CN116990146A - Safety evaluation and disposal method for throttled low-temperature service oil and gas pipeline - Google Patents
Safety evaluation and disposal method for throttled low-temperature service oil and gas pipeline Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
- G01N3/18—Performing tests at high or low temperatures
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- G—PHYSICS
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- G—PHYSICS
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
- G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
- G01N27/83—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws by investigating stray magnetic fields
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- G—PHYSICS
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- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/04—Analysing solids
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/30—Investigating strength properties of solid materials by application of mechanical stress by applying a single impulsive force, e.g. by falling weight
- G01N3/303—Investigating strength properties of solid materials by application of mechanical stress by applying a single impulsive force, e.g. by falling weight generated only by free-falling weight
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- G—PHYSICS
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0001—Type of application of the stress
- G01N2203/001—Impulsive
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0014—Type of force applied
- G01N2203/0016—Tensile or compressive
- G01N2203/0017—Tensile
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- G—PHYSICS
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/02—Details not specific for a particular testing method
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- G01N2203/0222—Temperature
- G01N2203/0228—Low temperature; Cooling means
Abstract
The invention discloses a safety evaluation and treatment method of a throttled low-temperature service oil-gas pipeline, which comprises the steps of determining whether the low-temperature service oil-gas pipeline subjected to toughness evaluation contains an out-of-standard defect, and detecting the stress of the low-temperature Duan Quexian part of the low-temperature service oil-gas pipeline containing the out-of-standard defect; establishing a low-temperature failure evaluation chart of the low-temperature service oil-gas pipeline material by using a stress-strain curve of a defect-free pipe sample at the temperature of a pipeline medium; according to the low-temperature failure evaluation graph, low-temperature applicability evaluation is carried out on the low-temperature service oil-gas pipeline which passes through toughness evaluation and contains the out-of-standard defects, if the low-temperature service oil-gas pipeline passes through the low-temperature applicability evaluation, a safety margin S is calculated, and if the low-temperature service oil-gas pipeline does not pass through the low-temperature applicability evaluation, the low-temperature service oil-gas pipeline containing the out-of-standard defects is repaired or replaced, so that the evaluation and treatment method can effectively solve the safety problem of the low-temperature service oil-gas pipeline, particularly the low-temperature service oil-gas pipeline containing the defects under the low-temperature service condition.
Description
Technical Field
The invention belongs to the technical field of pipeline operation and maintenance, and particularly relates to a method for evaluating and disposing safety of a throttled low-temperature service oil and gas pipeline.
Background
In the natural gas conveying process, after the gas passes through the pressure regulating device, the gas can be rapidly expanded at a throttling position to generate pressure dip. Since the pressure drop often occurs in a short period of time, the gas in the tube cannot sufficiently exchange heat with the outside, so that the temperature drops suddenly. This phenomenon is known as the Joule-Thomson effect, also known as the "throttling effect". The research shows that under the 'throttling effect', the temperature is reduced by about 4-5 ℃ when the pressure of the gas pipeline is reduced by 1 Mpa. According to investigation, after three-stage throttling of natural gas in a certain station in the southwest, the service temperature of the pipeline is lower than-34 ℃. When the temperature of natural gas conveyed in the low-pressure gas conveying pipeline is reduced to below zero after throttling, the pipeline is in a low-temperature service state for a long time, and frosting and even icing can occur on the surface of the ground pipeline and equipment; the soil body around the buried pipeline is continuously condensed at low temperature, the volume is increased, frost heaving phenomenon is caused on the soil body around the pipeline, the soil body after frost heaving can push up the underground horizontal gas pipeline upwards, the pipeline displacement deformation, the ground bulge near the pipeline, the phenomenon that the wall body is propped up to be split by the wall penetrating pipeline, the valve body and the valve seat near the buried pipeline are separated from each other when the pipeline is out of the station, when the upward displacement exceeds a safety value, the stress generated by the stress concentration area of the pipeline is increased to the limit that the pipeline material can bear, deformation failure is necessarily caused on part of the pipeline section, such as pipeline rupture, welded junction rupture, flange joint leakage and the like, and great potential safety hazards are brought to the safe operation of the pipeline.
