GB2473357A

GB2473357A - Method of manufacturing a flexible tubular structure

Info

Publication number: GB2473357A
Application number: GB1018411A
Authority: GB
Inventors: Alain Coutarel; Frodoric Demanze; Jean Hardy
Original assignee: Technip France SAS
Current assignee: Technip Energies France SAS
Priority date: 2008-05-13
Filing date: 2009-05-13
Publication date: 2011-03-09
Anticipated expiration: 2029-05-13
Also published as: GB2473357B; WO2009141538A2; AU2009248601A1; WO2009141538A3; AU2009248601B2; BRPI0912199A2; NO342067B1; FR2931098B1; FR2931098A1; FR2931099A1; DK178345B1; NO20101538A1; FR2931099B1; GB201018411D0; DK201070481A; MY150383A

Abstract

The invention relates to a flexible tubular pipe (10) intended for offshore oil production and to its method of manufacture, said pipe comprising an impermeable internal duct and at least one sheet (20) made of a thermoplastic elastomer polymer of defined thickness around said internal duct (18). According to the invention said thermoplastic elastomer polymer comprises a polypropylene-based thermoplastic polymer and an elastomer based on an ethylene-propylene-diene terpolymer and it further includes a compatibilizer for increasing the miscibility between said polypropylene-based thermoplastic polymer and said elastomer based on an ethylene-propylene-diene terpolymer.

Description

Method of Manufacturing a Flexible Tubular Structure The present invention relates to a method of manufacturing a flexible tubular structure of great length intended to transport fluids for offshore oil production, and to the structure obtained according to said method.

It more particularly concerns certain sheaths of the structure, which are made of polymer material.

In the present application, the term "flexible tubular structure" refers both to flexible subsea pipes and subsea umbilicals, as well as flexible tubular structures combining the functions of flexible pipes and subsea umbilicals.

Flexible subsea pipes are essentially used to transport the oil or qas extracted from an offshore deposit. They can also be used to transport pressurized seawater intended to be injected into the deposit in order to increase the production of hydrocarbons.

These flexible pipes are formed by a set of different layers, each intended to allow the pipe to withstand the stresses of offshore service or installation. These layers comprise in particular polymeric sheaths and reinforcing layers formed by windings of shaped metal wires, hoops or filaments made of composite material.

The flexible tubular pipes generally comprise, from the inside outward, at least one internal sealing tube intended to convey the transported fluid, reinforcing layers around the internal tube and an external polymeric protective sheath around the reinforcing layers. The internal sealing tube generally consists of a polymeric material, and in this case it may equally be referred to by the term "internal sealing sheath" or "pressure sheath". There are, however, flexible subsea pipes in which the internal sealing tube is a thin-walled corrugated metal tube, such as those described in the document W098/25063. In certain cases, an intermediate polymeric sheath is also provided between the internal sealing tube and the external protective sheath, for example between two reinforcing layers.

The flexible pipe may furthermore comprise a thermal insulation layer arranged between the reinforcing layers and the external protective sheath. This thermal insulation layer is generally made by helicoid winding of syntactic foam strips.

Such flexible pipes are described in the standardization documents published by the American Petroleum Institute (API), API 17J "Specification for Unbonded Flexible Pipe" and API RP 173 "Recommended Practice for Flexible Pipe".

Subsea umbilicals are mainly used to transport fluids, power and signals to subsea equipment, for example valves, wellheads, collectors, pumps or separators, with a view to supplying power and monitoring and remote controlling the actuators of these items of equipment. The fluids transported for these applications are generally hydraulic control oils.

Subsea umbilicals may also be used to transport various fluids intended to be injected into a main hydrocarbon transport pipe, with a view either to facilitating the flow of said hydrocarbon, for example by injecting chemical agents aiming to prevent the formation of hydrate plugs, or methane so as to make it easier for the oil to rise to the surface ("gas lift" method), or to ensure maintenance of said main pipe, for example by injecting corrosion inhibitors.

A subsea umbilical consists of an assembly of one or more internal sealing tubes, and optionally electrical cables and/or fiber-optic cables, said assembly being made by helicoid or S/Z winding of said tubes and cables so that the umbilical is flexible; said assembly may be surrounded by reinforcing layers and an external polymeric protective sheath. These internal sealing tubes, the function of which is to transport the aforementioned fluids, generally have a diameter very much less than the external diameter of the umbilical.

An internal sealing tube of an umbilical generally consists of either a metal sealing tube or an impermeable polymeric tube surrounded by one or more reinforcing layers.

Such subsea umbilicals are described in the standardization documents published by the American Petroleum Institute (API), API 17E "Specification for Subsea Umbilicals".

The document US6J02077 discloses a flexible tubular structure combining the functions of a flexible subsea pipe and a subsea umbilical. At its center, the structure comprises a flexible pipe with a large diameter used to transport hydrocarbons, said central flexible pipe being surrounded by a plurality of peripheral tubes with a small diameter assembled helicoidally or in S/Z fashion around the central flexible pipe, said peripheral tubes being used for functions similar to those of umbilicals, in particular hydraulic control or injection of fluids. Such flexible tubular structures are known to the person skilled in the art by the names "Integrated Subsea Umbilical" and "Integrated Production Bundle". These structures with a large diameter are generally surrounded by an external polymeric sheath.

