Construction method of ultimate bearing capacity model for copper-nickel alloy pipelines with groove defects

Abstract

The present invention provides a method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects. The method uses the outer diameter D and the wall thickness t of the pipeline to represent the pipe path ratio K, and obtains the ultimate bearing capacity of the lossless pipeline to consider the influence of the wall thickness on the critical state of failure. Based on the influence of the wall thickness on the length correction coefficient Q, the first correction coefficient v is used to calculate the maximum bearing capacity of the pipeline. 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 influence factor term R, the third correction coefficient v G3 The defect influence factor term R is corrected; an ultimate bearing capacity model is constructed based on the ultimate bearing capacity of the lossless pipeline, the length correction coefficient Q and the defect influence factor term R; and the ultimate bearing capacity model is solved by numerical simulation analysis using a dynamic explicit finite element method to obtain a corrected ultimate bearing capacity model.

Description

Technical Field

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

[0002] The assessment of corrosion-prone piping strength has long been a hot topic of research. With age, seawater erosion causes defects in the form of metal loss on the inner surfaces of seawater pipelines. These defects reduce the pipeline's load-bearing capacity, severely impacting the reliability and safety of equipment. Failure analysis by British offshore engineering operator BRITOIL revealed that 33% of all ship equipment failures were caused by corrosion. Corrosion in seawater piping can range from mild to severe, causing leaks and potentially threatening navigation safety. The rupture of engine room seawater piping is caused by a sudden increase in internal pressure, which in areas containing corrosion, cracks, and pinholes exceeds the pipeline's load-bearing capacity, leading to rupture.

[0003] While domestic and international researchers have conducted extensive research on the ultimate load-bearing capacity of pipelines, these efforts have focused on thin-walled pipeline steel. Some researchers have conducted preliminary research on the evaluation of seawater pipelines containing corrosion defects, but the results have been subject to significant errors. Existing standards are based on thin-walled pipelines and, to ensure universality, do not account for variations in defect shape. Consequently, the error between evaluation results and test results is generally large, remaining around 20%.

[0004] Existing standards are based on thin-walled oil and gas pipelines. The extent to which wall thickness affects pipeline strength, and how this impact is measured, are still under investigation. The selection of failure modes for thick-walled pipelines, the modification of existing prediction models, and the development of a safe and reliable evaluation method for copper-nickel alloy pipelines containing corrosion defects are all areas of ongoing research.

[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 containing groove defects, so as to solve the technical problem that the existing ultimate bearing capacity prediction model has low accuracy when submarine copper-nickel alloy pipelines contain groove defects.

[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 containing groove defects, comprising the following steps:

[0008] Step S1: The pipe path ratio K is expressed using the pipe outer diameter D and the wall thickness t to obtain the ultimate bearing capacity of the non-destructive pipe, taking into account the influence of the wall thickness on the critical failure state;

[0009] Step S2: Based on the influence of wall thickness on the length correction coefficient Q, the first correction coefficient 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 influence factor term R, the third correction coefficient v G3 Correcting the defect impact factor item R;

[0010] 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;

[0011] Step S4: solving the ultimate bearing capacity model by numerical simulation analysis using a dynamic explicit finite element method to obtain a revised ultimate bearing capacity model.

[0012] Preferably, in step S1, the expression of the ultimate bearing capacity p0 of the non-destructive pipeline is:

[0013]

[0014] Where, η represents the material yield strength ratio; σ b Indicates the tensile strength of the material; t indicates the wall thickness, and D indicates the outer diameter of the pipe.

[0015] Preferably, in step S2, the length correction coefficient Q is expressed as:

[0016]

[0017] Where, L Z It represents the length of 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 impact factor term R is:

[0019]

[0020] Where N is the corrosion depth ratio; Q is the length correction factor.

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

[0022]

[0023] Where p0 represents the ultimate bearing capacity of the intact pipeline; R represents the defect influence factor; η represents the material yield strength ratio; σ b represents the tensile strength of the material; t represents the wall thickness; D represents the outer diameter of the pipe; N represents the corrosion depth ratio; and Q represents the length correction factor.

