Method and inspection device for examining the cathodic corrosion protection of metal pipelines

Abstract

The invention relates to a method for examining the cathodic corrosion protection of a metal pipeline with the aid of an inspection device that is capable of passing through pipelines and is in particular of medium-driven configuration, the inspection device having a magnetising device having at least one magnetic pole positioned adjacent to a wall of the pipeline, a measuring device with at least two capacitor plates arranged on both sides of the at least one magnetic pole as viewed in a direction of advance of the inspection device in the pipeline and with a voltage measuring device for determining a potential difference between the two capacitor plates, together with a storage means for recording the measurement results. The method comprises generating a magnetic field in the wall of the pipeline using the magnetising device, which magnetic field is oriented at least in part transversely to a cathode protection current flowing in the pipeline; measuring a potential difference between the capacitor plates using the voltage measuring device; and recording the measurement result using the storage means.

Description

[0001] Method and inspection device for investigating the cathodic corrosion protection of metallic pipelines

[0002] The present invention relates to a method and an inspection device for investigating the cathodic corrosion protection of a metallic pipeline.

[0003] Metallic pipelines are often laid underground or underwater. In these environments, pipelines are exposed to an increased risk of corrosion. To counter this risk, pipelines are regularly passively protected against corrosion. Passive corrosion protection is achieved by coating the pipeline with an external protective layer. Such a protective layer can be a coating of paint. In the case of pipelines for oil or gas transport, the pipes are typically encased in a plastic sheathing, which also acts as passive corrosion protection.

[0004] Additionally, pipelines can be equipped with active corrosion protection. The pipeline can be electrically connected at regular intervals to a so-called sacrificial anode made of metal. The pipeline and the sacrificial anode are further electrically connected via the surrounding environment, which typically contains an electrolyte. This creates a closed circuit between the sacrificial anode and the pipeline. The sacrificial anode has a lower electrochemical potential than the pipeline material. Therefore, the sacrificial anode rusts or corrodes instead of the pipeline.

[0005] For pipelines that are permanently exposed to moisture and / or extend over long distances, as is typically the case with pipelines in the oil and gas industry, cathodic corrosion protection using impressed current can be employed instead of sacrificial anodes. In this system, an inert anode is electrically connected to the pipeline. A direct current source placed in this connection applies a protective voltage to the pipeline. This voltage is designed to prevent corrosion processes on the pipeline. The current intensity of the protective current depends on the surface to be protected and can be on the order of 50 A. Inert electrodes are used, which can be made of graphite or mixed metal oxides (MMOs) such as titanium oxide or silicon oxide. The circuit is completed via the surrounding environment between the pipeline and the anode, which typically contains an electrolyte.

[0006] The protective current generates cathodic polarization of the pipeline and prevents metal ions from being released from the pipe surface. If this so-called cathodic protection fails, corrosion can develop very rapidly at defects in the passive corrosion protection within the pipeline walls. The effectiveness of the cathodic corrosion protection is therefore typically checked at regular intervals, usually every few months. These inspections also help to detect and repair defects in the protective layer of the passive corrosion protection in a timely manner. The cathodic corrosion protection is generally checked at several critical measuring points by measuring the system-soil potential. The corresponding devices are permanently installed at these measuring points. Additionally, the potential field can be measured at the surface of the pipeline and in the pipeline's surroundings.Potential measurements for offshore pipelines are carried out using remotely operated underwater vehicles. Such potential measurements are generally very expensive and involve a great deal of manual effort.

[0007] Alternatively or additionally, cathodic corrosion protection can be determined by measuring the cathodic protection current flowing in the pipe wall using a pipe-walkable inspection device, also known as a pig. This approach allows measurements along the entire pipeline and represents a cost-effective and reliable method for assessing the efficiency of cathodic corrosion protection and for detecting defects in the protective layer of a passive corrosion protection system.

[0008] Methods and pipe-mounted inspection devices are known from US 7,104,147 B2 and US 7,821,247 B2, in which the cathodic protection current is determined by measuring a potential difference along an axial extension of the pipeline. For this purpose, the inspection device has at least two contacts spaced apart along the longitudinal extension of the pipeline and brought into contact with the pipeline wall. The potential difference between the two contact points is determined using a voltage measuring device. The potential difference, or voltage difference, between the two axially spaced contact points results from the ohmic resistance of the pipeline material.Since the distance between the contact points is limited by practical constraints on the length of the inspection device, and since the walls of pipelines have a comparatively large cross-sectional area, the resistance of the pipeline in the respective section under investigation is low. The potential difference measured by such an inspection device is in the microvolt range. Reliable measurement requires reliable contact between the inspection device and the pipeline wall at both contact points. Furthermore, such an inspection device cannot be used in pipelines with an electrically insulating coating on the inside.

[0009] Alternatively, the cathodic protection current can be determined based on the magnetic field generated by the flowing charges. However, this is only easily possible outside the pipeline. No magnetic field exists inside the pipeline.

[0010] From EP 4334703 A1 and WO 2024 100059 A1, inspection devices and methods for investigating cathodic corrosion protection are known, in which an inspection device is used that locally imposes a primary magnetic field, designed as an alternating magnetic field, on the wall of the pipeline. In this local area, a secondary magnetic field generated by the cathodic protection current emerges from the wall of the pipeline into the interior of the pipeline, where it can be measured by a magnetic field sensor of the inspection device.

[0011] The present invention is based on the objective of providing an alternative method and an alternative inspection device for examining the cathodic corrosion protection of a pipeline, enabling a robust and preferably non-contact examination of the cathodic corrosion protection. The method according to the invention provides for the use of a pipeline-compatible and, in particular, medium-driven inspection device. The inspection device comprises at least one magnetizing device having at least one magnetic pole positioned adjacent to a wall of the pipeline. The at least one other magnetic pole, which is always necessary, can be arranged at a distance from the wall of the pipeline. Positioned adjacent to the wall of the pipeline means that the magnetic pole is preferably in direct contact with the wall of the metallic pipeline.When the magnetic pole comes into contact with the pipeline, a magnetic field with particularly high field strengths is generated. In practice, however, the magnetic pole is typically positioned at a distance from the pipeline wall, especially to prevent damage or wear to the magnetic pole during the process. The distance between the magnetic pole and the pipeline wall should be kept as small as possible. Typically, this distance is on the order of a few millimeters. The inspection device also includes a measuring device with at least two capacitor plates arranged on both sides of at least one magnetic pole, viewed along a feed direction of the inspection device in the pipeline. Furthermore, the measuring device includes a voltage measuring device for determining a potential difference between the two capacitor plates.Furthermore, the inspection device includes a storage device for recording the measurement results. To carry out the procedure, a magnetic field is generated in the pipe wall via the magnetizing device, which is oriented at least partially perpendicular to any cathodic protection current flowing in the pipe. Additionally, a potential difference between the capacitor plates is measured using the voltage measuring device, and the measurement result is recorded in the storage device.

