METHOD AND INSPECTION DEVICE FOR EXAMINING THE CATHODIC PROTECTION OF A, MORE PARTICULARLY FERROMAGNETIC, PIPELINE

Abstract

A method is provided for examining the cathodic protection of a metallic and in particular ferromagnetic pipeline. An inspection device is also provided for examining the cathodic protection of a pipeline, in particular of a ferromagnetic pipeline. The inspection device is formed to be able to pass through the pipeline and in particular be driven by the medium, and includes a magnetizing device serving to create an alternating magnetic field. A magnet unit and a measuring device are provided, and includes at least one magnetic field sensor serving to measure a magnetic field formed on the inner side of the wall of the pipeline.

Claims

1. A method for examining the cathodic protection of a metallic pipeline, the method comprising the steps of: creating a primary alternating magnetic field and a local change in permeability in a wall of the pipeline by a magnetizing device of an inspection device moved through the pipeline, wherein a secondary DC magnetic field is caused by a DC current of cathodic protection being formed in the wall of the pipeline; using a measuring device moved through the pipeline to measure a resultant magnetic field that emerges from superposition of the primary alternating field and the secondary DC magnetic field; using a computing device to analyze signal components of at least the secondary magnetic field giving consideration to the change in the permeability; and deriving a magnitude of the DC current on the basis of the signal components of the secondary magnetic field.

2. The method as claimed in claim 1, wherein the measurement takes place while the alternating field is created.

3. The method as claimed in claim 1, wherein a spectrum of the secondary magnetic field is determined in the computing device.

4. The method as claimed in claim 3, wherein at least one signal component which is an even multiple of a frequency of a directional change of the magnetic field is selected in the analysis for determining the DC current.

5. The method as claimed in claim 1, wherein a current intensity is determined by one or more regression functions in the computing device on the basis of an amplitude of at least one even multiple of a frequency of a directional change of the magnetic field.

6. The method as claimed in claim 1, wherein measurement conditions are determined in the computing apparatus on the basis of a spectrum of the primary alternating field.

7. The method as claimed in claim 1, wherein a voltage of the cathodic protection is varied and at least increased for a measurement run of the inspection device.

8. The method as claimed in claim 1, wherein a multiplicity of data from a measurement run are fused in order to determine the current intensity.

9. The method as claimed in claim 1, wherein the measurement is implemented by at least one magnetic field sensor which is positioned at least substantially at a fixed distance from the wall at the measurement time.

10. The method as claimed in claim 9, wherein the at least one magnet unit revolves on a carrier in a longitudinal direction of the pipeline.

11. The method as claimed in claim 1, wherein, during a measurement run, an additional current is applied to the inner wall side of the pipeline by two contacts which are spaced apart in the longitudinal direction of the pipeline.

12. An inspection device for examining the cathodic protection of a pipeline, the inspection device being formed to be able to pass through the pipeline, the inspection device comprising: a magnetizing device serving to create an alternating magnetic field, the magnetizing device having: at least a magnet unit and a measuring device including at least one magnetic field sensor and serving to measure a magnetic field formed on an inner side of a wall of the pipeline, at least one carrier which is able to be rolled through the pipeline in a longitudinal direction thereof in an operational state, the said carrier being provided with an at least substantially circular perimeter in a section running transversely to an axis of rotation and, along the perimeter, including at least one magnet units for creating an alternating magnetic field, magnetic field directions of which run at least partially against one another.

13. The inspection device as claimed in claim 12, wherein the magnetic field directions of the at least one magnet unit are formed parallel or radially to the axis of rotation of the carrier.

14. The inspection device as claimed in claim 12, wherein, along the perimeter of the carrier, at least one magnetic field sensor is arranged on or in the carrier.

15. The inspection device as claimed in claim 14, wherein the magnetic field sensor is a coil with windings running in a circumferential direction.

16. The inspection device as claimed in claim 14, further including a multiplicity of magnetic field sensors arranged next to one another along the perimeter.

17. The inspection device as claimed in claim 12, wherein a part of the measuring device is arranged in a holder for arrangement close to the wall.

