NO347041B1

NO347041B1 - Petroleum fluid-conveying flexible pipe comprising a barrier against diffusion

Info

Publication number: NO347041B1
Application number: NO20190755A
Authority: NO
Inventors: Frédéric Demanze; Xavier Lefebvre; Michael Martinez; Marie-Helene Klopffer; Jean-Marc Rousseau; Carol Taravel-Condat; Cécile Plennevaux
Original assignee: Technip France; Ifp Energies Now
Priority date: 2016-12-22
Filing date: 2017-12-11
Publication date: 2023-04-24
Also published as: MY198092A; GB201910039D0; BR112019012614B1; DK180841B1; AU2017384001B2; GB2572120B; AU2017384001A1; CN110177969A; FR3046208B1; FR3046208A1; NO20190755A1; DK201970447A1; GB2572120A; WO2018114418A1; BR112019012614A2

Description

PETROLEUM FLUID-CONVEYING FLEXIBLE PIPE COMPRISING A BARRIER AGAINST DIFFUSION

The present invention relates to a flexible tubular pipe for the transportation of petroleum fluid used in the field of the offshore oil production.

The flexible pipes targeted by the present invention are formed of an assembly of different concentric and superimposed layers and are said to be of unbonded type as these layers exhibit a degree of freedom of movement with respect one another. These flexible pipes satisfy, inter alia, the recommendations of the prescriptive documents API 17J, "Specification for Unbonded Flexible Pipe" (4th edition, May 2014), and API 17B, "Recommended Practice for Flexible Pipe" (5th edition, March 2014), which are published by the American Petroleum Institute. The constituent layers of the flexible pipes comprise in particular polymeric sheaths, generally providing a leaktightness function, and reinforcing layers intended to take up the mechanical forces and formed by windings of strips, of wires made of metal, of various bands or of profiled elements made of composite materials.

These flexible pipes are used in particular to transport hydrocarbons of oil or gas type from subsea equipment located on the seabed, for example a well head, to a floating production unit located at the surface. Such pipes can be deployed at great depth, commonly at a depth of more than 2000 m, and they thus have to be capable of withstanding a hydrostatic pressure of several hundred bar. In addition, they also have to be able to withstand the very high pressure of the hydrocarbons transported, it being possible for this pressure itself also to be several hundred bar.

When the flexible pipe is in service, it can be subject to high static and dynamic loads, which can result in a phenomenon of fatigue. The most severe loadings are generally observed in the upper part of the risers connecting the seabed to the surface. This is because, in this region, the flexible pipe is subjected to a high static tensile stress related to the weight of the pipe, to which are added dynamic tensile and transverse bending stresses related to the movements of the floating production unit under the effect of the swell and the waves. As regards the part of the flexible pipe stretching over the seabed (flowlines), the loads applied are essentially static.

The flexible pipes of unbonded type most widely used in the offshore oil industry generally comprise, from the inside toward the outside, an internal carcass consisting of a shaped strip made of stainless steel wound helically with a short pitch in turns which interlock with one another, said internal carcass acting mainly to prevent the flexible pipe from collapsing under the effect of the external pressure, an internal leaktightness sheath made of polymer, a pressure vault consisting of at least one metal wire of interlocked form helically wound with a short pitch, said pressure vault acting to take up the radial forces related to the internal pressure, tensile armor plies formed of helical windings with a long pitch of metal wires or composite threads, said tensile armor plies being intended to take up the longitudinal forces which the flexible pipe is subjected to, and finally an external leaktightness sheath intended to protect the reinforcing layers from seawater. In the present patent application, winding with a short pitch is understood to mean a winding having a helical angle, the absolute value of which is close to 90°, in practice between 70° and 90°. The term winding with a long pitch for its part denotes any winding, the helical angle of which is less than or equal to, in absolute value, 55°.

In the present patent application, the terms "internal leaktightness sheath" and "pressure sheath" have the same meaning and are used without distinction.

The internal carcass makes it possible for the flexible pipe to have a collapse resistance sufficient to allow it to withstand high external pressures, in particular the hydrostatic pressure when the flexible pipe is immersed at great depth (1000 m, indeed even 2000 m, or more), or also the external contact pressures sustained during offshore handling and installation operations. A flexible pipe comprising an internal carcass is said to be a "rough bore" pipe as the innermost element is the internal carcass which forms a rough bore due to the butt spaces between the metal turns of the interlocked strip.

