AU2014276646A1

AU2014276646A1 - Method for manufacturing a freestanding body for thermal insulation of a vessel for storing a fluid and freestanding body produced thereby

Info

Publication number: AU2014276646A1
Application number: AU2014276646A
Authority: AU
Inventors: Gery Canler; Benoit Capitaine; Sebastien Delanoe; Bruno Deletre; Florent OUVRARD; Olivier Perrot; Nicolas WALKER
Original assignee: Gaztransport et Technigaz SA
Current assignee: Gaztransport et Technigaz SA
Priority date: 2013-06-07
Filing date: 2014-06-02
Publication date: 2015-12-24
Anticipated expiration: 2034-06-02
Also published as: WO2014195614A2; KR102263150B1; FR3006661A1; FR3006661B1; JP6336051B2; ES2785383T3; EP3004719B1; AU2014276646B2; CN105263826B; EP3004719A2; CN105263826A; WO2014195614A3; JP2016524679A; KR20160016866A

Abstract

The invention relates to a method for manufacturing a freestanding body intended for thermal insulation of a vessel, with moisture barrier, for storing a fluid, said method comprising: providing a plurality of supporting webs (14) made of a composite material comprising a thermoplastic matrix reinforced by fibres; providing a bottom panel (10) and a cover panel (11); inserting the supporting webs (14) between the bottom panel (10) and the cover panel (11), such that the bottom panel (10) and the cover panel (11) are separated in the thickness direction of the body and that the supporting webs (14) extend vertically in the thickness direction; attaching the supporting webs (14) to the bottom panel (10) and to the cover panel (11); filling a plurality of compartments (15) arranged between the supporting webs (14) with a heat-insulating filler; wherein the bottom panel (10) and the cover panel (11) each comprise at least one thermoplastic element (21, 22) for attaching the supporting webs (14); and wherein the supporting webs (14) are attached to the bottom panel and to the cover panel by a thermoplastic welding operation performed at the interface areas (25) between the supporting webs (14) and the thermoplastic elements (21, 22) of the bottom panel (10) and the cover panel (11). The invention also relates to a body manufactured thereby, and to a sealed, thermally insulating vessel comprising a thermal-insulation barrier including a plurality of bodies such as mentioned above.

Description

1 Method for manufacturing a freestanding body for thermal insulation of a vessel for storing a fluid and freestanding body produced thereby Technical field 5 The invention relates to field of fluid-tight, thermally insulated tanks with membranes, for storage and/or transport of fluid such as a cryogenic fluid. Fluid-tight, thermally insulated tanks with membranes are used in particular for the storage of liquefied natural gas (LNG) which is stored at around -1620C at atmospheric pressure. These tanks may be installed on land or on a floating 10 installation. In the case of a floating installation, the tank may be intended for the transport of liquefied natural gas or to receive liquefied natural gas serving as fuel for propulsion of the floating installation. Technological background Document FR 2 877 639 describes a fluid-tight, thermally insulated tank 15 comprising a tank wall which is fixed to the supporting structure of a floating installation and presents successively, in the thickness direction from the inside to the outside of the tank, a primary fluid-tight barrier intended to be in contact with the liquefied natural gas, a primary insulating barrier, a secondary fluid-tight barrier and a secondary insulating barrier anchored to the supporting structure. 20 The insulating barriers comprise a plurality of adjacent, parallelepipedic, heat-insulating cases. The parallelepipedic cases comprise a plywood base panel, a plywood cover panel and a plurality of bearing webs interposed between the base panel and the cover panel. The bearing webs are undulating so as to ensure good resistance to compressive forces in the direction perpendicular to the base and cover 25 panels, and thus resist the hydrostatic pressure exerted by the liquid contained in the tank. The cases are also filled with heat-insulating linings extending inside the compartments arranged between the bearing webs.

