WO2007055583A1

WO2007055583A1 - Flexible duct for cryogenic fluids

Info

Publication number: WO2007055583A1
Application number: PCT/NO2006/000397
Authority: WO
Inventors: Mallen Herrero JOSÉ
Original assignee: Advanced Production And Loading As
Priority date: 2005-11-09
Filing date: 2006-11-06
Publication date: 2007-05-18
Also published as: NO20055260D0; NO329279B1; EP1945983A1

Abstract

The invention relates to a flexible duct for a cryogenic fluid comprising, from the inside to the outside, an inner duct (10), a first insulation and sealing layer (20), a second insulation and sealing layer (30) and an outer layer (40) characterized in that the inner duct (10) is made of a at least on carcass (1), the first insulation and sealing layer (20) comprising a sub-layer (21) made of an insulation means (2) combined with a sealing means (3), said sub-layer (21) being located above and in direct contact with the carcass (1).

Description

Flexible duct for cryogenic fluids

Field of the invention

The invention relates to a flexible duct for cryogenic fluids. More specifically, the present invention relates to a flexible duct used in marine and/or submarine environments in order to carry cryogenics fluids such as liquefied natural gas or the like between two sets, such as, for instance, a fixed structure and a floating structure or two floating structures or a floating structure and the seabed.

Prior art

Due to the low temperatures involved by the presence of cryogenic fluids, the flexible duct comprises insulation means to avoid heat transfer between the inner duct, wherein the cryogenic fluid flows, and the outside of the duct which is at ambient temperature, that is to say at sea temperature. When the duct is dived in sea water, the temperature of the outer part of the wall of the duct has to be higher than the temperature of solidification of sea water. Indeed, if it was not the case, the ice generated at the outer part of the duct would have a negative impact on the flexibility of the duct and could break it.

Such a flexible duct further comprises sealing means to prevent both the mass transfer of the cryogenic fluid and of sea water through the wall of the duct. These sealing means are respectively settled in the vicinity of the inner duct and on the outer part of the wall of the duct.

The man skilled in the art knows many solutions to meet the above-mentioned requirements. A well-known solution is a structure which comprises, from the inside to the outside of the duct, an inner yielding duct or tube such as a corrugated duct (bellow), a heat insulation surrounding the inner duct and an outer protective wrapper or sheath having a sealing function, surrounding this heat insulation. In the working conditions prevailing at sea, this kind of solutions has not proved to be fully effective because of the behaviour of such a structure to cyclic fatigue. Thus, other structures have been proposed. The document FR 2475185 discloses a fluid-conveying, flexible pipe built up in particular for carrying liquefied natural gas and which comprises an inner tubular duct consisting of a first metal wire helically wound with mutually spaced turns, an intermediate layer of yielding composite material surrounding the inner layer, and a second metal wire helically wound about the intermediate layer. The second metal wire have its turns longitudinally offset by one half winding pitch with respect to the first metal wire and the intermediate layer of winding composite material is interposed between the first and the second metal wires. The intermediate layer consists of a superposition of offsets bands with each other, ensuring a sealing function to avoid any leakage of the cryogenic fluid. This structure avoids in particular the problems encountered with the cyclic fatigue. The structure disclosed in this document further comprises, as it is well-known in the related art, an insulation layer surrounding the inner tubular duct and an outer protective sheath or wrapper which has a sealing function to avoid a flood of sea water into the duct. Insulation and sealing functions are realized by two separate layers.

The solution based on the superposition of offsets bands with each other, forming a set of baffles, has proved to be effective to limit the cryogenic leakages. For instance, the paper entitled "Offshore LNG transfer - a new flexible cryogenic hose for dynamic service" prepared for presentation at the Offshore Technology Conference, Houston, Texas, USA, 3-6 May 2004 discloses conclusive tests made with an experimental set-up on the structure disclosed in FR 2475185. However, this solution is quite complex and other kinds of effective structures, in particular more simple to manufacture, can be proposed.

