WO2025056853A1 - Storage system and method for storing fluids comprising access means for performing maintenance operations - Google Patents

Abstract

This is an underground storage system (1) for storing fluids. It comprises a cavity (2) made in the ground (3), said cavity (2) having a bottom (6), a support element (4) comprising at least one opening, at least one tank (5), said tank (5) having a longitudinal axis, a lower end (13) closed by a first closing means (12), and an upper end (14) closed by a second closing means (15), and means of access from the outside of the cavity (2) comprising at least one of the following: means for accessing each tank (5); means for accessing the inside of the tanks (5); and means for accessing the content of the cavity (2).

Description

DESCRIPTION

TITLE: STORAGE SYSTEM AND METHOD FOR STORING FLUIDS INCLUDING ACCESS MEANS FOR CARRYING OUT MAINTENANCE OPERATIONS

Technical field

The present invention relates to underground storage and in particular the storage of fluids, for example hydrogen or oxygen. More particularly, the invention relates to the field of maintenance of a fluid storage system and, in particular, of a high-pressure fluid storage system, i.e. for pressures which may be between 100 bar and 1200 bar, more particularly between 200 bar and 500 bar.

The invention also relates to a method for underground storage of fluids.

A fluid storage system must be checked regularly, especially when the fluid presents significant risks, including explosion or fire.

In an underground fluid storage facility that presents risks of explosion or fire, different levels of maintenance must be expected. This involves maintaining the upper level of the storage system, which may be located outside, the storage system's tanks, and also the recess in which the tanks are located.

Maintenance operations therefore require different access points that are not easy to arrange when the storage system to be inspected is compact and/or installed at depths of up to several tens or hundreds of meters. In addition, certain maintenance operations, such as the replacement of certain tank components with a limited lifespan, are complex to implement. Statement of the invention

The invention aims to propose a solution ranging from inspection to replacement of the components of a fluid storage system, by optimizing the ease of access to the tanks and/or the recess. The invention also aims to optimize the replacement rate of certain components of the tank linked to wear. More particularly, the invention also aims to optimize the service life of the sensitive components of the storage tank and consequently to limit the handling necessary for the replacement of said sensitive components.

The invention relates to an underground storage system for storing fluids. The system comprises a recess arranged in a ground, said recess having a bottom, a support element comprising at least one opening, at least one reservoir, said reservoir having a longitudinal axis, a lower end closed by a first closing means, and an upper end closed by a second closing means. The upper end comprises a support piece, said upper end being assembled to the support element by means of the support piece so that the reservoir is suspended inside the recess. The storage system also comprises means of access from outside the recess comprising at least one of means of access to each reservoir, means of access to the interior of the reservoirs, and means of access to the contents of the recess.

Advantageously, the means of access to each tank include a passage provided on the periphery of the support element.

According to one embodiment, this passage surrounds the support element. In other words, this passage is then formed around the support element. According to one embodiment, this passage is provided on an upper surface of the support element, for example on a collar forming the external contour of the support element. Providing the passage on such a collar ensures the presence of a lower collar surface sufficient to allow the support element to rest reliably on the ground surface while providing an upper collar surface free of any obstacle since there is no opening for the tanks.

In one embodiment, this passage may be jointly formed around and on the upper surface of the support member.

Advantageously, the means of access to each tank comprise a corridor located on the support element. Such a corridor advantageously allows access from the support element to one or more tanks not accessible from the passage provided on the periphery of the support element.

Such a passage or corridor are surfaces free of obstacles and allowing an operator to move around to have direct access to the tanks located along said free surfaces. Typically, thanks to such a passage or corridor, the operator can directly access a tank in order to directly manipulate the first closing means.

The means of access to the interior of the tanks comprise a sacrificial emergency part inserted between the first closing means and the support part.

Advantageously, the sacrificial backup part is removable. Thus, this sacrificial backup part can be removed from the tank to access the interior of the tank. In addition, this sacrificial backup part constitutes an easily replaceable element, for example after three screwing/unscrewing with the closing means during maintenance operations, allowing the support part to have a longer life expectancy.