[0012] The magnetic field introduced into the pipe wall by the magnetizing device causes a Lorentz force to be applied to the electric charges of the cathodic protection current flowing in the pipe. This deflects the electric charges in the region of the magnetic field oriented perpendicular to the cathodic protection current flowing in the pipe. As a result, a zone of charge accumulation and a zone of charge depletion form on the inner surface of the pipe wall in the immediate vicinity of at least one magnetic pole. The capacitor plates are positioned in the region of these zones, spaced as close as possible to the inner surface of the pipe wall. Each capacitor plate forms a capacitor with the inner surface of the pipe wall.The gap between the capacitor plate and the inside of the pipe wall is filled with a dielectric.

[0013] Typically, the dielectric is the medium flowing in the pipeline.

[0014] One of the capacitor plates is positioned opposite the zone of charge accumulation, and the other capacitor plate opposite the zone of charge depletion. A potential difference forms between the two capacitor plates, which is measured using the voltage measuring device. The magnitude of the potential difference is directly proportional to the strength of the magnetic field generated by at least one magnetic pole in the pipe wall, perpendicular to the cathodic protection current flowing in the pipe, and to the current of the cathodic protection current. If the field strength of the magnetic field in the pipe wall is known, the current of the cathodic protection current can be directly determined from the voltage difference. Otherwise, at least qualitative changes in the current due to the voltage difference between the capacitor plates can be determined.The measured potential difference depends on the strength of the magnetic field generated in the pipe wall. The distance of at least one magnetic pole from the inside of the pipe wall must be dimensioned such that the magnetic field generated in the pipe wall is sufficiently strong.

[0015] Furthermore, the potential difference depends on the capacitance of the capacitors formed by the capacitor plates. This, in turn, depends on the area of ​​the capacitor plates, the distance of the capacitor plates from the inside of the pipe wall, and the dielectric material located in the gap between the capacitor plate and the pipe wall.

[0016] Pipelines are typically constructed using ferromagnetic steel. In ferromagnetic steels, the magnetic flux density induced by a magnetic field is particularly high. Therefore, a particularly strong Lorentz force acts on a cathodic protection current flowing in the pipe wall of a ferromagnetic material. For such pipelines, with a suitable selection of the field strength of the magnetic field introduced into the pipe wall—which depends, among other things, on the distance of the magnetic pole from the pipe wall, as well as the size of the capacitor plates and their distance from the inside of the pipe—potential differences in the millivolt range can be measured between the capacitor plates at typical cathodic protection current levels.The potential difference is therefore several orders of magnitude greater than in known methods that measure the potential difference directly between two points spaced apart in the longitudinal direction of the pipeline. The method according to the invention thus enables a particularly robust determination of the cathodic protection current.

[0017] Operators of such pipelines are particularly interested in changes in the cathodic protection current over time. Such changes indicate damage to the passive corrosion protection. Depending on the results of the procedure, a pipeline operator can initiate measures to inspect and, if necessary, repair or renew the passive corrosion protection.

[0018] The inspection device may include a storage medium and an evaluation unit for assessing the measurement results. In this case, the results of the evaluation determined by the evaluation unit represent the measurement results recorded in the storage unit. The inspection device may therefore include appropriate electronic data processing means with a processing unit, an input, an output, and optionally, a further storage medium. The inspection device may also include an electrical energy storage device, such as a battery or accumulator, from which the electrical energy required for operating the inspection device can be supplied. Preferably, the method includes a step for determining the current of the cathodic protection current based on the measured potential difference.The current strength of the cathodic protection current is the parameter of interest to a pipeline operator. This determined current strength is then stored as a measurement or evaluation result in the storage medium.

[0019] Preferably, the magnetizing device comprises two magnetic yokes, each with two magnetic poles. The magnetic yokes are oriented such that, viewed in the feed direction of the inspection device, the two magnetic poles of each yoke are arranged side by side and adjacent to the pipe wall in the circumferential direction of the pipeline. One of the capacitor plates is arranged between the two magnetic poles of each magnetic yoke. The magnetic fields generated by the magnetic yokes in the pipe wall have oppositely directed magnetic field lines. The capacitor plates arranged between the magnetic poles of each magnetic yoke are positioned on both sides of the respective magnetic poles when viewed in the feed direction.

[0020] The use of two magnetic yokes, each with two magnetic poles, makes it possible to generate a magnetic field in the pipe wall that is oriented circumferentially in the area between the two magnetic poles. The magnetic field lines between the two magnetic poles of the two magnetic yokes run essentially perpendicular to the cathodic protection current flowing in the pipe wall. The cathodic protection current flowing in the pipe wall is uniformly distributed around the circumference of the pipe. The use of such magnetic yokes makes it easy to generate a magnetic field in the pipe wall whose magnetic field lines are at least partially perpendicular to the cathodic protection current flowing in the pipe. The cathodic protection current flowing in these areas experiences a deflection in the region of one of the magnetic yokes towards the inner surface of the pipe wall.In the area of ​​the other magnetic yoke, the cathodic protection current is deflected away from the surface of the pipeline. A zone of charge accumulation thus forms between the magnetic poles of one magnetic yoke. A zone of charge depletion forms between the magnetic poles of the other magnetic yoke.

