METHOD AND INSPECTION DEVICE FOR INVESTIGATION OF THE CATHODAL PROTECTION OF A PARTICULARLY FERROMAGENTIC PIPELINE

Description

[0001] The present invention relates to a method for investigating the cathodic protection of a metallic and, in particular, ferromagnetic pipeline. The invention further relates to an inspection device for investigating the cathodic protection of a pipeline, in particular a ferromagnetic pipeline, wherein the inspection device is designed to be pipe-compatible and, in particular, medium-driven, and comprises a magnetizing device for generating an alternating magnetic field with at least one magnet unit and a measuring device comprising at least one magnetic field sensor for measuring a magnetic field formed on the inside of the pipeline wall.

[0002] Metallic pipelines are often laid in the ground or in water. These environments typically represent an electrolytic medium. At defects in the pipeline's coating, charge transport occurs from the metal of the pipeline towards the electrolyte. This transport of metal ions causes corrosion of the pipeline. To prevent corrosion, prior art techniques involve applying a direct current to the pipelines to be protected. This protective current creates cathodic polarization of the pipeline and prevents metal ions from being released from the pipe surface.

[0003] If this so-called cathodic protection fails, corrosion can develop very rapidly at the coating defects in the pipe walls. Therefore, the cathodic protection is inspected at relatively short intervals of one or a few months. Furthermore, such inspections help to detect and repair coating defects in a timely manner. A leak in a pipeline, such as an oil or gas pipeline, can lead to devastating environmental damage.

[0004] Cathodic protection is typically verified at several critical measuring points by measuring the system-soil potential. The appropriate devices are permanently installed at these measuring points. Additionally, the potential field can be measured at the surface along the pipeline and in the pipeline's surrounding area. For offshore pipelines, potential measurements are carried out using remotely operated underwater vehicles (ROVs). Potential measurement is generally very expensive and involves considerable manual effort.

[0005] Another method for verifying cathodic protection is based on measuring the direct current directly in the pipe wall using a pipe-walkable inspection device, also known as a pig. This approach allows measurements along the entire length of the pipeline and represents a cost-effective and reliable method for assessing the efficiency of the cathodic protection and detecting coating defects. However, the measurement requires a robust procedure that can be used even under the challenging measurement conditions of a pipeline, such as an oil pipeline.

[0006] A known prior art method for measuring current with a pig is based on measuring potential values ​​in the pipe wall upstream and downstream of an inspection device moving longitudinally through the pipe. The currents in the pipe wall and the potential distribution along the pipe can be determined from the measured potential difference. However, this approach requires good galvanic contact with the wall for reliable measurement. Therefore, the pipe must be thoroughly cleaned for the measurement. Such a measurement is generally not possible in pipes with an internal coating and / or containing an oil medium.

[0007] WO 2014 / 096942 A2 discloses an inspection device with a magnetizing device that is part of an odometer.

[0008] WO 2019 / 156584 A1 discloses a method for measuring cathodic protection in which a magnet inside the pipeline induces an axially asymmetric magnetic saturation. The cardboard protection is estimated from the resulting magnetic field.

[0009] The object of the present invention is to provide a cost-effective and reliable method and an associated inspection device for investigating the cathodic protection of a pipeline.

[0010] The problem is solved by a method according to claim 1 and by an object according to claim 12. Advantageous embodiments of the invention can be found in the respective dependent claims and in the following description.

[0011] In the inventive method for investigating the cathodic protection of a ferromagnetic pipeline, a primary alternating magnetic field, and thus a local change in permeability in the pipeline wall, is generated by means of a magnetizing device of an inspection device moved through the pipeline. Simultaneously, a secondary direct current magnetic field, caused by a direct current from the cathodic protection, is generated in the pipeline wall. Furthermore, a resulting magnetic field, which arises from the superposition of the primary alternating field and the secondary direct current magnetic field, is measured by means of a measuring device moving through the pipeline, which includes at least one magnetic field sensor and is preferably also part of the inspection device.Furthermore, the signal components of at least the secondary DC magnetic field are analyzed, taking into account the change in permeability, using a computer device. The magnitude of the DC current is derived based on the signal components of the secondary magnetic field. This analysis typically takes place after the data has been read from the inspection device after it has completed its inspection run and transferred to the computer device. Suitable computer devices include laptops used in the field, desktop PCs, and servers, each running dedicated analysis software. This software has access to the respective working and permanent storage, the processors, and interfaces for data transfer between the required computer systems. Databases can also be connected locally or in the cloud, containing the data necessary for the analysis, particularly calibration data.

