Method for storing fluid underground at maximum pressure p

A method for storing fluid underground at maximum pressure p, comprising the steps of digging, at a depth at which the lithostatic pressure generated by the weight of the abovejacent soil formations is at least p, an underground cavity in which is made a tight deformable casing anchored to the cavity wall at some places only, while it can freely expand or contract in every direction between said anchoring places, then of injecting pressured fluid into said casing in order that the latter be fully applied against the cavity wall, the possible movements of which said casing subsequently follows by gliding, the pressure of said fluid being, at every moment, counterbalanced by the lithostatic pressure of the cavity walls transmitted to said fluid by the thus expanded casing.

It is known to make underground tanks in which pressurized fluids can be 
left in transit or be stored until they are recovered later on. The idea 
of an underground storage of heat through pressurized hot water was 
expounded, in particular, during the 8th World Conference on Energy 
(Bucarest, June 28, 1971). 
Other modes of storage, e.g., compressed air, gas or radioactive waste, 
have already been contemplated or put into practice. 
In the present state of the art, it is most often resorted to the 
hydrostatic pressure of the water contained in the ground to 
counterbalance the pressure within the cavity, in order that a lack of 
tightness, always to be feared in view of stresses already existing or 
generated on coatings, cannot give rise to important leakages. Therefore, 
in the case of high pressures, the depth required soon becomes 
unrealistic. 
By way of example, in the case of water to be stored at 300.degree. C. 
(570.degree. F.), pressure being then of about 90 bars, the crown of the 
cavity would have to be at a depth of about 1000 meters (3300 ft). 
Moreover, in the prior art, the fluid is usually in contact with the cavity 
walls as in French Pat. No. 2,231,277, viz, with the rock or a protective 
concrete, which induces chemical and thermal phenomena likely to alter 
both the cavity behaviour and the fluid composition. In order to obviate 
such drawbacks, it has already been suggested to store a liquid, namely 
superheated water, in a metal tank open in the upper portion thereof and 
built within the cavity under air, or gas pressure. This solution has the 
double drawback of requiring a large investment of money for building a 
whole tank underground and high operation expenses in view of the 
necessitating to maintain the cavity atmosphere under constant pressure, 
in spite of the volume variations of the stored material due to the 
temperature changes thereof (variations amounting to about 20% in the case 
of water raised from 20.degree. C. to 200.degree. C.). 
In other instances, lithostatic pressure is resorted to for balancing 
internal pressure in various storage tanks, pressure-sustaining ducts or 
nuclear reactors (refer, in particular, to "Mecanique des roches et ses 
applications" published by Dunod, Paris 1967, pages 377 to 389). 
Lithostatic pressure as used in the art is the pressure which may be 
tolerated by an underground cavity sunk into rock at a certain depth 
without cracking. The lithostatic pressure depends directly on the height 
of the rocks situated above the cavity. 
In French Pat. No. 2,286,260, it is suggested to store hot water in 
underground tanks situated at a depth that is sufficient to enable the 
weight of the rocks above the tank to generate a lithostatic pressure 
adapted to counter-balance the pressure of the water to be stored. In said 
patent, it is suggested, for achieving tightness, to use either a plastic 
material foil or a metal tank constituted by rings of U-shaped 
cross-section. 
In either case, the solution advocated leads, in practice, to severe 
troubles, especially if the storage of hot fluids under high pressure is 
contemplated. Indeed, a plain foil of plastic material, in general, is not 
sufficiently resistent for sustaining the high pressures involved (e.g., 
of about 100 bars and upwards) without being torn, when hot; as for the 
metal tank, since it is made of rings and merely rests on the cavity 
floor, it is allowed but a longitudinal expansion (in only one privileged 
direction), which therefore precludes the possibility, for said tank, to 
adhere by all its points to the cavity wall under the pressure of the 
contents thereof and, later on, to follow the various "breathing" 
movements of said wall due, in particular, to thermal stresses generated 
in the sub-soil formations. 
