WO2009147162A1

WO2009147162A1 - A cryogenic container, and method of using the same

Info

Publication number: WO2009147162A1
Application number: PCT/EP2009/056783
Authority: WO
Inventors: Franciscus Antonius Henri Janssen; Levinus Marinus De Mul
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2008-06-03
Filing date: 2009-06-03
Publication date: 2009-12-10

Abstract

The present invention provides a cryogenic container (10) comprising walls (20) and a base (30), said walls comprising: a first liquid barrier layer (50); a second liquid barrier layer (100); one or more spacer elements (150), disposed between the first and second liquid barrier layers (50, 100), to separate the first and second liquid barrier layers (50, 100), thereby providing an annulus (200); one or more fluid vents (250), in direct fluid communication with the annulus (200) for venting fluid from the annulus (200), wherein at least one of the vents (250) is connected to a pressure differential device (320); sealing means (260) to contain the annulus (200); a pressure measuring device connected to the annulus for monitoring the pressure in the annulus over time; and a comparator for comparing at least two pressure measurements from the pressure measuring device.

Description

A CRYOGENIC CONTAINER, AND METHOD OF USING THE SAME

The present invention relates to a cryogenic container for storing a liquefied gas, such as liquefied natural gas (LNG), liquefied nitrogen, oxygen, carbon dioxide or hydrogen. Several kinds of cryogenic containers are known.

Usually, the container comprises an outer shell, an inner tank to contain the liquefied gas and any vapours, and insulation provided between the structural outer shell and the inner tank to reduce leakage of heat into the interior of the container. The inner tank usually comprises one or more liquid barrier layers, which may be combined with the insulation or provided as separate layers. Suitably the liquefied gas is stored in the container at approximately atmospheric pressure. A problem of the known containers for storing liquefied gas is that voids between layers of containment material forming the walls and/or base of the container can fill with fluid, for instance through leakage of liquefied gas through cracks in the liquid barrier layer material or diffusive permeation therethrough.

Accumulation of such leaked fluid any voids can cause a number of safety issues. The leaked liquefied gas at cryogenic temperatures can lead to the structural weakening of the container or its surroundings, for instance if the container is adjacent to the metallic hull of an ocean-going vessel, which can become brittle.

US Patent No. 3,031,856 discloses a vessel for transporting low temperature liquids in bulk. The vessel contains a number of cryogenic storage tanks . The inner one of two cargo liquid tank shells both independent of the hull structure of the vessel comprises a polyester sheet material. The outer shell is made of steel. The insulation between the two inner and outer tank shells is a prefoamed plastic material. The vessel is provided with an inert gas main, which is connected to compartments between the outer steel shell of the tanks and the hull of the vessel. The inert gas main can condition the atmosphere in the compartments to prevent the creation of a combustible mixture in case of leakage. A suitable sniffing connection may be provided for the compartments to allow sampling of the atmosphere therein. The compartments are provided with valves of the spring- loaded variety to protect the compartments against being overpressured by inert gas. US Patent No. 3,339,783 discloses an insulated container for low temperature liquids made from two separate and distinct layers of insulating panels. The panels consist of polyurethane foam encapsulated in fiberglass re-inforced polyester resin. The outer panels are attached to a rigid steel structure, the joints between the panels being made liquid-tight with plug pieces of similar material to the panels. A second layer of insulating inner panels is attached to the outer layer and spaced a small distance away therefrom by foot elements molded into the second layer of panels and of sufficient strength to maintain a predetermined spaced relation with the outer panels. The space between the two layers of panels is adapted to be monitored for the detection of gas, the presence of which would be in indication of leakage through the second layer.

Such a leakage may present a hazardous situation, and it is an object of the present invention to reduce the magnitude of any hazard. In a first aspect, the present invention provides a cryogenic container, for storing a cryogenic liquefied gas, the container comprising one or more walls and a base, capable of holding a load of the cryogenic liquefied gas, said one or more walls comprising at least : a first liquid barrier layer; a second liquid barrier layer; one or more spacer elements, disposed between the first and second liquid barrier layers, to separate the first and second liquid barrier layers, thereby providing an annulus; and one or more fluid vents, in direct fluid communication with the annulus for venting fluid from the annulus .