The main function of the pressure vault is to make it possible for the internal leaktightness sheath to withstand, without bursting, the pressure exerted by the petroleum fluid transported by the pipe, the external face of the internal leaktightness sheath resting against the internal face of the pressure vault. The pressure vault also contributes to improving the collapse resistance of the internal carcass, in particular as it limits the possibilities of deformation of the internal carcass under the effect of the hydrostatic pressure.

The main function of the tensile armor plies is to take up the longitudinal forces, in particular those related to the suspended weight of the flexible pipe when the latter is installed on the seabed from a laying ship located at the surface. In the case of a riser permanently connecting an installation positioned on the seabed to an item of equipment floating at the surface, these longitudinal forces related to the suspended weight are exerted permanently. When the pipe is immersed at great depth, the longitudinal forces related to the suspended weight during the installation and/or in service can reach several hundred tonnes.

The petroleum fluid transported by the flexible pipe comprises the hydrocarbons extracted from some oilfields which can be extremely corrosive. This is the case in particular with multiphase hydrocarbons comprising high partial pressures of hydrogen sulfide (H2S), typically at least 2 bar, and/or of carbon dioxide (CO2), typically at least 5 bar, and additionally exhibiting a high concentration of chlorides, typically at least 50000 ppm. Such fluids are generally highly acidic (pH < 4.5). In addition, their temperature can exceed 90°C, indeed even 130°C.

The internal carcass is in direct contact with these corrosive fluids and thus has to be made of a material which is highly resistant to corrosion, for example a stainless steel. On the other hand, the pressure vault and the tensile armors are isolated from these fluids by virtue of the internal leaktightness sheath and are thus found in an environment which is markedly less corrosive than that of the internal carcass, as only some corrosive gases can slowly diffuse through the internal leaktightness sheath. Consequently, the pressure vault and the tensile armors can be made of carbon steel, which is markedly less expensive than the stainless steel used for the internal carcass.

Although the pressure sheath (internal leaktightness sheath) is leaktight to hydrocarbons and to the other fluids transported, such as water, small amounts of gas may slowly diffuse through the sheath, in particular when the temperature and the pressure are high. This phenomenon mainly relates to small-sized molecules, in particular water in the vapor phase, and the gases of carbon dioxide (CO2), of hydrogen sulfide (H2S) and of methane (CH4). Thus, when the petroleum fluid contains one or more of these gases, this or these gases may diffuse through the pressure sheath and accumulate in the annular space located between the pressure sheath and the external leaktightness sheath. The presence of these diffusion gases in the annular space can generate a medium which is corrosive for the metal wires of the pressure vault and of the tensile armors. These migration and corrosion mechanisms are accentuated in particular by the temperature and by the contents of acidic entities.

It is possible to anticipate the effect of the corrosion by a rigorous selection of the grades of steel used (sour service steel) and by steel thicknesses suitable for the pressure vault and for the tensile armors. It is known that a grade of steel for an acidic medium (sour service) is more expensive to buy then a grade of steel for a substantially neutral medium (sweet service) and results in a heavier flexible structure because, at an equal section, a grade of sour service steel has poorer mechanical properties than a sweet service steel.

The document EP 844429 provides for the introduction, into a sheath made of polymer material, of products which are chemically active with the acid compounds (H2S and/or CO2), so as to irreversibly neutralize the corrosive effects of said acid compounds and so as to prevent the corrosive effects on the metal parts of the pipe.

Other solutions provide for the use of continuous metal sheaths, the permeability to gases of which can be regarded as zero; however, the use of these flexible pipes can be very complicated and the flexibility constraint of the pipe results in complex solutions of the metal liner type generally exhibiting insufficient resistance over time.

The document US 4 903 735, which describes the winding of metal bands over a sheath made of polymer, the edges of the band of which are adhesively bonded together, is known. However, during the winding of the pipe over a reel, the adhesive bonds may not remain integrated, which provides leaks at the metal band. This is because a metal-metal contact is not leaktight, except in static configuration, after meticulous preparation of the surfaces.

The present invention intends to improve this principle by optimizing a diffusion-barrier layer.