18. The inspection device as claimed in claim 17, wherein the carrier is rotatably mounted on the holder.

19. The inspection device as claimed in claim 12, wherein the magnetizing device and magnetic field sensor are movable relative to one another.

20. The inspection device as claimed in claim 17, wherein the carrier is provided via the holder with a supporting element being a roller wheel, a diameter of which is no more than 50% of the diameter of the carrier.

21. The inspection device as claimed in claim 12, further including at least one disk or cup for propulsion purposes.

22. The inspection device as claimed in claim 12, further including at least one propulsion element.

23. The method as claimed in claim 1, wherein the magnitude of the DC current is derived based on a database with calibration data.

24. The method as claimed in claim 5, wherein the current intensity is determined additionally with analysis data of the secondary magnetic field being normalized and/or calibrated by analysis data of the primary magnetic field.

25. The method as claimed in claim 9, wherein the distance of at least one magnet unit of the magnetizing device from the inner side of the wall varies.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Reference is now made more particularly to the drawings, which illustrate the best presently known mode of carrying out the invention and wherein similar reference characters indicate the same parts throughout the views.

[0062] FIG. 1 shows the result of a numerical simulation of the magnetic field strength induced by DC current.

[0063] FIG. 2 shows a configuration of permanent magnets in a trial setup.

[0064] FIG. 3 shows the magnetic flux created by the setup according to FIG. 2.

[0065] FIG. 4 shows a section of a secondary magnetic field in the case of the setup according to FIG. 2.

[0066] FIG. 5 shows a setup of a magnetizing device of an object according to the invention.

[0067] FIG. 6 shows a simplified representation of the curve of the magnetic permeability ? and of the primary and secondary magnetic field (H.sub.p and H.sub.s).

[0068] FIG. 7 shows an amplitude of the magnetic field measured on the inner side of the wall of the pipeline and a spectrum of the measured signal.

[0069] FIG. 8 shows a Fourier spectrum of the difference between magnetic fields, measured with and without DC current.

[0070] FIG. 9 shows an object according to the invention.

[0071] FIG. 10 shows a detailed view of the object according to FIG. 9.

[0072] FIG. 11 shows a detailed view of a further object according to the invention.

[0073] FIG. 12 shows a detailed view of a further object according to the invention.

[0074] FIG. 13 shows a detailed view of a further object according to the invention.

[0075] FIG. 14 shows a further object according to the invention.

[0076] FIG. 15 shows a further object according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0077] The features of the exemplary embodiments according to the invention, as explained hereinbelow, may also be part of the subject matter of the invention on their own or in other combinations than presented or described, but always at least in combination with the features of one of the independent claims. If advantageous, parts having the same functional effect are provided with the same reference signs.

[0078] A numerical simulation was initially performed in order to verify the method according to the invention; in the scope of said simulation, a DC current is simulated in a wall 1 of a pipeline. Arrows 2 serve to label the magnetic field strength and the direction thereof. The larger an arrow 2, the greater the magnetic field strength at the corresponding position. The magnetic field within the pipeline is zero.

[0079] For further verification, the magnetic permeability of the pipeline wall 1 was now varied using a permanent magnet setup according to FIG. 2. The arrangement of three permanent magnets 3 leads to the formation of a strong magnetic flux through the wall 1 of the pipeline. On a local scale, this greatly changes the magnetic permeability in the wall 1 of the pipeline. This leads to the ingress of the secondary magnetic field created by the DC current in the wall 1 of the pipeline, illustrated by the associated arrows 2 on the inner side of the wall 1 of the pipeline.