One object of the present invention is a method for storing a fluid under a 
maximum pressure p underground, said method obviating the above drawbacks, 
while in addition providing a few extra advantages. 
Said storage method essentially comprises the steps of digging, at a depth 
at which the lithostatic pressure resulting from the weight of the soil 
formations is at least p, an underground cavity in which is made a 
deformable tight casing, fixed to the cavity wall by a few points only 
while being free to expand or retract in every direction between said 
points, then injecting the pressurized fluid into said casing in order 
that the latter lie flat by all the points thereof against the wall of the 
cavity, all the possible movements of which it follows subsequently by 
sliding, the pressure of said fluid being thus counterbalanced, at every 
moment, by the lithostatic pressure of the cavity wall transferred to the 
fluid through the thus-expanded casing. 
The use of a very special tight diaphragm endowed, as a matter of fact, 
with two degrees of expansibility, namely the possibility of sliding along 
the rock-wall between its points of attachment to that wall and the 
possibility of "breathing" at right angles to said wall, in particular, by 
following the movements of the latter resulting from thermal expansions or 
contractions, permits to obtain the storage of pressurized fluid together 
with a uniform distribution of the pressure at every moment and over the 
whole area of the cavity. 
In case of need, moreover, a smoothing coat can be inserted between the 
deformable casing and the cavity wall. 
The present invention also relates to a tank for carrying out the above 
method, characterized in that it comprises, in an underground cavity dug 
at a depth at which the lithostatic pressure resulting from the weight of 
the soil formations is at least p, a deformable tight casing, fixed to the 
cavity-wall by a few points only while being free to expand or retract in 
every direction between said points, said casing being adapted to receive 
the pressurized fluid in order that said casing lie flat by all the points 
thereof against the wall of the cavity, all the possible movements of 
which it follows subsequently by sliding, the pressure of said fluid being 
thus counterbalanded, at every moment, by the lithostatic pressure of the 
cavity wall transferred to the fluid through the thus-expanded casing. 
The tank forming the object of the present invention advantageously turns 
to profit the fact that the pressure within the cavity is distributed over 
the surrounding sub-soil formations against which the deformable tight 
wall is applied. Accordingly, the internal pressure in said cavity no 
longer needs to be counterbalanded by the surrounding hydrostatic pressure 
as in the prior art. It is only sufficient that the lithostatic pressure, 
namely the pressure exerted by the sub-soil formations themselves, 
counterbalances said internal pressure. The minimum safety depth is thus 
divided by a coefficient that is at least equal to the mean specific 
weight of the above-jacent formation, viz, by about 2. 
In the above mentioned example, relating to water at the temperature of 
300.degree. C. (570.degree. F.) under a pressure of 90 bars, the crown of 
the cavity would have to be at a depth of about 500 meters (1650 ft) 
instead of 1000 meters as in the prior art, and, in addition, the digging 
operation that required a lot of technical skill in the prior art and was 
moreover quite expensive although uncertain as to the results obtained, 
becomes, according to the present invention, a conventional application of 
the mining technique, provided geological conditions be normal. Moreover, 
in the case of smaller pressures, the digging of the cavity that would 
have required underground operations in the prior art, can now be carried 
out from the ground level, according to the so-called "covered trenches" 
method or to any other appropriate method. 
Preferably, the tight deformable casing comprises a set of metal plates 
that are arc-welded or welded according to any suitable method capable of 
providing a very good tightness. Said metal plates are provided with a 
suitable number of appropriately shaped ribs ensuring a free play of the 
plates between the anchoring points. 
Said metal plates are constituted by steel sheets, the thickness of which 
is determined by the internal pressure and the radius of curvature of the 
corrugations forming said ribs, which permits said plates to withstand the 
pressure at the ribs. Stresses on the flat portions applied against the 
sub-jacent material have not to be taken into account for determining the 
thickness, since the internal pressure is balanced by the reaction of the 
sub-jacent material. Another advantageous feature of the present invention 
is to be noted, resulting from the fact that it is possible to make use of 
relatively thin metal sheets, since the reduction of thicknesses is 
limited only by the fact that the corrugations, of necessity, must have 
feasible radiuses of curvature, providing a sufficient play of the 
coating, and that account must be taken of welding requirements as well as 
of resistance to corrosion and abrasion. 
