WO2007101931A1

WO2007101931A1 - Method for laying and protecting an underground pipe

Info

Publication number: WO2007101931A1
Application number: PCT/FR2007/000378
Authority: WO
Inventors: Eric Puech
Original assignee: Eric Puech
Priority date: 2006-03-06
Filing date: 2007-03-02
Publication date: 2007-09-13
Also published as: FR2898178B1; FR2898178A1

Abstract

The invention relates to a method for laying and protecting an underground metal pipe used to carry fluid, in which: the pipe (1) is placed in a metal sleeve (2); the annular space (3) separating the pipe from the sleeve is closed off using a plugging ring (12, 13) at each longitudinal end of the sleeve; the annular space is filled with a filler material. The method is characterized in that: use is made, by way of filler material, of a water-based slurry (6) comprising water and, per 900 litres of water, 85 to 105 kilograms of a blast furnace slag, 8 to 13 kilograms of Portland clinker and 120 to 160 kilograms of bentonite; and the pipe is connected to an electric circuit comprising a cathodic protection current take-off or a sacrificial anode.

Description

METHOD FOR INSTALLING AND PROTECTING AN INFRINGED DRIVING

The invention relates to a method for installing and protecting a buried fluid transport line (liquid or gas). The invention applies more particularly to metal pipes, in particular to steel pipes, and, more generally, to pipes made of an electrically conductive material.

A buried metallic pipe is likely to be subjected to various forms of aggressions, among which mechanical aggression (stresses exerted on the ground and transmitted by the ground surrounding the pipe, instability of the ground ...) and chemical or electrolytic aggression (corrosion).

At the crossing of an obstacle such as a road, a railway, a river or an inhabited area, the pipe is placed in a sheath, for reasons of ease of installation. Indeed, unlike driving, the sheath can be placed by drilling, without having to dig a trench. The pipe is then easily threaded into the sheath. This technique is therefore essential when crossing a river. It is also advantageous for crossings of inhabited areas or roads and railways, since it does not involve any demolition or interruption of traffic (the installation of the sheath is thus done at a lower cost). In addition, the sheath is a mechanical protection of the pipe, appreciable when the stability of the ground is not guaranteed and / or that the pipe is likely to undergo significant mechanical stress.

There are concrete sheaths and metal sheaths. Although much more expensive, concrete sheaths are also more difficult to implement than metal sheaths, because of their design: a concrete sheath is formed of a succession of nested sections, which tend to to dissociate or to deviate from the planned trajectory during the installation by sinking the sleeve underground. In addition, the centering collars used to wedge the pipe in the sheath are often deteriorated by the rough surface condition of the concrete when the pipe is threaded into the sheath. This The last operation proves more difficult when the concrete sections do not define, at the bottom of the sheath, a rectilinear and regular water thread (the interlocking structures of the sections forming a rib at the junction of two successive sections). For all these reasons, metal sheaths are preferred over concrete sheaths. But the use of a metal sheath makes it necessary to take measures to prevent any direct contact between the sleeve and the pipe (such contact causing rapid corrosion of the pipe) and to limit the chemical or electrolytic attacks suffered by driving inside the sheath. It is therefore recommended to close the annular space separating the pipe and the sheath and to fill this space with a filling material.

US 4469469 recalls the adverse effects of moisture or water infiltrating into the annular space. This document first teaches introducing, in the annular space, a substance capable of mixing with the residual water and neutralizing the effects thereof, and then injecting a corrosion-resistant filling material suitable for remove the water thus treated and fill the annular space so as to prevent further infiltration or condensation of water. The filling material is a wax based on petroleum or bitumen.

