DK178465B1

DK178465B1 - Flexible tubular pipe for transporting corrosive hydrocarbons

Info

Publication number: DK178465B1
Application number: DK201470596A
Authority: DK
Inventors: Thomas Epsztein; Carol Taravel-Condat
Original assignee: Technip France
Priority date: 2012-03-01
Filing date: 2014-09-29
Publication date: 2016-04-04
Also published as: BR112014021437A2; WO2013128097A1; BR112014021437B1; BR112014021437B8; GB2517842A; GB2517842B; FR2987666A1; DK201470596A; FR2987666B1; GB201414924D0

Abstract

A flexible tubular pipe (1) of the unbonded type intended for transporting fluids in the field of offshore oil production, comprising at least, from the inside outward, a pressure sheath (3), an intermediate sheath (4), tensile armor layers (6, 7) and an external sheath (8), characterized in that the water permeance of said intermediate sheath (4) is at least two times smaller than the water permeance of said pressure sheath (3), and in that the hydrogen sulfide permeance of said intermediate sheath (4) is at least two times greater than the hydrogen sulfide permeance of said pressure sheath (3), and in that the carbon dioxide permeance of said intermediate sheath (4) is at least two times greater than the carbon dioxide permeance of said pressure sheath (3), and in that the methane permeance of said intermediate sheath (4) is at least two times greater than the methane permeance of said pressure sheath (3).

Description

Flexible tubular pipe for transporting corrosive hydrocarbons

The present invention relates to a subsea flexible pipe for transporting hydrocarbons that comprise corrosive agents, especially water and acid gases.

The present invention relates more particularly to flexible pipes of the unbonded type, as described in the normative documents API 17J “Specification for Unbonded Flexible Pipe” and API RB 17B “Recommended Practice for Flexi

These flexible pipes usually comprise, from the inside outward, an internal carcass, an internal sealing sheath, a pressure vault, several tensile armor layers and an external protective sheath.

The main role of the internal carcass is to take up radial buckling forces, for example those linked to the hydrostatic pressure. It is made from a profiled stainless steel strip that is wound in a short pitch in order to form touching turns that are interlocked with one another. The internal sealing sheath, generally referred to as a pressure sheath, is an extruded polymer sheath, the role of which is to confine the fluid flowing in the pipe. As regards the pressure vault, it is generally formed of a metal wire wound in a short pitch with touching turns around the internal sealing sheath. It thus makes it possible to take up the radial forces linked to the pressure of the fluid flowing in the pipe. The role of the tensile armor layers is to take up the tensile forces that are exerted on the pipe. These layers consist of armor wires, which are generally metal wires, wound helically in a long pitch around the pressure vault. Finally, an external protective sheath made of polymeric material is extruded around the tensile armor layers.

In the present application, the term “in a short pitch” characterizes the helical windings that have a helix angle with an absolute value of between 70° and 90°. The term “in a long pitch” itself characterizes helical windings that have a helix angle with an absolute value of less than or equal to 60°. The internal carcass and the pressure vault have helix angles with an absolute value that is generally close to 85°. The flexible pipes generally comprise one or two pairs of crossed tensile armor layers, said layers having a helix angle with an absolute value typically of between 20° and 60°, and advantageously between 25° and 55°.

Although the internal pressure sheath is leaktight with respect to hydrocarbons and the other fluids transported such as water, small amounts of gas may slowly diffuse through the latter, especially when the temperature and pressure are high. This phenomenon mainly concerns molecules of small size, especially water in the vapor phase, and carbon dioxide (C02), hydrogen sulfide (H2S) and methane (CH4) gases. Thus, when the hydrocarbon contains one or more of these gases, the latter may diffuse through the pressure sheath and accumulate in the annular space located between the pressure sheath and the external protective sheath. The presence of these diffusion gases in the annular space may generate a corrosive medium for the metal wires of the pressure vault and of the tensile armor layers.

Moreover, when the pipe is in service, these metal wires may be subjected to high static and dynamic loads, which may bring about a fatigue phenomenon. The most severe loadings are generally observed in the upper part of risers connecting subsea equipment located at great depth to a floating production support located at the surface. Specifically, in this zone, the tensile armor layers are subjected to a high static tensile stress linked to the weight of the pipe, to which are added dynamic transverse bending and tensile stresses linked to the movements of the production unit under the effect of swell and waves.

The wires of the pressure vault and of the tensile armor layers must therefore be designed in order to lastingly withstand the phenomena of corrosion and hydrogen embrittlement under applied stress, it being possible for said stress to be static or dynamic depending on the loading conditions.

Furthermore, for applications at great depth, it is necessary for the tensile armor layers to have high mechanical characteristics, otherwise the structure, made heavy by its great length, proves difficult to install and requires an oversized floating production support compared to conventional supports, which leads to very large additional costs.

Generally however, in the case where these armor layers are made of carbon steel or low-alloy steel, the increase in the mechanical characteristics takes place at the expense of the corrosion resistance, which makes it difficult to develop a flexible pipe intended for operating at very great depth, 2000 m and beyond, and that can withstand corrosive hydrocarbons.

In order to solve this problem, solutions are known that consist in producing the tensile armor layers and/or the pressure vault with special stainless steels. These solutions, which are in particular disclosed in patent applications EP 2 228 578 and WO 2010/128238, however have the drawback of being expensive to implement.

In addition, solutions are also known that consist in covering the armor wires with a coating that protects them from corrosion. Thus, document WO 03/074206 discloses a flexible pipe comprising carbon steel armor layers covered with an anti-corrosion coating made from titanium, titanium alloy, stainless steel, nickel or else nickel alloy. However, the process for producing these plated wires is complex and costly.