Other features of the invention will appear from the following description, 
given merely by way of exemplar, of the tank according to the invention 
with respect to the accompanying drawing, in which: 
FIGS. 1a, 1b and 1c show three possible embodiments, respectively, 
corresponding to various uses; 
FIG. 2 is a general view of a deformable tight casing formed of welded 
ribbed metal plates; 
FIG. 3a is a detail view of a typical portion of said metal plates, and 
FIGS. 3b and 3c represent the main fittings; 
FIGS. 4 to 6 are various cross-sections of the coating, showing how the 
tight deformable casing is anchored; and 
FIGS. 7a to 7c are detail views relating to advantageous fittings forming 
part of the coating according to FIG. 6.

As shown in FIG. 1a the tank for pressurized fluids according to the 
invention comprises a cavity 1 dug underground via mineshaft 2 and 
galleries 3. FIG. 1a shows the shape of cavity 1 and an arrangement of 
galleries 3 corresponding to the storage of a pressurized hot liquid 
according to the so called "balancing" conventional method, i.e., as 
follows: 
The cavity being filled up to the top with hot liquid surmounting the cold 
liquid, of higher specific weight, filling the cavity-bottom, the whole 
unit operates through transfer of the cold liquid from the cavity lower 
portion towards transfer-and-heating means M, and transfer of the water 
heated by said means M towards the upper portion of cavity 1, in the 
storage step, on the one hand, and reverse flow through ducts 4 with 
absorption of heat by said means M, in the exhaust step. 
Cavity 1, if necessary, is provided with a suitable coat 5; tight 
deformable casing 6 spread over the cavity wall contains a liquid 7. The 
connection of means M with a surface network providing the transfer of a 
coolant fluid from a source S to a station of use, is obtained through 
ducts 8. FIG. 1b shows a variant corresponding to lower pressures, the 
caving being dug directly from the ground-level according to the so-called 
"covered trench" method. Cavity 1 is closed, at the upper portion thereof, 
by a veil of concrete or a metal structure 9, capable of withstanding the 
weight of filling earth 10 and leaning against moulded walls 11. Tight 
deformable casing 6 is applied against said moulded walls 11 and veil 9 
and also against the bottom of the excavation, either directly or through 
a suitable coat 5. 
The above described means M of FIG. 1a are mounted in a hole 12 defined by 
a moulded wall 13. 
FIG. 1c shows a possible embodiment of tank for pressurized gas according 
to the invention. A cavity 1 has been dug via a shaft 2 and a gallery 3 
and is no more than a horizontal gallery of larger cross-section. 
Tight deformable casing 6 is spred over the wall of cavity 1, either 
directly or through a suitable coat 5. Pressurized gas is fed into, and 
from, cavity 1 by means of a duct 14 connecting the tank to filling and 
exhaust means M' via shaft 2. 
While FIGS. 1a to 1c comprise but a single cavity, quite obviously a tank 
according to the invention can comprise a plurality of cavities of various 
shapes and arranged in a number of possible ways; in fact, the main 
feature of the tank according to the invention lies in the presence of 
tight deformable casing 6, whatever the use, shape and size of the thus 
defined space may be. 
In FIG. 2 is shown a possible embodiment of a tight deformable casing, 
constituted by a plurality of metal sheets, arc-welded or welded according 
to any suitable method, comprising an appropriate number of ribs of 
suitable shape adapted to provide the free-play of said plurality of 
plates between their anchoring points. Preferably, said plurality of metal 
plates is constituted by an assembly of embossed metal sheets provided 
with ribs that comprise one or several corrugations, said sheets being of 
two different types, viz, anchoring sheets 15 and connecting sheets 16, 
the latter being joined by means of welds 17, either butt-welds or 
lap-welds, or by a metal strip (not shown). 
