Non-excavation weak magnetic detection method for circumferential weld of natural gas long-distance pipeline

Abstract

The invention discloses a non-excavation weak magnetic detection method for a circumferential weld of a natural gas long-distance pipeline. The non-excavation weak magnetic detection method comprises the following steps: S1, collecting basic information of a target pipeline; s2, determining the trend of a target pipeline and the burial depth of the pipeline by using a pipeline detector; s3, collecting a weak magnetic signal of a target pipeline by utilizing pipeline weak magnetic detection equipment; s4, the collected weak magnetic signal data are analyzed, and the circumferential weld position of the target pipeline is determined according to weak magnetic signal waveform characteristics; the method is carried out under the trenchless condition, the pipeline is not damaged, the detection safety is high, circumferential weld positioning is accurate, and the defect recognition accuracy is high. And compared with an internal detection technology, the pipeline does not need to be stopped, the defect identification accuracy is high, the positioning precision is high, the pertinence of excavation and repair is improved, and therefore the economic cost of circumferential weld risk investigation is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of girth weld detection for long-distance pipelines, and in particular to a trenchless weak magnetic detection method for girth welds for long-distance natural gas pipelines. Background Art

[0002] During the service of long-distance natural gas pipelines, pipeline safety accidents caused by girth weld defects are common. Taking the West-East Gas Pipeline II, Mo-Da Line, and Shaanxi-Beijing Line III as examples, more than 30 girth weld cracking and leakage accidents occurred during the initial pressure test and commissioning of the pipelines, of which more than 70% were caused by girth weld defects. Since the welding of long-distance pipelines (φ1016) is the weak link of the current long-distance pipelines, the quality of girth welds of long-distance pipelines is the focus of attention of pipeline operation management units. At present, there are two main means for pipeline companies to conduct risk investigation of girth welds of long-distance pipelines.

[0003] First, excavation sites are selected based on radiographic data taken during the construction period, or excavation sites are selected for black holes for which there is no radiographic data taken during the construction period. This inspection method is inefficient and the work is relatively blind and inaccurate, which directly affects the overall efficiency of the girth weld inspection work and seriously wastes manpower and economic resources.

[0004] Second, the leakage magnetic internal detection generally accepted by the pipeline industry is used as the main means of risk investigation of circumferential welds. The excavation point is determined based on the leakage magnetic detection results, and then other non-destructive detection methods are combined to accurately identify the risk welds and determine whether to repair them. However, the leakage magnetic internal detection can only detect volumetric defective circumferential welds, such as round defects, strip defects, concave and undercuts, while area defects and other types of defects (lack of fusion, lack of penetration, axial cracks) cannot be detected at present, and it is precisely these area defects that are most likely to cause pipeline failure. At the same time, pipeline defect repair requires accurate geographical location information on the ground. Due to various reasons, the leakage magnetic internal detection cannot accurately give the ground position, but can only give the approximate distance relative to the mileage pile. Moreover, according to the excavation results, there is an irregular deviation between the relative distance given by the internal detection and the relative distance of the actual excavation.

[0005] The weak magnetic detection technology of pipeline girth welds is a trenchless detection technology developed for ferromagnetic pipelines with a certain burial depth and magnetized by the geomagnetic field. It can provide dangerous girth welds with high stress levels based on the characteristics and size of the collected magnetic signals without interrupting the pipeline operation and without excavation. It can also provide the specific geographical location of defective girth welds, thereby reducing the blindness of excavation, reducing costs, and improving operational and economic efficiency. Summary of the invention

[0006] The purpose of the present invention is to provide a trenchless weak magnetic detection method for the girth weld of a long-distance natural gas pipeline, which can collect the magnetic field distribution generated by the girth weld of the long-distance natural gas pipeline under the action of the geomagnetic field under trenchless conditions, and realize trenchless identification and positioning of the girth weld of the long-distance natural gas pipeline.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A trenchless weak magnetic detection method for girth welds of a long-distance natural gas pipeline comprises the following steps:

[0009] S1. Collect basic information of target pipeline;

[0010] S2. Use pipeline detector to determine the direction and buried depth of target pipeline;

[0011] S3, using pipeline weak magnetic detection equipment to collect weak magnetic signals of the target pipeline;

[0012] S4. Analyze the collected weak magnetic signal data and determine the target pipeline girth weld position according to the weak magnetic signal waveform characteristics; the weak magnetic signal waveform has the following characteristics:

[0013] (3) The pipeline magnetic induction intensity gradient Bzz horizontally perpendicular to the pipeline direction has a peak or valley value; the magnetic field gradient modulus Cz horizontally perpendicular to the pipeline direction has a peak value and is always positive;

[0014] (4) When there is a defect in the pipeline ring weld, the amplitude of the magnetic induction intensity gradient Bzz and the magnetic field gradient modulus Cz at this location is greater than the amplitude of the corresponding magnetic induction intensity gradient Bzz and the magnetic field gradient modulus Cz on other ring welds.

[0015] Preferably, the basic information of the target pipeline includes the pipeline name, pipeline operating unit, pipeline operation time; pipeline length, pipeline wall thickness, pipeline outer diameter; operating pressure, design pressure; pipeline material; pipeline burial depth range; conveying medium; type and thickness of anti-corrosion layer; inspection status and pipeline maintenance status.

[0016] Preferably, in S2, after the pipeline direction and buried depth are determined, the pipeline to be measured is divided into several sections using carrier phase differential RTK, and the starting point and end point of each section are marked and located, and the actual mileage of the starting point and end point of each section relative to the reference object is recorded.

[0017] Preferably, in S3, during the magnetic signal acquisition process, the pipeline weak magnetic detection device must be located directly above the pipeline axis, and must remain stable and at the same height as the body, while maintaining a constant speed.

[0018] Preferably, in S4, analyzing the collected weak magnetic signal data includes the following steps:

[0019] Eliminate the background geomagnetic field of the collected weak magnetic data and perform secondary filtering at the same time;

[0020] According to the magnetic signal characteristics at the pipeline girth weld, the collected data is analyzed to identify and locate the girth weld, and the degree of damage to the girth weld is preliminarily assessed and evaluated in combination with the basic information of the pipeline.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention can collect the magnetic field distribution generated by the girth weld of the natural gas long-distance pipeline under the action of the geomagnetic field under trenchless conditions, realize the functions of real-time reception of magnetic field signals and real-time display of waveforms through a computer, and realize trenchless identification and positioning of the girth weld of the natural gas long-distance pipeline.

[0023] The present invention is carried out under trenchless conditions, does not damage the pipeline, has high positioning accuracy, has high detection safety, accurately locates the girth weld, and has a high defect identification accuracy rate.

[0024] Compared with the internal detection technology, the present invention does not require the pipeline to be shut down, has high defect identification accuracy, high positioning precision, improves the targetedness of excavation and repair, and thus reduces the economic cost of girth weld risk inspection. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of a pipeline weak magnetic detection device of the present invention detecting a pipeline;

[0026] Figure 2 Schematic diagram of magnetic induction intensity gradient Bzz in Embodiment 2 of the present invention;

[0027] Figure 3 Schematic diagram of magnetic induction intensity gradient Cz in Embodiment 2 of the present invention;

[0028] Figure 4 Schematic diagram of magnetic field induction gradient Cz and magnetic induction intensity gradient Bzz in Embodiment 2 of the present invention;

[0029] Figure 5 Schematic diagram of magnetic induction intensity gradient Bzz in Embodiment 3 of the present invention;

[0030] Figure 6 Schematic diagram of magnetic induction intensity gradient Cz in Embodiment 3 of the present invention;

