Method for predicting deep-sea pipeline low-cycle fatigue crack initiation by using cyclic cohesion model

Abstract

The application discloses a kind of deep-sea pipeline low-cycle fatigue crack initiation cyclic cohesion force model prediction methods, comprising: establishing the pipeline finite element calculation framework containing cyclic cohesion force model;Based on pipeline finite element calculation framework, the structural stress and deformation data at the scheduled position of pipe at the current time point are calculated and imported into the cyclic cohesion force model;Cyclic cohesion force model calculates the cyclic damage related to fatigue load and the monotonic damage related to low-cycle fatigue according to the input model parameters and structural stress and deformation data, obtains the total cumulative damage of pipeline at the current time point;According to the total cumulative damage of pipeline, when fatigue crack initiation occurs, the corresponding load number is output as the low-cycle fatigue crack initiation life of deep-sea pipeline.The application introduces the analysis of the material characteristics of pipeline steel and the damage mechanism of the material in low-cycle fatigue in the cyclic cohesion force model, realizes the prediction of fatigue crack initiation process.

Description

Technical Field

[0001] This invention relates to the field of pipeline low-cycle fatigue crack initiation prediction technology, and in particular to a cyclic cohesion model prediction method for low-cycle fatigue crack initiation in deep-sea pipelines. Background Technology

[0002] Deep-sea oil and gas pipelines operate in complex marine environments, facing challenges such as pipeline overhang and overall buckling. These issues lead to irreversible plastic deformation of the pipeline materials, resulting in low-cycle fatigue failure. To ensure the structural safety of deep-sea oil and gas pipelines, low-cycle fatigue assessment is necessary. The low-cycle fatigue process consists of two stages: fatigue crack initiation and fatigue crack propagation. Since the fatigue crack initiation life accounts for a larger proportion of the total low-cycle fatigue life, predicting the low-cycle fatigue crack initiation life is crucial for low-cycle fatigue assessment of deep-sea oil and gas pipelines.

[0003] Traditionally, strain-life curve methods, represented by the Manson-Coffin formula, are used to assess low-cycle fatigue crack initiation in deep-sea oil and gas pipelines. However, this method relies on extensive experimental data and empirical fitting, resulting in insufficient prediction accuracy and certain limitations. In recent years, the cyclic cohesive zone model (CCZM) has shown great potential in simulating material fatigue failure. Unlike traditional fatigue analysis methods, the CCZM is a phenomenological fatigue damage analysis model that considers the fatigue process as an irreversible continuous process. In the CCZM, the rate of accumulation of material fatigue damage is related to the stress and material deformation at the crack tip. Since the stress and material deformation at the crack tip continuously change over time, the rate of accumulation of material fatigue damage also changes accordingly. The CCZM quantifies the impact of crack tip stress and material deformation on material fatigue damage at each moment during the fatigue process, enabling cycle-by-cycle analysis of material fatigue damage and the ability to predict steady-state crack propagation. Furthermore, the CCZM can also simulate the degradation effect of material fatigue damage on material stiffness, predicting the continuous change in material stiffness during fatigue.

[0004] Roe and Siegmund applied a cyclic cohesive model to analyze fatigue crack propagation in specimens such as double cantilever beams. This method calculates stress and material deformation at the crack tip using a finite element model, and then calculates material damage and crack propagation rate based on the cyclic cohesive model. The study found that the cyclic cohesive model reproduces the exponential relationship between crack propagation rate and energy release rate, indicating that the cyclic cohesive model has the ability to predict steady-state crack propagation.

[0005] Most existing cyclic cohesion models revolve around the fatigue crack propagation stage. The model establishment process and parameter acquisition are all based on the fatigue crack propagation stage. The material characteristics of the fatigue crack initiation stage are not considered in the model, so the fatigue crack initiation and propagation stage cannot be accurately predicted.

[0006] CN107832492A discloses a method for calculating corrosion fatigue damage of steel structures. This method first establishes a cohesive model and links it with finite element software to develop a finite element model containing the cohesive model. Then, structural stress analysis is performed based on this finite element model. Finally, the analysis results determine whether the structure has failed. If the structure fails, the calculation ends; if the structure has not yet failed, the obtained stress analysis results are processed, and the fatigue damage of the cohesive model is calculated using the real-time rainflow counting method and Minner's linear cumulative damage theory. The finite element model is then updated, and stress analysis is repeated until structural failure occurs. This method uses the linear cumulative damage rule to calculate damage instead of a damage evolution equation and cannot predict the low-cycle fatigue process of pipeline steel.

