Evaluation method for remaining strength of elbow defects and method for determining safety factor for evaluation

Abstract

The present invention provides a method for evaluating the remaining strength of elbow defects and a method for determining the safety factor for evaluation. The determination method includes: determining the correction factor for the safety factor of the mountain pipeline with elbow defects; combining the correction factor to determine the range of the safety factor SF of the mountain pipeline; and determining the safety factor N for evaluation according to the range of the SF. PSF , where N PSF = N P × SF. The evaluation method may include the steps of: determining the working pressure P of the pipeline w ; determining the ultimate internal pressure load P of the pipeline L ; determining the safety factor N for evaluation according to the determination method described above PSF ; judging whether P w < P L / N PSF holds, and performing the evaluation according to the judgment result. By adding a correction factor for the safety factor of the mountain pipeline, the present invention performs more stringent constraints, can more accurately reflect the pipeline condition of the mountain pipeline under the influence of elbow defects, and is beneficial to the safe operation of the pipeline.

Description

Technical Field

[0001] The present invention relates to the field of pipeline evaluation, and in particular, to a method for evaluating the remaining strength of elbow defects and a method for determining the safety factor for evaluation. Background Art

[0002] The southwestern region of China is the area where mountains are most concentrated, and mountain disasters occur most frequently. The topography and geomorphology of the areas through which the pipeline passes are mountain valleys, with large undulations in terrain. Most of the sections where the pipeline crosses are characterized by high mountains and deep valleys, crisscrossed rivers, and frequent geological disasters. In the long-distance mountain pipeline project in the southwest, the mountain slopes are large and the corners are numerous, and various elbow defect problems will inevitably be encountered. When the wall thickness of the elbow fittings of the buried pipeline in the mountain area is thinned due to corrosion, erosion, etc., the method for evaluating the remaining strength of elbow defects can be used to calculate the allowable working pressure of the pipe fittings with wall thickness reduction, so as to evaluate its remaining strength.

[0003] Currently, most scholars determine the plastic limit load of the structure by studying the theory of plastic limit analysis and using the ideal elastoplastic material model; and then determine the allowable working load of the structure through the corresponding safety factor; it can be used to prevent the decrease in bearing capacity caused by wall thickness reduction and ultimately lead to the evaluation of the remaining strength of the structure under static loads when the structure undergoes plastic instability collapse failure. This evaluation method can be used for the situation where the wall thickness of the elbow fittings of the buried pipeline in the mountain area is thinned due to corrosion, erosion, etc.

[0004] The above evaluation method is of great significance for studying the remaining strength evaluation of elbow-defected pipelines, but if parameters for the complex load characteristics around mountain pipelines are not given, certain evaluation error problems will occur. The reasons are as follows: 1) There are complex loads around mountain pipelines, which are different from the loads around general pipelines; 2) In the remaining strength evaluation method based on the theory of plastic limit analysis and through the corresponding safety factor, there is no distinction between mountain pipelines and general pipelines, that is, there is no pertinence to mountain pipelines, so errors are easily introduced and there is a possibility of inaccurate evaluation. Summary of the Invention

[0005] The purpose of the present invention is to solve at least one of the above-mentioned deficiencies existing in the prior art. For example, one of the purposes of the present invention is to more accurately evaluate the pipe sections with elbow defects in areas prone to geological disasters.

[0006] To achieve the above purpose, on the one hand, the present invention provides a method for determining the safety factor for evaluating the remaining strength of elbow-defected mountain pipelines.

[0007] The determination method may include the following steps: determining a correction factor for the safety factor of the elbow defect mountain pipeline; determining the range of the safety factor SF of the mountain pipeline in combination with the correction factor; and determining the safety factor N for evaluation according to the range of SF PSF , where N PSF = N P ×SF, and N P is the safety factor of the elbow defect pipeline without considering the mountain safety factor.

[0008] Furthermore, the step of determining the correction factor may include: selecting risk factors to form a data set; performing normalization using the MIPCA model; comprehensively analyzing the risk factors of the elbow defect using the WASPAS method; and determining the correction factor according to the comprehensive analysis result.

