NO178554B - Thermally insulated tank and wall module element for use in building the tank - Google Patents

Description

The invention relates to a thermally insulated tank, particularly suitable for receiving low-temperature liquids, including a supporting outer structure, an inner shell formed by double-curved membrane elements at a distance from the supporting outer structure, a number of support structures between the elements and the supporting outer structure, and thermal insulation in the space between the inner shell and the supporting outer structure.

The invention also relates to a wall module element for use in the construction of a thermally insulated tank, particularly suitable for the absorption of low-temperature liquids. The invention has been particularly developed in connection with the desire and need for the transport of liquefied gases, such as LNG and LPG. Several hydrocarbons have boiling points and critical temperatures so low and critical pressures so high that it is not practical and/or theoretically possible to keep them liquid at ordinary temperatures by the application of pressure alone.

For many years, a lot of work has been done on problems in the area of storage and transport of hydrocarbons in a liquid state and under mainly atmospheric pressure. Under such conditions, the cold liquids are placed in thermally insulated tanks and allowed to evaporate or boil off as heat leaks in through the tank wall. The vapors that appear can either be taken directly to the atmosphere, used as gas fuel, or can be recondensed with the help of suitable freezing equipment and returned as liquid to the insulated tank. It is clear that the efficiency and economy of such storage and transport of low-temperature liquids will depend to a large extent on the thermal insulation used for the tank.

Another problem encountered in addition to evaporation loss is that the metal structural components in and around the tank tend to become brittle and lose strength under the influence of low temperatures. At temperatures in the order of magnitude of that of liquid methane under atmospheric pressure or liquid propane at atmospheric pressure, ordinary ferrous materials such as low carbon steel will lose a large degree of their energy-absorbing capacity under high voltage stresses, i.e. loss of impact resistance. Storage tanks in general and storage tanks on board ships in particular may be exposed to shock or impact stresses. When low-boiling materials such as methane or natural gas with a high methane content, or propane are liquefied for storage and transport, particular attention must therefore be paid to the risk of loss of impact resistance in steel walls and the like.

Physical failure of a wall in a tank that contains cold liquid hydrocarbons, for example liquid methane, could cause great danger to both life and property. To reduce the risk of such failure, it is known to store gases such as liquid methane or propane or other cold materials in steel tanks or containers provided with internal thermal insulation of a substantial thickness. By placing the insulation on the inside of the tank wall instead of on the outside, the tank wall material will be able to be kept at an essentially normal ambient temperature across the tank wall thickness, even if the tank is fully loaded with cold liquid. The solution enables the use of low-carbon steel, i.e. relatively cheap steel in a tank structure, so that one does not have to use expensive steel alloys or other materials that retain significant impact resistance at low temperatures.

It will usually be undesirable to leave the thermally insulating material directly exposed to the cold liquid being stored or transported. To avoid such direct exposure, it has therefore been proposed to place a lining or an inner tank shell, usually of metal, within the insulation. Such a lining can, for example, be made of aluminum or another metal such as stainless steel, which retains a significant impact resistance even under very low temperatures. However, such a lining can in itself present problems in a conventional tank construction. Although the liner may conform well to the insulation - usually insulation blocks - at room temperature, it will tend to move away from the insulation, at least in certain boundary layers when the tank structure is cooled during filling, as various thermal contractions may occur below. Such pulling away will create unwanted voids in the tank structure.

As mentioned, in recent years there has been great interest in the transport of liquefied gas on board suitable tankers. Safe and efficient storage and transport of liquefied natural gas (LNG), for example liquefied methane, presents several problems when it comes to transport on board ships. Such problems have many causes, in particular static and dynamic movements, bending and deformations in the ship's structure as a result of cargo and wind and wave forces, and as a result of stresses caused by extreme temperature variations in the tank during loading and unloading, which extreme variations already mentioned will be able to cause serious thermal expansions and contractions in the tank structure. Such stresses will be transferred between the ship's hull and the tank via the tank's support system.