In the present application, the term "internal sealing tube" covers equally the internal polymeric sheath or the pressure sheath of a flexible subsea pipe or the metal or polymeric fluid transport tubes of a subsea umbilical. The term "impermeable internal pipe" in turn refers to a part of the flexible tubular structure assembly, said part comprising at least one internal sealing tube. These definitions also apply to flexible tubular structures combining the functions of a flexible subsea pipe and a subsea umbilical.

The document W003083344 in the name of the Applicant teaches the use of a thermoplastic elastomer polymer (TEP) for producing the external sheath or the intermediate sheath of flexible subsea pipes.

Specifically, these polymers are well suited to such applications while being less expensive than the polyamides used in the past. These polymers are also naturally adapted for the production of external or intermediate polymeric sheaths of subsea umbilicals, or flexible tubular structures combining the functions of a flexible subsea pipe and a subsea umbilical.

The methods for producing these tubular pipes are well known; in the case of the aforementioned sheaths made of polymer material, and more precisely the outer sheath for the intermediate sheath, they are extruded directly around the reinforcing layers of the pipe.

Thus, the first step is to surround the internal pressure sheath with at least one traction-resistant layer and most often with at least one other pressure-resistant layer. The next step is to hot-extrude a softened layer of a thermoplastic elastomer polymer, including an olefin polymer mixed with an elastomer, and finally said layer is cooled in order to obtain at least one second polymeric sheath with a determined thickness around the reinforcing layers. The extrusion is carried out by means of an extruder equipped with a crosshead.

Until now, these polymeric sheaths have had a relatively small thickness, typically less than 10 mm, and the method of using the aforementioned thermoplastic elastomer polymer has in fact made it possible to obtain economical sheaths offering good mechanical strength. However, owing to the increase in water depths and the outer diameter of the flexible tubular structure, as well as thermal insulation requirements, attempts are quite naturally being made to increase in particular the thickness of the thermoplastic elastomer polymer sheaths. Unfortunately, it has been found that, overall, the mechanical properties of the thermoplastic elastomer sheaths then obtained with a large thickness are unsatisfactory. In particular, the elongation at break of the thermoplastic elastomer sheaths produced according to the previous method tends to decrease greatly when the thickness of the sheath is more than 15 mm, which has the drawback of increasing the minimum radius with which the flexible pipe can be bent without the risk of tearing said thick sheaths.

The document EP1619218 teaches a solution aiming to solve this problem. This solution consists principally in adding a nucleating agent to the thermoplastic elastomer polymer, and furthermore in taking care of the extruded sheath so that it does not have too great a hardness. However, the only test results presented on page 9 of this document relate to samples of extruded strips with a thickness of 2 mm, that is to say a small thickness, and therefore pertain to cases not representative of those in which the aforementioned problem arises. Furthermore, the author deduces from these nonrepresentative tests that this solution would make it possible to solve the fragility problem of external sheaths with a thickness of more than 5 mm, or mm or even 15 -. However, the Applicant has carried out full-scale tests on this solution and found that the elongations at break of the thick external sheaths produced in this way remain insufficient, so that the risk of said sheaths rupturing has not been overcome.

Also, a technical problem which then arises, and which the present invention aims to solve, is to provide a flexible tubular structure and a method for producing such a pipe, in which at least the external protective sheath or an intermediate sheath, made of thermoplastic elastomer polymer, withstands the working stresses of the structure even though the thickness of the sheath is increased in comparison with the thickness of the

sheaths produced according to the prior art.

To this end, and according to a first aspect, the present invention provides a method of manufacturing a flexible tubular structure intended for offshore oil production, said method being of the type according to which: an impermeable internal pipe and a thermoplastic elastomer polymer are first provided; a softened layer of said thermoplastic elastomer polymer is then formed by hot extrusion around said impermeable internal pipe; next, said softened layer is cooled in order to obtain at least one polymeric sheath with a determined thickness around said impermeable internal pipe; furthermore, according to the invention said thermoplastic elastomer polymer comprises a thermoplastic polymer based on polypropylene (PP) and an elastomer based on a terpolymer of ethylene-propylene-diene monomers (EPDM) and a compatibilizing agent is furthermore provided for mixing together said thermoplastic elastomer polymer and said compatibilizing agent before said hot extrusion, so as to increase the miscibility of said thermoplastic polymer based on polypropylene and said elastomer based on a terpolLymer of ethylLene-propylene-diene monomers (EPDN) during the extrusion, by means of which the elongation at break of said polymeric sheath is increased.

