A simulation method for random changes in fluid density in oil and gas pipelines

Abstract

The present invention discloses a method for simulating random variations in fluid density within an oil and gas pipeline. The method comprises: constructing a random variation model for the fluid density within the pipeline based on random parameters related to the transport of oil and natural gas within the pipeline; obtaining the probability distribution of the relevant random parameters through a gas-liquid two-phase flow physical model experiment and performing random value selection; coupling the dynamic control equations for the vibration of the flexible pipeline structure and the randomly varying density fluid based on the random variation model for the fluid density within the pipeline and the random values of the relevant random parameters; and solving the dynamic control equations using the finite element method or the finite difference method to obtain the dynamic response results of the pipeline structure vibration and the time-domain response results of the internal stress and strain of the structure. This simulation method makes the results of the random analysis of the gas-liquid two-phase flow within the pipeline more realistic, thereby improving the reliability and effectiveness of fatigue service life prediction for marine pipeline structures.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas transportation, and in particular to a method for simulating random changes in fluid density in an oil and gas transportation pipeline. Background Art

[0002] 71% of the Earth's surface is covered by ocean, and the ocean floor holds abundant reserves of oil and natural gas. As terrestrial oil and gas resources gradually become depleted and production dwindles, the exploitation of offshore oil and gas resources has intensified. Pipelines are often used to transport oil and natural gas, and therefore have gained widespread application, such as submarine pipelines, top tension risers, steel catenary risers, and slow-wave risers.

[0003] Due to variations in seabed geology, chemical reactions, and historical changes, the storage of submarine oil and gas resources may vary across different sea areas. Some sea areas hold large quantities of natural gas, others hold large quantities of oil, and still others contain both. These oil and gas resources are typically transported via pipelines, which offer advantages such as high efficiency, continuity, large throughput, and low cost. When pipelines transport submarine natural gas, fluctuations in ambient temperature and pressure can cause some heavy hydrocarbons to liquefy into liquids, resulting in a two-phase flow within the pipeline. Conversely, when pipelines transport submarine oil, fluctuations in ambient temperature and pressure can cause some light hydrocarbons to vaporize into gas, resulting in a two-phase flow within the pipeline. For wells producing both oil and natural gas, the pipelines typically transport both gas and liquid.

[0004] As can be seen, gas-liquid two-phase flow, consisting of gas and liquid, is common in offshore oil and gas pipelines. When flowing within pipelines, gases and liquids are prone to deformation, separation, and mixing. These flow patterns can exhibit a variety of instability, including bubble flow, slug flow, foam flow, annular mist flow, and mist-like flow. These flow patterns are highly unstable and easily altered by external disturbances. Parameters such as the total density, velocity, and pressure of the gas-liquid two-phase flow within pipelines can undergo random variations. Offshore oil and gas pipelines can be hundreds or even thousands of meters long and possess significant flexibility. Random excitation within these fluids can cause vibrations, leading to cyclical stress and strain within the pipeline structure. Under prolonged excitation, the pipeline structure is susceptible to fatigue cracks and even fatigue failure. Once fatigue failure occurs, the oil and natural gas within the pipeline will leak, causing major safety incidents, severe environmental pollution, and enormous economic losses.

[0005] Existing technologies have made significant simplifications to the gas-liquid two-phase flow composed of oil and natural gas in pipelines. They generally use a single-phase flow model to describe the flow of the fluid in the pipeline, or a stable slug flow model to simulate the effect of the gas-liquid slug flow on the pipeline. These methods do not consider the random changes in the mixed flow of gas and liquid inside the pipeline, the random changes in the total density of the fluid in the pipeline, or the coupling effect between the random changes in fluid density and the vibration of the pipeline structure. Summary of the Invention

[0006] The present invention aims to overcome the shortcomings of the prior art and, to this end, provides a method for simulating random variations in fluid density within an oil and gas pipeline. The method takes into account the amplitude, frequency, velocity, and initial phase of random variations in total fluid density when natural gas and oil flow together within the pipeline, and couples the flow of the fluid with randomly varying density with the motion of the pipeline structure. This improves upon prior art methods for simulating the randomness of gas-liquid two-phase flow within the pipeline, making the analysis results of the randomness of the gas-liquid two-phase flow within the pipeline more accurate, and improving the reliability and effectiveness of fatigue service life prediction for marine pipeline structures.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] The present invention provides a method for simulating random changes in fluid density in an oil and gas pipeline, comprising the following steps:

[0009] According to the random parameters related to oil and natural gas transportation in pipelines, a random variation model of fluid density in pipelines is constructed;

[0010] Through a gas-liquid two-phase flow physical model experiment, the probability distribution of the relevant random parameters is obtained and randomly selected;

[0011] According to the random density variation model of the fluid in the pipe and the random values of the relevant random parameters, the dynamic control equations of the flexible pipe structure vibration and the fluid with random density variation are coupled;

[0012] The dynamic control equation is solved by using the finite element method or the finite difference method to obtain the dynamic response results of the pipeline structure vibration and the time domain response results of the internal stress and strain of the structure.

