AU2013366322B2

AU2013366322B2 - Sealed, thermally insulating vessel

Info

Publication number: AU2013366322B2
Application number: AU2013366322A
Authority: AU
Inventors: Gery Canler; Guillaume Leclere; Nicolas THENARD
Original assignee: Gaztransport et Technigaz SA
Current assignee: Gaztransport et Technigaz SA
Priority date: 2012-12-21
Filing date: 2013-12-03
Publication date: 2017-12-14
Anticipated expiration: 2033-12-03
Also published as: CN104870882A; WO2014096600A1; CN104870882B; KR101994435B1; FR3000042A1; FR3000042B1; AU2013366322A1; KR20150096681A

Abstract

The invention relates to the secondary insulating barrier of a sealed, thermally insulating vessel which comprises a set of heat-insulating, generally parallelepiped-shaped elements (6) added to the bearing structure such as to form a substantially uniform supporting surface for the secondary sealing membrane, and retaining members attached to the bearing surface between the added heat-insulating elements and engaging with the heat-insulating elements such as to retain the heat-insulating elements against the bearing structure. An heat-insulating element of the secondary insulating barrier comprises a first parallelepiped sub-assembly (11, 13, 14) extending parallel to the secondary sealed membrane, and a second parallelepiped sub-assembly (16, 18) that has a shorter length such as to leave an upper surface (21) uncovered. A retaining member comprises an attachment element having peripheral portions engaging with the uncovered portion of the upper surface (21) of the intermediate panels of the heat-insulating elements (6) between which the retaining member is placed such as to retain the heat-insulating elements against the bearing surface.

Description

SEALED, THERMALLY INSULATING VESSEL

The invention relates to the field of sealed and thermally insulating tanks arranged in a bearing structure for containing a cold fluid, notably to membrane-lined tanks for containing liquefied gases.

Sealed and thermally insulating tanks may be used in various industries to store hot or cold products. For example, in the field of energy, liquefied natural gas (LNG) is a liquid with a high methane content which may be stored at atmospheric pressure at around -163°C in land-based storage tanks or in tanks carried on board floating structures.

Such a tank is disclosed for example in FR-A-2867831. In this known tank, a primary insulating barrier and a secondary insulating barrier are made up in modular juxtaposed parallelepipedal wooden boxes. The boxes are filled with a lagging packing of expanded perlite or aerogel materials. FR-A-2798902 discloses another LNG tank arranged in the hull of a ship in which tank a primary insulating barrier and a secondary insulating barrier are each made up of a single layer of boxes filled with foam blocks of a low density of the order of 33 to 40 kg/m3 bonded to spacer pieces made of plywood.

According to one embodiment, the invention provides a sealed and thermally insulating tank arranged inside a bearing structure for containing a cold fluid, in which one wall of the tank comprises in succession a primary sealed membrane intended to be in contact with the fluid, a primary insulating barrier, a secondary sealed membrane parallel to the primary sealed membrane, and a secondary insulating barrier arranged between the secondary sealed membrane and the bearing structure, in which the secondary insulating barrier comprises a set of lagging elements of parallelepipedal overall shape juxtaposed on the bearing structure to form a substantially uniform support surface for the secondary sealed membrane, and retaining members attached to the bearing structure between the juxtaposed lagging elements and collaborating with the lagging elements in order to hold the lagging elements against the bearing structure, in which a lagging element of the secondary insulating barrier comprises : a first parallelepipedal subassembly extending parallel to the secondary sealed membrane, the first parallelepipedal subassembly comprising a first high-density polymer foam block, a bottom panel bonded under the first high-density polymer foam block, an intermediate panel bonded to the first high-density polymer foam block, and a plurality of small cross section stiffening posts arranged between the bottom panel and the intermediate panel and extending in the direction of the thickness of the first high-density polymer foam block, and a second parallelepipedal subassembly extending parallel to the secondary sealed membrane, the second parallelepipedal subassembly comprising a second polymer foam block bonded to the intermediate panel and a cover panel bonded to the second polymer foam block, the second parallelepipedal subassembly having a length shorter than the length of the first parallelepipedal subassembly and being substantially centered on the first parallelepipedal subassembly so as to leave a top surface of the intermediate panel uncovered at the two opposite end portions of the first parallelepipedal subassembly, and in which a retaining member comprises: a rod oriented in the direction of the thickness of the secondary insulating barrier and having a lower end portion attached to the bearing structure, and a fastener connected to an upper end portion of the rod, the fastener having an underside surface and a top surface which are parallel to the secondary sealed membrane and separated by a distance equal to the thickness of the second parallelepipedal subassembly, the underside surface of the fastener having peripheral parts that collaborate with the uncovered portion of the top surface of the intermediate panels of the lagging elements between which the retaining element is arranged in order to hold the lagging elements against the bearing structure, the top surface of the fastener lying flush with the top surface of the cover panels of the lagging elements between which the retaining member is arranged in order to form, with said cover panels, the substantially uniform support surface for the secondary sealed membrane.