Such wax has many disadvantages: - it results from a fossil resource (oil) expensive and exhaustible;

- it constitutes a polluting material;

- its implementation is very restrictive; the waxes based on petroleum or bitumen are liquid only at high temperature (at least 100-130 ^° C.) and it is therefore necessary to provide a heavy installation on site to prepare the wax before injection;

- the injection operations are not without risk considering the injection temperature and the toxicity of the wax; for all the above reasons, the process of US 4469 469 proves to be particularly expensive; - The hot wax tends to congeal quickly in contact with the sheath and cold pipe, the process of US 4469 469 is reserved for sleeves of less than 60 meters;

- there are two main ways, which can be added, to fight against the electrolytic corrosion of a metal pipe: the implementation of a passive cathodic protection, which consists in preventing the formation of electric currents on the surface of the pipe , and the establishment of an active cathodic protection (which until now has been used only for portions of pipe buried directly in the ground, without sheath), which consists in transforming the pipe into a cathode for the purpose of prevent its oxidation, either by connecting it to a sacrificial anode or by drawing off current. Bitumen or petroleum-based waxes are electrically non-conductive materials. Once hardened, the wax injected into the sheath electrically isolates the pipe from the surrounding soil and the sheath, thereby providing passive cathodic protection of the pipe. This property of the wax may seem a priori advantageous; but it should be borne in mind that it is incompatible with the introduction of active cathodic protection of conduct, which is generally more reliable and more effective than passive protection.

US Pat. No. 4,932,810 recommends injecting an inert gas, such as argon, into the annular space in order in part to eliminate any moisture in the annular space and, on the other hand, to provide an electrical barrier between the pipe and the sheath. The injection of this gas thus pursues an objective contrary to the establishment of active cathodic protection of the pipe. In addition, the gas injected as "material" for filling the annular space provides no support or mechanical protection to the pipe; and it is not excluded that the sheath comes into electrical contact with the pipe due to deformation of the sheath under the effect of mechanical stresses or displacement of the sheath and pipe caused by a movement of the surrounding terrain. Finally, the implementation of the gas requires the use of complex devices to hermetically close the annular space, which significantly affect the costs of implementation. JP 2000120923 teaches on the one hand to fill the annular space by means of a cement mortar in order to provide mechanical protection and electrical insulation of the pipe, and secondly to coat the inner surface of the sleeve by a conductive layer (metal foil) to deflect any underground currents likely to reach the sleeve and thus overcome any defect in the electrical insulation made by the mortar. Again, and usually in the case of a pipe protected by a sleeve, the objective is contrary to the establishment of active cathodic protection. In addition, this method of installation and protection is not reversible: the mortar, once cured, can not be removed or destroyed without damaging the pipe, so that it is impossible, in case of problems, to proceed to inspection or repair operations of the pipe in the sheath.

The invention aims to overcome these disadvantages, by proposing a method of installing and protecting a buried pipe using a filling material compatible with the establishment of active cathodic protection of the pipe.

The invention also aims to propose a process that is easy to implement by means of a light and very simple installation. An object of the invention is to propose a filler material which can have a viscous liquid-like state at ambient temperature, with a view to its injection into the annular space, then a consolidated state after injection, capable of ensuring the setting and stability of the pipe in the sheath. Another object of the invention is to provide a filling material that can be handled by an operator without special protection, with the exception of a possible mask. Another object of the invention is to provide a filling material which does not tend to be washed or to be diluted in a residual water possibly present in the sleeve before injection of the material.

Another object of the invention is to provide an aqueous filling material, which develops a negligible resurgence of water during its consolidation and therefore has a volume substantially identical to the liquid state and the consolidated state. Another object of the invention is to provide a non-polluting filling material in the consolidated state, and whose composition is close to the geological elements that can be encountered in the soil.

Another object of the invention is to provide an inexpensive filling material, and in particular much less expensive than waxes based on petroleum or bitumen.

Another object of the invention is to provide a method of installation and reversible protection, by providing a filling material which, in the consolidated state, can be destroyed or removed from the annular space without risk of deterioration of the conduct.

The invention relates to a method for installing and protecting a buried fluid transport pipe, said pipe being made of an electrically conductive material. According to this method: the pipe is placed in a sheath of an electrically conductive material (metal sheath for example, in particular steel), which follows a longitudinal direction substantially parallel to a longitudinal direction of the pipe, the pipe and the sheath delimiting between them an annular space that extends mainly along these longitudinal directions; the annular space is closed at each longitudinal end of the sleeve by means of a so-called closure ring; the annular space is filled with a filling material.