[0007] CN113916705A discloses a method for obtaining and simulating crack propagation parameters based on a cyclic cohesive model. This method determines the damage parameters and quasi-static parameters required for the cyclic cohesive model based on crack closure tests and force-displacement curve measurement tests. Based on these parameters, the damage evolution equation in the cyclic cohesive model is modified, which can calculate the cumulative fatigue damage of the material based on the stress and deformation at the crack tip. By calling finite element subroutines such as UAMP, UMAT, URDFIL, and UEXTERNALAD, this cyclic cohesive model is embedded into the finite element model, and the changes in stress at the crack tip, material deformation, and material fatigue damage during fatigue crack propagation are calculated using finite element technology. Crack propagation assessment is then performed based on the finite element calculation results. However, this method cannot be used to predict the fatigue crack initiation process.

[0008] In summary, existing cyclic cohesion models can predict the fatigue crack propagation process of structures, but there is no mature technology that can be used to predict the fatigue crack initiation process. Existing cyclic cohesion models do not consider the influence of pipeline steel materials or the damage characteristics of materials during low-cycle fatigue when making predictions. Summary of the Invention

[0009] The purpose of this invention is to address the problems in the prior art by providing a cyclic cohesive model prediction method for low-cycle fatigue crack initiation in deep-sea pipelines. This method incorporates the analysis of the material characteristics of pipeline steel and the damage mechanism of materials in low-cycle fatigue into the cyclic cohesive model, overcoming the shortcomings of the prior art and achieving reasonable prediction of the fatigue crack initiation process.

[0010] The technical solution adopted to achieve the purpose of this invention is:

[0011] A method for predicting the cyclic cohesion model of low-cycle fatigue crack initiation in deep-sea pipelines includes the following steps:

[0012] S1. Establish a finite element calculation framework for pipelines containing a cyclic cohesive force model;

[0013] S2. Based on the established pipeline finite element calculation framework, the structural stress and deformation data at the predetermined location of the deep-sea pipeline at the current time point are calculated and imported into the cyclic cohesion model;

[0014] S3. The cyclic cohesion model calculates the cyclic damage related to fatigue load and the monotonic damage related to low-cycle fatigue based on the input model parameters, and obtains the total cumulative damage of the deep-sea pipeline at the current time point.

[0015] S4 determines the fatigue crack initiation life of the deep-sea pipeline based on the total cumulative damage of the deep-sea pipeline and outputs the corresponding number of loads when fatigue cracks initiate at a predetermined location in the deep-sea pipeline.

[0016] Specifically, when the total cumulative damage of the deep-sea pipeline material reaches a predetermined threshold under each time increment, fatigue crack initiation is considered to have occurred at a predetermined location in the deep-sea pipeline. The number of load loadings at this time is recorded, and the number of load loadings is determined as the low-cycle fatigue crack initiation life of the deep-sea pipeline.

[0017] If it is determined that fatigue cracks have not initiated at the predetermined location of the deep-sea pipeline, then proceed to the next time increment, update the cumulative material damage, and return to step S2.

[0018] The present invention provides a method for predicting the initiation of low-cycle fatigue cracks in deep-sea pipelines using a cyclic cohesive force model. This method incorporates the analysis of the material characteristics of pipeline steel and the damage mechanism of materials during low-cycle fatigue into the cyclic cohesive force model, overcoming the shortcomings of existing technologies and achieving reasonable prediction of the fatigue crack initiation process. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the prediction method for the cyclic cohesive force model of low-cycle fatigue crack initiation in deep-sea pipelines according to an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram illustrating the separation of the unloading and reloading process from material plasticity in the cyclic cohesive force model of this invention.

[0021] Figure 3 This is a front view schematic diagram of the test device for low-cycle fatigue crack initiation in deep-sea pipeline steel according to an embodiment of the present invention.

