A safety evaluation and disposal method for oil and gas pipelines serving at low temperature after throttling

Abstract

The application discloses a kind of safety evaluation and disposal method of throttling low-temperature service oil and gas pipeline, determine whether containing over-standard defect on low-temperature service oil and gas pipeline by toughness evaluation, detect the stress of low-temperature section defect of low-temperature service oil and gas pipeline containing over-standard defect;The stress-strain curve of defect-free pipe material sample at pipe medium temperature is used to establish the low-temperature failure evaluation diagram of low-temperature service oil and gas pipeline material;According to the low-temperature failure evaluation diagram, the low-temperature suitability evaluation of low-temperature service oil and gas pipeline containing over-standard defect and passing through toughness evaluation is carried out, if low-temperature service oil and gas pipeline passes through low-temperature suitability evaluation, then calculate safety margin S, if low-temperature service oil and gas pipeline does not pass through low-temperature suitability evaluation, then repair or replace pipe to low-temperature service oil and gas pipeline containing over-standard defect, the evaluation and disposal method of the application can effectively solve the safety problem of low-temperature service oil and gas pipeline, especially low-temperature service oil and gas pipeline containing defect under low-temperature service condition.

Description

Technical Field

[0001] This invention belongs to the field of pipeline operation and maintenance technology, specifically to a method for safety evaluation and handling of low-temperature oil and gas pipelines after throttling. Background Technology

[0002] During natural gas transportation, when gas passes through a pressure regulating device, it expands rapidly at the throttling point, causing a sudden pressure drop. Because this pressure decrease often occurs within a short period, the gas inside the pipeline cannot fully exchange heat with the outside environment, resulting in a rapid temperature drop. This phenomenon is known as the Joule-Thomson effect, also called the "throttling effect." Studies show that under the "throttling effect," for every 1 MPa decrease in pipeline pressure, the temperature drops by approximately 4°C to 5°C. According to a survey, after three stages of throttling, the service temperature of the natural gas pipeline at a station in Southwest China is below -34°C. When the temperature of the natural gas transported in the low-pressure gas pipeline drops below zero after throttling, the pipeline operates in a low-temperature state for a long time, and frost or even ice will appear on the surface of the ground pipeline and equipment. Meanwhile, the soil moisture around the buried pipeline will continuously condense due to the low temperature, and the volume will increase, causing frost heave in the soil around the pipeline. The frost heave will push the underground horizontal gas pipeline upward, causing pipeline displacement and deformation, ground heave near the pipeline, cracking of walls by pipelines passing through walls, and separation of valve bodies and valve seats near buried pipelines leaving the station. In severe cases, when the upward displacement of the pipeline exceeds the safety value, the stress generated in the stress concentration area of ​​the pipeline increases to the limit that the pipeline material can withstand, and some pipe sections will inevitably deform and fail, such as pipeline rupture, weld cracking, and leakage at flange connections, which will bring great safety hazards to the safe operation of the pipeline.

[0003] Currently, most countries focus their research on material selection for pipelines operating at low temperatures, and impose restrictions on manufacturing and structure. Regarding the safety evaluation of defects in pressure equipment, countries such as the United States, the United Kingdom, Japan, South Korea, and the Netherlands have conducted research, and some standards also involve defect evaluation methods, such as PD6493, API RP579, and BS7910. However, none of these standards cover methods and content for evaluating the suitability of operation at low temperatures, let alone handling methods. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a method for safety evaluation and treatment of low-temperature oil and gas pipelines after throttling, effectively solving the safety problems of low-temperature oil and gas pipelines, especially those with defects, under low-temperature service conditions.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for safety evaluation and handling of low-temperature oil and gas pipelines after throttling, the specific steps of which are as follows:

[0006] S1 performs toughness evaluation on low-temperature oil and gas pipelines, performs defect detection and stress detection on low-temperature oil and gas pipelines that pass the toughness evaluation, determines whether there are excessive defects on the low-temperature oil and gas pipelines according to the defect type and defect size, and detects the stress at the defect location in the low-temperature section of the low-temperature oil and gas pipeline containing excessive defects.

[0007] S2 establishes a low-temperature failure assessment diagram for oil and gas pipeline materials in low-temperature service based on the stress-strain curves of defect-free pipe samples at the temperature of the pipeline medium.

