Repair method for circumferential weld of sulfur-containing gas field long-term service low-alloy steel gathering and transportation pipeline

Abstract

The invention relates to a method for repairing circumferential welds of a sulfur-containing gas field long-term service low-alloy steel gathering and transportation pipeline, and belongs to the technical field of metal pipe repairing. The repairing method comprises the following steps that dehydrogenation heat treatment and destressing annealing heat treatment are sequentially conducted on the low-alloy steel gathering and transportation pipeline circumferential weld to be repaired, and then the low-alloy steel gathering and transportation pipeline circumferential weld to be repaired is repaired through carbon fiber cloth. According to the repairing method disclosed by the invention, the circumferential weld is repaired from two aspects of material and stress aiming at the situation that the sulfur resistance of the circumferential weld of the low-alloy steel gathering and transportation pipeline serving in the sulfur-containing gas field for a long time is reduced and the stress cracking risk of sulfide is improved; according to the method, the sulfur resistance of the circumferential weld can be effectively repaired quickly and conveniently, the stress cracking risk of sulfide is reduced, the problem that an existing pipeline circumferential weld repairing method only aims at the defects of corrosion and the like is solved, and powerful technical support is provided for guaranteeing long-term service safety of a sulfur-containing gas field low-alloy steel gathering and transportation pipeline.

Description

Technical Field

[0001] The invention relates to a method for repairing a girth weld of a low-alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service, and belongs to the technical field of metal pipe repair. Background Art

[0002] Due to the significant advantages of natural gas in terms of low carbon, environmental protection, green and clean, the development volume of natural gas in China has also shown a good development trend of increasing year by year in recent years, and China's sour natural gas has become the backbone of natural gas production capacity. The development of such gas reservoirs faces production safety issues such as high sulfur corrosion, harsh environment, high toxicity, and serious consequences of accidents. China has formed a set of anti-corrosion process technology for ground gathering and transportation pipelines in sour gas fields, and the electrochemical corrosion of anti-sulfur carbon steel pipelines can be effectively controlled.

[0003] Low alloy steel for gathering and transportation is widely used in gathering and transportation pipelines due to its good economy, excellent mechanical properties and excellent weldability. Welding is the laying process of gathering and transportation pipelines. During welding, steel undergoes a series of complex non-equilibrium physical and chemical processes, which results in defects such as uneven chemical composition, coarse grains, and segregation of the weld zone. The failure risk of the weld is higher. In a hydrogen sulfide environment, the weld's resistance to sulfide stress cracking is lower than that of the parent material, which is a weak link in the safe operation of the pipeline.

[0004] During the service of oil and gas pipelines, corrosion and other damages may occur, resulting in many defects in the pipelines. The consequences are reduced pressure bearing capacity of the pipelines, leakage of the medium inside the pipelines, further safety hazards and affecting normal production. It is impossible to stop using oil and gas pipelines as soon as defects appear. In order to ensure the safe operation of defective pipelines and extend their service life, it is necessary to use appropriate methods to repair defective pipelines. The repair methods used for defective pipelines at home and abroad can be roughly divided into three categories: (1) Welding type: cladding, patching, and sleeves, which have the risks of welding penetration, hydrogen embrittlement and cold embrittlement, and are not suitable for pipelines containing sulfur media; (2) Clamp type: Clamp epoxy injection and clamp, which are suitable for emergency repairs but not for long-term service applications; (3) Fiber composite material type: glass fiber material and carbon fiber material, which have strong applicability and relatively wide application. For the repair of defective pipelines, fiber composite material type repair methods and processes have formed a series of standards, scientific and technological literature and patent literature. Carbon fiber is the preferred repair fiber composite material, which has the following two major technical advantages: (1) Mechanical performance advantage: The elastic modulus of carbon fiber is very close to that of steel (about 207 GPa), and the reinforcement layer and the steel pipe have very good deformation synergy; carbon fiber has high tensile strength. For pipeline steel with a yield strength of 400 to 500 MPa, the repair thickness is only about 1 / 6 to 1 / 7 of the steel thickness; even if the pipeline reaches the elastic deformation limit (R t0.5), the corresponding deformation is still only 36% of the deformation limit that carbon fiber can withstand. (2) Performance advantages: No welding or fire, greatly reducing the risk of operation; targeted design and flexible laying of thickness, number of layers, fiber distribution, etc. can be carried out according to the on-site pipeline conditions and stress conditions; the thickness of carbon fiber cloth is thin, does not increase the burden on the pipeline, and does not affect the original external anti-corrosion and insulation measures of the pipeline; carbon fiber has good creep resistance, and its strength remains basically unchanged for more than 10 years; the construction operation is relatively simple, does not require large-scale lifting equipment, and the operation time is short, and it can be completed in about 1 day. The standards "Q / SY GD 0215.1-2011 Technical Specifications for Carbon Fiber Repair and Reinforcement of Pipeline Defects", "Q / SY1592-2013 Technical Specifications for Oil and Gas Pipeline Body Repair", "GB / T30671-2018 Guidelines for Repair of Buried Steel Pipeline Body Defects" and "SY / T 6649-2018 Technical Specifications for Repair of Oil and Gas Pipeline Body Defects" standardize the applicable methods for pipeline defect repair according to the defect types and degrees of different pipelines, including repair materials, repair processes, and design of the number of repair fiber composite material layers.

