OA19925A - Method and system for direct electric heating of a jacketed pipe for the transport of fluids. - Google Patents

Abstract

The invention relates to a method and system for direct electric heating of a jacketed pipe for transporting fluids, the method comprising mechanically connecting the inner steel casing (2) to the outer casing (6). steel at different intervals of the pipe, the installation of an electrical and thermal insulation between the inner casing and the outer casing, the application of an alternating electric current between an outer surface of the inner casing and an inner surface of the outer casing over the entire length of the pipe so as to heat the inner casing of the pipe by Joule effect, and the placing on the outer surface of the inner casing of at least one layer of resistive and ferromagnetic material (18) so as to increase the ratio of electric power transmitted to the internal envelope.

Description

Method and system for direct electric heating of a jacketed pipe for the transport of fluids

Background of the invention

The present invention relates to the general field of the electric heating of metal pipes for transporting fluids, and in particular underwater pipes resting on the seabed and providing a connection between wells for the production of submarine hydrocarbons, in particular oil. and gas, and a surface installation, for example a floating production, storage and unloading unit.

It relates more precisely to the electric heating of submarine pipes with a double jacket of the "Pipe In Pipe" or PIP type, that is to say "pipe in a pipe", in which an internal envelope which transports the fluids and a pipe. external envelope coaxial with the internal envelope which is in contact with the ambient medium, that is to say with sea water.

In the same offshore hydrocarbon production field, it is common to operate several wells which can be separated from each other by several kilometers, or even tens of kilometers. The fluids coming from these different wells must be collected by underwater metal pipes (typically steel) laid on the seabed and transferred by bottom / surface connection pipes to a surface installation, for example a platform, a ship or a land collection point, which will collect them in order to store them (and possibly process them).

The fluids from production wells tend to cool very quickly as they travel the many kilometers of subsea pipelines. However, if no measures are taken to maintain a minimum threshold temperature inside these pipes, there is a significant risk that the gas molecules, in particular methane, contained in the fluids transported combine with the water molecules. to form hydrate crystals at low temperature. These can stick to the walls, agglomerate there and lead to the formation of plugs capable of blocking the circulation of fluids inside the pipes. Likewise, the solubility in petroleum of compounds with high molecular weight, such as paraffins or asphaltenes, decreases when the temperature drops, which gives rise to solid deposits which are themselves capable of blocking circulation.

It is known to resort to passive solutions to try to remedy this problem. These passive solutions essentially constitute positioning an insulating material in the annular space delimited between the internal casing and the outer casing of the pipe, this insulating material possibly being a dry, wet or else vacuum material.

It is also known to have recourse to active solutions to try to remedy this problem. One of these solutions consists in heating the internal casing of the pipe by means of electric cables, round or flat, which are arranged around the inner casing over its entire length in order to heat it by the Joule effect. The electric power which is supplied to the electric cables comes from an external electric generator connected to the cables by an umbilical. This electric heating solution which is called "tracing heating" or "tracing heating" ("heat tracing" in English) keeps the fluids transported in the pipes at a temperature above a critical threshold throughout their path from the well. from production to surface installation.

Another known active solution in which the invention is more particularly interested consists in heating the underwater pipe by applying an alternating electric current directly to the internal steel casing of the pipe, the outer casing also. made of steel being used as a conductor for the return path of the electric current. The alternating electric current which passes through the internal envelope thus makes it possible to heat it by the Joule effect. More precisely, the heating of the internal envelope is produced by the Joule effect by the current which passes through it; the heat produced is largely transmitted to the fluids in the internal envelope, the thermal losses through the insulation being relatively reduced. This electric heating solution which is called in English "Direct Electrical Heating" (or DEH) for direct electric heating has the advantage of being relatively simple in design compared in particular with the traced heating solution.

On the other hand, the main drawback of the direct electric heating solution lies in its low efficiency, typically of the order of 50% to 60% approximately. In fact, the heat dissipated by the Joule effect by the current flowing through the outer casing is largely lost to the environment since the outer casing is in direct contact with sea water. Documents GB 2 084 284 are known. , WO 2015/148162, US 6,264,401 and US 6,371,693 such direct electric heating solutions.

Purpose and summary of the invention

The main object of the present invention is therefore to alleviate such drawbacks by proposing a method and a direct electric heating system which has a much higher efficiency.