[0009] Furthermore, the following formula is used for the comprehensive analysis:

[0010] where Q i is the comprehensive evaluation value of the i-th observation point, λ is the contribution degree of , λ = 0, …, 1, w j is the weight of the j-th attribute set, is the score of the i-th observation point in the standardized j-th attribute set C j .

[0011] Furthermore, the correction factor is determined according to the following formula:

[0012] where a is the correction factor, min is the minimum value among the comprehensive evaluation values of all observation points, and max is the maximum value among the comprehensive evaluation values of all observation points.

[0013] Furthermore, the is determined according to the following formula:

[0014] where c ij represents the score of the i-th observation point in the j-th attribute set C j .

[0015] Furthermore, the step of performing normalization may include: calculating the mutual information matrix of the risk factors; calculating the eigenvalues of the mutual information matrix, arranging them, and finding the corresponding eigenvectors; calculating the principal components of the mutual information; calculating the contribution rate of the principal components, and then determining the dimension of the features.

[0016] Furthermore, the range of the safety factor is determined according to the following formula:

[0017]

[0018] Among them, SF is the safety factor, P is the design pressure, P H is the minimum hydrostatic test pressure, MAOP is the maximum allowable operating pressure, P0 is the operating pressure, F is the design coefficient, and a is the correction factor.

[0019] Furthermore, the mountain conditions where the mountain pipeline is located include: the laying in high hilly terrain accounts for 75 - 80%, and the laying in plain valley terrain accounts for 20% - 25%. For example, the laying in high hilly terrain accounts for 78%, and the laying in plain valley terrain accounts for 22%.

[0020] On the other hand, the present invention provides a method for evaluating the remaining strength of a mountain pipeline with elbow defects.

[0021] The method may include the following steps:

[0022] Determine the working pressure P of the pipeline w ; Determine the ultimate internal pressure load P of the pipeline L ; According to the method for determining the safety factor for evaluating the remaining strength of a mountain pipeline with elbow defects as described above, determine the safety factor N for evaluation PSF ; Judge whether P w < P L / N PSF holds, and perform the evaluation according to the judgment result.

[0023] In this embodiment, the step of determining the ultimate internal pressure load P L includes: determining the material flow stress σ according to Equation 1 f ; Determine the ultimate internal pressure load P according to Equation 2 L ;

[0024] Equation 1 is:

[0025] Among them, σ s is the material yield strength at the evaluation working condition temperature, σ b is the material tensile strength at the evaluation working condition temperature, and φ is the pipeline welded joint coefficient;

[0026] Equation 2 is:

[0027] Among them, t c is the wall thickness used when calculating the ultimate internal pressure load, r m is the average radius of the elbow, G is the enhancement coefficient of the straight pipe sections at both ends of the elbow to the elbow, and R is the bending curvature radius of the elbow.

[0028] Compared with the prior art, the beneficial effects of the present invention include at least one of the following:

[0029] (1) The present invention imposes more stringent constraints by adding a correction factor for the safety factor of mountain pipelines, which can more accurately reflect the pipeline condition of mountain pipelines affected by elbow defects.

[0030] (2) Compared with the conventional local evaluation method, the evaluation of the present invention for pipelines has higher accuracy.

[0031] (3) The present invention fully considers the characteristics of the complex loads surrounding mountain pipelines, and selects a conservative safety factor calculation method, adding a correction factor for the mountain safety factor, which can more accurately evaluate the elbow defect pipe sections in areas prone to geological disasters. Therefore, the evaluation method of the present invention can be an important part of integrity management, which can help formulate maintenance plans and is beneficial to the safe operation of pipelines. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Through the following description in conjunction with the drawings, the above and other objects and / or features of the present invention will become clearer, wherein:

[0033] Figure 1 A flowchart showing a method for determining the safety factor for evaluating the remaining strength of an elbow-defected mountain pipeline according to the present invention is shown.

[0034] Figure 2 A flowchart showing a method for evaluating the remaining strength of an elbow-defected mountain pipeline according to the present invention is shown. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] In the following, the method for evaluating the remaining strength of an elbow defect and the method for determining the safety factor for evaluation according to the present invention will be described in detail in conjunction with exemplary embodiments.