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

[0025] Preferably, in step S4, when performing numerical simulation analysis and 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; when modeling, establish a simulation model by calling the plug-in formed by the Abaqus GUI script; modify the defect shape parameters and the maximum internal pressure load value to achieve model adjustment.

[0026] Preferably, step S4 includes:

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

[0028] 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;

[0029] Step S43: Using the least square method, v G =[ν G1 ,ν G2 ,ν G3 ] T The design variables are solved to obtain the revised ultimate bearing capacity model.

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

[0031]

[0032] Where p FEA represents the simulation results, p CG Represents the calculation results of the ultimate bearing capacity model.

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

[0034]

[0035] Where p0 represents the ultimate bearing capacity of the intact pipeline; R represents the defect influence factor; η represents the material yield strength ratio; σ b Indicates the tensile strength of the material; t indicates the wall thickness, D indicates the outer diameter of the pipe; N indicates the corrosion depth ratio; Q indicates the length correction factor; L Z Indicates the length of pipeline defects.

[0036] The beneficial effects of the present invention include at least: based on the established ultimate bearing capacity evaluation and prediction model of copper-nickel alloy pipelines containing groove defects, the present invention considers the influence of wall thickness on the critical state of failure, adopts the pipeline outer diameter D and wall thickness t to represent the pipe path ratio K, and corrects the ultimate bearing capacity of the non-destructive pipeline; in the case of correction of the ultimate bearing capacity of the non-destructive pipeline, the influence of wall thickness and corrosion position is considered at the same time, and the length correction coefficient and defect influence factor terms are corrected; then, a numerical calculation model close to the actual seawater pipeline characteristics is established based on the finite element method of explicit dynamics, and the burst failure characteristics of the pipeline under the action of internal pressure of the pipe are analyzed according to the numerical simulation results, and the influence of various pipeline parameters on the ultimate bearing capacity is studied. The ultimate bearing capacity evaluation and prediction model of copper-nickel alloy pipelines containing groove defects is corrected, and the calculation results of the obtained ultimate bearing capacity model are more accurate and effective for the calculation of the ultimate bearing capacity of copper-nickel alloy pipelines containing groove defects on the seabed. 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 pressure loading range;

[0039] Figure 3 This is a schematic diagram of the burst morphology of a groove defect pipeline;

[0040] Figure 4 This is the equivalent stress cloud diagram of G16# pipeline under different internal pressures;

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

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

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

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

[0045] Figure 9 This is the stress cloud diagram of G16# pipeline at 20.7MPa;

[0046] Figure 10 is a schematic diagram of the von Mises stress of the 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 defect length and ultimate bearing capacity;

[0049] Figure 13 Schematic diagram of the correction effect of the correction formula under different defect lengths of groove defects. DETAILED DESCRIPTION

[0050] The following is a clear and complete description of the technical solutions in the embodiments of the present invention, in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts are within the scope of protection of the present invention.

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

[0052] Step S1: The pipe path ratio K is expressed using the pipe outer diameter D and the wall thickness t to obtain the ultimate bearing capacity of the non-destructive pipe, taking into account the influence of the wall thickness on the critical failure state;

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

[0054] 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;

[0055] Step S4: Through numerical simulation analysis, the ultimate bearing capacity model is solved using the dynamic explicit finite element method to obtain a revised ultimate bearing capacity model.

[0056] The embodiments of the present invention are described in detail below:

[0057] Complex ocean conditions lead to corrosion in seawater pipelines, resulting in irregular shapes. Corrosion typically results in only metal loss defects, typically in the form of pits. For single external corrosion defects in oil and gas pipelines, four regular shapes—spherical, flat-bottomed, grooved, and locally uniform—were used to simulate actual corrosion defects. Strength evaluation requires defect parameterization and measurable geometric dimensions. Therefore, the actual corrosion defect should first be simplified into a geometric model.