This goal is achieved by means of a method of direct electric heating of a double jacket pipe for transporting fluids, the pipe comprising an internal steel jacket intended to transport fluids and an external steel jacket positioned around the envelope. internal by being coaxial with it in order to define therewith an annular space, the method comprising the mechanical connection of the internal casing to the outer casing at different intervals of the pipe, the installation of an electrical and thermal insulation between the inner shell and the outer shell along the entire length of the pipe, and applying an alternating electric current between an outer surface of the inner shell and an inner surface of the outer shell along the entire length of the pipe. the pipe so as to heat the internal casing of the pipe by the Joule effect, a method further comprising, in accordance with the invention, the placing on the internal surface rne of the outer casing over the entire length of the pipe of a jacket made of conductive and non-magnetic material, and / or the placement on the outer surface of the inner casing over the entire length of the pipe at least a layer of resistive and ferromagnetic material so as to increase the ratio of electric power transmitted to the internal casing,, χ

The inventors have observed that the application on the internal surface of the external envelope of a jacket made of a conductive and non-magnetic material makes it possible to significantly improve the ratio of electric power transmitted to the internal envelope, and thus to obtain an efficiency. at least 90% and up to 95% depending on the electromagnetic characteristics of the materials present. Likewise, the application on the external surface of the internal envelope of a layer of resistive and ferromagnetic material has the effect of substantially increasing the ratio of electric power transmitted to the internal envelope in order to obtain an efficiency of at least 85% and up to 90% depending on the electromagnetic characteristics of the materials present. Finally, when both the jacket of conductive and non-magnetic material and the layer of resistive and ferromagnetic material are applied, it is possible to obtain an efficiency of up to 98%.

The alternating electric current which circulates on the external surface of the internal envelope by skin effect and on the internal surface of the external envelope by proximity effect over the entire length of the pipe can have a frequency of 50 Hz or 60 Hz .

Alternating electric current can be applied at a single point in the pipe. Alternatively, the alternating electric current can be applied to several points of the pipe, each corresponding to at least one section of pipe, each section of pipe comprising an internal casing section and an external casing section.

The installation of the jacket of conductive and non-magnetic material may include the insertion of a tube of conductive and non-magnetic material inside the outer casing, then the pressure expansion of the tube against the internal surface of the tube. outer envelope.

Correlatively, the invention also relates to a direct electric heating system of a double-cased pipe for transporting fluids, the pipe comprising an inner steel casing intended to transport fluids and an outer steel casing positioned around it. the internal casing being coaxial with it to define therewith an annular space, the system comprising a plurality of mechanical connections between the inner casing and the outer casing positioned at different intervals of the pipe, an electrical and thermal insulation between the inner shell and the outer shell positioned along the entire length of the pipe, and an alternating electric current generator applied between an outer surface of the inner shell and an inner surface of the outer shell along the length of the pipe. the pipe so as to heat the internal casing of the pipe by Joule effect, and further comprising, in accordance with the invention, a jacket of conductive and non-magnetic material 10 positioned on the inner surface of the outer casing over the entire length of the pipe, and / or at least one layer of resistive and ferromagnetic material positioned on the outer surface of the internal envelope over the entire length of the pipe so as to increase the ratio of electric power transmitted to the internal envelope.

The jacket of conductive and non-magnetic material is advantageously made of aluminum, copper, bronze, brass or zinc. Preferably, it has a thickness of between 1 and 6mm.

As for the layer of resistive and ferromagnetic material, it is advantageously made of electrical steel or of amorphous metal.

Preferably, it has a thickness of between 1 and 3mm.

The mechanical connections between the inner casing and the outer casing of the pipe may be annular shoulders regularly spaced over the entire length of the pipe.

The inner casing and the outer casing of the pipe may each comprise a plurality of casing sections connected end to end to each other by weld beads, the jacket of conductive and non-magnetic material and the layer of resistive material and ferromagnetic showing discontinuities at the weld beads.

