WO1989002864A1 - A plant for pressurized storing of natural gases - Google Patents

Abstract

A plant for pressurized storing of natural gases includes one or more rock cisterns with a rock cover where the rock mass has a weight at least corresponding to the inner maximum cistern pressure against the cistern roof. The inner rock surfaces (2) which are reinforced through anchoring bolting or similar are lined with a sprayed concrete layer (4-8), a sealing layer including at least one reinforced (14) and one unreinforced plastics layer (16) being provided thereon via a binding layer (12).

Description

A plant for pressurized storing of natural gases.

The present invention relates to a plant for pressurized storing of natural gases.

For storing petroleum products and other liquids in concrete cisterns or in uncovered or sprayed concrete reinforced rock cavities there are since long well tried sealing systems which are built from plastics and elastomers, e.g. according to Swedish patent specification 185422.

Such a system copes well with the stresses normally occurring at storing petroleum products, where the storing pressure is moderate, usual not above 1 MPa. This results in making the demands on static resistance of the sealing moderate. Leakage also becomes neglectable due to the fact that the permeability of the liquid petroleum products is ery low and the pressure gradient through the plastics layers is low. For pressurized storing of natural gases, at first hand those mainly consisting of methane and ethane, and of so called air gases, i.e. nitrogene, argon etcetera, high storing pressures are required in order to make the plants economical. The pressures involved can be 10 to 20 MPa. There are today, and will be in future, a great need for storing large quantities of natural gas. In existing methods for this purpose the gas is stored in the form of condensed gas at atmospheric pressure. An example is the storing of LNG at a temperature of appr. -160°C. For the gas industry it would be desirable to be able to store great quantities of natural gas in gas form for seasonal storing, operational reserve storing or short time regulation, at first hand in connection with the gas nets. One should then be able to store the gas at high pressure only by means of a compressor plant and then feed the gas into the conduit directly from the store. One has, however, thus far not been able to solve the problem as to how great storing plants shall be designed for the high pressures, which are necessary in order to obtain good storing economy.

The present invention has for its aim to provide a gas storing plant that fulfils the just mentioned desires and solves said problem. This has been attained in that the gas storing plant according to the invention has obtained the features stated in claim 1.

According to one of the features of the invention the gas store is thus executed with one or more rock cisterns, where the rock cover may be determined by different criteria of which one is that the rock above the cisterns must have a weight at least corresponding to the inner maximum cistern pressure against the cistern roof. This is calculated in accordance with current geotechnical calculating methods and the depth is selected with sufficient safety margins.

Also other criteria, such as tensile strength criteria for the loaded surrounding rock mass can be used for controlling that the rock has the ability to carry the load from the gas pressure in the cistern with acceptable tensions and movements. Thus, the rock mass will be entirely responsive for the carrying function, i.e. take the inner pressure that exists in every point of the cistern surface. The sealing function lies upon the plastics coating of the sealing lining. In order to obtain an even support for the plastics cover and for securing the sealing against falling rock portions, if any, the rock surfaces are provided with the necessary anchor bolting and with a stabilizing, smoothed sprayed concrete.

On the sprayed concrete the plastics sealing is applied which in every point becomes locally anchored in the sprayed concrete which in turn is well anchored locally in the rock surface.

The plastics layer becomes exposed to different forces and the layer must be able to cope with all forces with very good margins, since the plastics layer by virtue of its sealing function is of a decisive importance for the safety of the plant "and the environment. The forces to which the sealing layer design is exposed or may be brought to be exposed to are the following.

Pf = inner gas overpressure (max 10 a 20 MPa). T = tangential stresses (tensile stresses) in the layer due to the diameter increase resulting from the compressibility of the rock. (Insignificant, mostly neglectable forces. At single curved surfaces stresses in one direction and double curved surface stresses in two directions.)

Py = adhesion tensions (tensile stresses) in the sealing layer due to underpressure in the cistern or if an outer gas overpressure should appear in the surrounding rock. (An entirely theoretical case).