[0008] S3 evaluates the low-temperature suitability of oil and gas pipelines that passed the toughness evaluation in step S1 but have defects exceeding the standard, based on the low-temperature failure assessment diagram. If the low-temperature oil and gas pipeline passes the low-temperature suitability evaluation, the safety margin S is calculated, and safe operation or monitoring operation is selected based on the value of the safety margin S. If the low-temperature oil and gas pipeline fails the low-temperature suitability evaluation, the low-temperature oil and gas pipeline with defects exceeding the standard is repaired or replaced.

[0009] Furthermore, in step S1, non-destructive testing is used to detect defects in low-temperature oil and gas pipelines to obtain the defect type and defect size. The measured defect size should include the height of the defect itself. The defect exceeding the standard refers to a defect that is judged to be level II or above by non-destructive testing.

[0010] Stress testing was performed using non-destructive stress testing methods to determine the stress at the low-temperature section of a cryogenic oil and gas pipeline containing defects exceeding the standard.

[0011] Furthermore, in step S1, the specific steps for toughness evaluation are as follows:

[0012] 1) The ductile-brittle transition temperature T of cryogenic oil and gas pipelines is determined by impact tests or drop hammer tear tests at a series of temperatures. D-C The lowest value of the series of temperatures must be lower than the pipeline medium temperature T of the low-temperature oil and gas pipeline:

[0013] 2) When the temperature of the pipeline medium T > T D-C If so, the low-temperature service oil and gas pipeline passes the toughness evaluation;

[0014] 3) When the temperature of the pipeline medium T≤T D-C If the low-temperature oil and gas pipeline is at risk of low-temperature brittle fracture, it needs to be replaced or a heating device needs to be added before the pipeline is throttled.

[0015] Furthermore, in step S1, when replacing the pipe, the T value of the pipe to be replaced is required. D-C <T; The added heating device is a phase change heating furnace, a water jacket heating furnace, a heat medium heating furnace, or an electric heat tracing heating belt.

[0016] Furthermore, in step S1, when the low-temperature service oil and gas pipeline containing defects exceeding the standard is a welded pipe, the ductile-brittle transition temperature T of five regions—the pipeline base material, the weld and heat-affected zone, and the circumferential weld and heat-affected zone—should be measured separately. D-C The temperature of the pipeline medium in any region is T≤T D-C If the defects exceed the standard, the low-temperature oil and gas pipeline in service cannot pass the toughness evaluation.

[0017] Furthermore, in step S2, a low-temperature tensile test is conducted using a defect-free pipe sample identical to the low-temperature service oil and gas pipeline material containing excessive defects, to obtain the stress-strain curve of the defect-free pipe sample at the pipeline medium temperature, and to establish a low-temperature failure assessment diagram.

[0018] Furthermore, in step S2, the low-temperature failure assessment diagram is established based on the R6 standard "Integrity Assessment of Defective Structures" published by the Central Electricity Council (CEGB) of the United Kingdom. The horizontal axis is the load ratio Lr, which characterizes the degree of plastic instability of the structure, and the vertical axis is the toughness ratio Kr, which characterizes the degree of fracture failure.

[0019] Furthermore, in step S2, the stress and strain used when establishing the low-temperature failure assessment diagram are the true stress and true strain obtained by converting the measured engineering stress and strain curves.

[0020] Furthermore, in step S3, the specific steps for evaluating the low-temperature suitability of the defects are as follows:

[0021] 1) Obtain the defect type, defect size, and stress at the defect location in the low-temperature section of the low-temperature oil and gas pipeline containing defects exceeding the standard.

[0022] 2) Combine the existing evaluation criteria for pipelines with the defect type, defect size and stress obtained in step 1) to calculate the fracture failure degree Kr and structural plastic instability degree Lr of low-temperature oil and gas pipelines with defects exceeding the standard, and obtain the evaluation point coordinates (Lr, Kr).

[0023] 3) Substitute the evaluation point coordinates (Lr, Kr) into the low-temperature failure assessment diagram constructed in step S2. When the evaluation point coordinates (Lr, Kr) are located in the area enclosed by the low-temperature failure assessment curve and the coordinate axis, the low-temperature suitability evaluation is passed; otherwise, the low-temperature suitability evaluation is not passed, and repair or pipe replacement is required.