The method according to the invention is characterized in that:

- a hydraulic grout containing water is used as filler and, for 900 liters of water, 85 to 105 kilograms of a blast furnace slag, 8 to 13 kilograms of Portland clinker and 120 to 160 kilograms of bentonite,

the pipe is connected to an electrical circuit comprising a current draw-off station or a sacrificial anode, with a view to setting up an active cathodic protection of the pipe.

"Slurry" is understood to mean a slurry comprising on the one hand an aqueous liquid phase and, on the other hand, a granular solid phase suspended in the liquid phase, the solid phase consisting essentially of fine particles having diameters less than or equal to 100 μm, unlike a "mortar" which comprises larger particles -forming a charge- such as sands.

While all known prior art aims to electrically isolate the pipe sleeve, the inventor wished to implement active cathodic protection of the pipe in the sheath. Unlike all the known filling materials (and in particular the mortar of JP 2000120923), the grout according to the invention remains electrically conductive after consolidation. And it is at the cost of months of trials and experiments that the inventor was able to develop a grout that, against all odds, has all of the following characteristics, some of which seemed a priori incompatible:

- Grout can be consolidated to provide mechanical protection of the pipe; it is stable and durable; it is particularly able to maintain the surrounding lands after a slow decomposition of the sheath;

in the consolidated state, the grout produces an electrolyte making it possible to implement active cathodic protection of the pipe. In particular, the slurry has a basic pH. Furthermore, surprisingly, the consolidated slurry has a low electrical resistivity, does not oppose the flow of electrolytic currents and allows in particular ion exchange, despite its solid state. In fact, mixing blast furnace slag, portland clinker and bentonite leads, by synergy, to obtaining a moist solid structure, forming a stable and resistant microscopic network in which the water added to the mixture is imprisoned; - The consolidation of the grout is done at constant volume (or almost constant), without resurgence of water (or with a very small and negligible resurgence) or risk of spinning.

In addition, the grout according to the invention has, in the consolidated state, a sufficiently low hardness to allow its destruction by means of a hydrocureuse, while offering sufficient mechanical strength to to guarantee an effective mechanical protection of the pipe, in particular in case of movement of the ground.

The filling material according to the invention is a hydraulic slurry, which is liquid (or viscous, of sufficiently low viscosity to allow its injection) at room temperature. Advantageously and according to the invention, the slurry is prepared by mixing its various constituents at room temperature. This preparation does not require any heavy installation: the mixture of constituents of the grout is carried out in a simple kneader, at room temperature; the grout is injected into the annular space by means of a usual pump.

The contact of the grout with the sheath and the pipe does not accelerate the setting of the grout (the latter being liquid at ambient temperature), so that it is possible to implement the method according to the invention in the case of traverses (and therefore sleeves) of great length (especially greater than 60 m). Note that the grout according to the invention has a curing time of 3 days and it reaches its final consolidated state only about 30 days.

In the consolidated state, the grout according to the invention is impervious to the charged waters brought by the surrounding ground, with which it can be in contact after a possible decomposition of the sheath. The grout and the process according to the invention are inexpensive. In addition, in the consolidated state, the grout according to the invention is a non-polluting and non-toxic material.

Advantageously and according to the invention, in a preferred version, use is made of a slurry comprising, for 900 liters of water, 95 kilograms of blast furnace slag, between 10 and 11 kilograms of Portland clinker and 145 kilograms of bentonite.

Advantageously and according to the invention, a slurry consisting exclusively of water, blast furnace slag, Portland clinker and bentonite is used. Advantageously and according to the invention, use is made of a sleeve having a diameter greater than or equal to 1.5 times the diameter of the pipe. By for example, for a pipe of 800 mm in diameter, a sheath of 1200 mm in diameter is used. A consolidated grout layer 200 mm thick is then obtained. The inventor has found with surprise that, despite its thickness, the consolidated grout layer has a sufficiently low electrical resistance to allow the establishment of effective cathodic protection of the pipe.

