SYSTEM AND METHOD FOR STORING FLUIDS, INCLUDING RISK REDUCTION MEASURES

Abstract

An underground storage system (1) for the storage of fluids. It comprises a recess (2) formed in ground (3), said recess (2) having a bottom (7), a support element (4) comprising at least one opening (9), an assembly element (6) inserted into the opening (9) of the support element (4), the assembly element (6) having ventilation vents to allow the fluid contained in the tank (5) to escape in the event of a leak, and at least one tank (5), said tank (5) having a longitudinal axis, a lower end (11) closed by a first closure means (10), and an upper end (12) closed by a second closure means (13). Figure for the abbreviation: Fig 1

Description

Title of the invention: SYSTEM AND METHOD FOR STORING FLUIDS INCLUDING RISK REDUCTION MEASURES technical field

[0001] The present invention relates to underground storage and in particular to the storage of fluids, for example hydrogen or oxygen. More particularly, the invention relates to the field of risk reduction in a fluid storage system and, in particular, in a high-pressure fluid storage system, which may be between 100 bar and 1200 bar, more particularly between 200 bar and 500 bar.

[0002] The invention also relates to a method of underground storage of fluids. Technological background

[0003] The risks of such a system are primarily related to explosion and / or fire caused by a leak of the fluid contained in the tanks. Indeed, an explosive fluid such as hydrogen is highly flammable and instantly causes a powerful detonation. This type of incident results in structural damage and / or potentially fatal injuries.

[0004] The solutions provided today relate in particular to the reduction of risks in the event of fluid leakage and propose, for example, a pressure relief valve which opens if a quantity of fluid is released into the cavity, to direct the fluid into a pipe and evacuate it.

[0005] This type of installation, however, is not reliable because the drainage system pipes are susceptible to blockages. Similarly, over time the valve may cease to function effectively. This necessitates maintenance and, if necessary, component replacements. Furthermore, since the fluid is not directly diluted in the air, a pressure increase could cause an explosion and subsequent damage to the drainage system. Description of the invention

[0006] The invention therefore aims to overcome the aforementioned disadvantages by proposing an effective solution that is not subject to mechanical constraints leading to additional maintenance operations.

[0007] The invention relates to an underground storage system for the storage of fluids comprising a recess formed in the ground, said recess having a bottom, a support element comprising at least one opening, an assembly element inserted into the opening of the support element, the assembly element comprising ventilation vents to allow the fluid contained in the tank to escape in the event of a leak, at least one tank, said tank 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 is assembled to the support element via the assembly element so that the tank is suspended inside the recess and an axial clearance capable of absorbing axial thermal expansion of said tank remains between the first closing means of the tank and the bottom of the recess.

[0008] Advantageously, the support element separates an upper cavity from a lower cavity, said upper cavity being open and comprising a first diameter larger than a second diameter of the lower cavity. Thus, the tank is entirely buried below ground level, including the second closure means that closes the upper end of the tank. Such a configuration of the storage system makes it possible to guide a deflagration occurring in the recess and thus to reduce the deflagration angle in the event of an explosion.

[0009] Advantageously, the lower cavity is filled with a flame-resistant fluid and / or solid. Since the lower cavity is already filled, it cannot be filled with oxygen. Therefore, the amount of oxygen in the lower cavity is zero or negligible. Consequently, in the event of a leak from a tank, the risk of explosion is considerably reduced, or even eliminated.

[0010] According to one embodiment, the explosion-proof fluid and / or solid comprises antibacterial agents and / or anti-corrosion agents.

[0011] Advantageously, the upper end of the tank includes a lifting cap system for handling the tank, and a system for connecting valves and piping for storing and distributing the fluid contained in the tank.

[0012] Advantageously, the reservoir comprises at least one metal tube, said metal tube having at least one termination having at least one threaded portion.

[0013] The tank comprises at least two metal tubes assembled by screwing, so as to form a column of tubes.