[0031] Figure 7 Schematic diagram of magnetic field induction gradient Cz and magnetic induction intensity gradient Bzz in Embodiment 3 of the present invention;

[0032] Figure 8 Schematic diagram of magnetic induction intensity gradient Bzz in Embodiment 4 of the present invention;

[0033] Fig. 9 Schematic diagram of magnetic induction intensity gradient Cz in Embodiment 4 of the present invention;

[0034] Fig.10 Schematic diagram of the magnetic field induction gradient Cz and the magnetic induction intensity gradient Bzz in the fourth embodiment of the present invention. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0036] Embodiment 1:

[0037] See also Figure 1-Figure 10 , a trenchless weak magnetic detection method for girth welds of a natural gas long-distance pipeline, comprising the following steps:

[0038] S1. Collect basic information of target pipeline;

[0039] According to the "Weak Magnetic Detection Method for Nondestructive Testing of Pipelines" GB / T 35090-2018, data on the pipeline to be tested is collected. It mainly includes: pipeline design parameters (specifications, pressure, material, medium, anti-corrosion layer type and thickness), pipeline operation time, pipeline burial depth range, recent pipeline inspection status, pipeline maintenance status and other relevant information (parallel pipelines, cross pipelines), etc.

[0040] S2. Use pipeline detector to determine the direction and buried depth of target pipeline.

[0041] First, the path of the pipeline to be tested is visually inspected, and the debris on both sides of the pipeline axis is cleared. At the same time, the terrain and buildings around the pipeline are recorded to facilitate the subsequent detection and calibration of the pipeline path; then a pipeline detector is used to detect the direction of the target pipeline and the buried depth of the pipeline; finally, the carrier phase difference technology (RTK) is used to divide the pipeline.

[0042] The pipeline to be tested is divided into several small sections, and the starting and ending points of each section are marked and located. At the same time, the actual mileage of the starting and ending points of each section relative to the reference object (usually mileage pile) is recorded. When dividing and locating the pipeline to be tested, if marker piles, elbows, embankments, cables and magnetic field interference sources are encountered, their position coordinates and relative mileage are recorded.

[0043] S3, using pipeline weak magnetic detection equipment (Grad-13 gradient magnetometer) to collect weak magnetic signals of the target pipeline;

[0044] The inspector holds the pipeline weak magnetic detection equipment and walks along the route calibrated by the pipeline detector directly above the pipeline axis to scan and collect the magnetic signal of the pipeline. The pipeline weak magnetic detection equipment automatically collects the pipeline weak magnetic signal through the sensor and saves the collected signal to the device storage module. The collected magnetic signal is the spatial magnetic induction intensity directly above the pipeline.

[0045] like Figure 1 The figure shows the position and direction of the pipeline weak magnetic detection equipment relative to the buried pipeline. In the figure, the pipeline weak magnetic detection equipment is equipped with two sets of sensors, each set of sensors has a three-axis magnetic sensor, and the x, y, and z directions of the sensors indicate the directions of the detection equipment, which are perpendicular to each other and conform to the right-hand rule.

[0046] For the direction of the weak magnetic detection equipment for pipelines, the three directions of x, y, and z are perpendicular to each other. The axial direction of the probe is z as shown in the figure, which is the direction of the z-axis connecting the two sets of sensor axes. The axial direction of the probe is the same as the horizontal and vertical pipeline direction; the walking direction of personnel during probe detection is x, and the walking direction that is the same as the flow direction of the pipeline medium is the positive direction of the x direction, which is also the same as the axial direction of the buried pipeline. The height direction of the probe from the ground is y, and the vertical upward direction is the positive direction. During detection, the axial direction of the probe remains horizontal, that is, the two sets of sensors are at the same height from the ground.

