METHOD AND DEVICE FOR INSPECTING A SECTION OF PIPELINE

Description

[0001] The present invention relates to the inspection of pipelines, and more particularly to the control of the integrity of pipelines buried in the ground. BACKGROUND OF THE INVENTION

[0002] Made up of sections of pipe welded together, pipelines are widely used to transport fluids (liquid, liquefied or gaseous) over long distances.

[0003] While their burial allows for high flow rates to be ensured discreetly, it is also problematic when maintenance is required, particularly to deal with the various attacks to which these transport structures are subjected: aggressive environment, damage by third parties or by ground movements, corrosive transported fluid, etc.

[0004] To prevent leak risks and ensure public and environmental safety, pipeline operators must regularly check the condition of their network. To this end, tools have been developed to monitor the condition of pipelines.

[0005] It is particularly known to carry out so-called "internal" checks using a robot introduced into the pipeline. This robot, also called a pig or PIG (from the English " Pipeline Inspection Gauge » ) , is driven by the pressure of the fluid flowing inside the pipeline and is equipped with sensors connected to a recording unit configured to detect corrosion points and deformations of the pipeline. However, this type of control is expensive and difficult to deploy. What's more, the robot is not suitable for all types of pipeline, particularly those with restricted diameters, constrictions, bent sections, etc.

[0006] It is also known to carry out so-called "external" inspections using a trolley moving above the pipeline and on which a plurality of magnetic sensors are mounted. The sensors are configured to record a signal representative of the magnetic field induced by the Earth's magnetic field in the section of pipeline being inspected, said section being made of ferromagnetic material (for example steel). While this type of inspection makes it possible to geolocate pipelines relatively precisely, it generally does not offer sufficient precision to quantify points of corrosion or deformation of the pipeline. Indeed, if at the level of such damage the component of the magnetic field induced by the Earth's magnetic field is close to that of said Earth's magnetic field, the induced magnetic field substantially overlaps the Earth's magnetic field, which can make corrosion or deformation difficult to detect.Furthermore, only a transverse component of the magnetic field induced by the Earth's magnetic field is usually detected, so the transverse direction perpendicular to this component represents a blind spot. Furthermore, magnetic remanence of the pipeline can distort monitoring and prevent detection of pipeline damage.

[0007] Document WO 2019 / 083923 A1 describes a system for detecting a ferromagnetic material in a structure, the detection system comprising: a plurality of directional magnetic field sensors, a plurality of magnetic field sources or generators, and a hardware processing system in electrical communication with the plurality of magnetic sensors and sources.

[0008] The hardware processing system is programmed with one or more software routines executing on the hardware processing system to: 1) activate each of the plurality of magnetic field sources; 2) measure the intensity of the directional magnetic field by each of the magnetic sensors; 3) deactivate each of the plurality of magnetic field sources; 4) measure the magnetic remanence of the inspected structure; 5) take the difference between the first and second measurements; and 6) use the difference to determine the amount and location of the ferromagnetic material in or within the structure. SUBJECT OF THE INVENTION

[0009] The invention therefore aims to propose a method and a device for inspecting a section of pipeline making it possible to at least partially overcome the aforementioned drawbacks. SUMMARY OF THE INVENTION

[0010] To this end, the invention proposes according to independent claim 1 a method for inspecting a section of pipeline made of ferromagnetic material and extending below a ground surface along a longitudinal axis. The method comprising the following steps: a) recording, using magnetometers arranged above the ground surface and the pipeline section, first signals representative of a first magnetic field combining the Earth's magnetic field and a magnetic field induced by the Earth's magnetic field in the pipeline section, the magnetometers being aligned along an axis substantially orthogonal to the longitudinal axis of the pipeline section and substantially parallel to the ground surface; b) generating, using a first coil traversed by a first current having at least a first non-zero value and a second non-zero value different from the first value, a first additional magnetic field having a polarization direction different from that of the magnetic field induced by the Earth's magnetic field in the pipeline section;c) recording, using the magnetometers, second signals representative of a second magnetic field combining the first magnetic field, the first additional magnetic field and a magnetic field induced by said first additional magnetic field in the pipeline section; d) determining, from the second signals and without using the first signals, third signals representative of the combination of the additional magnetic field and the magnetic field induced by said additional magnetic field in the pipeline section; e) comparing the first signals with first predetermined thresholds; f) comparing the third signals with second predetermined thresholds; g) deducing from the comparisons made in steps e) and f) the existence of at least one defect in the pipeline section. ;

[0011] Recording the first signals allows passive probing of the pipeline section, and therefore the detection of the possible presence of a defect in the pipeline section. However, as we saw in the introduction, such a survey may include blind spots. The generation of an additional magnetic field allows active probing of the pipeline section under conditions different from those of the passive survey, which allows the detection of a defect that would not have been detected during the passive survey.