Thus, one characteristic of the invention resides in the use of a compatibilizing agent adapted to increase the stability of the thermoplastic polymer and the elastomer in one another during the extrusion, one being based on polypropylene (PP) and the other based on a terpolymer of ethylene-propylene-diene monomers (EPDN) . In this way, it is found that the mechanical properties of the sheath are more homogeneous throughout its thickness. Also, such a sheath is less susceptible to tearing and more generally is stronger despite the more severe working conditions.

It has not previously been possible to observe such a benefit of the compatibilizing agent because the sheaths with a small thickness obtained according to the previous methods have had homogeneous mechanical properties throughout their thickness. In fact, the use of a sheath with a large thickness, for example more than 15 -, has brought to light a technical problem which did not arise in the past. Specifically, it has been discovered that the mechanical characteristics of the thermoplastic elastomer polymer depend greatly on its cooling conditions after extrusion. Thus, the outermost sheath portions and consequently those which are cooled most rapidly, in air or in water after the formation of the softened layer of thermoplastic elastomer polymer around the impermeable internal pipe, have superior mechanical characteristics compared with the sheath portions lying in the vicinity of said impermeable internal pipe. The latter sheath portions are quite clearly cooled more slowly. A correlation has therefore been established between the cooling rate of the softened polymer and the mechanical properties obtained after cooling. Now, there is no economically viable industrial means of cooling the softened layer after extrusion homogeneously in its entire thickness.

Since optical analysis shows the formation of large crystallites in the outer skin and small crystallites in the inner skin, an attempt was first made to incorporate a nucleating agent in order to homogenize the crystalline structure. Against all expectation, however, this solution (moreover taught by the document EP1619218) has not made it possible to increase the mechanical properties of the sheath sufficiently, despite homogenization of the crystalline structure.

Also, and this is one of the advantages of the invention, it was then envisaged for the compatibilizing agent which allows better cohesion between the phases of the two types of polymer, and consequently better mechanical strength of the cooled material, to be incorporated into the thermoplastic elastomer polymer before extrusion. This result is surprising because in the absence of the nucleating agent the extruded sheath has strong crystalline heterogeneities, in particular when the thickness of said sheath is more than 10 -, and these heterogeneities seem to be affected little by the presence of the compatibilizing agent. Furthermore,

according to the prior art and in particular that

disclosed in EP1619218, it was thought that the fragility problem of these thick sheaths was essentially linked with too low a cooling rate after extrusion, this overly slow cooling causing excessive growth of the crystallites. The natural solution for solving what was thought to be the main problem was therefore to add a nucleating agent. However, it was discovered after tests that this problem of crystallite size and heterogeneity is in fact secondary, and that another technical effect associated with another main problem conditions the quality of the sheath, and that adding a compatibilizing agent makes it possible to solve this other main problem. This other main problem seems to be linked with the large thermal gradient in the thickness of the sheath during its cooling just after extrusion. This is because the cooling of the sheath at the extrusion output takes place mainly through the exterior, the heat essentially being removed by the air of the workshop and by the water of the cooling tanks through which the pipe passes after it emerges from the extrusion head. In the case of a sheath with a large thickness, the thermal gradient resulting from this phenomenon has a large amplitude, the external face of the sheath being cooled much more rapidly than its internal face. The molten polymer solidifies rapidly in the vicinity of the external face of the sheath, then the solidification front progresses toward the internal face with an increasingly slow speed. In the mixture constituting the polymer of the sheath, the elastomer is in the state of solid nodules dispersed in a thermoplastic matrix of polypropylene, said matrix being in the molten state during the extrusion then solidifying progressively in the course of cooling. During the radial progression of the solidification front, the solid EPDM nodules tend to be displaced radially in the sheath and become concentrated in the vicinity of the internal face of the sheath, that is to say the zone which solidifies last. The reason for this displacement is not known, but it seems to be linked with the combination of the strong thermal gradient and the low cooling rate in the vicinity of the internal face. The result observed

within the prior art solutions is a significant

difference in composition of the thermoplastic elastomer between the zones lying in the vicinity of the internal and external faces. In the case of a thick

sheath according to the prior art, for example,

respective polypropylene and EPDM concentrations by mass of 70% and 5% have been observed in the vicinity of the external face, and 65% and 10% in the vicinity of the internal face, the polypropylene concentration dropping radially by 5% and the EPDM concentration increasing radially by the same order of magnitude.

These chemical composition gradients do not of course on their own explain the reason for the problem, but they seem to constitute an indicator revealing another underlying problem probably linked with the morphology and the structure of the polymer. Furthermore, it has been observed that adding a compatibilizing agent of the SEBS or polyolefin type makes it possible to significantly reduce this chemical composition gradient in the thickness of the sheath, and that this reduction in the chemical composition gradient is accompanied by an improvement in the mechanical properties of the sheath, in particular its elongation at break. It seems that the presence of this compatibilizing agent has had the effect of reducing the mobility of the EPDM nodules during the solidification phase, with the compatibilizing agent performing an anchoring role between the polypropylene molecules and the EPDM nodules -Advantageously, a polymer selected from polymers based on polyole fin or styrene-ethylene-butadiene-styrene (SEBS) is provided as the compatibilizing agent. In this way, such a polymer, which has on the one hand chemical functions or groups with an affinity for the elastomer and on the other hand chemical functions or groups with an affinity for the thermoplastic polymer, allows greater miscibility of the two types of polymer with one another when they are hot-extruded.