Furthermore, solutions have also been devised that consist in draining the water vapor and the corrosive gases present in the annular space toward one of the two end fittings of the pipe, then in discharging them to the outside or to the inside. These solutions, which are in particular described in patent applications WO 2005/019715 and WO 00/17479, use pumping means and optionally means for injecting an inert gas of nitrogen type inside the annular space. These solutions are however complex to implement.

Solutions are also known that consist in filling all or part of the annular space with a dewatering fluid, for example based on glycol and methanol, and/or with a corrosion inhibitor fluid. Such solutions are in particular described in patent application WO 2011/026801, and also in the article entitled “Technical solutions applied for the treatment of damaged dynamic risers” written by T.S. Taylor, M.W. Joosten and F. Smith, and published under the reference OMAE2002-28371 in the Proceedings of the 21st International Conference on Offshore Mechanics and Arctic Engineering, June 23-28, 2002, Oslo, Norway. These solutions themselves also have the drawback of being difficult to implement.

Patent applications EP 844 429 and WO 2009/153451 describe solutions that consist in introducing, into the pressure sheath made of polymer material, products that are chemically active with H2S and/or C02 so as to neutralize these corrosive gases during their diffusion through said sheath, so that they do not reach the annular space. These solutions are complex and have a limited lifetime, insofar as the barrier effect against diffusion stops as soon as all of the chemically active products have been consumed by reaction with the corrosive gases.

Patent application WO 2005/028198 discloses a solution that consists in adding to the pipe an intermediate layer that is both flexible and not very permeable to gases so as to reduce the diffusion of gases toward the annular space. This intermediate layer located beneath the pressure vault consists of a thin metal film that has been adhesively bonded to an extruded sheath made of polymer material. The adhesive bonding between this film and this sheath makes it possible to prevent the formation of gas pockets at their common interface, it being possible for the presence of such gas pockets to have a detrimental effect on the integrity of the inner layers of the pipe, especially during shutdown and rapid decompression phases (risk of collapse of the internal carcass and of the pressure sheath). However, due to the low ductility of the film, this pipe cannot be wound with a small radius of curvature, otherwise the adhesive bonding between the film and the sheath would be likely to break, which would then promote the formation of gas pockets. The same problem could well also occur in the dynamic zones where the pipe experiences high bending stresses, especially in the upper part of a riser.

US 2011/0232798 A1 discloses a flexible tubular pipe of the unbound type intended for transporting fluids in the field of offshore oil production comprising at least from the inside outward a pressure sheath capable of forming a barrier against outflow of a fluid which is conveyed through the pipe, an intermediate sheath, tensile armor layers and an external sheath.

US 2011/0120583 A1 discloses a flexible tubular pipe intended for carrying a petroleum effluent comprising at least one of the acid compounds C02 and H2S. The pipe comprises at least one metal element and a tubular sheath made of a polymer material, the metal element being provided outside the sheath. The sheath consists of a mixture of a polymer material with a predetermined amount of products chemically active with said acid compounds so as to irreversibly neutralize the corrosive effects of said compounds and to limit the corrosive effects on said metal elements.

Therefore, one problem that the present invention faces and aims to solve is to provide an inexpensive flexible pipe that is easy to manufacture and that can transport corrosive hydrocarbons on a long-term basis while being subjected to high static and dynamic loads.

For the purpose of solving this problem, the present invention proposes a flexible tubular pipe of the unbonded type intended for transporting fluids in the field of offshore oil production, said pipe comprising at least, from the inside outward, a pressure sheath, an intermediate sheath, tensile armor layers and an external sheath, characterized in that the water permeance of said intermediate sheath is at least two times smaller than the water permeance of said pressure sheath, and in that the hydrogen sulfide permeance of said intermediate sheath is at least two times greater than the hydrogen sulfide permeance of said pressure sheath, and in that the carbon dioxide permeance of said intermediate sheath is at least two times greater than the carbon dioxide permeance of said pressure sheath, and in that the methane permeance of said intermediate sheath is at least two times greater than the methane permeance of said pressure sheath.

The permeance of a polymeric sheath to a molecule is equal to the ratio between, on the one hand, the permeability coefficient of said sheath with respect to said molecule and, on the other hand, the thickness of said sheath. It characterizes the ability of this sheath to be passed through radially by this molecule. All other conditions having been set moreover, in steady-state, the mass flow of a stream of molecules passing radially through said sheath is substantially proportional to the permeance of said sheath to this molecule.

For the implementation of the present invention, the permeability coefficients with respect to water, hydrogen sulfide, carbon dioxide and methane are measured in a laboratory on samples of sheath with the equipment described below. The conditions for measuring the permeability coefficients, namely the temperature and pressure, are themselves also defined below.

The feature according to which the intermediate sheath has a water permeance at least two times smaller than the water permeance of the pressure sheath has the technical effect of greatly limiting the diffusion of water toward the annular space located between the intermediate sheath and the external sheath, so that the partial pressure of water that has diffused into the annular space remains below the saturation vapor pressure on a long-term basis. Thus, the addition of such an intermediate sheath makes it possible to prevent the phenomenon of condensation of the water that has diffused, and therefore to avert the corrosion of the tensile armor layers. Specifically, corrosion in the annular space only occurs in the presence of water in liquid form. Whilst the water remains in the vapor phase, the corrosion remains negligible, even in the presence of gases of the H2S and/or C02 and/or CH4 type.

