Apparatus for the storage of gases

Abstract

A storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises: a shaft (8) of substantially circular cross section provided in a rock mass (22); and within the

Description

FIELD OF THE INVENTION The present invention relates to apparatus for the storage of gases, especially apparatus for the underground storage of fuel gases. In particular, the invention relates to a lined rock shaft suitable for the storage of pressurised fuel gases, and especially for the storage of pressurised gaseous hydrogen. BACKGROUND OF THE INVENTION There are several types of facility for the storage of gas including storage underground. One example of this is storage in existing geological formations, such as salt caverns and depleted oil and gas fields. Alternatively, pressurised or liquified gas can be stored in lined rock caverns (LRCs). The lined rock caverns are formed in hard rock and a hydrogen-impermeable lining prevents leakage of gas. An example of a gas storage system of this type is described in US 2001 / 0002969, which relates to a lined rock cavity for storing natural gas at high pressures. The lining consists of an outer concrete layer and an inner impermeable lining layer, typically formed from steel plate, with a sliding layer between them. The sliding layer allows relative movement of the concrete layer and the lining layer. The concrete layer also comprises a welded mesh reinforcement layer which is intended to control the size and distribution of cracks in the concrete. US 2013 / 0336721 describes a storage system for a fuel gas such as methane in a lined underground reservoir. This reservoir is a cavity in a crystalline bedrock which is lined with steel and concrete which enables the cavity to serve as a storage vessel for fluids such as natural gas or air at a high pressure. The lining is composed of an inner steel layer and an outer layer of reinforced concrete. There is a sliding layer between the inner and outer layers which allows for relative slippage between them and reduces local strains. EP2832666A also relates to an installation for storage of compressed gas such as air. The installation comprises a cavity for containing the gas having an impermeable flexible lining, a sliding intermediate layer and a crack-controlling concrete outer layer adjacent to the rock. These storage systems are designed for the storage of methane or compressed air. It is an object of the present invention to provide a storage system which is suitable for a wide range of gases, but particularly hydrogen. PROBLEM TO BE SOLVED BY THE INVENTION Known systems of storing gases underground require the surrounding rock to be a hard or impermeable rock. The present invention aims to provide a robust and cost-effective storage vessel suitable for containing hydrogen, which is larger than conventional above-ground storage and which is suitable for use in a wide range of rock types, some of which are significantly softer than the rock of traditional storage facilities. This significantly reduces the cost of constructing a shaft in the rock. Furthermore, the storage vessels of the invention can be installed at locations which would not be possible for traditional storage facilities, meaning that they can be located close to the end user. A further aim of the invention is to reduce the cost of the materials used for the hydrogen-impermeable lining of the storage vessel by providing a liner which is thinner than that seen in conventional storage systems such as lined rock caverns. Because hydrogen is an extremely small molecule, it is significantly more difficult to prevent diffusion of hydrogen from a storage vessel than is the case with a gas such as methane, in which the molecules are much larger in size. It is therefore important to ensure that the lining of the storage apparatus is impermeable to hydrogen. The present invention makes provision for a storage apparatus in which the dimensions of each layer of the liner can be varied according to the properties of the rock type in which the apparatus is installed. Given the danger of explosion when hydrogen is brought into contact with air, it is important to ensure that, if hydrogen does leak from the inner cavity of the storage apparatus, it does not accumulate in the layers surrounding the inner cavity. A solution to this problem is also provided by the present invention. SUMMARY OF THE INVENTION In accordance with a first aspect of the invention, there is provided a storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises: a shaft (8), preferably of substantially circular cross section, provided in a rock mass (22); and within the shaft: an inner cavity (12) for the storage of a gas; a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity; a sliding layer (16) surrounding the hydrogen-impermeable inner liner; a structural liner (18) surrounding the sliding layer; and an overburden (26) above the upper cap (34); wherein the hydrogen-impermeable inner liner (14) is a thin-walled cylinder in which: d' 1 <20 wherein t is the wall thickness of the hydrogen-impermeable inner liner (14); d' is the internal diameter of the hydrogen-impermeable inner liner (14), and wherein the hydrogen-impermeable inner liner (14) is adapted to deform without failing on application of pressure transferred from the rock mass (22) via the structural liner (18) and the sliding layer (16) such that the thickness (by which it is meant wall thickness), t, of the hydrogen-impermeable inner liner (14) is expressed by the inequality: Pi x d' t >--- 2cra wherein: pi is the internal pressure in the hydrogen-impermeable inner liner (14);and Oa is the pennissible stress of the hydrogen-impermeable inner liner material. In a second aspect of the invention, there is provided a storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises: a shaft (8) of substantially circular cross section provided in a rock mass (22); and within the shaft: an inner cavity (12) for the storage of a gas; a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity; a sliding layer (16) surrounding the hydrogen-impermeable inner liner; and a structural liner (18) surrounding the sliding layer; and an overburden (26) above the upper cap (34); wherein the shaft (8) has a depth, i.e. wall height, (h) of 30 m to 500 m and a diameter (D) of Im to 18m; and wherein h >2D; and the hydrogen-impermeable inner liner (14) is formed from steel and has a thickness, t, of from 5 mm to 20 mm; and the overburden (26) has a height of 1 m to 50 m. In a third aspect of the invention, there is provided a storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises: a shaft (8), of preferably substantially circular cross section, provided in a rock mass (22); and within the shaft: an inner cavity (12) for the storage of a gas; a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity; a sliding layer (16) surrounding the hydrogen-impermeable inner liner; and a structural liner (18) surrounding the sliding layer; and an overburden (26) above the upper cap (34); wherein the gas-permeability of the structural liner (18) is higher than the gaspermeability of the inner liner (14) and is optionally lower than the gas-permeability of the overburden (26). In a further aspect, there is provided a method for constructing a storage vessel suitable for the storage of hydrogen, tire method comprising: sinking a shaft (8) in a rock mass (22) or using an existing shaft (8) in a rock mass (22); optionally providing a filling against a wall of the shaft 8 to form a filling layer 20; providing a hydrogen-impermeable inner liner 14 coated with a sliding layer 16 within the shaft 8, the hydrogen impermeable inner liner having an upper cap; filling an annular space between the wall of the shaft 8 and the sliding layer 16 with material to form a structural liner 18; and backfilling a space between the upper cap 34 and ground level with an overburden 26. Preferably, according to this aspect, the method further comprises providing an outer supporting layer 24, preferably made of steel, in the shaft between the hydrogen-impermeable inner liner 14 and the wall of the shaft 8 and preferably between the structural liner 18 and the wall of the shaft 8; and preferably the method further comprises filling an annular space between the wall of the shaft 8 and the outer supporting layer 24 with material to form a filling layer 20. ADVANTAGES OF THE INVENTION The present invention provides the advantages that the storage vessel can be constructed in a wide variety of bedrock types, not just in the hard rocks or impermeable geology in which known types of storage facilities were constructed. Storage vessels according to the invention have a different configuration from known lined rock caverns as they are significantly smaller in diameter for any given volume of gas stored. This means that it lias proved possible to construct a gas-tight storage vessel in which the inner liner is considerably thinner than that currently used in lined rock cavern storage systems, while still being able to withstand the internal pressure from the stored gas, the changes in pressure when the storage vessel is filled or emptied and the external pressure arising from ground pressure and hydrostatic pressure in the surrounding rock mass. In addition, the smaller diameter means that the height or quantity of the overburden required to withstand the internal pressure in the vessel is less, which means that less excavation is required, leading to a reduction in the cost of construction. In embodiments in which the hydrogen-impermeable inner liner surrounding the inner cavity is adapted to deform without failing on application of pressure transferred from the bedrock via the structural liner and the sliding layer, the leakage of hydrogen or other gas from the inner cavity is prevented if cracks are formed in the surrounding bedrock. Even if the hydrogen-impermeable inner liner does have some leakage, in embodiments in which tine gas-permeability of the structural liner is higher than the gas-permeability of the inner liner and is optionally lower than the gas-permeability of the overburden, hydrogen or other gas leaking from the inner cavity will not accumulate in or between the layers forming the lining of the storage vessel. Instead, any leaking hydrogen will diffuse through the layers forming the lining to the outermost layer, where it may rise to the surface or diffuse into the surrounding bedrock. In some cases, the storage vessel may be part of a system which comprises means for capturing any escaped gas (e.g. hydrogen) which rises to the surface after leaking from the inner cavity and passing into the structural liner. In some embodiments, it is possible to calculate the necessary minimum depth of the overburden in order to minimise the costs of drilling the shaft (8) and constructing the storage vessel. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partial horizontal cross section of a storage vessel according to one embodiment the invention. Figures 2A to 2C are a representation of the construction of a storage vessel according to one embodiment the invention. Figure 2A shows the boring of the shaft to the required depth and diameter using a large diameter rotary drill rig and shows that the shaft is filled with a drilling fluid; Figure 2B shows an inner liner being lowered into the shaft as it is constructed and shows that the buoyancy of the inner liner can be adjusted by adjusting the level of the drilling fluid in the shaft and in the inner liner; and Figure 2C is an illustrative representation of the completed inner liner in place with the end caps in place where the space surrounding the inner liner has been backfilled with material forming the structural liner.. Figure 3 is a schematic representation of one embodiment of a vessel of the invention positioned in a shaft with a backfill above the hemispherical upper cap of theinner liner and shows the hydrogen-impermeable inner liner, the structural liner and the filling layer as well as the overburden, which is held with anchor bolts. Figure 4 is a zoomed-in view of the upper part of Figure 3 showing in greater detail the upper cap of the inner liner, tire structural liner and the filling layer as well as the overburden and anchor bolts. Figures 5A to 5P illustrate as a sequence of drawings one embodiment of the method of construction and installation of a storage vessel for the storage of hydrogen, according to another aspect of the invention. DETAILED DESCRIPTION OF THE INVENTION Definitions In the present specification, except where the context requires otherwise due to express language or necessary implication, the word ‘"comprises”, or variations such as “comprise” or “comprising” is used in an inclusive sense i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. In the context of the present invention, the term “overburden” relates to the plug of backfilling material positioned above the upper cap of tire hydrogen-impermeable inner liner of the vessel, which is necessary to prevent the uplift from the operational pressure range of the vessel. The height of tire overburden therefore corresponds to the depth of the upper cap below ground level, i.e. if the vessel is positioned at a depth of 1 m to 50 m below ground level, the height of the overburden is 1 m to 50 m. The height of overburden required will depend on die operational pressure, the overburden density, the shaft radius, groundwater, properties of the plug material and rock / plug interface properties. In the context of the present invention, the term “rock mass” refers to the bedrock in which the reaction vessel of the invention is constructed. In tire context of the present invention, the terms “impermeable to hydrogen” and “hydrogen-impermeable” used in reference to the inner liner (14) mean that the permeability coefficient Pm is less than 5.0 x 10’". where Pm is defined as hydrogen loss through an inner liner of thickness 1cm and surface area of 1000 m2 at 1.0 MPa gas pressure and a temperature of 20 °C. Polymers such as polyester resin, polyurethane and epoxy resin have Pm of from about 1.4 x 10’11 to 4.5 x 10’11. Stainless steel is significantly less permeable to hydrogen than such polymers and has aPm of about 4.0 x IO’17 to 5.0 x IO47. The term pt refers to the internal pressure in the hydrogen-impenneable inner liner 14, i.e. the gas pressure in the inner cavity 12 transferred to the inner liner 14 and then to the surrounding rock mass 22. The term d' refers to the internal diameter of the hydrogen-impermeable inner liner 14, i.e. the diameter of the inner cavity 12. The term oa refers to the hoop stress in the hydrogen-impermeable inner liner 14, i.e. tensile stress which acts perpendicular to the longitudinal axis of the vessel and arises from the pressure of the gas inside the vessel. The term o« refers to the permissible stress in the hydrogen-impermeable inner liner 14 i.e. the maximum allowable design stress. Hie permissible stress will generally be determined by the designers of the vessel, who may consider factors including the materials used for the inner liner 14, the sliding layer 16 and the structural liner 18 and the layer geometry, as well as safety factors, as it is necessary to ensure that the inner liner 14 does not rupture in use. The term yield limit refers to the point on a stress-strain curve for a material which is the limit of elastic behaviour, i.e. below which the material will deform elastically and will return to its original shape. Above the yield limit, at least a part of the deformation will be permanent and non-reversible. In the present application, the rock mass deformation modulus (Erm) is used to define the rock deformation under stress caused by (pi) the internal pressure in the hydrogen-impermeable inner liner 14. The rock mass deformation modulus is defined empirically based on the estimated Geological Strength Index (GSI) values, intact rock strength properties and disturbances caused by excavation. The rock mass deformation modulus (Em) may be determined according to the equation (4) proposed by Hoek and Diederichs (2006). Erm = MR ■ ac • (0.02 +---....................... 1 ^60 + 151 / -05 / ^ \ 1+ / n 9 wherein oc is the uniaxial compressive strength of the intact rock; and MR is the modulus ratio; and GSI is the Geological Strength Index; and D is a factor which depends upon the degree of disturbance due to blast damage and stress relaxation. The higher the GSI and the uniaxial compressive strength (oc) of the intact rock, the higher rock mass deformation modulus (Erm) whereas increased disturbance (i.e. induced damage in the rock mass during excavation) may reduce stiffness. This application classifies rocks as very stiff (high deformation modulus), moderately stiff (medium deformation modulus), very weak (low deformation modulus) and the table below, shows how these classifications are related to rock mass deformation for an internal pressure pi. Rock Mass Deformation Modulus Rock Deformation (as % of shaft diameter) when subjected to internal pressure pi. Very Stiff High <0.02% Moderately Stiff Medium 0.02% - 0.34% Very Weak Low >0.34% Uniaxial compressive strength is used to define the strength of a rock mass. Uniaxial compressive strength, also known as unconfmed compressive strength (oc), is defined as the highest stress that a rock specimen can carry when a unidirectional stress is applied, normally in an axial direction to the ends of a cylindrical specimen. It represents the maximum load supported by a specimen during the test divided by the cross-sectional area of the specimen. Thus, oc is determined by a laboratory' test on an axially loaded sample of the rock, not a test on the entire rock mass. The oc of rock materials may be determined according to the standard BS EN ISO 14689:2018 Geotechnical investigation and testing. In the following description, embodiments of the invention relating to a component of the storage vessel of the invention may be combined with embodiments relating to other components of the invention. For example, an embodiment relating to features of the hydrogen-impermeable inner liner 14 may be combined with one or more embodiments relating to the structural liner 18 and / or one or more embodiments relating to the upper and lower end caps. Reference in the description and claims herein to reference numerals in the drawings is intended for illustrative purposes only, to aid in understanding, and is not intended to be limiting. In the storage vessel of the invention, the inner cavity 12, the hydrogen-impermeable layer 14, the sliding layer 16 and the structural liner 18 surrounding the sliding layer are all positioned within the shaft 8. The overburden 26 may also be positioned within tine shaft but, in some cases, may extend out of the shaft 8 above ground level. In some embodiments, storage vessels of the invention contain the features of two or all three of the first, second and third aspects of the invention. For example, in the first aspect of the invention, the hydrogen-impermeable inner liner 14 is a thin-walled cylinder in which: wherein t is the wall thickness of the hydrogen-impermeable inner liner); and d' is the internal diameter of the hydrogen-impermeable inner liner 14. Suitably, this condition also applies to the storage vessels of all other aspects of the invention. In the storage vessels 10 of the first, second and third aspects of the invention, the hydrogen or other gas is intended to be stored at a pressure of up to 30 MPa, more usually up to 25 MPa and typically up to 20 MPa. It is also important for the vessel to be able to withstand a reduction in pressure as it is emptied. The operating pressure may therefore be from 0 to 30 MPa, more usually 1 to 25 MPa or 3 to 20 MPa. In the second aspect of the present invention, the shaft 8 has a depth, i.e. wall height, (h) and a diameter (D) wherein: h >2D. Suitably, this