[0047] As shown in Table 1, for Figure 1 The buried pipeline shown in the figure can still be described in the three directions of x, y and z. The three directions are perpendicular to each other and conform to the right-hand rule. Their specific indications are different from those of the detection equipment. Among them, the pipeline axis direction is the x direction, and its positive direction is the flow direction of the medium in the pipeline; the vertical pipeline direction (pipeline burial depth direction) is the y direction, and its positive direction is the vertical upward direction, which is perpendicular to the earth's horizontal plane; the horizontal vertical pipeline direction is the z direction, and its positive direction conforms to the right-hand rule with the positive directions of x and y. The xz plane is parallel to the earth's horizontal plane, the xy plane is perpendicular to the earth's horizontal plane, and the yz plane is perpendicular to the earth's horizontal plane.

[0048] Table 1 Schematic diagram of the detection equipment and buried pipelines in the three directions of x, y and z

[0049]

[0050] It should be noted that since the pipeline magnetic signal is a weak magnetic signal, the detection equipment must be located directly above the pipeline axis during the magnetic signal acquisition process, and must remain stable and at the same height as the body, while maintaining a constant speed. Since the pipeline weak magnetic detection equipment uses a very sensitive sensor, during the scanning process directly above the pipeline axis, it is necessary to ensure that the detection personnel have no ferromagnetic accessories or other electronic products on their body to avoid interference with the detection caused by the external magnetic field.

[0051] S4, analyzing the collected weak magnetic signal data, and determining the target pipeline girth weld position according to the weak magnetic signal waveform characteristics; comprising the following steps:

[0052] aData preprocessing

[0053] First, the collected weak magnetic data needs to be processed to eliminate the background geomagnetic field and perform secondary filtering to improve the accuracy and reliability of the data.

[0054] b Girth weld identification

[0055] According to the magnetic signal characteristics at the pipeline girth weld, the collected data is analyzed to identify and locate the girth weld, and a preliminary assessment of the degree of damage to the girth weld is made in combination with the basic information of the pipeline.

[0056] The pipeline weak magnetic detection equipment is used to collect pipeline magnetic signals along the pipeline axis. When there are defects in the girth weld, the magnetic signal directly above the girth weld of the natural gas long-distance pipeline has the following characteristics:

[0057] (1) Just above the circumferential weld, the pipeline magnetic induction intensity gradient Bzz, which is horizontally perpendicular to the pipeline direction, has a peak or valley value, and the magnetic field gradient modulus Cz, which is horizontally perpendicular to the pipeline direction, has a peak value (always positive).

[0058] Specifically, the closer to the girth weld, the larger the amplitude of Bzz and Cz of the pipeline horizontally perpendicular to the pipeline direction, and they increase steadily; away from the girth weld, Bzz and Cz decrease steadily.

[0059] (2) When there is a defect in the pipeline girth weld, the Bzz and Cz characteristics are the same as those in characteristic (1). In addition, the Bzz amplitude and Cz amplitude in the girth weld area are greater than those in the surrounding normal girth welds. The more serious the girth weld defect, the greater the absolute value of the Bzz amplitude and the Cz amplitude, and the greater the degree of change.

[0060] Generally, for For long-distance natural gas pipelines, when the absolute value of the magnetic induction intensity gradient value Bzz, which is horizontal and perpendicular to the direction of the pipeline, is ≥5000nT at a normal burial depth (2.5 meters), it is considered that there is a serious girth weld defect at that location and planned repair or immediate repair is required; when the absolute value of Bzz is <5000nT, it is considered that there is a minor girth weld defect at that location and it has basically no impact on the safe operation of the pipeline and no treatment is required.

[0061] C-girth weld evaluation

[0062] According to the weak magnetic detection results and the detection database, the damage degree of the girth weld of the pipeline is evaluated. And according to the evaluation results of the girth weld damage degree, scientific and reasonable treatment suggestions are provided for the operation and maintenance of the pipeline.