The device according to the invention

The invention relates to a flexible pipe for transporting a petroleum effluent comprising water and at least one acid compound from carbon dioxide CO2 and hydrogen sulfide H2S, said pipe comprising at least one mechanical reinforcing metal element and a pressure sheath, said metal element being positioned on the outside of said pressure sheath, a diffusion barrier being positioned between said sheath for resistance to pressure and said mechanical reinforcing metal element, characterized in that said diffusion barrier comprises at least one metal band wound around said sheath for resistance to pressure so as to provide a metal overlapping over a distance of between 35% and 75% of the width of said metal band and in that a polymer material is inserted into this overlapping.

According to one embodiment, said metal band is covered, at least partially, with a layer of said polymer material on at least one face.

Alternatively, said diffusion barrier is formed by said metal band and by a band of said polymer material.

Advantageously, said band of said polymer material has a width substantially equal to the width of said metal band.

In accordance with one implementation, said metal band is made of stainless steel, made of carbon steel or made of alloy of nickel, of titanium or of aluminum.

Advantageously, said polymer material of said diffusion barrier is an elastomer or a thermoplastic polymer chosen from the following grades: NBR (butadiene/acrylonitrile copolymer), CR (polychloroprene, neoprene), EPDM (ethylene/propylene/diene monomer), CO (polychloromethyloxirane), TFE (tetrafluoroethylene), PU (polyurethane), silicone, PE (polyethylene), PA (polyamide) or PVDF (polyvinylidene fluoride).

According to one implementation option, said diffusion barrier comprises at least two layers of metal band wound flat, the second layer of metal band forming said overlapping.

Alternatively, said diffusion barrier comprises at least one layer of metal band wound in S fashion in order to form said overlapping.

In an alternative form, said diffusion barrier comprises at least one layer of metal band wound in "double S" fashion in order form said overlapping.

In accordance with one embodiment, said diffusion barrier has a width of between 25 and 150 mm.

According to one implementation of the invention, the layer of said polymer material of said diffusion barrier has a thickness of between 0.5 mm and 4 mm.

According to one embodiment, said metal band of said diffusion barrier has a thickness of between 0.2 mm and 2 mm.

In accordance with one implementation option, an elastomer layer is inserted between said diffusion barrier and said mechanical reinforcing metal element.

Brief presentation of the figures

Other characteristics and advantages of the process according to the invention will become apparent on reading the description below of nonlimiting implementational examples, with reference to the appended figures described below.

Figure 1 diagrammatically illustrates, in perspective, a flexible pipe according to the prior art.

Figures 2a, 2b and 2c illustrate, in section, along an axial plane, three embodiments of a flexible pipe according to the invention.

Figure 3 diagrammatically represents, in perspective, an embodiment of metal bands according to the invention.

Figure 4 shows a geometric scheme used for the theoretical evaluations of the effectiveness of the diffusion barrier according to the invention.

A flexible pipe according to the prior art is represented by figure 1. This pipe consists of several layers described below from the inside toward the outside of the pipe. The flexible pipe is of unbonded type and meets the specifications defined in the prescriptive document API 17J.

The internal carcass 1 consists of a metal band wound around a helix with a short pitch. It is intended for the resistance to collapse under the effect of the external pressure applied to the pipe.

The internal leaktightness sheath 2 is produced by extrusion of a polymer material, generally chosen from polyolefins, polyamides and fluoropolymers.

The pressure vault 3 made of interlocked or nestable metal wires provides the resistance to the internal pressure in the pipe.

The tensile armor plies 4 consist of metal wires helically wound along angles having an absolute value of between 20° and 55°. The pipe advantageously comprises two superimposed and crossed tensile armor plies 4, as represented in figure 1. For example, if the internal tensile armor ply is wound with a helical angle equal to 30°, the external tensile armor ply is wound with a helical angle equal to -30°. This angular symmetry makes it possible to balance the pipe in torsion, so as to reduce its tendency to revolve under the effect of a tensile force.

When the two superimposed and crossed tensile armor plies 4 are wound with a helical angle substantially equal to 55°, the pressure vault 3 can optionally be dispensed with as the helical angle of 55° confers, on the tensile armor plies 4, a good resistance to the internal pressure.

The external leaktightness sheath 5 made of polymer forms an external protection for the pipe.