An objective of the present invention is to propose such a structure. Summary of the invention

The objective delineated hereinabove is reached in the present invention thanks to a Flexible duct for a cryogenic fluid comprising, from the inside to the outside, an inner duct, a first insulation and sealing layer, a second insulation and sealing layer and an outer layer characterized in that the inner duct is made of a at least one carcass, the first insulation and sealing layer comprising a sub-layer made of an insulation means combined with a sealing means, said sub-layer being located above and in direct contact with the carcass

The flexible duct of the present invention will be able to have at least one of the following characteristics: - the insulation and sealing layer comprises a low temperature plastic sheath located above and in direct contact with the first sub-layer; the insulation means is a band, said band being embedded in the sealing means and helically wounded around the carcass; the sealing means is made of at least one polymeric film comprising a protuberant part extending beyond the band of insulation; the sealing means is made of at least one metallic film; - the insulation means is a band helically wounded around the carcass thus forming different bandwidths and in that the sealing means is made of several bands staggered around the insulation means; the insulation means and the sealing means are both formed of several layers, these layers being distinct ones from the other; the insulation means is a band helically wounded around the carcass thus forming different bandwidths and in that the sealing means is a polymerizable silicone layer, said bandwidths being stuck with each other thanks to the polymerizable silicone layer; the insulation means is a band helically wounded around the carcass thus forming different bandwidths and in that the sealing means is a Velcro^® band, said Velcro^® band fastening the different bandwidths with each other; the sealing means comprises a first polymeric or metallic film laid out in contact with the insulation means and a second polymeric film laid out in contact with the first film; the second polymeric film is a band helically wounded around the carcass on several layers thus forming a set of layers, alternatively disposed with layers formed by a combination of the first polymeric or metallic film with the insulation means; the second polymeric film is made of polyurethane or polyester; the insulation means is a band, said band being embedded in the first polymeric or metallic film; - the insulation means is a band helically wounded around the carcass thus forming different bandwidths, and in that the first polymeric or metallic film is a polymerizable silicone layer or a Velcro^® band to stick or respectively fasten the bandwidths; - the insulation and sealing layer further comprises at least one band of composite material made of polymer/metal/polymer; the insulation and sealing layer further comprises at least one thin metallic film in direct contact with the inner duct; - the carcass is an interlocked strip; the interlocked strip comprises two faces and a packing facet located between the two faces of the interlocked strip, said packing facet having a role of seal; the carcass is made of folded-seam strips; the folded-seam strips are chosen among one or a combination of the U, T, S or Z-shaped wires; the said at least one carcass is made of two carcasses, the first one being an interlocked strip carcass and the second one being a folded-seam strips carcass; a sealing means is interposed between the two carcasses; the inner duct comprises an interlocked strip carcass and a pressure arch in direct contact with the carcass; - a sealing means is interposed between the carcass and the pressure arch; the second insulation and sealing layer comprises, from the inside to the outside, a sub-layer made of an insulation means, and an intermediate plastic sheath, said sub-layer being located above and in direct contact with the first insulation and sealing layer; the outer layer comprises, from the inside to the outside, a pressure arch, tensile armours and an outer plastic sheath; the flexible duct is used for a floating application; - the flexible duct is used for a seabed application.

Brief description of the drawings

Further objectives, characterizing features, details and advantages of the present invention will appear by reading the detailed description which follows, and with regard to the enclosed drawings, given by way of non-limiting examples of several embodiments, and wherein:

Fig. 1 is a fragmentary view in partial longitudinal section, showing the structure of the wall of a flexible duct in conformity with the invention;

Fig. 2a is a fragmentary view in partial longitudinal section, drawn on a larger scale, and showing a sub-layer of the wall of the duct comprising an insulation means combined with a sealing means;

Fig. 2b is a perspective view of the sub-layer shown in Fig. 2 before it is settled on the wall of the flexible duct; - Fig. 3a is a fragmentary view in partial longitudinal section, drawn on a larger scale, and showing a sub-layer of the wall of the duct comprising an insulation means combined with a sealing means according to an alternative embodiment of that one shown in Fig. 2; - Fig. 3b is a fragmentary view in partial longitudinal section, drawn on a larger scale, and showing a sub-layer of the wall of the duct comprising an insulation means combined with a sealing means according to an alternative embodiment of that one shown in Fig. 2 - Fig. 4 is a fragmentary view in partial longitudinal section, showing an embodiment of the inner duct of the pipe; Fig. 5a, 5b, 5c and 5d are fragmentary views in partial longitudinal section, showing different embodiments of the inner duct of the pipe, - Fig. 6 is a fragmentary view in partial longitudinal section, showing the structure of the wall of the cryogenic core of the flexible duct in an alternative embodiment of the invention.