2 This document provides the machining of grooves in the inner face of the base and cover panels in order to join the bearing webs to the base and cover panels by interlocking forms. However, such an assembly requires additional machining operations. 5 It is also known to join the bearing webs to the base and cover panels by staples. However, such staples are unsuitable when the bearing webs are made of composite material. In fact, the fixing of the composite material walls to the base and cover panels with staples tends to reduce the strength of the webs. Summary 10 A concept on which the invention is based is to propose a method for production of a self-supporting case for thermally insulating a fluid-storage tank, wherein the fixing of the bearing webs to the base and/or cover panel is achieved in a simple and reliable manner. According to one embodiment, the invention provides a method for producing 15 a self-supporting case intended to provide thermal insulation for a fluid-storage tank with a fluid-tight membrane, said method comprising: - providing a plurality of bearing webs made of composite material comprising a fiber-reinforced thermoplastic matrix; - providing a base panel and a cover panel; 20 - interposing the bearing webs between the base panel and the cover panel such that the base panel and the cover panel are spaced apart in a thickness direction of the case and the bearing webs extend in the thickness direction; - fixing the bearing web to the base panel and/or to the cover panel; - lining a plurality of compartments arranged between the bearing webs with a 25 heat-insulating lining; wherein the base panel and the cover panel each comprise at least one thermoplastic element for fixing the bearing webs; and wherein the bearing webs are fixed to the base panel and the cover panel by a thermoplastic welding operation performed at interface zones between the bearing webs and the thermoplastic elements of the base 30 panel and the cover panel. Thus the bearing webs may be joined to the base panel and/or cover panel 3 in a simple and reliable manner since the fixing elements do not degrade the structural integrity of the bearing webs, such that their strength is not reduced by the fixing to the base panel and/or cover panel. According to embodiments, a method may comprise one or more of the 5 following characteristics: - the base panel and the cover panel each have an inner face turned towards the inside of the case, and an outer face, the inner faces of the base panel and cover panel being covered with a thermoplastic film for fixing the bearing webs. 10 - the inner face of the base panel and/or the cover panel is fully covered with thermoplastic film. - the method comprises a step of arranging removable protective masks on the inner face of the base panel and/or of the cover panel between the interface zones prior to the welding operation. Thus the films are 15 protected outside the fixing zones of the bearing webs. - the outer face of the base panel and/or the cover panel is also covered with thermoplastic film. Thus the thermal flexion of the panels is balanced when the tank is chilled. - the inner face of the base panel and/or the cover panel is partially 20 covered with thermoplastic film, a plurality of thermoplastic film strips each being arranged in an interface zone with a bearing web. - the base panel and/or the cover panel comprises a plywood body, the thermoplastic film being glued to the plywood body. - the thermoplastic film is a composite material comprising a fiber 25 reinforced thermoplastic matrix. Such a thermoplastic film increases the flexion strength and the puncture resistance of the base and/or cover panels. - the base panel and/or the cover panel comprises a body made of composite material comprising a fiber-reinforced thermoplastic matrix, 30 said body forming a thermoplastic element for the fixing of the bearing webs. - the base panel and/or the cover panel comprises a wooden body 4 impregnated with a thermoplastic matrix for the fixing of the bearing webs. - the thermoplastic elements are thermoplastic studs inserted inside bores arranged in the base panels and/or cover panels and inside bores 5 arranged in the bearing webs. - the thermoplastic elements for fixing the bearing webs comprise a thermoplastic matrix identical to the thermoplastic matrix of the bearing webs. This facilitates the welding operations. - the thermoplastic welding is performed by a method selected from 10 welding by infrared radiation, ultrasonic welding, induction heating, friction welding, welding by addition of filler material, hot air jet welding, and flaming. - the production method provides that: o the bearing webs have an upper end intended to support a cover panel 15 and provided with a fixing strap intended for anchoring a weld support for the welding of metal strakes of a fluid-tight membrane which are intended to rest against the self-supporting case, said fixing strap comprising a folded longitudinal edge intended to cooperate with a folded longitudinal edge of the weld support; 20 o the cover panel has parallel grooves extending over the entire thickness of the cover panel; o the bearing webs are arranged between the base panel and the cover panel such that the fixing straps each extend inside a groove of the cover panel. 25 - the fixing straps are set into the mass of the bearing webs during their forming. - a bearing web comprises a main body with an upper end provided with a housing, and an insert comprising thermoplastic material molded over a fixing strap, said insert being positioned in the housing of the upper end of 30 the main body of the bearing web and fixed to said main body by a thermoplastic welding operation. According to one embodiment, the invention also provides a self-supporting 5 case intended for thermal insulation of a fluid-storage tank with a fluid-tight membrane, said case comprising: - a base panel and a cover panel spaced apart in a thickness direction of the case; 5 - a plurality of bearing webs interposed between said base panel and said cover panel and extending in the thickness direction so as to define a plurality of compartments, said bearing webs being made of a composite material comprising a fiber-reinforced thermoplastic matrix; and - a heat-insulating lining extending inside said compartments arranged 10 between the bearing webs, wherein - the base panel and the cover panel each comprise at least one thermoplastic element; and - the bearing webs are fixed to the base panel and/or the cover panel by 15 thermoplastic welding in the interface zones between the bearing webs and the thermoplastic elements of the base panel and the cover panel. According to embodiments, such a case may comprise one or more of the following characteristics: - the bearing webs have load distribution plates extending along the two 20 edges of said bearing webs arranged facing the base panel and the cover panel respectively. - the bearing webs have a plurality of undulations, the axis of which extends perpendicular to the base panel and cover panel. According to one embodiment, the invention also provides a method for 25 production of a self-supporting case intended for thermal insulation of a fluid-storage tank with a fluid-tight membrane, said method comprising: - providing a plurality of bearing webs made of a composite material comprising a fiber-reinforced thermoplastic matrix; said bearing webs having an upper end intended to support a cover panel and provided with a fixing 30 strap intended for anchoring a weld support for the welding of metal strakes of a fluid-tight membrane which are intended to rest against the self supporting case, said fixing strap comprising a folded longitudinal edge 6 intended to cooperate