In this way, the invention makes it possible in particular to solve the problem of corrosion fatigue in the upper part of dynamic risers. This phenomenon, which occurs when the wires are fatigue-stressed in the presence of a corrosive medium comprising water in liquid form, determines to a large extent the characteristics of the tensile armor layers especially in the case of risers intended to be operated at great depth (2000 m and beyond). By preventing this phenomenon, the present invention therefore makes it possible to push back the usage limits of carbon steel wires.

The features relating to the H2S, C02 and CH4 permeances have the technical effect of preventing an accumulation of these gases at the interface between the pressure sheath and the intermediate sheath. Specifically, given that the permeance of the intermediate sheath with respect to these gases is significantly greater than that of the pressure sheath, it follows therefrom that such gases that have already diffused through the pressure sheath will then readily diffuse through the intermediate sheath in order to reach the annular space located between the intermediate sheath and the outer sheath. These gases may then, if necessary, flow along the pipe in order to reach one of the two end fittings of the pipe in order to be discharged to the outside. Specifically, due to the small helix angle of the tensile armor layers, the pressure drops are low and the gases can therefore easily flow along the annular space. Furthermore, means for discharging these gases to the outside may be easily introduced at the end fittings of the pipe, it being possible for these means to furthermore be equipped with valves so as to limit and control the pressure of the gases in the annular space.

In addition, according to one preferred embodiment of the invention, the intermediate sheath is in contact with the pressure sheath. According to this embodiment, the intermediate sheath is extruded directly onto the pressure sheath. These two sheaths are not adhesively bonded to one another, since the flexible pipe is of unbonded type. This absence of adhesive bonding between these two sheaths has the advantage of facilitating the manufacture of the pipe and of increasing its reliability in service. This makes it possible to avoid having to add an intermediate bonding layer. This also makes it possible to avoid the coextrusion process, a process that can indeed be envisaged but that is complex to implement in order to produce sheaths of large diameter. The two sheaths are advantageously extruded onto one another, in two successive steps, with conventional extrusion means. Furthermore, the particular permeance properties of these two sheaths make it possible to prevent the formation of gas pockets at the interface between these two sheaths, without it being necessary to adhesively bond them together. Thus, the invention is not dependent on the quality of adhesive bonding and its service life, which gives it a better reliability than that of the solutions from the prior art which use an adhesive bonding to prevent the formation of gas pockets.

The concept of the present invention is that of a selective screen with respect to permeation phenomena, so as to greatly limit the diffusion of water in order to prevent the condensation thereof in the annular space, while allowing H2S, C02 and CH4 to diffuse readily toward the annular space. Indeed, these gases frequently have high partial pressures in hydrocarbons, and it is therefore simpler to facilitate their diffusion and then to discharge them at the end fittings than to try to confine them while taking the risk of gas pockets forming. On the other hand, in these same hydrocarbons, water is either in the liquid phase with a very low saturation vapor pressure, or in the vapor phase with a partial pressure that is generally significantly lower than those of the other gases, so much so that in practice the partial pressure of the water vapor at the interface between the pressure sheath and the intermediate sheath remains low and far-removed from the risk zone.

Furthermore, the thickness of the intermediate sheath is preferably greater than or equal to 3 mm. Thus, this sheath may be easily extruded with conventional means.

Moreover, advantageously, the intermediate sheath consists of polyethylene. This material has a permeability coefficient with respect to water that is three to ten times lower than that of the other materials generally used for producing the pressure sheath of flexible pipes, namely mainly polyamides 11 or 12 and fluoropolymers based on vinylidene fluoride, and especially based on polyvinylidene fluoride (PVDF). In addition, polyethylene has a permeability coefficient with respect to C02, H2S and CFI4 that is substantially higher than that of the other materials used for producing the pressure sheath. Polyethylene therefore gives a good barrier effect against the permeation of water, while not excessively limiting the permeation of C02, H2S and CH4. Furthermore it is an inexpensive material that is easy to extrude.

According to another embodiment of the invention, the intermediate sheath consists of polypropylene.

Furthermore, the flexible tubular pipe may comprise an internal carcass located on the inside of the pressure sheath.

In addition, the flexible tubular pipe may comprise a pressure vault surrounding the pressure sheath. In this case, advantageously, the pressure vault surrounds the intermediate sheath. Moreover, according to one advantageous variant, the water permeance of the intermediate sheath is at least three times smaller than the water permeance of the pressure sheath, and the hydrogen sulfide permeance of the intermediate sheath is at least two times greater than the hydrogen sulfide permeance of the pressure sheath, and the carbon dioxide permeance of the intermediate sheath is at least two times greater than the carbon dioxide permeance of the pressure sheath, and finally the methane permeance of the intermediate sheath is at least two times greater than the methane permeance of the pressure sheath. In this way, the selective screen effect conferred by the intermediate sheath is even more favorable.

Furthermore, advantageously, the polymeric intermediate sheath does not comprise any chemically active filler capable of reacting with water and/or hydrogen sulfide and/or carbon dioxide and/or methane. In this way, the intermediate sheath produces barrier effects against the permeation of these molecules which are solely linked to diffusion and absorption/desorption phenomena. The result of this is that these barrier effects are stable and long-lasting, which is not the case when chemically active agents are added to the intermediate sheath.

Other distinctive features and advantages of the invention will emerge on reading several embodiments of the invention, and also from the appended drawings in which:

Figure 1 is a partial perspective view of a flexible pipe according to the invention.

Figure 2 is a cross-sectional view of a device for measuring the permeability coefficient of a sheath sample with respect to H2S or C02 or CFI4.

Figure 3 is an example of a measurement carried out with the device from figure 2.

Figure 4 is a cross-sectional view of a device for measuring the permeability coefficient of a sheath sample with respect to water.

