Method for constructing ultimate bearing capacity model of copper-nickel alloy pipeline with groove defect

Abstract

The invention provides a method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline with a groove defect, which comprises the following steps of: representing a pipe path ratio K by adopting a pipeline outer diameter D and a wall thickness t to obtain an ultimate bearing capacity of a lossless pipeline so as to consider the influence of the wall thickness on a failure critical state; based on the influence of the wall thickness on the length correction coefficient Q, correcting the length correction coefficient Q through a first correction coefficient vG1 and a second correction coefficient vG2; correcting the defect influence factor item R through a third correction coefficient vG3 based on the influence of the difference of the corrosion positions on the defect influence factor item R; constructing an ultimate bearing capacity model based on the ultimate bearing capacity of the lossless pipeline, the length correction coefficient Q and the defect influence factor item R; and solving the ultimate bearing capacity model by adopting a dynamic explicit finite element method through numerical simulation analysis to obtain the corrected ultimate bearing capacity model.

Description

Technical Field

[0001] The present invention relates to the technical field of predicting the ultimate bearing capacity of seawater pipelines, and particularly to a method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline with groove defects. Background Art

[0002] The problem of strength evaluation after pipeline corrosion has always been a hot research topic. With the increase of years, metal loss defects are formed on the inner wall surface of seawater pipelines under seawater erosion. The existence of defects reduces the bearing capacity of seawater pipelines and seriously affects the reliability and safety of equipment. The failure analysis of the British offshore engineering operation company BRITOIL shows that: among all the examples of ship facility failures, 33% are caused by corrosion. The corrosion of seawater pipe systems can cause corrosion leakage at worst and threaten navigation safety at worst. The reason for the bursting of the seawater pipeline in the engine room is the sharp rise of the internal pressure of the pipeline, which bursts beyond the bearing capacity of the pipeline in the areas containing defects such as corrosion, cracks, and sand holes.

[0003] Although domestic and foreign scholars have made a lot of explorations in the ultimate bearing capacity of pipelines, they are all concentrated on pipeline steel with thin-walled structures. Some scholars have made a preliminary study on the evaluation of seawater pipelines with corrosion defects, but the error of the evaluation results is relatively large. The existing standards are established based on thin-walled pipelines, and in order to meet the general evaluation requirements, the differences in defect shapes are not considered. Therefore, the error between the evaluation results and the test results is generally large, remaining at about 20%.

[0004] The existing standards are established based on oil and gas pipelines, that is, thin-walled pipelines. How much does the wall thickness affect the pipeline strength, and how to consider the influence of the wall thickness on the pipeline strength. How to select the failure mode of thick-walled pipelines, how to modify the prediction model of the existing system, and how to propose a non-overly conservative, safe and reliable evaluation method for copper-nickel alloy pipelines with corrosion defects, all of which need to be studied.

[0005] Therefore, how to improve the evaluation accuracy of the ultimate bearing capacity prediction model of pipelines with groove defects is a technical problem that needs to be solved urgently Summary of the Invention

[0006] The present invention proposes a method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline with groove defects to solve the technical problem of the low accuracy of the existing ultimate bearing capacity prediction model in the case of copper-nickel alloy pipelines with groove defects on the seabed.

[0007] To solve the above technical problems, the present invention provides a method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline with groove defects, including the following steps:

[0008] Step S1: Express the pipeline diameter ratio K using the outer diameter D and wall thickness t of the pipeline to obtain the ultimate bearing capacity of the non-damaged pipeline, so as to consider the influence of the wall thickness on the failure critical state;

[0009] Step S2: Based on the influence of the wall thickness on the length correction coefficient Q, correct the length correction coefficient Q through the first correction coefficient v G1 and the second correction coefficient v G2 ; Based on the influence of the difference in the corrosion position on the defect influence factor term R, correct the defect influence factor term R through the third correction coefficient v G3 ;

[0010] Step S3: Based on the non-destructive pipeline ultimate bearing capacity, length correction coefficient Q, and defect influence factor term R, construct an ultimate bearing capacity model;

[0011] Step S4: Through numerical simulation analysis, use the dynamic explicit finite element method to solve the ultimate bearing capacity model, and obtain the corrected ultimate bearing capacity model.

[0012] Preferably, in step S1, the expression of the non-destructive pipeline ultimate bearing capacity p 0 is:

[0013]

[0014] In the formula, η represents the yield ratio of the material; σ b represents the tensile strength of the material; t represents the wall thickness, and D represents the outer diameter of the pipeline.

[0015] Preferably, in step S2, the expression of the length correction coefficient Q is:

[0016]

[0017] In the formula, L Z represents the length of the pipeline defect; D represents the outer diameter of the pipeline; t represents the wall thickness.

[0018] Preferably, in step S2, the expression of the defect influence factor term R is:

[0019]

[0020] In the formula, N represents the corrosion depth ratio; Q represents the length correction coefficient.