At present, for low-temperature service pipelines, most countries focus on material selection and limit the manufacturing and structure of the pipelines; in terms of safety evaluation of defects of pressure equipment, research is conducted in the united states, united kingdom, japan, korea, the netherlands, and other countries, and some standards also relate to methods of evaluating defects, such as PD6493, API RP579, BS7910, and the like, but none relate to methods and contents of evaluating suitability for low-temperature operation, and even do not relate to a treatment method.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a method for evaluating and disposing the safety of a throttled low-temperature service oil and gas pipeline, which effectively solves the safety problem of the low-temperature service oil and gas pipeline, particularly the low-temperature service oil and gas pipeline containing defects under the low-temperature service condition.
In order to achieve the above purpose, the present invention provides the following technical solutions: a method for evaluating and disposing the safety of throttled low-temperature service oil and gas pipeline comprises the following specific steps:
s1, performing toughness evaluation on a low-temperature service oil-gas pipeline, performing defect detection and stress detection on the low-temperature service oil-gas pipeline subjected to toughness evaluation, determining whether the low-temperature service oil-gas pipeline contains an out-of-standard defect according to the defect type and the defect size, and detecting the stress of the low-temperature Duan Quexian part of the low-temperature service oil-gas pipeline containing the out-of-standard defect;
s2, establishing a low-temperature failure evaluation chart of the low-temperature service oil-gas pipeline material according to a stress-strain curve of the defect-free pipe sample at the temperature of a pipeline medium;
s3, evaluating low-temperature applicability of the low-temperature service oil-gas pipeline which passes through toughness evaluation and contains the exceeding defect in the step S1 according to a low-temperature failure evaluation chart, calculating a safety margin S if the low-temperature service oil-gas pipeline passes through low-temperature applicability evaluation, and selecting safe operation or monitoring operation according to the value of the safety margin S; if the low-temperature service oil and gas pipeline does not pass the low-temperature applicability evaluation, repairing or replacing the low-temperature service oil and gas pipeline with the exceeding defect.
Further, in step S1, a nondestructive inspection method is adopted to detect defects of the low-temperature service oil and gas pipeline, so as to obtain defect types and defect sizes, the determined defect sizes include the self-heights of the defects, and the out-of-standard defects refer to defects judged to be above grade II by the nondestructive inspection;
and (3) performing stress detection by adopting a nondestructive stress detection method, and measuring the stress at the low temperature Duan Quexian of the low-temperature service oil and gas pipeline containing the out-of-standard defects.
Further, in step S1, the specific step of toughness evaluation is:
1) The ductile-brittle transition temperature T of the low-temperature service oil-gas pipeline is measured through impact test or drop hammer tearing test at a series of temperatures D-C Wherein the lowest value of the series of temperatures is lower than the tubing medium temperature T of the low-temperature service oil and gas pipeline:
2) When the temperature T of the pipe transmission medium is more than T D-C Evaluating the toughness of the low-temperature service oil gas pipeline;
3) When the temperature T of the pipe conveying medium is less than or equal to T D-C And the low-temperature brittle failure risk of the low-temperature service oil gas pipeline exists, and a pipe is replaced or a heating device is added before the pipeline is throttled.
Further, in step S1, T of the pipe for replacement is required at the time of pipe replacement D-C < T; the added heating device is a phase change heating furnace, a water jacket heating furnace, a heating medium heating furnace or an electric tracing heating belt.
Further, in step S1, when the low-temperature service oil gas pipeline containing the out-of-standard defect is a welded pipe, the parent metal of the pipeline and the welding should be measured respectivelyDuctile-brittle transition temperature T of five regions of seam and heat affected zone, girth weld and heat affected zone D-C The temperature T of the pipe conveying medium in any area is less than or equal to T D-C The low-temperature service oil and gas pipeline containing the exceeding defect cannot pass the toughness evaluation.