[0012] In particular, the direction of polarization of the first additional magnetic field is substantially perpendicular to the longitudinal axis of the pipeline section.

[0013] Preferably, the direction of polarization of the first additional magnetic field is substantially perpendicular to that of the magnetic field induced by the Earth's magnetic field in the pipeline section.

[0014] According to a particular embodiment of the invention, the first current flowing through the first coil is alternating.

[0015] In particular, the first current flowing through the first coil is rectangular or sinusoidal in shape.

[0016] In particular, the method further comprises the following steps: h) deactivating the first coil and generating, using a second coil traversed by a second current having at least a first non-zero value and a second non-zero value different from the first value, a second additional magnetic field having a polarization direction different from that of the first additional magnetic field and that of the magnetic field induced by the Earth's magnetic field in the pipeline section; i) recording, using the magnetometers, fourth signals representative of a third magnetic field combining the first magnetic field, the second additional magnetic field and a magnetic field induced by said second additional magnetic field in the pipeline section;j) determining, from the fourth signals and without using the first signals, fifth signals representative of the combination of the second additional magnetic field and the magnetic field induced by said second additional magnetic field in the pipeline section; k) comparing the fifth signals with third predetermined thresholds; l) deducing from the comparisons made in steps e), f) and k) the existence of the defect in the pipeline section.

[0017] In particular, the direction of polarization of the second additional magnetic field is substantially perpendicular to the longitudinal axis of the pipeline section.

[0018] Preferably, the direction of polarization of the second additional magnetic field is substantially perpendicular to the direction of polarization of the first additional magnetic field.

[0019] The invention also relates according to independent claim 9 to a device for implementing such a method. The device comprises a chassis on which the magnetometers and the first coil are mounted, the magnetometers being connected to a recording and processing unit arranged to carry out steps a), c), d), e), f) and g), an alternating current generator connected to the first coil to generate the first current, and means for orienting a winding axis of the first coil.

[0020] The invention also relates according to independent claim 10 to a device for implementing such a method in which the second coil is mounted on the chassis, the recording and processing unit is arranged to further carry out steps i), j), k) and 1), the alternating current generator is further connected to the second coil to generate the second current, and the device further comprising means for orienting a winding axis of the second coil.

[0021] According to a particular characteristic, the magnetometers are triaxial fluxgate magnetometers. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention will be better understood in light of the following description, which is purely illustrative and not limiting, and must be read in conjunction with the appended drawings, among which: [ Fig. 1A ] there figure 1A is a schematic view of a device for inspecting a section of pipeline according to a first embodiment of the invention, in a first operating state; [ Fig. 1B ] there figure 1B is a view identical to that of the figure 1A , in which the inspection device is in a second operating state; [ Fig. 2A ] there figure 2A is a front view of the coil of the inspection device shown in figure 1A ; [ Fig. 2B ] there figure 2B is a side view of the coil shown in figure 2A ; [ Fig. 3 ] there figure 3 is a schematic view illustrating the implementation of the inspection method by the inspection device illustrated in figures 1A et 1B ; [ Fig. 4A ] there figure 4A is a schematic view of a device for inspecting a section of pipeline according to a second embodiment of the invention, in a first operating state; [ Fig. 4B ] there figure 4B is a view identical to that of the figure 4A , in which the inspection device is in a second operating state; [ Fig. 5A ] there figure 5A is a schematic view of a variant of the inspection device illustrated in figure 5A , in a first operating state; [ Fig. 5B ] there figure 5B is a view identical to that of the figure 5A , in which the inspection device according to this variant is in a second operating state. DETAILED DESCRIPTION OF THE INVENTION

[0023] In reference to the figures 1A et 1B , a section of pipeline T of a pipeline is buried in the ground at a depth p,here substantially between 1 and 2 meters relative to a flat surface S of the ground. This pipeline is intended for the transport of a pressurized fluid such as pressurized gas, oil, etc. The pipeline section T is cylindrical in shape and extends along a longitudinal Y axis substantially parallel to the surface S of the ground. It will be understood that only a cross-section of the pipeline section T, along a plane XZ perpendicular to the Y axis, is visible on the figures 1A et 1B .

[0024] The pipeline section T is made of ferromagnetic material (for example steel) so that the Earth's magnetic field induces, in said pipeline section T, a magnetic field Ft having a polarization direction Pt substantially perpendicular to the Y axis of the pipeline section T.