In an embodiment, at least one of the one or more vents are connected to a pressure differential device, and the apparatus further comprises: sealing means to contain the annulus; - a pressure differential device for pressurising the annulus to a pressure above atmospheric pressure; a pressure measuring device connected to the annulus for monitoring the pressure in the annulus over time; and a comparator for comparing at least two pressure measurements from the pressure measuring device.

The embodiment enables leak testing of the cryogenic container without damaging the materials thereof. The container of the invention enables non-destructive leak testing of synthetic material containers. Thus, the invention reduces expenses related to the construction and/or operation of cryogenic containers.

In a second aspect, the present invention provides the use of a cryogenic container according to the first aspect disclosed herein, for the venting of fluid from the annulus .

In a third aspect, the invention provides a method of leak testing a cryogenic container, the method comprising the steps of:

(a) pressurising the annulus to a pressure above atmospheric pressure by pumping gas into the annulus with a pressure differential device;

(b) monitoring the pressure in the annulus with a pressure measuring device to provide at least two pressure measurements at different times; and

(c) comparing the at least two pressure measurements from the pressure measuring device.

A decrease in the at least two pressure measurements, more preferably in the pressure measurements over time, is indicative of a leak in the cryogenic container. Thus, the container and method improve the safety by allowing leak testing even before cargo is introduced in the container. Also, the improved safety aspects will enhance the acceptance of the use of relatively affordable and/or lightweight synthetic materials for cryogenic containers instead of for instance steel.

The provision of one or more fluid vents allows the removal of any fluids which may accumulate in the annulus, before they can cause any harm such as for instance building up a unacceptable high pressure in the annulus or such as for instance degrading any material due to the presence of cryogenic fluid.

Hereinafter the invention will be illustrated with respect to the following non-limiting drawings.

Figure 1 shows a schematic representation of a cross- section of a cryogenic container according to a first embodiment of the invention. Figure 2 shows a schematic representation of a cross- section of a cryogenic container according to a second embodiment of the invention.

Figure 3 shows a schematic representation of a longitudinal-section of a cryogenic container according to a second embodiment of the invention.

The teachings disclosed herein can be used to provide a safe but light-weight cryogenic container for storing liquefied gas that can be constructed easily and quickly. It provides for one or more fluid vents, in direct fluid communication with an annulus between at least a first and a second liquid barrier layer in the walls and optionally also in the base, which allows for venting of fluids, in particular vapours, from the annulus. Such fluids or vapours may accumulate there, for example through leakage of the cryogenic liquid stored in the container, or diffusion of any vapour evaporated from the cryogenic liquid through at least the first liquid barrier layer and into the annulus . The cryogenic container described herein can be used for the containment, storage, processing, transport or transfer of a cryogenic fluid, such as a liquefied gas, including but not limited to LNG, liquefied nitrogen, oxygen, carbon dioxide and hydrogen. Such use can be temporary or permanent, on-shore or off-shore, above ground, above water, under water or underground, or a combination thereof. Such uses can also be up-stream and/or downstream of other apparatus, devices, units or systems of any part of a plant or facility for containing, storing, processing, transporting and/or transferring a cryogenic fluid. This includes one or more of a liquefaction plant, an export, loading, transport, unloading, import or end-use facility, or a part thereof . Such uses include but are not limited to the following applications; storage and transportation of cryogenic fluids (pure or blended) for the use at temperatures below -30 ⁰C, preferably at temperatures below -100 ⁰C, more preferably at temperatures below -150 ⁰C, including tanks (i.e. bulk storage) at export and import terminals and shipping; the containment of cryogenic fluids in onshore and offshore tanks of any geometric shape including (vertical) cylindrical tanks, prismatic tanks, ellipsoidal tanks, and spherical tanks; the onshore and offshore storage or transportation of cryogenic fluids in containers, portable containers, shop-fabricated containers, portable tanks and cargo tanks; the underground storage including caverns such as rock caverns or underground containers, (examples of which are discussed in the article by Eric Amantini, Emmanuel Chanfreau and Ho-Yeong Kim entitled "The LNG storage in lined rock cavern" in Gastech (2005), incorporated herein by way of reference) ; pressurised or non-pressurised vessels for the temporary or permanent storage of cryogenic fluids; and pressurised or non-pressurised vessels for the transport (on land, by sea or air by any means) of cryogenic fluids of any geometric shape including but not limited to (vertical) cylindrical, prismatic, ellipsoidal, spherical shapes.