D, S = tensile and shear stresses, respectively, appearing in the plastics lining over and in the vicinity of an opening or opened crack in the (sprayed concrete) support. Hd, Hs = tensile and shear stresses, respectively, appearing in the plastics lining due to height displacements between the edges of a crack. Bd, Bs = tensile and shear stresses, respectively, appearing in the plastics lining at side displacements between the edges of a crack, whereby most often buckling phenomena appear. I = impact loads from e.g. falling objects. Since the materials included in the plastics lining have different coefficients of elasticity, which usually are not linear, it is difficult to deal with the stress conditions at cracks in a theoretical way. It has, however, turned out that the layer designs and the dimensioning of the layers can build upon conclusions from analogous models and from practical tests.

In practice, with the dimensioning rules that should be used at dimensioning rock cavities and linings, the widening of existing or appearing cracks is limited to appr. 1,0 mm and appr. 2 mm as a maximum. This means that the resulting rock stresses in the completed plant must be carefully calculated and (most preferably) be checked in the blasted rock cavity. The plastics layer is, however, dimensioned so that (in laboratory tests) it can cope with crack widths of 20 mm or more at maximum cistern pressure.

Of the stresses and stress systems which can appear in a storing system according to the invention almost only those stresses are important, which appear in the plastics layer if a crack is opened in the underlying support. The stresses appearing at a height displacement or mutual displacement of the edges along the crack is of little or neglectable importance except in case of earth cracks or heavy military attacks.

The sealing lining provided according to the earlier mentioned Swedish patent 185.422 can cope with smaller cracks up to 5 to 10 mm as a maximum at moderate cistern pressures, say 1,0 to MPa as a maximum. The pressure applicable at storing of natural gas (or other gases with boiling point at or below the boiling point of natural gas) at normal rock temperature or environment temperature (appr. -30°C to +50°C) becomes essentially higher for economic reasons, viz. of an order of magnitude of 10,0 to 15,0 MPa. At the high loads on rock, concrete/sprayed concrete support and plastics layer then appearing, the safety against breakage in these designs must be essentially higher than for cistern designs for petroleum products or petroleum gases at low storing pressures. Since gas stores at high pressure imply that ve y great amounts of pressure energy is stored in each cistern unit it is decisive for the practicability and allowability of the storing method that the sealing function is so overdimensioned that it gives considerably higher safety factors than what is else common for cistern designs, also those which are pressurized.

A high degree of safety is also required due to the fact that it is not possible to establish the material properties of the rock mass with the same accuracy as for usual constructing materials such as steel and concrete. In this case it is therefore necessary to apply another philosophy of design than what is usual. The safety discussions build upon relatively coarse assumptions but lead to a sufficient safety factor without any important extra material costs. One then begins to calculate e.g. the maximum crack which can appear if all the crack movements appearing due to the tensile stresses within a certain zone, e.g. a corner, would be gathered to a single crack (which is unlikely per se). This total movement can e.g. in one case be calculated to amount to 2 mm. One thereafter arbitrarily determines that the lining at full operational pressure shall be able to bridge a crack that is 10 times wider, i.e. 20 mm. From logical reasons one then knows that the safety factor against bending tensile rupture for the crack width 2 mm is considerably higher than 10. In order to also cope with irregularities, if any, in the underlying support, e.g. hypothetical uneven yieldings in the support, the plant is dimensioned to also carry moderate such yielding movements.

In the relatively thin sealing design according to the invention it is difficult to theoretically dimension the lining for the loads appearing from said presupposed and unpressupposed movements.

Therefore the design is tested by loading it at still greater crack widths and still higher loads than those stated above. Thus, the plant is e.g. tested at a crack width of 40 mm instead of 20 mm and at a testing pressure 200 bar instead of 150 bar. By this it is possible to establish that also the safety factor against cutting off ruptures at the edges is high enough. The lining is presupposed also to be able to bridge a height displacement between the crack edges of 10 mm in a long-duration test.

