross section, D is the inner diameter of the pipe, f is the friction coefficient, a is the wave speed, K is the bulk elastic modulus of the liquid, e is the pipe wall thickness, and E is the Young's modulus of the pipe material.

[0072] Based on the deformation of the continuity equation and momentum equation, the equation group of water head H is obtained:

[0073]

[0074] In some embodiments, in order to quantitatively describe the two-phase flow in the GL pipe, the following definition of volume fraction is introduced:

[0075]

[0076] Among them, V g is the gas phase volume, V l is the volume of liquid phase, V m is the volume of the mixture.

[0077] You can use α g To measure the severity of liquid column separation. g When it is close to 1, it indicates that a more serious local liquid column separation has occurred. g When it is relatively small and close to 0, it indicates that the flow is in a diffuse flow or bubbly flow state. In the subsequent simulation analysis of liquid column pulling and liquid column separation in GL pipelines, the change of volume fraction can also be focused on.

[0078] Furthermore, in S2, in order to simulate the temperature change along the pipeline, the temperature drop along the pipeline is introduced on the basis of formulas (1) and (3). The temperature drop along the pipeline is calculated as follows:

[0079] T L =T0+(T R -T0)e -aL

[0080]

[0081] Where K is the total heat transfer coefficient, W / (m 2 ℃); T R is the starting temperature of the pipeline, ℃; T0 is the soil temperature around the pipeline, ℃; T L is the temperature at a distance L from the starting point, °C; D w is the outer diameter of the pipe, m; G is the mass flow rate of the oil-water mixture, kg / s; c is the specific heat of the oil, J / (kg℃).

[0082] Furthermore, in S3, the characteristic method is used for solving. This method is widely used in single-phase flow calculation. In order to simplify the calculation and obtain uniform time step and space step, this method generally ignores nonlinear terms. u sinθ. Then, we can get the compatibility relation on the characteristic line:

[0083]

[0084] Among them, C pc , B pc , R pc is a constant on the positive characteristic line, C mc , B mc , R mc is a constant on the negative characteristic line, Q pu is the upstream flow, Q p is the downstream flow, Q A , Q B is the flow on the positive and negative characteristic lines, and then, the following calculation format can be obtained:

[0085]

[0086] Among them, Δt is the time step, Δx is the space step, such as Figure 2 Shown is a schematic diagram of the characteristic line method.

[0087] Furthermore, in S4, in order to calculate transient multiphase flow (such as liquid column separation phenomenon, large drop liquid column emptying phenomenon), it is necessary to introduce strong assumptions as boundary conditions when solving equation group (3). The traditional MOC method needs to be used in conjunction with the discrete (centralized) vapor cavity model (DVCM) to consider the liquid column separation phenomenon. When the pressure at a certain point in the pipeline (node ​​on the computational grid) is lower than the saturation pressure, the pressure is set to the saturation pressure, that is:

[0088] p t =p v (7)

[0089] Among them, p v is the saturated vapor pressure.

[0090] The upstream and downstream flow rates are calculated using the compatibility relation (5), and the volume of the gas phase is described by the following formula:

[0091]

[0092]

[0093] in, is the weighting factor, V' is the gas phase volume at the current time step, V t-2Δt is the gas phase volume in the first two time steps, Q pu is the upstream flow, Q p is the downstream flow.

[0094] According to the discretized equation, the formula for iterative update of pressure drop can be obtained:

[0095] P (i+1) =P(i) +(150×(1-ε) 2 μ g ×ΔxV g ) / (ε 3 ×dp 2 ρg)+(1.75×(1-ε)×u l ×Δx) / (dp×ε 3 ×ρl)

[0096] Where P(i) represents the pressure in the i-th grid cell. P(i+1) represents the pressure value of the i-th grid cell in the next time step; ε is the volume fraction of the solid phase or liquid phase in the multiphase flow; μ g is the dynamic viscosity of the gas; μ l is the dynamic viscosity of the liquid; Δx is the length of the grid unit; V g is the flow rate of the gas; V l is the flow rate of the liquid; dp is the diameter of the particle or droplet; ρ g is the density of the gas; ρ l is the density of the liquid.