[0022] Figure 4This is a side view schematic diagram of the test device for low-cycle fatigue crack initiation in deep-sea pipeline steel according to an embodiment of the present invention. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0024] The method for predicting the cyclic cohesive force model for low-cycle fatigue crack initiation in deep-sea pipelines according to the embodiments of this application predicts the fatigue crack initiation life of deep-sea pipelines based on the cyclic cohesive force model and with the help of the finite element calculation framework.

[0025] The method for predicting the cyclic cohesive force model for low-cycle fatigue crack initiation in deep-sea pipelines according to the embodiments of this application predicts the fatigue crack initiation life rather than the fatigue crack propagation of deep-sea pipelines. When applying the cyclic cohesive force model to the fatigue assessment of deep-sea pipelines, the cyclic cohesive force model is modified according to the characteristics of deep-sea pipelines. Based on the basic material parameters of deep-sea pipelines and conducting low-cycle fatigue crack initiation tests on pipeline steel, the model parameters of the cyclic cohesive force model are obtained, and the effective cyclic cohesive force model parameters are determined.

[0026] The cyclic cohesive model of this application introduces monotonic damage related to the low-cycle fatigue damage mechanism of materials to simulate the low-cycle fatigue process of pipeline steel. Considering that the damage mechanism of materials in low-cycle fatigue is different from that in high-cycle fatigue, the introduction of low-cycle fatigue into the cyclic cohesive model needs to consider the irreversible plastic separation of materials and the material damage mechanism under low-cycle fatigue state. Based on this, the improved cyclic cohesive model is applied to predict the low-cycle fatigue process of pipeline steel.

[0027] refer to Figure 1 As shown, a method for predicting the cyclic cohesion model of low-cycle fatigue crack initiation in deep-sea pipelines adopts the following steps:

[0028] S1. Establish a linear finite element calculation framework containing a cyclic cohesive force model;

[0029] S2. Based on the established pipeline finite element calculation framework, the structural stress and deformation data at the predetermined location of the deep-sea pipeline steel at the current time point are calculated and imported into the cyclic cohesion model;

[0030] S3. The cyclic cohesion model calculates the cyclic damage related to fatigue load and the monotonic damage related to low-cycle fatigue based on the input model parameters, and obtains the total cumulative damage of the deep-sea pipeline material at the current time point.

[0031] S4. Based on the total cumulative damage of the deep-sea pipeline material, determine whether fatigue crack initiation has occurred at the predetermined location of the deep-sea pipeline. If it is determined that fatigue crack initiation has occurred at the predetermined location of the deep-sea pipeline, output the low-cycle fatigue crack initiation life of the deep-sea pipeline, i.e., the corresponding number of load applications.

[0032] In addition, if it is determined that the fatigue crack of the deep-sea pipeline has not yet been initiated, the next time increment is entered, the cumulative material damage is updated, and the process returns to step S2. The stress and deformation data of the deep-sea pipeline are calculated and re-imported into the cyclic cohesion model. Then, the process is carried out according to steps S2-S4 until the fatigue crack has been initiated based on the total cumulative material damage of the deep-sea pipeline. The low-cycle fatigue crack initiation life of the deep-sea pipeline is then output.

[0033] Specifically, when establishing a linear finite element calculation framework containing a cyclic cohesive force model, the framework can be established in the form of a material subroutine. The material subroutine is a UMAT subroutine, written in FORTRAN, and the cyclic cohesive force model is incorporated into the finite element calculation framework (finite element calculation model) in the form of a UMAT material subroutine.

[0034] In some embodiments, when the total cumulative damage of the deep-sea pipeline material at each time increment reaches a predetermined threshold, such as 1.0, the number of load loadings at this time is recorded. This number of load loadings is the low-cycle fatigue crack initiation life of the pipeline steel. Then, the low-cycle fatigue crack initiation life of the pipeline is output, and the process ends.

[0035] In the implementation example of this application, the unloading and reloading process of the cyclic cohesive model is as follows: Figure 2 As shown, the pipeline steel undergoes irreversible plastic deformation during reloading; unloading and reloading are constrained by the envelope of the cyclic cohesive model, so the unloading and reloading paths return to the degenerate envelope of the cyclic cohesive model.