[0025] Furthermore, during its manufacture and transport to its burial site, the section of pipeline T was lifted and manipulated using powerful electromagnets, which can locally induce a long-lasting remanent magnetic field (not shown) in said section of pipeline T.

[0026] It is also known to apply an electric current in the pipeline section T to lower its electrochemical potential below a given value to limit the corrosion of said pipeline. If necessary, a magnetic field (not shown) is also induced in the pipeline section T by this cathodic protection of the pipeline.

[0027] To inspect the pipeline, it is provided according to the method of the invention to arrange, on the surface S ground of the ground above the pipeline, a carriage configured to move along the pipeline in a direction substantially parallel to the axis Y so as to inspect successive sections of pipeline T. For this purpose, the carriage comprises a chassis which is for example carried by wheels and on which is mounted a network of magnetometers M 1 , M 2 , M 3 , M 4 aligned along an axis substantially perpendicular to the direction of advancement of the carriage and parallel to the surface S ground of the ground.

[0028] The network of magnetometers M 1 , M 2 , M 3 , M 4 comprises a central magnetometer M 1 , two first lateral magnetometers M 2 , two second lateral magnetometers M 3 and two third lateral magnetometers M 4 . The lateral magnetometers M 2 , M 3 , M 4 are distributed symmetrically on either side of the central magnetometer M 1 . The first lateral magnetometers M 2 are spaced from the central magnetometer M 1 by a distance d 1 . The second lateral magnetometers M 3 are spaced from the first lateral magnetometers M 2 by a distance d 2 greater than the distance d 1 . The third lateral magnetometers M 4 are spaced from the second lateral magnetometers M 3 by a distance d 3 greater than the distance d 2 . Preferably, the distance d 1 is substantially equal to an external radius r of the pipeline section T or to 10% of the depth pof burial of the pipeline section T. The distance d 2 is here substantially equal to twice the distance d 1 , and the distance d 3 is here substantially equal to three times the distance d 1 . More preferably, the third lateral magnetometers M 4 are spaced from the central magnetometer M 1 by a distance substantially equal to twice the depth p burial of the T pipeline section.

[0029] The magnetometers M 1 , M 2 , M 3 , M 4 are here triaxial fluxgate magnetometers which are connected to a recording and processing unit to measure and record the orientation and intensity of a magnetic flux between 0.1 nanotesla and 1 millitesla, with an accuracy of the order of nanotesla. The magnetometers M 1 , M 2 , M 3 , M 4 are here identical.

[0030] The carriage further comprises a coil C having a winding axis W which extends in a plane perpendicular to the direction of advancement of the carriage and in which the magnetometers M 1 , M 2 , M 3 , M 4 extend. The coil C is mounted on the chassis of the carriage via orientation means making it possible to adjust the orientation of the axis W of the coil C in said plane. It will be noted that the magnetometers M 1 , M 2 , M 3 , M 4 extend outside the coil C.

[0031] In reference to the figures 2A et 2B , the coil C comprises a rectangular frame c around which a conductive wire f is wound, here in copper, forming turns N. The axis W of the coil C is perpendicular to the frame c winding of the conductive wire f and passes through the center of said frame.

[0032] For a depth p burial of the section of pipeline T approximately equal to 1.5 meters, the frame cof the coil C has, for example, a length a substantially equal to 3 meters and a width b approximately equal to 1 meter, the length a extending parallel to the direction of travel of the carriage. The conducting wire f of the coil C has a diameter d substantially equal to 1 millimeter and the number of turns N is substantially equal to 100, so that the conductive wire f has a length l approximately equal to 800 meters. According to the following known formula (where ρ Cu is the resistivity of copper), the coil C has a resistance R coil substantially equal to 16 Ohm (Ω): R coil = ρ Cu 4 l πd 2 = 1.6 ∗ 10 − 8 4 ∗ 800 π 10 − 6 = 51.2 π ∗ 10 − 8 + 6 + 2 ≈ 16 Ω

[0033] The coil C is connected to a current generator G carried by the trolley. The current generator G here comprises an inverter connected to a battery delivering a direct voltage substantially equal to 24 volts, so that the coil C is traversed by an alternating current I having an amplitude substantially equal to 1.5 amperes. The coil C then generates a magnetic field F c having a variable intensity and a polarization direction P c extending along the axis W ( figure 1B ).

[0034] In view of the figure 3 , the inspection process for the T pipeline section will now be detailed.

[0035] The carriage is brought above the pipeline section T so that the direction of advance of the carriage is substantially parallel to the Y axis of the pipeline section T and the central magnetometer M 1 and said Y axis define a plane YZ substantially perpendicular to the ground surface S of the ground. The magnetometers M 1 , M 2 , M 3 , M 4 then extend along an axis substantially orthogonal to the Y axis of the pipeline section T and parallel to the ground surface S of the ground.

