Repair method of girth weld of low alloy steel gathering and transportation pipeline in long-term service in sour gas field

Abstract

The present invention relates to a method for repairing the girth weld of a low-alloy steel gathering and transportation pipeline that has been in service for a long time in a sulfur-containing gas field, and belongs to the technical field of metal pipe repair. The repair method of the present invention 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 with carbon fiber cloth. The repair method of the present invention is aimed at the girth weld of the low-alloy steel gathering and transportation pipeline that has been in service for a long time in a sulfur-containing gas field, which has a decreased sulfur resistance and an increased risk of sulfide stress cracking. The girth weld is repaired from both material and stress aspects, and can quickly and conveniently effectively repair the decreased sulfur resistance of the girth weld and reduce the risk of sulfide stress cracking. It solves 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 the sulfur-containing gas field.

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 long-term service in a sulfur-containing gas field, 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 emissions, environmental protection, and clean energy, China's natural gas development has shown a positive trend of annual growth in recent years. Sour natural gas has become the backbone of China's natural gas production capacity. Developing this type of gas reservoir presents production safety challenges such as high sulfur corrosion, harsh environments, high toxicity, and serious accident consequences. China has developed a comprehensive set of corrosion prevention technologies for surface gathering and transportation pipelines in sour gas fields, effectively controlling electrochemical corrosion of sulfur-resistant carbon steel pipelines.

[0003] Low-alloy steel for gathering and transportation is widely used in gathering and transportation pipelines due to its economical efficiency, excellent mechanical properties, and weldability. Welding is a process used in the laying of gathering and transportation pipelines. During welding, the steel undergoes a series of complex, non-equilibrium physical and chemical processes, resulting in defects such as uneven chemical composition, coarse grains, and structural segregation in the weld zone. This increases the risk of weld failure. In hydrogen sulfide-containing environments, welds have lower resistance to sulfide stress cracking than the parent material, making them a vulnerable link in the safe operation of pipelines.

[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 internal media in the pipelines, further safety hazards and impact on 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: surfacing, patching, and sleeves, which have the risk of weld penetration, hydrogen embrittlement and cold embrittlement, and are not suitable for pipelines containing sulfur media; (2) Clamp type: epoxy injection clamps and clamps, 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, as the preferred repair fiber composite material, 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 207GPa), and the reinforcement layer and the steel pipe have very good deformation synergy; carbon fiber has high tensile strength, and for pipeline steel with a yield strength of 400-500MPa, the repair thickness is only about 1 / 6-1 / 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) Advantages in performance: No welding or fire required, which greatly reduces the risk of operation; the thickness, number of layers, fiber distribution and other aspects can be designed and laid flexibly according to the on-site pipeline conditions and stress conditions; the 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; the 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 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 Specification for Carbon Fiber Repair and Reinforcement of Pipeline Defects", "Q / SY1592-2013 Technical Specification for Oil and Gas Pipeline Body Repair", "GB / T30671-2018 Guidelines for Repair of Defects in Buried Steel Pipelines", and "SY / T 6649-2018 Technical Specification for Repair of Defects in Oil and Gas Pipelines" specify applicable methods for pipeline defect repair, including repair materials, repair processes, and the design of the number of repair fiber composite material layers, based on the types and degrees of defects in different pipelines.

[0005] Chinese patent document CN 1853847 B provides a method for repairing and reinforcing weld defects using carbon fiber composite materials. The method is applicable to weld defects, particularly pipeline weld defects. The method includes wet-pasting carbon fiber sheets of a certain size and number of layers to the weld and the surrounding area of ​​the weld. The required number of layers of carbon fiber sheets are tightly wrapped and pasted to 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. The adjacent layers of composite materials are staggered in parallel, perpendicular, or at a certain angle. The thickness, width, and amount of reinforcing material used in the reinforcement layer are determined by a defect reinforcement parameter design method. This method can ensure that the pressure bearing capacity of the final composite repair layer reaches or exceeds the pressure bearing capacity of the original weld. Chinese patent document CN 106238970B discloses a composite material repair and reinforcement method for boss fillet welds. The method involves sequentially wrapping a steel pipe with cut composite fiber cloth and interlayer epoxy resin adhesive, cross-wrapping and spirally wrapping it according to the reinforcement width, number of reinforcement layers, and boundary groove requirements. During the spiral wrapping, the composite fiber cloth overlaps each other by 50%, and the boundary groove is smoothed to a slope of 15±5°. This method avoids safety accidents such as leakage during operation, especially cracks in the cross weld extending along the straight weld area of ​​the main pipeline, thus preventing serious disasters and losses. Chinese patent document CN 114909542A provides an oil and gas pipeline defect repair device and method, comprising: two semi-cylindrical barrels, the upper and lower parts of the two semi-cylindrical barrels are connected by a detachable connector to form a cylindrical repair barrel, an annular weld groove is provided in the repair barrel, which can quickly wrap the defective position of the pipe body, and the fixing object winding includes one or more of the following winding methods: polyethylene cold wrapping tape fixed wrapping, polypropylene cold wrapping tape fixed wrapping, iron chain winding and locking, epoxy resin coating, epoxy coal tar coating, glass fiber epoxy coal tar coating, and epoxy resin carbon fiber composite material coating. The repair barrel is installed by a detachable connection method, which can improve work efficiency.