In this case, the electrical continuity of the jacket of conductive and non-magnetic material and of the layer of resistive and ferromagnetic material can be achieved, either directly through the internal and external envelopes, or indirectly through a conductive annular ring or a conductive sleeve which are positioned at the weld seams, χ

Brief description of the drawings

Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate exemplary embodiments thereof without any limiting nature. In the figures:

FIG. 1 is a schematic view of a submarine pipe with a double jacket for the transport of hydrocarbons provided with a direct electric heating system according to one embodiment of the invention;

- Figure 2 is a magnifying glass of Figure 1;

- Figures 3 and 4 show alternative embodiments of the electrical continuity of the jacket of conductive and non-magnetic material and of the layer of resistive and ferromagnetic material at the weld beads; and

- Figure 5 is a schematic view of a submarine pipe with a double jacket for the transport of hydrocarbons provided with a direct electric heating system according to another embodiment of the invention.

Detailed description of the invention

The invention relates to the direct electrical heating (or "Direct Electrical Heating" in English) of any terrestrial or submarine metallic pipe for transporting fluids, and in particular to the direct electric heating of submarine double-cased steel pipes resting on the base. seabed and providing transport between subsea hydrocarbon production wells, including oil and gas, and a surface facility.

A submarine pipe with a double casing of the "Pipe in Pipe" (or "PIP") type as shown in FIGS. 1 and 2 is a pipe 2 which comprises an internal casing 4 centered on a longitudinal axis XX and intended to transport hydrocarbons from production wells, and an outer casing 6 arranged around the inner casing being coaxial with it and intended to be in direct contact with the surrounding seawater,

The internal 4 and external 6 envelopes of the submarine pipe are typically made of carbon steel. Of course, other electrically conductive materials could be used for these envelopes, in particular alloy steel (stainless steel type), aluminum, copper, carbon, etc.

This type of submarine pipe is typically used in the offshore production of hydrocarbons at great depth. In the context of such installations, the jacketed pipes can be assembled on land in several pipe sections 2a, each pipe section comprising an inner casing section 4a and an outer casing section 6a.

As shown in Figure 2, the inner casing sections 4a are connected end to end to each other by weld beads 8 to form the inner casing of the pipe. Likewise, the outer casing sections 6a are connected end to end to each other by weld beads 10 to form the outer casing of the pipe.

In addition, the inner casing sections 4a and the outer casing sections 6a define between them annular spaces 12 which include electrical and thermal insulation means (not shown in the figures). Typically, these insulation means can be in the form of a vacuum or filled with a thermal insulating material.

At each end of the inner and outer casings of the pipe, the annular spaces 12 are sealed by metal end partitions 13. These end partitions, commonly called “bulkheads” in English, provide a mechanical and electrical connection between the internal envelope and the external envelope.

Furthermore, at different intervals over the entire length of the pipe, the inner casing and the outer casing are connected to each other by intermediate links 15, commonly called "intermediate bulkheads" in English, which make it possible to keep the envelopes coaxial with each other. These intermediate links 15 are electrically isolated (no electric current passes through them). For example, it is possible to use rings made of elastomer or of polymer.

Further, as indicated above, the casing sections which form the inner casing and the outer casing of the pipe are connected to each other by weld beads 8, 10.

According to the invention, provision is made to place a jacket of conductive and non-magnetic material 16 on the internal surface of the external casing 6 over the entire length of the pipe and / or a layer of resistive and ferromagnetic material 18 on the pipe. the outer surface of the inner casing 4 over the entire length of the pipe to be heated.

As will be discussed below, the presence of such a jacket of conductive and non-magnetic material 16 and / or of such a layer of resistive and ferromagnetic material 18 makes it possible to increase the ratio of electric power transmitted to the envelope. internal when applying an electric current for heating the underwater pipe.

Still according to the invention, an alternating electric current is applied from a single-phase power supply 20 between the outer surface of the inner casing 4 and the inner surface of the outer casing 6 over the entire length of the pipe. so as to heat the internal casing of the pipe by Joule effect. In practice, one of the two cables of the phase is connected to the outside of the inner casing, and the other to the inside of the outer casing.

More precisely, the heating of the internal envelope of the pipe is obtained by a combination of interactive effects which include in particular effects of electrical resistance and electromagnetic effects (such as skin and proximity effects).

The electric current which circulates on the outer surface of the inner casing 4 by skin effect and on the inner surface of the outer casing 6 of the pipe by proximity effect is typically an alternating electric current (sinusoidal or not) having a frequency of the order of 50 Hz or 60 Hz.

The inventors have found that the efficiency of direct electric heating as described above is increased in two ways: by increasing for the inner shell the product of electrical resistivity per magnetic permeability and by decreasing for the outer shell. the product of electrical resistivity by magnetic permeability.