[0036] The coil C is oriented so that its winding axis W has a direction different from that of the polarization Pt of the magnetic field Ft induced in the pipeline section by the Earth's magnetic field. Preferably, the axis W of the coil C is perpendicular to the Y axis. More preferably, the axis W of the coil C is perpendicular to the Y axis and to the direction of the polarization Pt of the magnetic field Ft induced in the pipeline section T by the Earth's magnetic field.

[0037] Initially, coil C is not connected to current generator G and therefore does not generate the additional magnetic field F c ( figure 1A ). A so-called "passive" survey of the section of pipeline T is then carried out, which consists of recording, using magnetometers M 1 , M 2 , M 3 , M 4 , the first signals S 1 representative of a magnetic field BE combining: the Earth's magnetic field, the magnetic field Ft induced by the Earth's magnetic field in the pipeline section T, the remanent magnetic field in the pipeline section T (if applicable), the magnetic field induced by the current applied in the pipeline section T for its cathodic protection (if applicable).

[0038] When the section of pipeline T does not contain any defects (cracks, corrosion, deformation, etc.), the magnetic field BE has predictable propagation characteristics depending on the physical properties of the material from which said section of pipeline T is made. Each of the first signals S 1 then has, in the XZ plane perpendicular to the Y axis, a component substantially close to a first predetermined threshold depending on the predictable characteristics.

[0039] However, the presence of a defect in the pipeline section T causes an alteration in the propagation of the magnetic field BE in the form of anomalies called "magnetic flux leaks". The propagation characteristics are then different from the expected propagation characteristics and the component of at least one of the first signals S 1 is far from the first associated threshold.

[0040] Thus, by comparing the first signals S 1 with the first thresholds, the recording and processing unit can deduce the presence of a fault in the pipeline section T.

[0041] However, if the fault altering the propagation of the BE magnetic field in the pipeline section T is such that said BE magnetic field is superimposed on the Earth's magnetic field, it cannot be detected from the passive survey.

[0042] In a second step, the coil C is powered by the alternating current generator G and then generates the additional magnetic field F c ( figure 1B ). The magnetic field F c induces, in the section of pipe T, a magnetic field F i having a direction of polarization D i substantially identical to that of the polarization D c of the magnetic field F c generated by the coil C. The intensity of the magnetic field F i is inversely proportional to the distance between the coil C and said section of pipe T, and is directly proportional to the intensity of the current I passing through the coil C.

[0043] Therefore, the power supply to coil C generates a magnetic field B c combining the magnetic field F c and the magnetic field F i induced by said magnetic field F c in the section of pipe T. The magnetic field B c is thus proportional to the intensity of the current I delivered by the current generator G: B → C = ∂ B → C ∂ I I

[0044] We then carry out a so-called "active" survey of the section of pipeline T which consists of recording, using the magnetometers M 1 , M 2 , M 3 , M 4 , second signals S 2 representative of a magnetic field B combining the magnetic field BE and the magnetic field BC: B → = B → E + B → C = B → E + ∂ B → C ∂ I I

[0045] To detect, from the second signals S 2 (and without the help of the first signals S 1 ), the presence of a fault in the pipeline section T, it is necessary to decouple the magnetic field BC from the magnetic field BE .

[0046] A first method consists of configuring the inverter so that the current I passing through the coil C is rectangular in shape with a zero average value (and therefore with a non-zero low value and a non-zero high value), so that: B → C ± = ∂ B → C ∂ I I ± = ± ∂ B → C ∂ I I

[0047] Such a current I then generates two different states of the magnetic field B, namely: B → + = B → E + B → C + = B → E + ∂ B → C ∂ I I et B → − = B → E + B → C − = B → E − ∂ B → C ∂ I I

[0048] Subtracting these two states, we obtain: ∂ B → C ∂ I = B → + − B → − 2 I and therefore the decoupling of the magnetic field BC (without using the first signals S 1 ).

[0049] We will also note that by adding these two states, we obtain: B → E = B → + + B → − 2 and therefore the decoupling of the BE magnetic field, so that passive probing could be omitted. Nevertheless, it appears that passive probing allows more precise detection of the BE magnetic field and therefore of the defect.

[0050] A second method is to configure the inverter so that the current I flowing through coil C is sinusoidal in shape with a zero average value, so that: I t = I sin t ⇒ B → C t = ∂ B → C ∂ I I sin t

[0051] Preferably, the frequency of the current I is deliberately chosen to be low (for example around 1 Hertz, or even a few Hertz) so that any voltage induced by the temporal variation tcauses a negligible response compared to the magnetic fields Ft, F i induced in the pipeline section T.