[0058] Except for overall uniform corrosion, all other corrosion in seawater pipelines is local defects. Overall uniform corrosion can be regarded as uniform thinning of the pipeline wall thickness. When calculating strength, the thickness after thinning is used as a parameter, and it can be calculated as a non-destructive pipeline. The embodiment 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 with 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 effects of multiple corrosions cause defects in the pipe. Under electrochemical corrosion, scouring corrosion caused by muddy and sandy fluids is the most corrosive and harmful type of composite corrosion. The electrochemical corrosion process is the reaction of copper elements in Cl - The process by which electrons are lost under the action of seawater, resulting in a hydrolysis reaction and conversion into a copper oxide corrosion product film. The seawater scouring process is a mass transfer process on the solid surface caused by the mechanical action of seawater, mud, sand and sand impacting the material. The corrosion product film produced by copper-nickel alloys during the electrochemical process has a significant impact on the corrosion rate. Other factors change the corrosion rate of the material by affecting the protectiveness of the corrosion product film.

[0060] The ultimate bearing capacity of seawater pipelines is bursting or through-wall leakage. Therefore, the embodiments of the present invention use the bursting pressure of seawater pipelines as the ultimate bearing capacity. Based on this ultimate bearing capacity, a comprehensive analysis shows that elastic failure and plastic limit failure are too conservative, failing to account for the plasticity of the pipe and the effects of pipe wall thickness. Damage evolution failure is uncertain, making the assessment too risky. The plastic failure criterion is neither overly conservative nor overly risky. Choosing a criterion based on plastic failure as the basis for evaluating the bursting pressure of seawater pipelines is the most appropriate and practical choice.

[0061] Calculate the burst pressure of intact thick-walled pipelines. Pipeline internal pressure loads are the most critical operating loads for seawater pipelines. Determining the ultimate internal pressure load of intact pipelines is of great engineering significance in the design and condition assessment of seawater piping systems. Existing standards primarily target oil and gas pipelines, and the pipeline models are simplified to thin-walled models, without considering the impact of wall thickness.

[0062] There are many definitions for the classification of thin-walled pipes and thick-walled pipes. This paper is based on the description in "Mechanics of Materials I":

[0063]

[0064] It's generally assumed that stress in thin circular tubes is uniform across the wall thickness, and existing evaluation criteria are based on this assumption. For thick-walled cylinders, the wall thickness is no longer a negligible quantity compared to the radius, and the effect of radial stress caused by the wall thickness must be considered. When copper-nickel alloys are used in seawater piping systems, the pipe diameters are generally small, with a diameter-to-thickness ratio of less than 20, classifying them as thick-walled pipes. Therefore, the radial stress caused by the wall thickness cannot be ignored.

[0065] When a seawater pipeline is operating in a stable state, the operating pressure is the internal pressure load within the pipe. The pipe body is also subject to installation loads, impact loads, and vibration loads. Internal pressure is the primary load on the pipeline, while the effects of other loads on the seawater piping system are relatively small compared to internal pressure. Therefore, the effects of these other loads on the seawater piping system can be neglected, and the seawater piping system in a stable state can be considered as a thick-walled cylinder subjected only to internal pressure. Therefore, a seawater pipeline with corrosion defects is considered a thick-walled cylinder subjected to internal pressure and containing non-penetrating metal loss defects. Seawater pipelines are generally long and can be considered as axially infinite, with the effects of radial stress negligible.

[0066] However, the analytical solution for 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 at which the pipe wall material reaches full plasticity and the pressure at which it reaches the tensile limit, and is modulated by the material's yield strength ratio. The ultimate bearing capacity of thick-walled pipes commonly used in engineering is p 0C The empirical formula for calculation 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 yield strength of the material, 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; formula (2) ignores the plasticity and strain hardening of the material and cannot be calculated correctly; formula (4) has a negative calculation error band.