[0024] Furthermore, in step S3, the zero point in the low-temperature failure assessment diagram is denoted as O, the coordinate point (Lr, Kr) is denoted as A, the intersection of the extended line of OA and the low-temperature failure assessment curve is denoted as B, and the safety margin S is calculated as follows:

[0025] S = AB / OB

[0026] When S≥0.3, cryogenic oil and gas pipelines with defects exceeding the standard can operate safely without taking any measures; when S<0.3, cryogenic oil and gas pipelines with defects exceeding the standard can continue to operate, but it is necessary to monitor the stress changes around the defects or the dynamic expansion of the defects.

[0027] Compared with the prior art, the present invention has at least the following beneficial effects:

[0028] This invention provides a method for evaluating and handling cryogenic oil and gas pipelines. Combining toughness evaluation and cryogenic suitability evaluation, it offers a comprehensive evaluation method for cryogenic oil and gas pipelines after throttling. For pipelines with defects exceeding standards that pass both toughness and cryogenic suitability evaluations, continued operation is permitted, and a safety margin is further proposed. Based on the value of the safety margin, the method selects whether to implement safe operation or monitoring operation for pipelines with defects exceeding standards in subsequent operation and management. Pipelines that fail the cryogenic suitability evaluation are repaired or replaced. This invention is of great significance for ensuring the safe operation of cryogenic pipelines, avoiding unnecessary pipe replacements, maximizing their economic efficiency, and improving the level of refined pipeline management decision-making.

[0029] Furthermore, for pipelines that fail the toughness evaluation, in addition to pipe replacement, measures are provided to increase the temperature of the transported medium by adding heating devices such as phase change heaters, water jacket heaters, heat medium heaters, or electric heat tracing belts before pipeline throttling; for defective pipelines with a safety margin of <0.3, subsequent operation and management are supplemented with monitoring measures such as stress monitoring at the defect and dynamic expansion monitoring of the defect.

[0030] In summary, this invention combines toughness evaluation and low-temperature suitability evaluation to provide a comprehensive evaluation method for oil and gas pipelines operating at low temperatures after throttling. It also proposes handling methods under different conditions, which can effectively evaluate the safety of pipelines operating at low temperatures due to throttling effects, especially pipelines with defects. This provides guidance for pipeline integrity management and maintenance and fills a domestic technological gap. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the evaluation and handling process for low-temperature oil and gas pipelines after throttling.

[0032] Figure 2 The stress-strain curves obtained in an embodiment of the present invention are shown.

[0033] Figure 3 This is a low-temperature failure evaluation diagram (L485, -20℃) obtained in an embodiment of the present invention.

[0034] Figure 4 This is the result of the low-temperature applicability evaluation of the embodiments of the present invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0036] like Figure 1 As shown, a method for evaluating and addressing the safety of low-temperature oil and gas pipelines after throttling includes the following steps:

[0037] Step 1: Conduct toughness evaluation on cryogenic oil and gas pipelines, and perform defect detection and stress testing on cryogenic oil and gas pipelines that pass the toughness evaluation.

[0038] 1.1 Toughness evaluation of cryogenic oil and gas pipelines with excessive defects was conducted using conventional methods. Specifically:

[0039] Determination of the ductile-brittle transition temperature T of low-temperature oil and gas pipelines D-C :

[0040] When the temperature of the pipeline medium T > T D-C If the low-temperature oil and gas pipeline containing defects exceeding the standard passes the toughness evaluation, then the pipeline passes the toughness evaluation.

[0041] When the temperature of the pipeline medium T≤T D-C If a low-temperature oil and gas pipeline with excessive defects is deemed to have a risk of low-temperature brittle fracture, it needs to be replaced, or a heating device should be added before the pipeline is throttled to ensure that the temperature of the transported medium after throttling is T. H Higher than the ductile-brittle transition temperature T of the pipe D-C .

[0042] 1.2 Non-destructive testing was used to determine the defect type and size of the low-temperature section of the low-temperature oil and gas pipeline in service, and to determine whether the low-temperature oil and gas pipeline in service that has passed the toughness evaluation contains defects exceeding the standard.

[0043] Specifically, the defect size determined by non-destructive testing methods should include the height of the defect itself. Defects exceeding the standard refer to defects that are judged to be level II or above by non-destructive testing, such as X-ray testing, ultrasonic testing, magnetic particle testing, ultrasonic phased array testing, TOFD testing, etc.