[0012] The invention is based on the finding that applying a stronger magnetic field to the pipe wall changes the magnetic permeability in this area of ​​the pipe wall, and that, due to the inhomogeneous distribution of the magnetic permeability, the magnetic leakage flux of the direct current, which is otherwise not measurable within the pipe wall, can penetrate into the interior of the pipe and be measured from the inside of the pipe, i.e., by a measuring device moved in the pipe.

[0013] For the purposes of describing the present invention, the secondary direct current magnetic field will, for the sake of simplicity, also be referred to as the secondary magnetic field.

[0014] The resulting magnetic field in the pipeline is the sum of the magnetic field induced by the strong external magnetic field and the magnetic field induced in the pipeline wall by the direct current. The magnetizing device is designed, depending on the wall thickness and material, to achieve the largest possible change in permeability. This allows a usable signal to be generated by superimposing the primary alternating field with a secondary magnetic field generated by the direct current. Due to the local change in permeability and the associated inhomogeneous distribution of magnetic permeability, this secondary field extends into the interior of the pipeline with a (stray) flux component. The magnitude of the direct current is then determined based on the signal components of the secondary magnetic field.This allows the direct current to be measured and determined indirectly over the entire length of a pipeline in a simple way and without the previous limitations caused by, for example, oil-contaminated inner surfaces of the pipeline wall.

[0015] The magnitude of the direct current is derived in particular by comparing the determined signal components with the data of a calibration database, which includes the stray flux components of the secondary magnetic field that occur near the wall on the inside of a pipeline for a variety of magnetic field strengths, materials, pipe wall thicknesses and / or direct current quantities.

[0016] The strength of the locally generated alternating magnetic field by a magnetizing device is, in particular, at least 50 times, preferably at least 100 times, and more preferably 500 times greater than the strength of the direct current magnetic field. To achieve a sufficient change in magnetic permeability, a strong primary magnetic field is thus induced. The induced secondary magnetic field is very small compared to the primary magnetic field. However, because the magnetic permeability depends only on the amplitude of the applied magnetic field, it varies twice as fast as the primary magnetic field. The secondary magnetic field changes at the same rate as the magnetic permeability, i.e., it varies twice as fast as the primary magnetic field, which is used to identify the signal components of the secondary magnetic field.

[0017] Preferably, the frequencies of the applied alternating magnetic field are less than 500 Hz, and in particular, greater than 5 Hz. The effects are particularly easy to observe below 500 Hz.

[0018] In particular, the measurement of the magnetic field resulting from the primary and secondary magnetic fields takes place during the generation of the alternating field, so that the temporal course of the measured signal during the generation of the alternating field can be recorded and evaluated.

[0019] Preferably, the spectrum of the secondary magnetic field is determined in the computer device, whereby the entire signal can initially be analyzed, from which the signal components of the secondary field are then derived. The spectral analysis, known per se, for example in the form of a Fourier analysis, is now used to determine an external DC magnetic field that would otherwise not be measurable on the inside of the pipe wall.

[0020] Preferably, in the analysis for determining the direct current, at least one signal component is selected that is an even multiple of the frequency of the change in direction of the magnetic field. In particular, in the spectral analysis, all signal components corresponding to an even multiple of the frequency of the change in direction of the alternating field are considered, while the zero frequency is disregarded. The frequency of the strength of the secondary magnetic field Hs(t) is twice as high as the frequency of the strength of the primary magnetic field Hp(t). This difference in variation allows the signals Hp(t) and Hs(t) to be separated in the frequency domain. The current strength of the cathode protection is then derived from the signal of the secondary field.

[0021] For the purpose of accurate evaluation, signal lengths of 5 to 15 main periods of the applied alternating field are taken into account.

[0022] In particular, the current in the computer device is determined based on the amplitude of at least an even multiple of the frequency of the change in direction of the magnetic field using one or more regression functions. These one or more regression functions are determined, in particular, depending on the prevailing boundary conditions, especially the wall thickness of the pipeline, its material, the applied magnetic field, the speed of the measuring device, and / or the setup of the associated magnetic field generation device. Preferably, the analysis data of the secondary magnetic field are normalized and / or calibrated using the analysis data of the primary magnetic field.

[0023] To determine the current strength I To calculate the cathode protection, for example a regression function of the feature vector can be used. V = F 2 f m F f m F 4 f m F f m F 6 f m F f m F 8 f m F f m are determined where F( ) are the amplitudes of the respective spectrum. The individual values ​​from the spectrum are normalized to the principal frequency value from the signal spectrum. This normalization reduces the dependence on the variation of the magnetization strength and I = R ( V ), where R represents the regression function.