Further, in step S2, a low-temperature tensile test is performed on a non-defective pipe sample which is the same as the low-temperature service oil-gas pipeline material containing the out-of-standard defects, so as to obtain a stress-strain curve of the non-defective pipe sample at the temperature of the pipeline medium, and a low-temperature failure evaluation graph is established.
Further, in step S2, the low-temperature failure evaluation chart is built according to R6 criterion "integrity assessment of defect-containing Structure" issued by CEGB of the central electric power office in the united kingdom, the abscissa is the load ratio Lr, the degree of plastic instability of the structure is represented, the ordinate is the toughness ratio Kr, and the degree of fracture failure is represented.
Further, in step S2, the stress strain used in the low temperature failure evaluation chart is the true stress strain obtained by converting the actually measured engineering stress strain curve.
Further, in step S3, the specific steps for evaluating the low-temperature applicability of the defect are as follows:
1) Obtaining the defect type and defect size of a low-temperature section of the low-temperature service oil and gas pipeline containing the exceeding defect and the stress at the low temperature Duan Quexian of the low-temperature service oil and gas pipeline;
2) Combining the existing evaluation standard of the pipeline with the defect type, the defect size and the stress obtained in the step 1), and calculating the fracture failure degree Kr and the structural plastic instability degree Lr of the low-temperature service oil-gas pipeline containing the out-of-standard defects to obtain evaluation point coordinates (Lr, kr);
3) And (2) taking the evaluation point coordinates (Lr, kr) into the low-temperature failure evaluation graph constructed in the step (S2), wherein when the evaluation point coordinates (Lr, kr) are positioned in an area surrounded by the low-temperature failure evaluation curve and the coordinate axis, the low-temperature applicability evaluation is passed, and otherwise, the repair or the pipe replacement is required without the low-temperature applicability evaluation.
Further, in step S3, the zero point in the low-temperature failure evaluation graph is denoted as O, (Lr, kr) coordinate point is denoted as a, the intersection point of the OA extension line and the low-temperature failure evaluation curve is denoted as B, and the calculation of the safety margin S is as follows:
S＝AB/OB
when S is more than or equal to 0.3, the low-temperature service oil gas pipeline containing the out-of-standard defect can safely run without taking measures; s is less than 0.3, the low-temperature service oil and gas pipeline with the out-of-standard defect can continue to operate, but the stress change or the defect dynamic expansion condition around the defect of the low-temperature service oil and gas pipeline with the out-of-standard defect is required to be monitored.
Compared with the prior art, the invention has at least the following beneficial effects:
the invention provides an evaluation and treatment method of a low-temperature service oil and gas pipeline, which combines toughness evaluation and low-temperature applicability evaluation, provides a comprehensive evaluation method for the throttled low-temperature service oil and gas pipeline, allows continuous operation of an out-of-standard defect-containing pipeline which passes the toughness evaluation and the low-temperature applicability evaluation, and further provides a safety margin, and selects whether the out-of-standard defect-containing pipeline is subjected to safe operation or monitoring operation in subsequent operation management according to the value of the safety margin; and the pipelines which do not pass the low-temperature suitability evaluation are subjected to treatments such as repair and replacement. The invention has great significance for ensuring the safe operation of the low-temperature service pipeline, avoiding unnecessary pipe replacement, furthest playing the economic efficiency and improving the fine management decision level of the pipeline.
Furthermore, for the pipeline which does not pass the toughness evaluation, besides the pipe replacement, a treatment measure of increasing the temperature of a pipe transmission medium by adding heating devices such as a phase change heating furnace, a water jacket heating furnace, a heating medium heating furnace or an electric tracing heating belt and the like before the pipeline throttles is provided; and the defect pipeline with the safety margin less than 0.3 is assisted with monitoring measures such as stress monitoring at the defect part, defect dynamic expansion monitoring and the like in subsequent operation management.
