WO1998034061A1

WO1998034061A1 - Thermal insulation sheath, in particular for constructing underwater pipelines conveying oil products

Info

Publication number: WO1998034061A1
Application number: PCT/FR1998/000170
Authority: WO
Inventors: Philippe Marchal
Original assignee: Itp
Priority date: 1997-01-30
Filing date: 1998-01-30
Publication date: 1998-08-06
Also published as: FR2758872B1; AU6103998A; FR2758872A1

Abstract

The invention concerns an underwater pipeline comprising in particular a single metal tube (10) and an impermeable covering sheath for thermal insulation. The covering sheath comprises at least an external layer, made from a material with elastoplastic deformation, and a main layer (3) made of a fireproof microporous material with high porosity in open pores, maintained tight under pressure on a support layer relatively non-ductile, for instance consisting of a metal tube. In another embodiment the support layer is made of a non-elastic material so as to constitute an autonomous sheath to be mounted on site.

Description

THERMAL INSULATION ENCLOSURE, PARTICULARLY FOR THE

CONSTRUCTION OF SUBSEA PIPES

VEHICLE OF OIL PRODUCTS

The present invention relates to the production of a thermal insulation envelope covering at least partially the material to be insulated. It applies preferentially, but not limitatively, to the construction of subsea pipelines intended to transport petroleum products. It also applies to the manufacture of thermal insulation covers which are intended in particular, although not exclusively, to be used on such pipes, or on any other kind of material which must contain or receive products which, such as petroleum products retained immersed in the sea, must be kept there at a temperature significantly different from that of the environment.

If, to fix the ideas, one places oneself within the framework of the preferred application of the invention, namely underwater pipes intended to convey petroleum products, in the form of gas or liquid hydrocarbons kept hot during their transport, it should be remembered that pipes of this type are traditionally mounted from a series of identical pipes, manufactured in the factory, which are transported to the site and that there, they are assembled end to end, by connecting close by close a downstream tube already integrated in the pipeline to an upstream tube to be connected, as they are advanced, to let them plunge into the sea following the tubes already assembled.

The pipes are commonly made up each of coaxial steel tubes which form or form, in the running part of the pipe, a thermal insulation envelope around an internal tube delimiting the conduit for circulation of the fluids to be conveyed. French patent application FR-A-2 721 681 (ITP) describes an example of a method for producing a pipe of this type. It specifies how the assembly fittings are formed between two successive pipes, by means of a coupling sleeve which is positioned astride the junction zone and which is glued on either side thereof, on the outer peripheral faces of the end sections of the facing external tubes. The junction zone between two successive pipes, around the end parts of the internal tubes welded end to end, is thus closed and protected in mechanical and thermal continuity with the current parts of the pipes.

The pipelines thus produced are not free from drawbacks. First of all, the pipes constituting the elementary sections are heavy and expensive by the use of two coaxial steel tubes. In addition, the joints between pipes involve welding and the use of various auxiliary parts. This results in a non-negligible increase in the cost price and the high mechanical stresses to be collected.

It therefore appeared desirable to have a technology for producing thermal insulation envelopes making it possible to substantially reduce the cost price, while retaining advantageous technical characteristics, in mechanical strength for example, as required for this type of application.

In particular, it is imperative to maintain good thermal insulation of the inner tube guiding the fluid, whatever the overall structure of the pipe. Indeed, and by way of example, the submarine pipes carrying petroleum products are at cold temperature in the seabed (usually between 0 and 20 ° C), while the fluids conveyed are most often at temperature high (typically between 100 and 200 ° C, according to the usual prescriptions). Even when this temperature decreases during the life of a producing field, the fluid must maintain a minimum temperature (for example 40 ° C) up to the place of destination at the end of the pipeline, in order to avoid the formation of solid condensates.

On the other hand, the distances to be covered are considerable. They amount to tens of kilometers. The quality of thermal insulation must also continue for many years of use of the pipes. Among other imperatives of the practice, it can be emphasized that this thermal insulation capacity must not suffer from the operations of assembling the pipelines during which successive tubes are connected end to end, nor from the operations of placing d '' a pipeline thus formed, progressively submerged in the sea, nor yet the conditions of transport between a tube manufacturing plant and the site of laying the pipeline.