[0024] To define the regression function, a fit function or common machine learning methods such as linear regression, neural networks, decision trees, support vector machines, etc., can be used. Data for defining the regression function can be obtained during calibration. Depending on the device design, it may be necessary to define different regression functions for various measurement conditions, such as wall thickness. wtf, To define the magnetic properties of the piping material and the speed v of the measuring device. I = R 1 V wenn wt = wt 1 , v = v 1 , … R 2 V wenn wt = wt 2 , v = v 2 , … …

[0025] These dependencies can also be introduced into the regression function as additional parameters, i.e. I = R ( V , wt, v, ...). The current I It can therefore be defined as follows. I = R 1 V wt v für Stahl st 37 R 2 V wt v für Stahl st 52 …

[0026] Calibration can also be performed by changing the amplitudes of the odd frequencies and / or by using the BH curves of the odd frequencies.

[0027] The measurement conditions can be defined by the course of the primary magnetization. It may suffice to use the measured signal directly, neglecting the secondary magnetization component. It is also possible to use the frequencies 2 fm 4 fm , 6 fm , 8 fm... to filter out the secondary magnetic field from the signal in order to exclude its influence. In particular, according to a further advantageous development in the computer device, the measurement conditions are determined based on a spectrum of the primary alternating field, so that separate, additional work or measurement processes for determining the measurement conditions can be omitted.

[0028] Some of the measurement conditions may nevertheless be known or may be redundantly determined in advance by other measurement methods.

[0029] Preferably, in a further development of the inventive method, the voltage of the cathode protection is varied and at least increased for one or during a measurement run of the inspection device. As a result of the increase, the current in the pipe wall becomes greater and the secondary magnetic field increases. In particular, the voltage waveform can exhibit a signal pattern for better identification of the corresponding DC magnetic field component.

[0030] Another way to improve the separation of the primary and secondary magnetic field signals is to configure the device so that the primary magnetic field spectrum is mainly represented by the main frequency component in the Fourier spectrum. This makes it easier to separate the primary and secondary magnetic field signals in the Fourier spectrum.

[0031] In particular, determining the current intensity involves fusing a large number of data points from a measurement run. If the coating of a pipeline is undamaged, the current intensity changes very slowly and can only change abruptly in the case of coating defects or installation issues. This allows for a more accurate estimation of the current intensity through data fusion of data from a single run, e.g., using a Kalman filter.

[0032] Preferably, the collected measurement data from multiple runs can be fused to further improve the accuracy of the evaluation.

[0033] According to a further development of the invention, the resulting magnetic field is measured with at least one magnetic field sensor positioned at least substantially at a fixed distance from the wall, in particular wherein the distance of at least one magnet unit of the magnetizing device to the inside of the wall varies. "Positioned at least substantially at a fixed distance from the wall at the time of measurement" means that the sensor can be structurally held at a fixed distance from the wall at the time of measurement, but slight variations may occur due to the movement of the inspection device containing the magnetic field sensor through the pipeline, for example, due to uneven movement caused by contamination of the inner surface of the pipeline wall or due to the sensor lifting off due to weld seams.

[0034] For example, such a sensor can be formed by a coil winding along the rolling circumference of a carrier moving along the wall and rolling on it.

[0035] Alternatively or additionally, a ring of sensors can be arranged along the circumference, with each sensor receiving a measurement signal being the one that is closest to the wall.

[0036] The magnetizing device for generating the primary magnetic field can include a magnetic unit formed by at least one electromagnet or by at least one permanent magnet rotatably arranged in the pipeline. The use of an electromagnet allows for greater flexibility in controlling the course of the primary magnetization, but requires a significant amount of electrical energy.

[0037] Preferably, the at least two magnet units rotate longitudinally along the pipeline on a support, which is particularly designed to be rollable, so that an alternating field is generated in a simple manner. The axis of rotation of the support is located at least in the middle or always transversely to the longitudinal axis of the pipeline. For example, the magnetizing device is designed as a type of magnet wheel with the wheel as the support, wherein the magnetic field sensor(s) are additionally arranged on, in, or within the wheel or support. In addition, a stand-alone version of an inspection device designed with such a magnet and sensor wheel may include a support wheel, wherein the magnet wheel is at least three times larger than a support wheel.

[0038] In particular, the magnetization direction of the magnet acting on the wall changes along the circumference of the carrier and / or the rotation angle of the carrier, so that the change in permeability is detected in the spectrum as the carrier rolls.

[0039] Preferably, during a measurement run, an additional current is applied to the inner wall of the pipeline via two contacts spaced apart along the length of the pipeline to account for potential dependencies on changes in the measurement environment. This applies to pipelines without an insulating inner coating. The additional current can be measured and used, along with the signal change, to correct the evaluation.

[0040] The entire current I In a pipeline, for example, the calculation can be done as follows: I = k P + a , hier P = ∑ i c i V i .