In summary, the invention combines toughness evaluation and low-temperature applicability evaluation, provides a comprehensive evaluation method for throttled low-temperature service oil and gas pipelines, provides disposal modes under different conditions, can effectively evaluate the safety of pipelines in low-temperature service, particularly defective pipelines, due to throttling effect, provides guidance for the integrity management and maintenance of the pipelines, and fills up the domestic technical blank.
Drawings
FIG. 1 is a schematic diagram of a low temperature service oil and gas pipeline evaluation and disposal process after throttling.
FIG. 2 is a stress-strain curve obtained in an embodiment of the present invention.
FIG. 3 is a graph of low temperature failure evaluation (L485, -20 ℃ C.) obtained in the example of the present invention.
FIG. 4 shows the results of evaluation of low temperature suitability in the examples of the present invention.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the invention, as presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
As shown in FIG. 1, the method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline comprises the following steps:
step 1: performing toughness evaluation on the low-temperature service oil-gas pipeline, and performing defect detection and stress detection on the low-temperature service oil-gas pipeline subjected to the toughness evaluation:
1.1, evaluating toughness of a low-temperature service oil and gas pipeline containing an out-of-standard defect by adopting a conventional method, and specifically:
determination of ductile-brittle transition temperature T of low-temperature service oil-gas pipeline D-C ：
When the temperature T of the pipe transmission medium is more than T D-C The low-temperature service oil and gas pipeline containing the out-of-standard defects is evaluated through toughnessPrice;
when the temperature T of the pipe conveying medium is less than or equal to T D-C Judging that the low-temperature brittle failure risk exists in the low-temperature service oil-gas pipeline containing the exceeding defect, and replacing the pipeline or adding a heating device before throttling the pipeline to ensure that the temperature T of the throttled pipeline transportation medium H Is higher than the ductile-brittle transition temperature T of the pipe D-C 。
1.2, adopting a nondestructive inspection method to determine the defect type and defect size of a low-temperature section of the low-temperature service oil and gas pipeline, and determining whether the low-temperature service oil and gas pipeline subjected to toughness evaluation contains an out-of-standard defect or not;
specifically, the defect size measured by the nondestructive inspection method should include the self height of the defect, and the out-of-standard defect refers to a defect judged to be above grade II by nondestructive inspection, such as radial inspection, ultrasonic inspection, magnetic powder inspection, ultrasonic phased array inspection, TOFD inspection and the like;
1.3, measuring the stress at the low temperature Duan Quexian of the low-temperature service oil and gas pipeline containing the out-of-standard defects by adopting a nondestructive stress detection method.
Further, when the low-temperature service oil gas pipeline is a welded pipe, ductile-brittle transition temperatures T of a pipeline base metal, a welding line, a heat affected zone, a girth weld and five areas of the heat affected zone should be measured respectively D-C The temperature T of the pipe conveying medium in any area is less than or equal to T D-C The low temperature service oil and gas pipeline cannot pass the toughness evaluation.
Specifically, the ductile-brittle transition temperature can be measured by an impact test at a series of temperatures or a drop hammer tearing test at a series of temperatures, and the minimum value of the series of temperatures is lower than the tubing medium temperature T of the low-temperature service oil and gas pipeline.
Further, replacement of the pipe after no toughness evaluation is performed, and it is necessary to secure T of the pipe for replacement D-C ＜T。
Further, the heating device comprises a phase change heating furnace, a water jacket heating furnace, a heating medium heating furnace, an electric tracing heating belt or the like.
Step 2: and (5) establishing a low-temperature failure evaluation graph.
The low-temperature applicability evaluation is carried out on the low-temperature service oil gas pipeline which passes the toughness evaluation and contains the exceeding defect in the step 1, and the specific steps are as follows:
selecting a non-defective pipe sample which is the same as a low-temperature service oil and gas pipeline material containing the exceeding defect, carrying out a low-temperature tensile test to obtain a stress strain curve of the non-defective pipe sample at the temperature T of a pipeline medium, and establishing a low-temperature failure evaluation graph;
further, the stress strain used in the establishment of the low-temperature failure evaluation chart should be the true stress and true strain obtained by converting the actually measured engineering stress-strain curve.