The flexible tubular pipe (1) represented in figure 1 comprises, from the inside outward, an internal carcass (2), a pressure sheath (3), an intermediate sheath (4), a pressure vault (5), tensile armor layers (6, 7) and an external sheath (8).

The internal carcass (2) is made from a profiled stainless steel strip that is wound in a short pitch in order to form touching turns that are interlocked with one another. The main role of the internal carcass (2) is to take up radial buckling forces, for example those linked to the hydrostatic pressure or those exerted by external equipment, especially while the pipe is being laid offshore. The pipe (1) represented in figure 1 is referred to as a rough-bore pipe due to the presence and geometry of the internal carcass (2). Flowever, the present invention could also apply to a pipe that does not have a carcass, such a pipe then being referred to as a smooth-bore pipe, since its first layer, starting from the inside, is the pressure sheath (3), this sheath being in fact an extruded polymeric tube that has a smooth internal wall.

The pressure sheath (3) is an extruded polymeric sheath, the role of which is to confine the hydrocarbon flowing inside the pipe (1), since the internal carcass (2) is not leaktight. The polymer material constituting the pressure sheath (3) is chosen as a function in particular of the chemical composition, of the temperature and of the pressure of the hydrocarbon that the pipe must transport. The most commonly used polymers for producing the pressure sheath (3) are polyamide-11 (PA-11), polyamide-12 (PA-12), polyamide-6,12 (PA-6,12), crosslinked polyethylene and fluoropolymers based on vinylidene fluoride, and especially those based on polyvinylidene fluoride (PVDF).

When the temperature of the hydrocarbon is above 130°C, for example of the order of 150°C, other polymers may advantageously be used for producing the pressure sheath (3). Thus, for example, the latter may be made with a thermoplastic polymer that combines at least two fluoromonomers, one of these two monomers bearing at least one alkoxy function, for example a copolymer or terpolymer combining tetrafluoroethylene (TFE) and another fluoromonomer, especially a fluorinated cyclic ether or a fluorinated aldehyde, as is taught in patent application WO 96/30687. Such a polymer is especially sold by the company DuPont under the trademark Teflon® PFA. For high-temperature applications, the pressure sheath (3) could also be made with other high-performance engineering polymers, used alone or in combination, especially polyether ether ketone (PEEK), polyimide (PI), polysulfone (PSU), polyethersulfone (PES), polyphenylsulfone (PPSU), polyetherimide (PEI), polyphthalamide (PPA), polyphenylene sulfide (PPS), or liquid-crystal polymers (LCP). Patent application WO 2008/119677 teaches the use of a blend of PEEK and PPSU for producing the pressure sheath (3).

Thus, a large number of polymers and blends of polymers may be used for producing the pressure sheath (3), as is taught in particular by patent application WO 2005/028198 which mentions, inter alia, polyolefins, including polypropylene (PP), and polyurethane (PU).

The pressure vault (5) consists of one or more metal wires having a Z-, T-, C-, X- or K-shaped cross section, said wire or wires being wound helically in a short pitch and interlocked with one another. The main role of this layer is to take up the radial forces linked to the pressure of the hydrocarbon flowing in the pipe, the pressure sheath (3) not being capable of withstanding a high pressure by itself alone and having therefore to be supported by the pressure vault.

The pipe (1) also comprises a pair of tensile armor layers (6,7) consisting of metal wires wound in a long pitch, and having the role of taking up the tensile forces exerted on the pipe. The two armor layers are crossed, that is to say that they have substantially opposite helix angles, so as to balance the structure in torsion, that is to say to limit its tendency to turn under the effect of a tensile force. The tensile armor wires generally have a substantially rectangular cross section, but they may also have a circular or T-shaped cross section. For mainly economic reasons, these wires are advantageously made of highly work-hardened carbon steel, which gives them a great tensile strength. The ultimate tensile strength of the tensile armor wires is advantageously greater than 1000 MPa, more advantageously greater than 1200 MPa, preferably greater than 1400 MPa. The carbon content of the steels used is generally of the order of 0.3% to 0.4%. Steels with a high carbon content, typically of more than 0.8%, may also be used for the purpose of obtaining an ultimate tensile strength of greater than 1700 MPa, as is taught in particular by patent application WO 2011/105428.

The external sheath (8) is an extruded polymeric sheath, the role of which is to protect the inner layers of the pipe (1). It is generally made from polyethylene, thermoplastic elastomer or polyamide.

In order to economically reduce the weight of the pipe (1), it is particularly advantageous to produce the tensile armor layers (6,7) with carbon steels that have high mechanical strength, but it turns out that the corrosion fatigue resistance of such steels may prove insufficient in certain critical cases, especially in the upper part of certain dynamic risers. In order to solve this problem, the pipe (1) comprises a polymeric intermediate sheath (4) located between the pressure sheath (3) and the pressure vault (5). Furthermore, the thickness and the chemical composition of this intermediate sheath (4) are chosen so that firstly the water permeance of the intermediate sheath (4) is at least two times smaller than the water permeance of the pressure sheath (3), and so that secondly, the H2S permeance of the intermediate sheath (4) is at least two times greater than the H2S permeance of the pressure sheath (3), and so that thirdly the C02 permeance of the intermediate sheath (4) is at least two times greater than the C02 permeance of the pressure sheath (3), and so that fourthly, the CH4 permeance of the intermediate sheath (4) is at least two times greater than the CH4 permeance of the pressure sheath (3).

In this way, as is explained above, the presence of the intermediate sheath (4) limits the diffusion of water from inside the pipe toward the annular space located between the intermediate sheath (4) and the outer sheath (8). This makes it possible to prevent the phenomenon of condensation of the water that has diffused and consequently to solve the problem of corrosion fatigue of the tensile armor layers (6,7).