[0041] P results from the sum of the components Weof the feature vector V, multiplied by a weighting coefficient ci The coefficients k, a and ci These coefficients can be determined during the initial calibration of the device before the first run. ci will be assumed to be constant for all possible measurement environments, specific to the device. k and a They can be redefined for different measurement environments and stored in a database.

[0042] Due to the typically slow change in the magnitude of the cathode protection current, after an initial measurement section without additional current, a further measurement section with additional current applied, for example, by the inspection device, can be taken. Assuming that the cathode protection current has not changed, the change Δ I The current can be represented as follows: Δ I = k Δ P , hier Δ P = ∑ i c i Δ V i .

[0043] The coefficient k can be recalculated from the equation. The corrected k-value allows for an accurate calculation of the change in current value. The parameter a can be retrieved from the database with calibrated k - and a -values ​​are determined.

[0044] Calibration can be performed, in particular, if the amplitude and / or shape of the measured magnetic field has changed significantly due to altered environmental conditions, i.e., by at least 5% of the amplitude of the sinusoidal waveform of the entire measured magnetic field. During calibration, the measurement can continue, but the applied current must be subtracted from the measured current value.

[0045] The problem initially posed is also solved by an inspection device for examining the cathodic protection of a particularly ferromagnetic pipeline, wherein the inspection device is designed to be pipe-compatible and in particular medium-driven, and comprises a magnetizing device for generating an alternating magnetic field and a measuring device having at least one magnetic field sensor for measuring a magnetic field formed on the inside of the pipeline wall, and wherein the magnetizing device has at least one carrier that can be rolled through the pipeline in its longitudinal direction during operation, which is provided in a section extending transversely to an axis of rotation with a preferably at least substantially circular circumference and has at least two magnet units along the circumference for generating an alternating magnetic field.whose magnetic field directions at least partially and, in particular, exactly oppose each other. Such an inspection device is designed to generate the measurement data that are analyzed in the computer device for determining the direct current of the cathode protection in the method described above and / or below according to the invention.

[0046] As the carrier rolls along the inside of a pipe wall, it generates a strong alternating magnetic field at the point of contact. When using only one magnet unit with exactly one magnetic field generator, it should be a controllable magnet unit whose magnetic field direction changes with each revolution as it approaches the inside of the pipe. When using at least two magnet units, they can be permanent magnets with magnetic field directions that are angled relative to each other and, in particular, opposite to each other. The more frequently the magnetization directions change, the higher the main frequency of the induced magnetic field. Specifically, the magnet unit of the magnetizing device comprises four, six, or eight permanent magnets with opposite magnetic field directions.The main frequency of the primary magnetic field is derived from the circumference of the rollable carrier and the number of magnet units, together with the speed resulting from the rolling of the carrier on the inside of the (wall of the) pipeline.

[0047] For the purposes of the present invention, an inspection device is designed to be medium-driven if, during the measurement run, it can obtain at least part of the energy required for forward movement in the pipeline via the medium moving in the pipeline.

[0048] At the same distance along a pipe wall, a higher primary frequency allows for a more accurate measurement of the direct current. For example, a signal with 10 periods is generated by a wheel with four magnets whose magnetic field directions change along its circumference every 5 revolutions. The permeability changes twice as fast, as described above. However, too frequent a change in the magnetization direction can lead to a weakening of the magnetic field due to the interaction of the magnetic poles or to a greater distortion of the magnetic field due to eddy currents induced in the pipe. Therefore, the frequency of the primary magnetic field should be less than 500 Hz.

[0049] The magnetizing device can preferably be part of an inspection device, including a rolling carrier, that is propelled through a pipeline by means of cups or discs carrying the medium. Such a device can also have several such rolling carriers for simultaneous, parallel measurement. Alternatively, it can be a so-called "smart wheel," i.e., an inspection device that can be rolled along the pipeline, as described below.

[0050] Suitable magnetic field sensors include conventional sensors for measuring magnetic fields, particularly coils held at a fixed distance from the inside of the pipe wall. The rolling carrier rolls along its circumference, which is at least substantially circular, and in particular completely circular, along which the carrier's at least two magnetic units are arranged. A circumference is considered circular if it is suitable for the continuous rolling of the carrier in the longitudinal direction of the pipe. For example, a slightly elliptical wheel shape can also result in rolling.

[0051] Preferably, the magnetizing device comprises focusing elements made of magnetizable material, which, when permanent magnets are aligned parallel to the axis of rotation of the support, focus the magnetic field towards the pipeline in order to create a magnetic connection with it. For example, the magnets are bounded at least on one side by a frame comprising a magnetizable material and, in particular when viewed in the direction of the axis of rotation, preferably annular or annular segment-shaped and, in particular, magnetizable, which especially forms the circumference. At least one focusing element can be used for each pole of a permanent magnet.

