FR3066996A1 - COLLABORATIVE SYSTEM OF SUBAQUATIC VEHICLES FOR MONITORING IMMERGED LINEAR ELEMENTS AND METHOD IMPLEMENTING SAID SYSTEM - Google Patents

Abstract

The present invention relates to a collaborative system for underwater vehicles (10) adapted to follow an immersed linear element (1,33) capable of varying or producing a magnetic field comprising at least a first underwater vehicle (13, 37, 38, 39 ) for monitoring the position of the linear element (1,33), a second underwater vehicle (11,35) comprising at least a first system able to indicate its position and at least a first measuring means (28) of said magnetic field capable of indicating a first angle (26) measured between the vertical and a first direction (30) of said magnetic field, a third underwater vehicle (12, 36) comprising at least a second system capable of indicating its position and at least one second measuring means (29) of said magnetic field capable of indicating a second angle (27) measured between the vertical and a second direction (31) of said magnetic field, the first underwater vehicle (13, 37, 3 8, 39) being located vertically or behind the second and third underwater vehicles (11, 35, 12, 36), at least one of said underwater vehicles (13, 37, 38, 39, 11, 12) having a means calculating said position of said linear element (1, 33) and the first underwater vehicle (13, 37, 38, 39) being adapted to use the position of said linear element (1, 33) in order to follow it.

Description

Collaborative system of underwater vehicles for tracking submerged linear elements and method implementing this system

The present invention relates generally to a collaborative system of underwater vehicles for tracking linear elements such as pipelines, electric cables, optical fibers or anchor chains, preferably autonomous and not using references positioning on the surface of the water. Another subject of the invention is a method implementing this collaborative system.

The petroleum industry traditionally uses underwater vehicles linked to a ship and remotely piloted in order to inspect the pipes allowing the transportation of oil and gas coming from wells located in the seabed, more commonly called pipelines. These pipelines are generally laid on the seabed. The objective of these underwater vehicles is to detect faults on pipelines such as corrosion, cracks, nicks, openings or their movements or to monitor the state of elements associated with pipelines such as their anodes, their valves. or their stings. It is checked whether the anodes are present or the degree of corrosion thereof. Defects due to corrosion can lead to serious damage to pipelines, such as rupture, and therefore possible damage to the environment and significant financial loss since it is necessary to repair or even repair them. to replace them, resulting in production stoppage. Another industry also implements linear elements placed on the seabed, this is the telecommunications industry. These linear elements are, for example, electrically conductive cables, of copper or aluminum, or optical fibers surrounded by a metallic armor as well as their repeaters supplied by an electric cable, making it possible to transmit information in the form of signals. electric. These electric cables laid on the seabed are also used in other industries, such as the marine wind turbine industry in order to export energy.

It is therefore essential to implement, as a preventive measure, methods of inspecting efficient metallic linear elements in order to minimize these risks. The patent application US 2016/0231281 describes an example of an underwater vehicle allowing the inspection of pipelines. It describes a vehicle, commonly called ROV (Remotely Operated Vehicle), that is to say a vehicle controlled remotely and connected to a ship by a cable. This vehicle is placed directly on the pipeline to be inspected and moves along it by means of rollers 18, as can be seen in FIG. 1 of this patent application. These rollers allow, in particular, to follow the contours of the pipeline when the latter has non-rectilinear areas such as turns or spokes. This type of vehicle requires being in contact with the pipeline in order to be able to follow its contours. The major disadvantage of an ROV is that it must be constantly monitored, which involves human supervision. It is therefore necessary to set up a dedicated vessel and a team if we want to continuously inspect, 24 hours a day, a pipeline. These means represent a considerable cost. In addition, when the weather conditions are bad, in case of a storm for example, it is no longer possible to use the ROV for a period of up to several days. Inspection using such a vehicle is also very time consuming since the speed of the vessel to which it is connected is relatively low, i.e. less than 1 meter per second.

