Patent Publication Number: US-7900794-B2

Title: Sealed, thermally insulated tank with compression-resistant non-conducting elements

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending application Ser. No. 11/265,079 filed on Nov. 3, 2005, which claims priority to French Application No. 04 11968 filed on Nov. 10, 2004. The entire contents of each of the above-identified applications are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the production of sealed, thermally insulated tanks consisting of tank walls fixed to the load-bearing structure of a floating structure suitable for the production, storage, loading, ocean carriage and/or unloading of cold liquids such as liquefied gases, particularly those with a high methane content. The present invention also relates to a methane carrier provided with a tank of this type. 
     DESCRIPTION OF THE RELATED ART 
     Ocean carriage of liquefied gas at very low temperature involves an evaporation rate per day&#39;s sailing that it would be advantageous to minimize, which means that the thermal insulation of the relevant tanks should be improved. 
     A sealed, thermally insulated tank consisting of tank walls fixed to the load-bearing structure of a ship has already been proposed, said tank walls having, in succession, in the direction of the thickness from the inside to the outside of said tank, a primary sealing barrier, a primary insulating barrier, a secondary sealing barrier and a secondary insulating barrier, at least one of said insulating barriers consisting essentially of juxtaposed non-conducting elements, each non-conducting element including a thermal insulation liner arranged in the form of a layer parallel to said tank wall, and load-bearing elements that rise through the thickness of said thermal insulation liner in order to take up the compression forces. 
     For example, in FR-A-2 527 544 these insulating barriers consist of closed parallelepipedal caissons made from plywood and filled with perlite. On the inside, the caisson includes parallel load-bearing spacers interposed between a cover panel and a base panel in order to withstand the hydrostatic pressure exerted by the liquid contained in the tank. Non-load-bearing spacers made from plastic foam are placed between the load-bearing spacers in order to maintain their relative positioning. Manufacture of a caisson of this type, including the assembly of the outer walls made from plywood sections and the fitting of the spacers, requires a number of assembly operations, particularly stapling. Furthermore, the use of a powder such as perlite complicates the manufacture of the caissons because the powder produces dust. Thus, it is necessary to use high-quality and therefore expensive plywood so that the caisson is well sealed against dust, i.e. knot-free plywood. Furthermore, it is necessary to tamp down the powder with a specific pressure in the caisson, and it is necessary to circulate nitrogen inside each caisson in order to evacuate all the air present, for safety reasons. All these operations complicate manufacture and increase the cost of the caissons. Moreover, if the thickness of the insulating caissons is increased with an insulating barrier, the risk of the walls of the caissons and the load-bearing spacers buckling increases considerably. If it is desired to increase the anti-buckling strength of the caissons and of their internal load-bearing spacers, the cross section of said spacers has to be increased, which increases the thermal bridges established between the liquefied gas and the load-bearing structure of the ship by the same amount. Furthermore, if the thickness of the caissons is increased it is observed that, inside the caissons, gas convection currents arise that are highly detrimental to good thermal insulation. 
     FR-A-2 798 902 describes other thermally insulated caissons designed for use in such a tank. Their method of manufacture consists in alternately stacking a plurality of low-density foam layers and a plurality of plywood panels, placing adhesive between each foam layer and each panel until the height of said stack corresponds to the length of said caissons, in cutting the above-mentioned stack into sections in the direction of the height, at regular intervals corresponding to the thickness of a caisson, and in adhesively bonding a base panel and a top panel made from plywood on either side of each stack section thus cut, said panels extending perpendicularly to said cut panels, which serve as spacers. Although the result of this is a good compromise in terms of anti-buckling strength and thermal insulation, it has to be admitted that this manufacturing process also requires numerous assembly stages. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to propose a tank of this type while also improving at least one of the following characteristics without detriment to others of these characteristics: the tank&#39;s cost price, the ability of the walls to withstand pressure and the thermal insulation of the walls. A further object of the invention is to propose a tank of this type in which the non-conducting elements are easily adaptable in terms of their dimensions, without compromising the ability of the walls to withstand pressure and the thermal insulation of the walls. 
     To that end, a subject of the invention is a sealed, thermally insulated tank including at least one tank wall fixed to the hull of a floating structure, said tank wall having, in succession, in the direction of the thickness from the inside to the outside of said tank, a primary sealing barrier, a primary insulating barrier, a secondary sealing barrier and a secondary insulating barrier, at least one of said insulating barriers consisting essentially of juxtaposed non-conducting elements, each non-conducting element including a thermal insulation liner arranged in the form of a layer parallel to said tank wall, and load-bearing elements that rise through the thickness of said thermal insulation liner in order to take up the compression forces, characterized in that said load-bearing elements of a non-conducting element include pillars of small transverse section as compared with the dimensions of the non-conducting element in a plane parallel to said tank wall. 