Other difficulties are linked to the conditions of manufacture of the pipes, the inevitable dimensional tolerances for the tubes, taking into account their nature (generally steel) and their diameter (generally between 100 and 700 millimeters), and the realization of seals. One of the solutions which has been proposed to the petroleum industry consists in creating a vacuum in the annular space between the two coaxial tubes of a double-jacket pipe, but it is then necessary to ensure a very high degree of vacuum. pushed. It will be understood that, in this case, the production of the seals is particularly delicate, and that both the degree of vacuum and the thickness under vacuum required impose a heavy burden on the cost of the tubes.

The same imperatives of degree of thermal insulation and durability are found in other applications, in particular in situations where a temperature differential of the same order of magnitude is encountered between the interior and exterior of a enclosure of any shape, one way or the other. The object of the invention is, inter alia, to lead to an insulated pipe which does not have the drawbacks of the known art, while retaining the advantages thereof. It thereby meets the needs which are felt, in particular for applications which require keeping a hot product in an underwater environment for long periods of time, the installation also being subjected to high pressure conditions.

In the context described above, the invention results in practice in several forms of implementation, which nevertheless all have in common the implication of a main layer of microporous heat-insulating material with high porosity in open pores which is retained tightly under void between a relatively non-deformable support layer and an outer layer made of a material with elastic deformation and / or plastic.

The deformation capacity of the outer layer can be mainly exploited either during the factory manufacture of elementary pipes for continuous pipes, or on site during the installation of the thermal insulation envelope, or even under the action temperature and / or pressure conditions prevailing in the environment from which the material thus insulated must be isolated. In all cases, the invention has in particular the advantageous consequences of improving the thermal insulation by the combination of the microporous open-pore heat-insulating material with a degree of vacuum which does not need to be pushed, while improving the conditions for resumption of mechanical stresses and stresses, by constant support on the microporous material with open pores that jointly ensure the vacuum created both within the microporous material with open pores and around it and the relative deformation capacity of the outer layer.

In a first embodiment and according to one of its characteristics, the invention provides for not using

1 steel only for a single metallic pipe tube corresponding to the inner tube of a double jacket pipe. To ensure the insulation of this tube carrying the fluid to be transported, provision is then advantageously made to form all around it an envelope as a thermal insulation cover which, according to the invention, comprises at least one main layer of microporous heat-insulating material with open pores and an outer layer of a material which will be called elastoplastic here, while the substantially non-deformable support layer is formed directly by the single tube itself.

The microporous material with open pores being placed under partial vacuum, it completely fills an annular space which must be sealed. The tight closure can preferably be obtained either simply by virtue of the malleability of the external layer which is conformed to the ends of the envelope until it is fixed glued to said pipe tube, or by means of a relatively non-intermediate piece. deformable which moves the outer layer away from the support tube and avoids the harmful effects of thermal bridges.

In a preferred variant of the invention, the layer of open pore microporous heat-insulating material is placed in an envelope made of gas-permeable material, but not to this microporous open-pore material, made for example of a cotton canvas, so facilitate handling during construction of the envelope and vacuuming operations. The desirable mechanical cohesion is then ensured by the retention of the material between the two adjacent layers of the thermal insulation envelope produced.

According to a second embodiment of the invention, the thermal insulation envelope is produced in an autonomous structure, not subject to the presence of a tube or pipe conduction participating in its constitution. In this case, provision is advantageously made to have a third layer which forms the layer. support to replace the above pipe tube. This support layer is then made of a non-elastoplastically deformable material. It forms, with the outer layer of elastoplastic material, a double envelope enclosing the main layer of open pore microporous heat-insulating material, with a seal quality sufficient for vacuuming.