In a preferred embodiment disclosed herein, a cryogenic container comprising one or more walls and a base constructed substantially from polymeric materials, such as thermoplastic or thermosetting materials, is disclosed. By "substantially" is meant that the main structural elements, including at least the first and second polymeric liquid barrier layers, one or more spacer elements and optionally any thermally insulating layers, additional barrier layers or resilient layers are composed of polymeric material. Non-polymeric material, such as metallic connectors, can be used to join the individual layers of polymeric material forming the walls and base of the container. For instance, a cryogenic container may be provided in which at least the first and second liquid barrier layers and one or more spacer elements comprise polypropylene. Such a construction is advantageous because polypropylene can be recycled when the cryogenic container is no longer required.

Figure 1 shows a schematic representation of a first cryogenic container 10 for storing a cryogenic load 400, herein the form of a liquefied gas. The container comprises a wall 20 and a base 30. Under normal operational conditions, the base 30 will extend essentially horizontally, with the wall 20 protruding generally upwardly from the base 30 to form the container. The wall 20 and base 30 comprise a first and a second liquid barrier layer 50, 100 respectively. The first liquid barrier layer 50 comprises a wall portion 50a, and a base portion 50b, and is intended to be in direct contact with the cryogenic liquid load. The second liquid barrier layer 100, which functions as a back-up containment layer that under normal conditions is not intended to be in contact with the cryogenic liquid load 400, also comprises a wall portion 100a and a base portion 100b. One or both of these liquid barrier layers may be a polymeric liquid barrier layer. Preferred materials for the first and second liquid barrier layers 50, 100 will be discussed in greater detail below. The first and second liquid barrier layers 50, 100 are separated by spacer elements 150. The spacer elements 150 maintain a fixed distance from the first and second liquid barrier layers 50, 100, thereby separating these layers and creating an annulus 200 between them. At the top of the wall 20, the annulus 200 is in direct fluid communication with one or more fluid vents 250. It is preferred that the spacer elements 150 separate the walls defining the annulus by a distance of 1 to 100 mm, more preferably about 5 to 15 mm. The distance is preferred to be large enough to allow sufficient transport of fluids along the annulus 200 to the vent(s) 250 without causing too high of a dynamic pressure differential.

The spacer elements (150) are shown divided into groups of wall spacer elements 150a - separating wall portions 50a and 100a of the first and second liquid barrier layers 50 and 100 from each other - and base spacer elements 150b - separating base portions 50b and 100b of the first and second liquid barrier layers 50 and 100 from each other. The wall spacer elements 150a may be made of the same or different material from the base spacer elements 150b, and may be of the same or different shape. The spacer elements 150 may be shaped elements, the configuration of which will be dependent upon the shape of the cryogenic container 10. For instance, the shaped elements may be simple blocks, for example for a container with a quadrilateral foot-print, or annular elements, for example for a container with a circular foot-print. Alternatively, the shaped element may be a corrugated support or a mesh support.

The spacer elements 150 may be made of a material selected from the group comprising: a polymeric material, a layer of porous material, a fibrous material and a granular material. Where one or both of the first and second liquid barrier layers 50, 100 are formed out of a polymeric material, any polymeric material for the spacer elements 150 may be the same as the material used to form one or both of the first and second liquid barrier layers 50, 100. The polymeric material may be polyvinyl chloride, polyurethane or polypropylene. The layer of porous material may be an open cell foam, such as polyurethane or polyvinyl chloride. The fibrous material may be a mat of glass fibre. The granular material may be selected from the group comprising barite and alumina.

Optionally, the cryogenic container may further comprise one or more thermal insulating layers, which may be located inside the annulus 200 and/or outside of the second fluid barrier layer 100. An example of a cryogenic container including thermal insulating layers will be further detailed with reference to Figure 3, hereinbelow.

Also optionally, the cryogenic container may further comprise an optional mechanical support frame (not shown), e.g. surrounding or enveloping the first and second fluid barrier layers and any optional insulating layers . The support frame may be composed of wood, steel or a composite material. However, a self-supporting cryogenic container may also be provided, particularly when the first and second liquid barrier layers 50, 100 comprise a composite material.