[0044] 1.3 Non-destructive stress testing methods were used to determine the stress at the low-temperature section of the low-temperature oil and gas pipeline containing defects exceeding the standard.

[0045] Furthermore, when cryogenic oil and gas pipelines are welded pipes, the ductile-brittle transition temperature T0 should be measured separately for five regions: the pipeline base material, the weld and heat-affected zone, and the circumferential weld and heat-affected zone. D-C The temperature of the pipeline medium in any region is T≤T D-C If the oil and gas pipeline is operating at low temperatures, it will not pass the toughness evaluation.

[0046] Specifically, the ductile-brittle transition temperature can be determined by impact tests or drop hammer tear tests at a series of temperatures. The lowest value of the series of temperatures should be lower than the pipeline medium temperature T of the oil and gas pipeline in low-temperature service.

[0047] Furthermore, if pipe replacement fails the toughness evaluation, the T-value of the replacement pipe must be guaranteed. D-C <T.

[0048] Furthermore, the heating device includes a phase change furnace, a water jacket furnace, a heat transfer medium furnace, an electric heat tracing belt, or a similar device.

[0049] Step 2: Establish a low-temperature failure assessment diagram.

[0050] A cryogenic suitability evaluation is conducted on cryogenic oil and gas pipelines that passed the toughness evaluation in step 1 but contain defects exceeding the standard. Specifically:

[0051] Defect-free pipe samples, identical to those of low-temperature service oil and gas pipelines containing excessive defects, were selected for low-temperature tensile testing. Stress-strain curves of the defect-free pipe samples at pipeline medium temperature T were obtained, and a low-temperature failure assessment diagram was established.

[0052] Furthermore, the stress and strain used when establishing the low-temperature failure assessment diagram should be the true stress and true strain obtained by converting the measured engineering stress and strain curves.

[0053] Furthermore, the low-temperature failure assessment diagram was established based on the R6 criterion "Integrity Assessment of Structures with Defects" published by the UK Central Electricity Council (CEGB). The horizontal axis represents the load ratio Lr, characterizing the degree of plastic instability of the structure, and the vertical axis represents the toughness ratio Kr, characterizing the degree of fracture failure.

[0054] L r =σ ref / σ y

[0055]

[0056] K r =0 L r ≥L rmax

[0057]

[0058] In the formula:

[0059] L r —Load ratio;

[0060] K r —Toughness ratio;

[0061] σ ref —Reference stress (including circumferential stress caused by internal pressure and residual stress), MPa;

[0062] σ y — Yield strength of the material, MPa;

[0063] σ s —Tensile strength of the material, MPa;

[0064] K I —Stress intensity factor, MPa·m 1 / 2 ;

[0065] K IC — Fracture toughness value, MPa·m 1 / 2 ;

[0066] ε ref —The reference strain is the strain on the uniaxial tensile true stress-strain curve where the actual stress is equal to L. r σ y Real-time response;

[0067] E——Elastic modulus, MPa, E=σ / ε;

[0068] σ — Normal stress in the elastic stage, MPa;

[0069] ε—normal strain in the elastic stage;

[0070] L rmax —The cutoff line for the evaluation curve.

[0071] Step 3: Evaluation and handling of defects in low-temperature suitability.

[0072] Using existing pipeline evaluation standards, combined with the defect type, defect size obtained from non-destructive testing, and stress measured by non-destructive stress method, the fracture failure degree Kr and structural plastic instability degree Lr of the defects exceeding the standard are calculated. The calculated Kr and Lr are then substituted into the low-temperature failure assessment chart established in step 2 to conduct a low-temperature suitability evaluation of the pipeline containing defects exceeding the standard. That is, if the evaluation point coordinates (Lr, Kr) are located in the area enclosed by the failure assessment curve and the coordinate axis, the low-temperature suitability evaluation is passed; otherwise, the low-temperature suitability evaluation is not passed.

[0073] Existing evaluation standards for pipelines include API 579 "Compliance with Serviceability Assessment", GB / T 1962 "Safety Assessment of Defective Pressure Vessels in Use", and SY / T 6477 "Method for Evaluating Residual Strength of Defective Oil and Gas Pipelines".

[0074] For defects that pass the low-temperature suitability evaluation, calculate the safety margin S and select safe operation or monitoring operation; for defects that fail the low-temperature suitability evaluation, repair or replace the pipe.