As a marketable finished product, the subject of the invention is therefore in particular a heat-insulated pipe with respect to an ambient environment in which determined pressure conditions prevail, capable of constituting in particular an elementary pipe for the construction of underwater pipes. marine vehicles carrying petroleum products, characterized in that it comprises a substantially non-deformable internal tube, in particular of metallic material, and a covering envelope, thermally insulating and tightly bonded to said tube, which covers all or part of the periphery of the latter, and which comprises at least one main layer of microporous open-pore heat-insulating material retained clamped under vacuum between said tube forming its underlying support layer and an external layer of elastoplastic deformation material, of composition and thickness determined in such a way as to allow a recovery of mechanical forces and stresses ues due to said determined pressure prevailing in the ambient environment.

The subject of the invention is also a cover of a three-dimensional self-supporting structure to be placed in an ambient environment in which determined pressure conditions prevail, intended to constitute a thermal protection envelope in at least partial covering of a material to be insulated, in particular in an underwater environment, which is essentially characterized in that it has its own support layer of relatively non-deformable material to close with an external layer of elastoplastic deformation material, a vacuum space entirely occupied by a main layer consisting of a microporous open pore heat-insulating material, the outer layer having a composition and a thickness determined so as to allow a recovery of the forces and mechanical stresses due to said determined pressure prevailing in the ambient medium.

The invention has many advantages, among which the following are mainly noted:

By the elasto-plastic nature of the layer forming its external wall, the insulation envelope makes it possible to transmit the pressure and shear forces to be supported, through the microporous material with open pores, itself sensitive to the pressure, directly to the support layer forming the other wall, essentially non-elastoplastic, the latter possibly being the internal tube of a pipe, according to the variant embodiments. Indeed, the elastic deformation capacity of the microporous open pore materials used is low.

The fact of placing the microporous open pore heat-insulating material under partial vacuum has a double advantage. This allows the dimensional stability of the insulating jacket to be maintained during handling operations after construction. This also makes it possible to significantly increase the thermal insulation efficiency of the microporous material with open pores, this in a ratio which is equal to two for a pressure reduced to 0.1 bars (10 Pa) for example.

The invention will be better understood and other characteristics and advantages will appear on reading the description which follows and which refers to the appended figures, among which: the figure illustrates it schematically an example of pipe integrating a clean insulating cover envelope to constitute an elementary section for an insulated pipe according to a first embodiment of the invention; FIG. 1b is a detail figure illustrating more precisely one of the layers of the insulating covering envelope; Figure 2a schematically illustrates an alternative embodiment of the pipe and its insulating cover according to Figure 1; Figure 2b is an enlarged detail of one end of an elementary pipe pipe according to Figure 2a

- Figure 2c illustrates a particular procedure for producing the outer layer of one pipe cover envelope illustrated in Figure 2a; FIG. 3a schematically illustrates a second embodiment of a cover constituting an insulating envelope according to the invention; Figure 3b is a sectional view through the thickness of the insulating blanket of Figure 3a; Figure 4a schematically illustrates a thermal protection structure according to the invention which constitutes an alternative embodiment with respect to Figures 3a and 3b; and FIG. 4b illustrates, in cross-section in cross section, a pipe resting on the ground and provided with the insulating cover envelope according to FIG. 4a.

FIG. 1a illustrates a first example of implementation of the invention in the context of its application to the production of elementary pipes prefabricated in the factory or terminated on site for assembly into an underwater pipeline for petroleum products.

The elementary pipe 1 is a cylindrical steel tube, 12 or 24 m long, typically with a diameter of 300 mm and a wall thickness of 13 mm. In Figure la, there are shown two such butted tubes, 10 and 11, which are arranged substantially in line, on an axis common, and which, in a conventional manner are welded together, at their junction 100, by their radial end faces.

According to a first important characteristic of the invention, a single tube is used, as regards the main metallic structural components of the pipeline, and no longer two coaxial tubes as in the known art according to the teaching of the application FR-A-2,721,681 cited above.