[0036] There is a problem of elbow defects in mountain pipelines. When the wall thickness of the elbow pipe fittings of buried pipelines in mountainous areas is reduced due to corrosion, erosion, etc., it is necessary to evaluate the remaining strength of the elbow defects. However, the conventional remaining strength evaluation methods do not have parameters that fully consider the characteristics of the complex loads surrounding mountain pipelines. In this regard, the present invention considers the characteristics of the complex loads surrounding the Southwest Pipeline and selects a conservative safety factor calculation method, adding a correction factor for the mountain safety factor, which is used to more accurately evaluate the elbow defect pipe sections in areas prone to geological disasters.

[0037] Exemplary Embodiment 1

[0038] This exemplary embodiment provides a method for determining the safety factor for evaluating the remaining strength of an elbow-defected mountain pipeline.

[0039] Figure 1 A flowchart showing a method for determining the safety factor for evaluating the remaining strength of an elbow-defected mountain pipeline according to the present invention is shown.

[0040] As shown Figure 1 below, the determination method includes the following steps:

[0041] A10. Determine the correction factor of the safety factor of the mountain pipeline with elbow defects.

[0042] In this step, the MIPCA model and the WASPAS method are mainly used. Among them, MIPCA is used to screen out the risk factors related to pipeline scratch and depression defects; WASPAS determines the comprehensive evaluation values of different observation points according to the screened risk factors.

[0043] This step may include: selecting risk factors to form a data set; performing normalization processing using the MIPCA model; comprehensively analyzing the risk factors of elbow defects using the WASPAS method; and determining the correction factor according to the comprehensive analysis results.

[0044] In this embodiment, principal component analysis (PCA) is a multivariate statistical analysis method that selects a smaller number of important variables through linear transformation of multiple variables. However, in the actual data set, the relationship between variables is not only linear but also non-linear. Therefore, the mutual information method is introduced as a new way of feature processing. Since mutual information is based on information theory and has the advantage of reflecting all the information between variables, the combination of mutual information and principal component analysis has better variable selection advantages, thus forming the MIPCA model. The calculation process of MIPCA includes:

[0045] (1) Assume that a total of p risk factors (i.e., independent variables) are selected to form a data set X, X = [x1, x2,..., x p p], then the mutual information matrix between them is:

[0046]

[0047] (2) Calculate the eigenvalues of the mutual information matrix, arrange them in descending order, and find the corresponding eigenvectors. The formula is as follows:

[0048] B'∑I X B = Λ (2)

[0049] where B (B1, B2,..., B p p) is a matrix about the eigenvector Bβ, B' is the transpose of B, and Λ (μ1, μ2,..., μ p p) is a diagonal matrix containing the eigenvalues μ.

[0050] (3) Calculate the principal components of mutual information. The formula is as follows:

[0051] Z = B'X (3)

[0052] where Z(z1, z2, …, z p ) is a matrix of the principal components, and z k = B' k x k (k = 1, 2, …, p).

[0053] (4) Calculate the dimension m of the features, and the formula is as follows:

[0054]

[0055] where σ k is the contribution rate of the k-th principal component;

[0056]

[0057] where δ k is the sum of the contribution rates of the first k principal components. Generally speaking, when δ k reaches 85% - 95%, m = k.

[0058] In this embodiment, WASPAS mainly includes three steps, representing three optimizations respectively:

[0059] (1) It can achieve accurate evaluation of indicators, and the calculation formula is as follows:

[0060]

[0061] where w j is the weight of the j-th attribute set, and w j can be considered as the contribution degree σ k of the above principal components, or w j can be obtained by AHP;

[0062] n represents the total number of attribute sets; Q i (1) represents the first evaluation value of the i-th observation point; represents the score of the i-th observation point in the standardized j-th attribute set C j , and the calculation formula is as follows:

[0063]

[0064] where c ij represents the score of the i-th observation point in the j-th attribute set C j .

[0065] where the attribute set is the conditional attribute set with relatively high correlation with the decision attribute after the dataset is processed by the MIPCA model.

[0066] (2) It can highlight the contribution degree of the current data to the model accuracy, and the calculation formula is as follows:

[0067]

[0068] Among them, Q i (2) represents the second evaluation value of the i-th observation point.

[0069] (3) Add the results of (1) and (2) to combine the index evaluation and the data contribution degree, and improve the accuracy of the evaluation result. The calculation formula is as follows:

[0070]

[0071] Among them, Q i is the comprehensive evaluation value of the i-th observation point, λ and 1 - λ are the contribution degrees of Q i (1) and Q i (2) respectively, and λ = 0, …, 1.