[0075] Specifically, the safety margin S is calculated as follows: the zero point in the low-temperature failure assessment diagram is denoted as O, the coordinate point (Lr, Kr) is denoted as A, the intersection of the extension of OA and the low-temperature failure assessment curve is denoted as B, and S = AB / OB.

[0076] Furthermore, if S ≥ 0.3, the pipeline can operate safely without any intervention; if S < 0.3, the pipeline can continue to operate, but it is necessary to monitor the stress changes around the defects or the dynamic expansion of the defects in the cryogenic oil and gas pipeline containing excessive defects.

[0077] Furthermore, stress changes around the defect can be monitored using stress monitoring, and the dynamic expansion of the defect can be monitored using ultrasonic phased array or acoustic emission monitoring.

[0078] Example 1

[0079] This embodiment provides a low-temperature service evaluation method for a Φ610×10mm L485 straight seam submerged arc welded pipe. The internal medium temperature of the pipeline is -20℃, and the operating internal pressure of the pipeline is 2.96MPa. The method includes the following steps:

[0080] Step 1: Toughness Evaluation. A V-notch Charpy impact test was conducted at temperatures of 0℃, -10℃, -20℃, -30℃, -40℃, -50℃, and -60℃ to determine the ductile-brittle transition temperature of the material's tube body, weld, and heat-affected zone. The ductile-brittle transition temperature was <60℃ for the base metal, -28℃ for the weld, <60℃ for the heat-affected zone, -25℃ for the circumferential weld, and <60℃ for the heat-affected zone of the circumferential weld. Since the ductile-brittle transition temperatures of the weld and circumferential weld were below the service temperature, there was no risk of low-temperature brittle fracture, and the toughness evaluation was passed.

[0081] Step 2: Defect and stress detection. Ultrasonic phased array testing revealed an excessive defect at the 5 o'clock position of the circumferential weld, 5 mm in length and 5 mm in height. The residual stress at the defect was 45 MPa.

[0082] Step 3: Establishment of Low-Temperature Failure Assessment Chart. Using low-temperature tensile tests, the engineering stress-strain curves of the pipe material at -20℃ are obtained and converted to true stress-strain curves to establish a low-temperature failure assessment chart for the material, such as... Figure 2 ,3 As shown;

[0083] Step 4: Low-temperature suitability evaluation of defects. The evaluation was conducted using the circumferential semi-elliptical inner surface crack standard of SY / T 6477. Results are shown in [link to relevant documentation]. Figure 4 As can be seen, this defect can be evaluated for low-temperature suitability, and with S≥0.3, the pipeline can operate safely.

[0084] This invention establishes a comprehensive evaluation method that combines "toughness evaluation + low-temperature adaptability evaluation," and proposes clear evaluation and handling measures for low-temperature oil and gas pipelines that pass or fail the evaluation. It can effectively evaluate the safety of pipelines, especially those with defects, that are in low-temperature service due to throttling effects, and maximize their economic efficiency, providing guidance for pipeline integrity management and maintenance.