[0063] Embodiment 2:

[0064] The basic information of the target pipeline is as follows:

[0065] Pipeline wall thickness / mm: 17.5; Pipeline outer diameter / mm: 1016; Operating pressure / MPa: 8; Pipeline material: X70; Pipeline burial depth range / m: 1.5~2.5; Transport medium (main components and corrosiveness): natural gas; Anti-corrosion layer (type and thickness): 3 layers of PE.

[0066] Use the buried pipeline detector to determine the pipeline route (axis) and mark it on the ground. Use a geodetic surveying instrument to divide the ground pipeline into sections according to length, with each 120m being a detection section. The specific length can be adjusted according to the on-site environmental conditions.

[0067] Specifically, the path of the pipeline to be tested is first visually inspected, and the debris on both sides of the pipeline axis is cleared. At the same time, the terrain and buildings around the pipeline are recorded to facilitate the subsequent detection and calibration of the pipeline path; then a buried pipeline detector is used to detect the direction of the target pipeline and the buried depth of the pipeline; finally, the carrier phase difference technology (RTK) is used to divide the pipeline in combination with the on-site environmental conditions.

[0068] When dividing the pipeline to be tested into several small sections, it is necessary to mark and locate the starting and ending points of each small section, and record the actual mileage of the starting and ending points of each small section relative to the reference object (usually mileage pile). When dividing and locating the pipeline to be tested, if you encounter marker piles, elbows, embankments, cables and magnetic field interference sources, record their position coordinates and relative mileage.

[0069] The inspector holds the pipeline weak magnetic detection equipment and walks along the route calibrated by the pipeline detector directly above the pipeline axis to scan and collect the magnetic signal directly above the pipeline. The pipeline weak magnetic detection equipment is about 0.8-1.2m above the ground and is placed horizontally.

[0070] The starting point for recording data during the inspection process should be the location of the inspected pipeline with obvious markings (such as test piles, the starting point or end point of the pump station); the pipeline should be marked in sections by placing markers or spraying paint. During the inspection process, if there are environmental markers such as houses built above the pipeline, intersections with roads or gullies, and magnetic field interference sources, complete records should be made.

[0071] The data collected by the pipeline weak magnetic detection equipment is matched with the detection mileage and drawn into a waveform graph. The location of the pipeline girth weld is identified by analyzing the main characteristics of the magnetic signal in the weld area, and the girth weld is located based on the detection mileage identification point.

[0072] After data processing, the curve (waveform diagram) of magnetic induction intensity gradient and magnetic field gradient modulus and detection mileage in the vertical pipeline direction (Z direction) is drawn. The weak magnetic detection waveform diagram of this pipe section is as follows Figure 2 - Figure 4 As shown in the figure, according to the on-site environment, the detection length of this section is 80.5 meters and the pipeline is buried 2.5 meters deep. The ordinate in the figure is the magnetic induction intensity gradient (nT) and the abscissa is the detection mileage (m).

[0073] By analyzing the waveform, three girth welds were identified in the detection section, at 15m, 62m, and 73m. By analyzing the weak magnetic signal detection waveform, it was found that the amplitude of the characteristic point of the 62m girth weld was Bzz>5000nT, which was 5434nT, significantly higher than the amplitude of the characteristic points of the surrounding girth welds, and it was judged that the girth weld had a certain degree of serious damage.

[0074] C-girth weld evaluation

[0075] According to the weak magnetic detection results and combined with the detection database, the damage degree of the pipeline's girth weld is evaluated, and based on the girth weld damage degree evaluation results, scientific and reasonable treatment suggestions are provided for the operation and maintenance of the pipeline.

[0076] By excavating and verifying the girth weld at 62 meters (GPS coordinates: E: 113.3428255650, N: 30.1953686104), it was found that there was indeed a girth weld of the pipeline at the excavation site. After excavation, the girth weld was subjected to non-destructive testing and defect rating was performed according to SY / T4109-2020 "Non-destructive Testing of Petroleum and Natural Gas Steel Pipelines". Ultrasonic testing found that there were buried defects in the girth weld at this location, and radiographic testing found that the root of the girth weld was not fused and the interlayer was not fused (among which, the interlayer was not fused in size 60mm). The defect was determined to be a radiographic IV level defect (unqualified and needs to be repaired immediately).

