A method for monitoring magnetic stress of steel pipeline girth weld under exciting magnetic field

Abstract

The present invention discloses a method for monitoring magnetic stress of steel pipeline girth weld under excitation magnetic field, and the steps are as follows: collect basic information of steel pipeline, including pipeline size, buried depth and material information; install excitation device; determine the best excitation magnetic field suitable for target pipeline; correct the magnetic gradient signal deviation caused by sensor posture; calibrate the initial stress state of pipeline by ultrasonic equipment, determine the standard magnetic gradient signal strength; determine the installation state of pipeline under different external loads by signal rise model and signal fall model, and evaluate the risk of pipeline stress failure. Providing a monitoring scheme for the operation state of steel pipeline has important guiding significance for ensuring the safe and normal operation of pipeline.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas pipeline safety, and in particular to a method for monitoring magnetic stress of a steel pipeline girth weld under an excitation magnetic field. Technical Background

[0002] The total mileage of natural gas pipelines in Sichuan and Chongqing exceeds 50,000 kilometers, and the annual gas transmission capacity exceeds 40 billion cubic meters, forming a vertical-horizontal-circular trend. The pipelines are numerous, long, and wide, and most of them pass through hilly and mountainous areas. Some pipelines pass through V-shaped, large-drop mountainous areas, and geological disasters occur frequently. The complexity of the environment in which the pipelines are located is the highest in China. The abnormal external loads, cyclic fluctuating pressures, temperature loads, and soil pressures induced by geological disasters together constitute the complex internal and external load conditions of mountain pipelines, resulting in frequent pipeline safety failure accidents, among which girth weld failure is one of the main causes of pipeline failure. Therefore, it is necessary to conduct magnetic stress monitoring of the girth welds of steel pipelines to ensure the safe operation of the pipelines.

[0003] When a local force concentration occurs in a pipeline under the application of external loads, it will cause the irreversible reorientation of the magnetic domains inside the ferromagnetic pipeline, resulting in a sudden change in the magnetic signal. The magnetic stress monitoring technology for the girth weld of steel pipelines is to determine the stress state of the girth weld of the pipeline by collecting abnormal magnetic leakage signals of the pipeline.

[0004] However, there is no unified method for monitoring magnetic stress of steel pipeline girth welds at home and abroad. At present, the existing weak magnetic monitoring has the following disadvantages:

[0005] The quantitative change curve between the monitoring signal under weak magnetic conditions and the stress state of the girth weld will change under the influence of factors such as the geomagnetic field intensity, welding residual stress and initial magnetization intensity, making it difficult to carry out stress inversion based on magnetic signals. Summary of the invention

[0006] In view of the above problems, the present invention aims to provide a method for monitoring magnetic stress of girth weld of steel pipeline under excitation magnetic field. The method can not only enhance the monitoring signal strength, but also obtain magnetic force change curve with good repeatability, thereby realizing effective monitoring of stress state of girth weld of steel pipeline.

[0007] The technical solution provided by the present invention to solve the above technical problems is a method for monitoring magnetic stress of a steel pipeline girth weld under an excitation magnetic field, comprising the following steps:

[0008] S1. Collect basic information of steel pipelines, including pipeline material, diameter, wall thickness, operating pressure, burial depth, etc.

[0009] S2. Design and build a full-scale hydraulic test system according to the target pipeline, use the magnetic gradient tester to adjust the excitation magnetic field strength, and measure the change of the X component of the pipeline magnetic gradient signal under the cyclic pressure condition. The excitation magnetic field strength with the largest change of the X component of the magnetic gradient signal is selected as the excitation magnetic field strength of the pipeline material.