Detailed description

Figure 1 shows the structure of the wall of the flexible duct in conformity with the present invention. The flexible duct consists of, from the inside to the outside, an inner duct 10, a first insulation and sealing layer 20, a second insulation and sealing layer 30 and an outer layer 40.

The inner duct 10 is made of at least one carcass 1 generally tube- shaped. The cryogenic fluid, such as liquefied natural gas or the like, flows into the inner duct 10 according the longitudinal axis of the flexible duct, given by the direction Oz. In a preferred embodiment, a single carcass 1 is employed and it consists of an interlocked strip, as shown in figure 1, whose structure and nature is able to avoid the collapse of the inner duct 10. The material used is also able to challenge the severe requirements due to the very low temperatures involved (for instance - 162°C for liquefied natural gas). Alternative embodiments for the carcass 1 can be employed, as it is described hereafter with the support of figures 4 and 5a to 5d. The range of materials able to reach these requirements is quite restricted. It can be considered the use of austenitic steel such as 316L or the use of an alloy with Aluminium of the series 5000 and 6000. This alloy is known to be resistive to corrosion, what is important in a marine environment. It can also be considered an alloy with lead to make heavy the duct. This is particularly relevant in some applications wherein the buoyancy of the duct is too high or for seabed applications.

The first insulation and sealing layer 20 comprises, from the inside to the outside of the flexible duct, a first sub-layer 21 made of an insulation means 2 combined with a sealing means 3, and a low temperature plastic sheath 4. The insulation means 2 is a band of insulating material wound around the carcass 1 of the inner duct 10. More specifically, the insulation means 2 consists of at least one band wound in several layers wrapped one onto the other and in an offset manner so that an end 2a, 2b of a bandwidth of a lower layer of the band 2 is entirely overlapped by a bandwidth of an upper layer of the band 2. Within a layer, the band 2 is preferably wound so that the successive bandwidths, made by the successive turns of the band 2 around the carcass 1, are placed edge to edge. Typically, the thickness of the band 2 is comprised between 1 and 25mm and its bandwidth is comprised between 20 and 300mm. The insulation means 2 can be made of short fibres, spinning fibres or compacted powder.

The thickness of the sub-layer 21 is calculated so that the temperature at the level of the transition between the sub-layer 21 and the low temperature plastic sheath 4, which are in direct contact with each other, is high enough to fix such a plastic sheath 4 without any risk of rupture. It can easily be understood that the nature of the material making the low temperature plastic sheath 4 depends upon the thickness of the sub-layer 21, once given the nature of the insulating means 2. This low temperature plastic sheath 4 has a role of sealing with respect to the cryogenic fluid flowing into the inner duct 10.

For example, the low temperature plastic sheath 4 can be made of polyethylene for a temperature close to or higher than -50°C/-40°C. Indeed, above this temperature, the polyethylene has a mechanical behaviour which is no longer brittle. In that case, the thickness of the sub-layer 21 yields typically 25mm, 28mm or 30mm for an inner duct 10 of internal diameter 12.7 cm, 25.4cm or 50.8 cm respectively.

In the scope of the present invention, it can be considered other plastic material such as polyethylene tephthalate (PET), polypyromellitimide (PPMI), aromatic polyimides, polyethylene- 2, 6- naphthalenedicarboxylate (PEN) or polyphenylene sulphide (PPS) for the low temperature plastic sheath 4. In all cases, the thickness of the sublayer 21 depends upon the nature of the low temperature plastic sheath 4. The more resistive is the plastic sheath 4 to low temperatures, the thinner is the sub-layer 21 and conversely.

The sub-layer 21 further comprises a sealing means 3 which is combined with the insulating means 2. In a preferred embodiment, the insulating means 2 is embedded in the sealing means 3 which is made of at least one thin polymeric film, as shown in figure 1, 2a and 2b. This thin polymeric film 3 is sealed at the level of a protuberant part 3a, extending beyond the insulation means 2 as shown in figures 2a and 2b. In operation, the protuberant parts 3a of the thin polymeric film 3 create a set of baffles preventing any leakage of cryogenic fluid between the interstices existing between the bandwidths preferably placed edge to edge. It is the case for the part of the sub-layer 21 which is in direct contact with the carcass 1, but too between two layers of the combined insulating and sealing means 2, 3 wrapped one onto the other. Indeed, the staggered arrangement of these layers and the width of the protuberant part 3a of the thin polymeric film 3 make more difficult a leakage of cryogenic fluid. At the level of the protuberant part 3a, the thin polymeric film 3 can be thermo-fused, stuck with a special adhesive such as polyurethane, PTFE or stuck with any glue adapted to low temperatures.