[0052] By integrating over a complete cycle the magnetic field B multiplied by sin(t), we obtain: ∫ 0 2 π B → t sin t dt = ∫ 0 2 π B → E sin t + ∂ B → C ∂ I I sin 2 t dt = π ∂ B → C ∂ I I and therefore the decoupling of the magnetic field BC (without using the first signals S 1 ).

[0053] Such a decoupling method allows to avoid higher order harmonic signals and their associated distortion.

[0054] A third method is to configure the inverter so that the current I flowing through coil C is rectangular in shape with: a non-zero average value I avg; and a non-zero low value I min and a non-zero high value I max, with I avg = (I max + I min ) / 2.

[0055] Such a current I then generates two different states of the magnetic field B, namely: B → + = B → E + B → C + = B → E + ∂ B → C ∂ I I max et B → − = B → E + B → C − = B → E + ∂ B → C ∂ I I min

[0056] Subtracting these two states, we obtain: ∂ B → C ∂ I = B → + − B → − I max − I min and therefore the decoupling of the magnetic field BC (without using the first signals S 1 ).

[0057] It should also be noted that the two different states of the magnetic field B can be written in matrix form: B → − B → + = 1 I min 1 I max B → E ∂ B → C ∂ I

[0058] We then obtain: B → E ∂ B → C ∂ I = 1 I max − I min I max − I min − 1 1 B → − B → + ⇒ B → E = I max B → − − I min B → + I max − I min and therefore the decoupling of the BE magnetic field, so that passive probing could be omitted. Nevertheless, it appears that passive probing allows more precise detection of the BE magnetic field and therefore of the defect.

[0059] More generally, the current generator G may not be alternating and may be arranged so that the current I passing through the coil C has a first non-zero value I v1 during an instant t 1 and a second non-zero value I v2 during an instant t 2 , the first value I v1 being different from the second value I v2 .

[0060] Such a current I then generates two different states of the magnetic field B, namely: B → 1 = B → E + B → C 1 = B → E + ∂ B → C ∂ I I v 1 et B → 2 = B → E + B → C 2 = B → E + ∂ B → C ∂ I I v 2

[0061] Subtracting these two states, we obtain: ∂ B → C ∂ I = B → 2 − B → 1 I v 2 − I v 1 and therefore the decoupling of the magnetic field BC (without using the first signals S 1 ).

[0062] The two different states of the magnetic field B can be written in matrix form: B → 1 B → 2 = 1 I v 1 1 I v 2 B → E ∂ B → C ∂ I

[0063] We then obtain: B → E ∂ B → C ∂ I = 1 I v 2 − I v 1 I v 2 − I v 1 − 1 1 B → 1 B → 2 ⇒ B → E = I v 2 B → 1 − I v 1 B → 2 I v 2 − I v 1

[0064] and therefore the decoupling of the BE magnetic field, so that passive probing could be omitted. Nevertheless, it appears that passive probing allows more precise detection of the BE magnetic field and therefore of the defect.

[0065] A fourth method is to configure the inverter so that the current I passing through coil C is sinusoidal with: a non-zero average value I avg; and a non-zero low value I min and a non-zero high value I max with I avg = (I max + I min ) / 2; so that: It=Imax−Imin2sint+Imoy⇒B→Ct=∂B→C∂IImax−Imin2sint+Imoy

[0066] By integrating over a complete cycle the magnetic field B multiplied by sin(t), we obtain: ∫ 0 2 π B → t sin t dt = ∫ 0 2 π B → E + B → C sin t dt = ∫ 0 2 π B → E + ∂ B → C ∂ I I moy sin t dt + ∫ 0 2 π ∂ B → C ∂ I I max − I min 2 sin 2 t dt = ∂ B → C ∂ I I max − I min 2 ∫ 0 2 π sin 2 t dt = π ∂ B → C ∂ I I max − I min 2 and therefore the decoupling of the magnetic field BC (without using the first signals S 1 ).

[0067] Therefore, the recording and processing unit can, from the second signals S 2 (and without the aid of the first signals S 1 ), determine third signals S 3 representative of the combination of the magnetic field F c and the magnetic field F i induced by said magnetic field F c .

[0068] As previously, when the section of pipeline T does not include any defects (cracks, corrosion, deformation, etc.), the magnetic field BC has predictable propagation characteristics depending on the physical properties of the material from which said section of pipeline T is made. Each of the third signals S 3 then has, in a plane defined by the axis W of the coil C and the magnetometers M 1 , M 2 , M 3 , M 4 , a component substantially close to a second predetermined threshold depending on the predictable characteristics.