[0005] Chinese patent document CN 1853847 B provides a method for repairing and reinforcing weld defects with carbon fiber composite materials, which is applied to weld defects, especially pipeline weld defects. The method includes wet-pasting carbon fiber sheets according to a certain size and number of layers on the weld and the surrounding area of ​​the weld, so that the required number of layers of carbon fiber sheets are tightly wound and pasted on the weld to be treated and the surrounding area of ​​the weld along the axial direction, circumferential direction or at a certain angle to the axial direction of the pipeline, so that two adjacent layers of composite materials are staggered in parallel, vertically or at a certain angle. The defect reinforcement parameter design method determines the thickness, width and amount of reinforcement material of the reinforcement layer. The method can make the pressure bearing capacity of the final composite repair layer reach and exceed the pressure bearing capacity of the original weld. Chinese patent document CN 106238970B discloses a method for repairing and reinforcing a boss corner weld with composite materials, which includes, according to the reinforcement width, the number of reinforcement layers and the bevel requirements of the boundary, sequentially cross-winding and spirally winding the steel pipe with the cut composite fiber cloth + interlayer epoxy resin adhesive dipped adhesive, overlapping each other by 50% during the spiral winding, and smoothing the boundary bevel into a slope of 15±5°. This method avoids safety accidents such as leakage during operation, especially the phenomenon that cracks in the cross weld extend and crack along the straight weld area of ​​the performance main pipeline, thereby avoiding the occurrence of serious disasters and losses. Chinese patent document CN 114909542A provides an oil and gas pipeline defect repair device and method, including: two semi-cylindrical barrels, the upper and lower parts of the two semi-cylindrical barrels are connected by a detachable connecting piece to form a cylindrical repair barrel, an annular weld groove is provided in the repair barrel, and the defective position of the pipe body can be quickly wrapped, and the fixing object winding includes one or more of the following winding methods: polyethylene cold wrapping tape fixed winding, polypropylene cold wrapping tape fixed winding, iron chain winding and locking, epoxy resin brushing, epoxy coal tar brushing, glass fiber epoxy coal tar brushing, epoxy resin carbon fiber composite material brushing winding, and the repair barrel is installed by a detachable connection method, which can improve work efficiency.

[0006] For pipelines that transport sour natural gas, due to hydrogen sulfide, as the service time increases, the performance of the pipeline welds that transport hydrogen sulfide media will decline and the risk of cracking will increase. The study of the document "Study on the Plasticity and Toughness of X52 Gas Pipeline Steel in Service" (Natural Gas Industry, Vol. 26, No. 3, March 2003) shows that as the service time of X52 gas pipelines that transport H2S increases, their elongation and J integral value will decrease, and the decrease in the weld and its vicinity is greater than that of the parent material, and the weld has an increased risk of brittle bursting. The study of the document "Corrosion Detection and Safety Analysis of Wet Gas Transport Natural Gas Pipelines" (Oil and Gas Storage and Transportation, Vol. 25, No. 11, November 2006) shows that as the service time of No. 20 steel gas pipelines that transport H2S increases, the weld has a tendency to HIC, and the tendency of brittle cracking in the weld increases.