In order to reduce the inspection time, an underwater vehicle is used which is no longer in contact with the metallic linear element to be inspected. This type of vehicle has great flexibility of use because it can be launched from a ship and retrieved by the latter. Such a vehicle is described in patent application WO 2016/178045. The disadvantage of this type of vehicle is that it is not completely autonomous since it follows a predetermined path in order to follow the contours of the pipeline to be inspected, which implies knowing its geometry as well as its location on the seabed. Currently, it is difficult to know in real time the exact position of pipelines because they are in perpetual motion. Indeed, due to the tide phenomenon, sea currents or internal pressure, pipelines tend to move. This requires frequent position reports using, for example, a ship equipped with a multibeam sounder and keeping an up-to-date database which is extremely restrictive and complex. In addition, before each intervention, it is necessary to program the vehicle so that it has in memory the geometry of the pipeline to be checked. It is also possible to control the vehicle remotely, from the surface, using a wireless connection, but this solution has the disadvantage that it is necessary to mobilize a team in charge of piloting the vehicle. In addition, a wireless connection has a relatively long latency. Indeed, if the vehicle is moving at significant depths, for example of the order of 1500 meters, the time for the signal to arrive at the ship and return to the vehicle is of the order of 5 to 10 seconds. In such cases, the vehicle gets lost and it is then necessary for the vehicle to rise to the surface and be reprogrammed with a new trajectory. In addition, certain vehicles are programmed to perform S-shaped routes, perpendicular to their trajectory, in an attempt to find the pipeline. These operations take time and incur an additional cost. In addition, certain areas of the pipeline are not controlled. . These various problems have been partially resolved by the invention described in US Patent 9,511,831. It describes an underwater autonomous inspection vehicle implementing a system of lasers and multibeam sonars intended to guide the vehicle along the pipeline. This vehicle does not require remote control by a team located on a ship or on land. These systems allow the degree of curvature of the pipes to be detected. It is also known the possibility of implementing cameras making it possible to determine the geometry of the pipes, patent application US 2012/0048171 describes such an example. The disadvantage of these vehicles equipped with such systems is that they cannot follow pipes covered by sand or marine concretions. They then become invisible and underwater vehicles lose the line to be inspected. It is then necessary to recover them and to continue the inspection manually, which induces significant costs, as already mentioned previously. In order to solve these problems, it is known from the prior art to use an underwater vehicle equipped with magnetometers. These magnetometers will allow the detection of a metallic linear element which has become invisible due to an overlap, for example by sand, and traversed by an electric current. The magnetometers are installed on either side of the underwater vehicle. The disadvantage of this solution is that the risk of collision between the underwater vehicle and the element to be inspected is significant. When the magnetic field emanating from the element to be inspected is weak, the underwater vehicle must approach it in order to perform the inspection satisfactorily. The risk of collision therefore increases. Furthermore, this magnetometric configuration does not make it possible to precisely determine the position of the linear element, a significant uncertainty always remains. Thus, some parts are not inspected. It is then necessary to relaunch an inspection mission.

The objective of the present invention is therefore to improve the inspection systems for underwater vehicles of linear elements such as pipelines, cables, anchor chains, electrical or telecommunications cables.

To do this, the subject of the invention is in particular a collaborative system of underwater vehicles able to follow a submerged linear element capable of varying or producing a magnetic field comprising at least a first underwater vehicle intended for monitoring the position of the linear element, a second underwater vehicle comprising at least a first system capable of indicating its position and at least a first means of measuring said magnetic field capable of indicating a first angle measured between the vertical and a first direction of said magnetic field, a third vehicle underwater comprising at least a second system capable of indicating its position and at least a second means for measuring said magnetic field capable of indicating a second angle measured between the vertical and a second direction of said magnetic field, the first underwater vehicle being located vertically or behind the second and third v underwater vehicles, at least one of said underwater vehicles comprising means for calculating said position of said linear element and the first underwater vehicle being able to use the position of said linear element in order to follow it.

In addition to the main characteristics mentioned in the preceding paragraph, the collaborative system according to the invention may have one or more complementary characteristics among the following:

Preferably, at least one of the underwater vehicles is autonomous.

Preferably, the calculation means is located on the first underwater vehicle.

Preferably, at least one means for measuring said magnetic field is a magnetometer-gradiometer.

Preferably, the collaborative system also comprises at least one additional underwater vehicle comprising an optical sensor.

The subject of the invention is also a system of a collaboration of underwater vehicles capable of tracking a linear element capable of varying or producing a magnetic field comprising at least a first underwater vehicle intended for monitoring the position of the linear element, a second underwater vehicle comprising at least a first measuring means capable of indicating its position and at least a first measuring means capable of measuring a first amplitude of said magnetic field, a third underwater vehicle comprising at least a second system capable of indicating its position and at at least a second measurement means capable of measuring a second amplitude of said magnetic field, the first underwater vehicle being situated vertically or behind the second and third underwater vehicles, at least one of said underwater vehicles comprising means for calculating said position of the linear element and the first veh underwater icon being able to use the position of said linear element in order to follow it.