[0072] In this embodiment, the calculation formula of the correction factor of the safety factor is as follows:

[0073]

[0074] Among them, a is the correction factor of the safety factor, Q i is the comprehensive evaluation value of the i-th observation point, min is the minimum value among the comprehensive evaluation values of all observation points, and max is the maximum value among the comprehensive evaluation values of all observation points.

[0075] In summary, for the mountainous complex geographical environment, the present invention integrates other risk factors of elbow defects on the basis of pipeline-related standards, and uses the method combining MIPCA-WASPAS to correct the safety factor in ASME B31G. For example, it screens out the risk factors related to pipeline elbow defects, then sets up a scoring system for these risk factors according to relevant standards, and scores all risk factors at each point according to case data, and finally determines the correction factor of the safety factor.

[0076] A20. Combine the correction factor to determine the range of the safety factor SF of the mountain pipeline.

[0077] Considering the complex loads on mountain pipelines, the complex mountain geographical environment, and the greater threat variables to oil and gas pipelines, etc., the present invention determines the safety factor according to the more conservative ASME B31G-2012, and at the same time adds a correction factor for the safety factor of mountain pipelines for more stringent constraints, which more accurately reflects the pipeline condition of mountain pipelines under the influence of elbow defects, that is:

[0078]

[0079] Among them, SF is the safety factor, P is the design pressure, P H is the minimum hydrostatic test pressure, MAOP is the maximum allowable operating pressure, and P0 is the operating pressure;

[0080] P F is the predicted failure pressure, which is the ratio of the design pressure P to the design coefficient F, that is P F is P f the maximum value of.

[0081] A30. According to the range of the said SF, determine the safety factor N for evaluation PSF .

[0082] The safety factor of the mountain elbow defect pipeline after considering the mountain safety factor: N PSF = N P × SF.

[0083] Among them, N P is the safety factor of the elbow defect pipeline without considering the mountain safety factor.

[0084] Exemplary embodiment 2

[0085] This exemplary embodiment provides a method for evaluating the remaining strength of a mountain pipeline with elbow defects.

[0086] The method may include the following steps:

[0087] B10. Determine the working pressure P of the pipeline w .

[0088] B20. Determine the ultimate internal pressure load P of the pipeline L .

[0089] B30. According to the method for determining the safety factor for evaluating the remaining strength of the mountain pipeline with elbow defects described in item of Exemplary embodiment 1, determine the safety factor N for evaluation PSF .

[0090] B40. Judge whether P w < P L / N PSFWhether it holds, and perform the evaluation according to the judgment result.

[0091] In this embodiment, the working pressure P w includes at least one of the following working pressures: actual maximum working pressure; pressure test pressure; expected maximum working pressure.

[0092] In this embodiment, step B20 may include: determining the material flow stress σ according to Equation 1 f ; determining the ultimate internal pressure load P according to Equation 2 L .

[0093] Equation 1 is:

[0094] where σ s is the yield strength of the material at the evaluation working condition temperature, σ b is the tensile strength of the material at the evaluation working condition temperature, φ is the pipe welding joint coefficient;

[0095] Equation 2 is:

[0096] where t c is the wall thickness used when calculating the ultimate internal pressure load, r m is the average radius of the elbow, G is the enhancement coefficient of the straight pipe sections at both ends of the elbow to the elbow, and R is the bending curvature radius of the elbow.

[0097] In this embodiment, when P w <P L / N PSF holds, the evaluation result is qualified, otherwise it is unqualified.

[0098] Exemplary Embodiment 3

[0099] This exemplary embodiment provides a method for evaluating the remaining strength of a mountain pipeline with elbow defects. Figure 2 Shows a flowchart of the method for evaluating the remaining strength of a mountain pipeline with elbow defects of the present invention.

[0100] The method may include the following steps:

[0101] S10. Determine an evaluation model for the remaining strength of a pipeline with elbow defects.

[0102] In this embodiment, determine an evaluation model for the remaining strength of a pipeline with elbow defects.