[0021] Preferably, in step S3, the expression of the ultimate bearing capacity model p CG is:

[0022]

[0023] In the formula, p 0 represents the non-destructive pipeline ultimate bearing capacity; R represents the defect influence factor term; η represents the yield ratio of the material; σ bσ represents the tensile strength of the material; t represents the wall thickness, D represents the outer diameter of the pipeline; N represents the corrosion depth ratio; Q represents the length correction factor.

[0024] Preferably, when performing numerical simulation analysis in step S4, hexahedral meshes are used for division, and five layers of meshes are divided in the wall thickness direction of the corroded pipeline.

[0025] Preferably, when performing numerical simulation analysis in step S4, during parametric modeling, first write an Abaqus GUI script on the Python platform; encapsulate the written script into the Abaqus software in the form of a plug-in; establish a simulation model by calling the plug-in formed by the Abaqus GUI script during modeling; modify the defect shape parameters and the maximum internal pressure load value to achieve model adjustment.

[0026] Preferably, step S4 includes:

[0027] Step S41: Calculate the simulation result of the ultimate bearing capacity of the pipeline using the explicit finite element method;

[0028] Step S42: Take the sum of squares of the differences e(r) between the simulation result and the calculation result of the ultimate bearing capacity model as the optimization objective;

[0029] Step S43: Use the least squares method to solve with v G =[ν G1 ,ν G2 ,ν G3 T as the design variable to obtain the corrected ultimate bearing capacity model.

[0030] Preferably, the expression of the optimization objective e(r) is:

[0031]

[0032] In the formula, p FEA represents the simulation result, and p CG represents the calculation result of the ultimate bearing capacity model.

[0033] Preferably, in step S4, the expression of the corrected ultimate bearing capacity model is:

[0034]

[0035] In the formula, p 0 represents the ultimate bearing capacity of the non-damaged pipeline; R represents the defect influence factor term; η represents the yield ratio of the material; σ b represents the tensile strength of the material; t represents the wall thickness, D represents the outer diameter of the pipeline; N represents the corrosion depth ratio; Q represents the length correction factor; L Z represents the length of the pipeline defect.

[0036] The beneficial effects of the present invention at least include: Based on the established evaluation and prediction model for the ultimate bearing capacity of copper-nickel alloy pipelines with groove defects, considering the influence of wall thickness on the failure critical state, the pipe diameter ratio K is represented by the outer diameter D and wall thickness t of the pipeline, and the ultimate bearing capacity of the non-damaged pipeline is corrected; in the case of correcting the ultimate bearing capacity of the non-damaged pipeline, considering the influence of wall thickness and corrosion location at the same time, the length correction coefficient and defect influence factor terms are corrected; then, a numerical calculation model close to the actual characteristics of seawater pipelines is established based on the finite element method of explicit dynamics, and the bursting failure characteristics of the pipeline under internal pressure are analyzed according to the numerical simulation results, the influence law of each parameter of the pipeline on the ultimate bearing capacity is studied, and the evaluation and prediction model for the ultimate bearing capacity of copper-nickel alloy pipelines with groove defects is corrected. The calculation results of the obtained ultimate bearing capacity model are closer to the calculation of the ultimate bearing capacity of the copper-nickel alloy pipeline with groove defects on the seabed, and the calculation results are more effective. Brief Description of the Drawings

[0037] Figure 1 It is a schematic diagram of the explicit dynamics calculation process;

[0038] Figure 2 Schematic diagram of the pressure loading range;

[0039] Figure 3 It is a schematic diagram of the bursting morphology of the pipeline with groove defects;

[0040] Figure 4 It is the equivalent stress nephogram of G16# pipeline under different internal pressures;

[0041] Figure 5 It is a schematic diagram of the stress and strain conditions of each point along the wall thickness direction at the defect when two different pipelines burst;

[0042] Figure 6 It is a schematic diagram of the selection of two units in the wall thickness direction of G16# pipeline;

[0043] Figure 7 It is a schematic diagram of the stress and strain changes of two units under internal pressure of the pipeline;

[0044] Figure 8 It is a schematic diagram of the stress nephogram and strain condition of three states;

[0045] Figure 9 It is the stress nephogram of G16# pipeline at 20.7 MPa;

[0046] Figure 10 It is a schematic diagram of the von Mises stress of two paths;

[0047] Figure 11 It is a schematic diagram of the relationship between defect depth and ultimate bearing capacity;

[0048] Figure 12 It is a schematic diagram of the relationship between the defect length and the ultimate bearing capacity;

[0049] Figure 13 Schematic diagram of the correction effect of the correction formula under different defect lengths of the groove defect. Specific implementation manners

[0050] The following combines the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0051] The embodiment of the present invention provides a method for constructing a model of the ultimate bearing capacity of a copper-nickel alloy pipeline with groove defects, including the following steps:

[0052] Step S1: Use the outer diameter D and wall thickness t of the pipeline to represent the pipeline diameter ratio K to obtain the ultimate bearing capacity of the non-destructive pipeline, so as to consider the influence of the wall thickness on the failure critical state;

[0053] Step S2: Based on the influence of the wall thickness on the length correction coefficient Q, correct the length correction coefficient Q through the first correction coefficient v G1 and the second correction coefficient v G2 ; Based on the influence of the difference in the corrosion position on the defect influence factor term R, correct the defect influence factor term R through the third correction coefficient v G3 ;

[0054] Step S3: Based on the ultimate bearing capacity of the non-destructive pipeline, the length correction coefficient Q, and the defect influence factor term R, construct an ultimate bearing capacity model;

[0055] Step S4: Through numerical simulation analysis, use the dynamic explicit finite element method to solve the ultimate bearing capacity model to obtain a corrected ultimate bearing capacity model.