[0069] However, the presence of a defect in the pipeline section T alters the propagation of the magnetic field BC inside the pipeline section T and causes magnetic flux leaks. The component of at least one of the third signals S 3 is then far from the associated second threshold.

[0070] Thus, by comparing the third signals S 3 with the second thresholds, the recording and processing unit can deduce the presence of a fault in the pipeline section T, in particular when passive probing does not allow this fault to be detected.

[0071] However, if the fault altering the propagation of the magnetic field BC in the pipeline section T is such that said magnetic field BC is superimposed on the Earth's magnetic field, the fault cannot be detected from the active survey (but probably from the passive survey).

[0072] Therefore, active probing allows the detection of a fault that could not have been detected by passive probing.

[0073] The inspection method described above is of course implemented for each section of the pipeline to be inspected by moving the trolley along the pipeline in a direction substantially parallel to the Y axis.

[0074] In order to improve the detection of a defect in the pipeline section T, the inspection method described above can be extended by implementing two additional magnetic fields.

[0075] Thus, according to a second embodiment of the invention illustrated in figures 4A, 4B , the carriage comprises not one but two coils C 1 , C 2 of rectangular shape. The coils C 1 , C 2 have separate winding axes W 1 , W 2 which extend in a plane perpendicular to the direction of advancement of the carriage and in which the magnetometers M 1 , M 2 , M 3 , M 4 extend. It will be noted that the magnetometers M 1 , M 2 , M 3 , M 4 extend outside the coils C 1 , C 2 .

[0076] The coils C 1 , C 2 are mounted on the chassis of the carriage via orientation means making it possible to adjust the orientation of the axes W 1 , W 2 of the coils C 1 , C 2 so that said axes W 1 , W 2 have a direction different from that of the polarization Pt of the magnetic field Ft induced in the pipeline section T by the Earth's magnetic field. Preferably, the axes W 1 , W 2 of the coils are perpendicular to the Y axis. More preferably, the axes W 1 , W 2 of the coils are perpendicular to the Y axis and the axis W 1 of the coil C 1 is perpendicular to the axis W 2 of the coil C 2 . The coils C 1 , C 2 here extend symmetrically on either side of the plane YZ defined by the Y axis and the central magnetometer M 1 .

[0077] The coils C 1 , C 2 are selectively connected to the alternating current generator G to be traversed one after the other by an alternating current I 1 , I 2 having a non-zero minimum value and a non-zero maximum value, and thus generate one after the other an additional magnetic field F c1 , F c2 having a polarization direction P c1 , P c2 here substantially perpendicular to the axis Y of the pipeline section T.

[0078] The magnetic field F c1 induces, in the pipe section T, a magnetic field F i1 having a polarization direction P i1 substantially identical to that of the polarization P c1 of the magnetic field F c1 generated by the coil C 1 . The activation of the coil C 1 thus generates a magnetic field B C1 combining the magnetic field F C1 and the magnetic field F i1 induced by said magnetic field F c1 . As described previously, the magnetic field B C1 can be decoupled from the magnetic field BE .

[0079] Similarly, the magnetic field F c2 induces, in the pipe section T, a magnetic field F i2 having a polarization direction P i2 substantially identical to that of the polarization P c2 of the magnetic field F c2 generated by the coil C 2 . The activation of the coil C 2 thus generates a magnetic field B C2 combining the magnetic field F c2 and the magnetic field F i2 induced by said magnetic field F c2 . As described previously, the magnetic field B C2 can be decoupled from the magnetic field BE .

[0080] The independent decoupling of the magnetic fields B C1, B C2 makes it possible to combine passive probing with two active probings and therefore improve the detection of a possible fault in the pipeline section T.

[0081] It should be noted that, if the intensity of the current I 1 passing through the first coil C 1 is here substantially identical to the intensity of the current I 2 passing through the second coil C 2 , it can also be different.

[0082] According to a variant of the second embodiment of the invention illustrated in figures 5A, 5B , the carriage comprises two coils C 1 ', C 2 ' of rectangular shape. The coils have winding axes W 1 ', W 2 ' substantially perpendicular to the surface S ground of the ground. The coils C 1 ', C 2 ' extend substantially symmetrically on either side of the plane YZ and are here substantially adjacent.

[0083] Note that the magnetometers M 1 , M 2 , M 3 , M 4 extend outside the coils C 1 ', C 2 '.