According to some embodiments, such a tank may include one or more of the following provisions.

The stiffening posts may be provided in greater or smaller number and arranged in various ways in the first parallelepipedal subassembly. According to one embodiment, the stiffening posts of the first parallelepipedal subassembly are arranged between the bottom panel and the intermediate panel at the two opposite end portions of the first parallelepipedal subassembly. Such an arrangement allows any compressive loads transmitted via the fastener to the uncovered portion of the intermediate panel to be reacted effectively.

According to one embodiment, the first parallelepipedal subassembly comprises four stiffening posts which are arranged at the four corners of the first foam block. Such an arrangement makes it possible to create in a simple way a frame that is relatively rigid so as to stabilize the foam block, notably against bending loads caused by differential thermal expansion. For preference, the ends of the stiffening posts are attached, for example stapled, nailed and/or bonded, to the bottom panel and to the intermediate panel.

According to one embodiment, the first parallelepipedal subassembly has a recess extending over the entire thickness of the first parallelepipedal subassembly at each of the four corners of the first parallelepipedal subassembly in order to form clearances in which the retaining members are arranged, the recess having a depth equal to the depth of the uncovered end portion of the intermediate panel so that a lateral wall of the first parallelepipedal subassembly at the bottom of the recess is aligned with a lateral wall of the second parallelepipedal subassembly.

According to one embodiment, the recess has a rectangular cross section and the stiffening post arranged at the corner of the first foam block has two perpendicular wings running along two sides of the recess. Such an arrangement notably makes it possible to protect the corner of the foam block against accidental damage during the fitting of the retaining members.

According to one embodiment, the fastener comprises a lower metal mounting plate forming the underside surface, a top metal mounting plate forming the top surface and a block of rigid insulating material arranged between the bottom and top metal mounting plates. Such an arrangement makes it possible to produce a fastener that is relatively wide so as to spread the load transmitted to the intermediate panel engaging with the underside surface. Such an arrangement also makes it possible to produce a fastener that is relatively robust so that it can react any compressive load if an adjacent secondary lagging element becomes damaged, while at the same time limiting the thermal bridging to the bearing structure.

The panels may be made of various materials, for example of a composite material resistant to bending and to shear. According to one embodiment, the bottom panel, the intermediate panel and the cover panel are made of plywood. Such a material is economical and offers various possibilities for procurement.

According to one embodiment, the high-density polymer foam has a density in excess of 90 kg/m3, for example between 120 and 140 kg/m3. In particular, the high-density polymer foam may be chosen from the group consisting of polyurethane foam and glass fiber reinforced polyurethane foam.

According to one embodiment, beads of mastic arranged on an underside surface of the bottom panel rest against the bearing structure so as to compensate for any deficiencies in the flatness of the bearing structure.

According to one embodiment, the primary insulating barrier is made up of juxtaposed lagging elements, a lagging element of the primary insulating barrier comprising in each instance a box filled with an insulating packing essentially made up of mineral wool or of perlite.

According to one embodiment, the or each sealed membrane comprises parallel strips of sheet metal, the longitudinal edges of which are turned up to project toward the inside of the tank, and parallel welding flanges held on the underlying thermally insulating barrier and projecting toward the inside of the tank in each instance between two sheet-metal strips so as to form a sealed welded joint with the adjacent turned-up longitudinal edges.