Connecting sheets 16 are but plain flat sheets in which a median rib 18 has 
been embossed. Said connecting sheets are supported only by adjacent 
anchoring sheets and have no anchoring point whatever in the sub-jacent 
material. 
Anchoring sheets 15 are provided with ribs 19 and 20 at right angles that 
meet at the sheet center through a distributing rib 21 surrounding 
anchorage device 22. 
The tight deformable casing according to the latter embodiment has a number 
of advantages: 
The metal sheets can be formed merely by an embossing operation, contrary 
to those sheets for liquid-gas tanks, the latter sheets having to be 
treated according to more intricate methods, in view of the fact that they 
are subjected to temperatures at which the metal becomes brittle; 
Since but two types of metal-sheets are used, the present invention allows 
an easy prefabrication of said sheets; 
FIG. 2 corresponds to the coating of a developable surface, but it is easy 
matter to adjust a sheet to the accurate dimensions of the cavity and to 
cover non-developable surfaces. The anchoring sheets 15 are usually used 
in their entirely, but, in case of need, it is possible to tailor them, 
taking great care, however, not to cut distribution rib 21. 
On the other hand, connecting sheets 16, being of simple structure with but 
a single rib, can be adjusted to the cavity dimensions. By cutting out 
portions of, e.g., triangular shape, in said connecting sheets, it is 
possible to warp the whole metal-coating of the cavity and apply same on 
surfaces of spherical or ogival shape or the like. 
In FIG. 3a are shown distribution-rib 21 and anchorage device 22 more in 
detail. 
Distribution-rib 21 is in the shape of a polygon or a closed curved line, 
e.g., a circle as shown in the figure. It is substantially more portruding 
than adjoining perpendicular ribs 19 and 20. 
At four places, in the vicinity of the intersections of distribution rib 21 
with the bisectors of the angles defined by perpendicular ribs 19 and 20, 
said distribution rib 21 is provided with saddle-shaped carvings 23 
adapted to lower the level of its ridge so as to give the latter 
substantially the same height as the tops of perpendicular ribs 19, 20. 
Anchorage device 22 is constituted by a sleeve, or bushing, encircling 
anchorage hole 24. 
For clearness sake, all the ribs of sheets 15 and 16 including distribution 
rib 21, have been shown in the figure as comprising but a single 
corrugation, or wave; however, it is quite abvious that larger movements 
can be obtained without increasing neither the thickness (and, therefore, 
the weight) of the tight deformable casing, nor the cost and stiffness 
thereof, by using sheets with ribs comprising several parallel ribs. 
FIGS. 3b and 3c show a possible embodiment of weldable fittings for 
rendering tight the deformable casing according to FIGS. 2 and 3a. 
More precisely, FIG. 3b shows a connecting member to be used for applying 
the tight deformable casing agains non developable surfaces, assuming ribs 
with only one corrugation (such as ribs 18, 19 and 20) are used. Quite 
obviously connecting members with several corrugations might be 
contemplated. This connecting member is used as follows: the cuttings of, 
e.g., triangular shape, made in connecting sheets 16 in the case of non 
developable surfaces obviously cause anchorage sheets 15 to draw nearer to 
each other. Whenever their spacing might become too narrow, it is 
advisable to exchange some of them (as a rule, every second one) for 
connecting sheets 16. The latter are usually welded as an extension of 
anchorage sheets, except at interrupted rib 20 where, for ensuring 
tightness, it is necessary to provide a connecting member, such as that of 
FIG. 3b., welded astraddle both metal sheets. 
FIG. 3c shows a cup shaped sealing cap 25 of thick metal sheet, the 
diameter of which is the same as that of anchoring sleeve, or bushing, 22 
as shown in FIG. 3a. 