The annulus 200 formed inside, at least, the wall portions 50a and 100a of the first and second fluid barrier layers 50 and 100, with help of the one or more spacer elements 150, provides one or more voids through which any fluid that may have passed through the first liquid barrier layer 50 can be transported to the fluid vent(s) 250. The fluid may be any vapour from the liquefied gas 400 stored in the cryogenic container 10 which has diffused through the first liquid barrier layer 50. Alternatively, the fluid may be the liquefied gas itself, should the structural integrity of the first liquid barrier layer have been compromised such that channels, such as cracks, connecting the liquefied gas 400 and annulus 200 are formed.

The apparatus disclosed herein and the use thereof functions to remove any fluid, particularly gas, collected in the annulus 200. One or more fluid vents 250 which are in direct fluid communication with the annulus 200 are provided. These vents 250 allow the venting of the annulus, and any accumulated fluid. As shown in Figure 1, each vent may have an associated valve 300. Sealing means 260 can be provided, for instance at the top of the wall 20 such as at the top rim of the wall 20, to contain the annulus 200 whereby a gas-tight volume is formed within the annulus 200. Any material capable of forming a gas-tight seal may be used to form the sealing means 260. However, in embodiments wherein the first and/or second liquid barrier layers are polymeric, the sealing means 260 may be formed of the same material as the first or second polymeric liquid barrier layers 50, 100. The venting of the annulus 200 can be assisted by connecting at least one of the fluid vents 250 to a pressure differential device 320, such as a fan, a compressor or a pump, via vent line 310. In a first embodiment the pump 320 may be a vacuum pump, such that the annulus 200 is held under reduced pressure compared to ambient pressure. In this way, any fluid entering the annulus 200 will be removed by the action of the vacuum pump. In an alternative embodiment, the pressure differential device 320 passes an inert gas to the annulus 200, for example via at least one of the fluid vents 250. The inert gas can entrain any accumulated fluid, which will be removed with the inert gas through one or more fluid vents 250.

In this way, the cryogenic container 10 disclosed herein can be used in the venting of fluids, such as gas or the liquefied gas, from the annulus 200.

The annulus 200 may comprise one or more voids, the voids defined by the walls of the annulus e.g. the first and second liquid barrier layers, the one or more spacer elements 150, and optional sealing means 260. It will be apparent that at least one fluid vent 250 should be provided for every void forming the annulus 200 which is not in fluid communication with another void which has an associated vent 250. In this way, effective venting of the annulus 200 can be ensured. It is preferred that every void within the annulus 200 is in fluid communication, such that a minimum number of vents 250 can be used. For instance, a fluid permeable material such as an open cell foam, or a porous material can be used as the one or more spacer elements 150, such that fluid may travel between adjacent voids separated by such spacer elements 150. Alternatively, if materials of limited or no fluid permeability are used for the spacer elements 150, then the spacer elements 150 can be provided with channels to enable fluid transfer.

Optionally, a gas sensor (not shown) can be connected to one or more of the fluid vents 250. The gas sensor can monitor the gas vented from the annulus 200 for any gas evaporated from the liquefied gas 400. Any increase in the concentration of such a gas is indicative of a failure in the structural integrity of the first liquid barrier layer 50.

In a further embodiment, a method of leak testing a cryogenic container 10 is disclosed, the method comprising at least the steps of:

(a) pressurising the annulus 200 to a pressure above atmospheric pressure by pumping (inert) gas into the annulus 200 with a pressure differential device 320;

(b) monitoring the pressure in the annulus 200 with a pressure measuring device to provide at least two pressure measurements at different times; and

A significant decrease in the at least two pressure measurements, more preferably in the pressure measurements over time, is indicative of a leak in the cryogenic container 10.

In a practical embodiment, the annulus is pressurised to a pressure in the range of about 1.1 to 10 bar, for instance 2 to 5 bar, or about 3 to 4 bar.

The leak testing can be optionally complemented with detection means for detecting the location of the leak.

The location of the leak can for instance be determined by detecting the sound produced by the gas passing the leak in the liquid barrier. Thus, using a sound detection device it is substantially possible to determine the exact location of the leak, either in the first or the second liquid barrier.

Alternatively, the location of the leak can be determined by filling the annulus with a predetermined substance, and subsequently detecting said substance on the other side of the liquid barriers. The substance could for instance be helium or a similar detectable gaseous substance. Using a detector, the presence and location of the substance on the opposite side of the liquid barriers can be detected, thus indicating the location of the leak. Multiple leak tests may be conducted subsequently to improve the accuracy thereof.