The intermediate sheath (4) has been extruded directly onto the pressure sheath (3) with conventional extrusion means. These two sheaths are not adhesively bonded to one another, which facilitates the manufacture of the pipe (1). This absence of adhesive bonding is not however detrimental to the behavior of the pipe since the intermediate sheath (4) has high permeances with respect to H2S, C02 and CH4, which prevents the risk of the formation of gas pockets at the interface between the pressure sheath (3) and the intermediate sheath (4).

The thickness of the intermediate sheath (4) is advantageously greater than or equal to 3 mm, typically between 3 mm and 12 mm, preferably between 4 mm and 8 mm.

The intermediate sheath (4) advantageously consists of polyethylene, especially when the pressure sheath (3) consists of polyamide or PVDF. Examples are described in detail below.

Other layers not represented in figure (1) may be added if necessary, such as for example anti-wear strips inserted between the adjacent metal layers, thermal insulation layers, or else woven reinforcing strips. In certain applications, the pressure vault (3) may be eliminated, but in this case the armor layers (6,7) have a helix angle having an absolute value close to 55°, which gives the tensile armor layers the ability to withstand both the axial tensile forces and the radial forces linked to the internal pressure.

Figure 2 represents a device that makes it possible to measure the permeability coefficient of a polymeric sheath with respect to a gas of H2S or C02 or CH4 type. This device, referred to as a cell, comprises a hollow metal chamber (10) placed inside which is a sheath sample (11) in the form of a membrane of thickness L. The chamber (10) can be dismantled so as to be able to introduce the membrane-shaped sample (11) therein. The chamber (10) typically comprises two complementary housings (12, 13) joined together by screws (14, 15) or other equivalent fastening means. The membrane (11) separates the central cavity of the chamber (10) into two, so as to form two distinct cavities (18, 19) located on either side of the membrane (11). The two cavities (18, 19) are cylindrical, coaxial and have the same internal diameter D. The membrane (11) is positioned perpendicular to the axis of the two cavities (18,19). The perimeter of the membrane (11) is attached to the cell (10) by clamping between the two housings (12, 13) with the aid of screws (14, 15). Sealing means (16, 17) of the O-ring type are positioned between the perimeter of the membrane (11) and the housings (12, 13).

The first of the two cavities (18, 19), referred to as the upstream cavity (18), is connected via an upstream duct (20) to a reservoir (25) containing the gas with respect to which the permeability measurement must be made, namely H2S or C02 or CH4. This reservoir has a volume much greater than that of the upstream cavity (18), so that the pressure P1 of the gas in the upstream cavity (18) remains substantially constant despite the permeation of this gas through the membrane (11). An upstream pressure-measuring sensor (26) makes it possible to measure the pressure P1 in the upstream cavity (18).

The second of the two cavities (18, 19), referred to as the downstream cavity (19), is connected via a downstream duct (21) to the inlet of the valve (22), the opening and closing of which may be controlled by electric or pneumatic means that are not represented. The outlet of the valve (22) leads to evacuation and pumping means (23). A downstream pressure-measuring sensor (24) makes it possible to measure the pressure P2 in the downstream cavity (19). The downstream-pressure measuring sensor (24) is connected to the electric or pneumatic means for the controlling the opening and closing of the valve (22).

The upstream pressure P1 is far greater than the downstream pressure P2 so as to reproduce the operating conditions in a flexible pipe, namely conditions under which the pressure inside the pipe is far greater than the pressure in the annular space of the wall of the pipe. Consequently, the membrane (11) is not generally capable of withstanding the pressure difference P1-P2 by itself without damage. This is why the downstream cavity (19) is filled with a gas collector (28), this collector being a porous part that is highly permeable to gases and that is capable of mechanically supporting the membrane (11) to prevent it from being damaged under the effect of the pressure difference P1-P2. In practice, the collector (28) is a sintered stainless steel part that has a high compression strength. The permeability coefficients of the collector (28) with respect to H2S, C02 and CH4 are far greater than those of the membrane (11), typically at least a hundred times greater, so that circulation of these gases in the downstream cavity (19) is not disturbed by the presence of the collector (28).

Since the permeation phenomena are dependent on the temperature, the cell (10) is equipped with a sensor (27) for measuring the temperature T, and heating and temperature-control means (not represented). The temperature T and the pressures P1 and P2 are recorded as a function of time using means that are not represented.

Figure 3 illustrates a recording made by the cell from figure 2. The y-axis (41) corresponds to a pressure and the x-axis (42) to the time. The solid-line curve (40) is an example of a recording of the pressure P2 in the downstream cavity (19) as a function of the time t.

At the instant t0=0, the valve (22) is closed and the downstream cavity (19) is under partial vacuum, P2 being close to 0 bar. The cell (10) is then at the test temperature. Furthermore, at the instant to = 0, the upstream cavity (18) is rapidly filled with a gas (H2S or C02 or CFI4) under a pressure P1 that is high and constant, for example 50 bar, which pressure P1 then remains constant throughout the duration of the recording. The gas then begins to diffuse slowly through the membrane (11) from the upstream cavity (18) to the downstream cavity (19). The pressure P2 in the downstream cavity (19) firstly remains zero for a certain time then begins to increase once the first molecules of gas have passed through the membrane (11).

The means for controlling the valve (22) are programmed so as to automatically open the latter when the pressure P2 reaches a predefined threshold Ps (43), then to automatically close it once the downstream cavity (19) has been evacuated and the initial partial vacuum has been re-established. This evacuation operation is carried out automatically by evacuation and pumping means (23). A complete sequence of opening the valve (22), depressurization of the downstream cavity (19) and finally closure of the valve (22) lasts less than one minute, which remains negligible with respect to the total duration of the recording, which is of the order of several days due to the slowness of the diffusion phenomena.