According to another important characteristic of the invention, the single said internal tube, for example 10, is surrounded by thermal insulating means, which constitute with and on it a covering envelope. The latter provides a very good thermal insulation power, resistant in longevity despite the mechanical and thermal constraints, between the external environment and the interior of the tube 10, as well as good sealing. It should be understood that the other tubes of the pipe (for example the tube 11) are also provided with such a protective envelope or cover.

According to a first embodiment, this cover comprises two main layers of material, 2 and 3.

The external layer 2, subjected directly to the external environment and in particular to the high pressure which reigns there, constitutes here the so-called cold wall of the envelope, because it is intended to withstand permanent contact with the external environment of the bottoms under - sailors. It is made according to the invention, in an elastoplastic material. By this is meant a material, generally of an organic nature, which is known to be capable of malleability generating a permanent effect by its elastic or plastic deformation.

The main layer 3 is constituted by a microporous heat-insulating material, with a high structure WO 98/34061 "| Q PCT / FR98 / 00170

porosity in open pores, such as those offered commercially for other uses.

In reality, as shown in the detail figure 1b, in a preferred embodiment of the invention, the core 30 of the layer 3 is retained in an envelope 31 made of a nonwoven cotton fabric, which while being permeable to gases, does not allow particles of microporous material with open pores to pass through.

In the preferred application in which we have placed ourselves here, namely that of an underwater pipeline for petroleum products, the material of the outer closure layer 2 is chosen to have sufficient elasto-plastic properties so that the the forces exerted on it are transmitted without difficulty to the other face of the covering by means of the microporous material with open pores constituting the core of the envelope 3.

To fix the ideas, one can use a plastic material based on polyolefin resins, such as polyethylene, or vulcanized rubber.

In addition, it is necessary that the thickness of the layer 2 is sufficient to obtain the recovery of the abovementioned forces, that is to say to be able to transmit the pressure and shear forces supported to the support layer (the internal pipe 10 in the example described), via the layer of heat-insulating material 3, without risk of deterioration of the material constituting it and of degradation of the quality of thermal insulation. The thickness of the necessary external layer naturally depends on many parameters, in particular on the composition of the material of this layer, that is to say first of all its elastoplastic characteristics, of the maximum pressure prevailing in the ambient medium and the diameter of the pipeline. In the preferred application, for conventional immersion depths and for pipe diameters which have been WO 98/34061 -j -J PCT / FR98 / 00170

recalled, the thickness of layer 2 is typically in the range of 10 to 30 mm.

Before continuing the description of the structure of the pipe 1, it is useful to specify the nature of the microporous open-pore heat-insulating material constituting the main layer 30.

A particularly suitable material for this is represented by plates or strips of microporous insulation based on a ceramic material, and preferably based on silica, as they exist in commerce, where they are produced in particular by the company " Micropore International Ltd "under the brand name" Microtherm ".

The material of these plates or strips is made of a mixture of silica powder and ceramic reinforcing fibers, the whole compacted into a coherent three-dimensional structure of fine particles which is retained in an impermeable envelope. The latter is commonly made of a fabric of mineral fibers linked in a network of nonwoven cross fibers, in particular of glass fibers, but it will be more economical for the invention to prefer a cotton fabric, including here the case of cotton fibers tied together without real weaving.

From the chemical point of view, it is, at least for the silica microporous structure (without taking its envelope into account), a mixture, the major part of which is formed of silica, but which also contains a minor part of titanium.

In such a material based on fumed silica gel, the proportion of titanium dioxide can, for example, exceed 20% by weight of the total weight, until reaching approximately 30 to 35% by weight for 60 to 70% by weight. silica, if we neglect the minor parts of other mineral oxides which represent in total less than 5% by weight. These materials are also advantageous in the context of the present invention in that they are characterized by an open porosity and a pore diameter less than or at most equal to 0.1 micrometer. In the language of the trade used here, the porosity is said to be open when the open pores in communication with each other represent almost all of the pores that the microporous structure comprises, ie in practice of the order of 85 to 95% in volume of the overall pore volume, which is itself of the order of 80% of the apparent volume.