Figure 2 shows a schematic representation of a cross- sectional view of a second embodiment of the cryogenic container 10. A cryogenic storage cylinder is disclosed having a wall with a circular cross-sectional contour, comprising first liquid barrier layer 50a and second liquid barrier layer 150a. The wall portions of first and second liquid barrier layers 50a and 150a are separated by wall portion spacer element 150a. Again, the first fluid barrier layer 50a, and optionally also the second fluid barrier layer 100a, may be polymeric.

Wall portion spacer element 150a is a corrugated support. The corrugated support contains channels 155 allowing fluid communication between adjacent vertical voids formed between the corrugated support 150a and the wall portions of either the first or second liquid barrier layers 50a, 150a. In this way, all the voids forming the annulus 200 are linked and can thus be vented.

Figure 2 can be seen as the cross-section obtained from the sectional view 5 of Figure 1 when the container is cylindrical and wall portion spacer element 150a is a corrugated support .

In a preferred embodiment, one or both of the first and second liquid barrier layers 50, 100 are made from a polymeric material. The presence of the vented annulus 200 as described herein is particularly desirable when the first liquid barrier 50 is made of a polymeric material, since less experience with polymeric liquid barrier layers exist than certain other types of barrier layer materials, and polymeric liquid barrier layers may in some instances turn out to be more prone to permeation of gas molecules than certain other barrier layer materials such as metals.

Advantageously, the polymeric material composite material may have a tensile Young's modulus of less than 50 GPa. Preferably, the tensile Young's modulus is determined according to DIN EN ISO 527 at ambient conditions, that is standard atmospheric conditions according to ISO 554, in particular the recommended atmospheric conditions i.e. at 23 ⁰C, 50% relative humidity and at a pressure between 86 and 106 kPa.

The tensile strain at break of the composite is at least 5% at ambient conditions. Preferably the tensile strain at break at ambient conditions is above 8%, more preferably above 10%, and even more preferably above 15%. Typically, the tensile strain at break at ambient conditions is not more than 75%. The tensile strain at break is determined according to DIN EN ISO 527 at ambient conditions .

The stress of a material is related to its tensile Young's modulus and its coefficient of thermal expansion, and for cryogenic materials, it has hitherto been considered that low stress materials could not be used with cryogenic fluids due to the significant changes in temperatures experienced in use. However, it has been surprisingly found that the use of a composite material having a relatively low tensile Young's modulus can be used with cryogenic fluids. The use of such composite materials reduces the thermally induced stresses on one or both of the first or second liquid barrier layers 50, 100 as well as on any supporting structure, thereby enabling a wider range of materials to be selected for any supporting structure.

As used herein, a "composite material" is an engineered material made from two or more constituent materials with different physical or chemical properties and which remain separate and distinct on a macroscopic level within the finished structure. The tensile Young's modulus value of the composite material may depend on the relative amounts of the materials used. The person skilled in the art will readily understand how to vary the volume fractions of the various components of the composite material to tailor the desired properties.

In one embodiment disclosed herein, the composite polymeric material used as one or both of the first and second liquid barrier layers 50, 100 is a mono-material composite, i.e. a composite material formed from two layers comprising the same material, for example two layers of oriented thermoplastic material that are fused together at elevated temperature and pressure, thus forming thermoplastic matrix material interdispersed between and in the layers of oriented thermoplastic material. As is known by those skilled in the art, elevated pressure, in particular hydrostatic pressure, is important to control the melting temperature of the oriented thermoplastic material. Furthermore, in the mono-material composite material one or more additives may also be incorporated being chemically different.

In another embodiment disclosed herein, the composite polymeric material used as one or both of the first and second liquid barrier layers 50, 100 is a plastic matrix material reinforced by a reinforcer, preferably where the reinforcer is at least partially incorporated into the plastic matrix material. The plastic matrix material may thus function as a continuous solid phase in which the reinforcer is embedded. There are no specific limitations with respect to the ratio of plastic matrix material and reinforcer . The reinforcer may be in the form of chopped or continuous fibres, flakes or particles, but is preferably transformed into a material having a textile-like structure, such as felt, woven, roving, fabric, knit or stitched structure. Further it is preferred that the reinforcer is selected from the group consisting of natural and thermoplastic materials or a combination thereof. The natural material may comprise fibres including vegetable fibres such as coir, cotton, linen, jute, flax, ramie, sisal and hemp; and animal fibres such as sheep wool, horse hair, and silk.