The first depressurization sequence of the downstream cavity (19) takes place at the instant t1 and corresponds to the vertical curve section (44). Next, the pressure P2 starts to increase slowly again due to the diffusion phenomenon, which corresponds to the curve section (45). Next, as soon as P2 again reaches Ps (43), a second depressurization sequence is triggered automatically at the instant t2, which corresponds to the curve section (46). The phenomenon is thus repeated until the end of the recording, the pressure P2 varying in accordance with an alternating sawtooth curve of slow rises (45, 47, 49) and almost instantaneous drops (44, 46, 48, 51).

The predefined pressure threshold Ps (43) is much lower than the pressure P1, Ps (43) typically being equal to 2% of P1. Consequently, since P2 is always less than or equal to Ps (43), P2 remains much lower than P1, so much so that the pressure difference P1-P2 remains substantially equal to P1. Furthermore, since P1 is constant throughout the entire test, it follows therefrom that P1-P2 itself also remains substantially constant. Since the permeability coefficient of the membrane 11 is dependent, inter alia, on P1-P2, it is thus possible to regulate this influencing parameter and keep it constant.

Starting from the curve (40) of the pressure P2, the cumulative curve (50) of the pressure P2c is constructed in the following manner: tO < t < t1 => P2c (t) = P2 (t) t1 < t < t2 => P2c(t) = Ps + P2(t) t2 < t < t3 => P2c(t) = (2 x Ps) + P2(t) t3 < t < t4 => P2c(t) = (3 x Ps) + P2(t)

And more generally: t, < t < ti+1 => P2c(t) = (i x Ps) + P2(t)

The instant tO = 0 corresponds to the start of the test whereas the instants t1, t2, t3, t4, and so on correspond to the successive depressurization sequences of the downstream cavity (19).

The cumulative pressure P2c is proportional to the total number n of moles of gas that have diffused through the membrane (11) since the start of the test. Specifically, by applying the ideal gas law, the following formula is obtained:

R is the universal ideal gas constant, T is the temperature of the gas measured by the sensor (27), and V2 is the volume occupied by the gas in the downstream cavity (19) and in the downstream duct (21) when the valve (22) is closed. V2 is less than the internal volume of the downstream cavity (19) due to the presence of the collector (28), which reduces the volume available for the gas. V2 is measured prior to the test. In this way, it is possible to determine, at each instant t, the total number n of moles of gas that have diffused through the membrane since the start of the test.

The cumulative curve (50) of the pressure P2c generally comprises three parts. At the start of the test, the cumulative pressure P2c remains zero for a certain time. Then, when the first molecules of gas have finished passing through the membrane (11), P2c increases firstly in a non-linear manner. Finally, when steady state is achieved, P2c increases linearly.

The permeability coefficient Pe with respect to one of the gases H2S, C02 or CH4 is determined when steady state is achieved. It is given by the following formula:

A is the area of the membrane (11) through which the gas diffuses. L is the thickness of the membrane (11). P1 and P2 are respectively the pressures of the gas upstream and downstream of the membrane (11). Δη is the number of moles of gas that have passed through the membrane (11) during the time interval At.

In the present case, the area through which the gas diffuses is a disk of diameter D. Consequently:

In addition, as was explained above:

Furthermore, in steady state, it is deduced from equation [1] that Δη/Δί is proportional to ΔΡ2ο/Δί:

ΔΡ2ο/Δΐ is the slope of the curve (50) of the cumulative pressure P2c, this slope being constant in steady state.

By combining equations [2], [3], [4] and [5], the formula is obtained that makes it possible in practice to determine the permeability coefficient Pe from measurements carried out with the cell:

The slope ΔΡ2ο/Δί may be determined from the cumulative pressure curve (50). The pressure P1 and the temperature T are measured by the sensors (26) and (27). The thickness L of the membrane (11) is known and was able to be measured before the test. D and V2 are parameters linked to the dimensions of the cell, which are themselves known. R is the universal ideal gas constant, a constant that is itself also known.

R « 8.314472 J.K-1.mol'1 using SI units.

The SI units of a permeability coefficient Pe with respect to a gas of H2S or C02 or CH4 type are mol.S'1.nrr1.Pa·1, but it is generally preferred to use mol.S'1.cnrr 1.bar1 or cm3(STP).S'1.cnrr1.bar1 by replacing the moles with cm3(STP), 1 mole of gas corresponding to a volume of 22414 cm3 under STP conditions (temperature of 0°C and pressure of 1 atm = 1.013 bar).

Figure 4 represents a device that makes it possible to measure the permeability coefficient of a polymeric sheath with respect to water. In the case of water, the measurement method is simpler than in the case of the gases H2S, C02 and CH4. The measurement cell (30) comprises a cylindrical metal chamber (32) of internal diameter D and having a substantially vertical axis with respect to the ground. The metal wall of the chamber (32) extends only over its cylindrical side face (37) and over its flat upper face (33). The lower face of the chamber (32) is sealed by a sheath sample (31) in the form of a flat membrane with a thickness L. The membrane (31) is positioned perpendicular to the axis of the chamber (32), that is to say substantially parallel to the ground. The perimeter of the membrane (31) is attached to the chamber (32) by dismantlable means, for example a flange (36) held by screws (38, 39) that clamp the membrane (31) against the wall of the chamber (32). Sealing means (35) of the O-ring type are inserted between the perimeter of the membrane (31) and the chamber (32).