[0014] Advantageously, the axial clearance suitable for absorbing axial thermal expansion of the reservoir complies with the following inequality:

[0015] [Math.l] G £ (L2*^a) + [20*ct*80*( 1 - eo. 11 v.)|

[0016] Where: G is the axial clearance length expressed in meters, L represents the length of a reservoir expressed in meters, [3 represents the geothermal gradient expressed in degrees Celsius per meter, a represents the coefficient of thermal expansion of the metal expressed in meters per degree Celsius.

[0017] 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 assembled to the support element by means of an assembly element so that each tank is suspended inside the recess.

[0018] Optionally, the first closing means and / or the second closing means is suitable for closing the tank by screwing.

[0019] Advantageously, the recess includes at least one lining, said lining being made of concrete, cement, or steel.

[0020] 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: - creation of a recess in a plot of land, said recess having a bottom, - provision of a support element comprising at least one opening suitable for receiving an assembly element, the assembly element comprising ventilation vents to allow the fluid contained in the tank to escape in the event of a leak, - supply of at least one tank, said tank having a longitudinal axis, a lower end and an upper end, - provision of a first closing means suitable for closing said tank at its lower end, and a second closing means suitable for closing the tank at its upper end, - assembly of said upper end to the support element via the assembly element so that the tank is suspended inside the recess, and an axial clearance capable of absorbing axial thermal expansion of said tank remains between the first tank closure means and the bottom of the recess. Definitions

[0021] The term "lower end" of the tank refers to the end of the tank that 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 near the ground surface.

[0022] The term "axial clearance" refers to a length extending along the longitudinal axis of the tank, measured between the first tank closure means and the bottom of the recess. It should be noted that the position of a tank may not be perfectly vertical. In this case, the longitudinal axis of the tank is at an angle to the vertical in the (x; y) coordinate system. This angle has a maximum value of 15°. In this case, the axial play is measured by orthogonal projection onto the vertical axis passing through a point on the first closing mechanism located closest to the bottom of the recess. In other words, the axial play always corresponds to the shortest distance measured between the bottom of the recess and the first closing mechanism.

[0023] The term "threaded metal tube" means a tube comprising at least one end having at least one threaded portion, suitable for assembly with a threaded metal tube comprising at least one end having at least one additional threaded portion. The thread may be male or female.

[0024] The term "bottom of the excavation" refers to the surface at the bottom of the excavation. Thus, when the excavation is lined and the lining is cemented, the term "bottom of the excavation" designates the surface of the cement layer at the bottom of the excavation. When the lining is not cemented, the term "bottom of the excavation" simply designates the surface of the ground at the bottom of the excavation. Brief description of the drawings

[0025] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example and made with reference to the accompanying drawings in which:

[0026] - Fig. 1 is a schematic view of the general structure of a system of underground storage;

[0027] - Figure [Fig. 2] is a schematic view representing the assembly element of a underground storage system;

[0028] - Figure 3 is a schematic view representing the ventilation vents present on the assembly element of an underground storage system;

[0029] - Figure 4 is a schematic view of a support element separating a cavity in lower of an upper cavity in an underground storage system;

[0030] - Figure [Fig. 5A] is a schematic view of a tank in a storage system underground system including a lifting plug system; and

[0031] - Figure [Fig. 5B] is a schematic view of a tank in a storage system underground including a connection system. Detailed description

[0032] Fig. 1 illustrates a cross-sectional view of a storage system 1 according to an embodiment of the invention. As illustrated, the system is placed in a coordinate system (x; y) in which the x-axis of the coordinate system (x; y) is a horizontal axis, and the y-axis of the coordinate system (x; y) is a vertical axis.

[0033] The storage system 1 comprises a recess 2 made in a ground 3, a support element 4 placed on a ground surface S of the ground 3, six tanks 5 suspended from the support element 4 in the recess 2 (only four tanks 5 being visible in [Fig. 1]) and an assembly element 6 which includes vents to allow the fluid contained in the tank 5 to escape in case of a leak. In various embodiments, the storage system 1 generally comprises at least one tank 5.