[0013] Furthermore, the relevant random parameters for transporting oil and natural gas in the pipeline include:

[0014] The amplitude of random changes in the total density of the fluid when oil and natural gas mix and flow inside the pipeline, the frequency of random changes, the flow velocity of random changes and the initial phase of random changes.

[0015] Furthermore, the random variation model of the fluid density in the tube is:

[0016]

[0017]

[0018] Where: ρ(s,t) represents the total density of the fluid in the pipe, s represents the Lagrangian coordinate, is the distance from the current point along the pipe to the origin, and t represents time;

[0019] represents the average density of the fluid in the tube;

[0020] ε j represents the amplitude of the change in fluid density in the j-th tube;

[0021] exp[] represents the exponential function with the natural exponent e as the base;

[0022] i represents the imaginary unit;

[0023] k j It represents the wave velocity of the j-th tube fluid density change, and its value is related to the circular frequency ω j and flow rate u j related;

[0024] ω j represents the circular frequency of the j-th tube fluid density change;

[0025] θ j represents the initial phase of the j-th fluid density change in the tube;

[0026] u j It represents the flow rate of the fluid density change in the j-th tube.

[0027] Furthermore, through a gas-liquid two-phase flow physical model experiment, the probability distribution of the relevant random parameters is obtained and random values are taken; including:

[0028] Through gas-liquid two-phase flow physical model experiments, the amplitude of random changes in fluid density, the frequency of random changes in fluid density, the flow rate of random changes in fluid density, and the initial phase of random changes in fluid density are measured and statistically analyzed when oil and natural gas are mixed and flowed inside the pipeline.

[0029] Randomly select values based on the probability distribution of measurement statistics.

[0030] Furthermore, the dynamic control equations for coupling the vibration of the flexible pipe structure with the fluid with randomly varying density include:

[0031]

[0032] Where: m s Indicates the mass per unit length of the pipeline structure;

[0033] y represents the lateral displacement of the pipeline structure vibration;

[0034] c s Indicates the damping coefficient of the pipeline structure vibration;

[0035] A in Indicates the area of the internal cross-section of the pipe structure;

[0036] T represents the axial force of the pipeline structure;

[0037] EI represents the bending stiffness of the pipe structure.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] This method takes into account the influence of factors such as the amplitude of random changes in the total density of the gas-liquid two-phase flow, the random change frequency, the random change flow rate and the random change initial phase when natural gas and oil are mixed and transported inside the marine pipeline. Physical model experiments are used to determine the probability distribution of each random parameter, and the pipeline structure vibration is coupled with the fluid movement inside the pipe. Thus, a new simulation method for the transportation of fluids with randomly changing density in flexible pipelines is established, which provides a theoretical basis and approach for the safety, reliability and durability analysis of marine oil and gas pipelines. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A flow chart of a method for simulating random changes in fluid density in an oil and gas pipeline provided by an embodiment of the present invention;

[0041] Figure 2 This is a schematic diagram of a method for simulating random changes in fluid density in an oil and gas pipeline according to an embodiment of the present invention. DETAILED DESCRIPTION

[0042] In order to make the technical means, creative features, objectives and effects achieved by the present invention easier to understand, the present invention is further described below in conjunction with specific implementation methods.

[0043] In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," "the other end," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0044] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "provided with," "connected," etc., should be understood in a broad sense. For example, "connected" may refer to a fixed connection, a detachable connection, or an integral connection; it may refer to a mechanical connection or an electrical connection; it may refer to a direct connection or an indirect connection through an intermediate medium; it may refer to internal communication between two components. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0045] Pipelines are commonly used to quickly and efficiently transport oil and natural gas produced from submarine wells to offshore platforms. When transporting oil and natural gas, pipelines often involve a two-phase flow consisting of gas and liquid. This two-phase flow is extremely unstable and easily affected by external factors. Parameters such as the total mass, density, flow rate, and pressure of the fluid can fluctuate randomly. Offshore oil and gas pipelines typically connect submarine wells to offshore platforms, stretching for hundreds or even thousands of meters and possessing high flexibility. Stimulated by the random fluctuations of the internal oil and gas mixture, the pipeline vibrates. Over time, fatigue cracks and subsequent fatigue failure can easily develop within the pipeline structure. Conventional approaches to understanding the effects of the oil and gas mixture on the pipeline have simplified it as a single-phase flow, ignoring the random fluctuations of the mixture. Consequently, the calculated results differ significantly from actual conditions. During the design of offshore pipelines, a large safety factor is often required to ensure that the pipeline does not fail.