[0006] Due to hydrogen sulfide, the weld performance of pipelines transporting sour natural gas will decline and the risk of cracking will increase with service life. The study "Study on the Plasticity and Toughness of In-Service X52 Gas Pipeline Steel" (Natural Gas Industry, Vol. 26, No. 3, March 2003) shows that the elongation and J-integral values ​​of X52 gas pipelines transporting H2S-containing media decrease with service time, and the decrease is greater in the weld and its vicinity than in the base material, increasing the risk of brittle cracking at the weld. The study "Corrosion Detection and Safety Analysis of Wet Gas Transmission Natural Gas Pipelines" (Oil and Gas Storage and Transportation, Vol. 25, No. 11, November 2006) shows that the welds of 20-gauge steel gas pipelines transporting H2S-containing media tend to exhibit HIC over time, increasing the susceptibility to brittle cracking at the weld.

[0007] Degraded weld performance poses significant risks to the safe operation of pipelines transporting hydrogen sulfide-containing media. Current pipeline weld repair processes only address corrosion and other defects. Gathering and transportation pipelines in sour gas fields utilize a large number of prefabricated low-alloy, sulfur-resistant pipeline steel pipes, butt-welded to form girth welds. However, a method for repairing the degraded sulfur-resistant girth welds in pipelines that have long served to transport hydrogen sulfide-containing media remains lacking. Therefore, a method for repairing the sulfur-resistant girth welds in low-alloy steel gathering and transportation pipelines in sour gas fields is urgently needed to ensure the long-term service safety of these low-alloy steel gathering and transportation pipelines. 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 objectives, 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: sequentially performing a dehydrogenation heat treatment and a stress relief annealing heat treatment on the girth weld of the low-alloy steel gathering and transportation pipeline to be repaired, and then repairing the girth weld of the low-alloy steel gathering and transportation pipeline to be repaired using carbon fiber cloth.

[0011] The method for repairing girth welds of low-alloy steel gathering and transportation pipelines in sour gas fields that have been in service for a long time is designed to address the decline in sulfur resistance and increased risk of sulfide stress cracking of girth welds of low-alloy steel gathering and transportation pipelines in sour gas fields that have been in service for a long time. The method repairs the girth welds from both material and stress aspects, and can quickly and conveniently effectively repair the decline in sulfur resistance of the girth welds and reduce the risk of sulfide stress cracking. This solves the problem that existing pipeline girth weld repair methods only address defects such as corrosion, and provides strong technical support for ensuring the long-term service safety of low-alloy steel gathering and transportation pipelines in sour gas fields.

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

[0013] The temperature of the stress relief heat treatment is lower than the low alloy phase transition temperature. The temperature of the 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 the low alloy steel, the low alloy steel structure does not change significantly, and the beneficial properties of the 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 the 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 the stress relief annealing heat treatment is 620℃ and the time is 1 hour.

[0014] The temperature increase before the dehydrogenation heat treatment and the stress relief annealing heat treatment is linear. The temperature decrease after the annealing heat treatment is also linear. Furthermore, the heating rate to the dehydrogenation heat treatment temperature is ≤ 200°C / hour, and 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.

[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 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 at a 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 carbon fiber weave. Because the girth weld of the gathering and transportation pipeline is subject to radial pressure and axial tension within the pipeline, the carbon fiber cloth combination includes a layer of plain carbon fiber cloth and a layer of twill carbon fiber cloth. The advantages of the plain weave's excellent bending strength and tensile strength and the twill weave's excellent shear performance are utilized to enhance the ability of the girth weld of the gathering and transportation pipeline to withstand multi-directional tension. Bidirectional weaving here refers to carbon fiber cloth in which the carbon fiber bundles are woven in both the warp and weft directions.

[0017] Furthermore, 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 at an angle of 45°.