Further, the low-temperature failure evaluation chart is established according to an R6 criterion 'integrity assessment of a defect-containing structure' issued by CEGB of the central electric power office in the United kingdom, wherein the abscissa is a load ratio Lr, the degree of plastic instability of the structure is represented, the ordinate is a toughness ratio Kr, and the degree of fracture failure is represented, wherein:
L r ＝σ ref /σ y
K r ＝0 L r ≥L rmax
wherein:
L r -load ratio;
K r -toughness ratio;
σ ref reference stress (including hoop stress and residual stress due to internal pressure), MPa;
σ y -yield strength of the material, MPa;
σ s -tensile strength of the material, MPa;
K I stress intensity factor, MPa.m 1/2 ；
K IC Fracture toughness value, MPa.m 1/2 ；
ε ref -reference strain, isTrue stress on uniaxial tensile true stress strain curve is equal to L r σ y Real strain at that time;
e-modulus of elasticity, MPa, e=σ/epsilon;
sigma-elastic stage normal stress, MPa;
epsilon-positive strain at elastic stage;
L rmax -evaluating the cut-off line of the curve.
Step 3: and evaluating and disposing the low-temperature applicability of the defects.
The existing evaluation standard of the pipeline is adopted, the fracture failure degree Kr and the structural plastic instability degree Lr of the out-of-standard defect are calculated by combining the defect type, the defect size and the stress measured by a nondestructive stress method, which are obtained by nondestructive inspection, and the calculated Kr and Lr are brought into a low-temperature failure evaluation chart established in the step 2, and the low-temperature applicability evaluation is carried out on the pipeline containing the out-of-standard defect, namely: when the evaluation point coordinates (Lr, kr) are located in the area surrounded by the failure evaluation curve and the coordinate axis, the evaluation is performed by the low-temperature suitability, otherwise, the evaluation cannot be performed by the low-temperature suitability.
The existing evaluation standard of the pipeline can be specifically used as an API 579 'integrated usability evaluation', GB/T1962 'safety evaluation of an in-use defective pressure vessel', SY/T6477 'residual strength evaluation method of a defective oil and gas pipeline', and the like.
For the defects passing the low-temperature applicability evaluation, selecting safe operation or monitoring operation after calculating a safety margin S; and repairing or replacing the defects which do not pass the low-temperature suitability evaluation.
Specifically, the calculation of the safety margin S is: in the low-temperature failure evaluation graph, zero point is marked as O, a (Lr, kr) coordinate point is marked as an intersection point of a A, OA extension line and a low-temperature failure evaluation curve is marked as B, and S=AB/OB.
Furthermore, S is more than or equal to 0.3, and the pipeline can safely run without taking measures; s is less than 0.3, the pipeline can continue to run, but the stress change or the defect dynamic expansion condition around the defect of the low-temperature service oil gas pipeline with the exceeding defect is monitored. .
Furthermore, stress change around the defect can be monitored by adopting stress, and the defect dynamic expansion condition can be monitored by adopting an ultrasonic phased array or acoustic emission.
Example 1
The embodiment provides a low-temperature service evaluation method of a phi 610 multiplied by 10mm L485 longitudinal submerged arc welded pipe, wherein the medium temperature in the pipe is minus 20 ℃, and the running internal pressure of the pipe is 2.96MPa. Comprises the following steps:
step 1: and (5) toughness evaluation. And measuring the ductile-brittle transition temperature of the material pipe body, the welding line and the heat affected zone by adopting a V-shaped notch Charpy impact test, wherein the test temperature is 0 ℃, -10 ℃, -20 ℃, -30 ℃, -40 ℃, -50 ℃, -60 ℃. The ductile-brittle transition temperature of the base metal is less than 60 ℃, the ductile-brittle transition temperature of the weld joint is-28 ℃, the ductile-brittle transition temperature of the heat affected zone is less than 60 ℃, the ductile-brittle transition temperature of the circumferential weld joint is-25 ℃, and the ductile-brittle transition temperature of the heat affected zone of the circumferential weld joint is less than 60 ℃. The ductile-brittle transition temperature of the welding seam and the circumferential weld is lower than the service temperature, the risk of low-temperature brittle fracture is avoided, and the toughness evaluation is carried out.