The pipe represented by figure 1 is of the rough bore type, that is to say that the fluid moving through the pipe is in contact with the internal carcass 1.

Alternatively, the pipe can be of the smooth bore type. In this case, the pipe represented by figure 1 does not comprise an internal carcass 1. The polymer sheath 2 is directly in contact with the fluid moving through the pipe.

Detailed description of the invention

The flexible pipe according to the invention comprises at least one pressure sheath and at least one mechanical reinforcing metal element. In the present patent application, the term "mechanical reinforcing metal element" denotes any metal layer of the flexible pipe surrounding the pressure sheath, the function of which is to take up the mechanical forces to which the pipe is subjected. Thus, each tensile armor ply produced with metal armor wires constitutes a mechanical reinforcing metal element. In addition, the metal pressure vault itself also constitutes a mechanical reinforcing metal element. Furthermore, the flexible pipe according to the invention can advantageously comprise at least one of the other layers of the flexible pipe described with reference to figure 1, in particular an internal carcass, an external leaktightness sheath and/or other additional layers. Preferably, the flexible pipe according to the invention is of unbonded type and meets the specifications defined in the prescriptive document API 17J.

According to the present invention, the aqueous acidity within the mechanical reinforcing metal element is limited by positioning a diffusion barrier over the pressure sheath in order to limit the consequences of the relative permeability of said pressure sheath to acid gases. The diffusion barrier is positioned between the pressure sheath and the mechanical reinforcing element.

According to the invention, the diffusion barrier comprises at least one metal band wound around the pressure sheath so to provide a metal overlapping. Metal overlapping refers to the fact that one turn of the winding of the metal band F is superimposed on the preceding turn: two consecutive turns of the winding of the metal strip overlap. The fundamental principle of the diffusion barrier according to the invention is based on the interaction of an assemblage between several materials, each material having a specific function. A metal material, namely the metal band, is used for its virtually complete gastightness (for example stainless steel) and a polymer is used for its flexibility and its limited permeability. The term of polymers should be understood as meaning thermoplastic polymers and elastomers. The presence of a polymer in the overlapping, after the fashion of a seal, makes it possible to exert better control over the leaktightness (metal/polymer leaktightness).

Advantageously, the diffusion barrier can be formed directly on the external surface of the pressure sheath of the flexible pipe. With this aim, a metal layer is wound over the pressure sheath so as to retain a certain flexibility in the flexible pipe but with a sufficiently broad overlapping to create a masking of a part of the diffusion surface (and thus to reduce the flow rates towards the annular). The tortuosity created has the effect of increasing the length of the diffusion pathway and thus of reducing the flow rates. The empty spaces of this tortuosity are filled in with the polymer material, which is preferably weakly permeable.

Advantageously, the metal band F is wound with a helical angle having an absolute value of greater than 55°, preferably of greater than 70°.

According to a first embodiment of the invention, the diffusion barrier can comprise a metal band (also known as metal strip) covered at least partially with polymer on at least one of the faces of the metal band. Thus, the polymer is directly attached to the metal band. Such a metal band, at least partially covered with polymer, is denoted by the term "multilayer band" (preferably bi- or trilayers). In this way, the assemblage can take the form of a multilayer band which, for example, can be helically wound with overlapping around the pressure sheath of the flexible pipe during the manufacture. This embodiment makes it possible to facilitate the manufacture of the flexible pipe.

Several production techniques can be envisaged in order to provide a predetermined adhesion between a polymer and a metal strip:

- adhesive bonding (only for certain polymers/elastomers). This solution exhibits the advantage of being relatively inexpensive.

- "adherization"; this technique consists in using the polymer/elastomer directly on the steel strip. The adhesion is both chemical and physical, which ensures a better resistance and durability.

According to a second embodiment of the invention, the diffusion barrier can comprise a distinct metal strip and a distinct polymer band. These two bands are not assembled (they are not adhesively bonded or do not adhere to one another before they are added to the polymer sheath for resistance to pressure). The polymer band is arranged at least in the region of overlapping of the metal band. Advantageously, the width of the polymer band is substantially equal to the width of the metal band. Thus, it is possible to carry out the simultaneous positioning of the bands and in this way to facilitate the manufacture of the flexible pipe. Alternatively, the two bands (with or without a substantially equal width) can be wound independently. Preferably, the sufficient adhesion (the absence of relative movement) between each band can be provided by the laying tension during the manufacture and by the internal pressure in service.