     Small-cross section pillars of this type have the advantage that they can be distributed in the non-conducting element as a function of local requirements. By adapting the number and the distribution of the load-bearing pillars, the non-conducting element&#39;s compression strength can, in particular, be made more uniform than with prior-art spacers. It is also possible to prevent localized depression or pinching of a cover panel. Advantageously, said pillars are regularly distributed over the entire surface of the non-conducting element seen in a plane parallel to the tank wall. A further advantage of the non-conducting element with small-cross section pillars is that it allows the manufacture of a non-conducting element of any desired dimensions without loss of compression strength, at least insofar as these dimensions remain greater than or equal to the spacing between the pillars. A non-conducting element of small surface area may, in particular, be obtained by cutting an element of larger surface area. 
     According to a particular embodiment, said pillars are identically spaced apart in the length direction and in the width direction of the non-conducting element. 
     Pillars of this type may have a hollow or solid cross section, for which a number of shapes are possible. Preferably, said pillars have a closed hollow transverse section. Such hollow pillars with a closed transverse section, in particular tubes with a circular cross section, make it possible to obtain very good anti-buckling resistance while at the same time minimizing the effective thermal conduction cross section. 
     Advantageously, said pillars are produced from plastic or a composite. 
     Preferably, said insulation liner of the non-conducting element includes a block of synthetic foam. 
     According to one embodiment, said pillars are inserted in holes machined in said block of synthetic foam. 
     According to a further embodiment, said block of synthetic foam is obtained by pouring between said pillars so as to embed at least one height portion of said pillars, for example half or all their height, in said block of synthetic foam. 
     Advantageously, said non-conducting element includes a planar positioning element arranged parallel to said tank wall in the thickness of the insulation liner and having openings traversed by said pillars in order to define their mutual positioning. 
     Preferably, said non-conducting element includes at least one panel extending parallel to said tank wall on a side of said non-conducting element. In other words, in such a case, the non-conducting element comprises a base panel or a cover panel. By convention, “cover” is the name given to a panel on that side of the non-conducting element that faces toward the inside of the tank and “base” is the name given to a panel on the side of the non-conducting element that faces toward the load-bearing structure. The non-conducting element may also include both a base panel and a cover panel. Any fixing means may be used for fixing a panel of this type to the non-conducting element. 
     The non-conducting elements may be open or closed. Advantageously, the presence of a cover panel provides uniform support for the adjacent sealing barrier. However, a panel of this type is not mandatory because sufficient support of this type may also be obtained from the pillars alone. Advantageously, the presence of a base panel provides well distributed transmission of compression forces from the primary insulating barrier toward the secondary insulating barrier or from the secondary insulating barrier toward the hull. However, a panel of this type is not mandatory because this transmission may also be sufficiently guaranteed by the pillars alone. Panels of this type may be formed in several ways. One possibility is to form a load-bearing structure incorporating, as a single piece, a panel with the pillars. A further possibility is to fix a separate panel on a side of the non-conducting element. 
     Advantageously, the inner face of a said panel has recesses arranged in such a manner as to interact by flush-fitting with said pillars. This results in a particularly robust link. In such a case, the panel may have a thermal expansion coefficient that is different from that of said pillars so as to give rise to gripping between said panel and said pillars flush-fitted in the latter when the tank is cooled. 
     According to a particular embodiment, said non-conducting element has the form of a closed box with a base panel, a cover panel and peripheral walls extending between said panels along the edges of the latter. A design of this type allows the fitting of an insulation liner in the form of granular material. However, depending on the construction of the insulation liner, it is possible, also, to use non-conducting elements that do not have peripheral walls. 
     According to a further particular embodiment, said load-bearing elements of a non-conducting element are produced in the form of at least one load-bearing structure formed as a single piece including, on each occasion, linking means that rigidly link said load-bearing elements together and at least one height portion of said pillars. 
     A load-bearing structure of this type formed as a single piece combines very advantageous mechanical properties both in terms of stiffness and in terms of anti-buckling resistance in the direction of the thickness of the hollow elements, of ease of forming, of thermal insulation and of cost price. Indeed, for a given geometry of the pillars, their anti-buckling resistance is increased by the rigid integral links as compared to separate pillars. Furthermore, manufacture of the links between the pillars and pillars, i.e. at least one portion of their height, in the form of a single piece makes it possible to dispense with certain assembly operations, makes it possible to obtain a relatively rigid load-bearing structure without excessively increasing the cross section of the pillars and/or their thickness, and thus the thermal bridges, and simplifies fitting of the thermal insulation liner in the non-conducting element. 
     According to a further embodiment of the linking means, said linking means include arms extending between said pillars. Advantageously, said arms extend parallel to said tank wall along at least one side of said insulation liner. Positioned in this way, the arms offer a supplementary surface, in addition to that of the pillars, for the fixing of a possible base panel and/or cover panel formed independently of the load-bearing structure. 
     According to a preferred embodiment of the linking means, said linking means of a load-bearing structure include a panel extending parallel to said tank wall on a side of said non-conducting element, said pillars projecting from an inner face of said panel. 
     According to one embodiment of the non-conducting element, it has two load-bearing structures arranged in such a manner that their respective panels have said inner faces turned toward one another, the pillars projecting from said inner faces being assembled in pairs in the region of their ends located opposite said panels in order to form, on each occasion, a pillar of said non-conducting element. In other words, in such a case, the pillars of each of the two load-bearing structures are placed end to end in order to form, on each occasion, a pillar having two parts extending, respectively, through a portion of the thickness of the non-conducting element. In particular, it is possible to use two completely symmetrical load-bearing structures. 