[0036] In some embodiments, within the finite element calculation framework, the rigid pressure block and fixed support in the fatigue testing device of the component can be set as rigid bodies, and their contact with the straight pipe section of the pipeline steel under test can be defined. Specifically, when establishing a finite element calculation framework containing a cyclic cohesion model in the form of a material subroutine, the cyclic cohesion model is introduced at the fatigue crack initiation point using a material subroutine.

[0037] In the embodiment of the test of specimen 1 and specimen 2 using the prediction method and component test device of this application, the low-cycle fatigue crack initiation life of pipeline steel obtained based on the cyclic cohesion model is shown in Table 1 below. Table 1 shows the comparison between the predicted value and the experimental value of the low-cycle fatigue crack initiation life of pipeline steel based on the cyclic cohesion model.

[0038]

[0039] Table 1

[0040] In this embodiment of the application, the model parameters of the cyclic cohesion model are determined or calculated by acquiring the basic material parameters of the pipeline steel, or obtained through corresponding experiments. The model parameters of the cyclic cohesion model include a first parameter and a second parameter of the cyclic cohesion model.

[0041] Based on the established calculation method, this application implements a case study to predict the low-cycle fatigue crack initiation life of pipeline steel. The basic material data of the pipeline steel can be obtained from "Murthy, RA, Vishnuvardhan, S, Anjusha, KV, Gandhi, P, Singh, PK. Prediction of fatigue crack initiation life in SA312 Type304LN austenitic stainless steel straight pipes with notch. Nuclear Engineering and Technology. 2022 54(5):1588-1596."

[0042] The first parameter includes the cohesive strength σ. max,0 Cohesive length δ0, maximum separation displacement δ f and cohesive endurance limit σ f ;

[0043] Among them, based on the ultimate tensile strength σ of the pipeline steel u Or the fatigue strength coefficient σ of pipeline steel f ′ Determine the cohesive strength σ max,0 The cohesive length δ0 is determined based on the stiffness of the pipeline steel; the maximum separation displacement δ is determined based on the fracture toughness of the pipeline steel. f The cohesive endurance limit σ is determined based on the fatigue limit of the pipeline steel. f .

[0044] Generally, the cohesive strength σ max,0 The value can be the ultimate tensile strength σ of pipeline steel. u The value; if the pipeline steel has high strength, the cohesive strength σ max,0 The value can be the fatigue strength coefficient σ of pipeline steel. f ′ The value.

[0045] Among them, the ultimate tensile strength σ of the pipeline steel ufatigue strength coefficient σ of pipeline steel f ′ The basic material parameters for pipeline steel can be obtained through existing testing techniques or literature.

[0046] The value of the cohesive length δ0 can be determined according to the following formula:

[0047]

[0048] Where K is the stiffness of the pipeline steel, which is a basic material parameter of the pipeline steel and can be obtained through existing experimental techniques or literature.

[0049] Wherein, the maximum separation displacement δ f It can be determined based on the cohesive energy Φ0:

[0050] Φ0=f(σ max,0 ,δ0,δ f )

[0051] Among them, the value of cohesive energy Φ0 is related to the crack initiation toughness J of pipeline steel. i same:

[0052] J i =Φ0

[0053] Among them, crack initiation toughness J i These are the basic material parameters of the pipeline steel, which can be obtained through existing experimental techniques or literature. The function f can be determined according to the form of the traction separation law in the cyclic cohesion model: the traction separation curve can be determined according to the traction separation law, and the traction separation curve is related to the normal separation displacement δ. n The integral area between the axes (i.e., the area enclosed by the curve and the axis) is Φ0, therefore the normal traction force T n normal separation displacement δ n Integrating yields an expression for the function f. For example, under the exponential form of the traction separation law, the function f is f(σ). max,0 ,δ0,δ f )=σ max,0 δ0e.

[0054] When the crack toughness J i Cohesive strength σ max,0 Once the cohesive length δ0 is determined, the maximum separation displacement δ can be calculated using the above formula. f .

[0055] Wherein, the cohesive endurance limit σ f The value can be the fatigue limit of pipeline steel, which is a basic material parameter of pipeline steel and can be obtained through existing test techniques or literature.