[0007] The degradation of weld performance will bring great risks to the safe operation of pipelines transporting hydrogen sulfide media, and the current pipeline weld repair process is only for defects such as corrosion. A large number of low-alloy sulfur-resistant pipeline steel prefabricated pipelines are used in the gathering and transportation pipelines of sour gas fields to form girth welds by butt welding. There is still a lack of repair methods for the degradation of the anti-sulfur performance of the girth welds of pipelines that have been in service for a long time to transport hydrogen sulfide media. Therefore, there is an urgent need for a repair method for the anti-sulfur performance of the girth welds of low-alloy steel gathering and transportation pipelines that have been in service for a long time in sour gas fields to ensure the long-term service safety of low-alloy steel gathering and transportation pipelines in sour gas fields. Summary of the invention

[0008] The purpose of the present invention is to provide a method for repairing the girth weld of a low-alloy steel gathering and transportation pipeline in a sour gas field in long-term service, which can ensure the long-term service safety of the low-alloy steel gathering and transportation pipeline in the sour gas field.

[0009] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0010] A method for repairing the girth weld of a low-alloy steel gathering and transportation pipeline in long-term service in a sulfur-containing gas field comprises the following steps: performing dehydrogenation heat treatment and stress relief annealing heat treatment on the girth weld of the low-alloy steel gathering and transportation pipeline to be repaired in sequence, and then repairing the girth weld of the low-alloy steel gathering and transportation pipeline to be repaired by using carbon fiber cloth.

[0011] The method for repairing the girth weld of a low-alloy steel gathering and transportation pipeline in sour gas fields for a long time of service is designed to repair the girth weld from the aspects of material and stress, aiming at the decline in the anti-sulfur performance of the girth weld of the low-alloy steel gathering and transportation pipeline in sour gas fields for a long time of service and the increase in the risk of sulfide stress cracking. The method can quickly and conveniently effectively repair the decline in the anti-sulfur performance of the girth weld and reduce the risk of sulfide stress cracking, solve the problem that the existing pipeline girth weld repair method only targets defects such as corrosion, and provides strong technical support for ensuring the long-term service safety of the low-alloy steel gathering and transportation pipeline in sour gas fields.

[0012] Furthermore, the temperature of the dehydrogenation heat treatment is T1°C, 250°C≤T1≤350°C; the time of the dehydrogenation heat treatment is t1 hours, t1 satisfies: 1.829 hours≤t1≤5.76 hours; the product of T1 and t1 satisfies: 640°C·h≤T1×t1≤1440°C·h. For example, the temperature of the dehydrogenation heat treatment is 320°C and the time is 1 hour.

[0013] The temperature of stress relief heat treatment is lower than the phase transition temperature of low alloy steel. The temperature of stress relief heat treatment is lower than the phase transition temperature of low alloy steel. During the stress relief heat treatment, no phase transition occurs in low alloy steel, the structure of low alloy steel does not change significantly, and the beneficial properties of low alloy steel do not change significantly. Furthermore, the temperature of the stress relief annealing heat treatment is T2, 600℃≤T2≤630℃; the time of stress relief annealing heat treatment is t2 hours, t2 satisfies: 1.034 hours≤t1≤2.325 hours; the product of T2 and t2 satisfies: 620℃·h≤T2×t2≤1395℃·h. For example, the temperature of stress relief annealing heat treatment is 620℃ and the time is 1h.

[0014] The temperature rise before the dehydrogenation heat treatment and the stress relief annealing heat treatment is a linear temperature rise. The temperature drop after the annealing heat treatment is also a linear temperature drop. Furthermore, the temperature rise rate to the dehydrogenation heat treatment temperature is ≤200°C / hour, and the temperature rise rate from the dehydrogenation heat treatment temperature to the stress relief annealing heat treatment temperature is ≤200°C / hour. The temperature drop rate from the stress relief annealing heat treatment temperature to room temperature is ≤260°C / hour.

[0015] Furthermore, the method for repairing the girth weld of the low alloy steel gathering pipeline to be repaired using carbon fiber cloth comprises the following steps: sticking the carbon fiber cloth to the girth weld to be repaired and around the girth weld to be repaired and wrapping the girth weld to be repaired.

[0016] Furthermore, the carbon fiber cloth includes plain carbon fiber cloth and twill carbon fiber cloth. When pasting the carbon fiber cloth, the plain carbon fiber cloth and the twill carbon fiber cloth are laminated and pasted; a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are used as a carbon fiber cloth combination, and one carbon fiber cloth combination is pasted each time; after pasting, the fiber bundles of the plain carbon fiber cloth and the fiber bundles of the twill carbon fiber cloth in each carbon fiber cloth combination are cross-woven. The carbon fiber cloth is a bidirectional weave of carbon fiber. Since the girth weld of the gathering and transportation pipeline is subjected to radial pressure and axial tension in the pipeline, the carbon fiber cloth combination includes a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth, and utilizes the excellent bending strength and tensile strength of the plain weave and the excellent shear performance of the twill weave to enhance the ability of the girth weld of the gathering and transportation pipeline to withstand multi-directional tension. Bidirectional weaving here refers to a carbon fiber cloth in which the carbon fiber bundles are woven in both the warp and weft directions.