[0056] The following details the embodiments of the present invention:

[0057] Complex marine conditions lead to the corrosion of seawater pipelines, and the shape after corrosion is generally irregular. Corrosion generally only causes metal loss defects, and the shape is an erosion pit. For the single external corrosion defect of oil and gas pipelines, four regular-shaped defects of spherical type, flat-bottom type, groove type, and local uniform type are used to simulate actual corrosion defects. The premise of strength evaluation is defect parameterization, and its geometric dimensions can be measured. Therefore, first, the actual corrosion defect should be simplified into a geometric model.

[0058] Except for overall uniform corrosion, all other corrosion of seawater pipelines are local defects. Overall uniform corrosion can be regarded as uniform thinning of pipeline wall thickness. When calculating strength, the thickness after thinning is used as a parameter and can be calculated as a non-destructive pipeline. The embodiments of the present invention will not be explained here. The morphology of local corrosion defects in seawater pipelines can be summarized into the following three categories: (1) The bottom pit of the local uniform corrosion defect is approximately a rectangular uniform wall thickness defect; (2) If the corrosion morphology is a defect with deep corrosion in the middle and shallow corrosion on both sides in the circumferential direction and uniform axial direction, such as accumulation corrosion, it is generally simulated by groove defects; (3) The pits caused by selective corrosion, pitting, etc. are represented by spherical defects; for defects that may produce multiple spherical pits in the corrosion morphology, only axial double spherical defects of equal depth and diameter are considered. The present invention focuses on the analysis of the calculation of the ultimate bearing capacity of copper-nickel alloy pipelines containing groove defects.

[0059] There are many factors that affect the corrosion rate of copper-nickel alloy pipelines, mainly affected by fluid temperature, fluid pressure, seawater salinity, current environmental pH, seawater flow rate, scouring and water intrusion effects, and seawater oxygen content protective coating (corrosion product film). Due to the complexity of the corrosion environment, there is currently no model that can accurately predict the corrosion rate of seawater pipelines during service. During the long-term service of seawater pipelines, the combined action of multiple corrosions causes defects in the pipe. The scouring corrosion of muddy and sandy fluids under electrochemical corrosion is the most corrosive and harmful type of composite corrosion. The electrochemical corrosion process is the reaction of copper elements in Cl - The process of losing electrons under the action of water and then undergoing hydrolysis to convert into copper oxide corrosion product film. The seawater scouring process is the solid surface mass transfer process caused by the mechanical action of the material being impacted by seawater, mud, sand and sand. The corrosion product film produced by copper-nickel alloy in the electrochemical process has a great influence on the corrosion rate. Other factors change the corrosion rate of the material by affecting the protectiveness of the corrosion product film.

[0060] The limit state of the bearing capacity of seawater pipelines is bursting or wall leakage, so the embodiment of the present invention uses the bursting pressure of seawater pipelines as the limit bearing capacity. According to this limit state, comprehensive analysis shows that elastic failure and plastic limit failure are too conservative, and fail to take into account the plasticity of the pipe and the thickness of the pipe wall. The damage evolution failure has uncertainty and is too dangerous to be assessed. The evaluation of the plastic failure criterion is neither too conservative nor too dangerous. It is most appropriate and practical to choose the criterion based on plastic failure as the basis for judging the bursting pressure of seawater pipelines.

[0061] Calculate the bursting pressure of intact thick-walled pipelines. The internal pressure load of the pipeline is the most important working load of the seawater pipeline. Determining the ultimate internal pressure load of the intact pipeline has important engineering significance in the design and status assessment of the seawater pipeline system. The existing standard system is mainly proposed for oil and gas pipelines, and the pipeline models are simplified to thin-walled pipelines without considering the influence of wall thickness.

[0062] There are many definitions for the classification of thin-walled pipelines and thick-walled pipelines. The present invention is based on the description in "Mechanics of Materials I":

[0063]

[0064] It is generally believed that the stress of thin circular tubes is uniform along the wall thickness direction, and the existing evaluation criteria are based on the thin circular tube assumption. For thick-walled cylinders, the wall thickness is no longer a negligible quantity compared to the radius, and the influence of radial stress caused by the wall thickness must be considered. When copper-nickel alloy materials are used in seawater piping systems, the pipeline diameter is generally small, and the diameter-to-thickness ratio is less than 20, which is classified as a thick-walled pipeline, and the radial stress caused by the wall thickness cannot be ignored.