Once an anchoring sheet 15 has been fixed to the cavity wall (or to a 
suitable coat first applied to said wall), sealing cap 25 is welded to 
said sleeve of anchoring device 22. Fluid leakages through anchoring hole 
24 are thus prevented. 
It is to be noted that, in FIG. 3c, an externally threaded pin 26, provided 
with a nut, is welded to sealing cap 25; in fact, a ring or any other 
suitable fastening device might be used instead of said threaded pin. 
Such an optimal arrangement permits to use sealing caps 25 as fastening 
means for scaffolding or any devices used in the course of building the 
tank, or for maintenance operations or repairs. 
As shown in FIG. 4, tight deformable casing 6 (which can with advantage be 
of the type of FIGS. 2 and 3a) rests on a concrete lining 27 permitting to 
give a definitive shape to the cavity wall, applied before fixing tight 
deformable casing 6 by means of anchoring rods 28 passing through 
anchoring holes 24; anchoring nuts 29 are screwed on the externally 
threaded ends of said rods 28. Sealing cap 25 is subsequently welded to 
the sleeve of anchoring device 22. 
Again in FIG. 4 are shown anchoring bolts 30 provided with a distributing 
plate 31 and an anchoring device 32, the whole being arranged according to 
usual practice and constituting a paramount supporting means for such 
cavities as those used as tanks according to the invention. 
It can be contemplated to insert a lubricant substance 33 (or any other 
product likely to lessen friction forces at the operating temperatures 
involved), between tight metal casing 6 and the subjacent material. 
Such an arrangement, which is to be found in FIGS. 5 and 6, makes it 
possible to restrict abrasion resulting from friction between the 
deformable casing and the subjacent material, and therefore proves to be 
favorable whatever the shape of said casing and of the subjacent material 
may be. 
FIG. 5 shows another embodiment of the coat against which tight deformable 
casing 6 is applied. As in the case of FIG. 4, anchoring bolts 30 are 
provided, with a distributing plate 31 and an anchoring device 32, said 
bolt still constituting a paramount supporting means. The cavity walls are 
rendered smoother by means of a concrete lining 27 as above. However, in 
the present instance, between tight deformable casing 6 and concrete 
lining 27, is sandwiched a thermally insulating material 34 of appropriate 
thickness, capable of withstanding the tank internal pressure transmitted 
through casing 6. 
In FIG. 5, such thermally insulating medium, by way of example, is 
constituted by a concrete layer of low thermal conductivity, in which can 
be provided expansion joints 35 since tightness is ensured by casing 6. 
Said thermally insulating medium 34 is fastened to concrete lining 27 by 
means of hooks 36, whereas the tight deformable casing is fixed, as shown 
in FIG. 4, by means of anchoring rods 28 inserted into the thermally 
insulating material. Preferably, hooks 26 are not in alignment with 
anchoring bolts 30 and anchoring rods 30, nor in the immediate vicinity 
thereof, so as to avoid or restrict, thermal bridges. Such an arrangement 
permits to limit the thermal flow between the fluid in the tank and the 
surrounding sub-soil formations. 
FIG. 6 shows a more elaborate embodiment of the coat against which 
deformable casing 6 is applied. Generally of a structure similar to that 
shown in FIG. 5, the coat, between concrete lining 27 and thermally 
insulating material 34, additionally comprises a network of ducts 37 in 
which flows a coolant fluid. With a view to restricting the number of 
ducts 37, it is preferable to provide a metal grid, a lattice of expanded 
metal or any other suitable thermally conductive material 38, thermally 
connected to ducts 37 and hooks 36, e.g., by welding spots 39,40, 
respectively. Such grid, lattice or thermally conductive material 
constitute a substantially isothermal surface at the average temperature 
of the coolant fluid flowing in ducts 37. The circulation and cooling down 
of said fluid are obtained through pumping means and exchangers, e.g., air 
coolants or means for exchanging heat with a heat sink (a river or the 
sea), such auxiliary means being outside the cavity and not shown. 