These high demands cannot be fulfilled with any sealing design used today. The sealing plastics lining according to the present invention is, however, cab!able of doing this. In one embodiment the adhesion between the plastics lining and the support concrete/sprayed concrete is obtained by means of a primer layer of epoxy resin plant. Thereon one or more rounds of a soft binding layer, suitably of soft urethane elastomer with a hardness of 25° to 50° Shore A and a break elongation of 400 % or more.

On the binding layer one or more rounds of a reinforced sealing layer are applied. The layer is usually made from a relatively rigid although flexible epoxy resin and the reinforcement usually of glas fibres in the form of cut glass fibre rowing.

On the reinforced sealing layer there follows an unreinforced sealing layer which, with the material systems now available, is suitably made of a urethane elastomer which is more rigid than the material in the binding layer.

According to a modified embodiment the above-mentioned two sealing layers are replaced by a bending resistant double reinforced plate of thermosetting resin which is made with a reinforced thermosetting resin layer on each side of an unreinforced thermosetting plastics layer.

By designing the sealing layer in this case as a strong double reinforced plate the following is obtained.

On the one hand an extra safety against tensile breaks due to primarily those stresses which can appear in the section over a crack, if any, in the support.

On the other hand, a bending stiffness that copes with the movements which can appear e.g. due to different yieldings in the support due to different stiffness of different parts of the rock mass around the rock cavity. The bending stiffness is also needed over cracks appearing in the support, but also locally e.g. at concrete shutters towards drifts and shafts, where considerable differences in stress conditions and elasticity features can appear.

At last one or more often thin surface layers (approximately 0,25 mm) follow, which i.a. can serve to decrease the total permeability of the plant. In order to cope with crack movements without tearing away of the sealing layer the above-mentioned binding layer is made in the form of a soft binding layer which can be deformed by shearing at the appearance of a crack and then in the form of limited shear stresses can transfer the stresses between the support/sprayed concrete and the overlying sealing layer.

The invention will now be described more closely with reference to two embodiments shown on the attached drawings.

Both Figures schematically illustrate in perspective and not necessarily at scale, a section through a stabilizing and sealing layer in a plant according to the invention for pressurized storing of natural gases.

The plant according to the invention is composed of one or more rock cisterns with a rock cover, where the rock mass has a weight at least corresponding to the inner maximum cistern pressure against the cistern roof. The inner rock surfaces are reinforced by anchoring bolting.

The cisterns can be made in a manner known per se and need therefore not be shown or described more closer here. A suitable embodiment is, however, a system, known per se, of a number of mutually communicating rock caverns in the form of standing cylinders.

In Fig. 1 an even blasted rock surface is designated 2. A sprayed concrete layer on it includes an unreinforced smoothed surface layer 4, a steel fibre reinforced layer 6, and an unrein¬ forced smoothing layer 8.

The adhesion between the plastics lining and the support concrete/sprayed concrete is obtained by means of an approximately 0,75 mm thick primer layer 10 of epoxy resin in two rounds. On this one or more rounds of a soft binding layer 12 is arranged, suitably of soft urethane elastomer with a hardness of 25° to 50° shore A and a break elongation of 400 % or more. The layer in its normal appearance has a thickness of approximately 3 mm but where greater crack movements may be expected the layer can be made thicker so that a desired movement can be taken by the layer. On the binding layer a reinforced sealing layer 14 is applied. The layer is usually made from a relatively rigid although flexible epoxy resin and the reinforcement usually of glass fibres in the form of cut glass fibre roving. The layer is applied in one or more round to a thickness of normally 4 mm but the thickness can be increased at particularly loaded portions, in practice hardly to more than approximately 8 mm.

On the reinforced sealing layer 14 follows an unreinforced sealing layer 16 which, with material systems now available, is suitably made of a urethane elastomer which is more rigid than the material in the binding layer, approximately 60° to 80° Shore A. The break elongation should be approximately 200 %. The layer is sprayed in one, two or three rounds to a thickness of an order of magnitude of 3 to 4 mm.

At last follows one or more often thin surface layers 18 (approximately 0,25 mm) which i.a. can serve to decrease the total permeability of the plant and give the plant antistatic properties. The total layer thickness becomes, on an average, appr. 10 mm, but depending on storing pressure, type of gas etcetera, both thicker and thinner layers can be suitable.