[0017] Furthermore, after pasting, the fiber bundles of the plain carbon fiber cloth and the fiber bundles of the twill carbon fiber cloth in each carbon fiber cloth combination are cross-crossed at an angle of 45°.

[0018] Furthermore, the number of layers of the bonded carbon fiber cloth combination is determined according to the following formula:

[0019]

[0020] Where: n is the number of layers of carbon fiber cloth combination, rounded up, ≥2;

[0021] h c —Theoretical total thickness of a single layer of carbon fiber cloth after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated, mm;

[0022] α—safety factor, ranging from 1 to 2;

[0023] σ cfrp —The design value of tensile strength of a carbon fiber cloth combined with a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth, MPa;

[0024] t—pipe wall thickness, mm;

[0025] σ s —Minimum yield strength of pipeline steel, MPa.

[0026] In determining σ cfrp When a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are superimposed, the angle between the fiber bundles of the plain carbon fiber cloth and the fiber bundles of the twill carbon fiber cloth is the same as the angle between the fiber bundles of the plain carbon fiber cloth and the fiber bundles of the twill carbon fiber cloth in each carbon fiber cloth combination after pasting.

[0027] Furthermore, when pasting the carbon fiber cloth, with the circumferential weld to be repaired as the center, the pasting length of the carbon fiber cloth along the axial direction of the gathering and transportation pipeline (the width of the repair reinforcement layer) is determined according to the following formula:

[0028] l=β×(2l over +l defect +2l taper );

[0029] Where: l is the bonding length of the carbon fiber cloth along the axial direction of the gathering and transportation pipeline (the width of the repair and reinforcement layer), mm;

[0030] β—safety factor, ranging from 1 to 2;

[0031] l over —The length of the carbon fiber cloth extending outward along the axial direction of the pipeline within the area completely covered by the carbon fiber cloth in the pipeline circumferential direction from the edge of the pipeline welding joint area is calculated according to the following formula: over =2.5×(0.5Dt) 1 / 2 , where D is the outer diameter of the pipe, mm; t is the wall thickness of the pipe, mm; l over When the calculated value is less than 100mm, take 100mm; when the calculated value is greater than 100mm, follow the calculated value;

[0032] l defect —Length and axial width of the welded joint, mm. The welded joint includes the girth weld and the heat-affected zone;

[0033] l taper —Length of buffer area at the end of carbon fiber cloth, mm; l taper ≥5δ cfrp , δ cfrp It is the total design thickness of all carbon fiber cloths to be pasted.

[0034] The buffer area in the present invention refers to a conical area on the pipe surface where the carbon fiber cloth is pasted but not completely covered by each layer of carbon fiber cloth after the carbon fiber cloth is pasted. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below in conjunction with specific implementation methods.

[0036] Whether the girth weld of the low alloy steel gathering and transportation pipeline in the sour gas field needs to be repaired can be evaluated by the anti-sulfur performance. The specific method includes the following steps:

[0037] (1) Collection and analysis of basic parameters.

[0038] The basic parameters include the natural gas composition of high-sulfur gas fields (including hydrogen sulfide content, carbon dioxide content, etc.), natural gas water content (including water composition, etc.), gathering and transportation pipeline operation parameters (including temperature, pressure, flow rate, service time, etc.), gathering and transportation pipeline pipe materials (including steel model, composition, mechanical properties, etc.), gathering and transportation pipeline girth weld parameters (including welding process, welding joint performance, etc.), etc.

[0039] (2) Evaluation of sulfur resistance of girth welds.

[0040] Low-alloy gathering and transportation pipelines serving in sour gas fields are subject to sulfide stress cracking due to hydrogen sulfide, which is potentially harmful, sudden and catastrophic. Therefore, the sulfur resistance of girth weld joints is the most critical and core parameter of their performance. The sulfide stress cracking resistance of girth welds is used as an evaluation indicator of their sulfur resistance.