Because of these serious problems, authorities and class companies have issued rules for the marine transport of liquefied gas. As a result, many different carrying systems have been proposed for tanks on board ships intended for the transport of this type of cargo. It has been proposed to use a double-walled tank, where the tank system is resiliently stored in the ship's hull in order to thereby minimize stress transfers from the hull structure to the tank and to be able to absorb small relative movements between the ship's hull and tanks. Such arrangements are very expensive as double tank systems are required and the support structure will be relatively complicated. The construction of the tank itself is also complicated by the fact that the outer tank must be built around the inner tank and that, as soon as the tank walls are completed, certain welds in the structure can no longer be inspected and tested.

A well-known tank type for the transport of liquefied gas on board a ship is the so-called integrated tank or membrane tank. Such a cargo tank is built from a light or thin material which is not intended to take a particular load in its own plane. The load pressure is transferred normally (perpendicular to the membrane) on the structure of the ship's hull. Such tank structures are double-walled (primary barrier and secondary barrier) cargo systems.

One purpose of the present invention is to provide a thermally insulated tank, particularly suitable for receiving low-temperature liquids, where the tank wall itself will be able to take lateral load from the liquid and transfer this load to a supporting structure, which then in turn transfers the load to an external strength-bearing structure.

It is also a purpose of the invention to provide a thermally insulated tank designed so that small contraction forces occur during temperature changes.

Another purpose of the invention is to be able to give the tank wall a shape that is adapted to the supporting structure so that mainly membrane stresses will arise in the tank wall, i.e. minimal bending stresses.

It is also a purpose of the invention to enable the arrangement of an air layer between the tank wall and the insulation and channels in the insulation with a view to detecting and locating leaks in the tank wall.

A particular purpose of the invention is to enable the use of cost-effective panels or tank elements for the construction of a thermally insulated tank.

It is also a purpose of the invention to make a thermally insulated tank so that it can be easily attached to the outer supporting structure of a ship so that floating is avoided if liquid enters the ship.

According to the invention, a thermally insulated tank is therefore proposed, particularly suitable for receiving low-temperature liquids, including a supporting outer structure, an inner shell formed by double-curved membrane elements at a distance from the supporting outer structure, a number of support structures between the elements and the supporting outer structure and thermal insulation in the space between the inner shell and the supporting outer structure, which tank according to the invention is characterized by the fact that the inner shell transfers mainly as tensile stresses all forces due to the contents of the tank to the points where it is supported by the supports arranged in said space -structures, which transfer all compressive forces to the load-bearing outer structure, as the support structures are adapted to the elements in the inner shell so that thermal contraction can occur freely due to changes in geometry, and that the thermal insulation that fills the other spaces between the inner shell and the load-bearing outer structure has no load-bearing capacity.

In such a tank, the tank wall that encloses the liquid will be able to take lateral load from the liquid and transfer this load to the load-bearing outer structure, as the forces are transferred through the membrane-limiting support structures. The tank will consist of a number of membranes where the membrane forces will be relatively small. The insulation will not transfer significant forces from the inner shell to the supporting outer structure.

Advantageously, the support structures can be made of a thermally insulating material.

Particularly advantageously, the thermally insulating material can include a number of blocks of thermally insulating material lying in the wall plane, each of which is assigned to one of the aforementioned support structures. Particularly advantageously, the aforementioned blocks can be arranged closely together.

The individual block of thermally insulating material can particularly advantageously be included as a component in a tank wall element which in the tank wall from the inside out includes a membrane and an associated support structure. This provides a particularly advantageous option for building up the tank using larger or smaller elements.

Several continuous barriers can be built into the tank wall and, in a preferred embodiment, the load-bearing support structure can be divided in the wall plane with the intermediate raft structure, which will then represent a secondary barrier.

According to the invention, the load-bearing support structure can enclose the insulation material in a tank wall element, or the load-bearing support structure can penetrate the insulation material in a tank wall element.

In the inner shell, the aforementioned respective membranes can have a mutual distance in the wall plane, since there are inserted membranes filling the wall surface between them.