Advantageously, the selected styrene-ethylene-butadiene-styrene is furthermore a nongrafted linear triblock polymer.

Moreover, a nucleating agent is furthermore advantageously provided and said nucleating agent, said compatibilizing agent, said thermoplastic polymer based on polypropylene and said elastomer based on a terpolymer of ethylene-propylene--diene monomers are mixed together before said hot extrusion in order to increase the crystallization rate of said polymers and subsequently homogenize the crystalline structure in the thickness of said softened layer, then the sheath.

The nucleating agents make it possible to initiate the crystallization reactions of the polymers by promoting the formation of the crystallites as soon as the polymer is softened, at a temperature lying above its glass transition temperature. It will be noted that the cooling conditions of the thermoplastic elastomer polymer after extrusion directly influence the size and the density of the crystallized zones. However, the nucleating agent initiating the formation of the crystallites very quickly makes it possible to obtain a relatively homogeneous distribution of these crystallized zones both in the thickness of the sheath and furthermore over the circumference of said sheath.

It has also been observed that whenever there has been a cornpatibilizing agent, the addition of a nucleating agent makes it possible to further increase the elongation at break performance even though the same nucleating agent, without the presence of a compatibilizing agent, leads to no improvement in the mechanical characteristics and in particular the elongation at break. To our knowledge, this is the first time that this compatibilizing agent/nucleating agent synergy has been observed in the field of thick sheaths.

According to a particularly advantageous embodiment of the invention, a nucleating agent comprising a phosphate salt and/or a dicarboxylate salt is provided so as to obtain an even better distribution of the crystallized zones in the thickness of the sheath.

According to a preferred embodiment of the invention, a thermoplastic elastomer polymer comprising between 40% and 70% by weight of said thermoplastic polymer based on polypropylene, between 10% and 20% by weight of said elastomer based on a terpolymer of ethylene-propylene-diene monomers, and between 1% and 20% by weight of said compatibilizing agent, is provided. In this way, a sheath is obtained whose mechanical properties, in particular the elongation at break, make it possible to avoid the risks of the sheath tearing. Specifically, as will be explained in more detail below, the material of the sheath obtained according to the aforementioned method has an elongation at break of more than 200% irrespective of its position in the thickness of the sheath and irrespective of the value of this thickness, while the material of the sheaths obtained according to the prior art, and in the vicinity of the impermeable internal pipe when the sheath has a thickness of between 15 and 20 mm, has an elongation at break of between 7% and 50% depending on its surface condition at the interface with the internal pipe. According to

the prior art, under these conditions, when the

internal face of the sheath has a good surface condition, the elongation at break is of the order of 50%. Conversely, in the zones where the internal face of the sheath has indentations linked with the geometry of the underlying layer, the elongation at break may be degraded and may drop to around 7%. However, when the flexible tubular structure is bent to its allowed minimum radius, the external polymeric sheath may experience an elongation of the order of 5% to 7% along the most greatly stretched generatrix. Thus, the invention makes it possible to significantly increase the elongation at break and therefore move away from the risk zone.

Furthermore, a thermoplastic elastomer polymer comprising between 0.1% and 1% by weight of said nucleating agent is provided in order to initiate the crystallization reactions of the polymers when working with the polymer.

Advantageously, a softened layer of said thermoplastic elastomer polymer is formed by hot extrusion around said impermeable internal pipe so as to obtain a polymeric sheath with a thickness of more than 10 -around said impermeable internal pipe. By virtue of the compatibilizing agent as well as the nucleating agent, such a sheath has homogeneous mechanical characteristics in the thickness of the sheath obtained, even though the cooling rates of the extruded polymer layer are different.

According to a second aspect, the invention relates to a flexible tubular structure intended for offshore oil production, said flexible tubular structure comprising an impermeable internal pipe and at least one polymeric sheath with a determined thickness around said impermeable internal pipe, said sheath being obtained by hot extruding a softened layer of said thermoplastic elastomer polymer around said impermeable internal pipe and by cooling said softened layer; according to the invention, said thermoplastic elastomer polymer comprises a thermoplastic polymer based on polypropylene and an elastomer based on a terpolymer of ethylene-propylene-diene monomers, and it furthermore comprises a compatibilizing agent for mixing together said thermoplastic elastomer polymer and said compatibilizing agent before said hot extrusion, so as to increase the miscibility of said thermoplastic polymer based on polypropylene and said elastomer based on a terpolymer of ethylene-propylene-diene monomers during the extrusion, by means of which the elongation at break of said polymeric sheath is increased.

Thus, one characteristic resides in the use of an agent compatibilizing with the thermoplastic polymer and the elastomer during the extrusion, so as to obtain a thermoplastic elastomer polymer which, under the working conditions of pressure and temperature of the tubular structure, has uniform mechanical characteristics throughout the sheath so as to withstand the stresses which it experiences.