According to one embodiment, the means of access to the interior of the tanks comprise a plurality of sacrificial emergency parts interposed between the first closing means and the part removably supported, each sacrificial spare part increasing the durability of the support part

According to one embodiment, the means for accessing the contents of the recess comprise at least one fluid and/or solid suction pipe and at least one fluid and/or solid injection pipe. The injection pipe makes it possible to inject a fluid and/or a solid into the recess, typically so as to surround the reservoir(s) inserted into the recess with said fluid and/or solid. Similarly, the pumping pipe makes it possible to remove said fluid and/or solid from the recess so as to allow the pumping and circulation of the fluid and/or solid present at the bottom of the recess.

According to one embodiment, the means of access to the contents of the recess comprise a room outside the recess, the injection and/or suction pipes opening out and/or being controlled from this room outside the recess.

Advantageously, the tank comprises at least one metal tube, said metal tube having at least one termination provided with at least one threaded portion.

The tank consists of at least two metal tubes assembled by screwing, so as to form a column of tubes.

Preferably, the support piece comprises a lower threaded portion. The metal tube or column and the support piece are assembled by screwing a threaded portion of said or one of said metal tubes forming the column and the lower threaded portion of the support piece.

Preferably, the reservoir is suspended inside the recess so that an axial clearance capable of absorbing axial thermal expansion of the reservoir remains between the first means for closing the reservoir and the bottom of the recess.

Advantageously, the axial clearance capable of absorbing axial thermal expansion of the tank complies with the following inequality:

Où : G est la longueur du jeu axial exprimée en mètres, L représente la longueur d’un réservoir exprimée en mètres entre l’ élément de support et l’ extrémité inférieure du réservoir, P représente le gradient géothermique du terrain exprimé en degrés Celsius par mètre, a représente le coefficient de dilatation thermique du métal dans lequel est réalisé le réservoir exprimé en mètres par degré Celsius. [Math 1]

Where: G is the length of the axial clearance expressed in meters, L represents the length of a tank expressed in meters between the support element and the lower end of the tank, P represents the geothermal gradient of the ground expressed in degrees Celsius per meter, a represents the coefficient of thermal expansion of the metal in which the tank is made expressed in meters per degree Celsius.

Preferably, the support piece has an upper threaded portion and the sacrificial backup piece is tightly mounted on the support piece by screwing onto said upper threaded portion.

Optionally, the first closure means and/or the second closure means is capable of sealing the tank by screwing. For example, the first closure means is screwed onto an upper threaded portion of the sacrificial spare part.

According to one embodiment, the storage system comprises a plurality of tanks, each tank having a longitudinal axis, a lower end and an upper end, said upper end of each tank being adapted to be assembled to the support element by means of a respective support part so that each tank is suspended inside the recess.

Advantageously, the recess comprises at least one casing, said casing being made of concrete, cement, or steel.

The invention also relates to an underground storage method for storing fluids via a storage system as described above. The method comprises at least the following steps: providing a recess in a ground, said recess having a bottom, providing a support element comprising at least one opening capable of receiving a support part, providing at least one tank, said tank having a longitudinal axis, a lower end and a lower end upper end, said upper end comprising the support piece, providing a first closing means adapted to close said reservoir at its lower end, and a second closing means adapted to close the reservoir at its upper end, assembling said upper end to the support element by means of the support piece so that the reservoir is suspended inside the recess, and providing means of access from outside the recess comprising at least one of means of access to each reservoir, means of access to the interior of the reservoirs, and means of access to the contents of the recess.

Definitions

The term "lower end" of the tank means the end of the tank which is located near the bottom of the recess. This "lower end" is defined in contrast to the so-called "upper end" of the tank, which is located near the support element and therefore the ground surface.

"Axial clearance" means a length, which extends along the longitudinal axis of the tank, measured between the first closure means of the tank, which is proximal to the bottom of the recess, and the bottom of the recess. Note that the position of a tank may not be perfectly vertical. In this case, the longitudinal axis of the tank has an angle with respect to the vertical in the (x; y) reference frame. This angle has a maximum value of 15°. In this case, the measurement of the axial clearance is done by orthogonal projection on the vertical axis passing through a point of the first closure means located closest to the bottom of the recess. In other words, the axial clearance always corresponds to the shortest distance, measured between the bottom of the recess and the first closure means. The term "threaded metal tube" means a tube comprising at least one termination having at least one threaded portion, capable of being assembled to another element, for example another threaded metal tube, a closing means or even a support part, comprising at least one termination having at least one complementary threaded portion. The thread may be male or female.