with a folded longitudinal edge of the weld support; - providing a base panel and a cover panel, the cover panel having parallel grooves extending over the entire width of the cover panel; - interposing bearing webs between the base panel and the cover panel such 5 that the base panel and the cover panel are spaced apart in a thickness direction of the case and that the bearing webs extend in the thickness direction such that the fixing straps each extend inside a groove of the cover panel; - lining a plurality of compartments provided between the bearing webs with a 10 heat-insulating lining; - fixing the bearing webs to the base panel and to the cover panel. According to embodiments, a method for producing a self-supporting case may comprise one or more of the following characteristics: - the bearing webs are produced by forming from a composite material 15 comprising a fiber-reinforced thermoplastic matrix, and the fixing straps are set into the mass of the bearing webs during their forming. - a step of producing a bearing web comprises: o arranging in a mold a composite material comprising a fiber-reinforced thermoplastic matrix; 20 o inserting fixing straps intended for anchoring of a weld support for the welding of metal strakes of a fluid-tight membrane inside the mold; o forming the composite material, during which step the fixing straps are set into the mass of said bearing web. - the composite material is formed by thermoforming or thermocompression. 25 According to other embodiments, a method for producing a self-supporting case may comprise one or more of the following characteristics: - a bearing web comprises a main body with an upper end provided with a housing, and an insert comprising thermoplastic material molded over a fixing strap, said insert being positioned in the housing of the upper end of 30 the main body of the bearing web and fixed to said main body by a thermoplastic welding operation. - a step of producing a bearing web comprises: 7 o arranging in a mold a composite material comprising an upper end provided with a housing; o forming an insert by molding thermoplastic material over a fixing strap; o positioning said insert in the housing of the main body of the bearing web 5 and fixing said insert to the main body by a thermoplastic welding operation. The invention also concerns a self-supporting case with bearing webs having an upper end intended to support a cover panel, provided with a fixing strap intended for anchoring of a weld support for the welding of metal strakes of a fluid-tight 10 membrane. According to one embodiment, the invention also provides a fluid-tight, thermally insulated fluid-storage tank comprising a thermal insulation barrier comprising a plurality of the above-mentioned cases arranged next to each other, and a sealing membrane resting against the thermal insulation barrier. 15 Such a tank may form part of a land-based storage installation, for example for storing LNG, or be installed in a floating structure in coastal waters or off-shore, in particular an LNG tanker, a floating storage and regasification unit (FSRU), a floating production, storage and offloading unit (FPSO), and others. According to one embodiment, a ship for transporting a fluid comprises a 20 double hull and a tank as described above arranged in the double hull. According to one embodiment, the invention also provides a method for loading or unloading such a ship, wherein a fluid is conducted through insulated pipelines from or to a floating or land-based storage installation to or from the ship's tank. 25 According to one embodiment, the invention also provides a system for transferring a fluid, the system comprising said ship, insulated pipelines arranged so as to connect the tank installed in the ship's hull to a floating or land-based storage installation, and a pump for driving a fluid through insulated pipelines from or to the floating or land-based storage installation to or from the ship's tank. 30 8 Brief description of the figures The invention will be better understood and further aims, details, characteristics and advantages thereof will appear more clearly from the following description of several particular embodiments of the invention, given merely for 5 illustration and without limitation, with reference to the attached drawings. * Figure 1 is a simplified perspective view of a tank wall according to one embodiment. * Figure 2 is a simplified top view of an insulating case of the tank wall from figure 1. 10 0 Figure 3 is a side view of a bearing web. * Figure 4 is a cross-section view along plane IV-IV of figure 3. * Figures 5 and 6 illustrate the steps of production of a bearing web according to an embodiment. * Figure 7 is a diagrammatic cross-section view of a self-supporting 15 case according to a first embodiment. * Figure 8 is a diagrammatic cross-section view of a self-supporting case according to a second embodiment. * Figure 9 is a diagrammatic cross-section view of a self-supporting case according to a third embodiment. 20 0 Figure 10 is a detailed view of the assembly between the bearing web and a base panel according to a fourth embodiment. * Figure 11 is a cross-section view of a cover panel and a bearing web according to one embodiment, fitted with a fixing strap cooperating with a weld support for the welding of metal strakes of a fluid-tight 25 membrane. * Figure 12 is a perspective view illustrating the bearing web and the fixing strap according to the embodiment of figure 12. * Figure 13 is a cross-section view of a cover panel and the bearing 9 web according to another embodiment, provided with a fixing strap intended to cooperate with a weld support for the welding of metal strakes of a fluid-tight membrane. * Figure 14 is a perspective view illustrating the bearing web and the 5 fixing strap according to the embodiment of figure 13. * Figure 15 is an simplified diagrammatic depiction of a tank of an LNG tanker, and of a terminal for loading and unloading this tank. Detailed description of embodiments Figure 1 shows a wall of a fluid-tight, thermally insulated tank. The general 10 structure of such a tank is well-known and has a polyhedral form. Therefore only one zone of the tank wall will be described, given that all walls of the tank may have a similar general structure. The wall of the tank, from the outside to the inside of the tank, comprises a supporting structure 1, a secondary thermal insulation barrier 2 which is formed from 15 heat-insulating cases 3 arranged adjacent to each other on the supporting structure 1 and anchored thereto by secondary retention elements 4, a secondary sealing membrane 5 carried by the cases 3, a primary thermal insulation barrier 6 formed by heat-insulating cases 7 arranged next to each other and anchored to the secondary sealing membrane 5 by primary retention elements 8, and a primary sealing 20 membrane 9 carried by the cases 7 and intended to be in contact with the cryogenic fluid contained in the tank. The supporting structure 1 may in particular be a self-supporting metal plate, or more generally any type of rigid partition with suitable mechanical properties. The supporting structure may in particular be formed by the hull or double hull of a ship. 25 The supporting structure comprises a plurality of walls defining the general shape of the tank. The primary 9 and secondary 5 sealing membranes are for example formed from a continuous strip of metal strakes with raised edges, said strakes being welded by their raised edges to parallel weld supports fixed to the cover of the cases 3, 7. 30 The metal strakes are for example made of Invar@: i.e. an alloy of iron and nickel, the coefficient of expansion of which is typically between 1.2 . 10-6 and 2 . 10-6 K 1