[0085] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for safety evaluation and handling of cryogenic oil and gas pipelines after throttling, characterized in that, The specific steps are as follows: S1 performs toughness evaluation on low-temperature oil and gas pipelines, performs defect detection and stress detection on low-temperature oil and gas pipelines that pass the toughness evaluation, determines whether there are excessive defects on the low-temperature oil and gas pipelines according to the defect type and defect size, and detects the stress at the defect location in the low-temperature section of the low-temperature oil and gas pipeline containing excessive defects. S2 establishes a low-temperature failure assessment diagram for oil and gas pipeline materials in low-temperature service based on the stress-strain curves of defect-free pipe samples at the temperature of the pipeline medium. In step S2, a low-temperature tensile test is conducted using a defect-free pipe sample identical to the material of the low-temperature service oil and gas pipeline containing excessive defects. The stress-strain curve of the defect-free pipe sample at the pipeline medium temperature is obtained, and a low-temperature failure assessment diagram is established. The horizontal axis of the low-temperature failure assessment diagram is the load ratio Lr, which characterizes the degree of structural plastic instability, and the vertical axis is the toughness ratio Kr, which characterizes the degree of fracture failure. The stress and strain used in establishing the low-temperature failure assessment diagram are the true stress and true strain obtained by converting the measured engineering stress and strain curves. S3 evaluates the low-temperature suitability of oil and gas pipelines that have passed the toughness evaluation in step S1 but have defects exceeding the standard, based on the low-temperature failure assessment diagram. If the low-temperature oil and gas pipeline passes the low-temperature suitability evaluation, the safety margin S is calculated, and safe operation or monitoring operation is selected based on the value of the safety margin S. If the low-temperature oil and gas pipeline fails the low-temperature suitability evaluation, the low-temperature oil and gas pipeline with defects exceeding the standard is repaired or replaced. In step S3, the specific steps for evaluating the low-temperature suitability of the defects are as follows: 1) Obtain the defect type, defect size, and stress at the defect location in the low-temperature section of the low-temperature oil and gas pipeline containing defects exceeding the standard; 2) Combine the existing evaluation criteria for pipelines with the defect type, defect size and stress obtained in step 1) to calculate the fracture failure degree Kr and structural plastic instability degree Lr of low-temperature oil and gas pipelines with defects exceeding the standard, and obtain the evaluation point coordinates (Lr, Kr). 3) Substitute the evaluation point coordinates (Lr, Kr) into the low temperature failure evaluation diagram constructed in step S2. When the evaluation point coordinates (Lr, Kr) are located in the area enclosed by the low temperature failure evaluation curve and the coordinate axis, the low temperature suitability evaluation is passed; otherwise, the low temperature suitability evaluation is not passed and repair or pipe replacement is required. In step S3, the zero point in the low-temperature failure assessment diagram is marked as O, the coordinate point (Lr, Kr) is marked as A, the intersection of the extended line of OA and the low-temperature failure assessment curve is marked as B, and the safety margin S is calculated as follows: S=AB / OB When S≥0.3, cryogenic oil and gas pipelines with defects exceeding the standard can operate safely without taking any measures; when S<0.3, cryogenic oil and gas pipelines with defects exceeding the standard can continue to operate, but it is necessary to monitor the stress changes around the defects or the dynamic expansion of the defects.

2. The method for safety evaluation and handling of cryogenic oil and gas pipelines after throttling, as described in claim 1, is characterized in that... In step S1, non-destructive testing is used to detect defects in low-temperature oil and gas pipelines to obtain the defect type and defect size. The measured defect size should include the height of the defect itself. The defect exceeding the standard refers to a defect that is judged to be level II or above by non-destructive testing. Stress testing was performed using non-destructive stress testing methods to determine the stress at the low-temperature section of a cryogenic oil and gas pipeline containing defects exceeding the standard.

3. The method for safety evaluation and handling of cryogenic oil and gas pipelines after throttling, as described in claim 1, is characterized in that... In step S1, the specific steps for toughness evaluation are as follows: 1) The ductile-brittle transition temperature T of cryogenic oil and gas pipelines is determined by impact tests or drop hammer tear tests at a series of temperatures. D-C The lowest value of the series of temperatures must be lower than the pipeline medium temperature T of the low-temperature oil and gas pipeline: 2) When the temperature of the pipeline medium T > T D-C If so, the low-temperature service oil and gas pipeline passes the toughness evaluation; 3) When the temperature of the pipeline medium T≤T D-C If the low-temperature oil and gas pipeline is at risk of low-temperature brittle fracture, it needs to be replaced or a heating device needs to be added before the pipeline is throttled.

4. The method for safety evaluation and handling of cryogenic oil and gas pipelines after throttling, as described in claim 3, is characterized in that... In step S1, when replacing the pipe, the T value of the pipe to be replaced is required. D-C <T; The added heating device is a phase change heating furnace, a water jacket heating furnace, a heat medium heating furnace, or an electric heat tracing heating belt.

5. The method for safety evaluation and handling of cryogenic oil and gas pipelines after throttling, as described in claim 3, is characterized in that... In step S1, when the low-temperature service oil and gas pipeline containing defects exceeding the standard is a welded pipe, the ductile-brittle transition temperature T of five regions should be measured separately: the pipeline base material, the weld and heat-affected zone, and the circumferential weld and heat-affected zone. D-C The temperature of the pipeline medium in any region is T≤T D-C If the defects exceed the standard, the low-temperature oil and gas pipeline in service cannot pass the toughness evaluation.

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