RU2722333C1 - Method of determining mechanical stress in a steel pipe - Google Patents

Abstract

FIELD: measurement.SUBSTANCE: invention relates to assessment of technical condition of steel pipelines and can be used for determination of mechanical stress, for example, in steel pipelines of underground laying. Method comprises making a sample in the form of a hollow cylinder from a material similar to the material of the pipeline, step-by-step loading of sample by creation in it of excess internal pressure of liquid or gaseous medium and its bending, obtaining dependence of coercive force on value of mechanical stresses in sample. Two control points are set on the sample circumference: one is in the bending stretching zone and the second one is in the bending compression zone. Sample is loaded by simultaneous action of bending and internal pressure of medium. Coercive force is measured at control points by aligning the coercimeter sensor along the sample axis. Curves of dependence of coercive force Hon bending stresses are plotted σ, at different pressures medium PPipeline section with potentially high bending stress is determined. Pipeline circle control points are marked in selected cross-section, coercive force is measured at selected points by aligning coercimeter sensor so that direction of magnetic flux in sensor is aligned with pipeline axis. Selecting maximum and minimum among measured values, note here that said values must refer to diametrically opposite points of pipeline cross-section. Note that point with minimum coercive force value is related to zone of maximum tension with maximum value of maximum compression zone. An angle of the bending plane passing through the points of maximum tension and compression is determined, pressure in the pipeline is measured and bending stresses in the pipeline are determined using the obtained relationship for the corresponding pressure.EFFECT: possibility of determining mechanical stresses in the wall of a steel pipeline, taking into account simultaneous action of transverse bending and internal pressure of the transported medium, high reliability of the method and broader capabilities thereof.1 cl, 3 dwg

Description

The invention relates to the field of assessing the technical condition of steel pipelines and can be used to determine mechanical stresses, for example, in steel pipelines of an underground installation.

A known method of measuring mechanical stresses in pipelines operating under pressure, in which on a control sample of a pipeline with zero longitudinal stresses, for which a rectilinear underground section of the pipeline is chosen, values of the magnetic noise parameter are measured, the conversion coefficient of proportionality is determined, the value of the metal magnetic noise parameter is recorded the pipeline in the place of control and their values judge the stresses in the pipeline (RF Patent No. 2116635, IPC G01L 1/12, G01N 27/83. Publ. 27.07.98. Bull. No. 21, S. 342.).

The disadvantage of this method is the difficulty of choosing a section of the pipeline with zero longitudinal stresses, because the straightness of the section does not guarantee zero longitudinal stresses in the pipeline metal, which reduces the accuracy of measuring stresses.

A known method for determining stresses, based on obtaining tensile metal samples with different structural degradation, the dependences of the anisotropy of the coercive force on tensile stresses in the samples and the assessment of stresses in the structure using the obtained dependencies taking into account the actual metal structure (RF Patent No. 2281468 Claim 14.03.2005 Publish. 08/10/2006).

The disadvantage of this method is the inability to determine the voltage when the stress state of the metal structure. For example, pipelines are characterized by a plane-stressed state of the walls (axial and ring stresses).

Closest to the claimed method is a method for determining the stress state of steel structures, taken as a prototype (RF Patent No. 2439530, IPC G01N 3/08. Publ. 10.01.2012).

In a known solution, cylindrical hollow metal samples from a material similar to the construction material, the stress state of which must be determined, are loaded with a certain step by the internal pressure of a liquid or gas medium inside the cylinder to create a plane-stress state caused by tensile stresses in the axial and annular directions, or bend the sample to create axial tensile-compression stresses. For each loading step, the stresses in the sample are determined by calculation or in another way, at each loading step, the coercive force is measured, while the magnetic flux of the coercimetry sensor is oriented coaxially with the direction of the determined voltages. The dependence of the coercive force on the stresses in the sample is built. Then, the coercive force of the metal of the structure is measured, orienting the sensor in the direction of action of the estimated voltages, and stresses are determined using the obtained dependence.