[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] At the same time, in order to reduce the variable symbols in the formula, K in formula (5) is expressed as a function including the pipe outer diameter D and the wall thickness t, then formula (5) is expressed as:

[0073]

[0074] In this embodiment of the present invention, the principles of the DNV RP-F101 standard are followed to thoroughly analyze the various parameters in the model and establish an ultimate bearing capacity model for copper-nickel alloy pipelines containing groove defects. Assuming that the difference between thin-walled and thick-walled pipelines leads to different determinations of the critical failure state, which is reflected in the p0 calculation formula, the p0 calculated by formula (6) replaces the ultimate bearing capacity term p0 of the intact pipeline in the DNV standard.

[0075] Q reflects the effect of defect length on the ultimate bearing capacity of the pipeline. Due to the difference in wall thickness, the effect of defect length needs to be corrected. The calculation formula is still defined as the calculation formula in DNV, but the parameters are corrected.

[0076]

[0077] Where, L Z represents the length of pipeline defects; D represents the outer diameter of the pipeline; t represents the wall thickness; v G1 represents the first correction coefficient; v G2 Indicates the second correction coefficient.

[0078] The difference in the location of the corrosion in the pipeline is corrected by the third correction factor ν in the R term. G3 Improved, the R term is expressed as:

[0079]

[0080] Where N is the corrosion depth ratio; Q is the length correction factor.

[0081] In summary, the ultimate bearing capacity model of copper-nickel alloy pipelines with groove defects is expressed as:

[0082]

[0083] In step S4, the explicit analysis of dynamics is solved using an explicit method, which is generally used to calculate the transient impact response problem of high-speed structures. Because the explicit method adopts a forward solution method, it does not need to iteratively solve the tangential stiffness matrix, so there is no problem of difficult convergence. Therefore, it can simulate material stress degradation and failure that other finite element modules cannot complete, and can also effectively solve highly nonlinear quasi-static process problems. 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 next dynamic state based on the structural dynamic conditions of the current incremental step. The calculation process is as follows. Figure 1 shown. Figure 1 Where t' is the time increment; M' is the mass matrix; F is the external load; u is the displacement of a node; is the speed of a certain node; is the node acceleration.

[0084] Under load, copper-nickel alloys undergo three stages of transition: elasticity, plastic strengthening, and stiffness degradation. The changes within the elastic and plastic stages are defined by true stress-strain curves, while stiffness degradation is described using macroscopic methods from damage mechanics. The degradation process requires a clear understanding of ductile damage and the damage evolution patterns after it occurs.

[0085] The data in the true stress-strain curve are converted into true stress-strain values ​​and then input as the Abaqus material plasticity parameters. Other material properties are summarized in Table 1.

[0086] Table 1 Copper-nickel alloy material parameters

[0087]

[0088] Ductile damage uses the equivalent plastic strain at the time of damage failure To confirm, is a function of three-dimensional stress and strain rate:

[0089]

[0090] Where: 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. When the following equation holds true, the criterion for damage and failure of metal materials is met:

[0091]

[0092] In Abaqus / Explicit, the damage rate h is used to describe the damage evolution of metal materials. The value of depends on the unit length. Due to the uncertainty of the unit 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 expressed by the equivalent plastic displacement Or structural energy dissipation G f Here we define the failure displacement When h = 1, the material loses its ability to bear the load, that is, the unit equivalent plastic strain The unit fails when the unit length L is greater than the specified value.

[0093]

[0094] The grid is divided into four layers in the direction of wall thickness, and the accuracy of the calculation results meets the requirements and can save the calculation cost. In the embodiment of the present invention, the C3D8R hexahedral grid is used to divide the grid into five layers in the direction of the wall thickness of the corroded pipeline. During the finite element analysis, the effects of brazing, main engine vibration and fluid in the pipe on the strength of the pipeline are ignored, and only the contribution of the pressure in the pipe to the equivalent stress is considered. Fully 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 defects). In order to eliminate the influence of the fixed constraints at both ends on the equivalent pressure, avoid the two ends of the pipeline when setting the internal pressure, such as Figure 2 shown.