[0041] The standards "Q / SH 0248-2009 Specification for Welding Construction and Acceptance of High-Sulfur Natural Gas Pipeline Engineering" and "SY / T4117-2016 Technical Specification for Welding of Gathering and Transportation Pipelines in High-Sulfur Gas Fields" stipulate the requirements for the sulfide stress cracking resistance of the girth welds of low-alloy gathering and transportation pipelines in high-sulfur gas fields: According to the standard "NACE TM 0177 Standard Test Method for Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments", solution A is used, the weld specimen is loaded with stress by the four-point bending method (the weld is located in the middle of the specimen), the loading stress is 80% of the minimum specified yield strength of the parent material, the test period is 720 hours, the test temperature is 24±3℃, and the surface of the specimen is observed after the test for cracks and cracks.

[0042] Take the girth weld sample on site and judge according to the test results in full accordance with the above test method:

[0043] ① After the test, there is no cracking or cracking on the surface of the sample, indicating that the sulfur resistance of the girth weld still meets the requirements for the sulfide stress cracking resistance of the low-alloy gathering and transportation pipeline welds in sulfur-containing gas fields;

[0044] ② After the test, cracks or cracks appear on the surface of the specimen, indicating that the sulfur resistance of the girth weld has decreased, and there is a risk of sulfide stress cracking, which requires repair.

[0045] Note: In the case where it is not possible to take girth weld samples on site, a reference assessment can be made based on the evaluation results of similar working conditions.

[0046] Low-alloy gathering and transportation pipelines (including parent materials and welded joints) serving in high-sulfur gas fields react with hydrogen sulfide (corrosion-sensitive environment) to generate hydrogen, which penetrates into the pipe (including parent materials and welds) and migrates and accumulates in strong hydrogen traps (defects such as uneven chemical composition, coarse grains, and structural segregation in the weld area, which are strong hydrogen trap aggregation areas) under the action of stress (including the pipeline operating pressure borne by the pipeline itself, and residual stress caused by pressure, pressure fluctuations, external extrusion, etc. during the pipeline service process). When the amount of hydrogen and stress reach the critical value, sulfide stress cracking occurs.

[0047] Sulfide stress cracking of girth welds requires the coupling of three factors: hydrogen sulfide corrosion sensitive environment, materials, and stress, among which:

[0048] ① Hydrogen sulfide corrosion-sensitive environment: service conditions cannot be changed;

[0049] ②Material: The girth weld itself cannot be changed, but the hydrogen that has penetrated into the high-sulfur long-term service can be removed;

[0050] ③Stress: The operating pressure of the pipeline itself cannot be changed, and the residual stress caused by pressure, pressure fluctuation, external extrusion, etc. during the service of the pipeline can be removed; at the same time, a fiber composite material repair layer can be added to the outside of the pipeline girth weld to bear part of the operating pressure of the pipeline girth weld, thereby reducing the operating pressure borne by the pipeline girth weld body and partially removing the girth weld stress again.

[0051] The gathering and transportation pipeline of a high-sulfur gas field in China is welded with L360 low-alloy gathering and transportation pipes. It integrates the five elements of "sulfur-resistant pipes + corrosion inhibitors + corrosion monitoring + cathodic protection + intelligent pipe cleaning" and the anti-corrosion process technology of the high-sulfur wet gas gathering and transportation system. The average corrosion rate is maintained below 0.076mm / a and it has been in operation for 11 years.

[0052] (1) Sample specifications

[0053] Take the on-site pipeline girth weld section and process it into a sulfide stress cracking specimen (original specimen) according to Method E: Four-point bending test in the standard GB / T 4157-2017 Laboratory Test Method for Resistance of Metals to Sulfide Stress Cracking and Stress Corrosion Cracking in Hydrogen Sulfide Environments. The four-point bending specimen is a rectangular straight metal with uniform thickness and width. The specimen dimensions are: length 115.0±1.3mm, width 15.0±0.13mm, thickness 5.0±0.13mm, the weld is located in the center of the specimen, and the surface roughness Ra of the specimen is lower than 0.8μm specified in GB / T 1031.

[0054] (2) Evaluation of sulfur resistance of original samples

[0055] Solution A in 8.2 of GB / T 4157-2017 Laboratory Test Method for Metal Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in Hydrogen Sulfide Environment was used. The stress loading method of the weld specimen was the four-point bending method (refer to GB / T4157-2017 Laboratory Test Method for Metal Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in Hydrogen Sulfide Environment for details), the test period was 720 hours, and the test temperature was 24±3℃; the loading stress was 80% of the minimum specified yield strength of the parent material (L360), and the specimen cracked after the test; the loading stress was 72% of the minimum specified yield strength of the parent material (L360), and the specimen cracked after the test. This indicates that the sulfur resistance of the girth weld has decreased and the risk of sulfide stress cracking has increased. Solution A was prepared by dissolving 5.0% NaCl and 0.5% CH3COOH in distilled water, respectively, and the pH value was 2.6-2.8 before the solution came into contact with the specimen and before the H2S gas was saturated.