The inventors have also observed that the resistivity by permeability product of the internal envelope can be increased by adding to the external surface of the internal envelope a layer of resistive and ferromagnetic material. As for the resistivity product by permeability of the external envelope, it is increased by the addition to the internal surface of the external envelope of a jacket made of a conductive and non-magnetic material. Furthermore, it should be noted that these results are obtained even without adhesion of the layer of resistive and ferromagnetic material on the external surface of the internal casing and without adhesion of the liner of conductive and non-magnetic material on the internal surface of the outer casing. Likewise, the layer of resistive and ferromagnetic material may consist of a plurality of layers separated by a small space.

Furthermore, the jacket of conductive and non-magnetic material 16 is preferably made of aluminum, copper, bronze, brass or zinc, materials giving the best results in terms of increasing the ratio of electric power transmitted to the envelope. internal. With such materials, an efficiency of up to 95% is obtained depending on the electrical characteristics of the materials present.

In addition, it advantageously has a thickness of between 1 and 6 mm in order to further maximize the gain in the ratio of electric power transmitted to the internal envelope.

As for the layer of resistive and ferromagnetic material 18, it is preferably made of electrical steel or of amorphous metal or of an alloy of nickel and iron (mu-metal). More precisely, the material will be electric steel (grain oriented or not oriented) or Metglas® (amorphous metal alloy). With such materials, an efficiency of up to 90% is obtained depending on the electrical characteristics of the materials present.

In addition, it advantageously has a thickness of between 1 and 3 mm in order to further maximize the gain in the ratio of electric power transmitted to the internal envelope, p (

The two tables below show calculations of efficiency (that is to say of the ratio of electric power transmitted to the internal casing) for different jacketed pipes (nos. 1 to no. 8).

unity Line n ° l Conduct n ° 2 Conduct n ° 3 d i mm . 558.8 558.8 558.8 e i mm 31.75 31.75 31.75 d e mm 736.6 736.6 736.6 e e mm 34.925 34.925 34.925 rho i Ω.πί 0.00000013 0.00000013 0.0000012 mu i - 150 150 1500 rho e Ω.πί 0.00000013 2.80E-08 l, 30E-07 mu e - 150 1 150 eta - 0.544 0.969 0.920

unity Conduct n ° 4 Conduct n ° 5 Conduct n ° 6 d i mm 558.8 558.8 558.8 e i mm 31.75 31.75 31.75 d e mm 736.6 736.6 736.6 e e mm 34.925 34.925 34.925 rho i Ω.πί 0.0000012 0.00000058 0.00000058 mu i - 1500 750 750 rho e Ω.πί 2.80E-08 l, 30E-07 2.80E-08 mu e - 1 150 1 eta - 0.997 0.849 0.993

In the tables above:

“D_i” designates the external diameter of the internal envelope;

“E_i” designates the thickness of the internal envelope;

"D_e" designates the internal diameter of the outer shell;

"E_e" denotes the thickness of the outer shell;

“Rho_i” designates the resistivity of the internal envelope;

"Mu_i" designates the permeability of the internal envelope;

"Rho_e" designates the resistivity of the outer shell;

"Mu_e" designates the permeability of the outer shell;

“Eta” denotes the efficiency (between 0 and 1);

pipe n ° 1 corresponds to a steel pipe of the prior art described in publication WO 2015/148162;

pipe No. 2 corresponds to a steel pipe of the prior art described in particular in publication WO 2015/148162 and of which the internal surface of the external casing comprises an aluminum jacket according to the principle of the invention;

pipe no. 3 corresponds to a steel pipe of the prior art described in particular in publication WO 2015/148162 and whose outer surface of the internal casing comprises a layer of Metglas® according to the principle of the invention;

pipe 4 corresponds to a steel pipe of the prior art described in particular in publication WO 2015/148162 and whose internal surface of the outer casing comprises an aluminum jacket and the outer surface of the inner casing comprises a layer of Metglas® according to the principle of the invention;

pipe no. 5 corresponds to a steel pipe of the prior art described in particular in publication WO 2015/148162 and whose outer surface of the inner casing comprises a layer of mumetal according to the principle of the invention; and pipe No. 6 corresponds to a steel pipe of the prior art described in particular in publication WO 2015/148162 and of which the internal surface of the outer casing comprises an aluminum jacket and the outer surface of the casing. internal comprises a mu-metal layer according to the principle of the invention.