All the applications which have been recommended to date for microporous materials essentially consisting of silicic particles, in particular pyrogenic silica gel, exploit the fact that the diameter of the open pores is less than the mean free path of the air molecules, which provides essentially a thermal insulation capacity much higher than that of more traditional materials, in particular those which are manufactured so as to provide mainly closed pores. By way of nonlimiting example, in the preferred application of the invention relating to submarine pipelines, European patent application EP-A-0 220 122

(Hutchinson) discloses the use of a closed-pore heat-insulating material.

Unlike these known applications, according to a preferred variant, the present invention leads to further improving their performance by exploiting the fact that the pores are open to create a partial vacuum all within the material. At the same time, it follows from the invention that one can be satisfied with a partial vacuum, preferably resulting in a reduced pressure of between 0.5 millibar and 100 millibars (50 Pa to 10 ⁴ Pa).

By, among other things, the increase in the average free path of the gaseous molecules which results therefrom, it is possible to increase the insulating power by a factor ranging from 2 to 10, according to the value of reduced pressure chosen and according to the thermal conditions in positioning. The residual gas residual atmosphere under vacuum can be air or any other gas, argon in particular.

If we refer again to FIG. 1a, we can assimilate the tube itself, for example the tube 10, to the support layer of the thermal insulation envelope, which can be said here in the hot wall of the made of the environment of use, the so-called cold wall being constituted by the external elasto-plastic layer 2, the latter being exposed to the ambient environment in which low temperature conditions prevail (hence a significant temperature differential compared fluid circulating in the pipeline) and high pressure.

In this case, the protection and the insulation of the welded joint 100, formed at the connection of the tubes 10 and 11, can be carried out in a conventional manner, using a sleeve 40 threaded on the external wall of the pipe 1, that is to say here say the outer layer 2, and by filling the interior volume with an insulating filling material 41, as can be obtained by a polymerizable resin injected and hardened on site in a sleeve 40 made of plastic. It is however largely preferable, in application of the invention, to provide mechanical and thermal continuity at the junction between two successive elementary pipes, by a fitting formed at least in part, on the outside of the pipe, by means of a self-supporting structure of thermal insulation cover such as that which will be described later.

If the elastoplastic material constituting the outer layer 2 in the running part of each elementary pipe pipe has a temperature resistance compatible with the temperature prevailing on the surface of the metal tube 10, that is to say substantially that of the fluid conveyed, the wall 2 of elastoplastic material is directly fixed to the external wall of the tube 10, for example by gluing. This ensures the sealing the space between the two boundary walls of the envelope and preserving the partial vacuum created in the microporous open pore material of the main layer 3.

The layer 2 therefore comprises a main cylindrical wall 20 extending over most of the length of an elementary tube 10, and at the two ends of the frustoconical connections 21, extended by a portion of new cylindrical 22, matching the external wall of the tube 10 and glued to it. The total length of layer 2, cylindrical wall 20 and end fittings 21-22 included, is slightly less than the length of a tube 10.

There are situations where the seal provided by the material of the elastoplastic layer itself may prove to be insufficient over time. It is then advantageous to combine this layer with one or more films ensuring better sealing, in particular against gases, such as are known in particular of metallic, ceramic or polymeric nature.

In a particular embodiment corresponding to the invention, this is the case of an application to the production of an underwater pipeline which must offer a service life of the order of 20 years, or even more, in severe pressure and temperature conditions that have already been mentioned, while in addition that the annular space between the internal metal tube constituting the relatively non-deformable support layer and the outer elasto-plastic protective layer is completely occupied by the heat-insulating material to open micropores within which there is only residual pressure.

The invention therefore provides for integrating a sealing film with the elastoplastic layer, and preferably for embedding a sealing film (in particular in aluminum or aluminum alloy for reasons of cost and flexibility) in the very thickness of the elastic layer plastic. The latter is advantageously made of a material of a synthetic nature, therefore based on polymeric resins, belonging in particular to the family of polyolefins such as polyethylene or polypropylene, and more particularly based on polyethylene. In general, a laminate comprising several films interposed with several sheets of elasto-plastic layer would also be suitable, with the disadvantage of an often higher cost.