condition also applies to the storage vessels of all other aspects of the invention. In the present invention, therefore, the shaft 8 suitably has a depth of 30m to 500m, more suitably about 100m to 400m, still more suitably about 250m to 350m, for example about 300m. The shaft diameter is suitably about Im to 18m, more suitably 1.5m to 15m and still more suitably 3m to 8m. As noted above, the height of the overburden 26 will depend on a number of factors, including the shaft radius, the operational pressure, the overburden density, groundwater, properties of the plug material and rock / plug interface properties. A further factor in determining the height of the overburden 26 is whether anchor members, for example anchor bolts 48, are provided to hold the backfilling material forming the overburden in place in the shaft 8. The use of such anchor members will increase the uplift resistance of the overburden 26 so that a lower height of overburden 26 can be used. For a shaft with a diameter of Im to 18m, die height of the overburden 26 is suitably Im to 50m. As an example, for a shaft 8 having a depth of about 300m and a diameter of about 4m, the overburden is suitably about 25m in height, while a shaft 8 of depth about 300m and a diameter of about 6m will suitably have an overburden 26 of about 35m in height (when no anchor bolts are used). In the vessel of the first aspect invention, the minimum thickness of the inner liner is expressed by the inequality Pi *d' 2aa wherein: t is the thickness of the hydrogen-impermeable inner liner 14 pi is the internal pressure in the hydrogen-impermeable inner liner 14; d' is the internal diameter of the hydrogen-impermeable inner liner 14; and Ga is the permissible stress. Suitably, the maximum thickness, t of the hydrogen-impermeable inner liner 14 is expressed by the inequality: 1-^ „ 2 _ where pi is internal pressure in the hydrogen-impermeable inner liner 14 d' is internal diameter of the hydrogen-impermeable inner liner 14 t is the thickness of the hydrogen-impermeable inner liner 14 El is the Young’s modulus of the hydrogen-impermeable inner liner 14 v; is the Poisson’s coefficient of the inner liner 14; and 8d is radial rock mass displacement. These maximum and minimum values for t may also apply to other aspects of the invention. Suitably, as shown in Figures 1 and 4, in each of the storage vessels 10 of the first, second and third aspects of the invention, there is a filling layer 20 provided between the rock mass 22 and the structural liner 18. The filling layer 20 may be formed from grout or a similar material and lias the functions of providing a smooth surface on the rock mass 22, sealing cracks in the rock mass 22 and mitigating corrosion of the structural liner. In some embodiments, the filling layer may have a radial thickness of from about 150 mm to 450 mm, suitably from 200 mm to 400 mm, more suitably from 250 mm to 350 mm and typically about 300 mm. In many embodiments of the invention, the storage vessel 10 may also include an outer supporting layer 24. The outer supporting layer 24 may not be necessary when the rock mass is very stiff and rock deformation small (for example below 0.02% of the shaft diameter D) such that the rock mass will be able to do all of the work necessaty to resist the internal pressure (pi) of the stored gas. Also, if the hydrogen-impermeable inner liner 14 is extremely thick, such that it is capable of supporting all of the pressure from the stored gas inside the vessel 10, the outer supporting layer 24 may not be necessary. In most other cases, however, the outer supporting layer 24 will be present. The outer supporting layer 24 may either be positioned around the outer edge of the structural liner 18, for example between the structural liner 18 and the filling layer 20, when present, as shown in Figure 1, or may be disposed within the structural liner 18, e.g. by way of reinforcement. In some cases, the material forming the outer supporting layer may be a continuous sheet or plate, which is substantially impermeable to hydrogen, and in other cases, the material may have openings, which allow gas to pass through it. For example, it may be a mesh. Typically, the outer supporting layer 24 is formed from steel. In some embodiments, the outer supporting layer comprises a layer of sheet steel positioned between the structural liner 18 and the filling layer 20 and having a thickness of 15 mm to 35 mm, suitably 20 mm to 30 mm, for example about 25 mm. The outer supporting layer may be formed from a lower end cap and wall sections. Typically, there is no upper end cap of the outer supporting layer. The walls of the outer supporting layer may extend upward to about the level of tine top of the inner liner or higher still and optionally up to the surface. Preferably, however, the outer supporting layer does not extend upward to the surface, but ends about the level of the top of the inner liner. In storage vessels of the invention in which the gas-permeability of the structural liner 18 is higher than the gas-permeability of the inner liner 14, in the event of a leakage of gas from the inner cavity 12 through tine inner liner 14, the higher permeability of the structural liner 18 will allow the gas to diffuse through it. When the outer supporting layer 24 is gas-impermeable, for example when the material from which it is formed is a continuous sheet of material (for example steel), the gas-permeability of the overburden 26 is suitably higher than the gas-permeability of the structural liner 18 so that escaped gas does not accumulate in the structural liner but diffuses into and through the overburden 26. In some embodiments, the escaped gas may be collected at the ground surface. In such embodiments, the overburden may be provided with means to collect any escaped gas, for example pipes through which escaped gas may be conducted to the surface and collected. In storage vessels in which the outer supporting layer 24 is gas-permeable, for example when it is not a continuous sheet of material, i.e. it has openings or is in the form of a mesh, gas which has escaped from the inner cavity 12 may diffuse through the inner liner 14 and the structural liner 18. Suitably, such escaped gas will pass into a filling layer 20, which is positioned between the structural liner 18 and the rock mass 22. The filling layer 22 may be provided with means to collect any escaped gas, for example pipes through which escaped gas may be conducted to the surface and collected. In storage vessels in which the outer supporting layer 24 is gas-permeable, the overburden may also be of higher permeability than the structural liner 18 so that, in addition to passing into the filling layer 20, escaped gas may also pass into and through the overburden 26. In such cases, the escaped gas may be collected at the ground surface and, as above, the overburden 26 may be provided with means to collect escaped gas, for example pipes through which escaped gas may be conducted to the surface and collected. As described above, a method of an aspect the invention for constructing a storage vessel may comprise the steps of: sinking a shaft 8 in a rock mass 22 (or adopting an existing shaft); constructing an outer supporting layer 24, preferably of steel, in the shaft; filling an annular space between the shaft 8 and the outer supporting layer 24 with material to form a filling layer 20; constructing a hydrogen-impermeable inner liner 14 coated with a sliding layer (16) within the outer supporting layer (24); filling an annular space between the outer supporting layer (24) and the sliding layer (16) with material to form a structural liner (18); backfilling a space between the upper cap (34) and ground level with an overburden (26). As noted above, the outer supporting layer may be formed from a lower end cap and wall sections. The lower end cap 28 and wall sections 30 of the supporting layer 24 may be welded together either before lowering into the shaft or welded in situ in the shaft. Preferably, the wall of the supporting layer is formed of a plurality of annuli, preferably of steel, which may be welded together to form the wall section 30. The inner cavity 12 of the storage vessel 10 of the invention suitably has side walls 33 of height of 25m to 425m, more suitably 80m to 320m. The hydrogen-impermeable inner liner 14 prevents leakage of hydrogen or other gas from tire inner cavity 12 of the vessel, and preferably defines the inner cavity 12 of the vessel 10, and the material from which it is constructed may be steel, other metallic material (e.g. aluminium) or a polymeric material such as Kevlar® (poly-paraphenylene terephthalamide (K29)), polyester resin, polyurethane, epoxy resin or epoxy resin comprising one or more additives. The hydrogen permeability coefficient Ph2 of the material used for the inner liner (14) is suitably less than 5 x 10'11, more suitably less than 5 x 10’13, 5 x IO’15 or 5 x 1 O’17. Polymers such as polyester resin, epoxy resin, epoxy resin mixtures and polyurethane may have hydrogen permeability coefficients of from about 1.4 x 1011 to about 4.5 x 10’11. Steel is less permeable to hydrogen than these polymers and typically lias a hydrogen permeability coefficient of not greater than 5 x 10'17, for example about 4.0 x IO'17 to 5.0 x IO'17. For example, stainless steel has a hydrogen permeability coefficient of about 4.64 x IO"17. Suitably, the hydrogen-impermeable inner liner is formed from steel, especially stainless steel. The rock mass 22 is likely to be subject to geological strain and cracking, and cracks and pressure may be transmitted via the structural liner 18 and the sliding layer 16, and any filling layer 20 and any outer supporting layer (24) (if present) to the hydrogen-impermeable inner liner 14. In the vessels of tire invention, the hydrogen-impermeable inner liner 14 is adapted to deform without failing on application of pressure transferred from the rock mass 22 via the structural