Such an inserted, complementary membrane piece can advantageously have an edge bent in the direction outwards into the tank between two adjacent support structures, thereby providing channels.

Between the load-bearing outer structure and the support structures, fasteners are advantageously arranged, for retention and protection against floating.

A thermally insulated tank according to the invention is particularly well suited for use on board ships, where the load-bearing outer structure can then be part of the ship's hull.

As mentioned, the invention also relates to a wall module element for use in the construction of a thermally insulated tank, particularly suitable for receiving low-temperature liquids, and such a wall module element according to the invention is advantageously characterized in that it includes a hollow support structure with a first and second end and a closed circle,

a membrane carried by the support structure on said first end, and

a block of thermally insulating material, carried by the support structure, between said first and second ends.

In an advantageous embodiment, the support structure can enclose the block as a frame. In another advantageous embodiment, the support structure may advantageously extend through the block.

According to the invention, the support structure can be divided parallel to the membrane and between the membrane and the block, as there is a raft structure that is similar in shape to the block in coverage between the support structure parts. This fleet structure can be used as a secondary barrier.

Particularly advantageously, the block can have an approximately straight square shape, as this facilitates the assembly of the blocks in the tank wall.

Particularly advantageously, the frames can have bent or curved side edges in the wall plane. Regular or strictly right-angled square frames will, when put together, form a right-angled pattern. Upon cooling, the pattern structure will shrink, and there is a risk of cracking. If the sides of the frames are bent and the corners are attached, the sides will straighten when cooled.

At the aforementioned other end of the support structure, there can advantageously be arranged means for attaching the wall module element to a strength-bearing structure, for example part of a ship's hull.

The support structure is advantageously mainly filled with thermally insulating material between the membrane and the raft structure. The invention will now be explained in more detail with reference to the drawings, where: Fig. 1 shows a section of a ship's hull with a cut-through membrane tank according to the invention,

fig. 2 shows wall module elements assembled in the wall plane,

fig. 3 shows a section through those in fig. 2

assembled wall module elements,

fig. 4 shows a right-angled structural pattern, i

contrast to

fig. 5 which shows a structure pattern with bent

frame pages,

fig. 6 shows a section of a tank wall, with a

modified membrane wall design,

fig. 7 shows the special membrane wall in fig. 6 in

a build-up phase,

fig. 8-16 show various steps during the production of a wall module element as shown in fig. 17 and as

included in the wall in fig. 6 and 7,

fig. 17 shows a wall module element,

fig. 18 shows a complementary thermal insulation

element for use in the wall in fig. 6 and 7, fig. 19 shows a piece of membrane for use as a complementary element in the wall design in fig.

6 and 7,

fig. 20 shows schematically how channels can be provided in the tank wall and utilized for

detection of leaks,

fig. 21 shows a section of a ship's hull with a cut-through membrane tank according to the invention,

fig. 22 shows an enlarged section from the tank wall,

with an element according to the invention,

fig. 23 shows a partially cut-through element which

is used in the embodiment in fig. 21, and

fig. 24 shows a perspective view of a fastening device.

In fig. 1 shows a section of a membrane tank 1 in a ship's hull 2. The membrane tank is made up of a number of membranes 3 which are limited by and supported by respective support structures 4 against a supporting outer structure 5, which is part of the ship's hull 2. The membrane limiting support structures 4 are made of a thermally insulating material. Between the inner shell of the tank which is formed by the membranes 3, there is arranged thermally insulating material 6. A secondary barrier is indicated by the reference number 7.

The one in fig. 1 schematically shown tank design can for example be built up using wall element modules 8 of the type shown in fig. 2 and 3.

Each such module has an approximately right-angled square shape and includes, from the inside out, a corresponding square membrane 9, a support structure 10 with a square frame shape and a block 11 of a thermally insulating material.

The individual frame is advantageously made with bent or curved sides, as indicated in fig- 1. The purpose of this will be clear from a study of fig. 4 and 5. In fig. 4, ordinary square frames are put together and a structural pattern with straight lines in a right-angled pattern appears. When cooled, the structure will shrink, and there is a risk of cracking. By bending the frame sides and attaching the corners, as outlined in fig. 5, you will achieve that the sides align when cooling.