In an alternative embodiment, shown in figure 3a, the insulation means 2 is not embedded in a thin polymeric film 3, but is surrounded with several bands of thin polymeric films 3 which are more precisely staggered around the insulation means 2 so as to create a set of baffles ensuring sealing.

In another alternative embodiment shown in figure 3b, the insulation means 2 and the sealing means 3 are both formed of several layers, these layers being distinct ones from the other. More precisely, in this embodiment, the layers of insulator are alternated with the layers of thin polymeric films.

An alternative embodiment, which corresponds to a mixture between the embodiments illustrated on Fig. Ia and on Fig. 3b is also possible. In this embodiment, the general structure of the sub-layer 21 is that one of the Fig. 3b, and the insulation means 2 is- protected, embedded, wrapped in an impervious manner, by the sealing means 3 as in Fig Ia.

In another embodiment, it can be considered as sealing means 3 a silicone layer which is polymerizable at ambient temperature, said bandwidths preferably placed edge to edge being stuck with each other thanks to the polymerizable silicone layer 3.

In another embodiment, the sealing means 3 can be a Velcro^® band fastening the different bandwidths of the band 2 with each other thanks to indentations. Moreover, these indentations form a set of baffles limiting the cryogenic fluid from passing through the sub-layer

21. Nevertheless, with such a solution, sealing is not perfect.

For all embodiments cited above, the use of a carcass 1 in a flexible duct applied to cryogenic fluids conveying allows to combine the insulating means 2 with the sealing means 3 because the carcass 1 provides a support for the band of insulation 2.

In the prior art, the structures proposed for cryogenic applications do not allow such a combination of an insulating means with a sealing means (for the cryogenic fluid) because the sealing means are part of the structure forming the inner duct. It explains why, in the prior art, the sealing means and the insulating means form two separate layers.

Moreover, if the structure of carcass is known in the prior art, it is only used for non-cryogenic applications and the sealing means in direct contact with this carcass is always made of a semi-rigid, plastic sleeve or tube but not of a thin polymeric film as disclosed in the present invention. A semi-rigid, plastic sleeve presents disadvantages at ambient temperature because it can be pinched by the carcass (effect of the extrusion process) what can brings about a rupture of the semi- rigid, plastic sleeve. It is worse for cryogenic applications, because the material is more brittle.

In the scope of the present invention, the set formed by the combination of the insulating means 2 with the sealing means 3, provides it a minimum of rigidity, so that the sealing means 3 can be maintained in direct contact with the carcass 1 without being pinched by this carcass 1. In addition, this set remains flexible enough to follow the bending movements of the flexible duct.

The set formed by the carcass 1, the sub-layer 21 comprising the combined insulating and sealing means 2, 3, and the low temperature plastic sheath 4 is also called the cryogenic core of the flexible duct.

The second insulation and sealing layer 30 comprises, from the inside to the outside, a second sub-layer 31 made of an insulation means 5, and an intermediate plastic sheath 6. The insulation means 5 is a band wound around the cryogenic core of the flexible duct, in an analogous manner of the insulation means 2. Thus, the second sub-layer 31 is in direct contact with the low temperature plastic sheath 4. The thickness of the second sub-layer 31 is calculated as a function of the temperature of the cryogenic fluid which flows into the inner duct in order to obtain a temperature close to and preferably at least slightly above O⁰C at the level of the intermediate plastic sheath 6.

The intermediate plastic sheath 6 is useful if a crack or a rupture of the outer plastic sheath 9 occurred because, in that case, it would prevent water from flooding the insulation means 5, changing the insulation capacity of the insulation means 5. In addition, in such a situation, the level of temperature maintained in the intermediate plastic sheath 6 prevents sea water from freezing, so that the flexible duct keeps on normally conveying the cryogenic fluid. The outer layer 40 comprises, in a preferred embodiment, a pressure arch 7, tensile armours 8 and an outer plastic sheath 9 in direct contact with the outside of the flexible duct, that is to say with sea water. This structure of the outer layer 40 but also the materials that can be implemented respectively for the pressure arch 7, the tensile armours 8 and the outer plastic sheath 9 are well-known in the related art and will not be described in details. The pressure arch 7 and the tensile armours 8 have respectively a role of mechanical resistance (to avoid the collapse of the duct) and of reinforcement, the outer plastic sheath 9 preventing the flood of sea water inside the wall of the flexible duct. As a function of their weight and/or density, the pressure arch 7 and the tensile armours 8 can either be designed for seabed applications by making heavier the duct or for floating applications.