Such a tank may form part of a land-based storage facility, for example for storing LNG, or may be installed in an in-shore or off-shore floating structure, notably a methane tanker, a floating storage and regasification unit (FSRU), a floating production storage and offloading (FPSO) unit or the like.

According to one embodiment, a ship for transporting a cold liquid product comprises a double hull and an aforementioned tank arranged in the double hull.

According to one embodiment, the invention also provides a method for loading or offloading such a ship, in which method a cold liquid product is conveyed through insulated pipes from or to a floating or land-based storage facility to or from the tank of the ship.

According to one embodiment, the invention also provides a transfer system for transferring a cold liquid product, the system comprising the aforementioned ship, insulated pipes arranged in such a way as to connect the tank installed in the hull of the ship to a floating or land-based storage facility, and a pump for driving a flow of cold liquid product through the insulated pipes from or to the floating or land-based storage facility to or from the tank of the ship.

One idea underlying the invention is that of designing a tank wall structure, particularly a secondary insulating barrier structure, that offers advantageous properties in terms of thermal insulation, mechanical strength and cost.

Certain aspects of the invention are derived from the observation that when a sealed and thermally insulating tank is filled with liquefied natural gas, the difference in temperature between the outside of the tank and the inside of the tank generates a thermal gradient within the lagging elements. This thermal gradient may cause phenomena of differential expansion within the bonded assemblies of polymer foam with other rigid materials, for example plywood, which phenomena are likely to give rise to stresses that tend to cause the lagging elements to bend. This bending may notably occur when the means of attaching the lagging element in the tank are unable fully to react these stresses, for example when the lagging element is not bonded to the hull over the full surface area thereof but is simply attached at a plurality of attachment points.

Certain aspects of the invention start out from the idea of creating relatively thick lagging elements for a secondary insulating barrier while placing an intermediate panel within the thickness of the lagging element in order to limit bending stresses of thermal origin.

The invention will be better understood and further objects, details, features and advantages thereof will become more clearly apparent during the course of the following description of several particular embodiments of the invention which are given solely by way of nonlimiting illustration with reference to the attached drawings.

In these drawings: • Figure 1 is a perspective partial view with cutaway of a wall of a sealed and insulating tank. • Figure 2 is a perspective view of a secondary insulating element of the wall of figure 1. • Figure 3 is a planar view of the secondary lagging element of figure 2, in the direction of arrow III. • Figure 4 is an enlarged view in cross section on IV of figure 3 of a corner post of the secondary lagging element. • Figure 5 is a planar view of a coupler that can be used in the tank wall of figure 1. • Figure 6 is a schematic depiction with cutaway of a tank of a methane tanker and of a terminal for loading/offloading this tank. • Figures 7 and 8 are two diagrammatic representations of a phenomenon of differential contraction generating a bending moment.

The phenomenon of differential contraction will be recalled briefly using two simple examples schematically illustrated in figures 7 and 8. A plywood panel 37 is bonded to a thicker one-piece layer of polymer foam 36. The plywood panel 37 and the layer of polymer foam 36 are subjected to a downward thermal gradient 38. That means that the temperature at the plywood panel 37 is lower than the temperature at the underside surface 41 of the layer of polymer foam 36. The polymer foam has a higher thermal expansion coefficient than the plywood. Thus, the polymer foam contracts more with respect to ambient temperature than does the plywood panel 37 under the effect of the thermal gradient 38. Because the plywood panel 37 and the layer of polymer foam 36 are bonded together and because in addition the layer of polymer foam 36 has greater flexural rigidity than the plywood panel 37, the panel 37 and the layer of polymer foam 36 tend to flex into the convex curvature 39.

The same phenomenon may be observed in a second example illustrated schematically in figure 8, in which the plywood panel 37 is bonded underneath the layer of polymer foam 36. Flowever, in this case, the layer of polymer foam 36 and the panel 37 tend to flex into a convex curvature 40 that is the opposite of the convex curvature 39 described in the first example. Moreover, because the plywood panel 37 is situated underneath the layer of polymer foam 36, this panel 37 is subjected to a higher temperature than in the first example for the same thermal gradient 38. As a result, it contracts less than it does in the first example, leading to a convex curvature 40 that is more pronounced than the convex curvature 39 of the first example. This is because the difference in thermal contraction between the layer of polymer foam 36 and the panel 37 is greater than in the first example.