[0095] Combined with copper-nickel alloy tensile test data, copper-nickel alloy pipes will experience localized strong plastic deformation at the defect before fracture, and the process has obvious physical and geometric nonlinearity of the pipe material. To visualize the deformation and fracture process of the model, ductile damage and damage evolution are set to define material failure and delete failed units. Material failure is considered when the equivalent strain of the unit in the corrosion defect area reaches the fracture strain of the copper-nickel alloy pipe material. The Abaqus / Explicit module is used to simulate the quasi-static process and obtain the stress and deformation of a copper-nickel alloy pipe under internal pressure. For pipes with different defect types and similar defects but different defect parameters, 93 simulations were performed to complete the numerical analysis of 93 pipes.

[0096] The embodiment of this invention is developed based on the Abaqus GUI on the Python platform. The interactive language between Abaqus and the GUI is Python, and Abaqus software provides a pre-configured Python scripting interface for users. This interface enables pre-processing of numerical analysis, such as 3D modeling and meshing, as well as post-processing, such as accessing calculation results and custom modules. This interface gives Abaqus software a significant advantage in handling large numbers of similar model building processes and boundary condition simulations.

[0097] To complete the parametric modeling of the numerical analysis process of the burst of defective copper-nickel alloy pipelines, we first wrote an Abaqus GUI script on the Python platform. Then, we packaged the script written in this article into the Abaqus software as a plug-in. When modeling, we built the simulation model by calling the plug-in generated by the Abaqus GUI script. When the model needs to be adjusted, we only need to modify the defect shape parameters and the maximum internal pressure load value p in the plug-in. m , the modification, reconstruction and analysis of the numerical model can be easily realized.

[0098] The Python-based implementation of the numerical analysis process for bursting defective copper-nickel alloy pipes replaces the repetitive modeling, meshing, material assignment, and boundary condition application tasks. This parameterized, modularized, and simplified modeling process eliminates manual pre-processing operations, saving the time and effort associated with repetitive operations. This also reduces modeling time and improves pre-processing efficiency.

[0099] The failure characteristics of the bursting process of copper-nickel alloy pipes with groove defects were studied. The dimensionless corrosion depth ratio N = d / t and the corrosion length ratio were selected to describe the defect parameters. To describe the degree of corrosion, pipelines with different defect parameters in the finite element method were numbered. The ultimate bearing capacity of copper-nickel alloy pipelines with different numbers was calculated using the explicit finite element method. The corrosion defect dimensions for each numbered pipeline were designed as shown in Figure 2. The explicit finite element method was used for 27 simulations to complete the numerical simulation of the pipelines with different defect parameters listed in Table 2.

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

[0101]

[0102] The dynamic explicit finite element method was used to complete the numerical simulation of 27 pipelines according to the defect 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 and then breaks. The numerical simulation results are shown in Figure 2. Figure 3 As shown in the simulation diagram, local stress concentration will occur in the corroded area, forming a large rupture in the corroded area and starting to release pressure. During the continuous pressurization process inside the pipeline, there is no obvious bulging in the non-defective parts of the pipeline.

[0103] Figure 4 The cloud diagram shows the process of the pipe numbered G16# bursting under the action of internal pressure. After the pressure was applied, stress changes occurred at the defect, showing high stress areas on both sides of the defect in the circumferential direction and low stress areas on both sides of the axial direction. As the pipeline loading pressure increases, the center of the defect shows obvious stress concentration, from elastic deformation to plastic flow stage and finally reaching the tensile limit. Figure 4(d) It can be seen that the stress cloud diagram at the center of the defect begins to shrink, indicating that some units have entered the stress degradation stage and the stress degradation starts from the most central area of ​​the defect. Figure 4 (e) The stress degradation along the wall thickness direction has been completed at the defect. After necking occurs, the pipeline will suddenly burst, and a huge crack will appear along the axial direction of the pipeline.