[0056] The second parameter includes the model cumulative cohesive length δ. ∑ The determination of the method is made by conducting low-cycle fatigue crack initiation tests on pipeline steel and then basing the results on the error between the test results and the simulation results of the cyclic cohesion model. The low-cycle fatigue crack initiation test can be a four-point bending fatigue test of the pipe segment. During fatigue loading, ACDP (alternating current potential drop) technology can be used to monitor the fatigue crack initiation in the pure bending portion of the pipeline steel segment.

[0057] For example, the second parameter model accumulates the cohesive length δ ∑ The steps to determine this are as follows:

[0058] During the test, when cracks or defects of a certain length, such as 0.1 mm or more, are detected on the surface of the pipeline steel, it is considered that fatigue crack initiation has occurred in the pipeline steel, and the low-cycle fatigue crack initiation life of the pipeline steel is recorded.

[0059] During the experiment, the load stress ratio and stress range in the four-point bending fatigue test of the pipe section were modified, the four-point bending fatigue test of the pipe section was repeated, and the low-cycle fatigue crack initiation life of the pipeline steel under different load stress ratios and stress ranges were recorded.

[0060] Based on the experimental results, an initial cumulative cohesive length is set for the cyclic cohesive force model to simulate the low-cycle fatigue crack initiation process of pipeline steel in the above experiment. The value of the cumulative cohesive force length is continuously adjusted according to the difference between the simulation and experimental results. The cumulative cohesive force length is determined when the error between the simulation and experimental results reaches an acceptable threshold; this value is the final cumulative cohesive force length δ of the model. ∑ .

[0061] In this embodiment of the application, the fatigue crack initiation component test of the deep-sea pipeline can be carried out by a four-point bending fatigue test of the pipe section, or by a three-point bending test of the specimen, whichever is more specific.

[0062] In some embodiments, the pipeline steel low-cycle fatigue crack initiation test device can be as follows: Figure 3 --- Figure 4 As shown, the specimen includes a straight pipe section 1, a rigid pressure block 1 above the specimen 2, a rigid pressure block 2 above the specimen 3, a fixed support 1 below the specimen 4, and a fixed support 2 below the specimen 5.

[0063] During the low-cycle fatigue crack initiation test of pipeline steel, the fixed support 4 and the fixed support 5 below the specimen are kept fixed. A vertically downward fatigue load is applied to the rigid pressure block 2 and the rigid pressure block 3 above the specimen, so that they cyclically press the straight pipe section specimen 1 downward. The crack initiation status in the fatigue crack initiation monitoring zone 6 on the surface of the pipe section is recorded using ACDP technology.

[0064] When a short fatigue crack or fatigue crack pit with a length exceeding 0.1 mm appears on the straight pipe section specimen 1, it is considered that a fatigue crack has initiated, and the fatigue crack initiation life at this time is recorded. Based on the test results, the cumulative cohesion length δ of the model is determined. ∑ .

[0065] In this embodiment, a low-cycle fatigue damage mechanism is introduced into the damage evolution calculation of the cyclic cohesive model to analyze the material damage caused by low-cycle fatigue; at the same time, plastic separation of the material is introduced into the unloading and reloading calculation of the cyclic cohesive model to consider the influence of irreversible plastic separation on material damage during low-cycle fatigue and to calculate the total cumulative material damage D of the pipeline steel.

[0066] Wherein, the total cumulative material damage D caused by low-cycle fatigue is composed of monotonic damage D m and cyclic damage D c The composition, the total cumulative damage D of the material can be determined as follows:

[0067]

[0068] in, Indicates monotonic damage D m The derivative with respect to time t; Indicates cyclic damage D c The derivative of D with respect to time t. When D is 0, it means that the material has not failed; when D is 1.0, it means that the material has failed completely, that is, fatigue cracks have started.

[0069] The monotonic damage D m The degree of material fracture failure is defined by the following formula:

[0070]

[0071] Where, parentheses <δ n -δ0> indicates taking δ n -δ0 is a positive value, when δ n When -δ0≤0, <δ n -δ0>=0;δ n The normal separation displacement (relative separation displacement between two cohesive surfaces) at the fatigue crack initiation point can be calculated based on the structural stress and deformation data calculated using the pipeline finite element calculation framework.