According to the invention, the metal overlapping provided by the winding of the metal band F is produced so that the overlapping distance (along the longitudinal direction of the flexible pipe) is between 35% and 75% of the width of the metal band, in order to produce the property of the diffusion barrier. Thus, the positioning of the metal band is produced by the preceding winding of the metal band over a distance representing from 35% to 75% of the width of the metal band. The degree of overlapping can be chosen as a function of the width of the metal band. Above 75%, the thickness of the diffusion barrier will be too great (need for four layers of metal bands). Advantageously, in order to enhance the diffusion barrier property, the metal overlapping distance is greater than 40% of the width of the metal band, so that the masking is as great as possible. Preferably, in order to optimize this diffusion barrier property, the metal overlapping distance is between 45% and 55% of the width of the metal band. This is because a degree of overlapping of approximately 50% is close to the optimum for maximizing the diffusion barrier property.

Several embodiments can be envisaged for producing the diffusion barrier. Among these embodiments, mention may in particular be made of flat overlapping, S overlapping and "double S" overlapping (for this overlapping, the metal strip forms three substantially flat levels separated by two curved parts). Figures 2a to 2c diagrammatically show, along a section of a flexible pipe, and nonlimiting, these examples of implementations of the diffusion barrier according to the invention. In these figures, the elements similar to the elements of the flexible pipe of the prior art illustrated in figure 1 carry the same references.

A metal internal carcass 1 is arranged at the center of the pipe. This metal internal carcass 1 is covered with a pressure sheath 2. A diffusion barrier B is provided on this pressure sheath 2. Within this diffusion barrier B, metal strips F are wound with a polymer material P inserted between the overlapping of the metal bands. A metal pressure vault 3, metal tensile armor plies 4 and an external leaktightness sheath 5 made of polymer are arranged outside the diffusion barrier B. The reference T identifies the length of the overlapping or length of tortuosity filled with polymer material.

This winding can, for example, be produced flat in the form of two superimposed layers (metal strip and polymer) with longitudinal overlapping (figure 2a). The two layers have a substantially identical width. For this implementation, the overlapping is substantially equal to 50%.

According to figure 2b, it is also possible to produce an S positioning with longitudinal overlapping. For this configuration, a part of the metal band F directly overlaps the pressure sheath 2 and another part overlaps another winding of the metal strip F. For this implementation, the overlapping is substantially equal to 50%.

According to the embodiment of figure 2c, the winding is carried out according to a "double S" shape; this implementation relates to a metal overlapping of greater than 50% of the width of the band. In this way, three metal bands are superimposed. For this implementation, the overlapping is substantially equal to 66%.

The pipes represented in figures 2a, 2b and 2c are of the rough bore type, that is to say that the fluid moving through the pipe is in contact with the internal carcass 1.

Alternatively, the pipes can be of the smooth bore type. In this case, the pipes respectively represented by figures 2a, 2b and 2c do not comprise an internal carcass 1. The polymer sheath 2 is directly in contact with the fluid moving through the pipe.

The presence of a polymer in the tortuosity, after the fashion of a seal, makes it possible to optimize the leaktightness (metal/polymer leaktightness) of the diffusion barrier B. For this, the metal band F is preferably positioned with a degree of tension in order to provide contact between the different layers and with the pressure sheath 2 under all the conditions of use of the flexible pipe, whether in production or in shutdown. In service, the pressure internal to the flexible pipe contributes even more to providing the confinement of the layers.

The diffusion barrier B around the pressure sheath 2 can be manufactured in different ways. Preferably, it can be manufactured by helical winding of a metal strip F at least partially covered with a layer of polymer P on at least one face, that is to say by the winding of a multilayer band.

The metal band can exhibit different geometries. Figure 3 illustrates, diagrammatically and nonlimitingly, the winding of a metal band F over a sheath 2. The metal band F is covered with a polymer (not represented).

The final geometry of the tortuosity thus depends on the form of positioning (flat, S, double S or other) and on the geometry of the metal layer.