The disadvantages of this method are:

1. the inability to determine stresses in existing pipelines under the simultaneous effects of bending and internal pressure of the medium;

2. does not determine the angle of the bending plane, the determination of which is an almost important task;

3. insufficient reliability of the method, since the actual values of the coercive force of the metal of the pipe walls can be caused not only by the stress state of the pipeline in a given section, but also by the influence of external factors, as a result of which the interpretation of the results is incorrect.

As an explanation, we report the following.

The stress state of the walls of thin-walled pipelines is approximately biaxial with two main stresses: annular and longitudinal. Ring stresses in the pipe wall arise from the internal pressure of the transported medium and their determination does not cause difficulties by known dependencies, if the pressure of the medium is known. These stresses are conditionally constant around the circumference of the considered pipe section.

Longitudinal stresses are composed of stresses due to: 1. the internal pressure of the medium (calculated as a fraction of the ring); 2. thermal deformations of the grounded section of the pipeline (determined by calculation); 3. the bend caused by the curvature of the route for laying the pipeline (for example, the curvature of the trench). Also, a bend can be associated with non-design changes in the position of the pipeline (for example, as a result of its ascent, movement due to landslide processes, etc.).

Therefore, the main practical task of determining the stress state of pipelines is to assess bending stresses in potentially dangerous sections of the pipeline. Unlike the other stresses mentioned above, bending stresses are characterized by the fact that at different points around the circumference of the pipe section under consideration, their value is different, while they are maximum in absolute value at two diametrically opposite points of the section through which the bending plane passes. At one point, the bending stresses are negative (compressive), at the other - positive (tensile).

The presence of internal pressure changes the longitudinal stresses in the wall of the pipeline, which limits the application of the prototype method on existing pipelines operating under pressure.

The noted features of the formation of the stress state of the pipe walls uses the proposed method, which allows us to solve the technical problem.

An object of the invention is the determination of mechanical stresses in the wall of a steel pipeline taking into account the simultaneous effects of transverse bending and internal pressure of the transported medium, increasing the reliability of the method, expanding its capabilities.

The problem is solved in that in the method for determining bending stresses in steel pipelines, including the manufacture of a sample in the form of a hollow cylinder from a material similar to the material of the pipeline, stepwise loading of the sample by creating excessive internal pressure of a liquid or gas medium in it and bending it, obtaining a dependence of the coercive force from the magnitude of the mechanical stresses in the sample, according to the invention, two control points are assigned on the circumference of the sample: one in the tensile zone during bending, the second in the compression zone during bending, the sample is loaded by the simultaneous action of bending and internal pressure of the medium, and the coercive force is measured in the control points, orienting the coercimetry sensor along the axis of the sample, build graphs of the dependence of the coercive force H _c on bending stresses σ _ex , at various medium pressures P _int , determine the cross-section of the pipeline with potentially high bending stresses, designate points for monitoring the circumference of the pipeline and in the selected section, measure the coercive force at the selected points, orienting the coercimetry sensor so that the direction of the magnetic flux in the sensor coincides with the axis of the pipeline, select the maximum and minimum among the measured values, while these values should relate to diametrically opposite points of the section of the pipeline, consider that the point with the minimum value of the coercive force is associated with the zone of maximum tensile stresses, with the maximum value - with the zone of maximum compression, determine the angle of the bending plane passing through the points of maximum tensile and compression, measure the pressure in the pipeline and determine the bending stresses in the pipeline using dependencies for the corresponding pressure.

In FIG. 1 shows a cylindrical sample for stepwise loading by bending and internal pressure of the medium and measuring the coercive force at the control points. In FIG. 1 marked: 1 - cylindrical sample; 2 - end caps; 3 - concrete blocks; 4 - clamps; 5 - a jack; 6 - flexible pump hose; 7 - dynamometer; 8 - control section; 9, 10 - control points.

In FIG. 2 shows the dependence of H _c = ƒ (σ _mfd) the magnitude of the coercive force of the longitudinal stresses due to the action of transverse bending and internal pressure for the region of compression and expansion at a pressure medium P _ext = 4.5 MPa.