[0052] Preferably, the magnetic field directions of the at least one magnet unit are designed parallel or radial to the axis of rotation of the support in order to obtain a magnetic field that is as little closed as possible in the circumferential direction of the support, or in order to obtain the best possible introduction of the magnetic flux into the pipeline by means of the focusing elements.

[0053] Advantageously, an inspection device according to the invention is provided along the circumference of the support with at least one magnetic field sensor, which is arranged on or in the support. This sensor can be fixedly arranged on the support with respect to its distance from the wall, for example in a guide or holder of the support, so that part of the measuring device is arranged in a holder designed for a wall-proximity arrangement. A wall-proximity arrangement is an arrangement within a range that enables the acquisition of the desired signals, in particular an arrangement within a distance of 10 cm from the inside of the pipe wall.

[0054] In particular, the magnetizing device and the magnetic field sensor are arranged so that they can be moved relative to each other, allowing the same sensor to be used for each measurement. This is advantageous for evaluating and calibrating the inspection device. Specifically, the carrier is rotatably mounted on the holder, enabling a "smart wheel" configuration. For this purpose, the carrier can be additionally equipped with a support element in the form of a roller wheel, the diameter of which is at most 50% of the carrier's diameter. This creates a stable inspection device that can be driven forward by the medium through a pipeline.

[0055] Alternatively or additionally, according to the invention, a plurality of magnetic field sensors can be arranged along the circumference, which are moved with the carrier, wherein the approach to the pipe wall can be determined in particular via the measurement signals of the individual sensors and thus a measurement signal can be determined and stored at the moment of greatest approach to the wall.

[0056] In particular, the magnetic field sensor (or another sensor) is designed as a coil with one or more circumferential windings. Specifically, the coil windings are fully circumferential, providing complete coverage of a circle. By positioning the sensor equidistant with respect to the axis of rotation of the carrier on a carrier that is at least substantially circular along its outer circumference, measurements can be taken throughout the entire unwinding process of the carrier. Alternatively, two coils can be arranged side-by-side or one above the other with respect to the axis of rotation. Similarly, several coils can successively cover different angular ranges of the carrier in the circumferential direction, thus providing complete or partial coverage of 360° around the axis of rotation of the carrier.

[0057] According to a further advantageous embodiment of the invention, several magnetic field sensors are provided, which are in particular arranged spatially separated from one another and measure simultaneously, which allows for a better separation of the primary and secondary magnetic fields, since these are distributed differently in space.

[0058] In particular, for a design in the form of an inspection device that can be driven without cups or discs, this inspection device can have at least one propulsion element extending away from the axis of rotation. For example, in a "Smart Wheel" design, laterally projecting struts can act, at least temporarily, like a sail in the medium, enabling propulsion. These sails or struts can be arranged on the holder or on the rotating support and secure the inspection device against tipping over completely during operation.

[0059] Preferably, an inspection device according to the invention comprises one or more weights radially spaced from the axis of rotation and / or one or more magnets, in particular radially spaced from the axis of rotation, as stabilizing means, wherein a respective magnetic field of the magnet(s) interacts with the pipe wall in the operating case to generate the desired primary alternating field.

[0060] In particular, the magnets are heavier than areas of the inspection device offset radially from the axis of rotation. While weight alone increases the generated angular momentum and thus promotes adherence to the direction of rotation, one or more magnets around or along the circumference of the inspection device, in addition to potentially increasing angular momentum, at least increase the adhesion between the wall and the device, thus greatly facilitating slip-free operation. As the inspection device rolls, the magnetizable material of the pipeline is magnetized and demagnetized again when the magnet is removed during rolling. This creates eddy currents that counteract an increase in the device's running speed and thus significantly improve its running characteristics.The magnets that generate the primary alternating field thus promote a more stable operation of the device, both due to gravity and through magnetic interaction.

[0061] In particular, the support has a spherical or ellipsoidal envelope. An ellipsoidal shape is one that is either exactly an ellipsoid or one that deviates from an ellipsoid by a few centimeters, i.e., less than 10 cm, preferably less than 5 cm. Furthermore, preferably, in the ellipsoidal envelope, the lengths of two semi-axes, which are of equal length, are greater than the length of the third semi-axis lying in the axis of rotation.

[0062] The circumference of the carrier intended for rolling is preferably formed by at least one damping element, preferably ring-shaped and / or elastic, made at least partially of plastic, and / or by at least one rolling element, which ensures that the inspection device designed as a "Smart Wheel" runs as smoothly as possible.

[0063] Furthermore, the carrier containing magnetic field sensors can have a variable average density through fillable cavities of the inspection device and / or weight elements integrable into the carrier and / or through an interchangeable support structure, so that the inspection device can be used in different media.