Advantageously and according to the invention, the grout is injected into the annular space by means of a breather having an upper surface end, connected to an injection pump of the grout, and a buried lower end s opening in the sheath, the breather being arranged such that its lower end opens at the bottom of a section of the sheath. In known prior art, the filling material is injected into the upper part of a section of the sheath. According to the invention, the breather is arranged such that the grout penetrates into the sheath in the lower part thereof, near the bottom of the sheath. This arrangement is particularly advantageous when the sheath contains residual water, which must be evacuated. The inventor has indeed found, in the known techniques, a certain dilution of the filling material in the residual water, the extent of which varies according to the nature of the material, the conditions of its injection, the dimensional characteristics of the pipe and the sheath, the amount of residual water ... The inventor determined that this dilution could be due to bursting of the material out of the breather: the material pushed by the pump in the breather penetrates into the residual water by squirts and settles in the bottom of sheath. This contact with the residual water is quite violent (it is even more so if the injection speed is high), the material undergoes a light wash. The material exiting the breather does not come into direct contact with the material already injected until the annular space is almost completely filled. Conversely, when the lower end of the breather is arranged, according to the invention, in the lower part of the sleeve, in the residual water that may be present, the injected grout first enters the residual water by mass, and pushes the latter as it is injected, without becoming diluted. A few minutes or even a few seconds only after the start of the injection, the lower end of the breather, which extends at the bottom of the sleeve, is immersed in the grout already injected: the grout comes out of the breather in contact with the grout already injected, and not the residual water. Any risk of washing the grout is thus eliminated, and it is possible to provide high injection speeds.

The invention also relates to a method of installing and protecting a buried pipe, characterized by all or part of the characteristics mentioned above and below.

Other objects, features and advantages of the invention will appear on reading the following description, which refers to the appended figures representing a preferred embodiment of the invention, given solely by way of non-limiting example.

Figure 1 is a schematic top view of a buried pipe, some portions pass through structures and are therefore installed according to the invention. The structures are schematized in projection.

Figure 2 is a view of the buried pipe of Figure 1 at the crossing of a road. A first part of the pipe and the sheath is seen in perspective; a second part of the pipe is shown in perspective and devoid of the sheath; a third part of the pipe is shown in perspective and devoid of sheath and consolidated grout; a fourth part of the pipe is seen in perspective and in longitudinal section.

Figure 3 shows the diagram of Pourbaix iron in the presence of water.

FIG. 1 represents a buried pipe 1 which, on its course, crosses a road 14, a rural road 101, a railway 102 and a rural road 105. Under the road 14, the pipe 1 is placed in a metal sheath 2, according to the method of the invention. Similarly, under the track 102, the pipe is placed in a metal sleeve 104. In contrast, the installation of the pipe under the rural roads 101 and 105 is done by digging a trench across the road and burying the driven directly into the ground without a sheath. Figure 2 shows the pipe 1 to the right of the road 14, installed according to the method of the invention. According to this method, a sheath 2 is first of all arranged under the road 14. The sheath 2 consists of a succession of cylindrical sheath sections made of steel, each section having a circular section of 1200 mm in internal diameter and a length of 6 m. To set up the sheath, one dig first, in the ground 4, a working pit (not shown) on each side of the road. There is a horizontal drilling machine (not shown) in one of the two pits, said pit, the other pit being called arrival pit. Such a machine comprises in particular a guide rail, a thrust ring, a tube guide, worm and a drill head. A first sleeve section is arranged on the guide rail and is fixed to the thrust ring; it is kept centered on said rail by the tube guide. A driving collar is welded to the front of said sleeve section, in order to protect the latter and to ensure excavation of the ground to the outer diameter of the sleeve. The drill head is placed inside the sleeve section at the drive collar. A worm is arranged in the sleeve section. The cuttings torn off the ground by the drill head are evacuated by the worm to the pit. As the drilling progresses, the sheath section is pushed towards the arrival pit. When the first section of the sleeve is almost completely depressed underground, a second section of sleeve is placed in the extension of the first section and is welded thereto. A second worm is arranged in the second section. The drilling continues (by sequentially arranging the necessary number of sections of sheath and worm, depending on the length of the crossing to be made), until the first section opens into the arrival pit. The sheath 2 thus created is emptied of the cuttings it contains and is carefully cleaned.