[0065] When the seawater pipeline is in a stable working state, the working pressure is the internal pressure load in the pipe, and the pipe body will also be affected by the installation load, impact load, and vibration load. The internal pressure is the main load borne by the pipeline, and the effect of other loads on the seawater pipeline system is smaller than the internal pressure. Therefore, the effect of other loads on the seawater pipeline system can be ignored, and the seawater pipeline system in a stable working state can be regarded as a thick-walled cylinder that only bears the internal pressure load. Therefore, the seawater pipeline with corrosion defects is a thick-walled cylinder with non-penetrating metal loss defects under internal pressure. Seawater pipelines are generally long and can be regarded as axially infinitely long pipelines, and the influence of radial stress can be ignored.

[0066] However, the analytical solution of the bursting pressure of thick-walled cylinders under simple loading conditions is still difficult to obtain. The bursting pressure of thick-walled cylinders is between the pressure when the pipe wall material reaches full plasticity and the pressure when it reaches the tensile limit, which is modulated by the material yield strength ratio. The ultimate bearing capacity of thick-walled pipelines commonly used in engineering is p 0C The empirical formula for calculating is as follows:

[0067]

[0068] Where K is the pipe path ratio (ratio of outer diameter to inner diameter); η is the material yield strength ratio, η = σ s / σ b , σ s represents the material yield strength, σ b Indicates the tensile strength of the material.

[0069] Then, the embodiment of the present invention collects experimental data and shows that formula (3) is conservative when evaluating low yield ratio materials and dangerous when evaluating high yield ratio materials; while formula (2) ignores the plasticity and strain hardening of the material and cannot be calculated correctly; and the calculation error band of formula (4) is negative.

[0070] Therefore, the embodiment of the present invention is aimed at the copper-nickel alloy pipeline with groove defects. First, the ultimate bearing capacity p of the existing thick-walled pipeline is measured. 0C The improvements are as follows:

[0071]

[0072] Meanwhile, to reduce the variable symbols in the formula, K in Equation (5) is expressed as a function including the outer diameter D and wall thickness t of the pipeline, and then Equation (5) is expressed as:

[0073]

[0074] In the embodiment of the present invention, the idea of the DNV RP-F101 criterion is followed to deeply analyze each parameter in the model, and a limit bearing capacity model of a copper-nickel alloy pipeline with a groove defect is established. Assuming that the difference between whether the pipeline is thin-walled or thick-walled makes the determination of the failure critical state different, which is reflected in the p 0 calculation formula. Therefore, the p 0 calculated by Equation (6) is used to replace the limit bearing capacity term p 0 of the non-destructive pipeline in the DNV criterion.

[0075] Q reflects the influence of the defect length on the limit bearing capacity of the pipeline. Due to the difference in wall thickness, the influence level of the defect length needs to be corrected. The calculation formula is still defined in the form of the calculation formula in DNV, but the parameters therein are corrected, and the parameters are corrected as follows:

[0076]

[0077] In the formula, L Z represents the pipeline defect length; D represents the pipeline outer diameter; t represents the wall thickness; v G1 represents the first correction coefficient; v G2 represents the second correction coefficient.

[0078] The difference in the corrosion position of the pipeline is improved by the third correction coefficient ν G3 in the R term, and then the R term is expressed as:

[0079]

[0080] In the formula, N represents the corrosion depth ratio; Q represents the length correction coefficient.

[0081] To sum up, the limit bearing capacity model of the copper-nickel alloy pipeline with a groove defect is expressed as:

[0082]

[0083] In step S4, the dynamic explicit analysis uses an explicit method to solve, which is generally used to calculate the instantaneous impact response problem of high-speed structures. Since the explicit method for solving uses a forward-solving method and does not require iterative solution of the tangential stiffness matrix, there is no problem of difficult convergence. Therefore, it can simulate material stress degradation and failure that cannot be completed by other finite element modules, and can also effectively solve the problem of highly nonlinear quasi-static processes. In the embodiment of the present invention, the numerical analysis content belongs to the material failure problem of the quasi-static process. This analysis method uses the central difference method to perform explicit time integration on the motion equation, and derives the dynamic state of the next step based on the structural dynamics conditions of the current increment step. The calculation process is as Figure 1 shown. Figure 1 where t' is the time increment step; M' is the mass matrix; F is the external load; u is the displacement of a certain node; is the velocity of a certain node; is the node acceleration.

[0084] The copper-nickel alloy material will successively go through three stages of elasticity, plastic strengthening, and stiffness degradation under the action of load. Among them, the changes in the elastic and plastic stages are defined by the true stress-strain curve, and the stiffness degradation is expressed by the macroscopic method in damage mechanics. The degradation process needs to clarify ductile damage and the damage evolution law after damage occurs.

[0085] After converting the data in the true stress-strain curve into true stress-strain numerical values, they are input as the plastic parameters of the Abaqus material. Other material properties are summarized in Table 1.