In Fig. 2 the rock surface 2 and the layers 4, 6, 8 are shown again. As in Fig. 1 the stabilizing smoothed sprayed concrete support has been applied after the rock surfaces have been provided with the necessary anchoring bolting. On the concrete surface two primar layers 20, 22 are applied which seal the pores and capillaries of the concrete surface. Thereafter follows an elastomer binding layer 24 which is applied in preferably more rounds. The binding layer 24 is chosen also here so that it can be deformed by shearing when a crack occurs, and then can transfer stresses, in the form of limited shear stresses, between the sprayed concrete and the overlying sealing layer described below. On the binding layer 24 a composite sealing layer is arranged consisting of, in turn, a glass fibre reinforced thermo¬ setting plastics layer 26, an unreinforced thermosetting plastics layer 28, and a glass fibre reinforced thermosetting plastics layer 30. The glass fibre reinforcement is laid in the lower and upper layers 26 and 30, respectively, of the sealing layer for providing therein the best possible stiffening. The unreinforced intermediate layer 28 is usually provided with metal and glass flakes having for their purpose to lengthen the leakage passages for the gas through the layers and decreasing the permeability of the layers thereby. On the; compooed sealing layer 26 - 30 a surface layer 32 is finally applied consisting of pigmented thermosetting resin or a plastics with special properties in accordance with the earlier embodiment. In the table below a suitable thickness of the layers shown in Fig. 2 are given as examples.

First primer layer 20 0,3 mm

Second " " 22 0,3 mm-

Elastomer layer 24 213 mm Glass fibre reinforced thermo¬ setting resin layer 26 3 mm Unreinforced thermosetting resin layer 28 2 mm Glass fibre reinforced thermosetting layer 30 2 mm

Two surface layers 32 of thermosetting resin or any other corresponding resin, each 0,3 mm.

By making the sealing layer as a double reinforced plate in this way it is able to cope with the punching forces appearing over a broad crack. With the layer thicknesses given as an example the layer is able to cope with a crack width of 10 mm at an inner overpressure of 20 MPa without any risk for punching through or other type of breakage.

Claims

Claims.

1. A plant for pressurized storing of natural gases, characterized in that, in one or more rock cisterns with a rock cover where the rock mass has a weight at least corresponding to the inner maximum cistern pressure against the cistern roof, are the inner rock surfaces (2) which are reinforced by means of anchoring bolting or similar, lined with a concrete layer, such as sprayed concrete layer (4-8) or similar, on which sealing layers including at least one reinforced (14;26,30) and one unreinforced plastics layer (16;28).are applied via a binding layer (12;24).

2. A plant according to claim 1, characterized in that the binding layer (12;24) is a thick elastomer layer which, at the - appearance of a crack can be deformed by shearing and then transmits the stresses, in the form of limited shear stresses, between the concrete layer and the sealing layer.

3. A plant according to any of the preceding claims, characterized in that between the sprayed concrete lining (4-8) and the binding layer (24) a pri ar layer (10;20,22) is arranged that secures addition to the sprayed concrete lining.

4. A plant according to any of the preceding claims, characterized in that the sealing layer, as seen from the binding layer (12), consists of a glass fibre reinforced epoxy resin layer (14) in one or more rounds, and an unreinforced thermosetting resin layer (16) of urethane elastomer in one or more rounds.

5. A plant according to any of claims 1 - 3, characterized in that the sealing layer is made as a bending resistive double reinforced plate of thermosetting resin and including a reinforced thermosetting resin layer (26,30) on each side of an unreinforced thermosetting resin layer (28).

6. A plant according to claim 5, characterized in that the unreinforced thermosetting resin layer (28) includes metal, mineral, or glass flakes.

7. A plant according to any of the preceding claims, characterized in that a surface layer (18,32) is applied on the sealing layer.

8. A plant according to claim 7, characterized in that the surface layer (18;32) includes metal or glass flakes with the purpose at lengthening the leak passages for the gas through the layer.