The term "bottom of the recess" means the surface of the bottom of the recess. Thus, when the recess is lined and the said lining is cemented, the term "bottom of the recess" then designates the surface of the cement layer at the bottom of the recess. When the lining is not cemented, the term "bottom of the recess" simply designates the surface of the ground in which the recess is formed, located at the bottom of the recess.

Brief description of the drawings

Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely as a non-limiting example and made with reference to the appended drawings in which:

- [Fig. 1] is a schematic view of the general structure of an underground storage system;

- [Fig. 2] is a schematic view of the support element of an underground storage system;

- [Fig. 3] is a schematic view of the support element of an underground storage system comprising a corridor according to one embodiment of the invention; and

- [Fig. 4] is a schematic view of the upper end of a reservoir contained in an underground storage system according to one embodiment of the invention.

Detailed description Figure 1 illustrates a sectional view of a storage system 1 , in a (x; y) frame. The x axis of the (x; y) frame is a horizontal axis, and the y axis of the (x; y) frame is a vertical axis corresponding to the Earth's gravity axis.

The storage system 1 comprises a recess 2 made in a ground 3, a support element 4 placed on a surface of a ground S of the ground 3, means of access from outside the recess (2), and six tanks 5 suspended from the support element 4 in the recess 2 (only four tanks 5 are visible in FIG. 1). According to the embodiments, the storage system 1 comprises at least one tank 5.

The recess 2 has a bottom 6 and comprises a casing 7. The recess 2 can be obtained by drilling or by excavation. The recess 2 is of substantially circular cylindrical shape and has, for example, an average diameter of four meters. The casing 7 is made of cement and extends vertically from the surface of the ground S to the bottom 6 of the recess 2.

As shown in Figures 1 and 2, the support element 4 is a circular cylindrical plate having a central body 19 and a flange 20, said flange 20 having a lower surface resting on the ground S. The central body 19 has a first thickness and the flange has a second thickness, the first thickness being greater than the second thickness. The support element 4 further comprises an upper surface, said upper surface being opposite the lower surface of the flange 20.

As illustrated in Figure 2, the support element 4 also comprises six openings 8. The openings 8 are holes passing through the first thickness and opening onto the upper surface and the lower surface of the central body 19 of the support element 4. The support element 4 comprises at least one opening 8. Preferably, these openings 8 are cylindrical and have a generatrix perpendicular to the upper surface and to the lower surface. Each reservoir 5 may comprise a plurality of tubes A. In the embodiment illustrated in FIG. 1, the reservoirs 5 comprise several threaded tubes A. Thus, the tubes A are assembled by screwing so as to form a column C of tubes A. These tubes A and therefore the column C have a section of dimension smaller than the section of the opening 8 intended to receive the reservoir 5 comprising the tube or the column C so that the tube A or the column C of said reservoir 5 can be inserted or removed from the recess 2 through the opening 8.

Thus, the tanks 5 are generally tubular in shape, with a circular section, and each have a longitudinal axis, a lower end 13 and an upper end 14.

Each tank 5 is closed at its lower end 13 by a first closing means 12, and each tank 5 is closed at its upper end 14 by a second closing means 15. In the embodiment illustrated in FIG. 1, the lower end 13 and the upper end 14 of each tank 5 have a thread onto which the first closing means 12 and the second closing means 15 are screwed in a sealed manner respectively. Thus, the lower end 13 is closed in a sealed manner by screwing the first closing means 12, and the upper end 14 is closed in a sealed manner by screwing the second closing means 15.

In other words, each tank 5 is formed by an assembly comprising a column C, and is closed at its ends 13, 14 by closing means 12, 15. A tank 5 can also comprise a single tube A, and be closed at its ends 13 and 14 by closing means 12 and 15.

As explained below, the upper end of the tanks 5 comprises a support piece 17 interposed between the tube A, or the column C, and the second closing means 15. This support piece 17 cooperates with the support element 4 to suspend the tank 5 in the recess 2.

The means of access are intended to enable maintenance operations to be carried out by allowing access to the various elements of the installation from the outside. They comprise in particular at least one of means of access for each tank 5, means of access to the interior of each tank 5, and means of access to the contents of the recess 2.