.

10 The cases 3 of the secondary thermal insulation barrier 2 and the cases 7 of the primary thermal insulation barrier 6 may have identical or different structures and the same or different dimensions. With reference to figure 2, we will now describe the general structure of a 5 case 3, 7 of the secondary thermal insulation barrier 2 and/or the primary thermal insulation barrier 6. The case 3, 7 has substantially the form of a parallelepipedic rectangle. The case 3, 7 comprises a base panel 10 and a cover panel 11 which are parallel to each other. The base panel 10 and cover panel 11 are for example made of plywood. 10 A plurality of spacer elements is interposed between the base panel 10 and the cover panel 11, perpendicular thereto. The plurality of spacer elements comprises firstly two opposing side walls 12, 13, and secondly a plurality of bearing webs 14. The bearing webs 14 are arranged parallel to each other between the two side walls 12, 13 in a direction perpendicular to said side walls 12, 13. The bearing webs 14 15 extend in the thickness direction of the case 3, 7. In one embodiment, the cover panel 11 on its inner face has grooves for housing weld supports for the welding of the metal strakes of the fluid-tight membrane. In other embodiments which will be described below with relation to figures 11 to 14, the weld supports 31 are held by fixing straps 30 integral with the bearing webs 14 of 20 the cases 3, 7, and the cover panel 11 has through grooves 32 allowing attachment of the weld supports 31 to the fixing straps 30. Compartments 15 for receiving a heat-insulating lining are provided between the bearing webs 14. The heat-insulating lining may be made of any material with suitable thermal insulation properties. For example, the heat-insulating lining is 25 selected from materials such as perlite, glass wool, polyurethane foam, polyethylene foam, polyvinyl chloride foam, aerogels or other. The bearing webs 14 are undulating and protrude to both sides of their general longitudinal direction. Each undulation thus extends along an axis perpendicular to the base panel 10 and cover panel 11. In the embodiment shown, 30 the undulations are substantially sinusoidal. However, other forms of undulation are also possible. For example, the undulations may in particular take the form of 11 triangular teeth or rectangular slots. Thanks to their shape, such undulating bearing webs 14 have a high buckling resistance without needing a great thickness. Although undulations with a periodic structure allow good uniformity of compressive strength, it is also possible to provide non-periodic undulations in order to meet to certain 5 localized mechanical requirements. Figure 3 and 4 illustrate a bearing web 14. Along its edges extending against the base panel 10 and cover panel 11, the bearing web 14 comprises load distribution plates 16a, 16b. The upper plate 16a has a flat surface intended to rest against the cover panel 11, while the lower plate 16b has a flat surface intended to rest against 10 the base panel 10. The plates 16a, 16b have a width which is greater than the thickness of the wall of the bearing web 14 in its principal part extending between the two plates 16a, 16b. Thus the load distribution plates 16a, 16b prevent a concentration of stress on a particular zone, by offering a larger support surface between the bearing web 14 and the base panel 10 and cover panel 11. The load distribution plates may 15 have a parallelepipedic form as shown on figures 3 or 4. In this case, the width of the plates 16a, 16b may be equal to the amplitude of the undulations. In other embodiments, the load distribution plates 16a, 16b may themselves have undulations. The bearing webs 14 are made of composite material with a fiber- reinforced thermoplastic matrix. A method for producing the bearing webs 14 is described in 20 relation to figures 5 and 6. In a first phase shown on figure 5, an intermediate product is produced in the form of composite material plates. To do this, a double-belt press 17 is supplied with glass fibers 18 and thermoplastic resin 19, for example based on polypropylene. The thermoplastic resin 19 may be loaded into the double-belt press 17 in the form of 25 extruded films or powder. The glass fibers 18 are provided in the form of glass fiber coils cut to the desired length. The thermoplastic resin 19 and the glass fibers 18 are laminated together in the double-belt press 17. At the outlet from the double-belt press 17, a cutting device allows a plurality of plates to be obtained. Such plates have a composite structure comprising a thermoplastic matrix 30 and a glass-fiber felt or mat. Such composite structures are designated GMT, for glass-fiber mat reinforced thermoplastics.