Advantageously, said compatibilizing agent is a (co)polymer selected from the group of polymers comprising polymers based on polyolefin or styrene-ethylene-butadiene-styrene (SEES) . These polymers have the advantage of being widely produced in industry, and that they can be obtained at advantageous prices.

Furthermore, said thermoplastic elastomer polymer preferably comprises a nucleating agent intended to induce a homogeneous distribution of crystallites over said determined thickness during the cooling phase, and thus obtain spherolites distributed uniformly both in the thickness of the sheath and over the circumference of the sheath. Advantageously, said nucleating agent comprises a phosphate salt and/or a dicarboxylate salt.

According to an advantageous characteristic, said thermoplastic elastomer polymer comprises between 40% and 70% by weight of said thermoplastic polymer based on polypropylene, between 10% and 20% by weight of said elastomer based on a terpolymer of ethylene-propylene-diene monomers, and between 1% and 20% by weight of said compatibilizing agent so as to obtain satisfactory mechanical properties and making it possible to avoid the risks of tearing. Furthermore said thermoplastic elastomer polymer advantageously comprises between 0.1% and 1% by weight of nucleating agent. In addition, said polymeric sheath has a thickness of more than 10 mm around said impermeable internal pipe and it has identical mechanical properties at all points.

According to a particular embodiment of the invention, said impermeable internal pipe comprises, from the inside to the outside of said pipe, an internal * 15 polymeric sealing sheath and reinforcing filaments wound around said internal polymeric sealing sheath in order to form at least one reinforcing layer. A flexible tubular pipe commonly used for the offshore transport of hydrocarbons is thus produced. In the present application, the term reinforcing filament equally covers metal wires or filaments of composite material, for example based on glass or carbon fibers, and woven strips or reinforcing cables made of metallic material or of high-strength fiber, for example of the aramid type.

Other features and advantages of the invention will become apparent on reading the following description of a particular embodiment of the invention, given by way of indication but without implying limitation and with reference to the appended drawings, in which: -figure 1 is a partial schematic view in perspective and cutaway of a flexible tubular pipe obtained by the method according to the invention, and -figure 2 is a schematic perspective view of an installation for carrying out the method according to the invention.

Figure 1 partially illustrates a flexible tubular structure 10, which will be described first before describing its production method for which protection is also sought. This tubular structure is a pipe intended especially for the transport of hydrocarbons and it has, from the inside outward, an impermeable internal pressure sheath 12 inside of which a hydrocarbon can flow. This internal pressure sheath 12 is surrounded by an armor layer 14 formed by a short-pitch winding of a shaped metal wire which is clamped and intended to take up the internal pressure forces with the internal sheath 12. Around the armor layer 14, two layers of tensile armor 16, 18 are wound with a long pitch, these being intended to take up the longitudinal tensile forces to which the pipe is subjected. These two layers of tensile armor 16, 18 wound around the armor layer, itself lying around the internal pressure sheath, together constitute the impermeable internal pipe. Lastly, the flexible tubular pipe 10 has an external protective sheath 20 intended to protect the aforementioned reinforcing layers 14, 16, 18. The subject matter of the invention more precisely relates to this external protective sheath 20. It will, however, be noted that an intermediate sheath of the same type may be inserted between the reinforcing layers 14, 16, 18, and that it can be produced with the same polymeric materials and according to the same manufacturing method as will be described below.

Referring to figure 2,. the installation making it possible to manufacture a flexible tubular pipe of the aforementioned type, and more particularly the external protective sheath 20, will be described first.

Thus, the installation schematically comprises an extruder 22 having a drive screw 24, which comprises a loading hopper 26 at one of its ends and the injection nozzle 28 at the other of its ends. Unlike extruders equipped with an injection mold, the extruder 22 presented here has a crosshead 30 adapted for a pipe 32 being manufactured to pass through it and adapted to be connected to the injection nozzle 28. The pipe is moved from the upstream end 31 of the crosshead 30 to its downstream end 35. Conventionally, the polymers in the form of granules are introduced into the loading hopper 26 and they are heated and softened inside the drive screw 24 and driven in molten form through the crosshead 30, which is adapted to form a cylindrical layer 33 of softened polymer over the outermost reinforcing layer 18 of the pipe 32 being manufactured.

This layer of polymer has a thickness of more than 15 Beyond the downstream end 35 of the crosshead 30, the pipe coated with the cylindrical layer 33 of softened polymer is then cooled, either in air or by passing through a water bath. However, the change from the extrusion temperature, which is more than the glass transition temperature of the polymer and close to the melting temperature, to room temperature is not instantaneous. Thus, as soon as the layer starts to pass through the cooling zone, a radial thermal gradient is set up in the thickness of the external sheath, the outside being colder than the inside. Quite clearly, this gradient disappears after a certain length of time.