[0056] The girth welds used for repair in the following Examples 1 to 6 are formed by butt welding of low alloy steel L360 pipes (only 10% of the pipe wall thickness from the outer surface is welded, and 90% of the pipe wall thickness from the inner wall of the pipe is not welded) using a welding process of a combination of gas shielded welding and arc welding (the welding process is consistent with the welding process of a certain domestic high sulfur gas field gathering and transportation pipeline mentioned above). The outer diameter of the low alloy steel L360 pipe is 159 mm and the wall thickness is 15 mm. Both ends of the pipe after welding are sealed with welding blind plates.

[0057] Example 1

[0058] The method for repairing the girth weld of a low-alloy steel gathering and transportation pipeline in a sour gas field in long-term service of the present embodiment comprises the following steps:

[0059] 1) The girth weld to be repaired is heated to 320°C at a heating rate of 3°C / min and kept at that temperature for 2.0h for dehydrogenation heat treatment, and then the temperature is raised from 320°C to 620°C at a heating rate of 3°C / min and kept at that temperature for 1.0h for stress relief annealing heat treatment. After the insulation is completed, the temperature is lowered to room temperature at a rate of 4°C / min;

[0060] 2) Then, a plain carbon fiber cloth and a twill carbon fiber cloth are laminated and pasted on the ring weld to be repaired and its surroundings. The plain carbon fiber cloth and the twill carbon fiber cloth are both bidirectionally woven carbon fibers. After pasting each layer of plain carbon fiber cloth, a layer of twill carbon fiber cloth is pasted on the plain carbon fiber cloth. After pasting, the extension direction of one of the two groups of vertical fiber bundles in each plain carbon fiber cloth is consistent with the extension direction of the pipeline; each plain carbon fiber cloth to be pasted and the twill carbon fiber cloth to be pasted on the corresponding plain carbon fiber cloth are regarded as a carbon fiber cloth combination. After pasting, the fiber bundles of the plain carbon fiber cloth and the fiber bundles of the twill carbon fiber cloth in each carbon fiber cloth combination are obliquely crossed at 45 degrees.

[0061] The number of carbon fiber cloth combinations consisting of pasted plain carbon fiber cloth and twill carbon fiber cloth is calculated as follows:

[0062] a) Establish the calculation formula for the combined tensile strength of carbon fiber cloth:

[0063]

[0064] Where: cfrp —The design value of tensile strength of the carbon fiber cloth combination after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated (the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth are 45° oblique), MPa;

[0065] f fk —The standard value of tensile strength of a carbon fiber cloth combination after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated (the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth are cross-crossed at 45°), MPa;

[0066] γ f —The strength partial coefficient of the carbon fiber combination after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated (the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth are 45° oblique), take 1.4;

[0067] γ e —The environmental impact coefficient of the carbon fiber cloth combination after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated (the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth are obliquely crossed at 45 degrees) is taken as 1.2.

[0068] Based on standards: "GB / T 37195-2018 Technical Specification for Corrosion-Resistant Repair of Resin-Based Pipelines" and "GB50608-2020 Technical Standard for Engineering Application of Fiber-Reinforced Composite Materials"

[0069] b) Establish the calculation formula for the total circumferential thickness of carbon fiber cloth:

[0070]

[0071]

[0072] Where: n is the number of layers of carbon fiber cloth combination, rounded up, ≥2;

[0073] h c —Theoretical total thickness of a single layer of carbon fiber cloth after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated, mm;

[0074] δ cfrp —Total hoop thickness of carbon fiber cloth, mm;

[0075] α—safety factor, ranging from 1 to 2;

[0076] D—pipeline outer diameter, mm;

[0077] P total —The pressure that the pipeline is designed to withstand, MPa;

[0078] σ cfrp —The design value of tensile strength of the carbon fiber cloth combination after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated (the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth are 45° oblique), MPa;

[0079] t—pipe wall thickness, mm;

[0080] σ s —Minimum yield strength of pipeline steel, MPa.