The amount of heat thus dissipated varies only according to the temperature 
differential between coating 6 and the coolant fluid, to the thickness of 
thermally insulating medium 34 and to the thermal conductivity thereof. A 
coherent selection of the values of these parameters will make it possible 
to restrict the dissipation of heat to a reasonable value. In order to 
avoid troubles that might result from the thermal expansion of anchoring 
bolts 30. it is preferable to maintain said bolts at the same temperature 
as the coolant fluid. To this end is diagrammatically shown, in FIG. 6, a 
connecting box 41 connected to ducts 37 around the head of an anchoring 
bolt 30. Thermal continuity between anchoring bolt 30 and connecting box 
41 is preferably obtained through the contact of the latter with 
distributing box 31, of usual design, used for maintaining anchoring bolt 
30 in traction by means of nut 42. Connecting box 41 is maintained in 
position by means of a counter-plate 43 contributing to ensure the 
requested thermal continuity, said counter-plate being attached to 
anchoring bolt 30 by means of an anchoring nut 44. The type of cavity coat 
as shown in FIG. 6 is especially suited for high temperatures. Inserting a 
thermally insulating medium, although restricting the flow of heat, is not 
sufficient however for preventing the temperature of adjacent sub-soil 
formations from increasing gradually, which may give rise to troubles 
resulting from induced mechanical stresses. 
While the calculation of such stresses is already intricate in the case of 
a well known medium, it can lead to a severe disappointment when applied 
to a natural medium, the parameters of which are, of necessity, not very 
well known. The rise of the soil temperature therefore leads to hazards 
which, in practice, it is impossible to assess as regards the tank 
stability. The arrangement of FIG. 6 permits to delete, or at least 
lessen, the soil temperature increase. FIGS. 7a and 7b shown details of a 
possible embodiment of connecting box 41, in cross section and as seen 
from above, respectively. In FIG. 7a are show in dotted line an anchoring 
bolt 30, with its distributing plate 31 and its anchoring nut 42. Such a 
bolt is fixed as follows: after having drilled a bore of suitable length 
in the cavity wall, the bolt is provisionally fixed to the bore end. 
A flexible tube is forced into the annular space defined between the 
anchoring bolt and the soil; distribution plate 31 is then inserted and 
anchoring nut 42 is only partially screwed so as to allow the flexible 
tube to penetrate freely. A yoke (not shown) provided with two jacks 
applied against the sub-soil formations is previously screwed instead of 
anchoring nut 44, then anchoring bolt 30 is put in tension by means of 
said jacks. Then concrete is injected through the flexible tube, the 
latter being gradually extracted from the bore. 
Once a sufficient amount of concrete has been injected, the flexible tube 
is withdrawn and plate 31 is locked by means of nut 42. 
When the concrete is set, the jacks are released and the yoke is unscrewed. 
It is then possible to mount box 41 and to lock same by means of 
counter-plate 43 and anchoring nut 44. It is, then, only sufficient to 
make connections with ducts 37 by means of crooked fittings 45 (FIG. 7a). 
FIGS. 7a and 7b permit to understand the principle of box 41. In these 
figures said box 41 is assumed to be of circular shape and constituted by 
welded sheets; however, neither the shape of these boxes, nor the way they 
are manufactured, nor the material of which they are made, are specific 
features of the invention. 
In the example represented in FIGS. 7a and 7b, the box is made of thin 
metal sheet and it is all the thinner as thermally insulating material 34 
is thicker and stiffer. Box 41 is of annular shape and comprises several 
threaded ports in its upper surface. FIGS. 7a and 7b represent a box with 
four ports, each of which is provided with a crooked fitting 45 (only one 
of which is shown in the figure). 
FIG. 7c shows, seen from above, a set of two half boxes (41a, 41b) adapted 
to ensure the cooling of anchorage bolt 30 via two distinct circuits, 
which, in some cases, can be advantageous as regards the general tank 
safety.