[0080] Now, a device according to the invention comprising a magnet unit of a magnetization device having permanent magnets 4 was used for the method according to the invention. The left-hand part of FIG. 5 depicts the permanent magnets in an associated carrier 5, which, on its circumference, can roll through the pipeline in the longitudinal direction. The magnetic field direction of permanent magnets following one another in the circumferential direction is always opposite, with the result that the sequence of north poles and south poles, identifiable in FIG. 5, arises in the circumferential direction. The carrier 5 in which the magnets 4 are arranged may be formed from a nonmagnetic material and rotates about an axis of rotation 38. For the purpose of aligning the magnetic fields of a respective permanent magnet 4, focusing elements 6 are arranged on both sides on the north and south poles of a permanent magnet and direct the magnetic field flux in the direction of the inner surface of the wall 1 of the pipeline. These focusing elements are annular segment-shaped focusing elements 6 consisting of a magnetizable material. They are fastened to the carrier 5 on one side and, on the inner side, rest on a flange 7 of the carrier. The radially outwardly directed surfaces 8 of the focusing elements 6 protrude slightly beyond the outer perimeter of the carrier 5, with the result that the outermost surface of the latter does not make a connection with the wall of the pipeline, but only rolls on the latter along the circumference of said carrier.

[0081] When considered in the wall of the pipeline, the rolling of the magnetizing device in the form of a magnet wheel yields the profile of the primary magnetic field H.sub.p(t), as depicted in FIG. 6, with the progress of the wheel. Since the permeability u depends only on the magnitude of the primary magnetic field H.sub.p, the frequency of the signal is twice as high as that of the primary magnetic field. The time profile ?(t), as likewise depicted in FIG. 6, arises accordingly. Under the condition that the external constraints remain unchanged and, for example, no defects are present, the local change in the permeability then in turn yields the profile of the secondary magnetic field H.sub.s measurable on the inner side of the pipeline.

[0082] A magnetizing device according to FIG. 5 brings about the magnetic field measured on the inner surface of the pipeline wall in accordance with the left-hand part of FIG. 7, wherein no DC current of a cathodic protection was applied within the pipeline wall. A spectral analysis then exhibits the peak of the dominant frequency at N=1 in the right-hand part of FIG. 7. N is a multiple of the dominant frequency f.sub.m. The smaller peaks at N=2 and N=3 arise from the measurement inaccuracies as this is not an ideal system.

[0083] The Fourier spectrum depicted in FIG. 8 arises if a DC current is applied in the wall of the pipeline, the magnetic field arising on the inner surface of the pipeline wall is likewise measured again, and, subsequently, the difference of the field with the DC current and the field without the DC current is formed in the spectrum. The peaks at N=2, N=4, N=6, and N=8, which arise due to the secondary magnetic field which extends into the pipe interior on account of the local changes in the permeability, are clearly visible. With the exception of the zero frequency, the maximum value has a frequency corresponding to the frequency of the directional change of the magnetic field. In the case of the magnet wheel depicted in the figure, the magnetic direction changes four times per revolution. While the spectrum of the primary magnetic field is now determined by integer multiples of f.sub.m=2*v/(?*D) (where D is the diameter of the magnet wheel, v is the speed of the center of the magnet wheel), the spectrum of the secondary magnetic field is represented by frequencies which are multiples of 2*f.sub.m. The accuracy of the spectrum increases with the length of the signal cut out for the calculation. Preferably, signal lengths of 6 to 12 main periods are used for the evaluation.

[0084] The primary and secondary magnetic fields are distributed differently in space. A plurality of magnetic field sensors may be installed at different positions for the purpose of a better separation of the primary and secondary magnetic fields. The recorded signals can be used to construct a combined feature vector.

[00006]$V=\left[\frac{F_1\left(2f_m\right)}{F_1\left(f_m\right)},\frac{F_1\left(4f_m\right)}{F_1\left(f_m\right)},\frac{F_1\left(6f_m\right)}{F_1\left(f_m\right)},\frac{F_2\left(8f_m\right)}{F_1\left(f_m\right)},.Math.,\frac{F_n\left(2f_m\right)}{F_n\left(f_m\right)},\frac{F_n\left(4f_m\right)}{F_n\left(f_m\right)},\frac{F_n\left(6f_m\right)}{F_n\left(f_m\right)},\frac{F_n\left(8f_m\right)}{F_n\left(f_m\right)}\right],$ [0085] where F.sub.n, is the spectrum corresponding to the signal recorded by the n-th magnetic field sensor. The dimension of the vector V can be reduced, for example with the aid of principal component methods, before the regression function is formed.