Preferably the reinforcer comprises a thermoplastic material. Preferably the thermoplastic material for the reinforcer comprises a polyolefin selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, polyisobutene or a copolymer or terpolymer thereof, preferably polypropylene.

The reinforcer can also be selected from a broad range of materials including carbon fibres, glass fibres, and polymeric fibres as long as the resulting composite material has a tensile Young's modulus of less than 50 GPa and a tensile strain at break of at least 5%.

Preferably, the reinforcer has a tensile strain at break of at least 5% as determined according to DIN EN ISO 527 at ambient conditions, more preferably the tensile strain at break at ambient conditions is above 8%, even more preferably above 10%, and most preferably above 15%. Typically, the tensile strain at break at ambient conditions is not more than 75%.

The plastic matrix material can be selected from a broad range of materials such as polymer materials including polyester, polycarbonate, vinyl ester, epoxy, phenolic resins, polyimide, polyamide and others, as long as the resulting composite material has a tensile Young's modulus of less than 50 GPa. However, it is preferred that the plastic matrix material has a tensile Young's modulus of 0.1-5.0 GPa as determined according to DIN EN ISO 527 at ambient conditions.

The plastic matrix material preferably includes a thermoplastic material or a thermoset material.

According to an especially preferred embodiment the plastic matrix material is a thermoplastic material. An advantage of the thermoplastic material is that it can be easily shaped. Preferably, the thermoplastic material comprises a polyolefin selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, polyisobutene or a copolymer or terpolymer thereof, such as EPDM, preferably polypropylene .

The mono-material composite is preferably a thermoplastic material, in which both an oriented thermoplastic material, such as a reinforcing fibrous phase, and a matrix between the oriented thermoplastic material, comprises, preferably consists essentially of, more preferably consists of, the same thermoplastic polymer. Bonding is achieved due to controlled surface melting of the oriented thermoplastic material. The physical properties of the mono-material composite, such as tensile Young's modulus and coefficient of thermal expansion (CTE), can be controlled by the extent of melting effected in the process, which determines the oriented/not oriented thermoplastic material volumetric ratio, also referred to as the fibre/matrix ratio.

The way to manufacture these mono-material composites is known in the art and has for example been disclosed in US patent application publication No. 2005/0064163; B. Alcock et al . (2007), Journal of Applied Polymer Science, Vol. 104, 118-129; and B. Alcock et al . (2007), Composites: Part A (applied science and manufacturing), Vol. 38, 147-161, incorporated herein by reference.

The manufacturing process typically utilizes oriented thermoplastic polymer fibres in various forms: unidirectional lay-up, woven fabric or chopped fibres/non-woven felt. As is known in the art, it is important to control the fibres ' melting temperature by hydrostatic pressure. The fibres are heated under elevated pressure to a temperature that is below their melting point at the elevated pressure but above the melting temperature at a lower pressure. Reduction of pressure for controlled time results in melting of the fibres, which starts at the fibre surface. This surface melting under controlled pressure followed by crystallization produces the consolidated structure.

An alternative known process involves the use of a special co-extrusion of matrix material around oriented thermoplastic material strands, such as fibres. This process of co-extrusion and tape welding has advantages over the conventional sealing processes because of the large sealing window (130-180 ⁰C) without loss of material properties.

Preferably, the mono-material composite comprises, more preferably consists essentially of, even more preferably consists of, a polyolefin selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, polyisobutene or a copolymer or terpolymer thereof, such as EPDM, more preferably polypropylene. The composite polymeric material is to be used as used the first and second liquid barrier layer under cryogenic conditions, that is below -30 ⁰C, more preferably at temperatures below -100 ⁰C, or even below -150 ⁰C. Such a temperature (below -100 ⁰C, preferably below -150 ⁰C, typically, -160 ⁰C) is suitable for liquefied natural gas (LNG) .

For the purposes of this specification a liquefied gas has been liquefied by lowering the temperature to cryogenic conditions. A liquefied gas includes a cryogenic liquid, a gas that is kept under cryogenic conditions and a supercritical fluid that is kept under cryogenic conditions. A liquid barrier layer is a barrier suitable for cryogenic liquids. Cryogenic conditions for the purposes of this specification mean temperatures less than -30 ⁰C, preferably less than -100 ⁰C, more preferably less than -150 ⁰C. The composite material is preferably used at temperatures less than -100 ⁰C, more preferably less than -150 ⁰C because in that way the benefits of using composite materials with these properties are fully exploited.