[0086] Table 1 Copper-nickel alloy material parameters

[0087]

[0088] Ductile damage is determined using the equivalent plastic strain at the time of damage initiation, which is a function of the three-dimensional stress and strain rate:

[0089]

[0090] In the formula: λ = -σ p / σ von Mises is the stress triaxiality, σ p is the compressive stress, σ von Mises is the von Mises equivalent stress, is the plastic strain rate, and the criterion for the initiation of damage failure of the metal material is reached when the following formula holds:

[0091]

[0092] In Abaqus / Explicit, the damage evolution law of metallic materials is described by the damage rate h. The value of the equivalent plastic strain at failure depends on the element length. Due to the uncertainty of the element length L, the equivalent plastic strain cannot be used as a material parameter to define the damage evolution law. Therefore, the damage evolution law can be defined by the equivalent plastic displacement or the structural energy dissipation G f . Here, the failure displacement is defined as . When h = 1, the material loses its ability to bear the load, that is, when the product of the equivalent plastic strain of the element and the element length L is greater than the specified value, the element fails.

[0093]

[0094] The mesh is divided into four layers in the wall thickness direction, and the calculation result accuracy meets the requirements and can better save the calculation cost. In the embodiments of the present invention, C3D8R hexahedron meshes are used, and five layers of meshes are divided in the wall thickness direction of the corroded pipeline. During finite element analysis, the effects of brazing, host vibration, and internal fluid on the pipeline strength are ignored, and only the contribution of the internal pressure to the equivalent stress is considered. Full fixed constraints are added at both ends of the pipeline, and a linearly increasing pressure is applied to the inner wall of the pipeline (including the defect location). To exclude the influence of the fixed constraints at both ends on the equivalent pressure, the internal pressure is set to avoid the two ends of the pipeline, as Figure 2 shown.

[0095] Combined with the tensile test data of copper-nickel alloy, local severe plastic deformation will occur at the defect location of the copper-nickel alloy pipe before fracture, and the process has obvious physical and geometric nonlinearities of the pipe material. To visualize the model deformation and fracture process, ductile damage and damage evolution are set to define material failure and delete the failed elements. When the equivalent strain of the elements in the corrosion defect area reaches the fracture strain of the copper-nickel alloy pipeline material, the material is considered to have failed. The Abaqus / Explicit module is used to simulate the quasi-static process to obtain the stress and deformation conditions of a copper-nickel alloy pipeline under internal pressure. For pipelines with different defect types and different defect parameters of the same type of defect, 93 simulations are performed to complete the numerical analysis of 93 pipelines.

[0096] The embodiments of the present invention are developed based on the Abaqus GUI on the Python platform. The interactive language between Abaqus and the GUI interface is the Python language, and Abaqus software pre-sets a script interface for the Python language for users. This interface can complete the pre-processing of numerical analysis, such as three-dimensional modeling and mesh division of the model; post-processing such as accessing calculation results and customizing modules, etc. The existence of this interface makes Abaqus software have significant advantages in processing the modeling process of a large number of similar models and simulating boundary conditions.

[0097] To complete the parametric modeling of the numerical analysis process of the burst of defective copper-nickel alloy pipelines, an Abaqus GUI script is first written on the Python platform.

[97] , and then the script written in this paper is encapsulated into the Abaqus software in the form of a plug-in. When modeling, a simulation model is established by calling the plug-in formed by the Abaqus GUI script. When the model needs to be adjusted, only the defective shape parameter and the maximum internal pressure load value p m need to be modified in the plug-in, and the modification, reconstruction, and analysis of the numerical model can be conveniently realized.

[0098] Based on the programming implementation of the numerical analysis process of the burst of defective copper-nickel alloy pipelines on the Python platform, the work of repeated modeling, meshing, specifying materials, applying boundary conditions, etc. is replaced, and the modeling process is parameterized, modularized, and simplified, avoiding manual operations in the preprocessing process, thus saving the time and energy consumed by repeated operations. At the same time, the modeling time cost is saved, and the efficiency of the preprocessing is improved.

[0099] Study the failure characteristics of the burst process of copper-nickel alloy pipelines with groove defects. The dimensionless corrosion depth ratio N = d / t and the corrosion length ratio are selected as the defective parameters to describe the degree of corrosion. The pipelines with different defective parameters in the finite element are named in the form of numbers, and the ultimate bearing capacity values of copper-nickel alloy pipelines with different numbers are calculated by the explicit finite element method. The corrosion defect sizes of each numbered pipeline are designed as shown in Figure 2. Use the explicit finite element method to simulate 27 times to complete the numerical simulation of the pipelines with different defective parameters in Table 2.

[0100] Table 2 Number table of pipelines with groove defects

[0101]

[0102] Adopt the dynamic explicit finite element method to complete the numerical simulation of 27 pipelines according to the defective parameters designed in Table 2. Taking the G16# pipeline in Table 2 as an example, under the action of the internal pressure of the pipeline, the corrosion area bulges to fracture, and the numerical simulation results are as Figure 3 shown. It can be seen from the simulation diagram that local stress concentration will occur in the corrosion area, and a huge break will be formed in the corrosion area, and the pressure will start to be relieved. During the continuous pressurization process inside the pipeline, there is no obvious bulging at the non-defective position of the pipeline.