[0086] To this end, a plurality of measuring devices with corresponding magnetizing devices 10 and magnetic field sensors may be provided on an inspection device, in particular in accordance with the exemplary embodiment of FIG. 9. An inspection device designed as an inspection pig comprises the magnetizing devices 10, which are held via respective holders 12 on the further inspection device. During operation, a respective holder 12 is pressed against the inner side or surface of the wall 1 by means of a force-storing element 14 in the form of a spring, which is arranged on the holder 12 at one end and mounted on the central body of the inspection device at the other end. The holder 12 itself is pivotably hinged on the central body 16 of the inspection device. On account of the magnetic interaction of the individual magnet units with the wall 1 of the pipeline, the magnetizing devices 10 precisely roll on the said pipeline while the inspection device is propelled by the medium situated in the pipeline. To this end, the inspection device comprises sealing elements in the form of disks 18, which substantially completely cover the internal, free pipeline cross section. The sealing elements additionally center the inspection device and in part lead to a certain amount of cleaning of the pipeline wall 1. To be able to calibrate the results in improved fashion, a current is fed into the wall 1 of the pipeline via metal brushes 20.

[0087] There are a multiplicity of different options regarding the arrangement of magnetic field sensors. According to the exemplary embodiment of FIG. 9, a multiplicity of magnetic field sensors 22, represented in punctiform fashion, are arranged along the perimeter of the carrier 5. Alternatively, the magnetic field sensors 22 are fixedly arranged on the holder 12, with the result that the magnetic field sensors 22 remain at a fixed distance from the inner upper side of the wall 1 (FIG. 11). Combinations of these arrangements are also possible.

[0088] A measuring device, which preferably is a part of the inspection device, generally comprises electronics required for the readout and possibly required sensor control, associated storage means and power supply means.

[0089] In yet a further alternative, or cumulatively, a magnetic field sensor may be designed as a coil wound along the entire perimeter of the carrier 5, indicated by windings 24 in FIG. 12.

[0090] The scope of the invention likewise includes the combination of a magnetic field sensor 22 fixedly arranged on the holder 12 with a magnetic field sensor comprising windings 24 of a coil.

[0091] In the case of a measurement of the magnetic field by means of a coil along the entire perimeter of the carrier 5 in particular, said coil always has the same geometric configuration in relation to the contact point with the inner wall. As a result, it is possible to construct the magnetizing device with the magnetic field sensor comparatively easily.

[0092] According to FIG. 14, a further exemplary embodiment according to the invention is provided with a support element 32 in the form of a roller wheel, which is provided to support the inspection device against the pipeline wall and which is rotatably held on the holder 12 that serves as a fastening device. The carrier 5, only indicated using dashed lines, is rotatably held via sliding and/or roller bearings in a part 34 of the holder 12, which is hollow cylindrical in the exemplary embodiment above, and partly covered thereby. The axes of rotation of the roller wheel and of the carrier 5 run in parallel, and both run transversely to the pipeline longitudinal direction and direction of travel. The roller wheel is the leading element of the inspection device, the carrier 5 of which comprises propulsion elements 34 spiraling away from an axis of rotation 38. In a view in the direction of the axis of rotation 38, the carrier 5 is provided with a circular perimeter, which is formed by surrounds 36. The above-described focusing elements may be worked into the surrounds 36. Data storage and power supply means are preferably arranged in the carrier 5.

[0093] A further device according to the invention, according to FIG. 15, has a similar design to the variant according to FIG. 14 with laterally protruding propulsion elements 34, but lacks an additional support or roller wheel. Accordingly, the carrier 5 is not mounted in a holder 12 either. The carrier 5 preferably has a setup of magnetic field sensors 12 which corresponds to the arrangement according to the exemplary embodiments in FIG. 10 or 12. In this variant, too, the north-south axes of the permanent magnets arranged in the carrier are arranged parallel to the axis of rotation 38.