Thus, the thermoplastic elastomer polymer used for forming the aforementioned external sheath comprises a first mixture including a vulcanized elastomer based on a terpolymer of ethylene-propylene-diene monomers (EPDM) and a thermoplastic polymer based on polypropylene (22) . Advantageously, this first mixture has been manufactured by a dynamic vulcanization method making it possible to obtain a crosslinked elastomer phase distributed in the form of fine particles dispersed in the matrix of the thermoplastic polymer.

The mechanical properties of the sheath 20 are improved in this way. This method, which is known to the person skilled in the art, is described particularly in US3037954. This first mixture comprises, by weight, about 60% of thermoplastic polymer and 15% of elastomer. The formulation of this first mixture also includes plasticizers and fillers. It will be noted that this first mixture is marketed in particular under the brand "Santoprène 203-50" by "ExxonF'Jobil".

A second mixture including a compatibilizing agent, in the case in point styrene-ethylene-butadiene-styrene (SEBS), is incorporated into this first mixture. The selected SEBS is advantageously a linear triblock copolymer based on styrene and ethylene-butadiene, marketed under the brand "Kraton� G-1650" by "Kraton Polymers". This second mixture is generally referred to as a master batch. This second mixture may also include a nucleating agent comprising a phosphate salt or a dicarboxylate salt. For example, this nucleating agent is a bicyclo[2.2.l]heptane dicarboxylate salt. The latter is marketed by "Milliken" under the reference "HPN-68". Thus, according to a preferred embodiment, a second mixture including, by weight, 46% of compatibilizing agent in the form of a styrene-ethylene-butadiene-styrene and 8% of nucleating agent, the bicyclo[2.2.1]heptane dicarboxylate salt, is provided.

The first mixture and the second mixture are then mixed together in a ratio of 100 parts by weight of the first mixture to 8 parts of the second mixture, in order to form a third mixture. This operation may be carried out automatically upstream of the extruder with the aid of a known device known as a gravimetric mixing doser. The first and second mixtures are provided in the form of granules which feed the two inlets of the mixing doser, said mixing doser providing a homogeneous mixture of granules in the desired proportions at its outlet. The outlet of the mixing doser is directly connected to the reception part of the extruder.

Thus, the compatibilizing agent, in the example given here the styrene-ethylene-butadiene-styrene (SEBS), will make it possible to increase the ability of the thermoplastic polymer based on polypropylene and the elastomer when the two polymers are melted together. In fact, these two categories of polymer are not very miscible in one another when they are molten.

In this way, the first and second mixtures together constitute a third mixture incorporating 55.5% of thermoplastic polymer based on polypropylene, 14% of a terpolymer of ethylene-propylene-diene monomers, 3.4% of compatibilizing agent, i.e. the styrene-ethylene-butadiene-styrene, and 0.6% of nucleating agent, i.e. the bicyclo[2.2.1]heptane dicarboxylate salt.

It will be noted that the aforementioned formulation is an advantageous formulation, but that other formulations could be suitable. Also, the thermoplastic polymer composition is preferably between 40% and 70% by weight of the total formulation. As regards the elastomer, it is between 10% and 20%. The compatibilizing agent is for its part between 1% and 20%. The nucleating agent, if it is incorporated into the thermoplastic elastomer, is between 0.1% and 1%.

Quite clearly, the formulation also comprises conventional and well-known additives such as a thermal stabilizer, antioxidant, anti-UV, fillers or plasticizers.

Thus, the third mixture, or final formulation, which is in the form of a mixture of granules, is introduced into the reception hopper 26 of the installation represented in figure 2. In this way, a softened and homogeneous layer of the third mixture is formed by hot extrusion around the outermost reinforcing layer, then this layer is cooled in order to obtain a polymeric sheath with a determined thickness around the reinforcing layer.

Tests were carried out in order to verify the performance of this sheath and compare its properties with those of sheaths produced according to the prior art. These tests consisted in full-scale extrusion of an external sheath with a thickness varying between 10 mm and 20 mm around a flexible pipe core with a diameter of the order of 225 mm. Said core comprises all the conventional layers of a flexible pipe, apart from the external sheath. Single-screw extruders with a diameter of 150 mm and 155 mm were used. The prior art solutions tested are on the one hand the mixture polypropylene/EPDM without nucleating agent or compatibilizing agent, and on the other hand the mixture polypropylene/EPDM with a nucleating agent but without compatibilizing agent. The nucleating agent used was a bicyclo[2.2.1]heptane dicarboxylate salt according to the teaching of EP1619218.

Sheath samples were then taken and analyzed in the laboratory. It was first observed that the sheaths produced according to the present invention have satisfactory mechanical characteristics, even when the thickness of the sheath is more than 15 mm. It would in theory be better to use the value of elongation at the threshold in order to class and select suitable materials, but threshold elongation measurements are not reliable for these highly ductile polymers.

Furthermore, it was established with the same sampling and test method that the sheaths produced according to the present invention have a greater elongation at break than those produced according to the prior art, particularly in the case in which the sheath has a thickness of more than 15 mm, typically 20 mm, and when the traction test-pieces are taken in proximity to the internal face. In the most severe cases, the elongation at break of the material according to the present invention may be more than two times greater than that

of the materials according to the prior art.