[0081] When pasting the carbon fiber cloth, the circumferential weld to be repaired is taken as the center, and the pasting length of the carbon fiber cloth along the axial direction of the gathering and transportation pipeline (the width of the repair reinforcement layer) is determined according to the following formula:

[0082] l=β×(2l over +l defect +2l taper );

[0083]

[0084] Where: l—bonding length of carbon fiber cloth along the axial direction, mm;

[0085] β—safety factor, 1 to 2;

[0086] l over —The length of the carbon fiber cloth extending outward along the axial direction of the pipe within the area completely covered by each layer of carbon fiber cloth around the pipe, measured from the edge of the pipe welding joint area, l over When the calculated value is less than 100mm, take 100mm; when the calculated value is greater than 100mm, follow the calculated value;

[0087] l defect —Axial width of welded joint (including girth weld and heat-affected zone), mm;

[0088] l taper —The length of the buffer area at the end of the carbon fiber cloth, mm;

[0089] δ cfrp —Total design thickness of all carbon fiber sheets to be bonded, mm;

[0090] D—pipeline outer diameter, mm;

[0091] t—pipe wall thickness, mm.

[0092] Using the equal strength principle, for safety reasons, the total pressure bearing capacity of the carbon fiber cloth is designed according to the pressure of the pipeline 12MPa, thereby reducing the pressure on the welding joint. The specifications of plain carbon fiber cloth and twill carbon fiber cloth are both 12K, 400g / m 2 , the design thickness of plain carbon fiber cloth and twill carbon fiber cloth are both 0.35mm, so the theoretical total thickness of a single layer of carbon fiber cloth combined after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth is 0.7mm, and both are bidirectional. After calculation, n = 0.9, that is, 0.9 "a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth combined" can reach a pressure of 12MPa; the safety factor α is 1; according to the calculation formula for the total circumferential thickness of carbon fiber cloth, "the number of layers of carbon fiber cloth combination n, rounded up, ≥2", rounded up n = 2.

[0093] Axial width of welded joint (including girth weld and heat affected zone) l defect is 30mm, the safety factor β is 2, l over 78mm, 100mm, l taper It is 8.75mm, which is calculated to be 495mm.

[0094] The repaired girth weld pipe section was subjected to pressure treatment, and the results are shown in Table 1.

[0095] Table 1 Experimental parameters of “Carbon fiber composite material repair”

[0096]

[0097] The experimental results show that carbon fiber cloth can fully withstand the pipeline design pressure of 12MPa.

[0098] Embodiments 2 to 6

[0099] The difference between Examples 2 to 6 and Example 1 is only the holding time of the dehydrogenation heat treatment and the stress relief annealing heat treatment. The specific holding time of the dehydrogenation heat treatment and the stress relief annealing heat treatment of each example is shown in Table 2. The contents not mentioned are exactly the same as those of Example 1.

[0100] Table 2 Holding time of dehydrogenation heat treatment and stress relief annealing heat treatment

[0101] Example Holding time of dehydrogenation heat treatment Holding time of stress relief annealing heat treatment 2 2.5h 1.25h 3 3.0h 1.5h 4 3.5h 1.75h 5 4.0h 2.0h 6 4.5h 2.25h

[0102] Experimental example

[0103] The repaired girth weld pipe sections in Examples 2 to 6 were made according to the standards "Q / SH 0248-2009 Specification for Welding Construction and Acceptance of High-Sulfur Natural Gas Pipeline Engineering", "SY / T 4117-2016 Technical Specification for Welding of High-Sulfur Hydrogen Gas Field Gathering and Transportation Pipelines" and "NACE TM 0177 Standard Test Method for Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments". Solution A was used, and the stress loading method of the weld specimens was the four-point bending method. The test period was 720 hours, and the test temperature was 24±3°C; the loading stress was 80% of the minimum specified yield strength of the parent material (L360), and there was no cracking or cracking on the surface of the specimen after the test.

[0104] The experimental results show that the sulfur resistance of the girth weld has been repaired and restored to the requirements of the standards "Q / SH 0248-2009 Welding Construction and Acceptance Specifications for High-Sulfur Natural Gas Pipeline Projects" and "SY / T 4117-2016 Technical Specifications for Welding of High-Sulfur Hydrogen Gas Field Gathering and Transportation Pipelines".