The invention also relates to a method for monitoring a linear element, said method comprising the following steps:

1) implementing a collaborative system of underwater vehicles according to any one of claims 1 to 6;

2) calculating the position of said linear element based on said position of the second underwater vehicle, said position of the third underwater vehicle, the measurement of said first angle or of the first amplitude and the measurement of said second angle or of the second amplitude;

3) use the calculated position in order to follow the linear element by means of the at least first underwater vehicle.

The method according to the invention may have one or more additional characteristics among the following:

Preferably, the linear element is a pipeline.

Preferably, at least one of the underwater vehicles includes an induction means inducing an electric current in the linear element. Preferably, the induction means comprises at least one coil.

The invention will be better understood on reading the following description, made with reference to the appended figures, in which:

- Figure 1 is a schematic view in which a collaborative system of underwater vehicles is implemented according to an exemplary embodiment of the invention;

- Figure 2 is a block diagram of the measurement system implemented in an exemplary embodiment of the invention;

- Figure 3 is a schematic view of another example of a collaborative system, according to the invention, in which a plurality of underwater vehicles is implemented.

FIG. 1 shows a collaborative system of underwater vehicles 10 according to one embodiment of the invention. A linear element 1 to be inspected is placed on a seabed 2. This linear element 1 can be, for example, a pipeline making it possible to convey gas or crude oil coming from an oil platform. In this embodiment, the pipeline is made of steel. The invention can be applied to any other material capable of varying a magnetic field or of producing a magnetic field. It can be, for example, ferromagnetic materials or cables crossed by an electric current. A first underwater vehicle 13 is shown schematically in FIG. 1. The first underwater vehicle 13, as well as all the other underwater vehicles used in the various embodiments of the invention, can be autonomous and thus capable of inspect the linear element 1 without assistance from the surface, contrary to what is done for a R.OV. In order to move, the first vehicle 13, as well as all the other underwater vehicles described below, comprise a body in which a propulsion system is integrated (not shown). The propulsion system includes one or more propellers and a motor to supply them with mechanical energy. Alternatively, the propulsion system includes one or more turbines. The engine may be controlled by a computer. The first vehicle 13 may also include an Inertial Measurement Unit (IMU) configured to guide the vehicle 13 to a desired position. The IMU may also include accelerometers, gyroscopes and other motion sensors. The IMU is initially supplied with the current position of the vehicle 13 as well as its speed. This information comes from another source, for example, an operator, a GPS or another IMU located on a ship or on another underwater vehicle. The IMU then calculates its own position and speed based on information from its motion sensors and / or transducers. The first vehicle 13 can also include a compass, an altimeter, for measuring its altitude, or a pressure sensor for measuring its depth. Furthermore, the first vehicle 13 may also include a system for avoiding obstacles, a wireless communication system, for example by Wi-Fi, and a High Frequency modem system, acoustic or optical, in order to determine its positioning relative to to another underwater vehicle. The first vehicle 13 also includes ailerons, transverse, lateral and / or vertical thrusters in order to guide it to a desired position. These fins can be used in combination with the propulsion system. The first vehicle 13 also includes a flotation system so as to control its depth relative to the surface of the water. In addition, the first vehicle 13 may include an antenna and an associated low frequency acoustic system to communicate long distance with a ship. This acoustic system can be an acoustic modem capable of receiving acoustic waves and transforming them into electrical signals and vice versa. Alternatively, or in addition, the acoustic system includes an Ultra-Short Baseline system (USBL) or a Long baseline acoustic positioning system (LBL). These systems implement an acoustic underwater positioning process. A complete USBL system includes a transceiver, which is installed on a ship or other underwater vehicle, and a transponder on the first vehicle 13. The computer is used to calculate a position from the distances measured by the transmitter receiver. For example, an acoustic pulse is transmitted by the transceiver and is detected by the transponder, which itself responds with its own acoustic pulse. This return pulse is detected by the transceiver on the ship or on another underwater vehicle. The time between the transmission of the initial acoustic pulse and the detection of the response is measured by the USBL system and converted into a distance. In order to calculate the position of the first vehicle 13, the USBL system calculates the distance and the angle from the transceiver to the first vehicle 13. The angles are measured by the transceiver which includes a set of transducers. The transceiver includes, for example, at least three transducers at most 30 cm apart. An LBL system uses beacons placed on the seabed with a known position.