12 Then the composite material plates are formed as shown on figure 5. To do this, the composite material plates are heated in an oven 20 then arranged in a mold 21 in which they will be formed by application of pressure. The bearing webs 14 thus formed are then cooled. The bearing webs 14 are thus formed by thermocompression, 5 by heating the composite material plates then by deep-drawing them under pressure. In another embodiment, the bearing webs 14 may also be produced by thermoforming i.e. by flux of the composite material plates under conditions of temperature and vacuum. It is noted that the method of producing a bearing web 14 is described above 10 merely as an example, and the invention is not limited to bearing webs 14 produced in this way. In particular, the bearing webs 14 may also be obtained by injection molding, by extrusion or pultrusion. Also the thermoplastic matrix may comprise any suitable thermoplastic material such as polypropylene (PP), polyethylene (PE), polyamides (PA), polyetherimide (PEI), polyvinyl chloride (PVC), polyethylene 15 terephthalate (PET), polybutylene terephthalate (PBT), acrylonitrile-butadiene styrene copolymer (ABS), or others. Similarly, it is also conceivable to reinforce the thermoplastic matrix with carbon fibers or with a mixture of carbon and glass fibers. Figure 7 illustrates the assembly of the bearing webs 14 and base panel 10 and cover panel 11 according to one embodiment. The base panel 10 and cover panel 20 11 have a plywood body. The inner faces of the base panel 10 and cover panel 11 facing the inside of the case are covered with a thermoplastic film 21, 22. To allow the fixing of the bearing webs to the panels 10, 11, a plastic welding operation is performed in the interface zones 25 between the thermoplastic films 21, 22 and the bearing webs 14 of composite material. 25 The welding operation is for example performed by infrared radiation. It is however possible to use any other suitable method of plastic welding, such as ultrasonic welding, induction heating, friction welding, welding by the addition of filler, hot air jet welding or flaming. Note that in the case of induction welding, it is necessary to provide metal inserts on the bearing webs and/or base panel 10 and/or cover panel 30 11, at the interface between the bearing webs 14 and the base panel 10 and cover panel 11, so as to allow heating of the thermoplastic material.