[0104] Figure 5 The Von Mises stress and equivalent plastic strain at three points along the wall thickness at the defect center are shown as the internal pressure of the pipe continues to increase. Figure 5 (b) in Figure 5 The enlarged image of the black rectangular area in (a) is shown. In the late stage of plastic flow, the Von Mises stress of the inner wall point, the middle point and the outer wall point successively exceed the tensile limit and begin stress degradation and quickly reach the fracture condition. The residual wall thickness of the G14# pipeline is relatively thick. At the same internal pressure, the unit has failed after the stress degradation of the inner wall point ends, and the Von Mises stress of the middle point reaches the damage initiation limit and begins stress degradation. In other words, the inner wall point, the middle point and the outer wall point fail successively in a very short pressure range. Figure 5 Figure (b) shows a concentrated interweaving of stress and strain curves, macroscopically indicating brittle fracture. The G20# pipeline has a relatively thin residual wall thickness. After the unit stresses along the wall thickness exceed the tensile limit and begin to degrade, the equivalent plastic strains at the inner, middle, and outer wall points gradually increase to the fracture standard, resulting in plastic fracture. This indicates that although both are thick-walled copper-nickel alloy pipelines, plastic fracture occurs when the residual wall thickness is thin, while brittle fracture occurs when the residual wall thickness is thick.

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

[0106] Under the action of linearly increasing internal pressure, the defective pipeline goes through four stages: elasticity, yielding, strengthening, and necking. The deformation of the pipeline goes through elastic deformation, plastic deformation, and finally plastic instability and stiffness degradation occur locally. The pipeline eventually breaks and fails. This section uses the pipeline numbered G16# in Table 3 as the

[0107] The stress-strain change process is explained using the pipeline as an example. Figure 6 It shows two units along the wall thickness direction at the pipeline defect. Figure 7 The changes of von Mises equivalent stress and equivalent plastic strain of these two elements under the action of pressure in the pipe are described. Figure 8 Described Figure 7 The three state points in the curve correspond to the pipeline state. Figure 7Before point a, the pipeline defect undergoes elastic deformation but has not yet reached the yield point, so the equivalent plastic strain is zero. After point a, the unit at the defect enters the plastic deformation stage and begins plastic flow. The pipeline gradually bulges noticeably, and the equivalent plastic strain gradually increases nonlinearly. When the equivalent plastic strain at the pipeline defect reaches a critical value and the equivalent stress approaches the true tensile strength, local plastic instability occurs, and the pipeline instantly explodes. A huge rupture is visible after the explosion, and a severely deformed rupture is visible at the defect.

[0108] Depend on Figure 7 It can be seen that the von Mises equivalent stress and equivalent plastic strain values ​​of element 2 are greater than those of element 1 at the same internal pipe pressure, indicating that stress extends from the inner wall to the outer wall. A stress gradient does exist in the wall thickness direction, but the gradient is small. The difference is mainly reflected in the plastic deformation region, with element 2 entering the plastic flow region earlier than element 1. After the defect enters the late stage of plastic flow, although element 2 deforms slightly more than element 1 and reaches the critical equivalent plastic strain value earlier, the difference in equivalent stress values ​​at this time is extremely small. This indicates that the failure of the two elements in the pipe wall occurs sequentially within a very small pressure rise range, and the pipeline burst also occurs within this pressure range.

[0109] Figure 9 The stress cloud diagrams of the two sections of the G16# pipeline after it is cut along the circumferential and axial directions when the internal pressure of the pipeline is 20.7MPa are shown. Figure 10 The equivalent plastic stress of the elements along path 1 and path 2 under the action of the internal pressure of the pipeline is shown, where the horizontal axis of the coordinate is sorted from left to right along the path. Figure 9 and Figure 10 As can be seen from the curve, there is a peak in the middle region, a clear difference. This indicates that the equivalent stress at the defect is much greater than that at the intact pipe, indicating that the defect is a stress concentration area in the pipeline. Groove defects are axially longer defects, so the stress changes slowly near the peak along the axial path; the stress changes more rapidly in the circumferential direction of the pipeline, resulting in a narrower curve. Near the stress concentration area, low-stress areas exist on both the circumferential inner surface and the axial outer surface. However, high-stress areas exist on both axial inner surfaces, gradually merging with the defect. This provides a basis for the fact that cracking in pipeline hydrostatic burst tests always occurs along the axial direction.