[0103] Figure 4 shows the process cloud diagram of the burst of the pipeline numbered G16# under the action of internal pressure. After starting to pressurize, stress changes occur at the defect, manifested as high stress areas on both sides of the defect in the circumferential direction and low stress areas on both sides in the axial direction. As the loading pressure of the pipeline increases, obvious stress concentration appears at the center of the defect, from elastic deformation to entering the plastic flow stage and finally reaching the tensile limit. From Figure 4(d) It can be seen that the stress contour at the center of the defect begins to shrink, indicating that some elements have entered the stress degradation stage at this time, and the stress degradation starts from the central area of the defect. Figure 4 (e) Stress degradation has been completed along the wall thickness direction at the defect. After necking occurs, the pipeline will suddenly burst, showing a huge crack along the axial direction of the pipeline.

[0104] Figure 5 It shows the Von Mises stress and equivalent plastic strain of three points along the wall thickness direction at the center of the defect under the continuous increase of the internal pressure in the pipe, where Figure 5 (b) in Figure 5 is the enlarged view of the black rectangular area in (a) in Figure 5 . In the later stage of plastic flow, the Von Mises stresses of the inner wall point, the middle point, and the outer wall point successively exceed the tensile limit and start stress degradation, and quickly reach the fracture condition. The remaining wall thickness of the G14# pipeline is relatively thick. At the same internal pressure, after the stress degradation of the inner wall point is completed, the element has failed, and the Von Mises stress of the middle point just reaches the damage initiation limit and starts stress degradation. In other words, the inner wall point, the middle point, and the outer wall point fail successively within a very short pressure range, which is manifested as the stress curve and the strain curve concentrated and intertwined in (b) in

[0105] and macroscopically manifested as brittle fracture. The remaining wall thickness of the G20# pipeline is relatively thin. After the element stresses in the pipeline wall thickness direction all exceed the tensile limit and start stress degradation, the equivalent plastic strains of the inner wall point, the middle point, and the outer wall point gradually increase to the fracture standard, so it will show plasticity when fractured. It can be seen that although both are thick-walled copper-nickel alloy pipelines, when the remaining wall thickness is relatively thin, it shows plastic fracture, and when the remaining wall thickness is relatively thick, it shows brittle fracture.

[0105] The stress and deformation laws of copper-nickel alloy pipelines under internal pressure conditions can be described from two aspects: the stress-strain law of a certain point on the pipeline over time and the stress-strain law of a certain path at the same moment.

[0106] Under the action of linearly increasing internal pressure, the pipeline with defects successively goes through four stages: elastic, yield, strengthening, and necking. The deformation of the pipeline experiences elastic deformation, plastic deformation until local plastic instability and stiffness degradation occur, and the pipeline finally fails by fracture. In this section, the pipeline numbered G16# in Table 3 is used as an example to explain the stress-strain change process.

[0107] is taken as an example to explain the stress-strain change process. Figure 6 shows two elements along the wall thickness direction at the defect of the pipeline, Figure 7 describes the changes in the von Mises equivalent stress and equivalent plastic strain of these two elements under the action of the internal pressure in the pipe, Figure 8 describes Figure 7 the pipeline states corresponding to the three state points in the Figure 7Before point a, elastic deformation occurs at the pipeline defect, and the yield point has not been reached yet. Therefore, the equivalent plastic strain is zero. After point a, the elements at the defect enter the plastic deformation stage, the elements start plastic flow, and obvious bulging gradually becomes visible on the pipeline. The equivalent plastic strain gradually increases in a non-linear form. When the equivalent plastic strain at the pipeline defect reaches the critical value, and at the same time the equivalent stress approximately reaches the true tensile strength, local plastic instability occurs, and the pipeline bursts instantly. After bursting, a huge break can be seen, and a severely deformed break that opens outward can be seen at the defect.

[0108] It can be seen from Figure 7 that the von Mises equivalent stress and equivalent plastic strain values of element 2 are both greater than those of element 1 under the same internal pipe pressure, indicating that the stress extends from the inner wall to the outer wall, and there is indeed a stress gradient in the wall thickness direction, but the gradient is small. And the difference is mainly reflected in the plastic deformation area, and element 2 enters the plastic flow area earlier than element 1. After the defect enters the final stage of plastic flow, although the deformation degree of element 2 is slightly greater than that of element 1 and reaches the critical value of equivalent plastic strain earlier, the difference in equivalent stress values is extremely small at this time. This shows that the failure of the two elements of the pipe wall occurs successively within a very small pressure rise range, and the bursting of the pipeline also occurs within this pressure range.