A first means of access to the tanks comprises a passage provided on the periphery of the support element 4. This passage is for example formed around the support element 4 and/or on the upper face of the collar 20. Typically, such a passage allows an operator wishing to carry out a maintenance operation on a tank 5 located on the periphery of the support element 4 to have access to said tank 5 simply by moving around the periphery of the support element 4. Such a passage around the support element 4 can be made in the ground 3 surrounding the collar 20, for example on the surface S if the support element 4 rests on the ground S or on the bottom of a cavity (not shown) formed in the ground 3 and on which the support element 4 rests.

If there are many tanks 5 stored in the recess 2, some are located in the center of the assembly grouping the tanks 5 and are therefore inaccessible from a passage arranged on the periphery of the support element 4. It is however necessary to have access to them to carry out maintenance operations. The means of access to each tank 5 therefore comprise a second means of access to the tanks 5 in the form of a corridor 9, as illustrated in FIG. 3, extending radially around the central point of the support element 4. Depending on certain variables or constraints of the installation, such as the number of tanks 5 stored in the recess 2 and the dimensions of the support element 4, there may be one or more corridors 9 facilitating access to each tank 5.

Each tank 5 must be accessible by at least one operator and the configuration of the support element 4 must therefore allow access to each tank 5. Maximum compactness of the tanks is also preferred to optimize the number of tanks 5 in an installation. For this, the following equation defines the optimal level of compactness: [Math 2]

With j the index of each group of tanks 5 having a different diameter, ODsj the external diameter of the tank 5 and Am the area necessary to access the tanks 5 from the support element 4. Am is defined by considering a corridor 9 with a width between 0.6 meters and 1 meter. Access to the tanks 5 located at the end of the support element 4 is done directly from the outside of the support element 4 via the ad hoc passage.

In Figure 3, the storage system 1 comprises about fifty tanks 5 and, consequently, the support element 4 comprises about fifty openings 8, each tank 5 being suspended in the recess 2 via a respective opening 8 of the support element 4. This configuration is circular and contains a maintenance corridor 9 leaving two rows of tanks 5 on its radially external part and a row of seven tanks 5 on its radially internal part. The outer row of tanks 5 is accessible from the passage provided on the periphery of the support element 4, and the inner and central rows of tanks 5 are accessible from the circular corridor 9 on the support element 4. The tanks 5 are spaced on at least part of the two outer rows, to create an entrance 10 opening onto the corridor 9.

Of course, the support element 4 as well as the recess 2 can be of several shapes, for example circular or polygonal.

A circular area of the support element 4 is defined as follows: [Math 3]

A polygonal area of the support element 4 is defined as follows: [Math 4]

With: Ai the area of each triangle that makes up the area of the polygon.

Of course, the tanks 5 can be arranged on the support element 4 along a horizontal, vertical, offset line, aligned with a circle or any other geometric shape, and the tanks 5 can have different dimensions from each other.

As illustrated in Figure 1, the tanks 5 are suspended from the support element 4. Thus, for each tank 5 of the storage system 1, an axial clearance G remains between, on the one hand, the first closing means 12 which closes the lower end 13 of the tank 5, and on the other hand, the bottom 6 of the recess 2. This axial clearance G has the function of absorbing an axial thermal expansion of the tank 5, which occurs in particular during filling and emptying operations. Thus, for each tank 5, the dimensioning of the axial clearance G, and in particular its length, depends directly on the surrounding conditions of the storage system 1, in particular the temperature and pressure conditions, and the capacity of the tank 5 to elongate when it is subjected to variations in temperature and pressure, in particular during filling and emptying operations. Thus, the axial clearance G of any tank 5 of the storage system 1 responds to the following inequality:

[Math

Where: G is the length of the axial clearance expressed in meters. L represents the length of a reservoir 5 expressed in meters. P represents the geothermal gradient of the ground 3 expressed in degrees Celsius per meter. The geothermal gradient P varies according to the geological formation in which the storage system 1 is placed. Thus P is such that 0.02°/m < P < 2°/m. a represents the coefficient of thermal expansion of the metal in which the reservoir 5 is made expressed in degrees Celsius - 1. The coefficient of thermal expansion a varies according to the type of metal that makes up the tubes used to form a reservoir 5. Thus, a is such that 8 * 10-6 °C - 1 < a < 18 * 10-6 °C - 1. The means of access to the interior of each tank 5 comprise a sacrificial emergency part 16. This sacrificial emergency part 16, the support part 17 and the second closing means 15 together form the upper end 14 of the tank 5. The upper end 14 is shown in FIG. 4. The support part 17 comprises a metallic tubular body 21, of circular section, having a collar 22. A periphery of this collar 22 has dimensions greater than the dimensions of the opening 8 so that it rests on the upper surface of the support element 4 when the tank 5 is inserted into the opening 8. The collar 22 is thus able to suspend the tank 5 in the recess 2 via the support element 4.