[0110] Study on the influence of groove defect parameters on ultimate bearing capacity. Through explicit finite element analysis, the numerical simulation results of 27 pipes in Table 3 were obtained. From the numerical simulation results, the relationship between the corrosion defect depth ratio N and the ultimate bearing capacity can be obtained as follows: Figure 11 The depth ratio of the defect and the ultimate bearing capacity are approximately negatively correlated, and as the corrosion depth increases, the ultimate bearing capacity level decreases rapidly, indicating that the defect depth has a great influence 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 follows: Figure 12 As shown. The ultimate bearing capacity decreases exponentially with the axial length of the corrosion. The greater the axial length of the corrosion defect, the smaller the ultimate bearing capacity. However, as the axial length of the corrosion defect increases, the ultimate bearing capacity curve tends to be flat, and after exceeding a certain length, the ultimate bearing capacity tends to a stable value, indicating that the effect of the axial length of the corrosion defect on the ultimate bearing capacity of the copper-nickel alloy pipeline gradually weakens with the increase of the axial length. By comparing the effect of the defect length on the ultimate bearing capacity at different defect depths, it is found that when the defect depth is shallow, the defect length has limited influence on the ultimate bearing capacity, but when the defect depth is deep, the level of influence of the defect length on the ultimate bearing capacity is greatly improved, indicating that the effect of the defect length on the ultimate bearing capacity gradually increases with the increase of the defect depth.

[0112] It can be seen that the depth of the defect is the controlling factor that determines 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 deeper and the defect length is shorter.

[0113] The numerical simulation results of the present invention are calculated based on the true stress-strain curve, so the numerical simulation results can be considered to be reliable and highly accurate. Using the finite element results to correct the above-mentioned ultimate bearing capacity model can obtain a highly accurate corrected model.

[0114] The finite element method is used to calculate the ultimate bearing capacity of the 27 numbered pipelines in Table 2, using the parameter ν G1 ,ν G2 ,ν G3 The value of formula (9) is approached to the numerical simulation results. The sum of squares of the differences between the 27 sets of finite element calculation results obtained by simulation according to Table 3 and formula (9) is used as the optimization target:

[0115]

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

[0117] Using the least squares method, v G =[ν G1 ,ν G2 ,ν G3 ] T As the design variable, when the e(r) value is the smallest, it is determined that this set of design variables has the best fitting effect. The optimal design variable is v G =[0.5,0.86,1.1466] T , based on which the revised evaluation formula can be written

[0118]

[0119] The samples used in fitting this formula include data under various depth ratios, so it has good adaptability. The calculation results of DNV formula, modified formula and finite element method are compared, as shown in the following figure: Figure 13 As shown in the figure, based on the numerical simulation results, the data of each point calculated by the DNV formula is lower than the numerical simulation results, and the evaluation is conservative; the revised formula is in good agreement with the numerical simulation results.

[0120] In this example, bursting is the ultimate limit state for the seawater pipeline, so the ultimate bearing capacity is the bursting pressure of the seawater pipeline. This paper selects three seawater pipelines containing groove defects and applies this model to calculate the ultimate bearing capacity of actual pipelines, providing specific calculation examples.

[0121] Grooved defects are common when water or debris accumulates in pipelines for long periods of time. Under the influence of gravity, the lower portion of the pipeline, covered by the debris, corrodes severely, resulting in long, axially shaped pits. This example details the process of calculating the burst pressure using equation (14).