The first vehicle 13 can take several forms, for example a submarine shape having a substantially cylindrical or ellipsoidal cross section. Preferably, the body of the first vehicle 13 is made of carbon composite, glass or a material that does not conduct electricity. The first vehicle 13 comprises a buoyancy system which may include two chambers intended to be filled with or emptied of the surrounding water, in order to control the depth of the first vehicle 13. Furthermore, as seen above, the first vehicle 13 comprises a motor intended to rotate the propellers making it possible to produce a thrust. The propellers receive water via a conduit formed in the body of the first vehicle 13. They can also be arranged outside the vehicle 13. The conduit has an opening allowing the entry of water and an opening allowing the expulsion of water. These openings can be located on the front, rear or sides of the body of the first vehicle

13. The body of the first vehicle 13 may also include conduits or turbines in order to control its rotational and / or translation movements. We can also see in Figure 1, a second underwater vehicle 11, preferably autonomous, and a third underwater vehicle 12, preferably also autonomous, called respectively second vehicle 11 and third vehicle 12. The second and third vehicles have the same characteristics than those previously seen for the first vehicle 13. The second vehicle 11 is positioned on one side of the pipeline and the third vehicle 12 is positioned opposite this side so that the pipeline 1 is located between the second vehicle 11 and the third vehicle 12. Furthermore, and preferably, the longitudinal axes of the second and third vehicles 11,12 are substantially parallel to the direction of the pipeline 1 lying between the second and third vehicles 11,12. Each of the second and third vehicles 11, 12 includes at least one means for measuring the magnetic field (not shown), for example a three-axis magnetometer. Each magnetic field measurement means measures the direction of the magnetic field. FIG. 2 is an illustration of the configuration implementing the second vehicle 11 and the third vehicle 12 allowing this measurement. Underwater vehicles are positioned relative to each other using a positioning and acoustic communication network comprising at least one means of relative positioning of high precision, of the order of a few centimeters, with a range of communication of less than 200 meters and a high data transfer rate of around 100 bytes per second, thus allowing high-frequency comparison, that is to say several times per second, of data from the installed sensors on each vehicle, such as depth sensors or means for measuring the magnetic field. In Figure 2, we can see the water surface 20, the seabed 2 and the pipeline 1 in a partially buried position. We also see the second vehicle 11 and the third vehicle 12 positioned on either side of the pipeline 1. The second vehicle 11 is located at a first depth 21 and a first position XI, Yl, ZI relative to the surface of the water 20 and the third vehicle 12 is located at a second depth 22 and a second position X2, Y2, Z2 relative to the surface of the water 20. These depths and positions are determined by means of depth sensors conventionally used in the field, such as light sonars, IMU, GPS or the USBL system described above, but an absolute positioning of the vehicles is not necessary, a relative positioning of the vehicles between them is sufficient. Absolute positioning can be useful when you want, for example, to keep the pipeline position map up to date. The second vehicle 11 comprises a first magnetometer 28 indicating the direction of the pipeline 1 and the third vehicle 12 comprises a second magnetometer 29 indicating the direction of the pipeline 1. Indeed, the pipeline 1 varies the terrestrial magnetic field thus allowing the first and second magnetometers to determine the direction. More specifically, the first magnetometer 28 indicates a first angle 26 measured between the vertical and a first direction 30 and the second magnetometer 29 indicates a second angle 27 measured between the vertical and a second direction 31. Furthermore, the second and third vehicles 11 , 12 are spaced a distance 23. The distance 23 is determined by various positioning means, seen above, which are installed on each underwater vehicle. By performing a trigonometry calculation involving the first angle 26, the second angle 27 and the positions of the second vehicle 11 and the third vehicle 12, it is then possible to precisely determine the position of the pipeline 1. The pipeline is located at the intersection of the first direction 30 and of the second direction 31. This position is transmitted to the vehicle 13, called the measuring vehicle, which adapts its path, in depth and also in a horizontal plane relative to the surface of the water, as a function of this position in order to precisely follow the pipeline to carry out an inspection, for example by camera. The images are stored in the vehicle computer 13 or, if not, sent directly by acoustic waves, WI-FI, RF, GSM or Iridium to a ship so that they can be analyzed in real time. In another embodiment of the invention, it is possible to replace the directional magnetometers 28,29 described previously by scalar magnetometers 28 ', 29' mounted as a gradiometer. In this embodiment, the second vehicle 11 and the third vehicle 12 are located at the same depth. The second vehicle 11 comprises at least one scalar magnetometer 28 'measuring a first amplitude of the magnetic field and the third vehicle 12 comprises at least one scalar magnetometer 29' measuring a second amplitude of the magnetic field. Pipeline 1 is located vertical to the barycenter of the amplitudes measured. A configuration using magnetometers has been described, but it is also possible to mount the magnetometers 28,29 as a gradiometer, or any other means of measuring the magnetic field, such as an inductive system of the coil type. Preferably and in order to increase the measurement accuracy, it can be installed on each vehicle at least three magnetometers, operating as a gradiometer two by two: a first magnetometer at the end of a first fin and a second at the end a second fin and finally a third magnetometer approximately located in the middle of the underwater vehicle. It is also possible to install them transversely or longitudinally with respect to the body of the vehicle. An embodiment of the invention has so far been described making it possible to inspect a linear element varying the earth's magnetic field or producing a magnetic field, such as for example a pipeline, a pipe or even an electric cable supplied with electricity. On the other hand, when the linear element is of small dimensions, or if the latter is an unpowered electric cable, its magnetism is weak, or even zero, the embodiment described above no longer works correctly. Another embodiment of the invention solves this problem. At least one of the second and third vehicles 11,12 further comprises a system of coils making it possible to induce an electric current in the linear element 1 so that there is induced therein a magnetic field measurable by the first and second magnetometers 28.29. This embodiment is particularly advantageous when it is necessary to inspect electric cables that can possibly be broken, but it can also be used to inspect an anchoring chain, a small pipe, an optical fiber surrounded by '' a metallic armor or repeaters linked to an optical fiber. In this case, since the electric cable is broken, it is no longer crossed by an electric current and therefore no magnetic field is generated. The arrangement between the first vehicle 13, the second vehicle 11 and the third vehicle 12 is the same as that shown in FIGS. 1 and 2, except that the second vehicle 11 and / or the third vehicle 12 and / or the first vehicle 13 and / or a fourth vehicle comprises a system of coils capable of generating an electric current in the linear element 1.