[0109] Figure 9 Figure 9 shows the stress nephograms of two cross-sections after the G16# pipeline is cut along the circumferential and axial directions when the internal pipe pressure is 20.7 MPa. Figure 10 Figure 11 shows the equivalent plastic stress conditions of the elements along two paths, path 1 and path 2, under the action of the internal pipe pressure of the pipeline, where the horizontal coordinate axis is sorted from left to right along the path. It can be seen from Figure 9 and Figure 10 that there are peaks in the middle region of the curve, and there are obvious differences, indicating that the equivalent stress at the defect is much greater than that at the intact part of the pipeline, indicating that the defect is the stress concentration area of the pipeline. The groove defect is a defect with a relatively long axial length. Therefore, the change near the peak is relatively slow on the axial path; the stress changes quickly in the circumferential direction of the pipeline, so the curve is relatively narrow. Near the stress concentration area, there are low stress areas on the inner surfaces on both sides of the circumferential direction and the outer surfaces on both sides of the axial direction. But at the same time, there are high stress areas on the inner surfaces on both sides of the axial direction, and they gradually merge with the defect area, which provides a basis for the fact that the hydrostatic pressure burst test of the pipeline always cracks along the axial direction.

[0110] Research on the law of the influence of groove defect parameters on the ultimate bearing capacity. Through explicit finite element analysis, the numerical simulation results of 27 pipelines in Table 3 are obtained. From these numerical simulation results, the relationship between the corrosion defect depth ratio N and the ultimate bearing capacity can be obtained as Figure 11 shown. The depth ratio of the defect and the ultimate bearing capacity are approximately negatively correlated, and as the corrosion depth increases, the level of the ultimate bearing capacity drops rapidly, indicating that the defect depth has a very large impact on the ultimate bearing capacity.

[0111] The relationship between the axial length ratio Z of the corrosion defect and the ultimate bearing capacity is as Figure 12 shown. The ultimate bearing capacity decreases exponentially with the axial length of the corrosion. The larger the axial length of the corrosion defect, the smaller the ultimate bearing capacity. However, as the axial length of the corrosion defect increases, the change curve of the ultimate bearing capacity tends to be flat, and after exceeding a certain length, the ultimate bearing capacity tends to a stable value, indicating that the influence of the axial length of the corrosion defect on the ultimate bearing capacity of the copper-nickel alloy pipeline gradually weakens as the axial length increases. Comparing the influence of the defect length on the ultimate bearing capacity at different defect depths, it is found that when the defect depth is relatively shallow, the influence of the defect length on the ultimate bearing capacity is limited, but when the defect depth is relatively deep, the influence level of the defect length on the ultimate bearing capacity is greatly improved, indicating that the influence of the defect length on the ultimate bearing capacity gradually strengthens with the increase of the defect depth.

[0112] Thus, it can be seen that the size of the defect depth is the control factor determining the ultimate bearing capacity of the pipeline, and the influence of the defect length on the ultimate bearing capacity is mainly manifested when the defect is relatively deep and the defect length is relatively short.

[0113] The results of the numerical simulation of the present invention are calculated based on the true stress-strain curve. Therefore, it can be considered that the calculation results of the numerical simulation are reliable and high-precision results. Using the finite element results to correct the above ultimate bearing capacity model, a high-precision correction model can be obtained.

[0114] Using the finite element method to calculate the results of the ultimate bearing capacity of the 27 numbered pipelines in Table 2, using the parameters ν G1 , ν G2 , ν G3 to approximate the value of formula (9) to the numerical simulation results. Taking the sum of the squares of the differences e(r) between the 27 groups of finite element calculation results obtained according to the simulation in Table 3 and formula (9) as the optimization objective:

[0115]

[0116] In the above formula, p FEA is the finite element calculation result.

[0117] Using the least squares method, with v G =[ν G1 , ν G2 , ν G3 T as the design variable, when the value of e(r) is the smallest, it is considered that the fitting effect of this group of design variables is the best. The optimal design variable can be obtained as v G =[0.5, 0.86, 1.1466] T , and based on this, the corrected evaluation formula can be written

[0118] ​

[0119] The samples used in the formula fitting contain data under various depth ratios, so it has good adaptability. Compare the calculation results of the DNV formula, the modified formula, and the finite element method, as Figure 13 shown. It can be seen from the figure that based on the numerical simulation results, the data of each point in the calculation results of the DNV formula are lower than the numerical simulation results, and the evaluation is on the conservative side; the modified formula and the numerical simulation results are in good agreement.

[0120] In this embodiment, the bursting of the seawater pipeline is used as the limit state, so the ultimate bearing capacity is the bursting pressure of the seawater pipeline. The present invention selects 3 seawater pipelines with groove defects, applies this model to the calculation of the ultimate bearing capacity of the actual pipeline, and gives a specific calculation example.

[0121] Groove-type defects are common in the case of long-term water accumulation or sediment accumulation in the pipeline. Under the action of gravity, the lower half of the pipeline covered by dirt is severely corroded, and axial long strip-shaped corrosion pits appear. This example details the process of calculating the bursting pressure using formula (14).