Figure 1 depicts a sealed and thermally insulating wall in perspective with cutaway so as to show the structure of this wall. Such a structure may be employed over extensive surfaces of various orientations, for example to cover bottom, top and side walls of a storage vessel. The orientation of figure 1 is therefore nonlimiting in this respect.

The tank wall is attached to the wall of a bearing structure 1. By convention, "on top of" will mean a position situated further toward the inside of the storage vessel and "underneath" will refer to a position situated closer to the bearing structure 1, whatever the orientation of the tank wall with respect to the earth's field of gravity.

The tank wall comprises a secondary insulating barrier 2, a secondary sealed barrier 3 held on top of the secondary insulating barrier 2, a primary insulating barrier 4 held on the secondary sealed barrier 3 and a primary sealed barrier 5 held on top of the primary insulating barrier 4.

The secondary insulating barrier 2 is made up of a plurality of secondary parallelepipedal insulating modules 6 which are arranged side by side so as to cover substantially the internal surface of the bearing structure 1. In order to keep the sealed membranes flat, beads of mastic 7 are installed between the bearing structure 1 and the underside surface of the secondary insulating modules 6. These beads of mastic are, for example, bonded to the underside surface of the secondary insulating module 6. They do not adhere to the bearing structure 1 because kraft paper, not depicted, is fitted between the bearing structure 1 and the mastic. According to one embodiment, the beads of mastic may be wavy beads as described in FR-A1-2931535. Blocks 28 are also provided on the bearing wall to support the corners of the secondary insulating modules 6. A secondary insulating module 6 is depicted in greater detail in figures 2 and 3. It made up of two parts: a parallelepipedal subassembly 10 in the lower part near the bearing structure 1 and a parallelepipedal subassembly 20 of slightly shorter length in the upper part.

The subassembly 10 comprises a high-density polymer foam block 11, notably made of polyurethane with or without glass fiber, sandwiched between two flat plywood panels 13 and 14. The foam block 11 has a rectangular parallelepipedal overall shape with cutouts at the corners for the passage of the corner posts 12.

The bottom panel 13 and the intermediate panel 14 have an identical shape, namely a rectangle shape with a rectangular recess 15 in each corner to create clearances in the tank wall in the assembled state, at the interface between several juxtaposed secondary insulating modules 6. These clearances are there to house mechanical couplers 30 visible in figure 1.

Thus the cutout of the subassembly 10 is optimized so as to limit as far as possible the thermal chimneys present between the blocks of foam. For preference, the only clearances present are the assembly clearances and the passages for the mechanical couplers 30 in the corners.

The foam block 11 is bonded over its entire surface area to the bottom panel 13 and to the intermediate panel 14. At each corner, the foam block 11 is bonded over its entire height to the corner post 12. The corner post 12 is also attached to the bottom panel 13 and to the intermediate panel 14 by staples or the like.

Figure 4 shows the cross section of the corner post 12 in one embodiment. The internal surfaces 17 are bonded to the lateral wall of the foam block 11 whereas the external surfaces 15 are in the continuity of the rectangular recess 15 of the bottom panel 13 and of the intermediate panel 14. The corner posts 12 allow some of the compressive load to be reacted during use thus limiting the compression and creep of the foam 11.

The parallelepipedal subassembly 20 situated in the upper part of the secondary insulating module 6 comprises a second high-density polymer foam block 16, notably made of polyurethane foam, with or without glass fiber, sandwiched between the intermediate panel 14 and a cover panel 18 made of plywood. The foam block 16 is bonded over its entire surface area to the cover panel 18 and to the intermediate panel 14 and has no post, making it simpler to manufacture and to assemble. The foam block 16 is substantially less thick than the foam block 11, for example being approximately 1/3 of the thickness of the foam block 11. Because the parallelepipedal subassembly 20 is not as long as the subassembly 10, the two longitudinal ends of the intermediate panel 14 are uncovered and offer a clear top surface 21 in the form of a strip.