Another embodiment of the present invention is shown in Figure 3. There is shown a collaborative system of underwater vehicles 31, preferably autonomous. This collaborative system 31 comprises a first vehicle 32, called a scout, comprising an optical sensor, such as a camera, in order to detect the visible parts of the linear element 33, for example a pipeline, and thus determine its trajectory. For cases where the linear element 33 is not visible, since it is covered with sand for example, the first vehicle 32 does not provide information as to its trajectory. For such cases, and in order to improve the positioning of the collaborative system 31, a second vehicle 34 comprising an acoustic sensor, such as a sediment sounder, or a multibeam probe makes it possible to determine whether the linear element 33 is buried. A multibeam probe has several beams to measure depth simultaneously in several directions. In another embodiment, the first vehicle 32 may include a sediment sounder in place of the optical sensor or a multibeam probe or any combination of these three sensors. In order to determine the position of the covered linear element 33, a third vehicle 35 comprising a first magnetometer and a fourth vehicle 36 comprising a second magnetometer is also used in a configuration identical to that described above and as shown in the Figures 1 and 2. The information from the first vehicle 32, the third vehicle 35 and the fourth vehicle 36 are transmitted to the second vehicle 34 which merges and processes them in order to precisely determine the position of the linear element 33. This information is then transmitted to a fifth vehicle 37, called a measurer. The vehicle 37 is positioned relative to the first vehicle 32, the second vehicle 34, the third vehicle 35 and the fourth vehicle 36. It is responsible for carrying out the inspection of the linear element 33 by means, for example, of a multibeam probe. The fifth vehicle 37 then transmits the information which has been transmitted to it as well as the data acquired by itself to a sixth vehicle 38 and a seventh vehicle 39 each comprising, for example, side sonars in order to inspect the sides or below the linear element 33. In the case where the linear element 33 is a pipeline, this inspection below it will make it possible to determine if it is correctly placed on the seabed and not between two dunes . Indeed, this configuration induces stresses in the pipeline thus generating fatigue problems. In another configuration of the collaborative system 31, the information from the second vehicle 34 is transmitted directly to the sixth and seventh vehicles 38,39 without passing through the fifth vehicle 37. The information collected by the fifth vehicle 37 is transmitted to the sixth and seventh vehicles

38.39 and processed directly by them. In another embodiment, the sixth and seventh vehicles 38, 39 are slaves of the fifth vehicle 37 and follow exactly the same trajectory as the latter.