[0021] The inner surface of the pipe wall represents an interface for these zones of charge depletion or charge accumulation, across which charge equalization cannot occur. By using appropriate magnetic yokes, the potential difference measured by the capacitor plates, which are arranged between the magnetic poles of the yokes, is significantly greater under otherwise identical conditions than the potential difference when using only a single magnetic pole located near the wall. This method enables particularly clear measurement results and is therefore especially robust. Alternatively or additionally, the magnetizing device has at least one magnetic yoke with three magnetic poles arranged side by side and adjacent to the wall in the circumferential direction of the pipe, viewed in the feed direction. One of the magnetic poles is a central magnetic pole.The magnetic poles arranged laterally and circumferentially from the central magnetic pole are the outer magnetic poles. Between the central magnetic pole and the outer magnetic poles, magnetic fields are generated with magnetic field lines running in the wall in the opposite direction to the feed direction. One of the capacitor plates is positioned between each of the outer magnetic poles and the central magnetic pole. Such a magnetic yoke with three magnetic poles is functionally equivalent to two magnetic yokes with two poles each, with two of the magnetic poles of the same polarity being placed directly adjacent to each other. A magnetic yoke with three magnetic poles allows for a more compact inspection device compared to using two magnetic yokes with two poles each.

[0022] Preferably, the inspection device comprises at least four capacitor plates. Two of the capacitor plates are arranged side-by-side in the circumferential direction, as viewed in the feed direction, forming a capacitor plate pair. The voltage measuring device is configured to measure the potential difference between the two capacitor plates of a capacitor plate pair. The at least two capacitor plate pairs are spaced apart from each other in the pipeline in the feed direction of the inspection device. The potential differences between the capacitor plates of each capacitor plate pair are measured, and the largest measured potential difference is used.

[0023] In a pipeline with cathodic corrosion protection, several or a multitude of cathodic corrosion protection devices are arranged along the pipeline's longitudinal axis. The pipeline wall is thus connected to an anode at several or a multitude of points, each of which is supplied with direct current, particularly via a direct current source. The direct current propagates from the point where the electrode is located on the pipeline in both directions along the pipeline's longitudinal axis. As an inspection device is moved through the pipeline, the direction of the cathodic protection current in the pipeline wall can change when the inspection device passes a feed point—a point where the pipeline is connected to a direct current source and anode.

[0024] The magnetic field generated in the pipe wall causes the cathodic protection current to deflect due to the Lorentz force. This deflection is greater at the end of the magnetic pole furthest from the current flow. As the inspection device moves through the pipe, this furthest end can alternate between the area upstream of the magnetic pole and the area downstream of it. Using two pairs of capacitor plates spaced apart in the feed direction ensures that one of the pairs is always located in the section with a particularly high charge accumulation or depletion, and thus a particularly large potential difference between the plates of that pair.The device measures the potential differences between the two pairs of capacitor plates spaced apart in the feed direction. The larger potential difference is then used. This results in a particularly large potential difference. The method is especially robust.

[0025] Preferably, the magnetic field is generated as an alternating field. An alternating magnetic field can be easily generated by using at least one magnetic pole as a pole of an electromagnet excited by alternating current. The magnetic pole can be one end of a magnetic core around which an excitation coil is wound. In an alternating magnetic field, the direction of the magnetic field lines generated in the pipe wall changes periodically. Consequently, the zones of charge depletion and charge accumulation also alternate periodically. The periodically changing potentials on the inner surface of the pipe wall can be capacitively coupled into the capacitor plates located adjacent to the pipe wall. The potential difference can thus be determined with high reliability via the capacitor plates.The process is particularly robust.

[0026] The frequency of the alternating field is particularly preferably between 20 Hz and 5 kHz, especially between 100 Hz and 1 kHz.

[0027] Magnetic yokes for generating an alternating field are preferably constructed from stacks of transformer laminations that are narrow relative to their overall dimensions. These laminations typically have a thickness between 0.3 mm and 0.5 mm. The individual laminations within a stack are electrically insulated from one another. By using magnetic yokes with correspondingly thin and insulated transformer laminations, the occurrence of eddy currents can be suppressed and eddy current losses reduced.

[0028] Preferably, the potential difference between the capacitor plates is measured during the generation of the magnetic field.

[0029] Preferably, the impedance of the magnetizing device, in particular the impedance of an excitation coil of the magnetizing device, through which the alternating magnetic field is generated, is determined. For this purpose, the voltage and the current with which a corresponding excitation coil of the magnetizing device is excited are measured. Based on the impedance thus determined, and optionally after appropriate calibration by carrying out comparative measurements, the distance of the magnetic pole from the pipe wall can be determined. The distance of the magnetic pole from the pipe wall influences the strength of the magnetic field in the pipe wall. A greater distance of the magnetic pole from the pipe wall results in a weaker magnetic field in the pipe wall.With the same cathodic protection current in the pipe wall, a larger distance of the magnetic pole results in a smaller potential difference between the charge accumulation zone and the charge depletion zone. This could lead to a falsely determined decrease in the cathodic protection current. A change in the distance of the magnetic pole from the pipe wall is also known as lift-off. Such lift-off can occur particularly in the area of ​​welds in a pipeline. The measured potential difference can be corrected for this effect by measuring the impedance.

[0030] The effect is further amplified if the capacitor plates and the magnetic pole are arranged in fixed positions relative to each other, for example, in a common measuring head of the inspection device. In this case, an increase in the distance between the magnetic pole and the pipe wall also leads to an increase in the distance between the capacitor plates and the pipe wall. The capacitance of the capacitors formed by the capacitor plates and the pipe wall decreases. By determining the distance between the magnetic pole and the pipe wall through an appropriate impedance measurement, the distance of the capacitor plates from the pipe wall can also be determined directly or indirectly, and, in particular, the measured potential difference can be corrected for this effect.

[0031] The impedance of the magnetizing device, in particular the excitation coil, can be determined while the potential difference is being measured. Alternatively, it can also be determined before or after the potential difference measurement.

[0032] It is particularly preferred that the coil is subjected to an alternating voltage and the voltage and the resulting current are measured.

[0033] Furthermore, it is particularly preferred that the coil be excited at different frequencies for determining the impedance than for measuring the potential difference.

[0034] In particular, the frequency for determining the impedance is 100 kHz. The frequency can be between 10 and 500 kHz, preferably in the range of 200 kHz to 300 kHz. Determining the lift-off is particularly feasible in this frequency range.

[0035] It is particularly preferred that the impedance be determined at least twice at different frequencies. The impedance depends not only on the lift-off but also on the magnetic flux in the pipe material. By determining the impedance at two different frequencies, these two phenomena can be distinguished. Determining the impedance then allows not only the determination of the lift-off but also the determination of the magnetic flux in the pipe material. Since the Lorentz forces acting on the cathodic protection current, and thus the measured potential difference, depend not only on the current but also on the magnetic flux density in the pipe wall, a more precise determination of the magnetic flux density enables a more precise determination of the current from the measured potential difference.