The figure 4 shows an alternative embodiment for the interlocked strips. In this embodiment, the interlocked strip 1 comprises a packing facet 11, which is helically wound around the carcass 1 and which extends along this carcass 1. More specifically, the packing facet 11 is interposed between the two faces Ia and Ib of the interlocked strip 1. The packing facet 11 is employed as a seal.

Figures 5a, 5b, 5c and 5d show alternative embodiments to the interlocked strip for the carcass 1 which are different shapes of folded- seam strips. More specifically, figure 5a shows folded-seam strips made of S or Z-shaped 11 wires whereas T-shaped wires 12 and U-shaped wires 14 are respectively shown in figures 5b and 5d. A combination of these shapes of wires can be considered, as shown in figures 5d where two successive T-shapes wires 15a similar to wires 12 are joined by a U- shaped wire 15b and conversely. Moreover, it is obvious to the one skilled in the art that the materials which can be implemented are the same as those cited above for the interlocked strips.

In another embodiment (not shown), the said at least one carcass 1 can be made of a combination of two carcasses superposed, the first carcass being made of an interlocked strip and the second carcass being made of folded-seam strips and located above the first carcass. This combination is particularly relevant if the duct has to be made heavy, the first carcass consisting, in that case, of a material such as inox and the second carcass consisting of a dense material such as lead. This structure composed of two carcasses superposed is also interesting because a thin layer of a flexible material forming a seal can be added between the two carcasses. At last, this combination of two carcasses reinforces the mechanical behaviour to avoid the collapse of the duct.

On the basis of this last embodiment, one can be considered a first carcass made of an interlocked strip and a pressure arch instead of the second carcass with the same advantages. Furthermore, the sub-layer 21 can comprise at least one band of composite material, made of polymer/metal/polymer such as the Triplex^® product or the like to perfect the combination of the insulation means 2 with the sealing means 3. This band of composite material is preferably located between the sub-layer 21 and the low temperature plastic sheath 4, but it can be implemented, inside the sub-layer 21, between two layers of the insulation band 2.

This sub-layer 21 can also comprise at least one thin metallic film in addition of the combination of the insulation means 2 with the sealing means 3 as described above. In that case, the thin metallic film is located at the basis of the sub-layer 21 that is to say in direct contact with the inner duct 10 for all embodiments disclosed. The use of a thin metallic film in direct contact with the inner duct 10 is interesting to avoid the pinch of the film by the carcass 1. Besides, a thin metallic film can also be mixed with a thin polymeric film within the sealing means 3. For example, the metallic film can be made of Aluminium.

In a particular embodiment, said at least one thin polymeric film forming the sealing means 3 of the sub-layer 21 can also be replaced by at least one thin metallic film made of Aluminium, for example.

Finally, other embodiments can be considered for the structure of the wall which is located outside the cryogenic core. Thus, these embodiments concern modifications of the set formed by the second insulation and sealing layer 30 plus the outer layer 40, said set having a role of mechanical resistance, reinforcement and sealing.

More specifically, metallic or fibre composite wires for mechanical resistance mixed or not with a flexible and insulating product can be implemented between the low temperature plastic sheath 4 and the intermediate plastic sheath 6. Such a product can be for example the Vegaprene^® product. The reinforcement material added depends on the application desired. If the flexible duct is designed to float, then the reinforcement material is preferably a composite fibre as Kevlar^®. On the contrary, if the flexible duct is not designed to float, then metallic armours in the form of wires or stranded cables are preferable. In both cases, this reinforcement material can be mixed with the flexible and insulating product or can be separated from the flexible and insulating product. If the reinforcement material is separated from the flexible and insulating product then it can be implemented inside the sub-layer 31, that is to say between two layers of insulation, or outside this sub-layer 31, that is to say in direct contact with the intermediate plastic sheath 4. In all cases, the outer layer 40 is kept

In an alternative embodiment (figure 6), the sealing means 3 is made of a first polymeric or metallic film 310 as described hereinabove with the different embodiments illustrated on figures 1, 2a, 2b, 3a and 3b and of at least one second polymeric film 300 laid out in contact with the first film 310.