One of the advantages of these provisions is to avoid that slight leaks (which may in particular result from the natural porosity of the polymer or from cracking during life) lead to serious consequences by destroying the partial vacuum prevailing in the heat-insulating layer (or modifying the nature of the gas). In addition, they can be easily adapted to a production involving sealing films previously coated with a material to be hardened by polymerization which one chooses compatible with that of the elastoplastic layer proper, which is obtained in particular by the use of copolymerizable organic resins during the so-called hardening step ("curing"). In such an embodiment of the invention, the elastoplastic material is bonded by surface fusion with the coating of the sealing film when, in particular, a sheet of elastoplastic material is applied, close in close along the pipe, in the form of a strip of extruded material spirally winding around the heat-insulating material over a sealing film previously coated.

It should be noted that in all cases, the step of partial vacuum and the step of bonding the elastoplastic layer can be carried out in the factory, during the manufacture of the pipes constituting the elementary modules of the pipeline. On the other hand, the steps of welding and making the junctions 4 are carried out on site, the pipe 1 being built as one goes along and put into the water. In themselves, these operations are conventional and they have been described, for example, in the patent application. WO 98/34061 <\ g PCT / FR98 / 00170

French above filed in the name of the present plaintiff.

If the temperature proves to be too high, taking into account the material constituting the wall or layer 2, it is possible to use a thermal bridge insulating this wall 2 from the external wall of the metal tube 10. FIG. 2a illustrates such an arrangement.

The elements common to the previous figures have the same references and will only be re-described as necessary. The so-called "cold" wall, here referenced 2 ′, is reduced to a simple cylinder surrounding the insulating layer 3, made of microporous material with open pores.

The end seal is produced by using two metal members 5, of cylindro-conical shape with small opening, which are placed at each end of the tube 10. These members indeed comprise a cylindrical external zone 51, which extends by a conical end zone 50 whose circular opening 54 has a diameter equal to or slightly greater than the external diameter of the tube 10. The wall 2 'is glued to the metal of the cylindrical zone 51. The conical zone 50 is welded , in its opening 54, on the external wall of the tube 10.

Advantageously, the cylindrical zone 51 is provided with one or more annular grooves, for example two, referenced 52 and 53 as shown in detail in FIG. 2b. These grooves 52 and 53 will make it possible to ligate by a band 6a and / or to encircle by a strapping 6b, the layer 2 ′ of elastoplastic material, so as to fix it firmly bound to the intermediate member 5 and to prevent 1 the seal itself does not transmit shear forces.

Here again, these operations can be carried out in the factory, during the manufacture of the pipes constituting elementary sections of pipeline to be assembled end to end. In practice, the element 2 or 2 ′, made of elastoplastic material, can be produced from elastic bands which are wound tightly in a spiral and hardened on the spot by vulcanization if it is rubber, or with elastic bands already hardened or vulcanized which are wound and glued in a spiral.

FIG. 2c schematically illustrates, in cutaway, a section of pipe 1 produced according to this operating mode. It is clearly seen in this figure 2c the spiral winding of the strip b constituting the layer 2. A rate of recovery from one turn to another (dotted lines in the figure) is provided sufficient for a bonding ensuring 1 ' vacuum tightness.

The outer layer can also be produced simply by a cylindrical pipe made of elastoplastic material, of a diameter sufficient to allow it to be placed around the microporous material with open pores by threading one into the other. In a subsequent step, the diameter (shrunk) of the pipe is reduced by heating it under pressure so that it contracts. Provided that the material still has thermosetting properties, the shrinkage is definitive in compression of the microporous open-pore heat-insulating material. It is also possible to proceed directly by extrusion.