[0097] We can further get the iterative update format of the temperature:

[0098] T (i+1,j) =T0+(T (i,j) -T0)e -al

[0099] Among them, T0 is the initial temperature, l is the length of the pipeline, i is the horizontal coordinate of the grid unit, and j is the vertical coordinate of the grid unit.

[0100] The method of the present invention is further explained below in conjunction with specific embodiments.

[0101] The basic data of the pipeline in operation are: total length 1114.7km, pipe diameter D323.9mm, designed flow rate 130×10 4 t / a, design pressure 10~13MPa, average buried depth of pipeline center 1.6m, total heat transfer coefficient 0.43W / (m 2 ·K).

[0102] Three initial commissioning plans are proposed for the pipeline commissioning process, including:

[0103] (1) Direct water injection along the entire line;

[0104] (2) Direct oil injection across the entire line;

[0105] (3) Water injection at both the first and last stations / exhaust along the line.

[0106] Through comprehensive risk analysis, the focus is on the first commissioning plan (3). For the above commissioning plans, hydraulic and thermal numerical simulations of the entire commissioning process and the entire line are carried out. In addition, due to the complex operating conditions of the finished oil pipeline in the frozen soil area, it is necessary to evaluate the risks under the accident state. The specific steps of the mathematical model and numerical method used in the simulation include:

[0107] (1) Establishing a mathematical model: Based on the characteristics and assumptions of gas-liquid two-phase flow, a set of mathematical equations, including the mass conservation equation, energy equation, and state equation, are established to describe the behavior of the two-phase flow.

[0108] (2) Initialization parameters: Set the initial conditions, including initial velocity, initial temperature, initial concentration, etc. These initial conditions will be used as the starting point for the numerical solution.

[0109] (3) Characteristic line calculation: For each grid or unit, the slope and characteristic velocity of the characteristic line are calculated according to the initial conditions and the equations of the mathematical model. The characteristic line is a curve that describes the characteristic propagation of the equation.

[0110] (4) Characteristic line integration: Integrate the equations of the mathematical model along the characteristic line. For the solution of temperature drop and pressure drop, the energy equation and mass conservation equation are mainly concerned.

[0111] (5) Update solution: Update the solution in the grid or cell based on the solution calculated from the characteristic line. This involves interpolating the solution on the characteristic line to the grid nodes according to certain rules.

[0112] (6) Boundary condition processing: According to the specific boundary conditions of the problem, the solution on the boundary is processed to ensure the stability and accuracy of the numerical calculation, and finally the pressure change curve and temperature change curve along the grid are output.

[0113] Time stepping (if it is a time-dependent problem). If the problem is a time-dependent problem, repeat steps 3 to 6, updating the solution step by step according to the selected time step until the desired time point is reached.

[0114] The following is an example of a typical operating condition simulation method for the commissioning process.

[0115] 1. Hydraulic and thermal simulation of the process line of water injection at the first and last station and exhaust gas along the line

[0116] The flow rate of the first station of this production plan is 120m 3 / h, first inject 100km of water as isolation medium, then inject oil; the flow rate of the terminal station is also 120m 3 / h, reverse water injection 100km. Before the water flow arrives during the production process, exhaust operations are carried out at various stations, valve rooms and high points along the line.

[0117] The commissioning plan first injects water as an isolation medium, then injects oil, and reversely injects water at the terminal. Before the water flow arrives during the commissioning process, exhaust operations are performed at various stations, valve chambers, and high points along the line. Simulations are conducted for different exhaust conditions of the two sections of water injection at the first and last stations. Two schemes are considered: high-point exhaust (condition 1) and uphill valve chamber exhaust (condition 2).

[0118] According to the simulation results, the pressure variation range of key monitoring points along the line is summarized to determine whether the two exhaust schemes have overpressure. The high point pressure can be increased by isolating the ball and increasing the exhaust volume. In addition, the ambient temperature along the frozen soil section is the lowest ground temperature. Since the temperature along the line is sensitive to the total heat transfer coefficient, the commissioning plan should be determined in combination with the measured results and considering the safety margin.

[0119] 2. Full-line and full-process simulation of direct water injection and direct oil injection into production

[0120] The commissioning plan injects -20# diesel at the first station with a flow rate of 140m3 / h; the last station injects water in the reverse direction for 100km, with a flow rate of 140m3 / h. During the commissioning process, exhaust operations are performed at various stations, valve rooms and high points along the line before the water flow arrives.