[0103] (1) According to the evaluation requirements of different levels, determine the calculated wall thickness of the pipe fitting body. For problems such as corrosion and erosion thinning, the corrosion rate of the pipe body should also be determined, and the correlation between the calculated wall thickness of the pipe fitting body and the expected remaining operation time should be established.

[0104] (2) Determine the geometric dimensions of the local thinning defect according to the evaluation requirements at different levels. For corrosion and erosion thinning problems, the corrosion rate of the pipe body should also be determined, and the correlation between the depth of the local thinning defect and the expected remaining operation time should be established.

[0105] (3) Determine the material flow stress σ f ;

[0106]

[0107] where σ s is the yield strength of the material at the evaluation working condition temperature; σ s = R ef or σ s = R p0.2 , and the meanings of R ef and R p0.2 are well-known in the art, such as referring to the yield point and yield strength respectively; σ b is the tensile strength of the material at the evaluation working condition temperature, σ b = R m , and the meaning of R m is well-known in the art and refers to the tensile strength. φ is the pipe welding joint coefficient, which should be considered when the thinning part is located at the weld, otherwise φ = 1.0.

[0108] (4) Determine the working pressure P w and the corresponding safety factor n p .

[0109] The pipeline may operate under various working conditions. The evaluator can select all or typical working conditions to determine the working pressure and the corresponding safety factor. n p is the safety factor corresponding to the working pressure P W , and it has the same meaning as Np below.

[0110] The determined working pressure should include but not be limited to the following working conditions: the actual maximum working pressure; the pressure test pressure when the evaluator expects that the pipeline may be subjected to a pressure test during subsequent operation; the expected maximum working pressure when the evaluator expects that the pipeline may be subjected to a pressure higher than the actual maximum working pressure during subsequent operation.

[0111] For the determined working pressure, the evaluator shall determine the safety factor for the evaluation calculation according to the severity of the consequences of pipeline failure under the corresponding working conditions: the minimum safety factor shall not be less than 1.2; the maximum safety factor is not recommended to exceed 1.8; for pipelines with general failure consequences, the safety factor can be taken as 1.25; for pipelines with serious failure consequences, the safety factor can be taken as 1.50; for the pressure test with water as the medium, it can be considered as having general failure consequences; for the pressure test with gas as the medium, it shall be considered as having serious failure consequences; when the working medium is flammable, explosive, toxic liquid, or gas, liquefied gas, or medium with a certain temperature, it is recommended to consider the safety factor as having serious failure consequences.

[0112] (5) Calculate the limit internal pressure P L

[0113] ① Geometric model parameters of the fitting body

[0114] The geometric parameters of the fitting body required for elbow evaluation include: nominal diameter d0, nominal wall thickness t, and bending radius R.

[0115] ② Determine the calculated wall thickness of the fitting body

[0116] The evaluation adopts the overall uniform thinning model of the fitting, and takes the measured minimum wall thickness of the fitting as the nominal wall thickness of the fitting. For elbows, the equal-thickness model is adopted, and the nominal wall thickness takes a single value; the calculated wall thickness of the fitting body is determined by the following formula.

[0117] t c =t - nc (2)

[0118] Among them, t c is the wall thickness used when calculating the limit internal pressure load; t is the nominal wall thickness of the fitting body, t = t min ; t min is the minimum measured wall thickness obtained from the inspection; n is the expected remaining operation period; c is the corrosion rate during the remaining operation period.

[0119] ③ Calculate the limit internal pressure load

[0120] For elbow fittings, calculate their limit internal pressure load according to the following formula (3):

[0121]

[0122] Among them, r m is the average radius of the elbow,

[0123] G is the enhancement coefficient of the straight pipe sections at both ends of the elbow to the elbow, and is calculated according to the following formula (4):

[0124]

[0125] where: k is the elbow diameter ratio,

[0126] (6) Each working pressure shall satisfy:

[0127] P w <[p] = P L / N PSF (5)

[0128] where [p] is the allowable internal pressure load under a certain working condition, and P L is the ultimate internal pressure of the elbow with defects and can be calculated according to the following text.

[0129] If equation (5) holds, the evaluation result is qualified; otherwise, it is unqualified.

[0130] S20. Determine the safety factor N for evaluating the remaining strength of the mountain pipeline with elbow defects according to the method for determining the safety factor described in item of Exemplary Embodiment 1. PSF , N PSF = N P × SF.