[0036] The impedance can be determined using the same impedance measuring device, operated with different parameters.

[0037] Particularly preferred is the determination of the capacitor plate impedance. Especially when the capacitor plates are fixed relative to the magnetic pole and their distance from the pipe wall changes in the same proportion as the distance of the magnetic pole from the pipe wall, measuring the capacitor plate impedance can be used to detect changes in the dielectric between the capacitor plates and the pipe wall and to correct the potential measurement for such effects. Changes due to lift-off, including of the capacitor plates, are corrected by determining the impedance of the excitation coil. To determine the capacitor plate impedance, the capacitor plates are actively driven with an alternating voltage, and the resulting current is measured. The impedance is then calculated from these measurements.This step can be performed before or after measuring the potential difference due to the magnetic field introduced into the pipe wall. It can also be performed during the measurement of the potential difference due to the magnetic field introduced into the pipe wall if the frequency of the alternating voltage applied to the capacitor plates differs sufficiently from the magnetization frequency or the frequency of the alternating magnetic field. The dielectric between the pipe wall and the capacitor plates is typically the medium within the pipe. The dielectric changes, particularly when deposits are present on the pipe wall. The measured potential difference can be corrected for such changes in the dielectric.

[0038] In particular, the impedance of the capacitor plates is determined using a further impedance measuring device, which is different from the impedance measuring device used to determine the impedance of the magnetizing device.

[0039] Furthermore, the measured impedances and / or changes in the dielectric are also stored with particular priority. This information can provide indications of deposits on the pipe wall.

[0040] Preferably, the measured potential differences are filtered using a narrowband filter with the magnetization frequency as the center frequency. When measuring with an alternating magnetic field, the potential difference between the capacitor plates is frequency-dependent. Filtering the results with the magnetization frequency as the center frequency allows the isolation of frequency components that are similar to the frequency of the alternating field. This further improves the signal quality and thus the robustness of the method.

[0041] Preferably, the position of the inspection device within the pipeline is determined along the feed direction or along the longitudinal direction of the pipeline. This information is stored along with the measurement results. A pipeline operator is particularly interested in changes in the cathodic protection current over time, especially across multiple, time-spaced runs of the process. Furthermore, the operator is interested in identifying the section along the longitudinal direction of the pipeline where these changes occur. Such changes are typically indicators of damage to a passive corrosion protection layer. The operator can then specifically investigate areas where such damage to the passive corrosion protection is suspected and repair or renew the passive corrosion protection in these areas.

[0042] The problem is further solved by an inspection device for examining the cathodic corrosion protection of a metallic pipeline. The inspection device is designed to be pipe-compatible and, in particular, is medium-driven. A medium-driven inspection device has, in particular, a sealing element such as a sealing disc or a sealing sleeve, against which a fluid flowing in the pipeline can exert a force that propels the inspection device through the pipeline. The sealing element can bear against the pipe wall in sections or along its entire length. Such a sealing element then also serves to position the inspection device in the pipeline relative to the pipe wall. In particular, such a sealing element can create a distance between the inspection device and the pipe wall.In this case, the inspection device preferably has at least two sealing elements spaced apart from each other in the feed direction of the inspection device. The sealing elements can then determine not only the position of the inspection device, but also its orientation in the pipeline relative to the pipeline wall.

[0043] The inspection device comprises a magnetizing device having at least one magnetic pole that can be positioned adjacent to a wall of the pipeline. The magnetizing device is configured such that a magnetic field can be generated in the wall of the pipeline via the magnetic pole.

[0044] The inspection device further comprises a measuring device with at least two capacitor plates arranged on either side of the magnetic pole in the pipeline, viewed in the feed direction of the inspection device, and a voltage measuring device for determining a potential difference between the two capacitor plates. The inspection device also includes a storage device for recording the measurement results. A magnetic field can be generated in the pipeline wall via the magnetizing device. A cathodic protection current flowing in the pipeline wall is deflected by the magnetic field. This results in zones of charge accumulation and charge depletion. The capacitor plates arranged on either side of the magnetic pole react to these zones of charge depletion and charge accumulation, respectively.Charge arrangement is achieved by creating a potential difference between the capacitor plates, which can be determined via the voltage measuring device.

[0045] The inspection device may further include an evaluation unit for assessing the measurement results. This evaluation unit may comprise electronic data processing means. The electronic data processing means have at least one input and one output, as well as at least one computing unit connected to the input and output. The electronic data processing means are preferably connected to the storage medium so that the measurement results and / or the results of an evaluation of the measurements can be stored directly in the storage medium. The inspection device may further include an electrical energy storage device, in particular a battery or accumulator. This electrical energy storage device can provide electrical energy if required to operate the inspection device within a pipeline.The voltage measuring device may include a measuring amplifier to amplify the measured potential difference.

[0046] The inspection device can have a movable measuring head on which at least the magnetic pole and the two capacitor plates are arranged. The distance of the magnetic pole and the capacitor plates from the pipe wall can be easily adjusted using such a movable measuring head, and in particular, the magnetic pole can be positioned close to the wall.

[0047] Preferably, the magnetizing device comprises two magnetic yokes, each with two magnetic poles. The magnetic yokes are oriented such that the two magnetic poles of each yoke are positioned next to each other in the feed direction. The magnetic poles are oriented so that they can be positioned adjacent to a pipe wall. The magnetizing device is configured so that magnetic fields with opposing magnetic field lines can be generated in the pipe wall via the two magnetic poles of each magnetic yoke. The capacitor plates are arranged between the two magnetic poles of each magnetic yoke. When used in a pipe, the magnetic field lines then run at least partially in the circumferential direction of the pipe. They are perpendicular to a cathodic protection current flowing in the pipe wall.Alternatively or additionally, the magnetizing device has a magnetic yoke with three magnetic poles arranged side by side in the feed direction. One of the magnetic poles is a central magnetic pole. The other two magnetic poles are arranged laterally from the central magnetic pole and form outer magnetic poles. Magnetic fields with oppositely directed magnetic field lines can be generated between each of the central and outer magnetic poles. One of the capacitor plates is arranged between each of the outer magnetic poles and the central magnetic pole. As a result, a magnetic yoke with three magnetic poles is equivalent to two magnetic yokes, each with two magnetic poles, arranged directly adjacent to one another.