[0064] Further advantages and details of the invention can be found in the following description of the figures. The schematic representation shows: Fig. 1 the result of a numerical simulation of the magnetic field strength induced by direct current, Fig. 2 a configuration of permanent magnets of an experimental setup, Fig. 3 the magnetic flux passing through the setup according to Figure 2 Fig. 4 shows a section of a secondary magnetic field during setup after the process is generated. Figure 2 Fig. 5 shows a structure of a magnetizing device of an object according to the invention, Fig. 6 shows a simplified representation of the course of the magnetic permeability µ as well as the primary and secondary magnetic fields (Hp and Hs), Fig. 7 shows the amplitude of the magnetic field measured on the inside of the wall of the pipe as well as a spectrum of the measured signal, Fig. 8 shows a Furier spectrum of the difference between magnetic fields, measured with and without direct current, Fig. 9 shows an object according to the invention, and Fig. 10 shows a detailed view of the object according to the invention. Figure 9, Fig. 11 a detailed view of another object according to the invention, Fig. 12 a detailed view of another object according to the invention, Fig. 13 a detailed view of another object according to the invention, Fig. 14 another object according to the invention, Fig. 15 another object according to the invention.

[0065] The features of the embodiments of the invention explained below can also be the subject of the invention individually or in combinations other than those shown or described, but always at least in combination with the features of one of the independent claims. Where appropriate, functionally equivalent parts are provided with identical reference numerals.

[0066] To verify the method according to the invention, a numerical simulation was first carried out in which a direct current was simulated in a wall 1 of a pipeline. Arrows 2 serve to indicate the magnetic field strength and its direction. The larger an arrow 2 is, the greater the magnetic field strength at the corresponding position. The magnetic field inside the pipeline is zero.

[0067] For further verification, the magnetic permeability of the pipe wall 1 was measured using a permanent magnet setup according to Figure 2The arrangement of three permanent magnets 3 leads to the formation of a strong magnetic flux through the wall 1 of the pipe. This causes a significant local change in the magnetic permeability in the wall 1 of the pipe. This leads to the penetration of the secondary magnetic field generated by the direct current in the wall 1 of the pipe, as illustrated by the corresponding arrows 2 on the inside of the wall 1 of the pipe.

[0068] For the method according to the invention, a device according to the invention with a magnetizing unit comprising a permanent magnet 4 was used. In the left part of the Figure 5 The permanent magnets are shown in an associated carrier 5, which can roll longitudinally along its circumference through the pipeline. The magnetic field direction of successive permanent magnets in the circumferential direction is opposite, so that the Figure 5a recognizable sequence of north and south poles in the circumferential direction results. The support 5, in which the magnets 4 are arranged, can be made of a non-magnetic material and rotates about an axis of rotation 38. For the purpose of aligning the magnetic fields of each permanent magnet 4, focusing elements 6 are arranged on both sides of the north and south poles of a permanent magnet, directing the magnetic field flux towards the inner surface of the wall 1 of the pipeline. These focusing elements are annular segment-shaped focusing elements 6 made of a magnetizable material. They are attached to the support 5 on one side and rest on a flange 7 of the support on the inside. The radially outwardly directed surfaces 8 of the focusing elements 6 project slightly beyond the outer circumference of the support 5, so that its outermost surface does not come into contact with the wall of the pipeline.The pipeline does not come into contact with it, but only rolls along its circumference.

[0069] The rolling of the magnetizing device in the form of a magnet wheel results in a pattern within the pipe wall, following the path of the wheel. Figure 6 The depicted course of the primary magnetic field Hp(t) shows that, due to the permeability µ's dependence solely on the magnitude of the primary magnetic field Hp, the signal frequency is twice that of the primary magnetic field. The corresponding time course µ'(t) is as shown in Figure 6 The local change in permeability, provided that the external conditions remain constant and, for example, no defects are present, results in the course of the secondary magnetic field H s measurable on the inside of the pipeline.

[0070] A magnetizing device according to Figure 5The magnetic field measured on the inner surface of the pipe wall causes, according to the left part of the Figure 7 , whereby no direct current for cathodic protection was applied in the pipe wall. A spectral analysis then shows the peak of the main frequency in the right part of the at N=1. Figure 7 N is a multiple of the skin frequency fm. The smaller peaks at N=2 and N=3 result from measurement inaccuracies, as this is not an ideal system.