In FIG. 3 shows a petal diagram, which shows the results of changes in the coercive force around the circumference in the selected section, performed in increments of 1 hour (30 degrees).

The method is implemented as follows. From a pipe made of a material similar to the material of the pipeline, the stress state of which must be determined, make a test sealed sample.

Fill the sample with test medium. Select a control section on the sample, mark two control points, one of which is located in the zone of maximum tensile metal during bending, the other in the zone of maximum compression. Increase the bending force that creates the bend step by step. Using a coercimeter, the coercive force is measured at each loading step, while the magnetic flux of the coercimetric sensor is oriented coaxially with the axis of the pipeline (longitudinally).

Pump up the pressure of the medium in the sample. At the selected pressure of the medium step by step load the sample by bending, at each loading step in the control points measure the coercive force.

Step loading by bending and measuring the coercive force at the control points are performed for various values of the pressure of the medium.

For each step of loading, bending stresses are determined at control points, for example, by the calculation method or by the method of electrical strain measurements.

Build dependence of the coercive force measured at the control points of bending stresses H _c = ƒ (σ _mfd) for different values of the internal pressure P _ext. In FIG. Figure 2 shows an example of a relationship for pressure P _int = 4.5 MPa.

The cross section of the pipeline with potentially high bending stresses is determined, the metal stresses in which it is necessary to determine.

Prepare the surface of the pipeline for measuring the coercive force: dig out the pipeline (if necessary), remove the protective coating (if necessary).

In the selected section, several control points are noted, placing them around the entire circumference of the pipeline.

Measure the coercive force at the control points, placing the coercimetric sensor along the axis of the pipeline.

Among the measured values, the maximum and minimum are selected, while these measurements should relate to diametrically opposite points of the circumference of the pipeline: the point at which the coercive force value is minimal is associated with the zone of maximum tensile tension, and the point with the maximum value of coercive force is associated with the zone of maximum compression (Fig. . 3). A bending plane with an angle relative to the vertical y passes through these points.

The pressure P _{nn is} measured in the pipeline near the selected section.

Bending stresses are determined by the values of the coercive force at the control points using the dependence H _c = ƒ (σ _ex ) obtained on the sample for a similar pressure P _ext .

Example.

It is necessary to determine the longitudinal stresses in the existing underground oil pipeline (diameter 219 mm, pipe wall thickness 5 mm, material 17G1S steel) caused by the simultaneous action of bending and internal oil pressure. The test sample is made of pipe 1 (diameter 219 mm, pipe wall thickness 5 mm, material 17G1S steel) with end caps 2. The total length of the sample is 10 m (Fig. 1).

The sample is mounted on concrete blocks 3, the ends of the pipe are fixed to the blocks using metal clamps 4. A hydraulic bottle jack 5 is manufactured by Service Key LLC with a loading capacity of 10 tons under the center of the stand, a flexible hose 6 of the pressure testing pump NA-250 is connected (maximum pressure 250 atm.) (not shown in FIG. 1) to create internal pressure in the sample. Between the jack 5 and the pipe 1, an electronic portable DEPZ 1D-10R-00 dynamometer is installed (Fig. 1. pos. 7). Taking into account the overall dimensions of the KM 455.2 coercimetry sensor, select a control section 8 on the sample in the zone of maximum bending stresses, spaced 200 mm from the jack, mark two control points with an angular orientation of 6 hours (Fig. 1. pos. 9) and 12 hours ( Fig. 1. Pos. 10), corresponding to the zone of maximum tension (12 hours - the top of the pipe) and maximum compression of the metal (6 hours - the bottom of the pipe).

Fill test sample 1 with water.

Calculation method is determined that the maximum voltage at the control points 9 and 10 do not exceed 80% of the tensile strength of the steel grade used (tensile strength steels 17G1S equal _to σ = 510 MPa) with a force equal to the jack 5 13,5 kN, which corresponds to raising the jack rod by 72 mm, while the maximum possible pressure of the medium is 4.5 MPa.