[0048] Preferably, the width of a magnetic yoke transverse to the feed direction is a maximum of 50 mm. A smaller magnetic yoke allows for a compact magnetizing device. Furthermore, the magnetic poles of the yoke are then arranged closer together. This makes it easier to generate magnetic fields with locally high magnetic field strength. The resulting Lorentz forces and, consequently, the potential difference are then particularly large.

[0049] Preferably, the length of a magnetic pole in the feed direction is one to twice the width of the magnetic pole perpendicular to the feed direction. This applies particularly to the central magnetic pole of a magnetic yoke with three magnetic poles. A magnetic field with a correspondingly long magnetic pole in the feed direction, relative to its width, can be generated, resulting in a correspondingly large extent in the feed direction of the inspection device. This creates correspondingly long zones of charge depletion and charge accumulation in the feed direction. These zones of charge depletion and charge accumulation can be detected particularly well via the capacitor plates, and the potential difference between the capacitor plates can be determined with exceptional reliability. This results in a particularly robust signal.

[0050] Preferably, the magnetizing device is configured to generate an alternating magnetic field. In particular, the inspection device is configured to excite an alternating magnetic field with a frequency of at least 20 Hz and a maximum of 5 kHz, and especially with a frequency of at least 100 Hz and a maximum of 1 kHz. For this purpose, the inspection device specifically includes a frequency generator and a magnetizing output stage. The magnetizing device may include an electromagnet. The electromagnet may have a core that forms at least one magnetic pole or be formed by a magnetic yoke.

[0051] The core of the electromagnet, or magnetic yoke, is preferably formed by a stack of transformer laminations. These are thin laminations compared to their other dimensions. The thickness of the laminations is typically between 0.3 mm and 0.5 mm. The transformer laminations are arranged within the stack with electrical insulation from one another. An excitation coil is wound around the stack of transformer laminations. The use of transformer laminations makes it possible to largely suppress eddy currents within the stack and to reduce eddy current losses.

[0052] Preferably, the voltage measuring device includes a measuring amplifier.

[0053] Preferably, the measuring amplifier is configured as a lock-in amplifier. The potential difference between the two capacitor plates is frequency-dependent when excited by an alternating magnetic field. The lock-in amplifier is configured to receive the excitation frequency of the alternating magnetic field as a reference signal. It represents an extremely narrowband bandpass filter that outputs the result of the potential difference measurement in the frequency range of the alternating magnetic field excitation. This significantly improves the signal-to-noise ratio and thus the robustness of the measurement result.

[0054] Preferably, the inspection device includes an impedance measuring device for determining the impedance of the magnetizing device. The impedance measuring device measures the voltage and current of an excitation coil used to generate the alternating magnetic field. From the impedance thus determined, the distance of the magnetic pole from a wall of the pipeline, the lift-off, can be determined. This determined impedance can be used to correct the measured potential difference for the effect of the lift-off. Particularly preferably, the impedance measuring device is configured to determine the impedance of the capacitor plates. For this purpose, the capacitor plates are actively supplied with an alternating voltage, and the resulting current is measured. From this, the impedance of the capacitor plates is determined.

[0055] Alternatively, two different impedance measuring devices can be provided, wherein a first impedance measuring device determines the impedance of the magnetizing device and a second impedance measuring device determines the impedance of the capacitor plates.

[0056] The impedance of the capacitor plates, determined in this way, can also be corrected after determining the lift-off via the impedance of the magnetizing device. This correction is caused by a change in the dielectric in the area of ​​the capacitor plates. When using the inspection device, the dielectric is typically the medium flowing in a pipeline. Changes in the dielectric can occur if deposits are present on the inside of the pipeline being inspected.

[0057] Preferably, the inspection device has at least one metallic, and in particular ferromagnetic, shielding device between the capacitor plates and the at least one magnetic pole. The shielding device is configured to prevent direct coupling between the magnetic pole and the capacitor plates. Particularly preferably, the shielding device comprises several metallic, and in particular ferromagnetic, shielding elements that are isolated from one another. By using such a shielding device, direct coupling between the magnetizing device and the capacitor plates is at least reduced, and preferably completely prevented.

[0058] This further improves the quality of the potential difference measurement.

[0059] Preferably, the magnetic pole is provided with a wear-resistant pad designed to contact the pipe wall. This wear-resistant pad is preferably made of ceramic. The thickness of the wear-resistant pad is preferably between 0.5 mm and 2 mm, and preferably 1 mm. When used in the pipeline, the magnetic pole is positioned at a distance from the pipe wall equal to the thickness of the wear-resistant pad. The wear-resistant pad allows it to come into contact with the pipe. The magnetic pole then maintains a defined distance from the pipe wall.Even when using a magnetic pole equipped with a wear pad, the measurement itself is contactless, since no electrically conductive contact between the magnetizing device and / or the measuring device and a wall of the pipeline is required, and the measurement can also be carried out if the wear pad is not in contact with a wall of a pipeline to be examined with the inspection device.

[0060] Preferably, the device comprises at least one spacer, wherein the spacer is configured such that the distance between the capacitor plates and the wall of the pipe is between 0.5 mm and 2 mm, and more particularly 1 mm. In particular, the spacer is formed by the magnetic pole or a wear-resistant surface of the magnetic pole. This applies especially when the capacitor plates are arranged in a constant position relative to the magnetic pole in the inspection device.

[0061] Preferably, the inspection device includes a position detection unit for recording the position of the inspection device in the longitudinal direction of a pipeline, also referred to as the axial direction. The position detection unit can, in particular, be designed as an odometer. By using a suitable position detection unit, it is possible to assign a potential difference determined by the inspection device and / or a current intensity of a cathodic protection current determined from the determined potential difference to a specific section in the longitudinal direction of the pipeline. If a change in the potential difference or a measured change in the current intensity of the cathodic protection current gives rise to a suspicion of damage to the passive corrosion protection of the pipeline, the pipeline can be specifically inspected in the corresponding section and the passive corrosion protection can be repaired if necessary.be repaired or renewed.