[0071] If a direct current is now applied to the wall of the pipe, the magnetic field resulting on the inner surface of the pipe wall is measured again, and then the difference between the field with and without direct current is calculated in the spectrum, the following results: Figure 8The Fourier spectrum shown is clearly visible. The peaks at N=2, N=4, N=6, and N=8 are clearly recognizable; these result from the secondary magnetic field, which extends into the interior of the tube due to local changes in permeability. With the exception of the zero frequency, the maximum value has a frequency corresponding to the frequency of the change in direction of the magnetic field. In the case of the magnet wheel shown in the figure, the magnetic direction changes four times per revolution. While the spectrum of the primary magnetic field is determined by integer multiples of fm = 2*v / (π*D) (where D is the diameter of the magnet wheel and v is the velocity of the center of the magnet wheel), the spectrum of the secondary magnetic field is represented by frequencies that are multiples of 2*fm. The spectrum becomes more accurate the longer the signal extracted for the calculation. Preferably, signal lengths of 6 to 12 principal periods are used for evaluation.

[0072] The primary and secondary magnetic fields are distributed differently in space. To better separate the primary and secondary magnetic fields, multiple magnetic field sensors can be installed at different locations. The recorded signals can be used to construct a combined feature vector. V = F 1 2 f m F 1 f m , F 1 4 f m F 1 f m , F 1 6 f m F 1 f m , F 1 8 f m F 1 f m , … , F n 2 f m F n f m , F n 4 f m F n f m , F n 6 f m F n f m , F n 8 f m F n f m , where F n The spectrum corresponds to the signal recorded by the nth magnetic field sensor. The dimension of the vector V can be reduced before forming the regression function, e.g., using principal component methods.

[0073] For this purpose, several measuring devices with corresponding magnetizing devices 10 and magnetic field sensors can be mounted on an inspection device, particularly according to the exemplary embodiment of the Figure 9The inspection device, designed as an inspection pig, includes magnetizing devices 10, which are held on the inspection device by respective holders 12. Each holder 12 is pressed against the inner side or surface of the wall 1 during operation by means of a force storage element 14 in the form of a spring, which is arranged at one end on the holder 12 and mounted at the other end on the central body of the inspection device. The holder 12 itself is pivotally mounted on the central body 16 of the inspection device. Due to the magnetic interaction of the individual magnet units with the wall 1 of the pipeline, the magnetizing devices 10 roll precisely along it, while the inspection device is driven forward by the medium in the pipeline.For this purpose, the inspection device has sealing elements in the form of discs 18 that essentially completely cover the inner, free cross-section of the pipe. The sealing elements also center the inspection device and partially clean the pipe wall 1. A current is introduced into the pipe wall 1 via metal brushes 20 to improve the calibration of the results.

[0074] There are numerous different ways to arrange magnetic field sensors. According to the exemplary embodiment of the Figure 9 A multitude of point-like magnetic field sensors 22 are arranged along the circumference of the carrier 5. Alternatively, the magnetic field sensors 22 are fixedly arranged on the holder 12, so that the magnetic field sensors 22 remain at a fixed distance from the inner top surface of the wall 1 ( Fig. 11 Combinations of these arrangements are also possible.

[0075] A measuring device, which is preferably part of the inspection device, generally comprises electronics necessary for reading and, if necessary, controlling the sensors, as well as associated storage and power supply means.

[0076] Alternatively or additionally, a magnetic field sensor can be designed as a coil wound along the entire circumference of the carrier 5, indicated in Figure 12 through windings 24.

[0077] It is also within the scope of the invention to combine a magnetic field sensor 22 fixedly arranged on the holder 12 with a magnetic field sensor having windings 24 of a coil.

[0078] In particular, when measuring the magnetic field using a coil along the entire circumference of the support 5, this coil always has the same geometric configuration with respect to the contact point with the inner wall. This makes it possible to construct the magnetizing device with the magnetic field sensor relatively simply.

[0079] Another embodiment according to the invention is according to Fig. 14The inspection device is provided with a support element 32 in the form of a roller wheel, which is rotatably mounted on the holder 12 serving as a fastening device. The carrier 5, indicated only by dashed lines, is rotatably held in a hollow cylindrical part 34 of the holder 12 in the preceding embodiment via sliding and / or roller bearings and is partially covered by the holder. The axes of rotation of the roller wheel and the carrier 5 are parallel and both transverse to the longitudinal and travel direction of the pipeline. The roller wheel is the guiding element of the inspection device, the carrier 5 of which has drive elements 34 extending spirally away from a rotation axis 38. In a view in the direction of the rotation axis 38, the carrier 5 is provided with a circular circumference formed by recesses 36. The focusing elements described above can be incorporated into the recesses 36.Data and energy storage means are preferably arranged in carrier 5.

[0080] Another device according to the invention according to Fig. 15 is similar to the variant according to Figure 14 with laterally projecting drive elements 34, but without an additional support or roller wheel. Accordingly, the carrier 5 is also not mounted in a holder 12. The carrier 5 preferably has a setup of magnetic field sensors 12 according to the arrangement shown in the exemplary embodiments of the Figure 10 or 12 In this variant as well, the north-south axes of the permanent magnets arranged in the carrier are arranged parallel to the axis of rotation 38.