[0122] Step 1: Determine the defect length L Z of the seawater pipeline, the defect depth d, the outer diameter D of the pipe, the inner diameter D i of the pipe, and the wall thickness t of the pipeline. The test data are shown in Table 3:

[0123] Table 3 Seawater pipeline size data

[0124]

[0125] Step 2: Obtain the pipe material parameters. The raw materials for obtaining these material parameters come from the bursting test pipeline, and the data are shown in Table 4:

[0126] Table 4 Measured values of the tensile test of the seawater pipeline

[0127]

[0128] Step 3: Take the No. 1 pipeline as an example to calculate the bursting pressure. The calculation unit is based on the International System of Units SI (mm).

[0129] Yield strength ratio:

[0130] η = σ s / σ b = 141 / 341 = 0.4135 (15)

[0131] Corresponding bursting pressure value of the intact pipeline:

[0132]

[0133] Length correction coefficient Q:

[0134]

[0135] Corrosion depth ratio N:

[0136]

[0137] Then the calculated result of the bursting pressure of the pipeline with groove defect 1# is:

[0138]

[0139] Similarly, the calculated bursting pressures of pipelines 2# and 3# are 35.447 MPa and 22.097 MPa respectively.

[0140] The technical features of the above embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features in the above embodiments are described. Only the preferred embodiments of the present invention are expressed. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent of the present invention. As long as the combinations of these technical features do not conflict, they should be considered as the scope recorded in this specification.

[0141] It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent of the present invention should be subject to the appended claims.

Claims

1. A method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects, characterized in that: The following steps are involved: Step S1: The pipe path ratio K is expressed by the outer diameter D and the wall thickness t of the pipe to obtain the ultimate bearing capacity of the non-destructive pipe, so as to consider the influence of the wall thickness on the critical state of failure; Step S2: Based on the influence of wall thickness on the length correction factor Q, the first correction factor v G1 and the second correction factor v G2 The length correction coefficient Q is corrected; based on the influence of the difference in corrosion position on the defect influencing factor term R, the third correction coefficient v G3 Correcting the defect influence factor item R; Step S3: constructing an ultimate bearing capacity model based on the ultimate bearing capacity of the non-destructive pipeline, the length correction coefficient Q and the defect influence factor term R; Step S4: solving the ultimate bearing capacity model by numerical simulation analysis and using a dynamic explicit finite element method to obtain a revised ultimate bearing capacity model.

2. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 1, characterized in that: In step S1, the expression of the ultimate bearing capacity p0 of the non-destructive pipeline is: In the formula, η represents the material's yield strength ratio; σb represents the material's tensile strength; t represents the wall thickness, and D represents the outer diameter of the pipe.

3. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 1, characterized in that: In step S2, the expression of the length correction coefficient Q is: Where, L Z It represents the length of pipeline defect; D represents the outer diameter of the pipeline; t represents the wall thickness.

4. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 1, characterized in that: In step S2, the expression of the defect impact factor term R is: Where N represents the corrosion depth ratio; Q represents the length correction factor.

5. The method for constructing the ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to any one of claims 1 to 4, characterized in that: In step S3, the expression p of the ultimate bearing capacity model is CG for: In the formula, p0 represents the ultimate bearing capacity of the undamaged pipeline; R represents the defect influencing factor; η represents the material yield strength ratio; σb represents the material tensile strength; t represents the wall thickness, D represents the outer diameter of the pipeline; N represents the corrosion depth ratio; and Q represents the length correction coefficient.

6. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 1, characterized in that: In step S4, when performing numerical simulation analysis, hexahedral meshes are used for division, and five layers of meshes are divided in the direction of the wall thickness of the corroded pipeline.

7. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 1, characterized in that: In step S4, when performing numerical simulation analysis and parametric modeling, firstly, an Abaqus GUI script is written on a Python platform; the script is encapsulated into the Abaqus software in the form of a plug-in; when modeling, a simulation model is established by calling the plug-in formed by the Abaqus GUI script; and the defect shape parameters and the maximum internal pressure load value are modified to achieve model adjustment.

8. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 1, characterized in that: Step S4 includes: Step S41: using explicit finite element method to calculate the simulation result of the ultimate bearing capacity of the pipeline; Step S42: taking the sum of squares of the differences between the simulation results and the calculation results of the ultimate bearing capacity model, e(r), as the optimization target; Step S43: Using the least squares method, v G =[ν G1 ,ν G2 ,ν G3 ] T The design variables are solved to obtain the modified ultimate bearing capacity model.

9. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 8, characterized in that: The expression of the optimization objective e(r) is: In the formula, p FEA represents the simulation results, p CG Represents the calculation results of the ultimate bearing capacity model.

10. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 1, characterized in that: In step S4, the modified expression of the ultimate bearing capacity model is: In the formula, p0 represents the ultimate bearing capacity of the undamaged pipeline; R represents the defect influencing factor; η represents the material yield strength ratio; σb represents the material tensile strength; t represents the wall thickness, D represents the outer diameter of the pipeline; N represents the corrosion depth ratio; Q represents the length correction coefficient; L Z Indicates the length of pipeline defects.

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