13 In one embodiment, before performing the welding operations, protective masks are first arranged on the inner faces of the base panel 10 and cover panel 11 between the interface zones 25 between the bearing webs 14 and the panels 10, 11. When the welding operations have been performed, the protective masks may then 5 be removed. Thus the thermoplastic films 21, 22 are not damaged during the welding operations. Such protective masks are for example made of metallic or ceramic materials, and/or of glass. Such masks are advantageously provided with a cooling circuit for circulation of a fluid such as water, air or oil, in order to regulate the temperature of said masks. 10 In the embodiment shown, the outer face of the base panel 10 and cover panel 11 is also covered with thermoplastic films 23, 24. Such an arrangement allows balancing of the flexion of the cover panel 11 and base panel 10, in particular when subjected to high thermal stresses during chilling of the tank. In the embodiment shown on figure 8, the thermoplastic films 21, 22 only 15 partially cover the inner face of the base panel 10 and cover panel 11. In this case, strips 29 of thermoplastic film are arranged between the interface zones 25 with the bearing webs 14. The thermoplastic films 21, 22, 23, 24, 29 are for example made of a composite material comprising a fiber-reinforced thermoplastic matrix. Thus such 20 thermoplastic films 21, 22, 23, 24, 29 help increase the mechanical strength of the base and cover panels by increasing their flexion strength and improving their puncture resistance. Such thermoplastic films 21, 22, 23, 24, 29 typically have a thickness of the order of 0.5 to 5 mm. in one embodiment, the thermoplastic films 21, 22, 23, 24, 29 are fixed to the 25 body of the base panel and cover panel 11 by gluing. The glue used is for example an acrylic glue, a polyurethane glue or an epoxy glue. In another embodiment, the thermoplastic films 21, 22, 23, 24, 29 are fixed to the body of the panels by a hot pressing process. In such a case, it is conceivable to integrate the fixing of the thermoplastic films directly in the plywood production 30 process. To do this, the pre-glued layers of wood and the thermoplastic films are superposed, then the resulting stack is subjected to hot pressing. For example, for 14 such hot pressing, the stack is subjected to a temperature of the order of 190 to 2000C and to a pressure of the order of 0.2 MPa for a duration of 5 minutes. To facilitate the welding operations, the thermoplastic films 21, 22, 23, 24, 29 comprise a thermoplastic matrix identical to the thermoplastic matrix of the bearing 5 webs 14. In the embodiment shown on figure 9, it is the actual bodies of the base panel 10 and cover panel 11 which form the thermoplastic elements for fixing of the bearing webs 14. In a first variant, the base panel 10 and cover panel 11 comprise a body made of a composite material comprising a thermoplastic matrix identical to that of 10 the bearing webs 14, reinforced by fibers. According to a second variant, the base panel 10 and cover panel 11 are made of a wooden body impregnated with a thermoplastic matrix of the same nature as that of the bearing webs 14. The body may be produced by agglomeration of the fibers previously impregnated with a thermoplastic matrix. Alternatively, the body may 15 be made of plywood, the inner layer of which - and optionally the outer layer -are made of wood which is sufficiently porous to cause diffusion of the plastic matrix inside said layers under heat and pressure. Such a wood is for example selected from birch, pine, oak or other. Figure 10 shows the assembly of the bearing webs 14 and base panel 10 20 and cover panel 11 in another embodiment. In this embodiment, the base panel 10 and cover panel 10, in their interface zones 25 with the bearing webs 14, have through bores through which thermoplastic studs 26 are inserted. The thermoplastic studs 26 are provided with heads 27 which rest against the outer face of the base panel 10 and cover panel 11, and distal ends 28 extending inside the bores provided in the edges 25 of the bearing webs 14. Welding of the thermoplastic studs 26 inside the bores provided in the bearing webs 14 ensures the fixing of the bearing webs 14 to the base panel 10 and cover panel 11. In one embodiment, the welding operations are performed by friction, by driving the thermoplastic studs 26 in rotation. The movement of the thermoplastic studs 26 relative to the bearing webs 14 causes a heating of the 30 interface until local plastification of the thermoplastic material, and then welding. The heat-insulating lining may be positioned in the compartments 15 15 provided between the bearing webs 14 after the bearing webs 14 have been fixed to the cover panel 11 and base panel 10. However, the order in which this step is performed is irrelevant. In particular, it is also possible to pre-assemble the heat insulating linings and the bearing webs 14, and then fix the bearing webs 14 to the 5 base panel 10 and cover panel 11. In another embodiment, it is also possible to fix the bearing webs 14 to one of the base panel 10 or cover panel 11, then line the compartments 15 provided between the bearing webs 14, for example by projection of foam, then close the self-supporting case by fixing the other of the base panel 10 or cover panel 11 to the bearing webs 14. 10 In the embodiment shown on figures 11 and 12, the bearing web 14 at its upper end comprises a fixing strap 30 for retaining a weld support 31 (shown only on figure 11) for the welding of metal strakes of a fluid-tight membrane. The cover panel 11 shown on figure 11 has a plurality of grooves 32 positioned opposite the upper ends of the bearing webs 14. The fixing straps 30 thus extend inside the grooves 32 15 in order to allow the weld supports 31 to engage with the fixing straps 32. The weld support 31 and the fixing strap 30 are both made of Invar@ sheet metal and constitute a sliding joint. To achieve this, the fixing strap 30 comprises a longitudinal edge folded into a U-shape, forming a hook 33. Similarly, the weld support 31 has a longitudinal edge folded into a U-shape, forming a hook 34. The two hooks 33, 34 are interlocked 20 such that the weld support 31 is retained on the bearing web 14 via the fixing strap 30. A sliding joint created in this way allows the weld supports 31 to slide relative to the self-supporting case 3, 7 in the longitudinal direction. On figures 11 and 12, the fixing strap 30 is integrated in an insert 35 made by molding thermoplastic material over the fixing strap 30. The fixing strap 30 comprises an anchoring arm 36 extending 25 substantially perpendicular to the thickness direction of the self-supporting case 3, 7 and allowing a firm anchorage of the fixing strap 30 in the insert 35. The insert 35 is positioned in a housing formed at the level of the upper load distribution plate 16a, then fixed to the main body of the bearing web 14 by thermowelding. The thermowelding operation may be performed by any of the welding methods described 30 above. Such an integration of the fixing strap 30 in the bearing web 14 avoids the use of staples which are liable to damage the structure of the bearing webs 14, causing cracking. Also such integration, via an insert of thermoplastic material thermowelded 16 to the main body of the bearing web 14, facilitates the production and extraction from the mold of the bearing web 14. In another embodiment shown on figures 13 and 14, the fixing strap 30 is set into the mass of the bearing web 14 during its thermoforming. The fixing strap thus 5 presents an anchorage arm 36 extending substantially perpendicular to the thickness direction of the case 3, 7. The fixing strap 30 is arranged inside the mold 21 and is set into the mass of the bearing web 14 during its thermoforming. Such an integration of the fixing strap 30 in the bearing web 14 is simple. The technique described above for production of a self-supporting case may 10 be used in various types of tank, for example to produce the primary thermal insulation barrier and/or the secondary thermal insulation barrier of an LNG tank in a land-based installation or in a floating structure, such as an LNG tanker or similar. With reference to figure 15, a simplified view of an LNG tanker 70 shows a fluid-tight and insulated tank 71 of generally prismatic form, mounted in the double 15 hull 72 of the ship. The wall of the tank 71 comprises a primary fluid-tight barrier intended to come into contact with the LNG contained in the tank, a secondary fluid tight barrier arranged between the first fluid-tight barrier and the double hull 72 of the ship, and two thermal insulation barriers arranged respectively between the primary fluid-tight barrier and the secondary fluid-tight barrier, and between the secondary 20 fluid-tight barrier and the double hull 72. In a manner known in itself, loading/unloading pipelines 73 arranged on the upper deck of the ship may be connected by means of suitable connectors to a floating or port-based terminal, in order to transfer a cargo of LNG from or to the tank 71. Figure 15 shows an example of a floating terminal comprising a loading and 25 unloading station 75, an underwater pipeline 76, and a land-based installation 77. The loading and unloading station 75 is a fixed offshore installation comprising a mobile arm 74 and a tower 78 which supports the mobile arm 74. The mobile arm 74 carries a bundle of insulated flexible hoses 79 which can connect to the loading/unloading pipelines 73. The orientable mobile arm 74 can be adapted to all sizes of tanker. A 30 connecting pipe (not shown) extends inside the tower 78. The loading and unloading station 75 allows the tanker 70 to be loaded or unloaded from or to a land-based 17 installation 77. This comprises liquefied gas storage tanks 80 and connecting pipes 81 connected by the underwater pipeline 76 to the loading or unloading station 75. The underwater pipeline 76 allows the transfer of liquefied gas between the loading or unloading station 75 and the land-based installation 77 over a great distance, for 5 example 5 km, which allows the LNG tanker 70 to remain at a great distance from the coast during the loading and unloading operations. To create the pressure necessary for the transfer of liquefied gas, on-board pumps in the ship 70 are used, and/or pumps installed in the land-based installation 77, and/or pumps fitted to the loading and unloading station 75. 10 Although the invention has been described in connection with several particular embodiments, it is evident that it is in no way limited thereto and comprises all technical equivalents of the means described and their combinations if these fall within the scope of the invention. The use of the verb "comprise" or "contain" or "include" and its conjugated 15 forms does not exclude the presence of elements or steps other than those stated in a claim. The use of the indefinite article "a" for an element of step does not, unless specified otherwise, exclude the presence of a plurality of such elements or steps. In the claims, any reference symbol in brackets should not be interpreted as a limitation of the claim. 20