In accordance with one alternative embodiment of the invention, it is possible to envisage a method of manufacturing the diffusion barrier from bare (uncoated) metal bands. A first layer of metal band (strip made of steel) is wound around the flexible pipe and the polymer is extruded directly over the metal strip, the diffusion barrier is then formed by addition of a second band of strip made of steel with sufficient overlapping to form the tortuosity and, in the case of an elastomer, the crosslinking can be carried out after the positioning of the metal bands. This embodiment is more difficult to control in manufacture but exhibits the advantage of being able to produce a continuous elastomer or polymer sheath, without space between each metal band, which is entirely embedded in the polymer.

In accordance with one embodiment of the invention, the thickness of the metal band of the diffusion barrier can be between 0.2 and 2 mm. Such a range of thickness makes it possible to correctly provide the function of barrier to the diffusion of acid gases and makes it possible to carry out the positioning with a machine of relatively low power (for example a taping machine). In addition, such a thickness makes it possible to limit the weight and the cost of the flexible pipe.

In accordance with one embodiment of the invention, the thickness of the polymer layer (bonded or not bonded adhesively to the metal band) of the diffusion barrier can be between 0.5 and 4 mm. This thickness can be chosen according to the width of the polymer layer. In particular, it is possible to choose a thickness/width ratio of the polymer layer so as to guarantee the tortuosity of the diffusion barrier. Such a polymer thickness makes it possible to provide the effect related to the tortuosity (role of diffusion barrier) while guaranteeing the leaktightness.

Advantageously, the width of the metal band of the diffusion barrier is between 25 and 150 mm. Thus, it is possible to use the current production means used for the other layers of the flexible pipe.

A study by finished elements of numerous theoretical configurations has made it possible to determine the impact of the main geometric and physical parameters of the diffusion barrier on the reduction in permeability: impact of the width of the metal layer, of the thickness of the tortuosity, of its length, of its position, of its permeability, of the space between two successive bands, and the like. In the majority of cases, the reduction in permeability is high: by a factor of 10 to 10<6 >according to the configuration and the materials used. The choice of the best configuration is always a compromise which takes into account at least: the mode of use of the flexible pipe, the pressure and temperature conditions, the dimensions of the flexible pipe, the mechanical behavior of the flexible pipe and the costs.

First calculations were carried out starting from a theoretical diffusion barrier geometry according to the scheme of figure 4. A polymer matrix with a thickness of 4 mm, in which impermeable metal bands of length L are embedded, which bands are separated by a thickness e_T of polymer, are considered. The overlapping is in this instance of the order of 50%. The flow rate of a gas through this assemblage, Q, is compared with the flow rate of the same gas in the same thickness of polymer, Q0, without the metal sheets. The ratio Q/Q0 represents the reduction factor for the permeability of the assemblage and makes it possible to directly quantify the barrier effect of an assemblage. The smaller this ratio, the greater the reduction in permeability and the greater the barrier effect. The calculations carried out with regard to this geometry made it possible to determine the change in the barrier property according to the width of the strip and the thickness of the tortuosity, for a given degree of overlapping. It is also possible to vary the degree of overlapping and it has been shown, by calculation, that a degree of overlapping of approximately 50% is close to the optimum for maximizing the barrier property, everything otherwise being equal. However, in view of the other mechanical, manufacturing or cost constraints, it is possible to consider that, for a petroleum flexible application, a metal band width L of between 25 mm and approximately 150 mm is suitable, and the overlapping can be between 35% and 75%, preferably greater than 40%, although 50% is preferably the best compromise.

The main results of the calculations relate to the positioning of a diffusion barrier comprising several bimaterial bands (metal band coated with polymer according to the first embodiment of the invention), with a width of 100 mm, positioned flat or in S shape over a pressure sheath made of polymer (polyamide, PA) with a thickness of 5 mm. The calculations of flow rate Q with regard to these assemblages were compared with the flow rate Q0 in a pressure sheath made of polymer (PA) with the same total thickness (and without diffusion barrier according to the invention), i.e. 7 mm or 9 mm according to the thickness of barrier under consideration. For example, a diffusion barrier according to the invention composed of HNBR (hydrogenated acrylonitrile/butadiene rubber) and of stainless steel positioned flat in the form of two layers of 1 mm on a pressure sheath of 5 mm of PA (polymer) has a flow rate 40 times lower than the same total thickness of PA (7 mm in this case).