The number of steps for loading a test specimen is set: 10 steps for pumping internal pressure (from 0 to 4.5 MPa in increments of 0.5 MPa) and 10 steps for creating a bend (stroke of the jack rod is from 0 mm to 72 mm in increments of 8 mm).

Sample 1 is bent stepwise relative to its longitudinal axis by jack 5 (with a pitch of 8 mm), the reaction of the jack is determined using dynamometer 7 at each test step. The coercive force is measured at control points 9, 10 at each loading step using a coercimeter, while the magnetic flux of the device sensor is oriented coaxially with the direction of the determined voltages.

Inject stepwise (in increments of 0.5 MPa) the internal pressure of the water in the pipe. At the selected pressure, the test specimen is loaded stepwise by bending (with a step of 8 mm), at each loading step at the control points, the coercive force is measured at each control point along the tensile stresses.

Loaded with bend and measure the coercive force at the control points for the pressure values of the medium at each step.

The value of the value of bending stresses caused by the simultaneous action of bending and internal pressure on the sample wall for the compression and tension zones by the calculation method is determined.

According to the calculated and experimental data, the dependences of the coercive force, measured at control points, on the magnitude of the longitudinal stresses H _c = ƒ (σ _ex ), for various values of the internal pressure P _int (Fig. 2).

They dig a section of the pipeline with potentially high bending stresses, remove the insulation. It is established that at the time of measurement the pressure in the oil pipeline is 4.6 MPa.

Outline the cross section of the pipe for control. In the selected section, 12 control points are marked, located in increments of 1 hour (30 degrees) (12 hours - top of the pipe, 6 hours - bottom of the pipe) (Fig. 3). The coercive force is measured at each of the selected points, orienting the sensor along the axis of the pipeline.

Select the maximum and minimum value. Build a petal diagram (Fig. 3).

It is established that on the petal diagram (Fig. 3) there are two maximum values of the coercive force (Hc = 700 A / m) located in the orientation of 1 and 2 hours and one minimum (Hc = 575 A / m) located in the orientation of 7 hours . Since points 1 and 7 are diametrically opposite, the bending plane passes through these points with an angle relative to the vertical y.

To determine the stress state, the dependence of the coercive force on the magnitude of the longitudinal stresses H _c = ƒ (σ _ex ) is used for the internal pressure P _int = 4.5 MPa (Fig. 2).

According to the obtained dependence (Fig. 2), it is determined that the bending stresses in the compression zone are about 56 MPa, in the tensile zone - 110 MPa.

Claims (1)

A method for determining bending stresses in steel pipelines, including manufacturing a sample in the form of a hollow cylinder from a material similar to the material of the pipeline, stepwise loading the sample by creating excessive internal pressure of a liquid or gas medium and bending it, obtaining the dependence of the coercive force on the value of mechanical stresses in the sample, characterized in that two control points are assigned on the circumference of the sample: one in the tensile zone during bending, the second in the compression zone during bending, the sample is loaded by the simultaneous action of bending and internal pressure of the medium, the coercive force is measured at the control points, orienting the coercimeter meter along the axis of the sample, build graphs of the dependence of the coercive force H _s on bending stresses σ _ex , at various pressures of the medium P _int , determine the cross-section of the pipeline with potentially high bending stresses, designate points for monitoring the circumference of the pipeline in the selected section, measure the coercive the strength at the selected points, orienting the coercimeter sensor so that the direction of the magnetic flux in the sensor coincides with the axis of the pipeline, choose the maximum and minimum among the measured values, while these values should relate to diametrically opposite points of the cross-section of the pipeline, consider that the point with the minimum the coercive force value is associated with the maximum tensile zone, with the maximum value - with the maximum compression zone, the angle of the bending plane passing through the maximum tensile and compression points is determined, the pressure in the pipeline is measured and the bending stresses in the pipeline are determined using the obtained dependence for the corresponding pressure.

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