The second polymeric film 300 is wounded around the carcass 1 in several layers. The set of layers 210 thus formed by a winding operation extends between two layers 211, each made of the first polymeric or metallic film 310 (see the embodiments illustrated with figures 1, 2a, 2b, 3a and 3b) combined with, embedded or also laid out in contact with the insulation means 2. A first set of layers 210 of the second polymeric film 300 can be in direct contact with the carcass 1, a first layer 211 being positioned after the first set of layers 210 of the second polymeric film 300, considering the radial direction of the flexible duct, from the inside to the outside of the duct. On the contrary, a first layer 211 of the first polymeric or metallic film 211 can be in direct contact with the carcass 1, a first set of layers 210 of the second polymeric film 300 being positioned after the first film 211.

More precisely, the layers 211 and 210 are alternated in the radial direction of the duct. One can be understood that the second polymeric film 300 is also combined with the insulation means 2, within the sublayer 21.

Of course, the number of alternative layers 211, 210 is calculated so as to obtain an acceptable temperature at the level of the transition between the sub-layer 21 and the low temperature plastic sheath 4, as described previously.

The use of the second polymeric film 300, especially in many layers, together with the first film 310 improves the efficiency of the sealing of the sealing means 3.

Moreover, the second polymeric film 300 can be covered at least on one of its faces by a metallized surface, which can limit radiative heat transfer. The second polymeric film 300 can be made of polyurethane, polyester or of any convenient polymeric material.

Once again, the first polymeric film 310 can be replaced by a silicone layer which is polymerizable at ambient temperature or by a Velcro^® band.

Moreover, the sub-layer 21 can also comprise a wire 320 wounded with the second polymeric film 300 in order to drain static electricity. In addition, the sub-layer 21 can also comprise another wire 330, for instance an aramid fiber or a band, wounded around each layer of the layer 210 and also around the film 310 to maintain them correctly positioned with each other, and more generally with respect to the carcass 1 of the flexible duct.

Indeed, during the use or the installation of the flexible duct, this latter can be put in many positions, especially in a vertical position, position which could bring about for instance a displacement of the different layers of the layer 210 ones compared to the others. One can be understood that such a displacement is not desired, because it would diminish the performance (sealing) of the flexible duct.

To maintain the efficiency of the flexible duct in all the positions, a layer of the set of layers 210 will be formed by a winding operation with a pitch adapted to obtain a sufficient overlap. For instance, but not exclusively, the pitch of the winding can be chosen to have an overlap of about 2/3.

Characteristic dimensions of the different means used for the flexible duct are specified herebelow: the thickness of the carcass 1 is generally comprised between 0,5 and 2,5mm; the thickness of the films 300, 310 is in both cases comprised between 25μm et 200μm; the thickness of the intermediate plastic sheath 6 is comprised between 3mm and 10mm, and the thickness of the outer plastic sheath 9 is also comprised between 3mm and 10mm.

The invention described hereinabove is particularly interesting because it discloses many sealing barriers, especially in the cryogenic core wherein sealing means 3 and a low temperature plastic sheath 4 are considered. Alternative embodiments, disclosing additional means to seal the cryogenic core go on this way.

It is to be understood that the invention may be practiced in ways other than those specifically delineated hereinabove without departing from the scope of the invention.

Claims

1. Flexible duct for a cryogenic fluid comprising, from the inside to the outside, an inner duct (10), a first insulation and sealing layer (20), a second insulation and sealing layer (30) and an outer layer (40) characterized in that the inner duct (10) is made of a at least one carcass (1), the first insulation and sealing layer (20) comprising a sub- layer (21) made of an insulation means (2) combined with a sealing means (3), said sub-layer (21) being located above and in direct contact with the carcass (1).

2. Flexible duct according to claim 1, characterized in that the insulation and sealing layer (20) comprises a low temperature plastic sheath (4) located above and in direct contact with the first sub-layer (21).

3. Flexible duct according to one of the preceding claims, characterized in that the insulation means (2) is a band, said band being embedded in the sealing means (3) and helically wounded around the carcass (1).

4. Flexible duct according to claim 3, characterized in that the sealing means (3) is made of at least one polymeric film comprising a protuberant part (3a) extending beyond the band of insulation (2).