     Advantageously, an insulation piece having a thermal conductivity that is lower than that of said pillars is interposed, on each occasion, between the two assembled pillars. This makes it possible to improve the thermal insulation obtained by means of the non-conducting element. 
     The two load-bearing structures may be assembled by any means. Preferably, the pillars of the two load-bearing structures are assembled in pairs, on each occasion, by means of a linking piece having a thermal expansion coefficient that is different from that of said pillars so as to give rise to gripping between said linking piece and said pillars when the tank is cooled. As a variant embodiment, or in combination, the linking piece may also be flush fitted, adhesively bonded, snap-fitted, etc. 
     Preferably, the load-bearing structure or structures of a non-conducting element is (or are) manufactured using a process of molding, extrusion, pultrusion, thermoforming, blow-molding, injection-molding or rotational molding. The load-bearing structures may be manufactured from any material suitable for the above-mentioned processes, particularly plastics such as PC, PBT, PA, PVC, PE, PS, PU and other resins. Advantageously, the load-bearing structures are produced from a composite material. The use of this type of materials brings together the conditions necessary for obtaining load-bearing elements with a thinner wall thickness than with plywood, while at the same time offering better or equivalent thermal conductivity and a lower expansion coefficient. For example, said load-bearing structures may be produced from a polymer-resin-based composite material, for example polyester resin or another resin. Within the meaning of the invention, polymer-resin-based composite materials include polymers or mixtures of polymers with all kinds of fillers, additives, reinforcements or fibers, for example glass fibers or other fibers, providing sufficient rupture strength and rigidity and other properties. Additives may also be employed to reduce the material&#39;s density and/or improve its thermal properties, particularly reducing its thermal conductivity and/or its expansion coefficient. Use may also be made of a composite that includes a high proportional of sawdust with a synthetic binder. In certain embodiments, the load-bearing structure may also be made from laminated wood or plywood molded by hot compression. 
     According to a particular embodiment, said at least one insulating barrier consisting of said non-conducting elements is covered, on each occasion, by one of said sealed barriers that is formed from thin metal plate strakes with a low expansion coefficient, the edges of which are raised toward the outside of said non-conducting elements, said non-conducting elements having cover panels carrying parallel grooves spaced by the width of a plate strake in which weld supports are slideably retained, each weld support having a continuous wing projecting from the outer face of the cover panel and on whose two faces the raised edges of two adjacent plate strakes are welded in a leaktight manner. The sliding weld supports form gliding joints allowing different barriers to move relative to one another through the effect of differences in thermal contraction and movements of the liquid contained in the tank. 
     Advantageously, secondary retention members integral with the load-bearing structure of the ship fix the non-conducting elements forming the secondary insulating barrier against said load-bearing structure, and primary retention members linked to said weld supports of the secondary sealing barrier retain said primary insulating barrier against the secondary sealing barrier, said weld supports retaining said secondary sealing barrier against the cover panels of the non-conducting elements of the secondary insulating barrier. Thus, the primary insulating barrier is anchored on the secondary insulating barrier, with no effect on the continuity of the secondary sealing barrier interposed between them. 
     According to a preferred embodiment, said thermal insulation liner includes reinforced or unreinforced, rigid or flexible foam of low density, i.e. under 60 kg/m 3 , for example around 40 to 50 kg/m 3 , which has very good thermal properties. It is also possible to use a material of nanoscale porosity of the aerogel type. A material of the aerogel type is a low-density solid material with an extremely fine and highly porous structure, possibly with a porosity up to 99%. The pore size of these materials is typically in the range between 10 and 20 nanometers. The nanoscale structure of these materials greatly limits the mean free path of the gas molecules, and therefore also convective heat and mass transfer. Aerogels are thus very good thermal insulators, with a thermal conductivity, for example, below 20×10 −3  W·m −1 ·K −1 , preferably less than 16×10 −3  W·m· −1 ·K −1 . They typically have a thermal conductivity 2 to 4 times as low as that of other, conventional insulators, such as foams. Aerogels may be in different forms, for example in the form of powder, beads, nonwoven fibers, fabric, etc. The very good insulating properties of these materials make it possible to reduce the thickness of the insulating barriers in which they are used, which increases the useful volume of the tank. 