[0105] Based on the data of the above experimental examples, the girth weld repair method of "dehydrogenation heat treatment (material aspect)" + "stress relief annealing heat treatment (stress aspect)" + "fiber cloth repair (stress aspect)" can quickly and conveniently repair the degraded sulfur resistance of the girth weld and reduce the risk of sulfide stress cracking from both the material and stress aspects, thereby ensuring the long-term service safety of low-alloy gathering and transportation pipelines in high-sulfur gas fields.

Claims

1. A method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service, characterized in that: The following steps are involved: The low alloy steel gathering and transportation pipeline girth weld to be repaired is subjected to dehydrogenation heat treatment and stress relief annealing heat treatment in sequence, and then the low alloy steel gathering and transportation pipeline girth weld to be repaired is repaired with carbon fiber cloth.

2. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service according to claim 1, characterized in that: The temperature of the dehydrogenation heat treatment is T1°C, 250°C≤T1≤350°C; the time of the dehydrogenation heat treatment is t1 hours, t1 satisfies: 1.829h≤t1≤5.76h; the product of T1 and t1 satisfies: 640°C·h≤T1×t1≤1440°C·h.

3. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service according to claim 1, characterized in that: The temperature of the stress relief annealing heat treatment is T2, 600°C≤T2≤630°C; the time of the stress relief annealing heat treatment is t2 hours, t2 satisfies: 1.034h≤t1≤2.325h; the product of T2 and t2 satisfies: 620°C·h≤T2×t2≤1395°C·h.

4. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service according to claim 1, characterized in that: The heating rate from the dehydrogenation heat treatment temperature to the stress relief annealing heat treatment temperature is ≤200°C / hour, the heating rate from the dehydrogenation heat treatment temperature to the stress relief annealing heat treatment temperature is ≤200°C / hour; the cooling rate from the stress relief annealing heat treatment temperature to room temperature is ≤260°C / hour.

5. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long service according to any one of claims 1 to 4, characterized in that: The method for repairing the girth weld of the low alloy steel gathering pipeline to be repaired by using carbon fiber cloth comprises the following steps: sticking the carbon fiber cloth to the girth weld to be repaired and around the girth weld to be repaired and wrapping the girth weld to be repaired.

6. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service according to claim 5, characterized in that: The carbon fiber cloth comprises plain carbon fiber cloth and twill carbon fiber cloth. When pasting the carbon fiber cloth, the plain carbon fiber cloth and the twill carbon fiber cloth are stacked and pasted; a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are used as a carbon fiber cloth combination, and one carbon fiber cloth combination is pasted each time; after pasting, the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth in each carbon fiber cloth combination are obliquely crossed.

7. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service according to claim 6, characterized in that: After pasting, the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth in each carbon fiber cloth combination are cross-crossed at an angle of 45°.

8. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service according to claim 6, characterized in that: The number of layers of the bonded carbon fiber cloth combination is determined according to the following formula: Where: n is the number of layers of carbon fiber cloth, rounded up, ≥2; h c —Theoretical total thickness of a single layer of carbon fiber cloth after a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth are laminated, mm; α—safety factor, ranging from 1 to 2; σ cfrp —The design value of tensile strength of a carbon fiber cloth combined with a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth, MPa; t—pipe wall thickness, mm; σ s —Minimum yield strength of pipeline steel, MPa.

9. The method for repairing the girth weld of a low alloy steel gathering and transportation pipeline in a sulfur-containing gas field in long-term service according to claim 6, characterized in that: When pasting the carbon fiber cloth, the circumferential weld to be repaired is taken as the center, and the pasting length of the carbon fiber cloth along the axial direction of the gathering and transportation pipeline is determined according to the following formula: l＝β×(2l over +l defect +2l taper )； Where: l is the bonding length of the carbon fiber cloth along the axial direction of the gathering and transportation pipeline, mm; β—safety factor, ranging from 1 to 2; l over —The length of the carbon fiber cloth extending outward along the axial direction of the pipeline within the area completely covered by each layer of carbon fiber cloth in the pipeline circumferential direction starting from the edge of the pipeline welding joint area is calculated according to the following formula: over =2.5×(0.5Dt) 1 / 2 , Where D is the outer diameter of the pipe, mm; t is the wall thickness of the pipe, mm; l over When the calculated value is less than 100mm, take 100mm; when the calculated value is greater than 100mm, follow the calculated value; l defect —Length and axial width of the welded joint, mm. The welded joint includes the girth weld and the heat-affected zone; l taper —Length of buffer area at the end of carbon fiber cloth, mm; l taper ≥5δ cfrp , δ cfrp It is the total design thickness of all carbon fiber cloths to be pasted.

Images