[0122] Step 1: Determine the defect length L of the seawater pipeline Z , defect depth d, pipe outer diameter D, inner diameter D i , pipe wall thickness t, 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 the material parameters come from the burst test pipeline. The data are shown in Table 4:

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

[0127]

[0128] Step 3: Take pipeline 1# as an example to calculate the bursting pressure. The calculation unit shall be based on the International System of Units SI (mm).

[0129] Yield-to-strength ratio:

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

[0131] Corresponding intact pipeline bursting pressure value:

[0132]

[0133] Length correction factor Q:

[0134]

[0135] Corrosion depth ratio N:

[0136]

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

[0138]

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

[0140] The technical features of the above embodiments may be combined in any manner. To simplify the description, not all possible combinations of the technical features in the above embodiments are described. Only preferred embodiments of the present invention are presented. While the description is relatively specific and detailed, it should not be construed as limiting the scope of the present invention. As long as there are no contradictions in the combination of these technical features, they should be considered to be within the scope of this specification.

[0141] It should be noted that those skilled in the art may make various modifications and improvements without departing from the scope of the present invention, and these modifications and improvements fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be based on the appended claims.

Claims

1. A method for constructing an ultimate bearing capacity model for a copper-nickel alloy pipeline containing groove defects, characterized by: The following steps are involved: Step S1: The pipe path ratio K is expressed using the pipe outer diameter D and the wall thickness t to obtain the ultimate bearing capacity of the non-destructive pipe, taking into account the influence of the wall thickness on the critical failure state; Step S2: Based on the influence of wall thickness on the length correction coefficient Q, the first correction coefficient 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 influence factor term R, the third correction coefficient v G3 Correcting the defect impact 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 using a dynamic explicit finite element method to obtain a revised ultimate bearing capacity model; In step S1, the expression of the ultimate bearing capacity p0 of the non-destructive pipeline is: Where, η represents the material yield strength ratio; σ b Indicates the tensile strength of the material; t indicates the wall thickness, and D indicates the outer diameter of the pipe; In step S3, the expression p of the ultimate bearing capacity model is CG for: Where p0 represents the ultimate bearing capacity of the undamaged pipeline; R represents the defect influence factor; η represents the material yield strength ratio; σ b Indicates the tensile strength of the material; t indicates the wall thickness, D indicates the outer diameter of the pipe; N indicates the corrosion depth ratio; Q indicates the length correction factor; In step S4, the modified expression of the ultimate bearing capacity model is: Where p0 represents the ultimate bearing capacity of the undamaged pipeline; R represents the defect influence factor; η represents the material yield strength ratio; σ b Indicates the tensile strength of the material; t indicates the wall thickness, D indicates the outer diameter of the pipe; N indicates the corrosion depth ratio; Q indicates the length correction factor; L Z Indicates the length of pipeline defects.

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 S2, the length correction coefficient Q is expressed as: Where, L Z represents the length of pipeline defects; D represents the outer diameter of the pipeline; t represents the wall thickness.

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 defect impact factor term R is: Where N is the corrosion depth ratio; Q is the length correction factor.

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 S4, when performing numerical simulation analysis, hexahedral meshing is used for division, and five layers of meshes are divided in the direction of the wall thickness of the corroded pipeline.

5. 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, an Abaqus GUI script is first written on the Python platform; the script is encapsulated into the Abaqus software in the form of a plug-in; during 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.

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: Step S4 includes: Step S41: Calculating the simulation results of the ultimate bearing capacity of the pipeline using the explicit finite element method; 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 square method, v G =[ν G1 ,ν G2 ,ν G3 ] T The design variables are solved to obtain the revised ultimate bearing capacity model.

7. The method for constructing an ultimate bearing capacity model of a copper-nickel alloy pipeline containing groove defects according to claim 6, characterized in that: The expression of the optimization objective e(r) is: Where p FEA represents the simulation results, p CG Represents the calculation results of the ultimate bearing capacity model.

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