     The invention also provides a floating structure, in particular a methane carrier, characterized in that it comprises a sealed, thermally insulated tank according to the subject of the above invention. A tank of this type may, in particular, be employed in an FPSO (floating, production, storage and offloading) facility, used to store the liquefied gas with a view to exporting it from the production site, or an FSRU (floating storage and regasification unit) used to unload a methane carrier with a view to supplying a gas transportation system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and further objects, details, characteristics and advantages thereof will become more clearly apparent in the course of the following description of a particular embodiment of the invention that is given solely by way of non-limiting illustrative example with reference to the appended drawings, in which: 
         FIG. 1  is a stripped-back perspective view of a tank wall according to a general embodiment that is useful for understanding the invention; 
         FIGS. 2 and 3  show a primary retention member of the tank wall of  FIG. 1  seen in two perpendicular directions; 
         FIG. 4  is a transverse sectional view of a tank wall according to one embodiment of the invention; 
         FIG. 5  is an expanded perspective view of a non-conducting element of the tank wall shown in  FIG. 4 ; 
         FIG. 6  is a perspective view of a molding step for obtaining a non-conducting element according to the first embodiment of the invention; 
         FIG. 7  shows, in perspective, a load-bearing structure molded as a single piece; 
         FIG. 8  is a partial sectional view showing a variant embodiment of the load-bearing structure of  FIG. 7 ; 
         FIG. 9  is an expanded perspective view of two types of non-conducting element produced with the aid of the load-bearing structure of  FIG. 7 ; 
         FIG. 10  is a partial, sectional view showing the assembly of a non-conducting element of  FIG. 9 ; 
         FIGS. 11 and 12  are views similar to  FIG. 7 , showing other variant embodiments of the load-bearing structure; 
         FIG. 13  is a partial, sectional view of a non-conducting element according to a further embodiment of the invention; 
         FIG. 14  is a plan view of the load-bearing structure of the non-conducting element of  FIG. 13 ; 
         FIGS. 15 to 18  show further embodiments of load-bearing elements in the form of pillars, seen in transverse section; 
         FIG. 19  is a view similar to  FIG. 6 , showing an alternate molding method; 
         FIG. 20  is an expanded perspective view of a non-conducting element according to a further embodiment of the invention; 
         FIG. 21  shows, in perspective, a load-bearing structure thermoformed from a single piece; and 
         FIGS. 22 and 23  show in plan view and in sectional view on line XXIII a non-conducting element according to a further embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will be given below of several embodiments of a sealed, thermally insulated tank incorporated in and anchored to the double hull of a structure of the FPSO or FSRU type or of a methane-type carrier. The general structure of such a tank is well known per se and has a polyhedral form. Therefore, a description will be given only of a wall zone of the tank, it being understood that all the walls of the tank have a similar structure. 
     A description is now given of a general embodiment that is useful for understanding the invention, with reference to  FIGS. 1 to 3 .  FIG. 1  shows a zone of the double hull of the ship, denoted by  1 . The tank wall is composed, in succession, in its thickness, of a secondary insulating barrier  2  formed from caissons  3  juxtaposed on the double hull  1  and anchored to the latter by means of secondary retention members  4 , then a secondary sealing barrier  5  carried by the caissons  3 , then a primary insulating barrier  6  formed from juxtaposed caissons  7  anchored to the secondary sealing barrier  5  by primary retention members  48 , and finally a primary sealing barrier  8  carried by the caissons  7 . 
     The caissons  3  and  7  are parallelepipedal non-conducting elements with a mutually identical or different structure and mutually identical or different dimensions. 
     Secondary retention members  4  are fixed on pins  31  welded to the double hull  1  in a regular rectangular grid arrangement so that these retention members  4  can, on each occasion, hold four caissons  3 , whose corners meet. Also provided are two secondary retention members  4  in the central zone of each caisson  3 . However, depending on the size of the caisson, more or fewer than six anchoring points per caisson  3  may be necessary. 
     The secondary sealing barrier  5  is produced in accordance with the known technique in the form of a membrane consisting of Invar plate strakes  40  with raised edges. As may be seen better in  FIG. 3 , the cover panels  11  of the caissons  3  have longitudinal grooves, with an inverted-T-shaped cross section, denoted by  41 . A weld support  42  in the form of a strip of Invar folded in the form of an L, is inserted slideably in each groove  41 . Each plate strake  40  extends between two weld supports  42  and has two raised edges  43  welded, on each occasion, continuously by a weld bead  44  to the corresponding weld support  42 , as may be seen in  FIGS. 2 and 3 . The primary sealing barrier  8  is produced in the same manner. 
     Similarly, the caissons  7  of the primary insulating barrier are anchored, on each occasion, to the four corners and at two points in the central zone of the caisson  7 . To that end, use is made, on each occasion, of a primary retention member  48  shown in detail in  FIGS. 2 and 3 . The primary retention member  48  has a lower sleeve  49  integral with a lug  50  welded at several, for example three, points  51  of a weld support  42  above the raised edges  43  of the plate strakes  40 . A rod  52  made from Permali, a composite material based on resin-impregnated beech wood, has a lower end fixed in the lower sleeve  49  and an upper end fixed in a sleeve  54  integral with a support washer  53  that bears on the cover panels  11  of the caissons  7 , being accommodated in countersinks  28  at the corners of the caissons  7  and at the central shafts  30 . The sleeve  54  is threaded and is screwed onto a corresponding threaded end of the rod  52 . When the washer  53  has been thus positioned, immobilizing screws  56  are engaged through holes  55  provided in the washer  53  and screwed into the panel  11  in order thus to prevent any subsequent rotation of the washer  53 . In each insulating barrier, the caissons  3  and  7  are juxtaposed with a small intermediate space of the order of 5 mm. 