In all the embodiments, at least one of the underwater vehicles may include one or more sensors or devices such as hydrocarbon detectors or meters, in the case of pipeline inspection missions, lateral sonars, multibeam sonars, video cameras or profile meters or any other system making it possible to determine the geometry, the surface condition and the physicochemical parameters of the environment of the linear element (l, 33).

Claims (3)

1. Collaborative system of underwater vehicles (10) able to follow a submerged linear element (1.33) capable of varying or producing a magnetic field comprising at least a first underwater vehicle (13, 37, 38, 39) intended for the followed by the position of the linear element (l, 33), a second underwater vehicle (11,35) comprising at least a first system capable of indicating its position and at least a first means of measurement (28) of said magnetic field capable to indicate a first angle (26) measured between the vertical and a first direction (30) of said magnetic field, a third underwater vehicle (12, 36) comprising at least a second system capable of indicating its position and at least a second means of measurement (29) of said magnetic field capable of indicating a second angle (27) measured between the vertical and a second direction (31) of said magnetic field, the first underwater vehicle (13, 37, 38, 39) being located vertically hold or behind the second and third underwater vehicles (11, 35, 12, 36), at least one of said underwater vehicles (13, 37, 38, 39, 11, 12) comprising means for calculating said position of said linear element (1, 33) and the first underwater vehicle (13, 37, 38, 39) being able to use the position of said linear element (1, 33) in order to follow it. 2. Collaborative system (10) according to claim 1, characterized in that at least one of the underwater vehicles (13, 37, 38, 39, 11, 12) is autonomous. 3. Collaborative system (10) according to any one of the preceding claims, characterized in that the calculation means is located on the first underwater vehicle (13, 37, 38, 39). 4. Collaborative system (10) according to any one of the preceding claims, characterized in that at least one measuring means (28, 29) of said magnetic field is a magnetometer-gradiometer. 5. Collaborative system (10) according to any one of the preceding claims, characterized in that it further comprises at least one additional underwater vehicle (32) comprising an optical sensor. 6. Collaborative system of underwater vehicles (10) capable of tracking a linear element (1.33) capable of varying or producing a magnetic field comprising at least a first underwater vehicle (13, 37, 38, 39) intended for tracking from the position of the linear element (1,33), a second underwater vehicle (11, 35) comprising at least a first measuring means capable of indicating its position and at least a first measuring means (28 ') capable of measuring a first amplitude of said magnetic field, a third underwater vehicle (12, 36) comprising at least a second system capable of indicating its position and at least a second measuring means (29 ') capable of measuring a second amplitude of said magnetic field, the first underwater vehicle (13, 37, 38, 39) being situated vertically or behind the second and third underwater vehicles (11, 35, 12, 36), at least one of said underwater vehicles (13, 37, 38, 39, 11, 12) com carrying a means of calculating said position of the linear element (1, 33) and the first underwater vehicle (13, 37, 38, 39) being able to use the position of said linear element (1, 33) in order to follow it . 7. Method for monitoring a linear element (1.33), said method comprising the following steps: 1) implementing a collaborative system of underwater vehicles (10) according to any one of claims 1 to 6; 2) calculate the position of said linear element (1.33) based on said position of the second underwater vehicle (11, 35), said position of the third underwater vehicle (12, 36), the measurement of said first angle (26) 5 or of the first amplitude and the measurement of said second angle (27) or of the second amplitude; 3) use the calculated position in order to follow the linear element (1.33) by means of the at least first underwater vehicle (13, 37, 38, 39). 8. Method according to claim 7, characterized in that the linear element (1, 33) is a pipeline. 9. Method according to claim 7, characterized in that at least one of the underwater vehicles (11, 12, 35, 36) comprises an induction means inducing an electric current in the linear element (1, 15 33). 10. Method according to claim 9, characterized in that the induction means comprises at least one coil.