[0084] The coils C 1 ', C 2 ' are selectively connected to the alternating current generator G to be traversed one after the other by an alternating current I 1 ', I 2 ' having a non-zero minimum value and a non-zero maximum value, and thus generate one after the other a magnetic field F c1 ', F c2 ' having a polarization direction substantially perpendicular to the surface S ground of the ground. The magnetic fields F c1 ', F c2 ' respectively induce, in the pipeline section T, a magnetic field F i1 ', F i2 '. Activation of the coil C 1 ' thus generates a magnetic field B C1 ' combining the magnetic field F c1 ' and the magnetic field induced in the pipeline section T by said magnetic field F c1 '. Similarly, the activation of the coil C 2 ' generates a magnetic field B C2 ' combining the magnetic field F c2 ' and the magnetic field induced in the pipeline section T by said magnetic field F c2 '.

[0085] As described above, the independent decoupling of the magnetic fields B c1 ', B C2 ' makes it possible to combine passive probing with two active probings, and therefore to improve the detection of a possible fault in the pipeline section T.

[0086] To ensure optimal use of the signals delivered by the magnetometers M 1 , M 2 , M 3 , M 4 , it is necessary to know precisely the relative position and orientation of each of the said magnetometers M 1 , M 2 , M 3 , M 4 , but also to carry out their calibration for passive sounding and active sounding(s).

[0087] For passive probing, a first calibration of the magnetometers M 1 , M 2 , M 3 , M 4 is carried out by placing, in the absence of the coil(s) C, C 1 , C 2 , C 1 ', C 2 ', the network of magnetometers M 1 , M 2 , M 3 , M 4 near a standard subjected to the Earth's magnetic field without any other magnetic disturbance. The standard is a high-precision triaxial fluxgate magnetometer. The comparison of the signals delivered by the magnetometers M 1 , M 2 , M 3 , M 4 with those delivered by the standard allows a computing unit to determine, for each magnetometer M 1 , M 2 , M 3 , M 4 , calibration data aimed at correcting vector components and minor misalignments of the signals delivered by the magnetometer considered. The calibration data are stored in the processing unit which uses them to correct the signals delivered by the magnetometers M 1 , M 2 , M 3 , M 4 during passive sounding.

[0088] For the active sounding(s), a second calibration is carried out by placing, in the presence of the coil(s) C, C 1 , C 2 , C 1 ', C 2 ', the network of magnetometers M 1 , M 2 , M 3 , M 4 near the standard subjected to the Earth's magnetic field without any other magnetic disturbance. Initially, while the coil(s) is (are) deactivated, the computing unit compares the signals delivered by the magnetometers M 1 , M 2 , M 3 , M 4 with those delivered by the standard, and determines initial calibration data. Then the coil C or the coils C 1 , C 2 , C 1 ', C 2 ' are activated one after the other by a greatly reduced alternating current.The computing unit then decouples the magnetic field generated by the coil(s) from the Earth's magnetic field, then determines, for each magnetometer M 1 , M 2 , M 3 , M 4 , second calibration data aimed at correcting the vector components of the signals delivered by the magnetometer when each coil C 1 , C 2 , C 1 ', C 2 ' is activated. The first and second calibration data are stored in the processing unit which uses them to correct the signals delivered by the magnetometers M 1 , M 2 , M 3 , M 4 during the active sounding(s).

[0089] Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0090] Although the number of magnetometers here is seven, it can be less or more than seven, but must be more than two.

[0091] The number of coils can be more than two.

[0092] Although the magnetometers here are flux gate magnetometers, other types of magnetometers can be used (magnetoresistive magnetometers, Hall effect magnetometers, etc.).

[0093] Although the frame (on which the magnetometers and coil(s) are mounted) is carried by wheels here, it can be carried by any system that allows the frame to be moved along the pipeline.

[0094] Although the alternating current (I, I 1 , I 1 ' I 2 , I 2 ') flowing through the coil (C, C 1 , C 1 ', C 2 , C 2 ') is here rectangular or sinusoidal, it can be of other shapes (square, triangular, notched...).

[0095] The current (I, I 1 , I 1 ' I 2 , I 2 ') flowing through the coil (C, C 1 , C 1 ', C 2 , C 2 ') may not be alternating but it must then have at least a first non-zero value and a second non-zero value different from the first value (as is the case for an alternating current).