     Advantageously, a layer of nanoporous materials of the aerogel type, which are very good thermal insulators, is included as insulation liner in the caissons  3  and/or  7 . Aerogels also have the advantage of being hydrophobic, so absorption of the moisture from the boat into the insulating barriers is thus prevented. An insulation layer may be produced with aerogels, possibly pocketed, in textile form or in the form of beads. 
     Generally speaking, aerogels may be made from a number of materials, including silica, alumina, hafnium carbide and also varieties of polymers. Furthermore, in accordance with the manufacturing process, aerogels may be produced in powder, bead, monolithic sheet and reinforced flexible fabric form. Aerogels are generally manufactured by extracting or displacing the liquid of a gel of micronic structure. The gel is typically manufactured by means of chemical conversion and reaction of one or more dilute precursors. This results in a gel structure in which a solvent is present. Use is generally made of hypercritical fluids such as CO 2  or alcohol, to displace the gel solvent. Aerogels&#39; properties may be modified by using a variety of doping and reinforcement agents. 
     The use of aerogels as insulation liners significantly reduces the thickness of the primary and secondary insulating barriers. It is, for example, possible to conceive of barriers  2  and  6  having a thickness of 200 mm and 100 mm, respectively, by using an aerogel bed in textile form in the caissons  3  and  7 . The tank wall then has a total thickness of 310 mm. As a variant embodiment, it is possible to conceive of a tank wall having a total thickness of 400 mm by using, on each occasion, a layer of aerogel particles, particularly aerogel beads, in the caissons  3  and  7 . 
     With reference to  FIGS. 4 and 5 , a description will now be given of a first embodiment of a sealed, thermally insulated tank according to the invention. In the first embodiment, the primary and secondary insulating barriers are formed from non-conducting elements in the form of parallelepipedal caissons  60  whose structure is shown in  FIG. 5  and that are arranged and anchored in a similar manner to the caissons  3  and  7  of  FIG. 1 , so a further description is unnecessary in this regard. 
     The caisson  60  includes a block of low-density synthetic foam  63 , for example low-density polyurethane foam, optionally reinforced with fibers, sandwiched between a base panel  61  and a cover panel  62  that are fixed to its larger faces, for example by means of adhesive bonding. 
     Between the panels  61  and  62 , load-bearing pillars  65  in the form of hollow tubes with a circular cross section extend in holes  64  provided in the thickness of the block  63 . In the example shown, the pillars  65  are distributed in the form of a square-mesh grid, but other forms of distribution are possible. In the case of a non-conducting element with a 1.5-m-sided square cross section, provision is made, for example, for sixty-four pillars  65 . However, the density of the pillars may be modified, particularly as a function of the forces to be taken up and of the cross section of the pillars. The inside of the pillars  65  is filled with insulation, which is, for example, the same foam as that forming the block  63  between the pillars  65 , or another material, for example a material of higher density, in order to take up more compression forces. 
     In the embodiment of  FIG. 5 , the caisson  60  may be manufactured by means of the following steps: cutting a block of foam  63  from a bed of continuously-poured foam, machining holes  64  through the block  63 , inserting pillars  65  in the holes  64 , inserting plugs of insulation  66  in the pillars  65 , and adhesive bonding of the panels  61  and  62 . 
     An alternate manufacturing method corresponds to  FIG. 6 , in which the block of foam is omitted. In such a case, pillars  65  are placed in the cavity  68  of a mold  67  and then foam is poured between the pillars  65  so as to obtain a block of foam in which the pillars  65  are embedded. The pillars  65  may also be filled during the same pouring step if their diameter is fairly large, for example greater than 100 mm. In order to guarantee the positioning and holding of the pillars  65  in the cavity of the mold, a planar positioning element is used, in this case in the form of a grid or of a glass mat  69 , through which the pillars  65  are tightly fitted. The grid or glass mat  69  is also embedded in the thickness of the block of foam after molding, which makes it possible to reduce the expansion coefficient of the foam in this zone and thus to reduce the shear stresses between the panels  61  and  62  and the foam. Lastly, the panels  61  and  62  are adhesively bonded. Alternately, or in combination with this adhesive bonding, it is possible to fit the panels and the ends of pillars  65  together, which ends should, in such a case, extend beyond the block  63 . 
     It would also be possible to commence by fixing the pillars  65  on the panel  61  and placing this assembly in the mold  67  in order to pour the foam directly over the panel  61 , with or without the grid  69 . 
       FIG. 19  illustrates, using the same reference numerals as in  FIG. 6 , a further variant embodiment of the process in which the block of foam  63  is molded between the panels  61  and  62 , which panels are placed with the pillars  65  (and, as appropriate, the grid or glass mat  69 ) in the mold  67 , which is closed by a cover  59 . This results in a caisson  60  that is finished in a single operation. 
     The pillars  65  may be manufactured in a number of materials. Plastics such as PVC, PC, PA, ABS, PU, PE and the like are particularly suited to the molding of pillars of any form and have an advantageous cost price. Other possible materials are composites, wood, plywood or synthetic foams. The panels  61  and  62  may be produced from plywood, plastic resin or a composite material. For example, their thicknesses are 6.5 mm for the base and 12 mm for the cover. 