[0062] Further advantages and details can be found in the following figure description. Figure 1 schematically shows a view of a section of a pipe wall with a magnetic pole attached to it;

[0063] Fig. 2 shows a view of a section of a pipe wall with a magnetic yoke with three magnetic poles attached to it;

[0064] Fig. 3 shows the arrangement according to Fig. 2 in a top view;

[0065] Fig. 4 shows the conditions when the direction of the magnetic field lines changes;

[0066] Fig. 5 shows the conditions when the direction of the cathode protection current changes;

[0067] Fig. 6 shows an inspection device;

[0068] Fig. 7 shows a representation of the wiring of the elements of the inspection device according to Fig. 8;

[0069] Fig. 8 shows a flowchart of the method according to the invention.

[0070] Parts with identical or similar effects are provided with identical reference numerals, where appropriate. Individual technical features of the embodiments described below can be combined with the features of the independent claims as well as with the features of individual embodiments described above to form articles according to the invention.

[0071] Fig. 1 shows a section of a wall 2 of a pipeline 4. A magnetic pole 6 of a magnetizing device 8 is arranged on the wall 2. In the illustrated embodiment, the magnetic pole 6 is a north pole. The corresponding south pole of the magnetizing device 8 is not shown. The magnetizing device 8 generates a magnetic field in the wall 2 of the pipeline 4 via the magnetic pole 6. It can be seen that magnetic field lines 12 of the magnetic field are at least partially oriented perpendicular to the direction of flow of a cathodic protection current 14. In the illustrated embodiment, the direction of flow of the cathodic protection current 14 is oriented into the plane of the drawing. The cathodic protection current 14 is uniformly distributed in the wall 2. A Lorentz force 13 acts on the cathodic protection current 14 due to the magnetic field. This results in a zone of charge accumulation 10 in Fig.1 to the left of magnetic pole 6 and a zone of charge depletion 11 to the right of magnetic pole 6.

[0072] Figure 2 illustrates this for a magnetic yoke 16 with three magnetic poles 6. The magnetic field lines 12 of the magnetic field extend in Figure 2 from the central magnetic pole 18 to each of the outer magnetic poles 20. In the illustrated embodiment, the central magnetic pole 18 is a north pole. The two outer magnetic poles 20 are each south poles. Between the magnetic poles 6, a zone of charge accumulation 10 and a zone of charge depletion 11 are formed. In the illustrated embodiment, the zone of charge accumulation 10 is located to the left and the zone of charge accumulation 11 to the right of the central magnetic pole 18.

[0073] Fig. 3 shows a view of the wall 2 of the pipe 4 rotated by 90 degrees. The cathodic protection current 14 is depicted. It can be seen that the magnitude of the charge change increases in the charge accumulation zone 10 and the charge depletion zone 11 in the direction of flow of the cathodic protection current 14. This is because the cathodic protection current 14 is exposed to the acting Lorentz forces 13 for a longer period in these areas.

[0074] Fig. 4 shows how the conditions change when the magnetization direction 16 is reversed. Such a reversal of the magnetization direction occurs periodically in an alternating magnetic field.

[0075] In the upper part of Fig. 4, the central magnetic pole 18 of the magnetic yoke 16 is configured as the north pole, and the outer magnetic poles 20 are each configured as south poles. The lower part of Fig. 4 shows the situation with the magnetization direction reversed.

[0076] Capacitor plates 22 are arranged between the magnetic poles 6 of the magnetic yoke 16. The potential difference resulting from charge accumulation and depletion is measured across the capacitor plates 22. Fig. 4 shows that it can be advantageous to arrange the capacitor plates 22 as capacitor plate pairs 23, 23', at least two of which are spaced apart from each other in the direction of flow of the cathode protection current 14 or in the longitudinal direction of the pipe 4. In the embodiment shown in Fig. 4, the potential difference between the capacitor plates 22 of the rear capacitor plate pair 23' in the direction of flow of the cathode protection current 14 is greater than the potential difference between the capacitor plates 22 of the front capacitor plate pair 23.

[0077] The lower part of Fig. 4 shows that the charge accumulation zones 10 and charge depletion zones 11 are reversed when the direction of the magnetic field lines 12 of the magnetic fields is reversed. This occurs when the magnetizing device 8 generates an alternating magnetic field. When using a magnetizing device 8 that generates an alternating magnetic field, the potential changes between the charge depletion zones 11 and charge accumulation zones 10 can be capacitively coupled into the capacitor plates 22. The potential differences can thus be determined particularly easily and reliably.

[0078] Fig. 5 illustrates the situation when the direction of the cathodic protection current 14 is reversed instead of the direction of the magnetic field lines 22. The direction of flow of the cathodic protection current 14 along the longitudinal extent of the pipeline 4 depends on where the magnetizing device 8 is positioned relative to the points where the pipeline 4 is connected to a cathodic corrosion protection system. From a point where such a cathodic corrosion protection system is connected to the pipeline 4, the cathodic protection current propagates in both directions along the longitudinal extent of the pipeline 4.

[0079] The upper part of Fig. 5 corresponds to the upper part of Fig. 4. It can be seen that when the direction of the cathode protection current 14 changes, the charge accumulation zones 10 and charge depletion zones 11 are initially reversed in sign. Furthermore, the charge accumulation or depletion is now greatest on the side of the magnet yoke 16 opposite the direction of feed. Compared to the representation in Fig. 4, the potential difference in the lower part of Fig. 5 is now greatest between the capacitor plates 22 of the other capacitor plate pair 23.The use of two pairs of capacitor plates 23, 23', which are spaced apart from each other in the longitudinal direction of the pipe 2 and in the direction of flow of the cathodic protection current 14 respectively, results in a greater potential difference between the capacitor plates 22 of one pair of capacitor plates than between the capacitor plates 22 of the other pair of capacitor plates. The potential difference can thus be determined with particular reliability.

[0080] Fig. 6 shows an inspection device 24, which is designed to be pipe-compatible and medium-driven. It has a central body 25 on which sealing elements are arranged at the front and rear. One of the two

[0081] The sealing element is designed as a sealing disc 26 and the other as a sealing sleeve 27. The sealing element designed as a sealing disc 26 serves in particular to position the inspection device 24 within the pipeline 4 with respect to the distance from the wall 2 of the pipeline.

[0082] The inspection device 24 is equipped with an electrical energy storage device 28, an evaluation unit 30, and a storage medium 32. The inspection device 24 also includes a position detection unit 34. In the illustrated embodiment, the position detection unit 34 has several odometers 36.