[0131] S30. Establish an evaluation model for the remaining strength of the mountain elbow defect pipeline according to steps B10 and B20.

[0132] Specifically, substitute the safety factor N determined in step B20 PSF into the relevant calculation formula in step B10 to establish an evaluation model for the remaining strength of the mountain elbow defect pipeline.

[0133] Furthermore, the method may further include the step of: performing an evaluation using the evaluation model established in step B30.

[0134] The evaluation method of the present invention is applicable to at least one of the following pipelines: steel buried pipelines with good plasticity and toughness of materials, buried pipeline sections with backfilling conditions meeting requirements, good soil support for the pipe body, and mainly bearing internal pressure loads, pipe sections of 90° elbows with full penetration welding structure and the minimum remaining wall thickness of the pipe fittings being greater than 2 mm, pipeline with a certain volume of corrosion and thinning defects, and pipelines with a working temperature lower than 300°C.

[0135] Although the present invention has been described above in conjunction with exemplary embodiments and the accompanying drawings, those of ordinary skill in the art should clearly understand that various modifications can be made to the above embodiments without departing from the spirit and scope of the claims.

Claims

1. A method for determining the safety factor for evaluating the remaining strength of mountain pipelines with elbow defects, characterized in that, The method includes the following steps: Determine the correction factor for the safety factor of the mountain pipeline with elbow defects; Combine the correction factor to determine the range of the safety factor SF of the mountain pipeline; Determine the safety factor N for evaluation according to the range of the said SF PSF , where N PSF = N P × SF, and N P is the safety factor of the elbow defect pipeline without considering the mountain safety factor; The steps of determining the correction factor include: selecting risk factors to form a data set; performing normalization using the MIPCA model; comprehensively analyzing the risk factors of elbow defects using the WASPAS method; determining the correction factor according to the comprehensive analysis results; Perform the comprehensive analysis using the following formula: Among them, Q i is the comprehensive evaluation value of the i-th observation point, and λ is the contribution degree, λ = 0, …, 1, w j is the weight of the j-th attribute set, is the score of the i-th observation point in the j-th attribute set C j after standardization; Determine the correction factor according to the following formula: where a is the correction factor, min is the minimum value among the comprehensive evaluation values of all observation points, and max is the maximum value among the comprehensive evaluation values of all observation points; The said is determined according to the following formula: Among them, c ij represents the score of the i-th observation point in the j-th attribute set C j ; The steps of performing normalization include: calculating the mutual information matrix of risk factors; calculating the eigenvalues of the mutual information matrix, arranging them, and finding the corresponding eigenvectors; calculating the principal components of mutual information; calculating the contribution rate of the principal components, and then determining the dimension of the features; Determine the range of the safety factor SF according to the following formula: Among them, SF is the safety factor, P is the design pressure, P H is the minimum hydrostatic test pressure, MAOP is the maximum allowable operating pressure, P0 is the operating pressure, F is the design factor, and a is the correction factor.

2. An evaluation method for the remaining strength of a mountain pipeline with elbow defects, characterized in that The method includes the following steps: Determine the working pressure P of the pipeline w ; Determine the ultimate internal pressure load P of the pipeline L ; According to the method for determining the safety factor for evaluating the remaining strength of a mountain pipeline with elbow defects as described in claim 1, determine the safety factor N for the evaluation PSF ; Determine P w <P L / N PSF Whether it holds, and perform the said evaluation according to the judgment result.

3. The method for evaluating the remaining strength of a mountain pipeline with elbow defects according to claim 2, characterized in that, The step of determining the ultimate internal pressure load P L comprises: Determine the material flow stress σ according to Equation 1 f ; Determine the limit internal pressure load P according to Equation 2 L ; Equation 1 is as follows: Among them, σ s is the yield strength of the material at the evaluation working condition temperature, σ b is the tensile strength of the material at the evaluation working condition temperature, and φ is the coefficient of the pipeline welded joint; Formula 2 is as follows: Among them, t c is the wall thickness used when calculating the limit internal pressure load, r m is the average radius of the elbow, G is the enhancement coefficient of the straight pipe segments at both ends of the elbow to the elbow, and R is the bending curvature radius of the elbow.

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