     It will be noted that the caisson  60  may be manufactured, or, above all, easily cut out, in any form whatsoever in order to achieve precise connections when the tank is constructed or to take up tolerances. Indeed, it is easy to cut the panels  61  and  62  and the block  63  between the pillars  65  without compromising the cohesion and compression strength of each caisson part thus separated. As appropriate, it is also possible to cut hollow pillars  65  vertically. 
     The tank wall produced with the aid of the caissons  60  is shown in section in  FIG. 4 . In this example, thicker caissons are used for the secondary insulating barrier  2  than for the primary insulating barrier  6 . The detail of the primary  4  and secondary  48  anchoring members and of the sealing barriers  5  and  8  is not shown. Reference may be made to  FIGS. 1 to 3  in this regard. 
     As the geometry of the double hull  1  is irregular, provision is made for shims around the threaded pins  31 . The thickness of each shim is calculated by computer on the basis of a topographical survey of the inner surface of the double hull  1 . Thus, the base panels  61  of the secondary barrier  2  are positioned along a theoretical regular surface. Between the base panels  61  and the double hull  1 , provision is conventionally made for beads of mastic  70  that are adhesively bonded to the base panels  61  and are crushed against the double hull when the caissons  60  are fitted, so as to provide their support. To avoid this mastic adhering to the double hull, a sheet of Kraft paper (not shown) is provided between them. Preferably, the beads  70  are placed in line with the pillars  65  in order to prevent flexing of the panel  61  on account of the compression force, which is transmitted predominantly in the region of the pillars  65 . Furthermore, it would be possible to dispense with base panels and to rest the pillars  65  directly on the beads  70 . 
     According to a variant embodiment (not shown), provision is made for peripheral walls extending to the periphery of the caisson  60  between the panels  61  and  62  so as to form a closed box capable of containing granular insulation. These walls may be fixed to the panels by means of adhesive bonding, stapling, flush-fitting and other fixing means. The caisson  60  may also be assembled in monobloc fashion, for example by means of blow-molding or rotational molding. 
     According to a further variant embodiment, the panels  61  and/or  62  are replaced by panel portions that cover only zones of the block  63  at the end of the pillars  65 , not the entire surface of the block  63 . The weld supports  42  will then be housed in the cover-panel portions. 
     Provision may be made for oblique pillars  65 , i.e. pillars whose axis is not perpendicular to the base  61  and cover  62  panels. An inclination of this type makes it possible to take up not only shear forces but also overturning forces applied to the caisson  60 . 
     With reference to  FIGS. 7 to 12 , a description is given of further embodiments of non-conducting caissons or elements that can be used to form the insulating barriers of the tank wall, the general structure of which was described for  FIGS. 1 to 3 . The production of the sealing barriers and the attachment of the various barriers is similar to the preceding embodiments, there will be no point in describing them again here. 
       FIG. 9  shows, in expanded perspective view, a caisson  570  and a caisson  670  that are, respectively, manufactured with the aid of molded load-bearing structures  500 , a description of which will now be given with reference to  FIG. 7 . 
     The load-bearing structure  500  is an injection-molded piece made from any appropriate material. It has a flat plate  571  with chamfered corners, for example in the form of a 1.5-m-sided square or of a rectangular, from one face of which sixteen hollow circular cylindrical pillars  575  project, arranged in the form of a regular square grid, plus two tubes  581  of smaller cross section in the region of a central zone of the plate, and also four triangular cylindrical pillars  580  in the region of the four corners of the plate. The plate  571  is continuous in the region of the base of the pillars  575  and  580 , but pierced in the region of the base of the tubes  581  in order to allow the passage of a coupler rod. Furthermore, in the case of a caisson of the primary barrier  6 , the plate  571  is slit in order to allow through the weld supports  42  and the raised edges  43  of plate strakes of the secondary sealing barrier. The pillars  580  serve to receive the bearing forces of the coupling members used at each corner of the non-conducting elements. The cross section of the pillars  575  is, for example, 300 mm for a 1.5 m square plate. As for the insulating liner, the load-bearing structure  500  may be covered with a layer of low-density foam, which is poured between and into the pillars  575 . 
     The cross section of the pillars may be reasonably large, the important thing being to always make provision for several pillars per caisson. Thus, the dimensions of the pillars in terms of cross section may be ⅓ or even ½ of the corresponding dimensions of the caisson. 
     In order to form the caisson  570 , an independent panel  572  with the same dimensions as the plate  571  is fixed on the end of the pillars  575  opposite this plate. This panel may be fixed by any means (adhesive bonding, stapling, flush fitting, etc.). In  FIG. 9 , provision has been made for circular grooves  573  on the inner face of the panel  572 , for receiving the end of each pillar  575  tightly. 
     The materials of the structure  500  and of the panel  572  may be chosen so as to produce heat-shrinking of the pillars  575  in the panel. For example, with a piece  500  made from PVC and a panel  572  made from plywood, which exhibits less heat shrinkage, the end of the pillars  575  is made to grip the circular core delimited by the groove  573  when the tank is cooled. Conversely, gripping of the pillars  575  could also be obtained with a panel  572  that contracts more than the piece  500 . 