Claims

1. A method for producing a self-supporting case (3, 7) intended to provide thermal insulation for a fluid-storage tank with a fluid-tight membrane, said method 5 comprising: - providing a plurality of bearing webs (14) made of composite material comprising a fiber-reinforced thermoplastic matrix; - providing a base panel (10) and a cover panel (11); - interposing the bearing webs (14) between the base panel (10) and the cover 10 panel (11) such that the base panel (10) and the cover panel (11) are spaced apart in a thickness direction of the case (3, 7) and the bearing webs (14) extend in the thickness direction; - fixing the bearing webs (14) to the base panel (10) and/or to the cover panel (14); 15 - lining a plurality of compartments (15) arranged between the bearing webs (14) with a heat-insulating lining; wherein the base panel (10) and the cover panel (11) each comprise at least one thermoplastic element (21, 22, 26, 29) for fixing the bearing webs (14); and wherein the bearing webs (14) are fixed to the base panel and the cover panel by a 20 thermoplastic welding operation performed at interface zones (25) between the bearing webs (14) and the thermoplastic elements (21, 22, 26, 29) of the base panel (10) and the cover panel (11).

2. The production method as claimed in claim 1, wherein the base panel (10) and the cover panel (11) each have an inner face turned towards the inside of the 25 case, and an outer face, the inner faces of the base panel (10) and cover panel (11) being covered with thermoplastic film (21, 22, 29) for fixing the bearing webs (14).

3. The production method as claimed in claim 2, wherein the inner face of the base panel (10) and/or the cover panel (11) is fully covered with thermoplastic film (21, 22). 30

4. The production method as claimed in claim 3, comprising a step of arranging removable protective masks on the inner face of the base panel (10) and/or of the 19 cover panel (11) between the interface zones (25) prior to the welding operation.

5. The production method as claimed in claim 3 or 4, wherein the outer face of the base panel (10) and/or the cover panel (11) is fully covered with thermoplastic film (23, 24). 5

6. The production method as claimed in claim 2, wherein the inner face of the base panel (10) and/or the cover panel (11) is partially covered with thermoplastic film, a plurality of thermoplastic film strips (29) each being arranged in an interface zone (25) with a bearing web (14).

7. The production method as claimed in any of claims 2 to 6, wherein the base 10 panel (10) and/or the cover panel (11) comprises a plywood body and wherein the thermoplastic film (21, 22, 29) is glued to the plywood body or assembled to the body by hot pressing.