Partial vacuuming of the microporous material with open pores can be carried out in different ways, using a conventional apparatus per se, depending on the type of external layer used (external spiral layer, etc.). The physical characteristic "open pores" presented by the heat-insulating material is a very important characteristic because it allows the creation of a partial vacuum within the material, step by step. The operation of shrinking the outer layer is then advantageously concomitant with the evacuation, and progressive along the pipe. A second embodiment of pipes according to the invention is illustrated in Figures 3a to 4b. According to this embodiment, the insulating and waterproof cover, here referenced 7, is in the form of a strip of great length, typically from a few tens of meters to several kilometers. The elements common to the previous figures have the same references and will only be re-described as necessary.

The strip 7 consists of a double envelope, comprising an external wall known as cold, quite similar to the corresponding layer 2 of FIGS. 1a to 2c, made of elastoplastic material, and an internal wall 8 known as hot (see FIG. 3b , in section AA of FIG. 3a), made of non-elastoplastic material.

This non-elastic material is chosen to exhibit characteristics of potential elongation, under the conditions of use (that is to say in the context of a given specific application), much less than the maximum elastic elongation of the material. microporous with open pores. A metal such as a steel strip is suitable for this purpose, as well as a rigid plastic material, for example based on polyolefin resins embedding a frame of mineral fibers.

A notable difference between this second embodiment and the first therefore resides in the presence of an additional layer 8, the internal non-deformable wall or layer called hot no longer being formed directly by the tube 10 (see for example figures la and 2a). The additional layer 8 must be sealed so that the partial vacuum created in the layer of microporous material with open pores is maintained.

Figures 3a and 3b schematically illustrate a first alternative embodiment of the cover according to this second embodiment of the invention. More precise, FIG. 3b is a cross-section through the thickness of the cover according to "AA" in FIG. 3a.

As shown more particularly in this last figure, the strip 7 comprises a first layer 2 trapping the layer 3 by lateral, 23-24, or end zones, 26, glued to the lower layer 8, made of non-elastic material. The layer 3 of microporous material with open pores is quite similar to the layer 2 previously described with reference to Figures la to 2a.

Advantageously, as shown more particularly in FIG. 3a, the cover 7 is subdivided into elementary cover plates, 7a, 7b, 7c to 7x

(x representing the total number of "modules"). To do this, the strip 7 comprises zones for separating the elementary plates 25, in which the layer 2, made of elastoplastic material, is bonded to the lower layer 8, made of non-elastic material. This arrangement makes it possible, in the event of damage to the strip, to limit any leaks to a leak that remains localized.

As before, the laying of the strip 7 can be carried out by spiral winding around tubes (not shown in these figures) which are abutted to form the overall pipe. The holding in place can also be carried out by bonding during the vulcanization of rubber constituting the elastoplastic material.

According to an additional variant of this second mode, the strip can be placed astride the pipe, when the latter is already deposited at the bottom of the water or, more generally, on a given support soil, whether it is underwater or terrestrial.

Figures 4a and 4b illustrate this alternative embodiment. The strip proper, here referenced 7 ', has a configuration similar to the strip 7 of Figures 3a and 3b. It is particularly advantageous to provide, as before, a partition of the strip into modules, of which only four have been shown, 7 'a to 7'd.

Figure 4b, section "BB" of Figure 4a, illustrates the installation of the cover 7 ', straddling the pipe 1, having a section in "Ω". Line 1 is assumed to have been placed beforehand on support S. However, in order for this strip 7 ′ to remain in place, it is useful to ballast it. To do this, one can provide, in a variant not shown, chains attached laterally to the strip 7 'forming a cover.

However, according to a preferred variant, shown in FIGS. 4a and 4b, two lateral zones, 9a and 9b, are advantageously provided, constituted by ballast pockets, 90a and 90b. These zones surround the bonding zones 23 and 24. They are filled with heavy material, or form, for example, sandbags.

In this alternative embodiment, according to the second mode, the cover of the invention has the main advantage of being able to be produced over large, even very long lengths, and be wound on a drum to be simply launched by unrolling drum on board a laying boat.

On reading the above, it is easy to see that the invention achieves the goals it has set for itself. It should also be clear that, although particularly suitable for applications for transporting petroleum products by submarine pipelines, the invention cannot be confined to this single type of application.