Claims

1. A method for examining the cathodic protection of a ferromagnetic pipeline, by creating a primary alternating magnetic field and thereby a local change in the permeability in a wall (1) of the pipeline by means of a magnetizing device (10) of an inspection device moved through the pipeline, with a secondary DC magnetic field caused by a DC current of the cathodic protection being formed in the wall (1) of the pipeline, using a measuring device moved through the pipeline to measure a resultant magnetic field that emerges from the superposition of the primary alternating field and the secondary DC magnetic field, using a computing device to analyze the signal components of at least the secondary magnetic field giving consideration to the change in the permeability, and deriving the magnitude of the DC current on the basis of the signal components of the secondary magnetic field and preferably on the basis of a database with calibration data.

2. The method as claimed in claim 1, characterized in that the spectrum of the secondary magnetic field is determined in the computing device.

3. The method as claimed in claim 2, characterized in that at least one signal component which is an even multiple of the frequency of the directional change of the magnetic field is selected in the analysis for determining the DC current.

4. The method as claimed in any of the preceding claims, characterized in that the current intensity is determined by means of one or more regression functions in the computing device on the basis of the amplitude of at least one even multiple of the frequency of the directional change of the magnetic field, preferably with analysis data of the secondary magnetic field being normalized and / or calibrated by means of analysis data of the primary magnetic field.

5. The method as claimed in any of the preceding claims, characterized in that the measurement conditions are determined in the computing apparatus on the basis of a spectrum of the primary alternating field.

6. The method as claimed in any of the preceding claims, characterized in that the voltage of the cathodic protection is varied and at least increased for a measurement run of the inspection device.

7. The method as claimed in any one of the preceding claims, characterized in that a multiplicity of data from a measurement run are fused in order to determine the current intensity.

8. The method as claimed in any of the preceding claims, characterized in that the measurement is implemented by means of at least one magnetic field sensor (22) which is positioned at least substantially at a fixed distance from the wall (1) at the measurement time, in particular with the distance of at least one magnet unit of the magnetizing device from the inner side of the wall (1) varying.

9. The method as claimed in claim 8, characterized in that the at least one magnet unit revolves on a carrier (5) in the longitudinal direction of the pipeline.

10. The method as claimed in any of the preceding claims, characterized in that, 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.

11. An inspection device for examining the cathodic protection of a pipeline, in particular of a ferromagnetic pipeline, the inspection device being formed to be able to pass through the pipeline and in particular be driven by the medium, and comprising a magnetizing device (10) serving to create an alternating magnetic field and a measuring device comprising at least one magnetic field sensor (22) and serving to measure a magnetic field formed on the inner side of the wall (1) of the pipeline, characterized in that the magnetizing device (10) comprises at least one carrier (5) which is able to be rolled through the pipeline in the longitudinal direction thereof in an operational state, the said carrier being provided with a preferably at least substantially circular perimeter in a section running transversely to an axis of rotation (38) and, along the perimeter, comprising at least two magnet units for creating an alternating magnetic field, the magnetic field directions of which run at least partially against one another and in particular exactly against one another.

12. The inspection device as claimed in claim 11, characterized in that the magnetic field directions of the at least one magnet unit are formed parallel or radially to the axis of rotation (38) of the carrier (5).

13. The inspection device as claimed in claim 11 or 12, characterized in that, along the perimeter of the carrier (5), at least one magnetic field sensor (22) is arranged on or in the carrier (5).

14. The inspection device as claimed in claim 13, characterized in that the magnetic field sensor (22) is in the form of a coil with windings (24) running in the circumferential direction.

15. The inspection device as claimed in claim 13, characterized in that a multiplicity of magnetic field sensors (22) are arranged next to one another along the perimeter.

16. The inspection device as claimed in any of claims 11 to 15, characterized in that a part of the measuring device is arranged in a holder (12) designed for arrangement close to the wall.

17. The inspection device as claimed in claim 16, characterized in that the carrier (5) is rotatably mounted on the holder (12).

18. The inspection device as claimed in any of claims 11 to 17, characterized in that magnetizing device (10) and magnetic field sensor (22) are movable relative to one another.

19. The inspection device as claimed in any of claims 11 to 18 with the inclusion of claim 16, characterized in that the carrier (5) is provided via the holder (12) with a supporting element in the form of a roller wheel, the diameter of which is no more than 50% of the diameter of the carrier.

20. The inspection device as claimed in any of claims 11 to 19, characterized in that the inspection device comprises at least one disk (18) or cup for propulsion purposes.

21. The inspection device as claimed in any of claims 11 to 190, characterized in that the inspection device comprises at least one propulsion element (34) which extends away from the axis of rotation (38) in particular.