[0072] The cyclic damage D c Define the degree of failure of a material under cyclic loading; its value can be determined according to... The integral is determined. Determined by the following formula:

[0073]

[0074] Among them, T n This represents the normal traction force (the traction force perpendicular to the cohesive surface) at the point of fatigue crack initiation. H represents the derivative of the normal separation displacement at the fatigue crack initiation point with respect to time, and H is the Herveyd function. It can be determined by the following formula:

[0075]

[0076] Wherein, the normal traction force T at the fatigue crack initiation point n The equation is expressed as:

[0077]

[0078] Among them, T n,t and δ n,t These represent the normal traction force and normal separation displacement at the fatigue crack initiation point under the previous time increment, respectively.

[0079] The calculation of unloading and reloading of the cyclic cohesive model is constrained by the envelope of the cyclic cohesive model, which is expressed as follows:

[0080]

[0081] Where T e The traction force value on the envelope of the cyclic cohesive force model, regardless of the normal traction force T at the fatigue crack initiation point. n No matter how the value is changed, its size cannot exceed T. e The value of T. That is, the above function restricts the value of T. n The range of variation was used to determine the envelope of the cyclic cohesive force model during loading and reloading.

[0082] In the implementation examples of this application, the stress ratio or stress range of the load can be changed, and nonlinear finite element calculations can be carried out again, thereby enabling the prediction of the low-cycle fatigue crack initiation life of pipeline steel materials under different stress ratios and stress ranges.

[0083] The prediction method of this application uses a cyclic cohesive model to predict the fatigue crack initiation life. It closely follows the material characteristics in the fatigue crack initiation process and establishes a cyclic cohesive model based on the material characteristics of fatigue crack initiation, which can effectively achieve reasonable prediction of fatigue crack initiation life.

[0084] The prediction method in this application takes into account the irreversible plastic separation during low-cycle fatigue, and introduces monotonic damage and cyclic damage to quantify material damage during low-cycle fatigue, thereby realizing the simulation and prediction of material degradation during low-cycle fatigue.

[0085] The prediction method of this application provides a simpler method for determining model parameters, overcoming the problem that the method for obtaining model parameters of cyclic cohesion models is relatively complex in the prior art. The prediction method of this application, by utilizing the basic parameters of the material, can determine all model parameters of the cyclic cohesion model based on less experimental data, saving experimental costs, reducing the difficulty of obtaining model parameters, and improving the applicability of the prediction method of this application in practical engineering.

[0086] The prediction method of this application takes into account the material characteristics of deep-sea pipeline steel. The model establishment and model parameter acquisition are both centered around the pipeline steel material, which effectively improves the applicability of the prediction method of this application in predicting the initiation of fatigue cracks in pipeline steel.

[0087] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for predicting the cyclic cohesive force model of low-cycle fatigue crack initiation in deep-sea pipelines, characterized in that, Including the following steps: S1. Establish a finite element calculation framework for pipelines containing a cyclic cohesive force model; S2. Based on the established pipeline finite element calculation framework, the structural stress and deformation data at the predetermined location of the deep-sea pipeline at the current time point are calculated and imported into the cyclic cohesion model; S3. The cyclic cohesion model calculates the cyclic damage related to fatigue load and the monotonic damage related to low-cycle fatigue based on the input model parameters, and obtains the total cumulative material damage of the deep-sea pipeline at the current time point. S4. Based on the total cumulative material damage of the deep-sea pipeline, determine when fatigue crack initiation occurs at a predetermined location in the deep-sea pipeline, and output the corresponding number of loads as the fatigue crack initiation life of the deep-sea pipeline. The model parameters include a first parameter and a second parameter; The first parameter is obtained or determined by calculating based on the basic material parameters of the deep-sea pipeline, including cohesive strength. Cohesive length Maximum separation displacement and cohesive endurance limit ; The cohesive strength Based on the ultimate tensile strength of pipeline steel or fatigue strength coefficient of pipeline steel Sure; The cohesive length The value is determined according to the following formula: ; It refers to the rigidity of the pipeline steel. The maximum separation displacement According to cohesive energy Sure: ; The cohesive energy is a function determined according to the form of the traction separation law in the cyclic cohesive force model. The values ​​and crack initiation toughness of pipeline steel same; The cohesive endurance limit The value represents the fatigue limit of the pipeline steel. The second parameter was obtained through low-cycle fatigue crack initiation tests on deep-sea pipeline steel, including the model's cumulative cohesion length. .