Preferably, the composite material has a tensile strain at break of at least 3% as determined according to DIN EN ISO 527 at -196 ⁰C (in liquid nitrogen), more preferably at least 5%, even more preferably at least 6%, even more preferably above 8%, even more preferably above 10%.

The composite material preferably has a coefficient of thermal expansion less than 250 x 10^~6 m/m/°C at 40 ⁰C. More preferably, the composite material is oriented and the composite material has a coefficient of thermal expansion less than 250 x 10^~6 m/m/°C at 40 ⁰C in the direction of the orientation of the composite material.

Further, preferably, the composite material has a coefficient of thermal expansion less than 100 x 10^~6 m/m/°C at -60 ⁰C. More preferably, the composite material is oriented and the composite material has a coefficient of thermal expansion less than

100 x 10^~6 m/m/°C at -60 ⁰C in the direction of the orientation of the composite material.

The coefficient of thermal expansion can suitably be determined according to ISO11359-2 in the temperature range between -60 and +70 ⁰C by thermal mechanical analysis (TMA) .

An example of a suitable material for one or both of the first and second liquid barrier layers 50, 100 is a composite material composed of a polypropylene matrix material enforced with polypropylene fibres, i.e. a single-polymer composite material. Such a composite material can be obtained as such as e.g. CurvTM ClOOA (obtainable from Propex Fabrics, Gronau, Germany) and has a tensile Young's modulus of 3.2 GPa as determined according to DIN EN ISO 527 at ambient conditions.

Another example of a suitable composite material is a composite material composed of polypropylene fibres co- extruded with a polyethylene-polypropylene mixture. The co extruded material is molten to form the matrix for the composite, marketed under the tradename "PURE"

(obtainable from Lankhorst Pure Composites B. V., Sneek, the Netherlands) . Tensile Young's modulus at ambient conditions was about 6.4 GPa and the tensile strain at break was 10% for this composite material.

Other suitable composite materials have been described in A. Pegoretti et al . , "Flexural and interlaminar mechanical properties of unidirectional liquid crystalline single-polymer composites", Composite Science and Technology 66 (2006), pp. 1953-1962, the content of which is hereby incorporated by reference.

Table I below lists a number of properties of the above-mentioned composite material CurvTM ClOOA.

Table I

The coefficient of thermal expansion was determined according to ISO11359-2 in the temperature range between -60 and +70 ⁰C by thermal mechanical analysis (TMA) . Measurements were carried out in both the fibre direction and the direction perpendicular to the fibre direction. Other measurements were performed according to the methods set out herein above.

Reference is now made to Figure 3. Figure 3 schematically shows a longitudinal-sectional view of a "polymeric" cryogenic container 10 for storing liquefied gases such as LNG, LPG and liquid nitrogen. Cryogenic container 10 comprises a load bearing structural outer shell 120. The outer shell is preferably composed of a base plate 120b and a sidewall 120a. The load bearing outer shell 120 may be made from a metallic material such as nickel steel, or concrete, but is preferably made from a stiff plastic material such as carbon reinforced epoxy material or glass reinforced epoxy material.

The container 10 includes a roof 350 preferably composed of a load bearing layer 360, which can be made of the same material as the load bearing outer shell 120, and an insulating roof layer 370, which can be made of and insulating polymeric material such as polyurethane foam. In the embodiment shown in Figure 3, the roof 350 contains two channels 380 for the vents 250 from annulus 200. The roof 350 should provide a gas-tight seal with any suitable part of the walls 20 of the container, such as but not exclusively the sidewall 120a, and between channels 380 and the vents 250. In a further embodiment (not shown), the roof 350 may contain one or more gas outlets for the removal of gas evaporated from the liquefied gas 400, such as boil-off gas from LNG.

The container 200 further includes two polymeric liquid barrier layers 50, 100. The first polymeric liquid barrier layer 50 is in contact with the liquefied gas 400 contained in the container 10. The second polymeric liquid barrier layer 100 lies between the first polymeric liquid barrier layer 50 and the load bearing structural outer layer 120. The first and second liquid barrier layers 50, 100 can be made from the same materials as discussed above. The composite material defined herein, a suitable example of which is given in Table I above, is particularly preferred.