The average in fig. The tank wall shown in 3 is built up of two layers of wall module elements, with an intermediate raft structure 12, which forms a secondary barrier (the membrane 9 = primary barrier). One can also consider two superimposed frames 10 in fig. 3 as a support structure divided by means of the raft structure 12. The insulation in the support structure outside the secondary barrier 12 can optionally go right up to the raft structure or the secondary barrier. Lower requirements are usually placed on the secondary barrier than on the primary barrier, because the membrane forces will be completely different in the two raft structures. In a practical embodiment, for example, the raft structure/secondary barrier will be designed from a 1 mm thick GRP membrane, while the primary barrier or membrane 9 will be designed as a 10 mm thick GRP membrane (GRP = Glass Fiber-Reinforced Plastic). In the modular design shown and described, the tank is thus built up of modular elements, for example with a unit size of length x width x height = lmxlmx0.25m. The module elements are structurally joined by joining the membranes 9 and the raft structures 12 and the insulation is pressed together (and possibly glued) in the end faces, so that the insulation is also made continuous.

Fig. 6 shows a section of an inner tank wall or an inner shell formed by membranes in a different embodiment than that in fig. 2 and 3 showed. Fig. 7 shows such a tank wall under construction, before the complementary membrane pieces 15 are joined in between limited by support structures, in this case circular membranes 26. This type of tank wall is built up of modular elements 17 which are assembled in the wall plane. These wall module elements are built up in a special way, which will now be described in more detail with reference to fig. 8-17.

The individual wall module element 17, see fig. 17 is produced in the following way: A molding box 18 (fig. 8) is provided with a bottom in which a circular slot 19 is cut. The box can have a length and width of 1 m, and the slot can have a depth of 3 cm. A pipe element 20 is placed in the slot 19 (fig.

9), which will form a hollow support structure in the wall module element. In fig. 10, the casting box and the pipe support are shown cast, i.e. that thermal insulating foam material has been foamed in the box 18 (also inside the pipe support 20). Fastening elements 21 are mounted on the pipe support 20. The purpose of these will be explained in more detail below.

In the aforementioned practical design example with length x width = 1 m x 1 m for the box 18, the box can conveniently have a height of 15 cm, and the pipe support 20 can in the one in fig. 10 fixed-cast position project 7 cm above the box.

The element molded in this way is taken out of the mold (box) and turned over, as shown in fig. 11. The element is now in the form of a molded block 22 of a thermally insulating material, penetrated by the pipe support 20.

In fig. 12, the element is shown as in fig. 11. A raft structure 23 is placed on the element (fig. 13). This raft structure will form a later secondary barrier in the tank and is glued to the pipe support 20. Between the block 22 and the raft structure 23 there is a distance of 3 cm in the specified example of dimensions.

A further pipe element 24 with cast-in insulation 25 (fig. 14) is supplied with a membrane 26 (fig. 15), which is glued to the pipe element 24. This further pipe element 24 with associated membrane 26 is put on and glued firmly to the one in fig.

13 shown element, as shown in fig. 16.

The wall module element 17 provided in this way is shown on a larger scale in fig. 17. These elements are put together as shown in fig. 7, for the formation of larger or smaller panels, respectively for the formation/build-up of a thermally insulated tank with an inner shell as shown in fig. 6. Those in connection with fig. 8-16 mentioned membranes 26 correspond to the membranes 26 in fig. 6.

As mentioned, the wall module elements 17 are put together as shown in fig. 7, as the insulation blocks 22 are assembled and glued. The raft structures 23 are also glued together, so that a continuous raft structure is formed which will act as a secondary barrier in the tank. Between those in fig. 7 pipe connections 24 directed upwards and inwards in the tank, supplementary insulation blocks 27 are laid (see also fig. 18). These blocks are also glued in place.