[0121] Based on the pipeline multiphase flow model, a full-line simulation analysis was conducted on the commissioning process of pipeline water transport. The commissioning plan injected -20# diesel at the first station and reversely injected water at the last station. Before the water flow arrived during the commissioning process, exhaust operations were performed at various stations, valve rooms and high points along the line.

[0122] According to the simulation results, the pressure variation range of key monitoring points along the line is summarized to determine whether the two exhaust schemes have overpressure. In winter, due to the drop in ground temperature and the increase in diesel viscosity, the operating pressure of each station will change slightly. The viscosity of -20# diesel at different temperatures is obtained by linear interpolation.

[0123] Analysis of the final station of air column impact in three large drop sections

[0124] When there is a large drop section in the finished oil pipeline, the section will form a "water chasing gas" flow state. When the terminal is in a closed state, the water column continues to compress the gas under the action of gravity, forming a high-pressure area in the station entrance area, which poses an overpressure risk. By simulating this multiphase flow process, we can obtain the changes in pressure and velocity distribution along the line and the changes in water volume fraction distribution over time.

[0125] 4. Analysis of freezing risk in frozen soil sections

[0126] The low temperature of the frozen soil section is around -3℃ all year round, and there is a risk of freezing when the water transport is put into operation. Numerical simulation analysis is carried out on the changes of temperature, pressure and velocity of the frozen soil section over time during the water transport operation process.

[0127] Based on the known basic data, the heat release coefficient a of the soil on the outer wall of the pipe and the total heat transfer coefficient K are calculated. Due to the lack of measured data on the total heat transfer coefficient, the temperature distribution along the line at each moment can be obtained by numerical simulation of the combination of operating conditions (starting temperature, starting pressure, and total heat transfer coefficient). The temperature before the large temperature gradient section is mainly the water temperature, and the temperature after that is the air temperature. The large gradient area corresponds to the gas-liquid interface. Due to the small density and specific heat capacity of air, it quickly drops to ambient temperature. The risk assessment mainly refers to the water temperature distribution, that is, the temperature distribution before the large gradient point. The basis for judging whether there is an icing risk is whether the temperature of the gas-liquid interface reaching the end of the frozen soil section is lower than the freezing point of 273K.

[0128] 5. Evaluation of the safe time of shutting down non-condensable pipes during water transport

[0129] In order to evaluate the freezing risk of the water transport pipeline after it is shut down, a two-dimensional numerical simulation of the radial section of the pipeline was carried out. The problem setting is as follows: Figure 3 As shown. The 12D×6D (i.e. Lx=12D, Ly=6D) area near the pipeline is taken as the simulation area. The heat flux is given on the upper side of the calculation area, and the heat flux is calculated by the Newton cooling formula. The temperature zero gradient boundary conditions are given on the left and right sides of the calculation area, and the fixed temperature (soil temperature) boundary conditions are given on the lower side.

[0130] The coupling between the liquid in the pipe and the soil is described by the following equation:

[0131]

[0132] Where ρ is the liquid density, e is the internal energy, and T env is the soil temperature around the pipe, T is the liquid temperature in the pipe, d is the inner diameter of the pipe, and A is the cross-sectional area of ​​the pipe.

[0133] The pipe, water and environmental parameters used in the simulation are known. The initial water temperature comes from the one-dimensional direct water injection simulation results. The temperature at the second highest point when the gas-liquid interface is about to pass through the frozen soil section can be taken as the initial temperature for two-dimensional simulation analysis.

[0134] Taking the total heat transfer coefficient as a variable, numerical simulation is performed for different working conditions to obtain the temperature evolution in the tube corresponding to different total heat transfer coefficients. According to the simulation results, the safe shutdown time (non-freezing shutdown time) can be obtained.

[0135] 6. Evaluation of the safe time of diesel oil production and shutdown

[0136] In order to evaluate the freezing risk of direct diesel production pipeline after the shutdown, a two-dimensional numerical simulation of the radial section of the pipeline was carried out. The pipeline, diesel and environmental parameters used in the simulation calculation are known. Among them, the initial temperature of the diesel comes from the one-dimensional direct diesel simulation results. The temperature at the third highest point when the gas-liquid interface is about to pass through the frozen soil section is taken as the initial temperature for two-dimensional simulation analysis.