[0034] The recess 2 has a bottom 7 and includes a lining 8. The recess 2 can be obtained by drilling or excavation. The recess 2 is substantially cylindrical in shape and has an average diameter of 4 meters. The lining 8 is made of cement and extends vertically from the ground surface S to the bottom 7 of the recess 2.

[0035] The support element 4 is a circular cylindrical plate that separates the storage system 1 into an upper cavity C1 and a lower cavity C2. The support element 4 has a central body and a flange, said flange having a lower surface resting on the floor S. The central body has a first thickness and the flange has a second thickness. The support element 4 further comprises an upper surface, said upper surface being opposite the lower surface of the flange.

[0036] The support element 4 also includes one or more openings 9. The openings 9 are through holes, provided in the first thickness of the body of the support element 4. In the illustrated embodiment, the support element has six openings.

[0037] Each tank 5 is suspended from the support element 4 by means of an assembly element 6. Thus, for each tank 5 of the storage system 1, an axial clearance G remains between, on the one hand, the first closing means 10 which closes the tank 5 at its lower end 11 and, on the other hand, the bottom 7 of the recess 2. This axial clearance G serves to absorb 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 subjected to variations in temperature and pressure, particularly during filling and emptying operations.Thus, the axial clearance G of any tank 5 of the storage system 1 satisfies the following inequality: .

[0038] [Math.l] G > (L2T«) + [20*a*80*(l - 11¾)]

[0039] Where: G is the axial clearance length expressed in meters. L represents the length of a reservoir 5 expressed in meters. [3 represents the geothermal gradient expressed in degrees Celsius per meter. The geothermal gradient [3] varies depending on the geological formation in which the storage system 1 is placed. Thus, [3] is such that 0.02° / m < [3] < 2° / m. a represents the coefficient of thermal expansion of the metal expressed in degrees Celsius-1. The coefficient of thermal expansion a varies depending on 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.

[0040] The tanks 5 are tubular, of circular section, and each have a longitudinal axis, a lower end 11 and an upper end 12.

[0041] Each reservoir 5 is closed at its lower end 11 by a first closing means 10, and each reservoir 5 is closed at its upper end 12 by a second closing means 13. In the embodiment illustrated in [Fig. 1], the lower end 11 and the upper end 12 of each reservoir 5 are threaded ends, and the first closing means 10 and the second closing means 13 also have a thread, said thread being complementary to the thread of the lower end 11 and upper end 12. Thus, the lower end 11 is sealed by screwing it with the first closing means 10, and the upper end 12 is sealed by screwing it with the second closing means 13.

[0042] The second closing means 13 is equipped with sensors 14 such as pressure gauges, thermometers, and leak detectors. Of course, the first closing means 10 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 monitored.

[0043] Each reservoir 5 can 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 them together to form a column C of tubes A. Thus, each reservoir 5 is formed by an assembly comprising a column C, closed at its ends 11, 12 by closure means 10, 13. A reservoir 5 can also consist of a single tube A, closed at its ends 11, 12 by closure means 10, 13.

[0044] Each tank 5 is assembled to an assembly element 6 visible in [Fig. 2]. Each assembly element 6 is inserted into an opening 9 and is retained by a collar 15 which abuts against the upper surface of the support element 4. Each assembly element 6 is therefore suspended from the support element 4. In this way, each tank 5 is suspended from the support element 4 by means of the assembly element 6 to which it is assembled.

[0045] For example, an assembly element 6 is a tubular metal part, with a circular cross-section, which comprises a tubular body 16, a flange 15 and vents ventilation 17.

[0046] The body 16 of the assembly element 6 is fixed to the upper end 12 of a tank 5, preferably by screwing. The body 16 of the assembly element 6 has a male or female thread (not shown) that complements the thread of the upper end 12 of the tank 5 to which the assembly element 6 is assembled. The flange 15 comprises an upper surface 18 and a lower surface 19, with ventilation vents 17 passing through these two surfaces 18, 19. Alternatively, the body 16 of the assembly element 6 can be fixed by welding. In another embodiment, the body 16 of the assembly element 6 has a male thread that complements a female thread of the opening 9 of the support element 4, so that the assembly element 6 is screwed into the opening 9.