[0077] The test results show that the method of the present invention can effectively identify girth welds under trenchless conditions and determine whether the girth welds have defects.

[0078] Embodiment three:

[0079] The basic information of the target pipeline is as follows: Operation time (year and month): November 5, 2012; Pipeline wall thickness / mm: 14.6; Pipeline outer diameter / mm: 1016; Operating pressure / MPa: 8.8;

[0080] Design pressure / MPa: 10; Pipeline material: X70; Pipeline burial depth range / m: 2~2.5; Transport medium (main component and corrosiveness): natural gas; Anti-corrosion layer (type and thickness): 3 layers of PE.

[0081] According to the magnetic signal characteristics at the pipeline girth weld, the collected data is analyzed to identify and locate the girth weld, and a preliminary assessment of the degree of damage to the girth weld is made in combination with the basic information of the pipeline.

[0082] After data processing, the curve (waveform diagram) of the magnetic induction intensity gradient and magnetic field gradient modulus in the vertical pipeline direction and the detection mileage is drawn. The weak magnetic detection waveform diagram of this pipe section is as follows Figure 5 - Figure 7 As shown in the figure, according to the on-site environment, the detection length of this section is 120 meters, and the buried depth of the pipeline is 2 meters to 2.5 meters. Among them, there is a signboard at 115 meters (there is no signboard interference here). The vertical axis in the figure is the magnetic induction intensity gradient (nT), and the horizontal axis is the detection mileage (m).

[0083] By analyzing the waveform, it was determined that there were 3 girth welds in the detection section, at 72 meters, 85 meters, and 97 meters. By analyzing the weak magnetic signal detection waveform, it was found that the amplitude of the girth weld feature point was Bzz < 5000nT, and it was determined that the girth welds had a certain degree of slight damage.

[0084] By excavating and verifying the three girth welds, it was found that there were indeed pipeline girth welds at the excavation site. After excavation, the girth welds were subjected to non-destructive testing, and defect rating was carried out in accordance with SY / T4109-2020 "Non-destructive Testing of Petroleum and Natural Gas Steel Pipelines". It was found that the three girth welds had circular and internal bite defects. The defects of the three girth welds were determined to be Radiographic Level II defects (qualified, no treatment).

[0085] Embodiment 4:

[0086] The basic information of the target pipeline is as follows:

[0087] Time of operation: 2013; pipeline wall thickness / mm: 12.8 / 15.3; pipeline outer diameter / mm: 1016; operating pressure / MPa: 7; design pressure / MPa: 10; pipeline material: X70; pipeline burial depth range / m: 1.5~4.5; transportation medium (main component and corrosiveness): natural gas; anti-corrosion layer (type and thickness): 3 layers of PE.

[0088] According to the magnetic signal characteristics at the pipeline girth weld, the collected data is analyzed to identify and locate the girth weld, and a preliminary assessment of the degree of damage to the girth weld is made in combination with the basic information of the pipeline.

[0089] After data processing, the curve (waveform diagram) of the magnetic induction intensity gradient and magnetic field gradient modulus in the vertical pipeline direction and the detection mileage is drawn. The weak magnetic detection waveform diagram of this pipe section is as follows Figure 8 - Fig.10 As shown in the figure, according to the on-site environment, the detection length of this section is 120 meters, and the buried depth of the pipeline is 2.2 meters to 3 meters. Among them, there is a signboard at 115 meters. The vertical axis in the figure is the magnetic induction intensity gradient (nT), and the horizontal axis is the detection mileage (m).

[0090] By analyzing the waveform, four girth welds were identified in the test section. Figure 7 Through the analysis of the weak magnetic signal detection waveform, it is found that the amplitude of the characteristic points of the four girth welds is Bzz> 5000nT, which indicates that there is a certain degree of stress concentration in the four girth welds.