In reality, by analyzing the aforementioned advantageous formulation in comparison with the

formulations according to the prior art, a certain

decline in the mechanical properties of the sheath portions lying in the vicinity of the reinforcing layer was observed when the thickness of the sheath increases. Nevertheless, not only is the elongation at break of these sheath portions with the aforementioned advantageous formulation greater irrespective of the thickness of the sheath, but furthermore the difference between the sheaths obtained from the two formulations increases with the thickness in favor of the sheath produced with the advantageous formulation. Thus, for a sheath with a thickness of 5 mm, the elongation at break of the sheath portions lying in the vicinity of the reinforcing layer is 500% for the sheath obtained with the aforementioned advantageous formulation, and 450% for the sheath obtained with the formulation according to the prior art. When the thickness of the sheath is 10 mm, these elongations are respectively 300% and 200%. When the thickness of the sheath is 20 mm, these elongations at break are respectively found to be 200% and 50%. Furthermore, the two aforementioned solutions of the prior art lead to elongations at break of the same order of magnitude, and adding a nucleating agent does not seem to have a significant effect on the mechanical properties of the sheath, particularly in the case of thick sheaths. Thus, the elongation at break of the material according to the invention is at least 200% in the thickness of the sheath for thicknesses of between 15 and 20 mm, which considerably improves the flexibility of the sheaths produced in this way.

Tests were also carried out under conditions similar to those described above, on the one hand using the mixture polypropylene/EPDN with compatibilizing agent but without nucleating agent, and on the other hand the mixture polypropylene/EPDM with compatibilizing agent and with nucleating agent. When the thickness of the sheath is 20 mm, the elongations at break are then respectively found to be 200% and 250%. Thus, owing to the combination of a compatibilizing agent and a nucleating agent, the elongation at break of the material according to the invention is at least 250% in the thickness of the sheath, for thicknesses of between and 20 -, which considerably improves the flexibility of the sheaths produced in this way.

It was furthermore checked that the thickness measurement of the sheaths of thermoplastic elastomer material produced in this way could be carried out precisely and reliably with conventional methods of nondestructive inspection by ultrasound echography, which has not always been the case with the thermoplastic elastomer polymers of the prior art. This is because in the sheaths with large thicknesses produced according to the prior art, it was in fact difficult to obtain a reliable measurement of this thickness by these ultrasound methods because the propagation speed of the ultrasound was not homogeneous enough in the thickness of the sheath. Conversely, by virtue of the new material according to the invention, on the one hand the propagation speed of the ultrasound is much more homogeneous in the thickness of the sheath, and on the other hand the ultrasound attenuation is greatly reduced, so that the measurement by ultrasound echography is more reliable and more precise. This nondestructive inspection thus allows the thickness of the thermoplastic elastomer sheath to be measured continuously during the manufacture of the tubular structure.

According to another exemplary formulation, the compatibilizing agent was increased to 15% in the third mixture. It was then observed that the elongation at break properties of the sheath were increased. It will, however, be noted that conversely the thermal resistance of the sheath is substantially affected.

Now, in certain applications it is necessary to maintain strong thermal insulation properties of the flexible pipe. Thus, it has been shown that the formulation of the third mixture should advantageously include between 2% and 5% of compatibilizing agent based on styrene-ethylene-butadiene-styrene (SEBS) . In this way, the thermal conductivity of the polymer of the sheath thus obtained is less than 0.2 W/m.K., which allows the tubular flexible pipe produced in this way to be provided with good thermal insulation.

Under other circumstances, a different compatibilizing agent may be incorporated into the formulation so as to replace the polymer based on styrene-ethylene-butadiene-styrene (SEBS), in particular a polymer based on polyolefin and more precisely polyethylene. For example, the polyethylene marketed under the brand "Exceed 1O18CA" by "Exxon" has been used. This is a linear hexene copolymer belonging to the family of linear low density polyethylenes (LLDPE) It was also checked that within the 20% by weight limit of the formulation, the more the proportion of this compatibilizing agent is increased, the more the mechanical properties and in particular the elongation of the polymer of the sheath obtained are improved.

According to a particular embodiment of the invention, a tubular structure of the umbilical type is produced.

Thus, an internal sealing tube and electrical cables are assembled. Said assembly is carried out by helicoid winding of the tubes and cables so that the umbilical is flexible. The assembly, which then forms an impermeable internal pipe, is surrounded by an external sealing sheath of thermoplastic elastomer polymer according to the method of the invention as described above.