8. The production method as claimed in any of claims 2 to 7, wherein the thermoplastic film (21, 22, 23, 24) is a composite material comprising a fiber 15 reinforced thermoplastic matrix.

9. The production method as claimed in claim 1, wherein the base panel (10) and/or the cover panel (11) comprises a body made of composite material comprising a fiber-reinforced thermoplastic matrix, said body forming a thermoplastic element for the fixing of the bearing webs (14). 20

10. The production method as claimed in claim 1, wherein the base panel (10) and/or the cover panel (11) comprises a wooden body impregnated with a thermoplastic matrix for the fixing of the bearing webs, said body forming a thermoplastic element for the fixing of the bearing webs (14).

11. The production method as claimed in claim 1, wherein the thermoplastic 25 elements are thermoplastic studs (26) inserted inside bores arranged in the base panels (10) and/or cover panels (11) and inside bores arranged in the bearing webs (14).

12. The production method as claimed in any of claims 1 to 11, wherein the thermoplastic elements for fixing the bearing webs comprise a thermoplastic matrix 30 identical to the thermoplastic matrix of the bearing webs (14). 20

13. The production method as claimed in any of claims 1 to 12, wherein the thermoplastic welding is performed by a method selected from welding by infrared radiation, ultrasonic welding, induction heating, friction welding, welding by addition of filler, hot air jet welding, and flaming. 5

14. The production method as claimed in any of claims 1 to 13, wherein: - the bearing webs (14) have an upper end intended to support a cover panel and provided with a fixing strap (30) intended for anchoring a weld support (31) for the welding of metal strakes of a fluid-tight membrane (5, 9) which are intended to rest against the self-supporting case (3, 7), said fixing strap (30) comprising 10 a folded longitudinal edge (33) intended to cooperate with a folded longitudinal edge (34) of the weld support (31); - the cover panel (11) has parallel grooves (32) extending over the entire thickness of the cover panel; - the bearing webs (14) are arranged between the base panel (10) and the cover 15 panel (11) such that the fixing straps (30) each extend inside a groove (32) of the cover panel (11).

15. The production method as claimed in claim 14, wherein the fixing straps (30) are set into the mass of the bearing webs (14) during their forming.

16. The production method as claimed in claim 14, wherein a bearing web (14) 20 comprises a main body with an upper end provided with a housing, and an insert (35) comprising thermoplastic material molded over a fixing strap (30), said insert (35) being positioned in the housing of the upper end of the main body of the bearing web (14) and fixed to said main body by a thermoplastic welding operation.

17. A self-supporting case (3, 7) intended for thermal insulation of a fluid-storage 25 tank with a fluid-tight membrane, said case (3, 7) comprising: - a base panel (10) and a cover panel (11) spaced apart in a thickness direction of the case (3, 7); - a plurality of bearing webs (14) interposed between said base panel (10) and said cover panel (11) and extending in the thickness direction so as to define 30 a plurality of compartments (15), said bearing webs (14) being made of a composite material comprising a fiber-reinforced thermoplastic matrix; and 21 - a heat-insulating lining extending inside said compartments (15) arranged between the bearing webs (14), wherein - the base panel (10) and the cover panel (11) each comprise at least one 5 thermoplastic element (21, 22, 26); and - the bearing webs (14) are fixed to the base panel (10) and/or the cover panel (11) by thermoplastic welding in interface zones (25) between the bearing webs (14) and the thermoplastic elements (21, 22, 26) of the base panel (10) and the cover panel (11). 10

18. The case (3, 7) as claimed in claim 17, wherein the bearing webs (14) have load distribution plates (16a, 16b) extending along the two edges of said bearing webs (14) arranged facing the base panel (10) and the cover panel (10) respectively.

19. The case (3, 7) as claimed in claim 17 or 18, wherein the bearing webs (14) have a plurality of undulations, the axis of which extends perpendicular to the base 15 panel (10) and cover panel (11).

20. A fluid-tight, thermally insulated fluid-storage tank comprising a thermal insulation barrier (2, 6) comprising a plurality of cases (3, 7) as claimed in any of claims 17 to 19 arranged next to each other, and a sealing membrane (5, 9) resting against the thermal insulation barrier. 20

21. A ship (70) for transporting a fluid, the ship comprising a double hull (72) and a tank (71) as claimed in claim 20, arranged in the double hull.

22. A method for loading or unloading a ship (70) as claimed in claim 21, wherein a fluid is conducted through insulated pipelines (73, 79, 76, 81) from or to a floating or land-based storage installation (77) to or from the ship's tank (71). 25

23. A system for transferring a fluid, the system comprising a ship (70) as claimed in claim 21, insulated pipelines (73, 79, 76, 81) arranged so as to connect the tank (71) installed in the ship's hull to a floating or land-based storage installation (77), and a pump for driving a fluid through insulated pipelines from or to the floating or land based storage installation to or from the ship's tank.