liner 18 and the sliding layer 16. In storage vessels according to the first aspect of the invention, where the thickness t of the hydrogen-impermeable inner liner 14 and the internal diameter d' of the hydrogen-impermeable inner liner 14 obey the inequality: Hie hoop stress oh in the inner liner (14) is: Pi x d' Thus, for a permissible stress, oa, the thickness tcan be expressed by the inequality: t > wherein: t is the thickness of the hydrogen-impermeable inner liner 14 pi is the internal pressure in the hydrogen-impermeable inner liner 14; d' is the internal diameter of the hydrogen-impermeable inner liner 14; and Oa is the permissible stress in the hydrogen-impermeable inner liner 14. When the thickness, t, of the hydrogen-impermeable inner liner 14 complies with this inequality, the hydrogen-impermeable inner liner 14 will have a stiffness such that the transfer of the internal gas pressure to the rock mass 22 results in a deformation no greater than the stress and yield limits of the material from which the inner liner 14 is formed, whilst also having a stiffness great enough to resist the external pressure due to in situ stress conditions when the vessel has low internal gas pressure. The storage vessels of the invention can be designed for any type of rock mass and the type of the material used for the hydrogen-impermeable inner liner 14 and the required thickness of the material will depend upon the rock type of the rock mass 22, defined in terms of its deformation modulus (Em). For example, in a rock mass 22 with a high deformation modulus, rock mass deformation will be minimised (for example below 0.02% of the shaft diameter D) and the rock mass will be able to do all of the work necessary to resist the internal pressure of the stored gas (which corresponds to pi, i.e. the internal pressure in the hydrogen-impermeable inner liner), and any geological deformation will not exceed the yield limits of the material from which the inner liner 14 is formed, which means that m in the inner liner 14 is relatively low. Therefore, the thickness, t, of the hydrogen-impermeable inner liner 14 can be relatively small as the inner liner 14 will only need to have sufficient thickness to support the external pressure arising from the rock mass 22 or from construction or transport load. In this case, the inner liner 14 is suitably formed from steel. In a rock mass 22 with a medium deformation modulus and therefore causing limited deformation (for example between 0.02% - 0.34% of shaft diameter D) the rock mass will be able to do some, but not all, of the work necessary to resist the internal pressure (pi) of the stored gas, and any geological deformation could result in the yield limits of the material forming the inner liner 14 to be exceeded. In this case, ah will be higher than for a very stiff rock mass 22 and so the required thickness of the inner liner 14, e.g. made from steel, will be greater than that needed for a vessel formed in a very stiff rock mass 22 as it must be sufficient to bear a contribution of the internal load arising from the pressure of the stored gas. Alternatively, the inner liner 14 may be formed from a material such as a polymer with an increased value of oa or an increased Young’s modulus Ei. In a rock mass 22 with a low deformation modulus causing larger deformation (for example above 0.34% of shaft diameter D), the rock mass 22 will be able to do ven little of the work required to resist the internal pressure (pi) of the stored gas, and any geological deformation will result in the yield limits of the material forming the inner liner 14 to be exceeded. In this case, oa is also higher than that for a moderately stiff rock mass 22 and so either the thickness of a hydrogen-impermeable inner liner 14 formed from steel would need to be very high, or the hydrogen-impermeable inner liner 14 can be formed from a material, such as a polymer, with a higher value of oa and / or Et than steel. Since cost is a factor which must be considered in the design of the hydrogen-impermeable inner liner and since the cost of the vessel increases as the thickness of the inner liner 14 increases, the maximum thickness of the inner liner 14 should be considered. The maximum thickness for the hydrogen-impermeable inner liner 14 can be calculated on the basis of providing an inner liner which does not resist any deformation of the rock mass 22 and therefore exhibits lower stiffness than the rock mass 22, i.e. it obeys the inequality Edi — Ed where: 6d, is the radial lining displacement; and 6d is the radial rock mass displacement. Under hoop stress and when the lining is unsupported by the rock mass: 8d, where: pi is internal pressure in the hydrogen-impermeable inner liner 14; d' is internal diameter of the hydrogen-impermeable inner liner 14; t is the thickness of the hydrogen-impermeable inner liner 14; Et is the Young’s modulus of the hydrogen-impermeable inner liner 14; and vt is the Poisson’s coefficient of the inner liner 14. When rearranged, this gives the following inequality which enables the maximum thickness, t, of the hydrogen-impermeable inner liner 14 to be determined. Therefore, in more suitable vessels of the invention, the maximum thickness, t of the hydrogen-impermeable inner liner 14 can be determined by the following inequality: Therefore, suitably the maximum and minimum thickness, t, of the inner liner is determined by the following: In a rock mass 22 with a high deformation modulus and therefore limited rock deformation (5d), for example below 0.02% of the shaft diameter D, this condition is easy to satisfy for an inner liner 14 made from steel. In a rock mass 22 with medium deformation modulus, rock deformation (8d) is greater than for a rock mass of higher stiffness (for example between 0.02% -0.34% of the shaft diameter D) but it is still relatively easy to provide a steel inner liner (14) of a thickness which satisfies the inequality above. For a moderately stiff rock mass 22 with limited deformation (for example between 0.02% - 0.34% of the shaft diameter D), it is possible to provide an inner liner 14 made from a suitable metallic material such as aluminium or steel, but especially steel, which fulfils the conditions for both minimum and maximum thickness. However, as noted above, in a rock mass 22 with a low deformation modulus, rock deformation (3d) is still greater (for example above 0.34% of shaft diameter D) and so the lining must be very thick in order to support the internal pressure of the stored gas and, therefore, in a rock mass of this type, it may not be possible to provide a steel inner liner 14 which also satisfies the inequality: i.e. which satisfies the conditions for both the minimum and maximum thickness. Therefore, in a rock mass 22 with a low deformation modulus (Erm), it may be necessary to provide an inner liner 14 formed from a material with a higher yield strength or in which the Young’s modulus will be lower. In this case, the polymers discussed above may be more suitable, although it will be necessary to ensure that the permeability to hydrogen is sufficiently low. In some embodiments of the invention, the hydrogen-impermeable liner inner 14 has a thickness of from about 8 mm to about 16 mm, suitably 12 mm to 14 mm, for example about 12 mm. When the hydrogen-impermeable inner liner 14 is formed from steel, it suitably has a thickness of from 5 mm to 20 mm, for example 7 mm to 16 mm and more suitably from about 10 mm to 14 mm, for example about 12 mm. In rock types which are not sufficiently strong for a desired material to be used for the inner liner 14, i.e. the ground deformation falls above the yield limit of the inner liner 14, it is possible to increase the stiffness of the rock by pre-grouting. Pregrouting reduces ground deformation and therefore makes it possible to use a material for the inner liner 14 in a wider range of rock types. Pre-grouting can be achieved by drilling boreholes the length of the shaft 8 in which the vessel 10 is positioned and pumping grout into the boreholes under pressure. The sliding layer 16 may adhere to either the hydrogen-impermeable inner liner 14 or the structural liner 18 but for ease of construction will more suitably adhere to the hydrogen-impermeable inner liner 14. The sliding layer is suitably fonned from a material such that the structural liner 18 and the hydrogen-impermeable inner liner are able to move relative to one another such that the shear stress transmitted from the rock mass 22 to the hydrogen-impermeable inner liner 14 is minimised. Suitable materials for the sliding layer 16 include bitumen and polymers such as polytetrafluoroethylene. The structural liner 18 is suitably formed from concrete. The thickness of the structural liner will depend on the diameter and the depth of the shaft and also on the verticality of the shaft, the construction tolerance of the sections of the inner liner 14 and the pressures from the rock mass 22. Suitably however, a concrete structural liner will have a thickness of from about 200mm to 400 mm, more suitably 250 mm to 350 mm, for example about 275 mm. In vessels according to the third aspect of the invention, the gaspermeability' of the structural liner 18 is higher than the gas-permeability of the inner liner 14 and is optionally lower than the gas-permeability of the overburden 26. Suitably, this feature is also present in storage vessels 10 of the first and second aspects of the invention. Ure increasing permeability of the layers from hydrogen-impermeable inner liner 14 outwards and upwards to the overburden 26 ensures that any gas which escapes from the inner cavity 12 will not accumulate in the sliding layer 16, the structural liner 16 or the filling layer 20, if present, (or get trapped between those layers). This maximises the possibility of re-capturing the gas and minimises the possibility of a build-up of gas or of an explosion, which is especially important when the gas stored in the vessel is