Step 2: and (5) defect detection and stress detection. Ultrasonic phased array detection shows that the girth weld has an out-of-standard defect at the 5 o' clock position, the length is 5mm, and the height is 5mm. The residual stress at the defect is 45MPa.
Step 3: and (5) establishing a low-temperature failure evaluation graph. Obtaining an engineering stress strain curve of the pipeline material at the temperature of minus 20 ℃ by adopting a low-temperature tensile test, converting the engineering stress strain curve into a true stress strain curve, and establishing a low-temperature failure evaluation graph of the material, as shown in figures 2 and 3;
step 4: and evaluating the low-temperature applicability of the defects. As a result of evaluation by using SY/T6477 standard to obtain the annular semi-elliptical inner surface crack, referring to FIG. 4, the defect can be evaluated by low-temperature applicability, S is more than or equal to 0.3, and the pipeline can be safely operated.
According to the invention, by establishing a comprehensive evaluation method of toughness evaluation and low-temperature adaptability evaluation, and providing clear evaluation and treatment measures for the low-temperature service oil and gas pipeline passing/not passing evaluation, the safety of the pipeline in low-temperature service, particularly the pipeline containing defects due to the throttling effect can be effectively evaluated, the economic efficiency of the pipeline is furthest exerted, and guidance is provided for the integrity management and maintenance of the pipeline.
The above is only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited by this, and any modification made on the basis of the technical scheme according to the technical idea of the present invention falls within the protection scope of the claims of the present invention.
Claims (10)
1. A method for evaluating and disposing safety of throttled low-temperature service oil and gas pipelines is characterized by comprising the following specific steps:
s1, performing toughness evaluation on a low-temperature service oil-gas pipeline, performing defect detection and stress detection on the low-temperature service oil-gas pipeline subjected to toughness evaluation, determining whether the low-temperature service oil-gas pipeline contains an out-of-standard defect according to the defect type and the defect size, and detecting the stress of the low-temperature Duan Quexian part of the low-temperature service oil-gas pipeline containing the out-of-standard defect;
s2, establishing a low-temperature failure evaluation chart of the low-temperature service oil-gas pipeline material according to a stress-strain curve of the defect-free pipe sample at the temperature of a pipeline medium;
s3, evaluating low-temperature applicability of the low-temperature service oil-gas pipeline which passes through toughness evaluation and contains the exceeding defect in the step S1 according to a low-temperature failure evaluation chart, calculating a safety margin S if the low-temperature service oil-gas pipeline passes through low-temperature applicability evaluation, and selecting safe operation or monitoring operation according to the value of the safety margin S; if the low-temperature service oil and gas pipeline does not pass the low-temperature applicability evaluation, repairing or replacing the low-temperature service oil and gas pipeline with the exceeding defect.
2. The method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline is characterized in that in the step S1, a nondestructive inspection method is adopted to detect defects of the low-temperature service oil and gas pipeline to obtain defect types and defect sizes, the detected defect sizes comprise the self heights of the defects, and the exceeding defects refer to defects which are evaluated as being above grade II by the nondestructive inspection;
and (3) performing stress detection by adopting a nondestructive stress detection method, and measuring the stress at the low temperature Duan Quexian of the low-temperature service oil and gas pipeline containing the out-of-standard defects.