[0010] S3. Under the set excitation magnetic field strength, the axial stress value of the pipeline girth weld under different operating pressures is calculated, and a calculation model for the Mises stress variation of the girth weld is established;

[0011] The calculation model of the maximum Mises stress value change of the girth weld during the rising stage of the magnetic gradient x component is:

[0012]

[0013] Where: ΔG x is the change in the x component of the magnetic gradient, nT / m; Δσ Mises is the Mises stress variation of the girth weld, MP; a and b are coefficients, which are determined by fitting the experimental test data.

[0014] The calculation model of the maximum Mises stress value change of the girth weld during the decline of the magnetic gradient x component is:

[0015]

[0016] c, d are unknown coefficients, which are determined by fitting the experimental test data.

[0017] S5. Randomly select three measuring points along the circumferential direction of the pipeline monitoring section, use stress detection technology to test the axial stress value and the circumferential stress value, and calculate the maximum Mises stress value of the section as the initial value.

[0018] S6. Install the excitation device along the circumferential direction of the circumferential weld through a non-metallic fixed bracket at the circumferential weld position along the axial direction of the circumferential weld, with an installation spacing of θ. The calculation formula for θ is:

[0019] θ=360 / n (1)

[0020] Where: θ is the installation spacing of the excitation device, °; n is the number of excitation devices.

[0021] S7. Adjust the power intensity of the excitation device and adjust the excitation magnetic field intensity to the excitation magnetic field intensity determined by the experiment.

[0022] S8. Install a magnetic gradient signal monitoring system just above the girth weld to collect the magnetic gradient X component of the girth weld in real time under the excitation magnetic field. The magnetic gradient meter is placed horizontally and perpendicular to the pipeline, and the sensors at both ends are symmetrically distributed about the pipeline axis.

[0023] S9. Establish evaluation indicators for the magnetic stress monitoring method of steel pipeline girth welds.

[0024] As a preference, 80% of the Mises stress of the pipeline reaching the yield limit [σ] is selected as the critical threshold. The early warning index evaluation system divides the pipeline stress failure risk into three levels:

[0025] Level I is high risk, σ Mises >0.8[σ];

[0026] Level II is medium risk, 0.72[σ]<σ Mises <0.8[σ];

[0027] Level III is low risk, σ Mises <0.72[σ].

[0028] The magnetic stress monitoring method of the steel pipeline girth weld under an excitation magnetic field of the present invention can be applicable to the laying environment of most steel pipelines. The quantitative change curve between the monitoring signal and the girth weld stress state is little affected by factors such as magnetic field intensity, welding residual stress and initial magnetization intensity, and the stress of the pipeline girth weld can be inverted based on the magnetic signal.

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments will be briefly introduced below. Obviously, the drawings in the following description only relate to some embodiments of the present invention, but are not intended to limit the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of the magnetic stress monitoring method for steel pipelines.

[0031] Figure 2 The figure is a pipeline operating pressure-magnetic stress relationship curve diagram of an embodiment.

[0032] Figure 3 The non-contact online magnetic monitoring data of a mountain pipeline is an embodiment. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those commonly understood by those skilled in the art to which this application belongs.

[0034] S10. Collect basic information of steel pipelines: pipe diameter 1016mm, buried depth 1.5m, material X80.

[0035] S20. According to the diameter of the pipeline, 18 excitation devices are installed along the circumferential direction at the circumferential weld position, one every 20°.

[0036] S30, adding an excitation magnetic field to the girth weld, adjusting the pipeline operating pressure from 3MPa to 1MPa, and selecting the magnetic field intensity with the greatest impact on the magnetic gradient change as the optimal excitation magnetic field.

[0037] The calculation model of the maximum Mises stress value change of the girth weld during the rising stage of the magnetic gradient x component is:

[0038]

[0039] The calculation model of the maximum Mises stress value change of the girth weld during the decline of the magnetic gradient x component is:

[0040]

[0041] Preferably, 400 A / m is selected as the optimal excitation magnetic field.

[0042] S40, measuring the axial stress at the 3, 9, and 12 o'clock positions of the girth weld by ultrasonic monitoring, and calculating the stress state at the girth weld.