hydrogen. The upper 34 and lower 32 caps are suitably hemispherical and are sealably connected to the side walls (33) of the hydrogen-impermeable inner liner 14 so as to seal the upper and lower ends of the inner cavity 12 in which the gas is stored. They are suitably formed from the same material as the side walls 33 of the hydrogen-impermeable inner liner 14, for example a metal such as steel or suitable polymer. The lower cap 32 may form a lower bulkhead at the bottom of the shaft 8 and the structural liner 18 and, where present, the filling layer 20 extend upwards to at least the level of the upper cap (e.g. approximately the apex) and optionally to ground level and also extend under the lower bulkhead formed by the lower cap 32 so as to fill the lower end of the shaft 8. The upper cap 34 must be capable of withstanding die pressure exerted by the gas within the inner cavity of the storage vessel. Suitably, therefore, the shaft 8 is constructed such that the top of the upper cap 34 is below the level of the ground and is covered by the overburden 26, which may be formed from concrete. The height (or depth) of overburden 26 required will depend on the operational pressure, the overburden tensile strength and density, the shaft 8 radius, groundwater, properties of the plug material and rock / plug interface properties. The height of the overburden 26 may be calculated using an adaptation of the method of Kim et al (2012), which makes the assumption that the shape of the uplifted material can be represented as the frustrum of a cone. Other factors which may be taken into account when calculating the required height of the overburden 26 include the coefficient of friction between the concrete (or other material) used to form the overburden 26 and the rock mass 22, the cohesive strength of the interface between the rock mass 22 and the material forming the overburden 26, the total downward force exerted by the overburden 26 and the uplift force exerted by the gas inside the vessel 10. Suitably, the upper surface of the upper cap 34 is positioned at from 20 m to 40 m below ground level, more suitably from 25 to 35 m below ground level and typically at about 30 m below ground level. A closable access means 36 passes through the upper cap 34 and the overburden 26 to allow gas to pass into or out of the storage vessel 10. In use, the gas can be pumped into or out of the inner cavity 12 of the vessel via the closable access means 36. The vessel 10 is suitably positioned such that the top surface of the upper cap 34 is about 20 to 40 metres below ground level (mbgl), for example about 30 mbgl so that there is sufficient resistance exerted by the overburden to counteract the upward pressure exerted by the gas inside the inner cavity 12. As noted above, the invention further comprises a method for constructing a storage vessel 10 suitable for the storage of hydrogen, the method preferably comprising: sinking a shaft 8 in a rock mass 22); constructing an outer supporting layer 24, preferably of steel, in the shaft; filling an annular space between the shaft 8 and the outer supporting layer 24 with material to form a filling layer 20; constructing a hydrogen-impermeable inner liner 14 coated with a sliding layer (16) within the outer supporting layer 24; filling an annular space between the outer supporting layer 24 and the hydrogen-impermeable inner liner 14 with material to form a structural liner 18; backfilling a space between the hydrogen-impermeable liner 14 and ground level with an overburden 26. Hie shaft 8 may be constructed using any suitable shaft 8 sinking method. Drill and blast, sequential excavation, blind boring and use of a caisson or shaft 8 boring machinery' are all known methods for sinking shafts. Traditional drill and blast methods may be unsuitable for some rock ty pes as the blasting will induce damage in the rock mass that may reduce stiffness. In addition, drill and blast and sequential excavation methods require personnel to be present at the face during construction and therefore, for the sake of safety, it is often preferable to use caisson construction or a vertical shaft sinking machine. These techniques are suitable for soft to medium rock. For medium to hard rock, mechanised shaft boring technologies such as those provided by Herrenknecht AG, Germany. Blind boring is a particularly suitable technique as there is no blasting and therefore no induced damage of the rock mass, which could potentially affect the interaction between the liner and the surrounding geology. In addition, blind boring is particularly suitable for use with harder rock masses and, when blind boring is used, the shaft is filled with drilling fluid throughout the drilling and lining process and this prevents rock movement. In some cases, pre-grouting of the rock surface for the full shaft depth may be required in order to decrease the permeability of the rock for shaft sinking and improve the stiffness of the rock mass. Pre-grouting will generally take place before sinking the shaft 8 and can be achieved by drilling boreholes the length of the shaft 8 and pumping grout into the boreholes under pressure. Pre-grouting the shaft 8 increases the stiffness of the rock mass 22 and makes it possible to reduce ground deformation. The filling layer 20 is suitably formed from grout which may be injected or pumped into the space between the rock mass 22 and the outer supporting layer 24 after fonnation of the outer supporting layer 24, where present in the shaft (8). Preferably, according to a particular embodiment of the method of constructing a storage vessel aspect of the invention, the storage vessel 10 is constructed (or installed) by one or both of: i) assembling and installing a supporting layer (e.g. of steel, preferably sheet steel), the assembling and installing comprising: a. filling the shaft with drilling fluid; b. providing a lower end cap of a supporting layer, which is preferably hemispherical; c. affixing (e.g. welding) to the lower end cap of the supporting layer one or a cylindrical assembly of two or more supporting steel annuli (e.g. each of up to 3 m high, e.g. up to 2 m high) to form a partially assembled supporting section; d. lowering from the surface opening of the shaft, into the shaft, the lower end cap or the partially assembled supporting section, preferably by adjusting the buoyancy thereof by filling with a buoyancy fluid (e.g. water); e. successively affixing (e.g. welding) to the upper edge of the partially assembled supporting section successive further supporting steel annuli; f. repeat steps d and e until a desired extent of supporting layer is achieved and lowered into a desired position; and preferably g. pumping into a cavity between the support layer and the shaft wall a filling material (e.g. grout) to form a filling layer, causing the drilling fluid to be expelled (optionally by pumping); and ii) assembling and installing an inner liner (e.g. of steel, preferably stainless steel) within an assembled installed support layer within a shaft, the assembling and installing comprising: a. filling the support layer with a fluid; b. providing a lower end cap of an inner liner, which lower end cap is preferably hemispherical, the lower end cap preferably having a sliding layer formed on the outside thereof; c. affixing (e.g. welding) to the lower end cap of the inner liner one or a cylindrical assembly of two or more inner liner annular sections (e.g. each of up to 3 m high, e.g. up to 2 m high) to form a partially assembled inner liner section, each inner liner annular sections preferably having a sliding layer fonned on the outside thereof; d. lowering from the surface opening of the shaft, into the cavity formed by the supporting layer, the lower end cap or a partially assembled inner liner section formed in step c, preferably by adjusting the buoyancy thereof by filling with a buoyancy fluid (e.g. water); e. successively affixing (e.g. welding) to the upper edge of the partially assembled inner liner section successive further inner liner annular sections; f. repeating steps d and e until a desired extent of inner liner wall is achieved; g. affixing (e.g. welding) to the upper edge of the partially assembled inner liner section, provided by step f, an upper cap of the inner liner to produce an assembled inner liner; h. lowering the assembled inner liner into a desired position radially inward of the supporting layer; and preferably i. pumping into a cavity between the inner liner and the support layer a structural material (e.g. concrete) to form a structural liner, causing the buoyancy fluid therein to be expelled (optionally by pumping). A method for forming a storage vessel 10 according to one embodiment the invention, and by way of non-limiting example only, is shown in Figures 2A, 2B and 2C. Another and more detailed example of a method for forming a storage vessel 10 according to another embodiment of the invention is shown in Figures 5A to 5P. A storage vessel according to one embodiment of the invention, which may be constructed using the method of an aspect of the invention, is shown in Figures 3 and 4 and a storage vessel according to another embodiment, is shown in Figure 5P. In the example shown in Figures 3, 4 and 5P, there is preferably an outer supporting layer 24, preferably made of steel, at the outer surface of the structural liner 18, which is about 200 mm to 300 mm in thickness. As shown in Figure 2A, a vessel is constructed in a shaft 8, which may be bored to the required depth and diameter using a large diameter rotary drill rig 80. If necessary, the rock is pre-grouted before sinking the shaft 8 as described above. Tire shaft 8 is filled with drilling fluid 54, which helps to support the rock mass 22 during construction. After any pre-grouting and after excavation as described above, a steel inner liner 14, is lowered into the shaft 8 as shown in Figure 2B. The inner liner 14 suitably comprises a pre-fabricated