[0083] The inspection device 24, designed as a pig, has measuring heads 37, each of which has at least one magnetic pole 6 or a magnetic yoke 16 with two or more magnetic poles 6. The capacitor plates 22 are also located there. The measuring heads 37 are movably arranged so that, during operation of the inspection device 24, the measuring heads 37, and thus the magnetic poles 6 and the capacitor plates 22, can be positioned close to the wall 2 of the pipeline 4.

[0084] Fig. 7 shows a sensor arrangement 38 positioned near the wall 2 of a pipeline 4. The sensor arrangement 38 has a magnetic yoke 16 with three magnetic poles 6. The central magnetic pole 18 is equipped with an excitation coil 39. This allows an alternating magnetic field to be introduced into the wall 2 of the pipeline 4. Capacitor plates 22 are arranged between the magnetic poles 6. The magnetic poles 6 and the capacitor plates 22 are spaced apart from the wall 2. This can be achieved by means of suitable spacers and / or by a wear plate arranged at the ends of the magnetic poles 6 of the magnetic yoke 16. Such a spacer or wear plate is not shown in Fig. 7. The inspection device 24 has a frequency generator 50 and a magnetization output stage 60, which is connected to the excitation coil 39 arranged on the magnetic yoke 16.This stimulates the magnetic yoke 16 to induce an alternating magnetic field.

[0085] The inspection device 24 has a first impedance measuring device 40, which is configured to measure the impedance of the excitation coil 39. For this purpose, the voltage and the current with which the excitation coil 39 is excited are measured.

[0086] The capacitor plates 22 are connected to a voltage measuring device 42. In the illustrated embodiment, the voltage measuring device 42 is designed to measure an alternating voltage. It includes a measuring amplifier. This amplifier is configured to determine the measured alternating voltage in a narrowband manner with the excitation frequency of the alternating field as its center frequency. For this purpose, the frequency generated by the frequency generator 50 is provided to the voltage measuring device 42.

[0087] The inspection device 24 has a second impedance measuring device 44, which is configured to measure the impedance of the capacitor plates 22. For this purpose, the current and voltage are also measured here, and the impedance is determined. The results of the measurements by the voltage measuring device 42, the first impedance measuring device 40, and the second impedance measuring device 44 are transmitted to an evaluation unit 30. In addition, a signal from the position detection unit 34 is transmitted to the evaluation unit 30. This allows the data determined by the voltage measuring device 42 to be assigned to a position of the pipeline 4 in the feed direction or in the longitudinal direction of the pipeline 4. In the evaluation unit 30, the results of the voltage measuring device 42 can be corrected with the measurement results of the impedance measuring devices 40 and 44.The measurement results obtained in this way are stored in storage medium 32 along with the associated position data.

[0088] An exemplary result of such a measurement is shown in the form of a cathodic protection current locus curve. The x-axis represents the position of the inspection device 24 within the pipeline 4 during the respective measurements, and the y-axis represents the current intensity of the cathodic protection current 14 determined from the measurements. The cathodic protection current 14 decreases along the axial extent of the pipeline 4 until a point where a sharp increase in the current intensity 14 is observed. Subsequently, the cathodic protection current 14 decreases again. This increase in the current intensity of the cathodic protection current 14 is due to the fact that a feed point for the cathodic protection current 14 is provided at this point in the pipeline 4. The cathodic protection current locus curve of a single measurement run with the inspection device 24 is not very informative.Only by comparison with cathodic protection current locus curves obtained from earlier or later measurement runs can areas of the pipeline be identified where a greater drop in the cathodic protection current 14 occurs over time. These are the areas where damage to the passive corrosion protection may be present.

[0089] Fig. 8 shows a flowchart of the method according to the invention. For each measuring position in the pipe, in step 100, the wall 3 of the pipe 4 is locally magnetized by the magnetizing device 8. In step 110, the potential difference between the capacitor plates 22 of the capacitor plate pairs 23, 23' is detected by the voltage measuring device 42. When using alternating magnetization, the potential difference is measured as an alternating voltage.

[0090] In the illustrated embodiment, in a parallel step 120, the impedance of the excitation coil 39 of the magnetizing device 8 is determined, in this embodiment by the first impedance measuring device 40. Additionally, in a step 130, the impedance of the capacitor plates 22 is determined, in this embodiment by the second impedance measuring device 44. From the impedances thus determined, the lift-off of the sensor head and the local relative dielectric between wall 2 of the pipe 4 and the capacitor plates 22 are determined in a step 140 of the illustrated embodiment. Using these values, the local current of the cathode protection current 14 is calculated in a step 150 from the alternating voltage measured with the voltage measuring device 42.

[0091] In addition, the position of the inspection device 24 in the pipeline 2 is determined along the longitudinal direction, in the illustrated embodiment in step 160 by the position detection unit 42 using one or more odometers.

[0092] The position data, together with the associated data on the local current of the cathode protection current 14, are stored in step 170 in the storage medium 32 of the inspection device 24. Steps 100 to 170 are performed within the inspection device 24 in the illustrated embodiment, which is indicated by the dashed line.

[0093] The data obtained in this way can be represented in step 180 as a cathode protection current locus curve, as shown by way of example in Fig. 8. The cathode protection current locus curves in Fig. 7 and Fig. 8 are identical.

Claims

Claims 1. Method for investigating the cathodic corrosion protection of a metallic pipeline (4) using a pipeline-compatible and, in particular, medium-driven inspection device (24), which comprises a magnetizing device (8) having at least one magnetic pole (6) positioned adjacent to a wall (2) of the pipeline (4), a measuring device with at least two capacitor plates (22) arranged along a feed direction (V) of the inspection device (24) in the pipeline (4) on both sides of the at least one magnetic pole (6), and with a voltage measuring device (42) for determining a potential difference between the two capacitor plates (22), as well as a storage means (32) for recording the measurement results.wherein the method comprises generating a magnetic field in the wall (2) of the pipeline (4) at least partially oriented transversely to a cathodic protection current (14) flowing in the pipeline by the magnetizing device (8), measuring a potential difference between the capacitor plates (22) with the voltage measuring device (42) and recording the measurement result with the storage medium (32).

2. Method according to claim 1, characterized in that the current strength of the cathodic protection current (14) of the cathodic corrosion protection of the pipeline (4) is determined from the potential difference.