[0091] C-girth weld evaluation

[0092] According to the weak magnetic detection results and combined with the detection database, the damage degree of the pipeline's girth weld is evaluated, and based on the girth weld damage degree evaluation results, scientific and reasonable treatment suggestions are provided for the operation and maintenance of the pipeline.

[0093] By comparing the nondestructive testing results and repair conditions of the girth welds at 7 meters and 47 meters, it was found that both locations had undergone nondestructive testing and B-type sleeve replacement before weak magnetic testing (the nondestructive testing results were both level IV, so B-type sleeve replacement was performed), so that the interval between the two adjacent girth welds was less than 12 meters, that is, the welding residual stress of the two girth welds formed by sleeve replacement at 7 meters and 12 meters was concentrated, and the welding residual stress of the two girth welds formed by sleeve replacement at 47 meters and 52 meters was concentrated, and no treatment was required. The weak magnetic test results are consistent with the nondestructive testing and repair conditions.

[0094] The test results show that the method of the present invention can effectively identify girth welds under trenchless conditions and determine whether the girth welds have defects.

Claims

1. A trenchless weak magnetic detection method for girth welds of a natural gas long-distance pipeline, characterized in that: The following steps are involved: S1. Collect basic information of target pipeline; S2. Use pipeline detector to determine the direction and buried depth of target pipeline; S3, using pipeline weak magnetic detection equipment to collect weak magnetic signals of the target pipeline; S4. Analyze the collected weak magnetic signal data and determine the target pipeline girth weld position according to the weak magnetic signal waveform characteristics; the weak magnetic signal waveform has the following characteristics: (1) The pipeline magnetic induction intensity gradient Bzz horizontally perpendicular to the pipeline direction has a peak or valley value; the magnetic field gradient modulus Cz horizontally perpendicular to the pipeline direction has a peak value and is always positive; (2) When there is a defect in the pipeline girth weld, the amplitude of the magnetic induction intensity gradient Bzz and the magnetic field gradient modulus Cz at the defect is greater than the amplitude of the corresponding magnetic induction intensity gradient Bzz and the magnetic field gradient modulus Cz on other normal girth welds.

2. According to claim 1, a trenchless weak magnetic detection method for girth welds of a natural gas long-distance pipeline is characterized in that: The basic information of the target pipeline includes the pipeline name, pipeline operating unit, pipeline operation time; pipeline length, pipeline wall thickness, pipeline outer diameter; operating pressure, design pressure; pipeline material; pipeline burial depth range; conveying medium; type and thickness of anti-corrosion layer; inspection status and pipeline maintenance status.

3. The trenchless weak magnetic detection method for the girth weld of a long-distance natural gas pipeline according to claim 1 is characterized in that: In S2, after the pipeline direction and buried depth are determined, the pipeline to be measured is divided into several sections using carrier phase differential RTK, and the starting point and end point of each section are marked and located, and the actual mileage of the starting point and end point of each section relative to the reference object is recorded.

4. The trenchless weak magnetic detection method for the girth weld of a long-distance natural gas pipeline according to claim 1 is characterized in that: In S3, during the magnetic signal acquisition process, the pipeline weak magnetic detection equipment must be located directly above the pipeline axis, and must remain stable and at the same height as the body, while maintaining a constant speed.

5. The trenchless weak magnetic detection method for girth welds of a long-distance natural gas pipeline according to claim 1 is characterized in that: In S4, analyzing the collected weak magnetic signal data includes the following steps: Eliminate the background geomagnetic field of the collected weak magnetic data and perform secondary filtering at the same time; According to the magnetic signal characteristics at the pipeline girth weld, the collected data is analyzed to identify and locate the girth weld, and the degree of damage to the girth weld is preliminarily assessed and evaluated in combination with the basic information of the pipeline.

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