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

[0019]

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

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

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

[0023] σ cfrp —Design value of tensile strength of the carbon fiber cloth combination after one layer of plain carbon fiber cloth and one layer of twill carbon fiber cloth are laminated, 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 stacked, the angle between the fiber tows of the plain carbon fiber cloth and the fiber tows of the twill carbon fiber cloth is the same as the angle between 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 after pasting.

[0027] Furthermore, when pasting the carbon fiber cloth, with the girth 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 axial length of the carbon fiber cloth along the gathering and transportation pipeline (the width of the repair 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 pipe within the area completely covered by the carbon fiber cloth in the circumferential direction of the pipe, measured from the edge of the pipe 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 If the calculated value is less than 100mm, take 100mm; if the calculated value is greater than 100mm, follow the calculated value;

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

[0033] l taper —Length of the buffer area at the end of the 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 surface of the pipe where the carbon fiber cloth is pasted but is not completely covered by all layers 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 welds of low-alloy steel gathering and transportation pipelines in long-term service in sour gas fields need to be repaired can be evaluated based on their sulfur resistance. The specific method includes the following steps:

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

[0038] 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 operating 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 susceptible 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 performance requirements for the sulfide stress cracking resistance of girth welds of low-alloy gathering and transportation pipelines in high-sulfur gas fields: in accordance with 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", A solution is used, the weld specimen is loaded with stress using 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, and the test temperature is 24±3°C. After the test, the specimen surface is observed for cracks and fissures.

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

[0043] ① After the test, there was no cracking or fissure 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 low-alloy gathering and transportation pipeline welds in sour 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 that girth weld samples cannot be taken on site, 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. The hydrogen penetrates into the pipe (including parent materials and welds) and, under the action of stress (including the operating pressure of the pipeline itself, residual stress caused by pressure, pressure fluctuations, external extrusion, etc. during the service of the pipeline, etc.), 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 accumulation areas). When the hydrogen amount and stress reach a critical value, sulfide stress cracking occurs.

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

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

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

[0050] ③Stress: The pipeline operating pressure borne by 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 pipeline girth weld operating pressure, 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 pipe. It integrates the five elements of "sulfur-resistant pipes + corrosion inhibitors + corrosion monitoring + cathodic protection + intelligent pipe cleaning" and is an anti-corrosion process technology for high-sulfur wet gas gathering and transportation systems. 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 girth weld section of the on-site pipeline and process it into a sulfide stress cracking specimen (original specimen) according to Method E: Four-point bend 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 environment". The four-point bend 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, as specified in Section 8.2 of "GB / T 4157-2017 Laboratory Test Method for Resistance of Metals to Sulfide Stress Cracking and Stress Corrosion Cracking in Hydrogen Sulfide Environments," was used. The weld specimens were stressed using the four-point bending method (refer to the "GB / T 4157-2017 Laboratory Test Method for Resistance of Metals to Sulfide Stress Cracking and Stress Corrosion Cracking in Hydrogen Sulfide Environments" for details). The test period was 720 hours and the test temperature was 24±3°C. The specimens cracked after the applied stress of 80% of the minimum specified yield strength of the base metal (L360); and after the applied stress of 72% of the minimum specified yield strength of the base metal (L360). This indicates that the girth weld's sulfur resistance has decreased and the risk of sulfide stress cracking has increased. Solution A was prepared by dissolving 5.0% NaCl and 0.5% CH₃COOH, respectively, in distilled water. The pH of the solution was maintained at 2.6-2.8 before contact with the specimens and before saturation with H₂S gas.

[0056] The girth welds used in the repairs in Examples 1-6 were formed by butt welding low-alloy steel L360 pipes using a combination of gas shielded welding and arc welding (the welding process is consistent with the welding process used for a high-sulfur gas field gathering and transportation pipeline in China). Only 10% of the pipe wall thickness was welded from the outer surface, leaving the remaining 90% of the pipe wall thickness from the inner wall unwelded. The low-alloy steel L360 pipes had an outer diameter of 159 mm and a wall thickness of 15 mm. After welding, the pipes were sealed at both ends with welded blind plates.