A breather 7, 8 is then arranged at each end of the sheath. Each breather is a tube of 50 to 80 mm in diameter for example, PVC, polyethylene or other synthetic polymer material. In the illustrated example, the end 15 of the sheath is the lowest point of said sheath, the end 16 corresponding to its highest point. It is therefore it is preferable to inject the grout in the annular space 3 by the breather 7. The breather 7 is arranged to extend substantially vertically between the sheath 2, close to its end 15, and the surface of the natural ground 4. A bore 26 is formed in the sheath at its end 15, in the upper part of said sheath, to receive the breather 7. At this stage, the lower end 21 of the breather 7 is introduced into this bore 26 and is positioned so that extend in the upper part of the sheath, so as not to hinder the installation of the pipe. The fixing of the breather 7 in the sheath will be performed later.

The breather 8 is arranged to extend substantially vertically between the sheath 2, near its end 16, and the surface of the natural terrain. A bore 27 is formed in the sheath, at its end 16, in the upper part of said sheath. A cylindrical metal / plastic connector 28 is welded to the sheath opposite the bore 27. The weld is made carefully so as to obtain a tight connection between the sheath and the coupling. The lower end 22 of the breather 8 is then introduced into this fitting, and is attached thereto.

A metal pipe 1 is then inserted into the sleeve 2. Like the sleeve 2, the pipe 1 is formed of a succession of pipe sections. Each section is a cylindrical tube of circular section, 12 to 15 m long and 800 mm in internal diameter. Each tube is made of steel (a mixture of iron and carbon) and is covered (at the factory) with an outer protective coating made of polyethylene (or possibly polypropylene). The polyethylene coating provides passive cathodic protection of the pipe. However, this coating is fragile and it is not uncommon that it is damaged punctually during the installation of the pipe or it deteriorates over time. In the absence of active cathodic protection of the pipe, the areas of the pipe without polyethylene coating eventually corrode, until they are pierced. In support of the passive protection provided by the coating, active cathodic protection makes it possible to overcome any defect in the coating and to avoid any corrosion of the pipe. First equip a first pipe section with polyethylene centering collars 5, which will allow the centering and the 2. Each collar comprises a ring 23, intended to be applied to the pipe section and to encircle it firmly, and pads 24 extending radially projecting from the ring, distributed all around the ring. - this. The different collars 5 are distributed along the pipe section, being spaced 2 m for example. They are fixed to said section by clamping their ring.

The first section of pipe is put into the sheath from the pit. The centering collars 5 carried by the section not only make it possible to keep the pipe section away from the sheath once the section has been put in place in the sheath, but also to avoid any scraping of the pipe section against the sheath (and therefore any deterioration of the polyethylene coating) during threading of said section into the sheath.

When the first pipe section is almost fully inserted into the sheath, is arranged in the extension of the first section, a second pipe section, which is welded to the first. We radiograph the weld to ensure its tightness. At the junction of the two pipe sections, where the polyethylene coating was destroyed, a polyethylene strip is applied so as to coat the weld and thus ensure continuity of the insulating coating of the pipe. The two sections thus assembled are axially pushed into the sheath until the second section is in turn almost fully inserted into the sheath. A third section of pipe is assembled to the second, as previously explained, and so on until the first section of pipe comes out of the sheath in the arrival pit. Thus, a pipe 1 (or rather a portion of the pipe 1 more widely shown in FIG. 1) is formed, the length of which is greater than that of the sleeve 2, so that it projects from the sleeve at each longitudinal end 15, 16 of this last.