2. The method for predicting the cyclic cohesion model of low-cycle fatigue crack initiation in deep-sea pipelines according to claim 1, characterized in that, If it is determined that fatigue cracks have not initiated at the predetermined location of the deep-sea pipeline, proceed to the next time increment, update the cumulative material damage, and return to step S2.

3. The method for predicting the cyclic cohesion model for low-cycle fatigue crack initiation in deep-sea pipelines according to claim 1, characterized in that, When the total cumulative material damage of the deep-sea pipeline reaches a predetermined threshold at each time increment, fatigue crack initiation is considered to have occurred at a predetermined location in the deep-sea pipeline. The number of load loadings at this time is recorded, and the number of load loadings is determined as the low-cycle fatigue crack initiation life of the deep-sea pipeline.

4. The method for predicting the cyclic cohesion model for low-cycle fatigue crack initiation in deep-sea pipelines according to claim 1, characterized in that, The second parameter is determined based on the error between the experimental results of low-cycle fatigue crack initiation tests on deep-sea pipeline steel and the simulation results of the cyclic cohesion model, including: When conducting low-cycle fatigue crack initiation tests on deep-sea pipeline steel, bending fatigue tests are performed on the pipeline steel. When cracks or defects of a predetermined length are detected on the surface of the pipeline steel, it is considered that fatigue crack initiation has occurred in the pipeline steel, and the low-cycle fatigue crack initiation life of the pipeline steel is recorded. Modify the load stress ratio and stress range in the test, repeat the bending fatigue test of pipeline steel, and record the low-cycle fatigue crack initiation life of pipeline steel under different load stress ratios and stress ranges. Based on the experimental results, an initial cumulative cohesive length is set for the cyclic cohesive force model to simulate the low-cycle fatigue crack initiation process of pipeline steel during the experiment. The cumulative cohesive force length is continuously adjusted according to the difference between the simulation and experimental results until the error between the simulation and experimental results reaches a preset threshold. The final cumulative cohesive force length for the model is then determined at this point. ; Based on the damage evolution equation in the cyclic cohesive model, calculate the cyclic damage associated with fatigue load and the monotonic damage associated with low-cycle fatigue.

5. The method for predicting the cyclic cohesion model of low-cycle fatigue crack initiation in deep-sea pipelines according to claim 1, characterized in that, The total cumulative material damage consists of monotonic damage and cyclic damage, and the total cumulative material damage is expressed as follows: ； in, Indicates the total cumulative damage to the material. Indicates monotonic damage Regarding time The derivative; Indicates circulatory damage Regarding time The derivative of the monotonic damage The degree of material fracture and damage, the cyclic damage. Characterizing the degree of material damage under cyclic loading; when A value of 0 indicates that the material has not been damaged; A value of 1.0 indicates the initiation of fatigue cracks in the material.

6. The method for predicting the cyclic cohesion model for low-cycle fatigue crack initiation in deep-sea pipelines according to claim 5, characterized in that, The monotonic damage Determined by the following formula: ； in,< > indicates taking The positive value, when When ≤0, < >=0; This represents the normal separation displacement at the point where fatigue cracks initiate.

7. The method for predicting the cyclic cohesion model for low-cycle fatigue crack initiation in deep-sea pipelines according to claim 6, characterized in that, The cyclic damage The value is based on The integral is determined. Determined by the following formula: ； in, This represents the normal traction force at the point of fatigue crack initiation. This represents the derivative of the normal separation displacement at the fatigue crack initiation point with respect to time. For Herveside functions; Determined by the following formula: 。 8. The method for predicting the cyclic cohesion model for low-cycle fatigue crack initiation in deep-sea pipelines according to claim 7, characterized in that, The normal traction force at the fatigue crack initiation point The expression is as follows: ； in, and These represent the normal traction force and normal separation displacement at the fatigue crack initiation point under the previous time increment, respectively.

9. The method for predicting the cyclic cohesion model for low-cycle fatigue crack initiation in deep-sea pipelines according to claim 8, characterized in that, The calculation of unloading and reloading of the cyclic cohesive model is constrained by the envelope of the cyclic cohesive model, and the expression of the envelope of the cyclic cohesive model is: ； in This represents the traction force value on the envelope of the cyclic cohesive force model. Greater than or equal to .

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