Complementary membrane pieces 15 (fig. 6) are inserted between the membranes 26, which are joined to the membranes 26, thereby creating a continuous inner shell in the tank. Such a complementary membrane piece 15 is shown in a perspective view in fig. 19. As shown in fig. 19, such a membrane piece 15 has two folded edges 16 facing each other and running parallel to each other. The height of the edge is, in the dimension six, equal to the height of the part of the pipe support 24 that projects up from or out relative to the insulation blocks 27. These folded edges 16 will extend itself between adjacent pipe supports 24, as shown in fig. 20, where only the folded edges 16 of the membrane pieces 15 and not the membrane pieces themselves are shown. It can be seen that the folded edges 16 will form channels 16, as indicated by arrows and dashed lines, between the insulation and the inner membrane shell in the tank. Thereby, channels are formed where, for example, you can flush through nitrogen and thereby detect leakage in a manner known per se. If in the dimensioning example the pipe socket 24 has an axial dimension of 20 cm and the complementary insulation block 27 in fig. 18 has a thickness of 15 cm, the formed channels will have a height of 5 cm, which is then also the height dimension of the folded edge 16. The one in fig. 21, the membrane tank 31 shown in section in a ship's hull 32 is built up of elements 33 attached to the ship's inner skin 34. The elements are of the type shown in fig. 23.

The element 33 is built around a central cylindrical structure 37. This cylinder is made of an insulating material which at the same time has the necessary mechanical strength to transfer the loads from the membrane ends 39 and 40 to the inner skin 34. At the outermost end of the inner skin 34 is a layer of insulation 38 built in a suitable material, for example polyurethane foam. The insulating block 38 is attached to the cylinder 37 with glue or in some other way. On the liquid side of the insulation 38 is a liquid-tight secondary barrier 41 of a suitable material, for example glass fiber reinforced polyester. This barrier is continuous through the cylinder 37, which must therefore be produced in two halves which are joined by gluing/casting over the secondary barrier 41.

The secondary barriers in the different elements are molded together after the elements have been fixed to the inner skin.

For the purpose of providing an overlying seam, the secondary barrier in each individual element is larger than the outer dimensions of the element. On the liquid side of the secondary barrier is a new layer of insulation 42. It is composed of two blocks for each element. A 43 is inside the cylinder and attached to it. It has a height that is lower than the height of the cylinder, and allows the necessary curvature of the primary liquid barrier 40. The space 45 between the insulation 43 and the primary barrier 40 is connected to the corresponding space 46 outside the cylinder by the fact that there are small holes 44 in the cylinder.

The second insulation block 47 fills the space between the cylinders after the elements are assembled together. It is thus not part of the prefabricated element. The height of the block 47 is lower than that of the cylinder 37, so that it allows the necessary curvature of the primary barrier 39 and thus also a space 46 between the insulation and the primary barrier.

Via the holes 44, all rooms 45 and 46 communicate with each other. This can be used to monitor leaks through the primary barrier by flushing a gas inside the barrier. Detection of liquid from the tank, possibly an added tracer, in this gas means that the primary barrier is leaking.

The primary liquid barrier consists of two parts, both double-curved. A part 40 is attached to the cylinder 37 and forms part of the prefabricated element. The second part 39 has a geometry which means that together with part 40 it forms a continuous structure.

Installation and assembly of the elements takes place by attaching a prefabricated element, consisting of cylinder 37, four fittings 35, second insulation layer 38, secondary barrier 41, central part of first insulation layer 43 and primary barrier 40 to the inner skin (cf. fig. 22). The insulation layers 38 in the individual elements can be joined by gluing or the like. The secondary barriers 41 are molded together in the overlapping seam. The complementary block of first insulation layer 47 is installed between the elements, possibly attached to the neighboring blocks and the adjacent cylinders. Finally, the complementary part 39 which is prefabricated from the primary barrier is attached to the already installed part 40 by an overlapping molding.

In those parts of the tank where different parts of the inner skin meet at an angle, the elements are adapted by cutting the insulation at the ends 48 at an angle. Otherwise, the installation takes place in the same way. The primary barrier 39 between the cylinders will have to be adapted individually, e.g. by casting on site.