The cylindrical cavity (34) inside the chamber was completely or partially filled with water (52) before installation of the membrane (31). This filling with water is carried out at atmospheric pressure and at ambient temperature. During the test, the pressure outside the chamber (32) remains equal to atmospheric pressure.

Since the permeation phenomenon is dependent on the temperature, the cell (30) is equipped with means (not represented) of heating and of controlling and measuring the temperature T of the water and the membrane (31). The pressure inside the chamber (32) is not regulated and may vary as a function of the temperature T.

The water (52) located in the chamber (32) diffuses slowly through the membrane (31) which decreases the total mass of the cell (30). Using a precision balance that is not represented in figure 4, the change in mass m of the cell (30) is recorded as a function of the time t. Since the diffusion of water is very slow, the test lasts in practice of the order of one month, the measurements of m succeeding one another with a frequency of the order of one day. At the start of the test, as steady state has not been achieved, the mass m varies non-linearly as a function of the time t. Then, as soon as steady state is achieved, m decreases linearly as a function oft. The mass Am of water diffusing through the membrane (31) over a fixed interval of time At is therefore constant. In steady state, the curve of m as a function of t has the slope -(Am/At), this slope being negative since the total mass of the cell decreases. From the measurements, it is therefore possible to determine the mass flow Am/At of water diffusing through the membrane (31) in steady state.

The permeability coefficient Pe’ with respect to water is therefore given by the following formula:

[7] A is the area of the membrane (31) through which the water diffuses. L is the thickness of the membrane (31). In the present case, the surface through which the gas diffuses is a disk of a diameter D. Consequently:

[8]

The SI units of a permeability coefficient Pe’ with respect to water are kg.s-1.nrr1 but it is generally preferred to use g.S'1.cnrr1.

As a general rule, the membranes (11, 31) used to measure the permeability coefficients with respect to the gases H2S, C02 and CH4 and with respect to water were machined from samples removed from the sheath of the pipe during manufacture, after extrusion thereof. However, in the case where the sheath consists of a semicrystalline polymer, for example PVDF, the samples of membrane (11, 31) may also be produced by a compression molding process on condition that the same raw material is used as that used for extruding the sheath for which it is desired to measure the permeability coefficients. Specifically, in the case where the polymer is semicrystalline, the orientation of the molecular chains generated by the extrusion process has no significant influence on the permeability coefficients, so that it is possible to produce a representative sample by compression molding.

The diameter D of the membranes (11, 31) from figures 2 and 4 is typically between 50 mm and 100 mm, for example 70 mm. The thickness L of the membranes (11, 31) is typically between 0.5 mm and 3 mm, for example of the order of 2 mm for the measurements of the permeability coefficients with respect to H2S, C02 or CH4, and of the order of 1 mm for the measurement of the permeability coefficient with respect to water.

Table 1 specifies the conditions for measuring the permeability coefficients of the sheaths that make it possible to implement the present invention.

Table 1

Several examples of flexible pipes produced according to the present invention will now be described.

Example number 1:

The first example relates to a flexible pipe having an internal diameter of 250 mm that can be submerged to a depth of 1300 m and that can transport corrosive hydrocarbons having a pressure of 300 bar and a temperature of the order of 65°C.

This pipe essentially comprises, from the inside outward, an internal carcass having a thickness of 10 mm made of stainless steel, a pressure sheath having a thickness of 7 mm made of polyamide 11 (PA-11), an intermediate sheath having a thickness of 3 mm made of polyethylene (PE), a pressure vault having a total thickness of 17.5 mm, two crossed tensile armor layers having a total thickness of 8 mm and an external sheath made of polyethylene (PE) having a thickness of 9 mm. The pressure vault consists of the superposition of two layers wound in a short pitch, namely on the one hand an interlocked layer formed from a “zeta”-shaped wire having a thickness of 10 mm, and on the other hand a non-interlocked layer formed from a wire of rectangular cross section and having a thickness of 7.5 mm. The pressure vault and the tensile armor layers are both made with carbon steels that have a carbon content of the order of 0.35%. The intermediate PE sheath and the PA-11 pressure sheath are not adhesively bonded to one another, the structure being of unbonded type.

Table 2 provides the various permeability coefficients of the PA-11 and PE, these coefficients having been measured with the experimental means described above and under the measurement conditions described in table 1.

Table 2 also specifies the permeances of the pressure sheath and of the intermediate sheath of this flexible pipe with respect to H2S, C02, CH4 and H20. The permeance of a sheath with respect to a molecule is calculated by taking the ratio between, on the one hand, the permeability coefficient of this sheath with respect to this molecule and, on the other hand, the thickness of this sheath.

Table 2

The last line of table 2 makes it possible to compare, molecule by molecule, the permeances of the intermediate sheath and of the pressure sheath. In this first example, the H2S permeance of the intermediate sheath is equal to 3.57 times the H2S permeance of the pressure sheath. In addition, the C02 permeance of the intermediate sheath is equal to 4.51 times the C02 permeance of the pressure sheath. Moreover, the CH4 permeance of the intermediate sheath is equal to 12.72 times the CH4 permeance of the pressure sheath. Furthermore, the H20 permeance of the intermediate sheath is equal to 0.21 of the H20 permeance of the pressure sheath, that is to say that it is almost 5 times smaller than the latter.

Calculations were made in order to quantify the diffusion of water and acid gases through the wall of this flexible pipe. For this purpose, use was made of a finite element analysis software known as MoldiTm, software that is in particular described in an article entitled “MoldiTm: a Fluid Permeation Model to Calculate the Annulus Composition in Flexible Pipes” written by Z. Benjelloum-Dabaghi, JC de Flemptinne, J. Jarrin, JM Leroy, JC Aubry, JN Saas and C. Taravel-Condat, and published in 2002 in the journal “Oil & Gas Science and Technology” edited by the IFP Energies Nouvelles institute.