[0046] The present invention takes into account the influence of factors such as the amplitude of random changes in the total density of the mixed fluid composed of oil and natural gas inside the pipeline, the frequency of random changes, the flow rate of random changes and the initial phase of random changes, and proposes a simulation method for the random changes in fluid density inside the oil and gas pipeline.

[0047] Reference Figure 1-2 As shown, the present invention provides an embodiment of a method for simulating random changes in fluid density in an oil and gas pipeline, comprising the following steps:

[0048] S10. Construct a random variation model of fluid density in the pipeline based on relevant random parameters of oil and natural gas transported in the pipeline;

[0049] S20, obtaining the probability distribution of the relevant random parameters through a gas-liquid two-phase flow physical model experiment, and performing random value selection;

[0050] S30, coupling the dynamic control equations of the flexible pipe structure vibration and the fluid with randomly varying density based on the random density variation model of the fluid in the pipe and the random values of the relevant random parameters;

[0051] S40. Using the finite element method or the finite difference method, the dynamic control equation is solved to obtain the dynamic response result of the pipeline structure vibration and the time domain response result of the internal stress and strain of the structure.

[0052] The method involves establishing a model for random variations in fluid density within a pipeline, determining the model's random parameters based on physical model experiments, and coupling the pipeline's structural vibrations with the dynamic governing equations for the fluid flow within the pipeline. This method fully considers the influence of factors such as the amplitude, frequency, velocity, and initial phase of random variations in total fluid density during the transport of mixed gas and liquid within the pipeline. Physical model experiments are used to determine the probability distribution of each random parameter, and by coupling the pipeline's structural vibrations with the fluid's motion within the pipeline, a new simulation method for transporting fluids with randomly varying density within flexible pipelines has been established. This method provides a theoretical basis and analytical approach for the safety, reliability, and durability of offshore oil and gas pipelines.

[0053] The following is a detailed description of each of the above steps:

[0054] In step S10, the random variation model of the fluid density in the tube is:

[0055]

[0056]

[0057] Where: ρ(s,t) represents the total density of the fluid in the pipe, unit is kg / m 3 ; s represents the Lagrange coordinate, which is the distance from the current point along the pipeline to the origin, in meters; t represents time, in seconds;

[0058] Indicates the average density of the fluid in the pipe, unit: kg / m 3 ;

[0059] ε j It represents the amplitude of the change in the density of the fluid in the j-th tube, dimensionless unit;

[0060] exp[] represents the exponential function with the natural exponent e as the base;

[0061] i represents the imaginary unit;

[0062] k j It represents the wave velocity of the j-th tube fluid density change, in units of 1 / m, and its value is related to the circular frequency ω j and flow rate u j related;

[0063] ω j It represents the circular frequency of the density change of the fluid in the j-th tube, in rad / s;

[0064] θ j It represents the initial phase of the j-th fluid density change in the tube, in rad;

[0065] u j The flow rate of the j-th tube fluid density change, in m / s.

[0066] In step S20, a gas-liquid two-phase flow physical model experiment can be used to measure and statistically analyze the amplitude, frequency, velocity, and initial phase of random changes in the density of the fluid within the pipeline when oil and natural gas are mixed and flowing. By measuring and statistically analyzing their probability distributions, such as normal distribution, Poisson distribution, and Rayleigh distribution, random values are then assigned based on these probability distributions.

[0067] In step S30, the dynamic control equation of the flexible pipe structure vibration and the fluid with random density variation is coupled:

[0068]

[0069] Where: m s Indicates the mass per unit length of the pipeline structure, in kg / s;

[0070] y represents the lateral displacement of the pipeline structure vibration, unit is m;

[0071] c s Indicates the damping coefficient of pipeline structure vibration, unit is N·s / m;

[0072] A in Indicates the area of the internal cross-section of the pipe structure, in m 2 ;

[0073] T represents the axial force of the pipeline structure, unit is N;

[0074] EI represents the bending stiffness of the pipeline structure, in N·m 2 .

[0075] In step S40, the control equation is solved by using the finite element method, the finite difference method or other methods to obtain the dynamic response of the pipeline structure vibration, the time domain response of the internal stress and strain of the structure, etc., providing a basis for fatigue analysis and reliability analysis of the pipeline structure.