It emerges from these tables that the principle according to the invention of applying a jacket of conductive and non-magnetic material on the internal surface of the outer casing and / or a layer of resistive and ferromagnetic material on the outer surface of the casing. internal allows to significantly increase the efficiency of the electric heating of the pipe, this efficiency going from 0.544 for the steel pipe of the prior art as described in publication WO 2015/148162 to more than 0.84 for the pipes. same pipes provided with a jacket and / or a layer according to the invention.

Moreover, as shown in FIG. 2, the jacket of conductive and non-magnetic material 16 and the layer of resistive and ferromagnetic material 18 each have discontinuities 35 (respectively 16a and 18a) at the level of the weld beads 15a, 15b. These discontinuities 16a, 18a, which extend longitudinally on either side of the weld beads, are necessary to produce the weld beads between the inner and outer shell sections of the pipe.

These discontinuities 16a, 18a can be the source of a very small loss of efficiency. However, this loss is acceptable insofar as, in the absence of reconstitution of these discontinuities, the electrical continuity of the jacket of conductive and non-magnetic material and of the layer of resistive and ferromagnetic material is produced directly through the internal envelopes and of the pipe thanks to the circulation of electric current in the carbon steel wall of the internal and external envelopes.

According to an alternative, to avoid this very low loss of efficiency, it is possible, as shown in FIG. 3, to ensure the electrical continuity of the jacket 16 of conductive and non-magnetic material through a conductive annular ring 22 positioned at the bottom. level of weld seams.

This ring 22 of conductive material makes it possible to ensure, by means of flexible tabs or blades 24, an electrical continuity between the two ends of each discontinuity 16a.

According to another alternative shown in FIG. 4, the electrical continuity of the layer 18 of resistive and ferromagnetic material is produced through an annular conductive sleeve 26 positioned at the level of the weld beads 8a and connecting the ends of the discontinuities 18a.

Furthermore, there are different methods for placing the jacket of conductive and non-magnetic material 16 inside the outer casing 6 of the pipe.

One of these methods consists in inserting a tube of conductive and non-magnetic material inside the outer casing, then in carrying out its expansion under pressure (for example under a hydraulic pressure of several hundred bar) against the internal surface. of the outer casing to allow it to match the profile thereof.

Another method consists of inserting inside the outer casing a tube of conductive and non-magnetic material which is split and closed, then releasing it once in place in the outer casing to allow it to come against the inner surface. of it. This slotted tube χ can also be force-fitted into the outer casing by pulling it by one of its ends.

Yet another method consists of using longitudinal gutter strips made of conductive and non-magnetic material to slide them inside the outer casing and weld them to the ends thereof.

Yet another method consists in producing the jacket in a conductive and non-magnetic material by spraying powder under a plasma torch on the internal surface of the external casing of the pipe. This powder projection could also be carried out only at the ends of the outer casing to serve as a weldable zone for a tube or longitudinal strips inserted inside the outer casing.

Finally, yet another method consists in producing the jacket 15 in a conductive and non-magnetic material by thick galvanizing.

In the embodiment of FIG. 1, the alternating electric current for heating the casing is internal is applied to the middle of the pipe, that is to say at an equal distance from the two end partitions 13. The currents electrical cables of the electrical power supply are then divided into two currents of equal intensity each passing through one half of the pipe, between the midpoint of the latter and one of the ends through the body of the internal casing, then between the end and the middle of the pipe through the body of the outer casing, the electrical continuity between the two casings 25 being ensured by the end partitions 13.

FIG. 5 represents an alternative embodiment of the invention in which the pipe 2, and more precisely the internal casing 4 thereof, is heated by applying an alternating electric current to several points of the pipe.

Preferably, these points of application of the electric current each correspond to at least one pipe section 2a. In this case, the internal casing sections and the outer casing sections of each pipe section are electrically isolated from each other at the level for example of the end partitions 13 (or “bulkheads”). In this example of FIG. 5, three pipe sections 2a are each supplied with electric current independently and in their middle by the single-phase electric supply 20. Thus, the current of a pipe section passes from the casing. internal to the outer casing passing through the adjacent surface of the end wall, and the same for the adjacent pipe section. In other words, in this multi-feed configuration of the pipe, both internal faces of the end walls are traversed by. currents from each of the electrical networks on either side of the end partitions.