To make the tank wall easier to construct, the secondary insulating module 6 is preferably supplied in the form of a prefabricated element. In one embodiment, the following dimensions are provided:

Length of subassembly 10:118 cm

Length of subassembly 20:114 cm

Width of secondary insulating module 6: 100 cm

Thickness of foam block 11: 20 cm

Thickness of foam block 16: 6.5 cm

Thickness of cover panel 18: 1.5 cm

Thickness of bottom panel 13: 1 cm

Thickness of intermediate panel 14: 1 cm

Thickness of secondary insulating module 6: 30 cm

These insulating-barrier thicknesses are advantageous in that they conform to the dimensions of earlier designs and are therefore compatible with the components available in the marketplace, such as the anchoring systems, the sealing membranes and the various specialist zones that the dihedral and trihedral corners of the tanks represent.

Returning to figure 1, it may be seen that the mechanical couplers 30 are positioned at the corners of the secondary insulating modules 6, there being four mechanical couplers 30 per module 6. The clear surface 21 of the intermediate panel 14 allows the mechanical couplers 30 to be fitted so as to hold the secondary insulating modules 6 on the bearing wall 1. A mechanical coupler 30 is depicted in greater detail in figure 5. The mechanical coupler 30 comprises a socket 22, the base of which is welded to the bearing structure 1 at a position that corresponds to a clearance at the corners of four adjacent secondary insulating modules 6. The socket 22 carries a first rod 23 screwed into it. The rod 23 passes between the adjacent modules 6. A fastener piece is mounted on the rod 23 to clamp the modules 6 against the bearing structure 1 at the clear surfaces 21 of the intermediate panel 14. The fastener piece comprises a lower metal mounting plate 24, a top mounting plate 26 and a block of plywood 25 mounted on the mounting plate 24 to act as a spacer piece between the mounting plate 24 and the top mounting plate 26 and reduce the thermal bridging to the bearing structure 1. The height of this arrangement is determined so that the top mounting plate 26 lies flush with the cover panels 18 for supporting the secondary membrane 3. In other words, the thickness 29 of the fastener piece formed by the block 25 and the mounting plates 24 and 26 is equal to the thickness of the subassembly 20. In addition, the thickness of the subassembly 10 corresponds to the distance 50 between the block 28 the lower mounting plate 24.

The block of wood 25 has a housing 47 in which the top end of the rod 23 is engaged through a central drilling in the bottom mounting plate 24. The bottom mounting plate 24 is held on the rod 23 by a nut 48 with the interposition of a plurality of Belleville washers 49 in order to afford elastic clearance.

At the corners of the subassembly 10, the compressive load applied by the mechanical coupler 30 to the insulating module 6 is reacted by the corner posts 12.

The cover panels 18 of the insulating modules 6 further comprise a pair of parallel slots 19, more or less in the shape of an inverted T, to accept welding flanges in the shape of angle brackets. The part of the welding flanges that projects toward the top of the panels 18 allows the secondary sealing barrier 3 to be anchored. The secondary sealing barrier is made of a plurality of Invar® strakes with turned up edges, of the order of 0.7 mm thick. The turned-up edges of each strake are welded to the above-mentioned welding flanges using the known technique.

Mounted on the secondary sealing barrier is the primary insulating barrier 4 which is made up of a plurality of primary insulating boxes 33. Each primary insulating box 33 consists of a rectangular parallelepipedal box made of plywood which is filled with nonstructural insulating material such as perlite or glass wool. The primary insulating boxes 33 also have internal partitions, a bottom panel and a top panel 45. The top panel 45 also comprises two slots 46 in the overall shape of an inverted T, so that they too can accept a welding flange to which the turned-up edges of the strakes of the primary sealing barrier are welded. The separation between two slots 19 or 46 corresponds to the width of a strake. The separation between the slots and the adjacent edge of the same box corresponds to half the width of a strake, so that a strake straddles two adjacent boxes.

Should the primary membrane 5 rupture, the primary insulating barrier 4 finds itself flooded with LNG such that the secondary membrane 3 finds itself at the very cold temperature of the liquid. The secondary insulating module 6 is then subjected to a very steep thermal gradient. The intermediate plate 14 is arranged in the region of thickness at which this gradient is very steep and therefore performs the function of reducing the bending forces generated by the differential contraction of the materials, namely of the foam and of the plywood. In addition, the hydrostatic pressure load borne by the secondary membrane 3 is transmitted by the coupler 30 to the surface 21 of the intermediate panel 14 and can be reacted directly by the posts 12. Thus, the risk of the foam block 11 being caused to creep is reduced.