hemispherical base section (or lower cap) 32 and annular wall sections 64. Tremie pipes (not shown) are fitted to the outer surfaces of the lower cap and annular wall sections of the steel inner liner 14. The lower cap 32 and annular wall sections 64 are welded together either before lowering or in situ section by section. Hie depth of the inner liner 14 in the shaft 8 may be controlled by adjusting the depth of the drilling fluid 54 in the shaft and the amount of drilling fluid (or buoyancy fluid) 62 in the partially assembled inner liner so as to adjust the buoyancy of the partially assembled inner liner. Once the outer supporting layer 24 is in position, the gap between the rock mass 22 and the outer supporting layer 24 can be filled with grout via the tremie pipes to complete the filling layer 20. The annular wall sections 64 of the inner liner are coated with a sliding layer 16, preferably formed from polytetrafluoroethylene or a bituminous material. An upper end cap 34, which is also suitably hemispherical, may be fitted over the storage vessel, and, preferably, in particular, over the side walls 33 of the inner liner 14 to cooperate with the upper edges of the assembled annular wall sections of the inner liner. The installed inner liner 14 is illustrated in Figure 2C, which shows the inner liner, assembled without any outer support layer. Figure 2C illustrate the annular sections (e.g. 64) of the inner liner 14 which make up the inner liner wall 33 and the respective upper and lower end caps 32,34, and illustrates the backfill of concrete to form the structural layer 18, radially outward from the inner liner 14 and overburden 26 above or on top of the upper end cap 34. As is shown in Figure 2C and in Figures 3 and 4, the upper end cap 34 of the inner liner 14 is provided with a closable access means 36 which passes through the upper end cap 34 so that gas can be transferred into and out of the storage vessel 10. Concrete or grout forming the structural liner 18 may be pumped into the gap between the hydrogen-impermeable inner liner 14 and the shaft wall 56 via the tremie pipes. Once the upper hemispherical cap has been welded in place, drilling fluid is pumped out of the inner cavity 12 of the vessel 10, suitably via the closable access means 36. Tire overburden 26 is then formed by backfilling above the hemispherical cap 34 of the inner liner 14, for example using concrete. The vessel 10 is suitably positioned such that the top surface of the upper cap 34 is about 20 to 40 metres below ground level (mbgl), for example about 30 mbgl so that there is sufficient resistance exerted by the overburden to counteract the upward pressure exerted by the gas inside the inner cavity 12. In Figures 3 and 4, an embodiment of the storage vessel 10 is illustrated in which the inner liner 14, preferably of steel, is provided with a sliding layer (not shown) on the outside thereof and having radially outward and therebelow a structural liner 18 of concrete. Radially outward from the structural liner 18, a support layer (preferably of steel) 24 is provided which is separated from the outer wall 56 of the shaft by a fdler layer 20 of grout. In Figures 3 and 4, the support layer is illustrated as extending upward beyond the level of the upper cap 34 of the inner liner 14, optionally to the surface, but preferably extends upward only to a level similar to the upper edge or apex of the upper cap 34. In some cases, anchor members, for example anchor bolts 48, may be used to increase the uplift resistance. Hie height of overburden 26 required may be calculated using an adaptation of the method developed by Kim et al (2012). Since the vessel 10 is a thin-walled cylinder, the height of the overburden required is less than would be needed for a traditional LRC. As shown in Figures 3 and 4, the height of the overburden 26 may be reduced if anchor members 48 are employed to counter some of the upward force exerted by the gas inside the storage vessel 10. As previously, the inner liner 14, which defines the internal cavity 12 of the vessel 10, comprises a side wall 33 and upper and lower caps 34,32 and has a closeable access means 36, in the form of a pipe, extending from an upper portion thereof. In Figures 5 A to 5P, an example of the method of installation of a vessel 10 according to one embodiment of that aspect of the invention is illustrated. As can be seen in Figure 5 A, optional pre-grouting holes 38 may be drilled in the rock mass 22 and reinforcing material (not shown, but grout, for example) inserted or injected into the pregrouting holes 38 to create a competent ground volume 42 for the drilling of a shaft 8, as shown in Figure 5B. Optionally, foundations 44 may be built by digging a foundation recess 46 and installing a surface pad 50 and annular opening reinforcement 52, as shown in Figures 5C and 5D. A shaft 8 may be bored in the rock mass 22 or competent ground volume 42 formed by the pre-grouting step, and the shaft filled with a drilling fluid 54, such as water, to provide structural support to the shaft walls 56, as is illustrated in Figure 5E. Figures 5F to 51 show the installation in the shaft 8 of an outer supporting layer 24 of sheet steel. The outer supporting layer 24 is assembled sequentially at the surface during installation by providing a supporting lower end cap 28 and welding a first supporting steel annulus 58 thereto to form a partially assembled section 60, placing into the fluid-filled shaft 8 and adding buoyancy fluid (typically water or drilling fluid) 62 to the interior of the partially assembled section 60 to control (reduce) its buoyancy so that it can be successively lowered into the fluid-filled shaft 8 until in a suitable position for welding each successive supporting steel annulus 58 to the top of the partially assembled section 60 at the top of the shaft 8. Optionally, the partially assembled section 60 may be clamped or secured in position at the surface to prevent movement during the assembly and welding of a further supporting steel annulus 58. The assembly and lowering process for the outer supporting layer 24 continues until the supporting layer walls 30 have reached a desired height, which could be a height that, when in position, brings them to the surface of the shaft, but is more typically of a height that is some distance below the surface and typically at a position that corresponds to the expected level of the top of the inner liner 14 when in position. The assembled outer supporting layer 24 is then lowered into position as is illustrated in Figure 5H. Filling material, typically grout, is then poured into the cavity between the shaft wall 56 and the outer supporting layer 24, displacing the drilling fluid (e.g. water) which may optionally be pumped out and forming the filling layer 20 as is shown in Figure 51. An inner liner 14 is then installed into the outer structural layer 24, as illustrated in Figures 5J to 5M, in much the same way as the outer structural layer 24 was installed in the shaft 8 as described above. Firstly, a lower end cap 32 is placed in position at the surface of the shaft 8 and an inner liner annular section 64, typically of steel, is welded or otherwise sealed to the lower end cap 32 to form a partially assembled inner liner section 66. A buoyancy fluid 62 (e.g. drilling fluid or water) is added to the partially assembled inner liner section 66 to control the buoyancy thereof and to lower it into the fluid-filled outer supporting layer 24 until the upper edge thereof is suitably positioned for assembling the next inner liner annular section 64 thereto. This process is repeated until the partially assembled inner liner section 66 defines an inner liner side wall 33 of a desired height, at which point an upper cap 34 is welded onto the top edge 68 of the side walls 33 and the completed inner liner 14 lowered into position within the supporting layer 24. Hie upper cap 34 lias a closable access means in the form of an upstanding pipe 70 extending upwardly from an apex 72 thereof, which upstanding pipe 70 provides access into the inner cavity 12 defined by the inner liner 14 and which is, in Figures 5L and 5M filled with buoyancy fluid 62. Concrete is then poured into the space between the inner liner 14 and the outer supporting layer 24, displacing the buoyancy fluid 62 therein (which may optionally be pumped out), in order to form a concrete structural liner 18. Hie installation method may be completed as illustrated in Figures 5N to 5P. Firstly, as shown in Figure 5N, an overburden 26, typically of concrete, may be provided above the upper cap 34. Process equipment 74 may be installed at the head of the vessel at the surface, to include a fluid access line 76 extending through upstanding pipe 70 and sealed therein, leading to a valve arrangement 78 for the controlled opening and closing of the fluid access line, and pumping of fluid into and out of the inner cavity 12. The inner cavity 12 may then be emptied of fluid, using the fluid access line attached to a pump (not shown) or by pumping an inert gas into the vessel 10 and opening the access line to enable the removal of buoyancy fluid (or water) 62, until the vessel 10 is empty. The inner cavity 12 may optionally be purged with inert gas to facilitate the removal of moisture or other impurities ready for use in gas (e.g. hydrogen) storage. For the avoidance of doubt, the dimensions of the system in Figures 5 A to 5P are not intended to be to scale - for example, the height (or depth) of the overburden does is not necessarily shown in proportion to the vertical extent of the inner cavity. The present invention therefore provides a storage vessel for gases, and especially for hydrogen, which overcomes the problems presented by traditional storage vessels such as lined rock caverns. REFERENCES Kim et al, “Parametric sensitivity analysis of ground uplift above pressurized underground rock caverns”, Engineering Geology, (2012), 135-135, 