These calculations demonstrated the favorable technical effect of the intermediate PE sheath. Thus, in the absence of this intermediate sheath, that is to say according to practice prior to the present invention, a significant diffusion of water is observed from inside the pipe to the annular space, and also condensation of the latter in the annular space, which condensation phenomenon may occur very rapidly depending on the composition, temperature and pressure of the hydrocarbon, sometimes after using the pipe for only a few months. The same detrimental phenomenon also occurs, but slightly later, when the thickness of the PA-11 pressure sheath is increased, going for example from 7 mm to 10 mm.

Flowever, when the PE intermediate sheath is added, it is observed that the diffusion of water is greatly reduced, so much so that in practice the phenomenon of condensation of water in the annular space either occurs much later in the absence of draining the annulus, or may even be completely avoided throughout the service life of the pipe with the aid of simple drainage of the annular space. This drainage consists in discharging the content of the annular space to the outside from at least one of the pipe’s two end fittings. In the case of a riser, this discharging generally takes place into the open air via the upper end fitting located at the surface.

Due to the absence of condensed water in the annular space, the medium in which the pressure vault and the tensile armor layers are found is substantially less corrosive, so much so that the pressure vault and the armor layers may withstand the stresses to which they are subjected on a long-term basis. In addition, the problem of corrosion fatigue of the tensile armor layers is itself thus also resolved.

Example number 2:

The second example relates to a flexible pipe having an internal diameter of 150 mm that can be submerged to a depth of 1600 m and that can transport corrosive hydrocarbons having a pressure of 550 bar and a temperature of the order of 110°C.

This pipe essentially comprises, from the inside outward, an internal carcass having a thickness of 6 mm made of stainless steel, a pressure sheath having a thickness of 6.5 mm made of polyvinylidene fluoride (PVDF), an intermediate sheath having a thickness of 4 mm made of polyethylene (PE), a pressure vault having a total thickness of 13.7 mm, two crossed tensile armor layers having a total thickness of 8 mm and an external sheath made of polyethylene (PE) having a thickness of 6 mm. The pressure vault consists of the superposition of two layers wound in a short pitch, namely on the one hand an interlocked layer formed from a “zeta”-shaped wire having a thickness of 6.2 mm, and on the other hand a non-interlocked layer formed from a wire of rectangular cross section and having a thickness of 7.5 mm. The pressure vault and the tensile armor layers are both made with carbon steels that have a carbon content of the order of 0.35%. The intermediate PE sheath and the PVDF pressure sheath are not adhesively bonded to one another, the structure being of unbonded type.

Table 3 provides the various permeability coefficients of the PVDF and PE, these coefficients having been measured with the experimental means described above and under the measurement conditions described in table 1. Table 3 also specifies the permeances of the pressure sheath and of the intermediate sheath of this flexible pipe.

Table 3

In this second example, the H2S permeance of the intermediate sheath is equal to 15.02 times the H2S permeance of the pressure sheath. In addition, the C02 permeance of the intermediate sheath is equal to 2.58 times the C02 permeance of the pressure sheath. Moreover, the CH4 permeance of the intermediate sheath is equal to 18.80 times the CH4 permeance of the pressure sheath. Furthermore, the H20 permeance of the intermediate sheath is equal to 0.27 of the H20 permeance of the pressure sheath, that is to say that it is almost 4 times smaller than the latter.

In the same way as in the preceding example, calculations made with the MoldiTm software demonstrated the fact that the addition of the PE intermediate sheath makes it possible to limit the diffusion of water enough to prevent the phenomenon of condensation of water in the annular space and consequently to solve the problem of corrosion fatigue of the tensile armor layers.

Claims

A flexible tubular tube (1) of the loose type intended for the transport of fluids in the field of offshore oil production, comprising at least, seen from the inside and out, a pressure sheath (3), an intermediate sheath (4), wood reinforcement layer (6) , 7) and an outer sheath (8), wherein the intermediate sheath (4) hydrogen sulfide permeance is at least twice the pressure sheath (3) hydrogen sulfide permeance, and the intermediate sheath (4) carbon dioxide permeance is at least twice the pressure sheath (3) carbon dioxide permeance and the intermediate spoon (4) of methane permeans is at least twice the pressure of the intermediate spoon (3), characterized in that the water sperm of the intermediate spoon (4) is at least twice that of the pressure spoon (3).

Flexible tubular tube (1) according to claim 1, characterized in that the intermediate sheath (4) is in contact with the pressure sheath (3).

Flexible tubular tube (1) according to claim 1 or 2, characterized in that the thickness of the intermediate sheath (4) is greater than or equal to 3 mm.

Flexible tubular tube (1) according to any one of the preceding claims 1 to 3, characterized in that the intermediate sheath (4) consists of polyethylene.

Flexible tubular tube (1) according to any one of the preceding claims 1 to 4, characterized in that the flexible tubular tube (1) has an internal carcass (2) located on the inner impression sheath (3).

Flexible tubular tube (1) according to any one of the preceding claims 1 to 5, characterized in that the flexible tubular tube (1) has a pressure vault (5) surrounding the intermediate sheath (4).

Flexible tubular tube (1) according to claim 6, characterized in that the pressure vault (5) surrounds an intermediate sheath (4).

Flexible tubular tube (1) according to any one of the preceding claims 1 to 7, characterized in that the intermediate sheath (4) is of a polymer which does not comprise any chemically active filler capable of reacting with water. and / or hydrogen sulfide and / or carbon dioxide and / or methane.