[0047] In the embodiment illustrated in [Fig. 1], the tubular body 16 of each assembly element 6 is inserted into an opening 9 in the support element 4. Each assembly element 6 rests on the support element 4 via its lower surface 19, which is supported against the upper surface of the support element 4. In addition, each assembly element 6 is assembled to a tank 5 by screwing it to the upper end 12 of said tank 5.

[0048] The collar 15 therefore allows the assembly element 6 to rest on the upper surface of the support element 4. Thus, the assembly element 6 does not need to be fixed to the support element 4, for example by welding or screwing, which simplifies the assembly of the storage system 1, in particular for suspending the tanks 5.

[0049] Figure 3 illustrates a tank 5 suspended from the support element 4 via the assembly element 6. Indeed, the assembly comprising the assembly element 6 fixed to the upper end 12 of the tank 5 is inserted into the opening 9 and rests on the upper face of the support element 4. In this configuration, the opening 9 has a larger diameter than the outer diameter of the assembly element 6. There is therefore a radial gap between the support element 4 and the assembly element 6.

[0050] If there is no means of ventilation in such a storage system 1 and a leak occurs, the released fluid accumulates in the cavity and builds up pressure before causing an explosion, and / or a detonation, and / or a fire. In order to minimize these risks, ventilation vents 17 create an air circulation passage between the upper cavity Cl and the lower cavity C2. The storage system 1 is therefore not airtight, which allows the fluid released from the tanks 5 to easily escape to the outside of the storage system and the cavity in the event of a leak. The ventilation vents 17 are holes passing through the flange of the assembly element 6. The ventilation vents can be, for example, the holes in a ventilation grille, circular and / or rectangular holes. Of course, the ventilation vents may have other forms not described in this document.

[0051] The total ventilation area is expressed according to the following relationships:

[0052] [Math.2] A = N wind*Advent

[0053] A ïAgap*0.9

[0054] Where A denotes the total ventilation area, Nvent is the number of ventilation vents 17 greater than or equal to 1, Avent is the area of ​​a ventilation vent 17, Agap is the total area of ​​the radial space which is between the support element 4 and the assembly element 6, and which is calculated according to the following equation:

[0055] Agap = ^(d^-dô2)

[0056] In which Gap is the width of the radial space, which is calculated according to the following equation:

[0057] [Math.4] Gap = (d9 - d6) / 2

[0058] Where d9 is the diameter of the opening 9, and d6 is the outside diameter of the assembly element 6.

[0059] The support element 4 separates the upper cavity Cl from the lower cavity C2. According to an embodiment shown in [Fig.4], the upper cavity Cl is open and includes a first diameter DI greater than a second diameter D2 of the lower cavity C2.

[0060] In the event of a deflagration, this configuration makes it possible to guide the explosion and reduce the pressure induced by the deflagration.

[0061] Indeed, if there is a leak of fluid in the storage system 1 and the pressure rises before it can be properly evacuated through the ventilation vents 17, an explosion may occur. Such an explosion would cause structural damage and / or potentially fatal injuries. In order to reduce the extent of the deflagration, once the fluid is outside the recess 2, an upper cavity Cl is created, this cavity having a larger diameter than a lower cavity C2 into which the fluid is released.

[0062] In the example illustrated in [Fig. 4], the upper cavity Cl directs the overpressure gas flow outward from the storage system 1 in a conical shape with a solid angle α, the apex of which coincides with the bottom of the upper cavity Cl, which corresponds to the support element 4. Surrounding structures are thus avoided. The angle α is determined as a function of the depth d of the upper cavity Cl. If there is no upper cavity Cl, the angle α is equal to 90°, and the explosion risks damaging surrounding structures. Preferably, the depth of the upper cavity Cl is at least one meter.

[0063] Another way to reduce the risks of explosion and / or fire in the storage system 1 is to fill at least part of the recess 2, in particular to fill at least part of the lower cavity C2, with a material comprising at least one explosion-proof fluid and / or at least one explosion-proof solid.