3. The method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline according to claim 1, wherein in the step S1, the specific steps of evaluating the toughness are as follows:
1) The ductile-brittle transition temperature T of the low-temperature service oil-gas pipeline is measured through impact test or drop hammer tearing test at a series of temperatures D-C Wherein the lowest value of the series of temperatures is lower than the tubing medium temperature T of the low-temperature service oil and gas pipeline:
2) When the temperature T of the pipe transmission medium is more than T D-C Evaluating the toughness of the low-temperature service oil gas pipeline;
3) When the temperature T of the pipe conveying medium is less than or equal to T D-C And the low-temperature brittle failure risk of the low-temperature service oil gas pipeline exists, and a pipe is replaced or a heating device is added before the pipeline is throttled.
4. The method for evaluating and disposing the safety of a throttled low-temperature service oil and gas pipeline according to claim 3, wherein in the step S1, T of the pipeline for replacement is required during pipe replacement D-C < T; the added heating device is a phase change heating furnace, a water jacket heating furnace, a heating medium heating furnace or an electric tracing heating belt.
5. The method for evaluating and disposing the safety of a throttled low-temperature service oil and gas pipeline according to claim 3, wherein in the step S1, when the low-temperature service oil and gas pipeline containing the out-of-standard defects is a welded pipe, ductile-brittle transition temperatures T of five regions of a pipeline parent metal, a weld joint and a heat affected zone, a girth weld joint and a heat affected zone are measured respectively D-C The temperature T of the pipe conveying medium in any area is less than or equal to T D-C The low-temperature service oil and gas pipeline containing the exceeding defect cannot pass the toughness evaluation.
6. The method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline according to claim 1, wherein in the step S2, a low-temperature tensile test is performed by using a non-defective pipe sample which is the same as the material of the low-temperature service oil and gas pipeline and contains the exceeding defects, so as to obtain a stress-strain curve of the non-defective pipe sample at the temperature of a pipeline medium, and a low-temperature failure evaluation graph is established.
7. The method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline according to claim 6, wherein in the step S2, an abscissa of the low-temperature failure evaluation chart is a load ratio Lr, a structural plastic instability degree is represented, an ordinate is a toughness ratio Kr, and a fracture failure degree is represented.
8. The method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline according to claim 6, wherein in the step S2, the stress strain used in the establishment of the low-temperature failure evaluation chart is true stress obtained by converting an actually measured engineering stress-strain curve.
9. The method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline according to claim 1, wherein in the step S3, the specific steps for evaluating the applicability of the defect low temperature are as follows:
1) Obtaining the defect type and defect size of a low-temperature section of the low-temperature service oil and gas pipeline containing the exceeding defect and the stress at the low temperature Duan Quexian of the low-temperature service oil and gas pipeline;
2) Combining the existing evaluation standard of the pipeline with the defect type, the defect size and the stress obtained in the step 1), and calculating the fracture failure degree Kr and the structural plastic instability degree Lr of the low-temperature service oil-gas pipeline containing the out-of-standard defects to obtain evaluation point coordinates (Lr, kr);
3) And (2) taking the evaluation point coordinates (Lr, kr) into the low-temperature failure evaluation graph constructed in the step (S2), wherein when the evaluation point coordinates (Lr, kr) are positioned in an area surrounded by the low-temperature failure evaluation curve and the coordinate axis, the low-temperature applicability evaluation is passed, and otherwise, the repair or the pipe replacement is required without the low-temperature applicability evaluation.
10. The method for evaluating and disposing the safety of the throttled low-temperature service oil and gas pipeline according to claim 9, wherein in the step S3, zero points in a low-temperature failure evaluation graph are marked as O, (Lr, kr) coordinate points are marked as a, intersection points of an OA extension line and a low-temperature failure evaluation curve are marked as B, and the safety margin S is calculated as follows:
S＝AB/OB
when S is more than or equal to 0.3, the low-temperature service oil gas pipeline containing the out-of-standard defect can safely run without taking measures; s is less than 0.3, the low-temperature service oil and gas pipeline with the out-of-standard defect can continue to operate, but the stress change or the defect dynamic expansion condition around the defect of the low-temperature service oil and gas pipeline with the out-of-standard defect is required to be monitored.
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