[0057] Example 1

[0058] 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 in this 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 heated 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 holding period, 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 around the girth weld to be repaired. Both the plain carbon fiber cloth and the twill carbon fiber cloth are bidirectionally woven. After each layer of plain carbon fiber cloth is pasted, a twill carbon fiber cloth is pasted on top of the plain carbon fiber cloth. After pasting, one of the two groups of perpendicular fiber tows in each plain carbon fiber cloth extends in the same direction as the extension 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 considered a carbon fiber cloth combination. 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 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 a formula for calculating the combined tensile strength of carbon fiber cloth:

[0063]

[0064] Where: cfrp —The design value of the tensile strength of the carbon fiber cloth combination after one layer of plain carbon fiber cloth and one 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 degrees), in MPa;

[0065] f fk —The standard value of the tensile strength of the carbon fiber cloth combination after one layer of plain carbon fiber cloth and one 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 degrees), 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 cross-crossed at 45 degrees) is taken as 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 the 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 carbon fiber cloth layers, rounded up to the nearest integer, ≥ 2;

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

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

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

[0076] D—outer diameter of the pipe, mm;

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

[0078] σ cfrp —The design value of the tensile strength of the carbon fiber cloth combination after one layer of plain carbon fiber cloth and one 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 degrees), in MPa;

[0079] t—pipe wall thickness, mm;

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

[0081] When pasting carbon fiber cloth, take the girth weld to be repaired as the center, and the pasting length of carbon fiber cloth along the axial direction of the gathering and transportation pipeline (the width of the repair reinforcement layer) shall be 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 If the calculated value is less than 100mm, take 100mm; if 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 —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—outer diameter of the pipe, mm;

[0091] t—pipe wall thickness, mm.

[0092] Adopting the principle of equal strength, for the sake of safety, the total pressure bearing capacity of carbon fiber cloth is designed according to the pressure of 12MPa that the pipeline bears, thereby reducing the pressure on the welded joint. The specifications of plain carbon fiber cloth and twill carbon fiber cloth are both 12K, 400g / m 2 The design thickness of both plain and twill carbon fiber cloth is 0.35mm. Therefore, the theoretical total thickness of a single layer of carbon fiber cloth, formed by stacking one plain and one twill, is 0.7mm, and both are bidirectional. Calculations show that n = 0.9, meaning 0.9 "combinations of one plain and one twill carbon fiber cloth" can withstand a pressure of 12MPa. The safety factor α is 1. According to the formula for calculating the total hoop thickness of carbon fiber cloth, "the number of layers n, rounded up, ≥ 2," and 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 for “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] Examples 2 to 6

[0099] The only difference between Examples 2 to 6 and Example 1 is 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 sections in Examples 2 to 6 were tested in accordance with 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 Gathering and Transportation Pipelines in High-Sulfur Hydrogen Gas Fields", 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 weld specimens were loaded with stress using the four-point bending method. The test period was 720 hours, and the test temperature was 24±3°C. The loaded stress was 80% of the minimum specified yield strength of the base material (L360). After the test, no cracks or fissures were observed on the surface of the specimens.

[0104] The experimental results show that the sulfur resistance of the girth weld has been restored and returned to the requirements of the standards "Q / SH 0248-2009 Welding Construction and Acceptance Specifications for High-Sulfur Natural Gas Pipeline Engineering" 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, reduce the risk of sulfide stress cracking, and ensure the long-term service safety of low-alloy gathering and transportation pipelines in high-sulfur gas fields from both material and stress aspects.

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 by: The following steps are involved: The girth weld of the low-alloy steel gathering and transportation pipeline to be repaired is sequentially subjected to a dehydrogenation heat treatment and a stress relief annealing heat treatment, and then a carbon fiber cloth is pasted on the girth weld to be repaired and around the girth weld to be repaired, thereby wrapping the girth weld to be repaired; the carbon fiber cloth includes a plain carbon fiber cloth and a twill carbon fiber cloth, and 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 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; The number of layers of the carbon fiber cloth combination is determined according to the following formula: Where: n is the number of carbon fiber cloth layers, rounded up to the nearest integer, ≥ 2; h c —Theoretical total thickness of a single layer of carbon fiber cloth after laminating one layer of plain carbon fiber cloth and one layer of twill carbon fiber cloth, mm; α—safety factor, ranging from 1 to 2; σ cfrp —Design value of tensile strength of the carbon fiber cloth combination after one layer of plain carbon fiber cloth and one layer of twill carbon fiber cloth are laminated, MPa; t—pipe wall thickness, mm; σ s —Minimum yield strength of pipeline steel, MPa.

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, and 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-term service according to claim 1, 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 obliquely crossed at an angle of 45 degrees.

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 1, 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 axial length of the carbon fiber cloth along 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 pipe within the area completely covered by each layer of carbon fiber cloth in the circumferential direction of the pipe, starting from the edge of the pipe 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 If the calculated value is less than 100mm, take 100mm; if the calculated value is greater than 100mm, follow the calculated value; l defect —Length and axial width of the welded joint, in mm. The welded joint includes the girth weld and the heat-affected zone; l taper —Length of the buffer area at the end of the carbon fiber cloth, mm; l taper ≥5δ cfrp , δ cfrp It is the total design thickness of all carbon fiber cloths to be pasted.

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