[0043] S50, correcting the magnetic gradient signal according to the sensor posture, and correcting the collected signal to a magnetic gradient modulus standard signal.

[0044] S60, calibrate the pipeline magnetic gradient signal at that location, and use the signal as an initial reference magnetic signal.

[0045] S70, increase the pipeline operating pressure from 1MPa to 8MPa, and collect the pipeline magnetic gradient signal. At the same time, collect the local strain of the pipeline through the strain gauge to calculate the real stress of the pipeline. Figure 2 Figure 2 shows the relationship between pipeline operating pressure and pipeline magnetic gradient signal.

[0046] S80, long-term monitoring of the pipeline girth weld magnetic gradient signal, and then regular processing and summary of the magnetic signal, such as Figure 3 As shown, observe whether the girth weld has stress failure risk under external load.

[0047] S90. The above is only the technical solution of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement and improvement made according to the technical essence of the present invention without departing from the content of the technical solution of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for monitoring magnetic stress of a steel pipeline girth weld under an excitation magnetic field, characterized in that: The following steps are involved: S1. Collect basic information of steel pipelines, including pipeline material, diameter, wall thickness, operating pressure, burial depth, etc.; S2. Design and build a full-scale water pressure test system according to the target pipeline, use a magnetic gradient tester to adjust the excitation magnetic field strength, and measure the change of the x component of the pipeline magnetic gradient signal under cyclic pressure conditions; select the excitation magnetic field strength with the largest change of the x component of the magnetic gradient signal as the excitation magnetic field strength of the pipeline material; S3. Under the set excitation magnetic field strength, calculate the axial stress value of the pipeline girth weld under different operating pressures, and establish a calculation model for the Mises stress variation of the girth weld; The calculation model of the maximum Mises stress value change of the girth weld during the rising stage of the magnetic gradient x component is: Board Mises =alnΔG x -b (1) Where: ΔG x is the change in the x component of the magnetic gradient, nT / m; Δσ Mises is the Mises stress variation of the girth weld, MP; a and b are coefficients, which are determined by fitting the experimental test data; S4. Calculation model for the change in the maximum Mises stress value of the girth weld during the decline of the magnetic gradient x component: c, d are unknown coefficients, which are determined by fitting the experimental test data; S5. Use a three-axis high-precision magnetic gradient sensor to collect the magnetic gradient signal of the pipeline under the action of the geomagnetic field and stress excitation as a reference magnetic signal; select three measurement points arbitrarily along the circumferential direction of the pipeline monitoring section, use stress detection technology to test the axial stress value and the circumferential stress value, and calculate the maximum Mises stress value of the section as the initial value; S6. Install the excitation device along the circumferential direction at the circumferential weld position along the axial direction of the circumferential weld through a non-metallic fixed bracket, with an installation spacing of θ; the calculation formula of θ is: θ=360 / n (3) Where: θ is the installation spacing of the excitation device, °; n is the number of excitation devices; Adjust the power supply strength of the excitation device and adjust the excitation magnetic field strength to the excitation magnetic field strength determined by the experiment; S7. Install a magnetic gradient signal monitoring system just above the girth weld to collect the magnetic gradient x component of the girth weld in real time under the excitation magnetic field. The magnetic gradient meter is placed horizontally and perpendicular to the pipeline, and the sensors at both ends are symmetrically distributed about the pipeline axis. S8. Quantify the magnetic stress monitoring method of the girth weld of steel pipelines under the excitation magnetic field and establish a monitoring method evaluation system; Selecting 80% of the Mises stress of the pipeline reaching the yield limit [σ] as the critical threshold, the monitoring method evaluation system divides the pipeline stress failure risk into three levels: Level I is high risk, σ Mises >0.8[σ]; Level II is medium risk, 0.72[σ]<σ Mises <0.8[σ]; Level III is low risk, σ Mises <0.72[σ].

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