[0064] In the event of a leak from the tank 5, filling the lower cavity C2 directs the released fluid to the support element 4, thus preventing a pressure increase in the lower cavity C2. The released fluid is then rapidly diluted in the upper cavity Cl via the ventilation vents 17, and the risks of explosion and fire are avoided. Filling the lower cavity C2 also reduces or eliminates its oxygen content to prevent detonations in the event of a leak of a flammable fluid.

[0065] The explosion-proof material is a liquid and / or a solid, for example demineralized water, fresh water, salt water, sand, brine, or oil. It may also include antibacterial agents such as chlorine, an antibacterial filter, or hydrogen peroxide, and anti-corrosion agents.

[0066] This material must not induce pressure on the lower cavity C2 and on the reservoirs 5.

[0067] The volume of the material is characterized by the following equation:

[0068] [Math.5] Vm = Vc2 - VR

[0069] Where Vm is the volume of the material, Vc2 is the volume of the lower cavity C2, VR is the volume of the reservoirs contained in the lower cavity C2.

[0070] The volume of the lower cavity C2 is calculated according to the following equation:

[0071] [Math.6] Vc2- ^^rc^^hcl 4

[0072] Where rc2 denotes the radius of the lower cavity C2, and hc2 is the height of the lower cavity C2.

[0073] The volume of the tanks 5 is calculated according to the following equation:

[0074] [Math.7] VR = (^rR^hRV^R

[0075] Where rR is the radius of a tank 5, hR denotes the height of a tank 5, NR denotes the number of tanks 5 contained in the storage system 1.

[0076] The pressure of the material is characterized by this equation:

[0077] [Math. 8] Pm = hc2 * V Mm * g

[0078] Where Pm is the pressure of the material, VMm denotes the density of the material and g is the intensity of terrestrial gravity.

[0079] Figures 5a and 5b illustrate the closure means 13 of the upper end 12 of a tank 5. The upper part of the closure means 13 includes a male or female junction machined to allow the connection of valves and piping for the distribution of the fluid contained in the tank 5.

[0080] According to an embodiment illustrated in Figure 5a, a thread is machined on the outer diameter of the upper part of the closure means 13 to allow for the installation of a lifting plug 20. This embodiment is substantially used when handling the tank 5, typically during maintenance phases, for example, if components need to be replaced. The lifting plug 20 comprises a body 22 and a lifting means 24. The body 22 has a first female threaded recess 23, which is screwed into the male junction of the upper part of the closure means 13. The body 22 further comprises a second recess 25 axially opposite to the first recess 23. The second recess 25 is configured to receive and secure the lifting means 24. The lifting means 24 is inserted and secured into the second recess 25 by screwing. According to the embodiment illustrated in Figure 5a, the lifting means 24 is a lifting ring.According to another embodiment illustrated in Figure 5b, a thread is machined on the inner diameter of the upper part of the closing means 13, in order to ensure the installation of a connection system 21. This connection system allows the valves and piping of the tank 5 to be connected to external machines, allowing the distribution of the fluid contained in the tank 5 to be managed.

[0081] This system is advantageous because it allows the use of several nozzles as needed, depending on the operations performed on the storage system 1. In particular, the installation of a lifting plug 20 helps prevent the risk of injury or damage to the structure when handling the tank 5. Furthermore, the internal and external threads on the upper part of the same closure means 13 prevent the lifting plug 20 and the connection system 21 from being attached simultaneously to the same tank 5. Such a configuration would require the operator to empty the tank 5 before removing the connection system 21 and attaching the lifting plug 20 in order to handle the tank 5, for example, for maintenance. This prevents the operator from handling a full tank. A storage system with such features therefore presents fewer risks.

[0082] The storage system 1 is used to store any type of fluid, in particular explosive gases. The fluid can be hydrogen, oxygen, methane, nitrogen, ammonia; preferably, the fluid is hydrogen.

[0083] The invention also relates to an underground storage method for the storage of fluids by means of a storage system 1 as described above.