3. Method according to claim 1 or claim 2, characterized in that the magnetizing device (8) has two magnetic yokes (16) each with two magnetic poles (6), wherein the magnetic yokes (16) are each oriented such that the two magnetic poles (6) of each magnetic yoke (16) are arranged next to each other and adjacent to the wall (2) of the pipe (4) when viewed in the feed direction (V), wherein one of the capacitor plates (22) is arranged between the two magnetic poles (6) of a respective magnetic yoke (16) and wherein the magnetic fields generated by the magnetic yokes (16) in the wall (2) have oppositely directed magnetic field lines (12).

4. Method according to one of claims 1 to 3, characterized in that the magnetizing device (8) has at least one magnetic yoke (16) with three magnetic poles (6) arranged side by side and adjacent to the wall (2) in the circumferential direction of the pipeline (4) when viewed in the feed direction (V), wherein one of the magnetic poles (6) is a central magnetic pole (18) and the other two magnetic poles (6) arranged laterally spaced in the circumferential direction from the central magnetic pole (18) are outer magnetic poles (20), wherein magnetic fields with oppositely directed magnetic field lines (12) running in the wall (2) are generated between the central magnetic pole (18) and the outer magnetic poles (20), wherein one of the capacitor plates (22) is arranged between each of the outer magnetic poles (20) and the central magnetic pole (18).

5. Method according to one of the preceding claims, characterized in that the inspection device (24) has at least four capacitor plates (22), wherein two capacitor plates (22) are arranged next to each other in the circumferential direction when viewed in the feed direction (V) and form a capacitor plate pair (23, 23') and wherein the at least two capacitor plate pairs (23, 23') are spaced apart from each other in the feed direction (V), wherein the potential differences between the capacitor plates (22) of the respective capacitor plate pairs (23, 23') are measured and the largest measured potential difference is used as the measurement result.

6. Method according to one of the preceding claims, characterized in that the magnetic field is generated as an alternating field.

7. Method according to claim 6, characterized in that the magnetization frequency of the alternating field is between 20 Hz and 5 kHz, in particular between 100 Hz and 1 kHz.

8. Method according to one of claims 6 or 7, characterized in that the measurement of the potential difference is carried out during the generation of the magnetic field.

9. Method according to one of claims 6 to 8, characterized in that the impedance of the magnetizing device (8) is determined and the result of the measurement of the potential difference is corrected using the impedance.

10. Method according to claim 9, characterized in that the impedance of the capacitor plates (22) is determined and the result of the measurement of the potential difference is corrected using both impedances.

11. Method according to one of claims 6 to 10, characterized in that a narrowband filtering of the measured potential differences is carried out with the frequency of the alternating field as the center frequency.

12. Method according to one of the preceding claims, characterized in that the position of the inspection device (24) within the pipeline (4) is determined along the feed direction (V).

13. Inspection device for investigating the cathodic corrosion protection of a metallic pipeline (4), wherein the inspection device (24) is designed to be pipe-compatible and, in particular, medium-driven, comprising a magnetizing device (8) which has at least one magnetic pole (6) that can be positioned adjacent to a wall (2) of the pipeline (4) and via which a magnetic field can be generated in the wall (2), and a measuring device with at least two sensors each positioned in the feed direction (V) of the inspection device (24) in the pipeline (4). The capacitor plates (22) are arranged on both sides of the magnetic pole (6), a voltage measuring device (42) is used to determine a potential difference between the two capacitor plates (22), and a storage device (32) is used to record the measurement results.

14. Inspection device according to claim 13, characterized in that the magnetizing device (8) has two magnetic yokes (16) each with two magnetic poles (6), wherein the magnetic yokes (16) are each aligned such that the two magnetic poles (6) of each magnetic yoke (16) are arranged next to each other when viewed in the feed direction (V) and can be positioned adjacent to the wall (2) of the pipeline (4), wherein the magnetizing device (8) is arranged such that magnetic fields with oppositely directed magnetic field lines (12) can be generated via the two magnetic poles (6) of the two magnetic yokes (16), wherein one of the capacitor plates (22) is arranged between the two magnetic poles (6) of each magnetic yoke (16).

15. Inspection device according to claim 13 or claim 14, characterized in that the magnetizing device (8) has a magnetic yoke (16) with three magnetic poles (6) arranged side by side in the feed direction (V), wherein one of the magnetic poles (6) is a central magnetic pole (18) and the other two magnetic poles (6) arranged laterally spaced from the central magnetic pole (18) are outer magnetic poles (20), wherein magnetic fields with opposite directions are located between the central magnetic pole (18) and the outer magnetic poles (20). can be generated by the magnetic field lines (12), wherein one of the capacitor plates (22) is arranged between each of the outer magnetic poles (20) and the central magnetic pole (18).

16. Inspection device according to one of claims 13 to 15, characterized in that the measuring device has at least four capacitor plates (22), wherein two capacitor plates (22) form a capacitor plate pair (23, 23'), wherein the capacitor plate pairs (23, 23') are arranged spaced apart from each other in the feed direction (V), wherein the voltage measuring device (42) is configured to determine the potential difference between the two capacitor plates (22) of a capacitor plate pair (23, 23').

17. Inspection device according to one of claims 13 to 16, characterized in that the magnetizing device (8) is configured to generate an alternating magnetic field.

18. Inspection device according to claim 17, characterized in that the voltage measuring device (42) comprises a lock-in amplifier.

19. Inspection device according to claim 18 or claim 19, characterized in that the inspection device (24) has a Impedance measuring device (40) for determining the impedance of the magnetizing device (8) is included.

20. Inspection device according to claim 19, characterized in that the impedance measuring device (40) is configured to determine the impedance of the capacitor plates (22).

21. Inspection device according to one of claims 17 to 20, characterized in that the inspection device (24) has at least one metallic and in particular ferromagnetic shielding device between the capacitor plates (22) and the at least one magnetic pole (6).

22. Inspection device according to one of claims 12 to 20, characterized in that the magnetic pole (6) is provided with a wear pad designed to be in contact with the wall (2) of the pipeline (4).

23. Inspection device according to one of claims 12 to 21, characterized in that the inspection device (24) has at least one spacer which is arranged such that the distance of the condenser plates (22) from the wall of the pipeline (4) is between 0.5 mm and 2 mm, in particular 1 mm.

24. Inspection device according to one of claims 12 to 22, characterized by a position detection unit (34) for detecting the position of the inspection device (24) in a pipeline (4) in the feed direction (V).

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