Several items of information can be deduced from the calculations:

- the flat positioning appears to be more effective from a barrier viewpoint than the positioning in S shape, everything otherwise being equal;

- the positioning of four strips of 1 mm appears to be more effective than the positioning of two strips of 2 mm, everything otherwise being equal;

- the properties of the material filling the tortuosity strongly impact the reduction factor for the flow rate. If HNBR (elastomer) is replaced with PA (thermoplastic), the reduction factor changes from 22 to 165. In the calculations, the permeability of PA is 14 times lower than that of HNBR.

The combined calculations demonstrate the importance of the selection of the material present in the tortuosity. This material should have the lowest possible permeability while being capable of being significantly deformed elastically at each fatigue cycle. This is why the materials will preferably be chosen from elastomers. Furthermore, one criterion for choice of the polymer material can be its compatibility with the material of the polymer sheath.

This is because the repeated bendings of the flexible pipe, in the case in particular where the latter is used as riser and is for this reason subjected to numerous bending cycles, preferably cannot generate significant deformations in the metal bands. The deformations should remain less than 0.2%, preferably 0.1%, in order not to plasticize the metal and thus greatly reduce its lifetime. In some cases, the metal bands can accommodate the bending of the flexible pipe by "sliding" without being deformed or virtually without being deformed. The bands are then chosen to be thick enough to have sufficient stiffness. The sliding can either be true sliding, but this can generate wear and require lubrication, or be accommodated by a flexible intermediate layer consisting of a polymer material capable of being sheared without generating significant forces in the metal band or the substrate. This function is carried out by the polymer material inserted in the metal overlapping provided by the winding of the metal band.

In addition, it can be envisaged for an elastomer layer to be inserted between the pressure sheath and the metal band in order to prevent direct sliding between the two elements. In other cases, if the metal band is formed of a metal material of the superelastic type, this "sliding" is limited, indeed even prevented.

The polymer material can preferably also withstand the openings and closings between two metal bands, which will generate significant localized deformations in the elastomer.

A mechanical calculation makes it possible to define the optimum thicknesses of the metal bands and elastomer and the clearance between two bands. It will also make it possible to direct the choice of the elastomer.

The application targeted preferably takes place at the pipes for transportation of the petroleum effluent produced (flowlines and risers). The loadings to be taken into account on the pipe are:

- The loadings in manufacture and installation, characterized by high curvatures, low contact pressures, ambient temperature (-15°C to 50°C), a low number of bending cycles (a few thousand).

- The loading in production. It is limited to high contact pressures, high temperatures with low variations in the contact pressures and in temperature over time.

In the case of the application extended to risers, the dynamic loadings in production specific to risers, which can be low curvatures, high contact pressures, temperature, large number of bending cycles (several million), can be taken into account.

The polymer material present in the tortuosity can exhibit precise characteristics in terms of:

- Permeability;

- Swelling under the effect of the solubility of the gases and of the water;

- Mechanical properties (hardness, elastic deformation, fatigue, creep);

- Durability in the presence of the chemical compounds present in the annular.

The methods for determining these characteristics are on the whole fairly conventional: tests of permeability to the different gases and liquids, agings in representative atmospheres, mechanical tests (tensile, compression, creep, fatigue, hardness, and the like).

It should be remembered that the polymer of the diffusion barrier can be an elastomer or a thermoplastic polymer. For example, the characterization of the permeability of several families of elastomers shows that the Viton® products (The Chemours Company, USA) and the Butyl products are included among the least permeable grades and are thus suitable for the diffusion barrier according to the invention. It is found that the permeability decreases with the increase in the hardness. There is thus certainly a compromise to be found between permeability (the lowest possible) and mechanical properties (sufficient flexibility in shearing, fatigue strength). The price of these materials can also be a factor of choice. Mention may be made, inter alia, of: HNBR 80 or 90 Shore, Aflas 80 Shore ® (Seal and design, USA), Viton 75 or 80 Shore ® (The Chemours Company, USA), Hypalon 60 Shore ® (DuPont, USA), Butyl 60 Shore, Coflon XD ® (Technip, France).

In addition, the polymer material of the diffusion barrier can be chosen from the following grades: NBR (butadiene/acrylonitrile copolymers), CR (polychloroprene, neoprene), EPDM (ethylene/propylene/diene monomer), CO (polychloromethyloxirane), TFE (tetrafluoroethylene), PU (polyurethane), silicone, PE (polyethylene), PA (polyamide) or PVDF (polyvinylidene fluoride).