Claims (16)

1. A method of direct electric heating of a double-jacketed pipe for transporting fluids, the pipe (2) comprising an inner casing (4) of steel intended for transporting fluids and an outer casing (6) of steel positioned around of the internal envelope by being coaxial with it in order to define therewith an annular space (12), the method comprising: the mechanical connection of the inner shell to the outer shell at different intervals of the pipe; the installation of electrical and thermal insulation between the inner casing and the outer casing over the entire length of the pipe; and applying an alternating electric current between an outer surface of the inner casing and an inner surface of the outer casing over the entire length of the pipe so as to heat the inner casing of the pipe by Joule effect ; characterized in that it further comprises: placing on the outer surface of the inner casing over the entire length of the pipe of at least one layer of resistive and ferromagnetic material (18) so as to increase the ratio of electric power transmitted to the inner casing. 2. The method of claim 1, further comprising placing on the inner surface of the outer casing over the entire length of the pipe of a jacket of conductive and non-magnetic material (16). 3. Method according to one of claims 1 and 2, wherein the alternating electric current which circulates on the outer surface of the inner casing by skin effect and on the inner surface of the outer casing by proximity effect has a frequency of 50 Hz or 60 Hz. Λ 4. Method according to any one of claims 1 to 3, wherein the alternating electric current is applied at a single point of the pipe. 5. Method according to any one of claims 1 to 3, wherein the alternating electric current is applied at several points of the pipe each corresponding to at least one pipe section (2a), each pipe section comprising a section d. inner shell (4a) and an outer shell section (6a). 6. Method according to any one of claims 1 to 5, wherein the establishment of the jacket of conductive and non-magnetic material (16) comprises inserting a tube of conductive and non-magnetic material inside the liner. outer casing, then the pressure expansion of the tube against the inner surface of the outer casing. 7. Direct electrical heating system of a double-jacketed pipe for transporting fluids, the pipe (2) comprising an inner casing (4) of steel intended for transporting fluids and an outer casing (6) of steel positioned around it. of the internal envelope by being coaxial with it in order to define therewith an annular space, the system comprising: a plurality of mechanical links (15) between the inner shell and the outer shell positioned at different intervals in the pipe; electrical and thermal insulation between the inner casing and the outer casing positioned over the entire length of the pipe; and an alternating electric current generator (20) applied between an outer surface of the inner casing and an inner surface of the outer casing over the entire length of the pipe so as to heat the inner casing of the pipe by the effect of Joule; characterized in that it further comprises: at least one layer of resistive and ferromagnetic material (18) positioned on the outer surface of the inner casing over the entire length of the pipe so as to increase the ratio of electric power transmitted to the inner casing. X [--19925 8. The system of claim 7, further comprising a jacket of conductive and non-magnetic material (16) positioned on the inner surface of the outer shell over the entire length of the pipe. 9. System according to one of claims 7 and 8, wherein the jacket of conductive and non-magnetic material (16) is made of aluminum, copper, bronze, brass or zinc. 10. System according to any one of claims 7 to 9, wherein the jacket of conductive and non-magnetic material has a thickness between 1 and 6mm. 15 11. System according to any one of claims 7 to 9, wherein the layer of resistive and ferromagnetic material (18) is made of electrical steel or of amorphous metal. 12. System according to any one of claims 7 to 11, wherein the layer of resistive and ferromagnetic material has a thickness of between 1 and 3mm. 13. A system according to any one of claims 7 to 12, wherein the mechanical connections between the inner shell and the outer shell of the pipe are annular shoulders (15) regularly spaced over the entire length of the pipe. 14. A system according to any one of claims 7 to 13, wherein the inner casing and the outer casing of the pipe 30 each comprise a plurality of casing sections (2a) connected end to end to each other by. weld beads (8, 10), the jacket of conductive and non-magnetic material and the layer of resistive and ferromagnetic material having discontinuities (16a, 18a) at the weld beads. 15. The system of claim 14, wherein the electrical continuity of the jacket of conductive and non-magnetic material and of the layer of resistive and ferromagnetic material is produced directly through the internal and external envelopes. 16. The system of claim 14, in ieque! the electrical continuity of the jacket of conductive and non-magnetic material and of the layer of resistive and ferromagnetic material is achieved indirectly through a conductive annular ring (22) or a conductive sleeve (26) which are positioned at the level of the weld seams.