As mentioned, the tank 31 is made up of elements 33 attached to the ship's inner skin 34. The dimension of the element will be determined by practical assessments, e.g. that it must be able to be carried by one person. A typical measurement would be lmxlmx0.4m. The fastening mechanism consists of an element 50 attached to the inner skin 34. Each element is equipped with a fitting which is adapted to the fitting and fastening mechanism for the associated elements. The fitting is available as male (35a) and female 35b. Each element has two male and two female fittings. The female or male fittings have a length which means that they extend beyond the element's edge 36 and are accessible when the element is mounted. The fastening mechanism will lock the elements to each other and to the inner skin (cf. fig. 22 and 24).

It will appear that the liquid barrier is composed of double-curved surfaces, of which ball caps make up a significant proportion. Such a surface is well suited to dampen the wave energy that occurs in a tank in motion, e.g. a ship tank. Both the projecting parts of the barrier will slow down any fluid movement parallel to the wall, and also gas pockets trapped in the cavities will dampen pressure shocks that occur when a wave hits a wall perpendicularly.

Structurally, the tank consists of a wall that takes lateral forces without significant bending stresses occurring. This is achieved by the wall being concave double-curved, as close as possible to a ball cap where it is not stored. Such a geometry gives a clean stress pattern and is advantageous with regard to crack growth. The wall is supported on many points and it has a geometry that means that these points are not rigidly connected to each other via the wall. This means that small demands can be made for the supporting structure to be flat, and that negligible stresses occur in the wall if the supporting structure bends. Both of these conditions make the invention very suitable for installation on board a ship.

By the fact that the wall is stored at points in a supporting structure, it is achieved that the underlying insulation is not loaded with lateral forces from the contents of the tank. The geometry given to the tank wall, namely curvature, also means that contractions in the tank wall are taken as local radius changes. This will set up small local stresses, but global tensile stresses are avoided.

The invention enables a tank wall construction where the tank wall can take lateral loads. The inner tank shell is shaped so that mainly membrane stresses occur, and it is supported by a structure made of a material with the necessary strength and sufficient thermal insulation. Necessary insulation in the tank wall is attached to this supporting structure or support structure (the insulation does not take any load).

With the invention, it becomes possible to produce tanks with the desired properties with regard to geometry, shape, strength, insulation, secondary barriers, leak detection, etc., and a number of problems are avoided/reduced by the fact that the tank can be built without sharp corners, i.e. that the will be less prone to so-called sloshing. The support or bearing structure is not coherent, but in the form of a finely meshed network of support structures, and the tank wall is formed by curved surfaces, i.e. surfaces that are less exposed to contraction forces at low temperatures.

When using wall module elements that are used in the surfaces (bottom, wall and roof), you can appropriately create elements designed for corners and transitions between bottom-wall, -wall-ceiling etc. These corners and transitions are also made so that the tank wall absorbs the load on the same way as for the standard module elements. In this way, the thermally insulated tank can be built to fit an "arbitrary" external strength structure.

The fastening elements are used for fastening to the outer structure so that floating is avoided if liquid enters between the module elements and the outer structure.

Although two barriers are assumed in the design examples, the invention can be realized with one barrier.

The invention is not limited to the storage and transport of low-temperature liquids. For example, chemically reactive liquids will also be relevant.

The invention differs from existing construction solutions in several important respects.

Firstly, it is well suited for production using prefabricated elements. These can be manufactured almost completely in a suitable production facility. The installation work in the tank itself is thus reduced to a minimum. This results in low total costs. If special conditions require it, it can alternatively be built as a continuous structure at the installation site.

Secondly, the liquid-tight barrier(s) are designed so that they are well suited for construction in a laminated, non-metallic material. This results in simplified production and installation methods and generally reduced costs. If special conditions, e.g. the chemical properties of the liquid require it, however, the barrier(s) can be made of metallic materials.

When the invention is based on or built around a unit element, it is easy to adapt arbitrary geometries and overall dimensions, which is an advantage.