In accordance with a first implementational example of the invention, the polymer material used to produce the diffusion barrier of a flexible riser typically subjected to static and dynamic stresses can be chosen from elastomers, due to their good mechanical properties for shearing.

In accordance with a second implementation of the invention, the polymer material used to produce the diffusion barrier of a flexible pipe stretching over the seabed (flowline) typically subjected to static stresses can be chosen from polyamides, which are stiffer, making it possible to limit the sliding, and less expensive.

The specification of the metal strip (metal band of the diffusion barrier) can take into account the corrosiveness of the annular medium and the use of a stainless steel may not be sufficient. The corrosion resistance of numerous metal materials (stainless steels and nickel/titanium/aluminum alloys, and the like) is studied and the determination of the best grade of steel to use as a function of the operating conditions can be chosen from stainless steels, carbon steels, Duplex, Super-Duplex, Monel, Inconel® (Special Metals Corporation, USA), Hastelloy, or equivalent. Advantageously, the metal band can be made of characteristic low carbon steels exhibiting a good corrosion resistance. This is because the corrosion of this type of steel exists in the form of a generalized corrosion, generally slower than the pitting corrosion formed in some steels.

Claims

1. A flexible pipe for transporting a petroleum effluent comprising water and at least one acid compound from carbon dioxide CO2 and hydrogen sulfide H2S, said pipe comprising at least one mechanical reinforcing metal element (3, 5) and a pressure sheath (2), said metal element (3, 5) being positioned on the outside of said pressure sheath (2), a diffusion barrier (B) being positioned between said sheath for resistance to pressure (2) and said mechanical reinforcing metal element (3, 5), characterized in that said diffusion barrier (B) comprises at least one metal band (F) wound around said sheath for resistance to pressure (2) so as to provide a metal overlapping over a distance of between 35% and 75% of the width of said metal band (F) and in that a polymer material (P) is inserted into this overlapping.

2. The pipe as claimed in claim 1, in which said metal band (F) is covered, at least partially, with a layer of said polymer material (P) on at least one face.

3. The pipe as claimed in claim 1, in which said diffusion barrier (B) is formed by said metal band (F) and by a band of said polymer material (P).

4. The pipe as claimed in claim 3, in which said band of said polymer material (P) has a width substantially equal to the width of said metal band (F).

5. The pipe as claimed in one of the preceding claims, in which said metal band (F) is made of stainless steel, made of carbon steel or made of alloy of nickel, of titanium or of aluminum.

6. The pipe as claimed in one of the preceding claims, in which said polymer material (P) of said diffusion barrier (B) is an elastomer or a thermoplastic polymer chosen from the following grades: NBR (butadiene/acrylonitrile copolymer), CR (polychloroprene, neoprene), EPDM (ethylene/propylene/diene monomer), CO (polychloromethyloxirane), TFE (tetrafluoroethylene), PU (polyurethane), silicone, PE (polyethylene), PA (polyamide) or PVDF (polyvinylidene fluoride).

7. The pipe as claimed in one of the preceding claims, in which said diffusion barrier (B) comprises at least two layers of metal band (F) wound flat, the second layer of metal band (F) forming said overlapping.

8. The pipe as claimed in one of claims 1 to 6, in which said diffusion barrier (B) comprises at least one layer of metal band (F) wound in S fashion in order to form said overlapping.

9. The pipe as claimed in one of claims 1 to 6, in which said diffusion barrier (B) comprises at least one layer of metal band (F) wound in "double S" fashion in order to form said overlapping.

10. The pipe as claimed in one of the preceding claims, in which said diffusion barrier (B) has a width of between 25 and 150 mm.

11. The pipe as claimed in one of the preceding claims, in which the layer of said polymer material (P) of said diffusion barrier has a thickness of between 0.5 mm and 4 mm.

12. The pipe as claimed in one of the preceding claims, in which said metal band (F) of said diffusion barrier (B) has a thickness of between 0.2 mm and 2 mm.

13. The pipe as claimed in one of the claims, in which an elastomer layer is inserted between said diffusion barrier (B) and said mechanical reinforcing metal element (3, 5).