Claims

1. Thermal insulation envelope, characterized in that it comprises a covering structure, thermally insulating and waterproof, comprising at least one main layer (3) of microporous open-pore heat-insulating material, held tight under vacuum against a support layer

(10, 8) relatively non-deformable by an external layer

(2) made of an elastoplastic material, of composition and thickness determined so as to allow a recovery of the forces and mechanical stresses due to said determined pressure prevailing in the ambient medium.

2. Envelope according to claim 1, characterized in that said cover comprises an incorporated support layer (8), of relatively non-deformable material, and in that said outer layer (2) of elastoplastic material encloses said main layer (3 ) of microporous material with open pores, by bonding on its periphery (23-26) to said support layer (10, 8).

3. Envelope according to one of claims 1 or 2, characterized in that said microporous material with open pores is in the form of strips and in that it consists of a mixture of silica powder and ceramic reinforcing fibers, all compacted into a coherent three-dimensional structure with high porosity in open pores, made of fine particles and retained in an envelope that is not gas-tight.

4. Envelope according to any one of claims 1 to 3, characterized in that the pressure prevailing in the microporous material with open pores is a reduced pressure between 0.5 and 100 millibars.

5. Envelope according to any one of claims 1 to 4, characterized in that said support is an underwater pipe tube (10) conveying petroleum products, said underwater pipe being intended to be placed in an ambient environment in which determined pressure conditions prevail, and in that the outer layer (2) has a determined composition and thickness, so as to allow recovery of the forces and mechanical stresses due to said determined pressure prevailing in the ambient medium.

6. Envelope according to claim 5, characterized in that said external (2) and main (3) layers are in the form of two substantially coaxial cylindrical tubes substantially covering the length of said pipe tube (10), in that said outer layer (2) protrudes at the ends of said main layer (3), so as to form a cylindrical-conical profile, the cylindrical zone (22) of which surrounds said tube (10), and in that this zone is glued on the outer wall of this tube (10) to obtain said seal.

7. Envelope according to claim 5, characterized in that said outer (2) and main (3) layers are in the form of two substantially coaxial cylindrical tubes substantially covering the length of said pipe tube (10), and in that 'there is provided, at both ends of this tube (10), a cylindrical-conical intermediate metallic member (5), the cylindrical part (51) of which is disposed under said external layer (2) and bonded to this layer (2) , and the conical part (50) comprises a circular opening (54) of diameter substantially equal to the diameter of said tube (10), this circular opening (54) being welded to the outer wall of this tube (10), to obtain said seal .

8. Envelope according to claim 7, characterized in that said cylindrical part (51) further comprises, at least, an annular groove (52, 53), so allows ligating and / or strapping of said outer layer (2 ') on said intermediate cylindrical-conical metal member (5).

9. Envelope according to any one of the preceding claims, characterized in that, said main layer (3) completely surrounding said tube (10), said outer layer (2, 2 ') consists of a spiral winding of a rubber band, made on the main layer (3), and by its vulcanization and / or its bonding.

10. Envelope according to any one of the preceding claims, characterized in that said main layer (3) completely surrounding said tube (10), said outer layer (2, 2 ') consists of a cylindrical tube threaded on said layer (3) and shrunk by exposure to heat.

11. Envelope according to any one of the preceding claims, characterized in that said cover has the form of a strip (7) subdivided into a plurality of said waterproof envelopes (7a to 7x), each envelope being separated from the following by a bonding area (25) of said outer layer (2) on said support layer (8).

12. Envelope according to claim 11, characterized in that said strip-shaped cover

(7 ') subdivided into a plurality of said envelopes (7 * a to 7' d) is arranged astride said pipe (1).

13. Envelope according to claim 12, characterized in that said strip-shaped cover (7 ') subdivided into a plurality of said envelopes (7' a to 7'd) is further provided with a plurality of side pockets of ballast (90a, 90b), said lateral ballast pockets (90a, 90b) being filled with a material ponderous or forming sandbags, so as to maintain said pipe (1) on said surface (S) of pipe or other material.