Claims

1. A method for inspecting a section of pipeline (T) made of ferromagnetic material extending below a soil surface (Ssoil) along a longitudinal axis (Y), the method comprising the following steps: • a) record, by means of magnetometers (M1, M2, M3, M4) arranged above the ground surface and the section of pipeline, first signals (S1) representative of a first magnetic field (BE) combining the Earth's magnetic field and a magnetic field (Ft) induced by the Earth's magnetic field in the section of pipeline, the magnetometers being aligned along an axis substantially orthogonal to the longitudinal axis of the section of pipeline, and substantially parallel to the surface of the ground; • b) generate, by means of a first coil (C, C1, C1') carrying a first current (I, I1, I1') having at least a first value (Imin, Iv1) that is non-zero and a second value (Imax, Iv2) that is non-zero and different from the first value, a first additional magnetic field (Fc, Fc1, Fc1') having a direction of polarization (Pc, Pc1, Pc1') different from that (Pt) of the magnetic field (Ft) induced by the Earth's magnetic field in the pipeline section of pipeline; • c) record, using the magnetometers, second signals (S2) representative of a second magnetic field (B) combining the first magnetic field (BE), the first additional magnetic field (Fc, Fc1, Fc1') and a magnetic field (Fi, Fi1, Fi1') induced by said first additional magnetic field in the section of pipeline; • d) determine, from the second signals (S2) and without using the first signals (S1), third signals (S3) representative of the combination of the additional magnetic field and the magnetic field induced by the said additional magnetic field in the section of pipeline; • e) compare the first signals (S1) to first predetermined thresholds; • f) compare third signals to second predetermined thresholds; • g) Infer from the comparisons made in steps e) and f) that there is at least one defect in the section of pipeline.

2. The method according to claim 1, wherein the polarization direction (Pc, Pc1, Pc1') of the first additional magnetic field (Fc, Fc1, Fc1' ) is substantially perpendicular to the longitudinal axis (Y) of the section of pipeline (T).

3. The method according to claim 2, wherein the polarization direction (Pc, Pc1, Pc1') of the first additional magnetic field (Fc, Fc1, Fc1' ) is substantially perpendicular to that of the magnetic field (Ft) induced by the Earth's magnetic field in the section of pipeline (T).

4. The method according to any one of the preceding claims, wherein the first current (I, I1, I1') flowing through the first coil (C, C1, C1') is an alternating current.

5. The method according to claim 4, wherein the first current (I, I1, I1') flowing through the first coil (C, C1, C1') is rectangular or sinusoidal in shape.

6. The method according to any one of the preceding claims, further comprising the following steps: • h) deactivate the first coil (C, C1, C1') and generate, by means of a second coil (C2, C2') carrying a second current (I2, I2') having at least a first non-zero value and a second non-zero value and differing from the first value, a second additional magnetic field (Fc2, Fc2') having a direction of polarization (Pc2), Pc2') different from that (Pc, Pc1, Pc1') of the first additional magnetic field (Fc, Fc1, Fc1') and that (Pt) of the magnetic field (Ft) induced by the Earth's magnetic field in the section of pipeline; • i) record, using magnetometers (M1, M2, M3, M4), fourth signals representative of a third magnetic field combining the first magnetic field (BE), the second additional magnetic field (Fc2, Fc2') and a magnetic field (Fi2, Fi2') induced by said second additional magnetic field in the section of pipeline; • j) determine, from the fourth signals and without using the first signals (S1), fifth signals representative of the combination of the second additional magnetic field (Fc2, Fc2') and the magnetic field (Fi2, Fi2') induced by said second additional magnetic field in the section of pipeline; • k) compare the fifth signals to third predetermined thresholds; • 1) Infer from the comparisons made in steps e), f) and k) the existence of the defect in the section of pipeline.

7. The method according to claim 6, wherein the polarization direction (Pc2, Pc2') of the second additional magnetic field (Fc2, Fc2') is substantially perpendicular to the longitudinal axis (Y) of the section of pipeline (T) .

8. The method according to claim 7, wherein the polarization direction (Pc2) of the second additional magnetic field (Fc2) is substantially perpendicular to the polarization direction (Pc1) of the first additional magnetic field (Fc1).

9. An apparatus for carrying out the method according to any one of claims 1 to 5, comprising a chassis on which the magnetometers (M1, M2, M3, M4) and the first coil (C, C1) are mounted, the magnetometers being connected to a recording and processing unit arranged to perform steps a), c), d), e), f) and g), an alternating current generator (G) connected to the first coil (C, C1) to generate the first current (I, I1), and means of orienting a winding axis (W, W1, W1') of the first coil.

10. The apparatus according to claim 9 for carrying out the method according to any one of claims 6 to 8, wherein the second coil (C2, C2') is mounted on the chassis, the recording and processing unit is further arranged to perform steps i), j), k) and l), and the alternating current generator (G) is further connected to the second coil for generating the second current (I2), the apparatus also comprising means of orienting a winding axis (W2, W2') of the second coil.

11. The apparatus according to claim 9 or 10, wherein the magnetometers are triaxial fluxgate magnetometers.