     The panel  572  has holes  574  opposite the tubes  581  of the molded structure  500 . 
     In the caisson  670 , two identical molded structures  500  are arranged symmetrically and assembled together by causing their respective pillars  575  to bear against one another. This assembly may be produced by any means (adhesive bonding, welding, flush fitting, etc.). In  FIG. 9 , it is achieved with the aid of a linking ring  680  interposed, on each occasion, between two aligned pillars  575  and flush fitted over them. This assembly can be seen better in  FIG. 10 , where it will be observed that the linking ring  680  has an outer annulus  682  and an inner annulus  681  that are connected by means of a radial tongue  683 . The pillars  575  flush fit between the two annuli  681  and  682  and abut on either side of the tongue  683 . The material of the ring  680  may be chosen to have lower conductivity than that of the pillars  575 , in order to fulfill a thermal insulation function. They may also, alternately or in combination, be chosen to have an expansion coefficient that is different from that of the pillars  575  in order to fulfill a thermal assembly function. In a variant embodiment, two molded structures having pillars with complementary cross sections may be fixed together by means of direct nesting of the pillars together. 
     The foam-filled piece  500  may also be used alone without a supplementary panel by rotating the plate  571  toward the inside of the tank in order to support the adjacent sealing barrier. The non-conducting element thus formed rests via the pillars  575  on the secondary sealing barrier or on the strips of resin fixed to the hull. 
       FIGS. 11 and 12  show molded load-bearing structures  600  and  700  that make it possible to produce non-conducting elements in a manner similar to the structure  500  described previously. 
     In  FIG. 11 , identical reference numerals to those in  FIG. 7  denote identical elements. The structure  600  includes planar peripheral walls  601  extending continuously along the four edges of the plate  571 , forming a box capable of containing insulation in the form of powder, beads or the like. For example, a structure  600  containing aerogel beads may be combined with a structure  600  containing low-density foam to form a caisson  670  as shown in  FIG. 9 . 
     In  FIG. 12 , the planar plate  771  carries thirty-six hollow tubular pillars  775  of smaller cross section (for example 100 mm) than the above-mentioned pillars  575 , four hollow tubular pillars  780  with an even smaller cross section (for example 50 to 60 mm) in the region of its corners, and two tubular pillars  781 , similar to the pillars  780 , in the region of a central zone of the plate  771  in order to allow the coupling members serving to attach the insulating barrier to pass through. 
     The structures  500 ,  600  and  700  may be injection-molded. A similar structure may also be obtained by thermoforming from a plastic plate. This possibility is illustrated in  FIG. 8 . In such a case, the initially planar plate  571  is heated and deformed to match the impression of a female mold  560 . This results in load-bearing pillars  575  whose plate-side end is open and whose opposite end is closed by a wall  583 . In such a case, the space  582  located inside the pillars  575  is filled with, for example, foam from the face of the plate  571  opposite these pillars. 
     The walls  601  may also obtained by thermoforming. 
       FIG. 21  shows, in perspective, a thermoformed load-bearing structure  1300  that includes a plate  1371  that can act as base panel or cover panel for a caisson, and load-bearing pillars  1375  obtained in a similar way to the pillars  575  in  FIG. 8 . In the example shown, the pillars  1375  have a frustoconical shape, which facilitates their forming. For example, provision may be made for a pillar diameter that varies from 160 mm at the base to 120 mm at the top, over a height of approximately 100 mm. 
     In order to serve as base panel of a caisson of the primary insulating barrier, the plate  1371  is provided with two longitudinal ribs  1384  extending over the entire length of the plate  1371 . Each rib  1384  is obtained during the thermoforming operation by pushing the material in the same direction as the pillars  1375 , so as to form a V-shaped fold that is open on the planar face of the plate  1371 , the inner space  1385  of which allows the weld supports  42  and the raised edges  43  of the secondary sealing barrier to pass through. In the case of the secondary insulating barrier, the ribs  1384  are unnecessary. 
     A description was given previously of the load-bearing structures that include a plate acting as cover or base panel. A description is now given of a further embodiment of a non-conducting element  870  with reference to  FIG. 13 , in which the molded load-bearing structure  800  includes load-bearing elements  875  of small cross section connected by arms  890 . This load-bearing structure is in plan view in  FIG. 14 . The load-bearing elements  875  are hollow circular cylindrical pillars arranged in a regular grid and connected by arms  890  that are arranged in the form of a square-mesh grid. A cover panel  872  and a base panel  871 , for example made from plywood, plastic, composite or another material, are adhesively bonded on the two faces opposite the load-bearing structure  800 . The arms  890  are located at the end of the load-bearing elements  875  adjacent to the panel  872  and have a planar upper face, which may serve for adhesive bonding of the panel  872 . 
       FIG. 20  shows the non-conducting element  870  in expanded perspective view, in a version that its slightly modified in terms of the arrangement of the linking arms  890 . 