5. Flexible duct according to claim 3, characterized in that the sealing means (3) is made of at least one metallic film.

6. Flexible duct according to one of the claims 1 to 2, characterized in that the insulation means (2) is a band helically wounded around the carcass (1) thus forming different bandwidths and in that the sealing means (3) is made of several bands staggered around the insulation means (2).

7. Flexible duct according to one of the claims 1 to 2, characterized in that the insulation means (2) and the sealing means (3) are both formed of several layers, these layers being distinct ones from the other.

8. Flexible duct according to one of the claims 1 to 2, characterized in that the insulation means (2) is a band helically wounded around the carcass (1) thus forming different bandwidths and in that the sealing means (3) is a polymerizable silicone layer, said bandwidths being stuck with each other thanks to the polymerizable silicone layer.

9. Flexible duct according to one of the claims 1 to 2, characterized in that the insulation means (2) is a band helically wounded around the carcass, (1) thus forming different bandwidths and in that the sealing means (3) is a Velcro^® band, said Velcro^® band fastening the different bandwidths with each other.

10. Flexible duct according to one of the claims 1 to 2, characterized in that the sealing means (3) comprises a first polymeric or metallic film (310) laid out in contact with the insulation means (2) and a second polymeric film (300) laid out in contact with the first film (310).

11. Flexible duct according to the preceding claim, characterized in that the second polymeric film (300) is a band helically wounded around the carcass (1) on several layers thus forming a set of layers (210), alternatively disposed with layers (211) formed by a combination of the first polymeric or metallic film (310) with the insulation means (2).

12. Flexible duct according to claim 10 or 11, characterized in that the second polymeric film (300) is made of polyurethane or polyester.

13. Flexible duct according to one of the claims 10 to 12, characterized in that the insulation means (2) is a band, said band being embedded in the first polymeric or metallic film (310).

14. Flexible duct according to one of the claims 10 to 13, characterized in that the insulation means (2) is a band helically wounded around the carcass (1) thus forming different bandwidths, and in that the first polymeric or metallic film (310) is a polymerizable silicone layer or a Velcro^® band to stick or respectively fasten the bandwidths.

15. Flexible duct according to one of the preceding claims, characterized in that the insulation and sealing layer (20) further comprises at least one band of composite material made of polymer/metal/polymer.

16. Flexible duct according to one of the preceding claims, characterized in that the insulation and sealing layer (20) further comprises at least one thin metallic film in direct contact with the inner duct (10).

17. Flexible duct according to one of the preceding claims, characterized in that the carcass (1) is an interlocked strip.

18. Flexible duct according to claim 12, characterized in that the interlocked strip comprises two faces (Ia, Ib) and a packing facet

(11) located between the two faces (Ia, Ib) of the interlocked strip, said packing facet (11) having a role of seal.

19. Flexible duct according to one of the claims 1 to 16, characterized in that the carcass (1) is made of folded-seam strips.

20. Flexible duct according to claim 13, characterized in that the folded-seam strips are chosen among one or a combination of the U, T, S or Z-shaped wires.

21. Flexible duct according to one of the claims 1 to 16, characterized in that the said at least one carcass (1) is made of two carcasses, the first one being an interlocked strip carcass and the second one being a folded-seam strips carcass.

22. Flexible duct according to claim 16, characterized in that a sealing means is interposed between the two carcasses.

23. Flexible duct according to one of the claims 1 to 16, characterized in that the inner duct (10) comprises an interlocked strip carcass and a pressure arch in direct contact with the carcass.

24. Flexible duct according to claim 23, characterized in that a sealing means is interposed between the carcass and the pressure arch.

25. Flexible duct according to one of the preceding claims, characterized in that the second insulation and sealing layer (30) comprises, from the inside to the outside, a sub-layer (31) made of an insulation means (5), and an intermediate plastic sheath (6), said sub- layer (31) being located above and in direct contact with the first insulation and sealing layer (20).

26. Flexible duct according to one of the preceding claims, characterized in that the outer layer (40) comprises, from the inside to the outside, a pressure arch (7), tensile armours (8) and an outer plastic sheath (9).

27. Flexible duct according to one of the preceding claims, characterized in that it is used for a floating application.

28. Flexible duct according to one of the claims 1 to 26, characterized in that it is used for a seabed application.