In a variant, the pipe 1 is formed out of the excavation: a plurality of successive sections (welding, radiography, then application of a polyethylene strip) are assembled, as previously explained, until the pipe thus formed has a length greater than that of the sheath. The pipe is then threaded into the sheath. The breather 7, whose lower end 21 has previously been introduced into the bore 26 of the sheath, is then pushed down. Its lower end 21 is thus inserted into the annular space 3 and slid between the sheath and the pipe (on one side of said pipe), until it is positioned in the lower part of the sheath. The breather 7 is then fixed to the sheath by any appropriate means (since this means does not involve any risk of damaging the pipe in the sheath or its polyethylene coating). The piercing 26 is sealed by means of sealant so as to ensure a tight connection between the breather 7 and the sheath 2. The annular space 3 separating the pipe 1 from the sheath 2 is closed. To this end, at each end 15 ( respectively 16) of said sheath, is arranged a sealing ring 13 (respectively 12) bellows, rubber. Each closure ring 13 (respectively 12) has firstly a first portion whose diameter substantially corresponds to the outer diameter of the sleeve 2, which first portion is provided with a first clamping collar 18 (respectively 20) allowing the fasten firmly to the sheath. The closure ring has, on the other hand, a second portion whose diameter substantially corresponds to the external diameter of the pipe 1, which second portion is provided with a second clamping collar 17 (19 respectively) making it possible to fix it firmly to the 1. The closure ring finally comprises a third bellows portion of variable diameter, which connects the first and second portions. The sealing ring 13 (respectively 12) provides a seal between the pipe and the sleeve at the longitudinal end 15 (16 respectively) of the latter. A potential-measuring terminal 9 is installed on the surface, directly above the sleeve 2. The terminal comprises a first pair of connectors comprising, on the one hand, a connector connected to a reference electrode (copper electrode) placed in the ground and, on the other hand, a connector connected to the pipe 1 by a conductive cable 10 fixed on said pipe by brazing. The terminal 9 comprises a second pair of connectors comprising, on the one hand, a connector connected to a reference electrode and, on the other hand, a connector connected to the sleeve 2 by a conductive cable 11 fixed on said sleeve by brazing. The first pair of connectors makes it possible to measure the potential of the pipe by means of a voltmeter; the second pair of connectors makes it possible to measure the potential of the sleeve by means of a voltmeter. These measures make it possible in particular to verify that there is no direct contact between the sleeve and the pipe. It should be noted that other potential measuring terminals (or ports) are arranged along the path of the pipe 1. They are placed in accessible places and / or in special points (bushings, delivery station , insulating fittings ...) and are, as far as possible, regularly distributed (approximately every 2 km) along the pipe. In particular, a terminal 107 similar to the terminal 9 is placed in line with the sleeve 104, close to the railway line 102; terminals 106 and 108 are arranged near the rural roads 101 and 105. The terminals 106 and 108 differ from the terminals 9 and 107 in that they comprise a single pair of connectors, to raise the electrical potential of the pipe. All these terminals make it possible to control the potential of driving over the entire route, with a view to ensuring the effectiveness of active cathodic protection implemented.

The lands initially excavated are backfilled to make the two working pits, so as to fill said pits. The slurry according to the invention is prepared by mixing in a conventional mixer at room temperature water, a blast furnace slag, Portland clinker and bentonite. For 900 liters of water, 95 kg of blast furnace slag, 10 kg of Portland clinker and 145 kg of bentonite are added to the mixer. The quantities of water, blast furnace slag, Portland clinker and bentonite actually introduced into the mixer are calculated so as to obtain a volume of grout slightly greater than the volume formed by the annular space 3 and the breather 7 and 8 The order of introduction of these constituents is indifferent. The mixture obtained in the kneader has a viscosity sufficiently low to allow its injection. The grout is injected into the breather 7 by its upper end, by means of a usual pump. The injection continues until the annular space 3 is completely filled and the grout comes out of the upper end of the breather 8. Each breather is then closed at its upper end by a cover plate.

Line 1 is connected to a circuit 25 comprising a current rectifier 110 and a weir 111 (see FIG. 1) ₃ which constitute a current draw-off station. The rectifier 110 is connected to a mains power supply (low-voltage electrical line passing nearby). The spillway is formed by a buried anodic metal mass: for example, a recovery railroad track or a ferrosilicon or graphite bar is used. When the rectifier is powered, an electric current is generated. Between the pipe and the spillway, the current flows in the ground according to current curves 112, passes through the conducting sheath 2 (in the form of electrons), then flows (in the form of ions) through the consolidated grout between the sheath and driving. Positioned as shown in Figure 1, the filling station offers a large radius of action centered on the spillway 111. The position of the withdrawal station is also chosen according to the nature of the soil, which must have a low electrical resistivity. Moreover, to obtain both a large radius of action and effective protection, the spillway 111 is advantageously buried more than 50 m from the pipe and so as to extend orthogonally to said pipe. For a long pipe, it may be necessary to install several draw stations, spaced about 20 km.