In the embodiment where the collar 22 allows the reservoir 5 to rest on the upper surface of the support element 4, the reservoir 5 does not need to be fixed to the support element 4, which simplifies the assembly of the storage system 1. It is however possible, as a variant, to fix the collar 22 to the support element 4 for example by screwing screws or any other suitable means.

The support piece 17 is tightly assembled at its lower end to the tube A or to the column C by screwing. The support piece 17 is assembled at its upper end to the sacrificial emergency piece 16, and the sacrificial emergency piece 16 is assembled to the second closing means 15. These assemblies are made with threaded connections composed of at least one seal.

The maintenance operations inside the tank 5 are carried out via the second closing means 15 and/or via the sacrificial emergency part 16. Indeed, the second closing means 15 and/or the sacrificial emergency part 16 can be unscrewed and detached from the support part 17 in order to access the contents of the tank 5, for example to inspect the state of the inside of the tank 5. However, the screwing/unscrewing carried out during the maintenance operations causes damage to the parts which must be changed after a few manipulations. For safety reasons, the screwing/unscrewing of the parts is preferably limited to three operations. Indeed, after three screwing/unscrewing operations of the second closure means 15, the seal at the second closure means 15 does not guarantee the sealing of the tank 5 with certainty. It is then necessary to replace the parts cooperating to form this seal. This is why it is advantageous to put a sacrificial backup part 16 serving to extend the life expectancy of the support part 17. Indeed, when three screwing/unscrewing operations of the second closure means 15 are carried out on the tank 5, the second closure means 15 and the sacrificial backup part 16 are then replaced. When carrying out this replacement, a screwing/unscrewing is carried out between the sacrificial spare part 16 and the support part 17. Once the sacrificial spare part 16 and the closing means 15 have been replaced, three operations can be carried out before having to change the parts again.

Thus, the use of a sacrificial spare part 16 allows up to nine operations on the tank 5 without having to change the support part 17. Since replacing the support part 17 involves the complete removal of the tank 5 from the recess 2, additional work time and other commitments, the sacrificial spare part 16 makes it possible to improve the practicality of use of the storage system 1 and to facilitate maintenance operations.

Of course, the upper end 14 of the tank 5 may comprise more than one sacrificial spare part 16, making it possible to further increase the number of maintenance operations without having to remove the entire tank 5 from the well 2.

To enable the inspection of the tanks 5, and in particular the interior of the tanks 5, the storage system 1 is designed to enable a tank 5 to be checked without impacting the operation of the other tanks 5. Thus, a tank 5 can be rinsed, isolated, changed, without the other tanks 5 being affected.

The second closing means 15 is equipped with sensors 18 such as pressure gauges, thermometers and leak detectors. Of course, the first closure means 12 can also contain pressure gauges, thermometers and leak detectors. Other types of sensors can be used depending on the nature of the parameters to be controlled.

In addition to the tanks 5, the recess 2 is preferably filled with an explosion-proof material. In the event of a leak from the tank 5, the presence of ambient oxygen in the recess 2 is limited or even zero, reducing the risk of explosion. This explosion-proof material is a liquid and/or a solid, for example water, sand or the like. It preferably comprises antibacterial agents such as chlorine, an antibacterial filter, or hydrogen peroxide and anticorrosion agents in order to prevent the proliferation of bacteria in the environment of the tank 5 and the degradation of the tanks 5.

This material requires maintenance in order to retain its characteristics and avoid deterioration of the storage system 1.

During maintenance operations, it is therefore necessary to be able to access this material. For this, the means of access to the contents of the recess 2 comprise at least one material suction pipe and one material injection pipe, said pipes being connected to the room. Ideally, the means of access to the contents of the recess also comprise a room outside the recess 2, for example in which the suction and injection pipes open and/or the control members of the suction and injection pipes are located.