[0137] Taking the total heat transfer coefficient as a variable, numerical simulation is performed for different working conditions to obtain the temperature evolution in the tube corresponding to different total heat transfer coefficients. According to the simulation results, the safe stop time (non-condensing tube stop time) can be obtained.

[0138] The method of the present invention studies the efficient and high-precision solution algorithm of the simulation model of the production process of the finished oil pipeline. The established mathematical and physical model can describe the compressible multiphase flow diffusion process of the pipeline, which can be divided into the flow process and the diffusion process. Among them, the flow process is a nonlinear hyperbolic partial differential equation, and the diffusion process is a parabolic partial differential equation. The overall solution scheme is to split and solve the physical process of the control equation. For the flow process, the numerically robust Godunov finite volume method is used to solve the model, and the corresponding three-wave approximate Riemann problem solver (HLLC) is constructed for calculating the numerical flux. The monotonic MUSCL format and the high-order weighted essentially non-oscillating WENO format are used to improve the spatial accuracy of the numerical method, and the monotonic Runge-Kutta method is used to improve the time accuracy of the numerical method. For the diffusion process, an explicit format solution method with high parallel computing efficiency is used. At the same time, the applicability of the selected numerical method is analyzed from the two perspectives of accuracy and efficiency, and the calculation scheme is comprehensively optimized. Research on large-scale parallel solution technology for the model: Based on technologies such as information transfer interface (MPI), a parallel simulation program for multiphase flow of pipeline production is formed. The program can perform large-scale parallel computing, providing a basis for subsequent risk assessment and program optimization. It has achieved the simulation of the commissioning plan and risk assessment under accident conditions for the oil pipeline in the permafrost area with very complex conditions.

[0139] The above are exemplary embodiments disclosed in the present invention, but it should be noted that various changes and modifications may be made without departing from the scope disclosed in the embodiments of the present invention as defined in the claims. The functions, steps and / or actions of the method claims according to the disclosed embodiments described herein do not need to be performed in any particular order. In addition, although the elements disclosed in the embodiments of the present invention may be described or required in individual form, they may also be understood as multiple unless explicitly limited to the singular.

[0140] It should be understood that, as used herein, the singular forms "a", "an" are intended to include the plural forms as well, unless the context clearly supports an exception. It should also be understood that, as used herein, "and / or" refers to any and all possible combinations including one or more of the associated listed items.

[0141] The serial numbers of the embodiments disclosed in the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0142] A person skilled in the art will understand that all or part of the steps to implement the above embodiments may be accomplished by hardware or by instructing related hardware through a program, and the program may be stored in a computer-readable storage medium, and the above-mentioned storage medium may be a read-only memory, a disk or an optical disk, etc.

[0143] A person skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other changes in different aspects of the embodiments of the present invention as above, which are not provided in detail for the sake of simplicity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included in the protection scope of the embodiments of the present invention.

Claims

1. A simulation method for the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model, characterized in that: include: The continuity equation and momentum equation based on pipeline transient flow simulation are transformed to obtain the equation group of water head H; On the basis of the continuity equation and momentum equation, a temperature drop calculation formula along the pipeline is introduced in combination with temperature conditions; The computational grid is divided according to the finite volume method, and the characteristic line method is used to solve the head H equation group to obtain the compatible relationship, and obtain the uniform time step and space step; Based on the pressure boundary conditions and the compatibility relationship, the gas phase volume is described. The multiphase transient flow is calculated by combining the discretized equation, the single temperature assumption, and the single velocity assumption. The pressure change curve along the grid is output over time. The temperature change curve over time is output based on the temperature drop calculation formula.

2. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: The equations for the water head H obtained by deforming the continuity equation and momentum equation based on pipeline transient flow simulation include: According to the characteristics and assumptions of gas-liquid two-phase flow, a control equation is established to describe the behavior of gas-liquid two-phase flow. The control equation includes a continuity equation and a momentum equation. Based on the deformation of the continuity equation and the momentum equation, the equation group of the water head H is obtained: Among them, x and t are space and time coordinates respectively, u is the average flow velocity in the pipe cross section, D is the inner diameter of the pipe, f is the friction coefficient, and a is the wave speed.

3. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 2 is characterized in that: The continuity equation and the momentum equation are: Among them, x and t are space and time coordinates respectively, p is pressure, ρ is density, u is the average flow velocity in the pipe cross section, D is the inner diameter of the pipe, f is the friction coefficient, and a is the wave speed.

4. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: Also includes: Based on the volume of the gas phase and the liquid phase, the volume fraction calculation formula is obtained to quantitatively describe the two-phase flow in the pipeline. The volume fraction calculation formula is: Among them, V g is the gas phase volume, V l is the volume of liquid phase, V m is the volume of the mixture.

5. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: The temperature drop calculation formula along the pipeline introduced in combination with temperature conditions includes: The temperature drop calculation formula along the pipeline is introduced by combining the starting point temperature of the pipeline and the soil temperature around the pipeline. The temperature drop calculation formula along the pipeline is: T L =T0+(T R -T0)e -aL Where K is the total heat transfer coefficient, W / (m 2 ℃); T R is the starting temperature of the pipeline, ℃; T0 is the soil temperature around the pipeline, ℃; T L is the temperature at a distance L from the starting point, °C; D w is the outer diameter of the pipe, m; G is the mass flow rate of the oil-water mixture, kg / s; c is the specific heat of the oil, J / (kg℃).

6. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: The compatible relations obtained by solving the head H equations using the characteristic line method include: For each grid or unit, the characteristic line method is used to ignore the nonlinear terms in the head H equation group and solve them. The slope and characteristic velocity of the characteristic line are calculated, and the integration is performed along the characteristic line to obtain the compatibility relationship on the characteristic line: H P =C pc -B pc Q Pu +R pc Q Pu |Q A | H P =C mc +B mc Q P +R mc Q P |Q B | Among them, C pc , B pc , R pc is a constant on the positive characteristic line, C mc , B mc , R mc is a constant on the negative characteristic line, Q pu is the upstream flow, Q p is the downstream flow, Q A , Q B is the flow on the positive and negative characteristic lines, and then, the following calculation format can be obtained: Among them, Δt is the time step and Δx is the space step.

7. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: The pressure boundary condition is: when the pressure of any node on the computational grid in the pipeline is lower than the saturation pressure, the pressure is taken as the saturation pressure.

8. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: The description of the gas phase volume based on pressure boundary conditions and compatibility relations includes: The gas phase volume is described based on the following formula: in, is the weighting factor, V' is the gas phase volume at the current time step, V t-2Δt is the gas phase volume in the first two time steps, Q pu is the upstream flow, Q p is the downstream flow.

9. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: The output grid pressure curves along the line are: The pressure variation curve along the line over time is obtained by the pressure drop iteration formula, and the pressure drop iteration formula is: P (i+1) =P (i) +(150×(1-e) 2 m g ×ΔxV g ) / (e 3 ×dp 2 ρg)+(1.75×(1-ε)×u l ×Δx) / (dp×ε 3 ×ρl) Where P(i) represents the pressure in the i-th grid unit, P(i+1) represents the pressure value of the i-th grid unit in the next time step; ε is the volume proportion of the solid phase or liquid phase in the multiphase flow; μ g is the dynamic viscosity of the gas; μ l is the dynamic viscosity of the liquid; Δx is the length of the grid unit; V g is the flow rate of the gas; V l is the flow rate of the liquid; dp is the diameter of the particle or droplet; ρ g is the density of the gas; ρ l is the density of the liquid.

10. The method for simulating the commissioning of a finished oil pipeline in a frozen soil region based on a discrete cavitation model according to claim 1, characterized in that: The output temperature change curve over time based on the temperature drop calculation formula includes: Based on the temperature drop calculation formula combined with the discretization equation, the temperature iteration formula is obtained to output the temperature change formula over time. The temperature iteration formula is: T (i+1,j) =T0+(T (i,j) -T0)e -al Among them, T0 is the initial temperature, l is the length of the pipeline, i is the horizontal coordinate of the grid unit, and j is the vertical coordinate of the grid unit.

Images