Claims

-25 -CLAIMS1. A method of manufacturing a flexible tubular structure (10) intended for offshore oil production, said method being of the type according to which: -an impermeable internal pipe (12, 14, 16, 18) is provided; -a thermoplastic elastomer polymer is provided; -a softened layer (33) of said thermoplastic elastomer polymer is formed by hot extrusion around said impermeable internal pipe (12, 14, 16, 18); -said softened layer (33) is cooled in order to obtain at least one polymeric sheath (20) with a determined thickness around said impermeable internal pipe (12, 14, 16, 18); characterized in that said thermoplastic elastomer polymer comprises a thermoplastic polymer based on polypropylene (PP) and an elastomer based on a terpolymer of ethylene-propylene-diene monomers (EPDM), and in that a compatibilizing agent is furthermore provided for mixing together said thermoplastic elastomer polymer and said compatibilizing agent before said hot extrusion, so as to increase the miscibility of said thermoplastic polymer based on polypropylene and said elastomer based on a terpolymer of ethylene-propylene-diene monomers (EPDM) during the extrusion, by means of which the elongation at break of said polymeric sheath (20) is increased.
2. The method as claimed in claim 1, characterized in that a polymer selected from polymers based on polyolefin or styrene-ethylene-butadiene-styrene (SEBS) is provided as the compatibilizing agent.
3. The method as claimed in claim 1 or 2, characterized in that a nucleating agent is furthermore provided, and in that said nucleating agent, said compatibilizing agent, said thermoplastic polymer based -26 -on polypropylene and said elastomer based on a terpolymer of ethylene-propylene-diene monomers are mixed together before said hot extrusion.
4. The method as claimed in claim 3, characterized in that a nucleating agent comprising a phosphate salt and/or a dicarboxylate salt is provided.
5. The method as claimed in any one of claims 1 to 4, characterized in that a thermoplastic elastomer polymer comprising between 40% and 70% by weight of said thermoplastic polymer based on polypropylene, between 10% and 20% by weight of said elastomer based on a terpolymer of ethylene-propylene-diene, and between 1% and 20% by weight of said compatibilizing agent, is provided.
6. The method as claimed in claims 3 or 4 and 5, characterized in that a thermoplastic elastomer polymer furthermore comprising between 0.1% and 1% by weight of nucleating agent is provided.
7. The method as claimed in any one of claims 1 to 6, characterized in that said thermoplastic elastomer polymer is obtained according to a dynamic vulcanization method.
8. The method as claimed in any one of claims 1 to 7, characterized in that a softened layer of said thermoplastic elastomer polymer is formed by hot extrusion around said impermeable internal pipe (18) so as to obtain a polymeric sheath with a thickness of more than 10 mm around said impermeable internal pipe (18)
9. A flexible tubular structure (10) intended for offshore oil production, said flexible tubular structure comprising an impermeable internal pipe (12, -27 - 14, 16, 18) and at least one polymeric sheath (20) with a determined thickness around said impermeable internal pipe (12, 14, 16, 18), said sheath (20) being obtained by hot extruding a softened layer (33) of said thermoplastic elastomer polymer around said impermeable internal pipe (12, 14, 16, 18) and by cooling said softened layer (33); characterized in that said thermoplastic elastomer polymer comprises a thermoplastic polymer based on polypropylene and an elastomer based on a terpolymer of ethylene-propylene-diene monomers, and in that it furthermore comprises a compatibilizing agent for mixing together said thermoplastic elastomer polymer and said compatibilizing agent before said hot extrusion, so as to increase the miscibility of said thermoplastic polymer based on polypropylene and said elastomer based on a terpolymer of ethylene-propylene-diene monomers during the extrusion, by means of which the elongation at break of said sheath (20) is increased.
10. The flexible tubular structure as claimed in claim 9, characterized in that said compatibilizing agent is a polymer selected from the group of polymers comprising polymers based on polyolefin or styrene-ethylene-butadiene-styrene (SEBS), or a mixture of these polymers.
11. The flexible tubular structure as claimed in claim 9 or 10, characterized in that said thermoplastic elastomer polymer comprises a nucleating agent intended to induce a homogeneous distribution of crystailites over said determined thickness.
12. The flexible tubular structure as claimed in claim 11, characterized in that said nucleating agent comprises a phosphate salt and/or a dicarboxylate salt.-28 -
13. The flexible tubular structure as claimed in any one of claims 9 to 12, characterized in that said thermoplastic elastomer polymer comprises between 40% and 70% by weight of said thermoplastic polymer based on polypropylene, between 10% and 20% by weight of said elastomer based on a terpolymer of ethylene-propylene-diene monomers, and between 1% and 20% by weight of said compatibilizing agent.
14. The flexible tubular structure as claimed in claims 11 or 12 and 13, characterized in that said thermoplastic elastomer polymer furthermore comprises between 0.1% and 1% by weight of nucleating agent.
15. The flexible tubular structure as claimed in any one of claims 9 to 14, characterized in that said polymeric sheath (20) has a thickness of more than 10 mm around said impermeable internal pipe (12, 14, 16, 18)
16. The flexible tubular structure as claimed in any one of claims 9 to 15, characterized in that said impermeable internal pipe (12, 14, 16, 18) comprises, from the inside to the outside of said pipe, an internal polymeric sealing sheath (12) and reinforcing filaments wound around said internal polymeric sealing sheath in order to form at least one reinforcing layer (14, 16, 18)