FIG. 3 represents the iron Pourbaix diagram (pH / equilibrium potential of the reactions), which describes the various possible theoretical reactions between a metal and its ions in the presence of water. This diagram makes it possible to visualize three domains of situation of the metal, which depend on the pH of the surrounding solution and the electric potential of the iron:

a corrosion domain, where the iron dissolves in the solution and forms soluble salts and hydroxides, a passivation domain, where the iron is protected by a superficial film which isolates it from the solution. The iron is thus protected from corrosion, provided however that the film could have formed in a uniform manner, that it remains adhered and that it does not undergo any mechanical aggression, - a field of immunity, where the iron remains in the metallic state

(oxidation reactions are no longer possible) and can not corrode. This is the field of active cathodic protection.

The steel pipe 1 has, when directly buried in the ground, a potential of between -200 mV and -500 mV, measured with a copper reference electrode (Cu / CuSO ₄ ). It is therefore likely to corrode when its polyethylene coating has a defect and the soil pH is less than 6 (acid soil, very common especially in agricultural areas or industrial areas, where runoff water is acidified by the treatments imposed on crops or industrial waste). Inside the sheath 2, the pipe 1 is in contact with the consolidated grout 6. The latter having a basic pH (between 10 and 12), the conditions are met for the pipe to be passivated. Even before the introduction of active cathodic protection, the mere presence of grout thus reinforces the protection of the pipe by placing it in its passivation range. Energizing the rectifier 111 reduces the potential of the pipe to a value below -900 mV, generally of the order of -1200 mV. The conduct is then placed in its field of immunity. It is totally protected from corrosion, in the range of action of the filling station and including inside the sleeves 2 and 104. The invention may be subject to numerous variants with respect to the embodiment described above. and shown in the accompanying figures.

In particular, the invention also applies to a curved sheath (the pipe is also), the latter being implemented by means of a directional drilling machine. Moreover, the dimensions provided for the pipe and the sleeve illustrated are not limiting. The invention applies to any pipe for transporting water, gas, oil, etc., whose internal diameter generally varies between 100 mm and 1000 mm, the sleeves used having diameters ranging from 130 mm to 2000 mm. .

In addition, if the terrain surrounding the pipe has a low resistivity (less than 5000 Ω.m), the active cathodic protection can be achieved by means of a sacrificial anode, aluminum, magnesium or zinc. The anode is buried in the ground and is connected to the pipe by a conductive cable. A control terminal (or key fob) installed at the surface is preferably interposed between the sacrificial anode and the pipe. This method is preferably reserved for pipes of short length (for example 500 m).

Claims

1 / A method for installing and protecting a buried fluid transport pipe, said pipe (1) being made of an electrically conductive material, in which the pipe is placed in a sleeve (2) made of an electrically conductive material, which follows a longitudinal direction substantially parallel to a longitudinal direction of the pipe, the pipe and the sleeve defining between them an annular space (3) which extends mainly in these longitudinal directions, the annular space is closed at each longitudinal end (15 , 16) of the sheath by means of a ring (12, 13) called closure ring, the annular space is filled with a filling material, characterized in that:

- as a filler material, a hydraulic grout (6) comprising water is used and, for 900 liters of water, 85 to 105 kilograms of a blast furnace slag, 8 to 13 kilograms of Portland clinker and 120 to 160 kilograms of bentonite,

the pipe (1) is connected to an electrical circuit (25) comprising a current draw-off station (110, 111) or a sacrificial anode. 2 / A method according to claim 1, characterized in that a slurry comprising, for 900 liters of water, 95 kilograms of blast furnace slag, between 10 and 11 kilograms of Portland clinker and 145 kilograms of bentonite.

3 / A method according to one of claims 1 or 2, characterized in that a slurry exclusively consisting of water, slag and blast furnace, Portland clinker and bentonite.

4 / A method according to one of claims 1 to 3, characterized in that the slurry is prepared by mixing its various constituents at room temperature. 5 / A method according to one of claims 1 to 4, characterized in that a sheath (2) having a diameter greater than or equal to 1.5 times the diameter of the pipe.

6 / A method according to one of claims 1 to 5, characterized in that the grout injected into the annular space by means of a breather (7) arranged so that its lower end (21) opens in the lower part of a section of the sheath (2).