[0084] According to this process, in a first step, a recess 2 is made in a plot 3, the said excavation 2 presenting a bottom 7.

[0085] In the next step of the process, a support element 4 is provided comprising at least one opening 9 suitable for receiving an assembly element 6, the assembly element 6 comprising ventilation vents 17 to allow the fluid contained in the tank 5 to escape in the event of a leak.

[0086] In the next step, at least one reservoir 5 is then provided, said reservoir 5 having a longitudinal axis, a lower end 11 and an upper end 12, then a first closing means 10 suitable for closing said reservoir 5 at its lower end 11, and a second closing means 13 suitable for closing the reservoir 5 at its upper end 12.

[0087] The upper end 12 is then assembled to the support element 4 via the assembly element 6 so that the reservoir 5 is suspended inside the recess 2, and an axial clearance G suitable for absorbing an axial thermal expansion of the said reservoir 5 remains between the first closing means 10 of the reservoir 5 and the bottom 7 of the recess 2.

Claims

Claims

1. Underground storage system (1) for storing fluids, characterized in that said system (1) comprises: - a recess (2) arranged in a ground (3), said recess (2) having a bottom (7), - a support element (4) comprising at least one opening (9), - an assembly element (6) inserted into the opening (9) of the support element (4), the assembly element (6) comprising ventilation vents (17) to allow the fluid contained in the reservoir (5) to escape in the event of a leak, - at least one reservoir (5), said reservoir (5) having a longitudinal axis, a lower end (11) closed by a first closing means (10), and an upper end (12) closed by a second closing means (13),said upper end (12) being assembled to the support element (4) by means of the assembly element (6) so that the tank (5) is suspended inside the recess (2) and that an axial clearance (G) capable of absorbing an axial thermal expansion of said tank (5) remains between the first closing means (10) of the tank (5) and the bottom (7) of the recess (2).,

2. Storage system (1) according to claim 1, wherein the support element (4) separates an upper cavity (Cl) from a lower cavity (C2), said upper cavity (Cl) being open and comprising a first diameter (Dl) greater than a second diameter (D2) of the lower cavity (C2).

3. Storage system (1) according to claim 2, wherein the lower cavity (C2) is filled with a fluid and / or an explosion-proof solid.

4. A storage system (1) according to claim 3, wherein the explosion-proof fluid and / or solid comprises antibacterial agents and / or anti-corrosion agents.

5. Storage system (1) according to any one of claims 1 to 4, wherein the upper end (12) of the tank (5) comprises a lifting cap system (20) when handling the tank (5), and a system (21) for connecting the valves and pipes during storage and distribution of the fluid contained in the tank (5).

6. Storage system (1) according to any one of claims 1 to 5, wherein 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 reservoir (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, wherein the first closing means (10) and / or the second closing means (13) is capable of closing the reservoir (5) by screwing.

9. Storage system (1) according to any one of claims 1 to 8, which comprises a plurality of tanks (5), each tank (5) having a longitudinal axis, a lower end (11) and an upper end (12), said upper end (12) of each tank (5) being assembled to the support element (4) by means of an assembly element (6) so that each tank (5) is suspended inside the recess (2).

10. An 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: - providing a recess (2) in a ground (3), said recess (2) having a bottom (7), - providing a support element (4) comprising at least one opening (9) capable of receiving an assembly element (6), the assembly element (6) comprising ventilation vents (17) to allow the fluid contained in the reservoir (5) to escape in the event of a leak, - providing at least one reservoir (5), said reservoir (5) having a longitudinal axis, a lower end (11) and an upper end (12), - providing a first closing means (10) capable of closing said reservoir (5) at its lower end (11), and a second closing means (13) capable of closing the reservoir (5) at its upper end (12), assembly of said upper end (12) to the support element (4) by means of the assembly element (6) so that the tank (5) is suspended inside the recess (2), and that an axial clearance (G) capable of absorbing an axial thermal expansion of said tank (5) remains between the first closing means (10) of the tank (5) and the bottom (7) of the recess (2).

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