60-65. E. Hoek, M.S. Diederichs, Empirical estimation of rock mass modulus, International Journal of Rock Mechanics and Mining Sciences, Volume 43, Issue 2, (2006),203-215. Hoek E, Carranza-Torres CT, Corkum B. Hoek-Brown failure criterion-2002 edition. In: Proceedings of the 5 th North American Rock Mechanics Symp., 2002: 1: 267-73 All publications, including but not limited to patents and patent applications, cited in this specification are herein incorporated by reference as if each individual publication were specifically and individually indicated to be incorporated by reference herein as though fully set forth. Further aspects and / or embodiments of the invention are described in the following clauses: Clause 1. A storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises: a shaft (8) of substantially circular cross section provided in a rock mass (22); and within the shaft: an inner cavity (12) for the storage of a gas; a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity; a sliding layer (16) surrounding the hydrogen-impermeable inner liner; a structural liner (18) surrounding the sliding layer; and an overburden (26) above the upper cap (34); wherein the hydrogen-impermeable inner liner (10) is a thin-walled cylinder in which: d' 1 <20 wherein t is the wall thickness of the hydrogen-impermeable inner liner (14); and d' is the internal diameter of the hydrogen-impermeable inner liner (14), and wherein the hydrogen-impermeable inner liner (14) is adapted to deform without failing on application of pressure transferred from the rock mass (22) via the structural liner (18) and the sliding layer (16) such that the thickness, t, of the hydrogen-impermeable inner liner (14) is expressed by the inequality: Pt x d' t >—--- 2¾ wherein: pi is the internal pressure in the hydrogen-impermeable inner liner (14); d' is the internal diameter of the hydrogen-impermeable inner liner (14); and Oa is the permissible stress of the hydrogen-impermeable inner liner material. Clause 2. A storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises: a shaft (8) of substantially circular cross section provided in a rock mass (22); and within the shaft: an inner cavity (12) for the storage of a gas; a hydrogen-impenneable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity; a sliding layer (16) surrounding the hydrogen-impenneable inner liner; and a structural liner (18) surrounding the sliding layer; and an overburden (26) above the upper cap (34); wherein the shaft (8) has a depth, i.e. wall height, (h) of 30m to 500m and a diameter (D) of Im to 18m; and wherein h >2D; and the hydrogen-impermeable inner liner (14) is formed from steel and has a thickness, t, of from 5 mm to 20 mm; and the overburden (26) has a height of 1 m to 50 m. Clause 3. A storage vessel according to clause 1 or clause 2 wherein the gas-permeability of the structural liner (18) is higher than the gas-permeability of the inner liner (14) and is optionally lower than the gas-permeability of the overburden (26). Clause 4. A storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises: a shaft (8) of substantially circular cross section provided m a rock mass (22); and within the shaft: an inner cavity (12) for the storage of a gas; a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity; a sliding layer (16) surrounding the hydrogen-impenneable inner liner; and a structural liner (18) surrounding the sliding layer; and an overburden (26) above the upper cap (34); wherein the gas-permeability of the structural liner (18) is higher than the gaspermeability of the inner liner (14) and is optionally lower than the gas-permeability of the overburden (26). Clause 5. A storage vessel (10) according to clause 4, wherein the minimum thickness of the hydrogen-impermeable inner liner (14) is expressed by the inequality Pt xd' wherein: t is the thickness of the hydrogen-impermeable inner liner (14) p: is the internal pressure in the hydrogen-impermeable inner liner (14); d' is the internal diameter of the hydrogen-impermeable inner liner (14); and is the permissible stress. Clause 6. A storage vessel (10) according to any one of clauses 1 or 3 to 5, wherein the maximum thickness, t of the hydrogen-impermeable inner liner (14) is expressed by the inequality: Pi x d'2 Yi 2 Where: pi is internal pressure in the hydrogen-impermeable inner liner (14) d' is internal diameter of the hydrogen-impermeable inner liner (14) t is the thickness of the hydrogen-impermeable inner liner (14) Ei is the Young’s modulus of the hydrogen-impermeable inner liner (14) vt is the Poisson’s coefficient of the inner liner (14); and 8d is radial rock mass displacement. Clause 7. A storage vessel (10) according to any one of clauses 1 or 3 to 6, wherein the shaft (8) has a depth, i.e. wall height, (h) and a diameter (D) wherein: h >2D. Clause 8. A storage vessel according to any one of clauses 2 to 7, wherein the hydrogen-impermeable inner liner (14) is a thin-walled cylinder wherein: d’ 1 <20 wherein t is thel wall thickness of the hydrogen-impermeable inner liner (14); and d' is the internal diameter of the hydrogen-impermeable inner liner (14). Clause 9. A storage vessel (10) according to any one of clauses 1 to 8 wherein the structural liner (18) is formed from concrete and has a thickness of from 200mm to 400 mm. Clause 10. A storage vessel (10) according to any one of clauses 1 to 9 wherein the operating pressure is from 0 to 30 MPa. Clause 11. A storage vessel (10) according to clause 10, wherein the operating pressure is from 3 to 20 MPa. Clause 12. A storage vessel (10) according to any one of clauses 1 to 3 or 5 to 11 wherein the shaft (8) has a depth of 30m to 500m. Clause 13. A storage vessel (10) according to any one of clauses 1 to 12, wherein tire shaft (8) has a depth of 250m to 350m. Clause 14 A storage vessel according to any one of clauses 1 to 13, wherein the diameter of the shaft (8) is from Im to 18m. Clause 15. A storage vessel according to clause 14, wherein the diameter of the shaft (8) is from 3m to 8m. Clause 16. A storage vessel according to clause 15, wherein the height of the overburden (26) is from Im to 50m. Clause 17. A storage vessel according to any one of clauses 1 to 16, further including anchor members (48) to increase the uplift resistance of the overburden. Clause 18. A storage vessel according to any one of clauses 1 to 17 wherein a filling layer (20) is provided between the rock mass (22) and the structural liner (18). Clause 19. A storage vessel (10) according to clause 18 wherein the filling layer (20) is formed from grout. Clause 20. A storage vessel (10) according to clause 17 or clause 18 wherein the filling layer has a radial thickness of from about 150 mm to 450 mm. Clause 21. A storage vessel (10) according to any one of clauses 1 to 18 further comprising an outer supporting layer (24), wherein the outer supporting layer (24) is either positioned around the outer edge of the structural liner (18) or is disposed within the structural liner (18). Clause 22. A storage vessel (10) according to clause 21 wherein the outer supporting layer comprises a layer of sheet steel positioned between the structural liner (18) and a filling layer (20). Clause 23. A storage vessel (10) according to clause 22 wherein the outer supporting layer has a thickness of 15 mm to 35 mm. Clause 24. A storage vessel (10) according to any one of clauses 21 to 23 wherein: the gas-permeability of the structural liner (18) is higher than the gas-permeability of the inner liner (14); and the outer supporting layer (24) is gas-impermeable; and the gas-permeability of the overburden (26) is higher than the gas-permeability' of the structural liner (18). Clause 25. A storage vessel (10) according to clause 24 wherein the overburden (26) is provided with means to collect escaped gas. Clause 26. A storage vessel (10) according to any one of clauses 21 to 23 wherein: the gas-permeability of the structural liner (18) is higher than the gas-permeability of the inner liner (14); and the outer supporting layer (24) is gas-permeable; and the gas-permeability of the overburden (26) is optionally higher than the gaspermeability of the structural liner (18). Clause 27. A storage vessel according to clause 26 wherein: a filling layer (22) provided between the structural liner (18) and the rock mass (22) is provided with means to collect escaped gas; and / or when the gas-permeability of the overburden (26) is higher than the gaspermeability of the structural liner (18), the overburden (26) is provided with means to collect escaped gas. Clause 28. A storage vessel (10) according to any one of clauses 1 to 27 wherein the upper (34) and lower (32) caps are hemispherical. Clause 29. A storage vessel (10) according to any one of clauses 1 to 28 wherein a closable access means (36) passes through the upper cap (34) and the overburden (26) to allow gas to pass into or out of the storage vessel (10). Clause 30. A method for constructing a storage vessel (10) suitable for the storage of hydrogen, the method comprising: sinking a shaft (8) in a rock mass (22); providing a steel outer supporting layer (24) in the shaft; filling an annular space between the shaft (8) and the outer supporting layer (24) with material to form a filling layer (20); providing a hydrogen-impermeable inner liner (14) coated with a sliding layer (16) within the outer supporting layer (24); filling an annular space between the outer supporting layer (24) and the hydrogen-impermeable inner liner (14) with material to form a structural liner (18); backfilling a space between the hydrogen-impermeable liner (14) and ground level with an overburden (26). Clause 31. A method according to clause 27 further including the step of pre-grouting of the rock surface for the full shaft depth before sinking the shaft (8). Each of the embodiments defined by the above clauses may be incorporated, where appropriate in the context, into the aspects of the invention set out above.