The techniques described hereinabove for creating a sealed and insulated wall can be used in various types of storage vessel, for example to form the wall of an LNG storage vessel in a land-based facility or in a floating structure such as a methane tanker or the like.

With reference to figure 6, a view with cutaway of a methane tanker 70 shows a sealed and insulating tank 71 of prismatic overall shape mounted in the double hull 72 of the ship. The wall of the tank 71 comprises a primary sealed barrier intended to be in contact with the LNG contained in the tank, a secondary sealed barrier arranged between the primary sealed barrier and the double hull 72 of the ship, and two insulating barriers arranged respectively between the primary sealed barrier and the secondary sealed barrier and between the secondary sealed barrier and the double hull 72.

In a way known per se, loading/offloading pipes 73 arranged on the upper deck of the ship can be coupled, using suitable connectors, to a maritime or harbor terminal to transfer a cargo of LNG from or to the tank 71.

Figure 6 depicts an example of a maritime terminal comprising a loading and offloading station 75, an underwater pipe 76 and a land-based facility 77. The loading and offloading station 75 is a fixed off-shore facility comprising a mobile arm 74 and a tower 78 supporting the mobile arm 74. The mobile arm 74 carries a bundle of insulated flexible hoses 79 that can be connected to the loading/offloading pipes 73. The orientable mobile arm 74 adapts to suit all sizes of methane tanker. A connecting pipe, not depicted, extends down inside the tower 78. The loading and offloading station 75 allows the methane tanker 70 to be loaded and offloaded from or to the land-based facility 77. The latter comprises liquefied gas storage tanks 80 and connecting pipes 81 connected by the underwater pipe 76 to the loading or offloading station 75. The underwater pipe 76 allows the liquefied gas to be transferred between the loading or offloading station 75 and the land-based facility 77 over a large distance, for example 5 km, allowing the methane tanker 70 to be kept a long way offshore during the loading and offloading operations.

In order to generate the pressure needed for transferring the liquefied gas, use may be made of pumps carried on board the ship 70 and/or pumps with which the land-based facility 77 is equipped and/or pumps with which the loading and offloading station 75 is equipped.

Although the invention has been described in conjunction with a number of particular embodiments, it is quite obvious that it is not in any way restricted thereto and that it comprises all technical equivalents of the means described and combinations thereof where these fall within the scope of the invention.

The use of the verbs "comprise", "include" or "have" and conjugated forms thereof does not exclude the presence of elements or steps other than those listed in a claim. The use of the indefinite article "a", "an" or "one" for an element or step does not, unless mentioned otherwise, exclude there being a plurality of such elements or steps.

Any reference sign between parentheses in the claims must not be interpreted as limiting the claim.

Claims

The claims defining the invention are as follows:

1. A sealed and thermally insulating tank arranged inside a bearing structure for containing a cold fluid, in which one wall of the tank comprises in succession a primary sealed membrane intended to be in contact with the fluid, a primary insulating barrier, a secondary sealed membrane parallel to the primary sealed membrane, and a secondary insulating barrier arranged between the secondary sealed membrane and the bearing structure, in which the secondary insulating barrier comprises a set of lagging elements of parallelepipedal overall shape juxtaposed on the bearing structure to form a substantially uniform support surface for the secondary sealed membrane, and retaining members attached to the bearing structure between the juxtaposed lagging elements and collaborating with the lagging elements in order to hold the lagging elements against the bearing structure, in which a lagging element of the secondary insulating barrier comprises : a first parallelepipedal subassembly extending parallel to the secondary sealed membrane, the first parallelepipedal subassembly comprising a first high-density polymer foam block, a bottom panel bonded under the first high-density polymer foam block, an intermediate panel bonded to the first high-density polymer foam block, and a plurality of small cross section stiffening posts arranged between the bottom panel and the intermediate panel and extending in the direction of the thickness of the first high-density polymer foam block, and a second parallelepipedal subassembly extending parallel to the secondary sealed membrane, the second parallelepipedal subassembly comprising a second polymer foam block bonded to the intermediate panel and a cover panel bonded to the second polymer foam block, the second parallelepipedal subassembly having a length shorter than the length of the first parallelepipedal subassembly and being substantially centered on the first parallelepipedal subassembly so as to leave a top surface of the intermediate panel uncovered at the two opposite end portions of the first parallelepipedal subassembly, and in which a retaining member comprises: a rod oriented in the direction of the thickness of the secondary insulating barrier and having a lower end portion attached to the bearing structure, and a fastener connected to an upper end portion of the rod, the fastener having an underside surface and a top surface which are parallel to the secondary sealed membrane and separated by a distance equal to the thickness of the second parallelepipedal subassembly, the underside surface of the fastener having peripheral parts that collaborate with the uncovered portion of the top surface of the intermediate panels of the lagging elements between which the retaining element is arranged in order to hold the lagging elements against the bearing structure, the top surface of the fastener lying flush with the top surface of the cover panels of the lagging elements between which the retaining member is arranged in order to form, with said cover panels, the substantially uniform support surface for the secondary sealed membrane.
2. The tank as claimed in claim 1, in which the stiffening posts of the first parallelepipedal subassembly are arranged between the bottom panel and the intermediate panel at the two opposite end portions of the first parallelepipedal subassembly.
3. The tank as claimed in claim 2, in which the first parallelepipedal subassembly comprises four stiffening posts which are arranged at the four corners of the first foam block.
4. The tank as claimed in claim 3, in which the first parallelepipedal subassembly has a recess extending over the entire thickness of the first parallelepipedal subassembly at each of the four corners of the first parallelepipedal subassembly in order to form clearances in which the retaining members are arranged, the recess having a width equal to the width of the uncovered end portion of the intermediate panel so that a lateral wall of the first parallelepipedal subassembly at the bottom of the recess is aligned with a lateral wall of the second parallelepipedal subassembly.
5. The tank as claimed in claim 4, in which the recess has a rectangular cross section and the stiffening post arranged at the corner of the first foam block has two perpendicular wings running along two sides of the recess.
6. The tank as claimed in one of claims 1 to 5, in which the fastener comprises a lower metal mounting plate forming the underside surface, a top metal mounting plate forming the top surface and a block of rigid insulating material arranged between the bottom and top metal mounting plates.
7. The tank as claimed in one of claims 1 to 6, in which the bottom panel, the intermediate panel and the cover panel are made of plywood.
8. The tank as claimed in one of claims 1 to 7, in which the high-density polymer foam has a density in excess of 90 kg/m3.
9. The tank as claimed in one of claims 1 to 8, in which the high-density polymer foam is chosen from the group consisting of polyurethane foam and glass fiber reinforced polyurethane foam.
10. The tank as claimed in one of claims 1 to 9, in which beads of mastic arranged on an underside surface of the bottom panel rest against the bearing structure so as to compensate for any deficiencies in the flatness of the bearing structure.
11. The tank as claimed in one of claims 1 to 10, in which the primary insulating barrier is made up of juxtaposed lagging elements, a lagging element of the primary insulating barrier comprising in each instance a box filled with an insulating packing essentially made up of mineral wool or of perlite.
12. The tank as claimed in one of claims 1 to 11, in which each sealed membrane comprises parallel strips of sheet metal, the longitudinal edges of which are turned up to project toward the inside of the tank, and parallel welding flanges held on the underlying thermally insulating barrier and projecting toward the inside of the tank in each instance between two sheet-metal strips so as to form a sealed welded joint with the adjacent turned-up longitudinal edges.
13. A ship for transporting a cold liquid product, the ship having a double hull and a tank as claimed in one of claims 1 to 12 placed inside the double hull.
14. A method of using a ship as claimed in claim 13, in which method a cold liquid product is conveyed through insulated pipes from or to a floating or land-based storage facility to or from the tank of the ship in order to load or offload the ship.
15. A transfer system for transferring a cold liquid product, the system comprising a ship as claimed in claim 13, insulated pipes arranged in such a way as to connect the tank installed in the hull of the ship to a floating or land-based storage facility, and a pump for driving a flow of cold liquid product through the insulated pipes from or to the floating or land-based storage facility to or from the tank of the ship.