According to one embodiment, the injection pipe is located above the level of the material, typically in the upper part of the recess 2, and the suction pipe is located at the bottom of the recess 2. These pipes allow the pumping, injection and therefore the circulation and/or replacement of the material in the recess 2.

Storage system 1 is used to store any type of fluid, especially explosive gases. The fluid can be hydrogen, oxygen, methane, nitrogen, ammonia. Preferably, the fluid is hydrogen.

The invention also provides an underground storage method for storing fluids via a storage system 1 such as as described above. The method comprises at least one step of arranging a recess 2 in a terrain 3, said recess 2 having a bottom 6, a step of providing a support element 4 comprising at least one opening 8 capable of receiving a support piece 17, a step of providing at least one reservoir 5, said reservoir 5 having a longitudinal axis, a lower end 13 and an upper end 14, said upper end 14 comprising the support piece 17, a step of providing a first closing means 12 capable of closing said reservoir 5 at its lower end 13, and a second closing means 15 capable of closing the reservoir 5 at its upper end 14, a step of assembling said upper end 14 to the support element 4 by means of the support piece 17 so that the reservoir 5 is suspended inside the recess 2 and preferably has an axial clearance G capable of absorbing an axial thermal expansion of said reservoir 5 remaining between the first closing means 12 of the reservoir 5 and the bottom 6 of the recess 2, and a step of providing means of access from the outside of the recess (2), comprising at least one of means of access to each reservoir 5 from the support element 4, means of access to the inside of the reservoirs 5, and means of access to the contents of the recess 2.

Claims

1. Underground storage system (1) for storing fluids, characterized in that it comprises: - a recess (2) arranged in a piece of land (3), said recess (2) having a bottom (6), - a support element (4) comprising at least one opening (8), - at least one reservoir (5), said reservoir (5) having a longitudinal axis, a lower end (13) closed by a first closing means (12), and an upper end (14) closed by a second closing means (15), said upper end comprising a support piece (17), said upper end (14) being assembled to the support element (4) by means of the support piece (17) so that the reservoir (5) is suspended inside the recess (2) and - means of access from outside the recess (2) comprising at least one of means of access to each reservoir (5), means of access to the interior of the reservoirs (5), and means of access to the contents of the recess (2). 2. Storage system (1) according to claim 1, in which the means of access to each reservoir (5) comprise a passage provided on the periphery of the support element (4) and/or a corridor (9) located on the support element (4). 3. Storage system (1) according to any one of claims 1 and 2, in which the means of access to the interior of the tanks (5) comprise a sacrificial emergency part (16) interposed between the second closing means (15) and the support part (17). 4. Storage system (1) according to claim 3, wherein the sacrificial spare part (16) is removable. 5. Storage system (1) according to any one of claims 1 to 4, in which the means for accessing the content of the recess (2) comprises at least one fluid and/or solid suction pipe and at least one fluid and/or solid injection pipe. 6. Storage system (1) according to any one of claims 1 to 5, in which the reservoir (5) comprises at least one metal tube (A), said metal tube (A) having at least one termination provided with at least one threaded portion. 7. Storage system (1) according to any one of claims 1 to 6, in which the tank (5) comprises at least two metal tubes (A) assembled by screwing, so as to form a column of tubes (C). 8. Storage system (1) according to any one of claims 1 to 7, in which the first closing means (12) and/or the second closing means (15) is capable of closing the tank (5) in a sealed manner by screwing. 9. Storage system (1) according to any one of claims 1 to 8, comprising a plurality of tanks (5). 10. Underground storage method for storing fluids via a storage system (1) according to any one of claims 1 to 9, characterized in that it comprises at least the following steps: - arrangement of a recess (2) in a piece of land (3), said recess (2) having a bottom (6), - providing a support element (4) comprising at least one opening (8) capable of receiving a support part (17), - providing at least one tank (5), said tank (5) having a longitudinal axis, a lower end (13) and an upper end (14), said upper end (14) comprising the support piece (17) - providing a first closing means (12) capable of closing said reservoir (5) at its lower end (13), and a second closing means (15) capable of closing the reservoir (5) at its upper end (14), - assembling said upper end (14) to the support element (4) by means of the support piece (17) so that the tank (5) is suspended inside the recess (2), and - arrangement of means of access from outside the recess (2) comprising at least one of means of access to each tank (5), means of access to the inside of the tanks (4), and means of access to the contents of the recess (2).