Claims

1. A storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises:a shaft (8) of substantially circular cross section provided in a rock mass (22); and within the shaft:an inner cavity (12) for the storage of a gas;a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity;a sliding layer (16) surrounding the hydrogen-impermeable inner liner;a structural liner (18) surrounding the sliding layer; andan overburden (26) above die upper cap (34);wherein the hydrogen-impermeable inner liner (10) is a thin-walled cylinder in which:d' t —20whereint is the wall thickness of the hydrogen-impermeable inner liner (14); andd' is the internal diameter of the hydrogen-impermeable inner liner (14),and wherein the hydrogen-impermeable inner liner (14) is adapted to deform without failing on application of pressure transferred from the rock mass (22) via the structural liner (18) and tire sliding layer (16) such that the thickness, t, of the hydrogen-impermeable inner liner (14) is expressed by the inequality:t Piwherein:pt is the internal pressure in the hydrogen-impermeable inner liner (14);d' is tire internal diameter of the hydrogen-impermeable inner liner (14); andOa is the permissible stress of the hydrogen-impermeable inner liner material.

2. A storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises:a shaft (8) of substantially circular cross section provided in a rock mass (22); andwithin the shaft:an inner cavity (12) for the storage of a gas;a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity;a sliding layer (16) surrounding the hydrogen-impermeable inner liner; anda structural liner (18) surrounding the sliding layer; andan overburden (26) above the upper cap (34);wherein the shaft (8) has a depth, i.e. wall height, (h) of 30m to 500m and a diameter (D) of Im to 18m; and whereinh, >2D; andthe hydrogen-impermeable inner liner (14) is formed from steel and has a thickness, t, of from 5 mm to 20 mm; andthe overburden (26) has a height of 1 m to 50 m.

3. A storage vessel according to claim 1 or claim 2 wherein the gas-permeability of the structural liner (18) is higher than the gas-permeability of the inner liner (14) and is optionally lower than the gas-permeability of the overburden (26).

4. A storage vessel (10) suitable for the storage of hydrogen, wherein the storage vessel (10) comprises:a shaft (8) of substantially circular cross section provided in a rock mass (22); and within the shaft:an inner cavity (12) for the storage of a gas;a hydrogen-impermeable inner liner (14) surrounding the inner cavity and comprising side walls (33) and upper (34) and lower (32) caps sealing the upper and lower ends of the inner cavity;a sliding layer (16) surrounding the hydrogen-impenneable inner liner; anda structural liner (18) surrounding the sliding layer; andan overburden (26) above the upper cap (34);wherein the gas-permeability of the structural liner (18) is higher than the gaspermeability of the inner liner (14) and is optionally lower than the gas-permeability of the overburden (26).

5. A storage vessel (10) according to claim 4, wherein the minimum thickness of the hydrogen-impermeable inner liner (14) is expressed by the inequalityPt xd'wherein:t is the thickness of the hydrogen-impermeable inner liner (14)pt is the internal pressure in the hydrogen-impermeable inner liner (14);d' is the internal diameter of the hydrogen-impermeable inner liner (14); and is the permissible stress.

6. A storage vessel (10) according to any one of claims 1 or 3 to 5, wherein the maximum thickness, t of the hydrogen-impermeable inner liner (14) is expressed by the inequality:Where:pi is internal pressure in the hydrogen-impermeable inner liner (14) d' is internal diameter of the hydrogen-impermeable inner liner (14) t is the thickness of the hydrogen-impermeable inner liner (14)Ei is the Young’s modulus of the hydrogen-impermeable inner liner (14) v( is the Poisson’s coefficient of the inner liner (14); and 8d is radial rock mass displacement.

7. A storage vessel (10) according to any one of claims 1 or 3 to 6, wherein the shaft (8) has a depth, i.e. wall height, (h) and a diameter (D) wherein:h >2D.

8. A storage vessel according to any one of claims 2 to 7, wherein the hydrogen-impermeable inner liner (14) is a thin-walled cylinder wherein:whereint is thel wall thickness of the hydrogen-impermeable inner liner (14); and d' is the internal diameter of the hydrogen-impermeable inner liner (14).

9. A storage vessel (10) according to any one of claims 1 to 8 wherein the structural liner (18) is formed from concrete and has a thickness of from 200mm to 400 mm.

10. A storage vessel (10) according to any one of claims 1 to 9 wherein the operating pressure is from 0 to 30 MPa, preferably from 3 to 20 MPa.

11. A storage vessel (10) according to any one of claims 1 to 3 or 5 to 10 wherein the shaft (8) has a depth of from 30m to 500m, preferably from 250m to 350m.12 A storage vessel according to any one of claims 1 to 13. wherein the diameter of the shaft (8) is from Im to 18m, preferably from 3m to 8m.

13. A storage vessel according to claim 12, wherein the height of the overburden (26) is from Im to 50m.

14. A storage vessel according to any one of claims I to 13, further including anchor members (48) to increase the uplift resistance of the overburden.

15. A storage vessel according to any one of claims 1 to 14 wherein a filling layer (20) is provided between the rock mass (22) and the structural liner (18), preferably wherein the filling layer (20) is formed from grout.

16. A storage vessel (10) according to claim 15 wherein the filling layer has a radial thickness of from about 150 mm to 450 mm.

17. A storage vessel (10) according to any one of claims 1 to 16 further comprising an outer supporting layer (24), wherein the outer supporting layer (24) is either positionedaround the outer edge of the structural liner (18) or is disposed within the structural liner (18).

18. A storage vessel (10) according to claim 17, wherein the outer supporting layer comprises a layer of sheet steel positioned between the structural liner (18) and a filling layer (20).

19. A storage vessel (10) according to claim 18 wherein the outer supporting layer has a thickness of 15 mm to 35 mm.

20. A storage vessel (10) according to any one of claims 17 to 19 wherein:the gas-permeability of the structural liner (18) is higher than the gas-permeability of the inner liner (14); andthe outer supporting layer (24) is gas-impermeable; andthe gas-permeability of the overburden (26) is higher than the gas-permeability of the structural liner (18).

21. A storage vessel (10) according to claim 20 wherein the overburden (26) is provided with means to collect escaped gas.

22. A storage vessel (10) according to any one of claims 17 to 19 wherein:the gas-permeability of the structural liner (18) is higher than the gas-permeability of the inner liner (14); andthe outer supporting layer (24) is gas-permeable; andthe gas-permeability of the overburden (26) is optionally higher than the gaspermeability of the structural liner (18).

23. A storage vessel according to claim 22 wherein:a filling layer (22) provided between the structural liner (18) and the rock mass (22) is provided with means to collect escaped gas; and / orwhen the gas-permeability of the overburden (26) is higher than the gaspermeability of the structural liner (18), the overburden (26) is provided with means to collect escaped gas.

24. A storage vessel (10) according to any one of claims 1 to 23 wherein the upper (34) and lower (32) caps are hemispherical.

25. A storage vessel (10) according to any one of claims 1 to 24 wherein a closable access means (36) passes through the upper cap (34) and the overburden (26) to allow gas to pass into or out of the storage vessel (10).

26. A method for constructing a storage vessel (10) suitable for the storage of hydrogen, the method comprising:sinking a shaft (8) in a rock mass (22);providing a steel outer supporting layer (24) in the shaft;filling an annular space between the shaft (8) and tire outer supporting layer (24) with material to form a filling layer (20);providing a hydrogen-impermeable inner liner (14) coated with a sliding layer (16) within the outer supporting layer (24);filling an annular space between the outer supporting layer (24) and the hydrogen-impermeable inner liner (14) with material to form a structural liner (18);backfilling a space between the hydrogen-impermeable liner (14) and ground level with an overburden (26).

27. A method according to claim 26 further including the step of pre-grouting of the rock surface for the full shaft depth before sinking the shaft (8).

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