[0076] The present invention overcomes many deficiencies of the prior art and is characterized by:

[0077] (1) Considering the random variation of the total density of the oil-gas mixture

[0078] When transporting oil and natural gas through offshore pipelines, two-phase flows, consisting of gas and liquid, are prone to occur within the pipelines. This flow is highly unstable and susceptible to changes due to external factors. This invention considers the random variations in the total density of the gas-liquid two-phase flow and establishes a theoretical calculation model for this random variation in fluid density. This model satisfies the fluid continuity criterion and can rationally analyze the density variations of the gas-liquid two-phase flow composed of oil and natural gas within the pipeline.

[0079] (2) Determine the probability distribution of randomness parameters through physical model experiments

[0080] The varying compositions of oil and natural gas within a pipeline result in varying amplitudes, frequencies, flow velocities, and initial phases of the total density of the mixed fluid. This paper considers the random variations in fluid density and employs a physical model experiment of gas-liquid two-phase flow to measure the probability distribution of factors such as the amplitude, frequency, flow velocity, and initial phase of the random variations in fluid density. These parameters are then assigned values within the probability distribution, resulting in a more reliable simulation of random variations in fluid density.

[0081] (3) Solved the problem of excitation of pipeline structure vibration caused by random changes in fluid density in the pipe

[0082] Marine oil and gas pipelines have a large aspect ratio and are highly flexible, making them susceptible to vibration under the influence of the internal fluid. Prolonged vibration can lead to fatigue failure. This paper, based on the Euler-Bernoulli beam model, couples the random variations in fluid density within the pipeline with the vibration of the pipeline structure to establish a dynamic control equation. Solving this dynamic control equation reveals the dynamic response of the pipeline structure's vibration, stress and strain changes, and other parameters, enabling further assessment of the pipeline structure's fatigue life and reliability.

[0083] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A method for simulating random changes in fluid density in an oil and gas pipeline, characterized in that: The following steps are involved: According to the relevant random parameters of oil and natural gas transportation in the pipeline, a random variation model of the fluid density in the pipeline is constructed; the random variation model of the fluid density in the pipeline is: Where: ρ(s,t) represents the total density of the fluid in the pipe, s represents the Lagrangian coordinate, is the distance from the current point along the pipe to the origin, and t represents time; represents the average density of the fluid in the tube; ε j represents the amplitude of the change in fluid density in the j-th tube; exp[] represents the exponential function with the natural exponent e as the base; i represents the imaginary unit; k j It represents the wave velocity of the j-th tube fluid density change, and its value is related to the circular frequency ω j and flow rate u j related; ω j represents the circular frequency of the j-th tube fluid density change; θ j represents the initial phase of the j-th fluid density change in the tube; u j represents the flow rate of the fluid density change in the j-th tube; Through a gas-liquid two-phase flow physical model experiment, the probability distribution of the relevant random parameters is obtained and randomly selected; According to the random density variation model of the fluid in the pipe and the random values of the relevant random parameters, the dynamic control equations of the flexible pipe structure vibration and the fluid with random density variation are coupled; The dynamic control equation is solved by using the finite element method or the finite difference method to obtain the dynamic response result of the pipeline structure vibration and the time domain response result of the internal stress and strain of the structure; The dynamic control equations for coupling the vibration of the flexible pipe structure with the fluid with randomly varying density include: Where: m s Indicates the mass per unit length of the pipeline structure; y represents the lateral displacement of the pipeline structure vibration; c s Indicates the damping coefficient of the pipeline structure vibration; A in Indicates the area of the internal cross-section of the pipe structure; T represents the axial force of the pipeline structure; EI represents the bending stiffness of the pipe structure.

2. The method for simulating random changes in fluid density in an oil and gas pipeline according to claim 1, characterized in that: The relevant random parameters of oil and natural gas transportation in the pipeline include: The amplitude of random changes in the total density of the fluid when oil and natural gas mix and flow inside the pipeline, the frequency of random changes, the flow velocity of random changes and the initial phase of random changes.

3. The method for simulating random changes in fluid density in an oil and gas pipeline according to claim 2, characterized in that: Through a gas-liquid two-phase flow physical model experiment, the probability distribution of the relevant random parameters is obtained and randomly selected; include: Through gas-liquid two-phase flow physical model experiments, the amplitude of random changes in fluid density, the frequency of random changes in fluid density, the flow rate of random changes in fluid density, and the initial phase of random changes in fluid density are measured and statistically analyzed when oil and natural gas are mixed and flowed inside the pipeline. Randomly select values based on the probability distribution of measurement statistics.

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