     Other arms may be provided in the region of the lower end of the pillars  875 . The arms may also be placed in another region of the load-bearing pillars (for example half way up). 
     The inner space of the caisson  870 , i.e. the inner space  880  of the pillars  875 , and the space  876  between the pillars is filled with one or more types of insulation. When low-density foam is used, the caisson may be manufactured by placing a structure  800  of rectangular form in plan view in a mold, pouring the foam into the mold so as to embed the structure  800  in a parallelepipedal block of foam, then fixing the panels  872  and  871  to this block. The base panel  871  is not always necessary. One of the panels may also be molded as a single piece with the structure  800 . 
     Although a description has been given of hollow load-bearing pillars of circular cross section in the caisson  60  and the load-bearing structures  500 ,  600 ,  700  and  800 , the load-bearing pillars may have any other form in terms of cross section and any type of regular or irregular spatial distribution. For example  FIG. 15  shows a load-bearing pillar  975  consisting of a plurality of concentric cylindrical walls  976 . In the pillar  1075  of  FIG. 16 , the cylindrical walls  1076  have a square cross section. 
       FIG. 17  shows pillars  1175  distributed in lines in the form of a regular figure and with a hollow, square cross section with chamfered corners. In  FIG. 18 , pillars  1275 , for example solid circular cylinders, are distributed in a staggered arrangement. Other cross sections are also achievable, i.e. rectangular, polygonal, I-shaped, solid or hollow, dihedral, etc. cross sections. The load-bearing pillars may also have a cross section that varies over their height, for example frustoconical pillars. 
     In all cases, such pillars may be molded so as to project from a plate and/or be linked by arms and/or by any linking means. When use is made of low-density foam as thermal insulation liner layer, it is particularly advantageous to pour this foam in a single step over the entire surface area of the linking plate, between and possibly into the load-bearing pillars. Another possibility is to machine wells in a block of foam formed in advance and to insert the load-bearing pillars into the wells formed for that purpose. 
     In the case of a granular insulation, it is necessary to use a non-conducting element with peripheral walls that are preferably formed as a single piece with the load-bearing structure, as in  FIG. 11 . By virtue of the form of the load-bearing elements of small cross section, the inner space of the box between them is not compartmentalized, and therefore the granular material is easier to distribute over the entire surface area of the non-conducting element. The granular material may also be inserted into hollow pillars. 
     Load-bearing pillars of very small cross section, for example smaller than 40 mm, may be left empty without detriment to the thermal insulation. Hollow pillars of small cross section may also be filled with a flexible-PE foam cone or with glass wool. 
     In the load-bearing structures  500 ,  600 ,  700  and  800  described previously, some pillars may also be replaced by partitions creating compartments inside the load-bearing structure. 
     With reference to  FIGS. 22 and 23 , a description is now given of an embodiment of a non-conducting element that comprises a monobloc hollow caisson  1470  produced by rotational molding or by injection blow-molding. This caisson has the form of a closed hollow envelope  1477  that includes eight frustoconical pillars  1475  formed so as to project from the base wall  1471  of the envelope and each having a top wall  1483  capable of bearing against the top wall  1472  of the envelope in order to take up the compression forces. 
     To fix the caisson, six frustoconical shafts  1480  are provided, arranged at the periphery of the envelope and open through the top wall  1472 . These shafts each have a base wall capable of bearing against the base wall  1471  in order take up the compression forces and capable of being pierced in order to receive a fixing rod, shown diagrammatically at  1431 , which is, for example, a pin welded to the hull or a coupling device fixed to an underlying sealing barrier. 
     The inner space  1476  of the caisson and the inner space  1482  of the pillars  1475  may be filled with any suitable insulation, for example by injection of foam. Similarly, the shafts  1480  may be filled with insulation, for example PE foam or glass wool, after the caisson is fixed. 
     To mold the caisson  1470 , use may be made, for example, of high-density PE, polycarbonate, PBT or another plastic. The shafts  1480  may also be dispensed with if use is made of another method of attaching the caissons, for example coupling members passing between the caissons to be attached and bearing on the top wall  1472  in the manner of the retention members  48  of  FIGS. 2 and 3 . Base and/or cover panels may also be fixed to the walls of the envelope in order to reinforce it. 
     Although a description has been given of essentially parallelepipedal, right-angled non-conducting elements, other forms of cross section are possible, notably any polygonal form capable of rendering a planar surface discrete. 
     Of course, the insulation liner of a non-conducting element may include an number of layers of material. 
     When one of the primary and secondary insulating barriers is produced with the aid of the non-conducting elements described above, it is possible, but not necessary, to produce the other insulating barrier in an identical manner. Non-conducting elements of two different types may be used in the two barriers. One of the barriers may consist of prior-art non-conducting elements. 
     The caissons of the secondary insulating barrier and of the primary insulating barrier may be anchored to the ship&#39;s hull in a different way from the example shown in the figures, for example with the aid of retention members engaged on the base panel of the caissons. 
     Although the invention has been described in connection with a number of particular embodiments, it is obviously not limited to these in any way and includes all technical equivalents of the means described and also combinations thereof if they fall within the scope of the invention.