In contrast to conventional membrane-tank design, where the joint between the individual parts/paths of the liquid barrier consists of a weld joint, the joints in this invention (in its non-metallic design) can be performed as an overlapping casting. The barrier thus becomes completely continuous.

Claims (16)

1. Thermally insulated tank, particularly suitable for receiving low-temperature liquids, comprising a supporting outer structure (5), an inner shell formed by double-curved membrane elements (3;9;26,15) at a distance from the supporting outer structure, a number of support structures (4 ; 10;20,21;37) between the elements and the supporting outer structure, and thermal insulation (6;11;22,25, 27) in the space between the inner shell and the supporting outer structure, characterized in that the inner shell transfers mainly as tensile stresses all forces due to the contents of the tank to the points where it is supported by the support structures arranged in the aforementioned space (4;10;20 ,24;37 ), which transfer all compressive forces to the supporting outer structure (5) , as the support structures are adapted to the elements (3;9;26,15) in the inner shell so that thermal contraction can occur freely due to changes in geometry, and that the thermal insulation (6;11;22,25,27) which fills the other spaces between the inner shell and the load-bearing outer structure not h ar some load-bearing capacity. 2. Thermally insulated tank according to claim 1, characterized in that the support structures (4;10;20,24;37) are made of a thermally insulating material. 3. Thermally insulated tank according to claim 1 or 2, characterized in that the thermal insulation in the space between the inner shell and the outer structure includes a number of blocks (11;22;38) lying in the wall plane of thermally insulating material, each assigned to one of the mentioned support structures (10;20;37). 4. Thermally insulated tank according to claim 3, characterized in that the individual block (11;22;38) of thermally insulating material is included as a component in a tank wall element which in the tank wall from the inside out includes a membrane (9;26;40) and an assigned support structure (10;20; 37 ). 5. Thermally insulated tank according to one of the preceding claims, characterized in that the load-bearing support structure (10;20,24;37) is divided in the wall plane by an intermediate raft structure (12;23;41). 6. Thermally insulated tank according to claim 4 or 5, characterized in that the supporting support structure (10) encloses the insulation material (11) in a tank wall element. 7. Thermally insulated tank according to claim 4 or 5, characterized in that the supporting support structure (20; 37) penetrates the insulation material in a tank wall element. 8. Thermally insulated tank according to one of the preceding claims, characterized in that the respective membranes (26; 40) are spaced apart in the wall plane, with wall surface-filling membranes (15; 39) inserted between them. 9. Thermally insulated tank according to claim 8, characterized in that an inserted, wall-filling membrane (15) has an edge (16) bent in the direction outwards into the tank between two adjacent support structures (24). 10. Wall module element for use in the construction of a thermally insulated tank, particularly suitable for receiving low-temperature liquids as specified in claim 1, characterized in that it includes a hollow support structure (10;20,24;37) with a first and second end and a closed circuit, a membrane (9;26;40) carried by the support structure on the first end, and a block (11;22;38) of thermally insulating material, carried by the support structure, between said first and second ends. 11. Wall module element according to claim 10, characterized in that the support structure (10) encloses the block (11) as a frame, preferably as a frame with curved sides. 12. Wall module element according to claim 10, characterized in that the support structure (20;37) extends through the block (22;38). 13. Wall module element according to claim 12, characterized in that the support structure is divided parallel to the membrane (26), between the membrane (26;40) and the block (22;38), and in that between the support structure parts (20,24) there is a raft structure ( 23; 41). 14. Wall module element according to one of claims 10-13, characterized in that the block (22) has a right-angled square shape. 15. Wall module element according to one of the preceding claims 10-14, characterized by means (21; 50,35) arranged on the other end of the support structure (4,-10;20;37) for attaching the wall module unit to a strength-bearing structure (5;34 ). 16. Wall module unit according to one of the preceding claims 13-15, characterized in that the support structure (24; 37) is partially filled with thermally insulating material (25; 47) between the membrane (26; 40) and the raft structure (23; 41).