At least two thermally insulating layers, i.e. a first or 'inner' thermally insulating layer 60 and a second or 'outer' thermally insulating layer 110 are provided. The first thermally insulating layer 60 can be provided inside the annulus 200, either directly adjacent to the first polymeric liquid barrier layer 50, on the side opposite to the liquefied gas 400, or respecting some space between the first polymeric barrier layer 50. When the first thermally insulating layer 60 is directly adjacent to the polymeric liquid barrier layer 50, it is essential that at least the first thermally insulting layer 60 allows the transport of any fluid which has passed through the first polymeric liquid barrier layer 50 into the annulus 200. Thus, the first thermally insulating layer 60 may be fluid permeable, such as an open-cell foam. If the first thermally insulating layer is nevertheless wholly or partially fluid impermeable, such as a closed-cell foam, channels or other pathways may be provided in the first thermally insulating layer 60 between the first liquid barrier layer 50 and the annulus 200 to allow fluid transport.

The second thermally insulating layer 110 can be secured to the inner surface of the structural shell 120, and forms a layer between the structural shell 120 and the second polymeric liquid barrier layer 100.

Each thermally insulating layer 60, 110 may include panels of an insulating material, for instance a foamed plastics material such as polyvinyl chloride (PVC) or polyurethane (PUR) .

In a further embodiment not shown in Figure 3, an additional liquid barrier layer can be applied directly adjacent to the inner wall of the structural outer layer 120 i.e. between structural outer layer 120 and the second thermally insulating layer 110. The additional liquid barrier layer may be present to prevent water penetration from outside the structural outer layer 120 into the container. The additional liquid barrier layer may be formed of a material selected from the group comprising: steel, particularly carbon steel, epoxy resin, polyethylene, polypropylene and a composite material, such as the composite material disclosed herein.

Annulus 200 is provided between the first thermally insulating layer 60 and the second polymeric liquid barrier layer 100. The first thermally insulating layer 60 and second polymeric liquid barrier layer 100 are separated by spacer elements 150.

It is apparent that spacer elements 150 separate the first and second polymeric liquid barrier layers 50, 100. However there is no requirement that the first and second polymeric liquid barrier layers 50, 100 provide the walls which define annulus 200.

In the embodiment shown in Figure 3, a plurality of spacer elements 150 are provided between wall portions 60a and 100a and between base portions 60b and 100b of the first thermally insulating layer 60 and second polymeric liquid barrier layer 100 respectively. The spacer elements 150 can be simple shaped blocks of polymeric material, such as the material used to form the first polymeric liquid barrier layer, or porous material, such as an open-cell foam, for instance the same material as the second thermally insulating layer e.g. open-cell polyurethane foam.

The spacer elements 150 are preferably fluid permeable such that one continuous void is formed in annulus 200. In the event that the spacer elements 150 are not fluid permeable, and they are located in such a way that multiple, unconnected voids are formed within the annulus 200, then fluid communicating channels linking neighbouring voids should be provided in such spacer elements 150, or each void should contain a dedicated vent 250.

The annulus 200 is contained by sealing means 260. In the embodiment of Figure 3, the sealing means provides a gas-tight seal between the first thermally insulating layer 60 and the second polymeric liquid barrier layer 100, which define the opposite sides of the annulus 200. The sealing means may be a polymeric material such as the material used for the polymeric liquid barrier layers 50, 100, or a closed-cell foam such as may be used for the thermally insulating layers 60, 110.

Vents 250 are provided through the sealing means 260, or at further positions as required (not shown) to ensure that the entire annulus 200 can be vented, thus removing any accumulated fluid, such as evaporated gas from the liquefied gas. For instance, a vent to drain accumulated fluid may be provided at one or more of the lowest points of the annulus, such as in the base portion of the container, to drain any accumulated fluid, especially liquid.

The first and second liquid barrier layers 50, 100, thermally insulating layers 60, 110, spacer elements 150 and outer shell 120 may be fixed to each other by any suitable means such as spraying, gluing, mechanical fixation, fusion welding etc., as is known in the art. In a further embodiment (not shown), the cryogenic container 10 may further comprise an optional resilient layer. The resilient layer may be placed between two adjoining layers forming one or both of the walls 20 and base 30 of the container 10. The resilient layer is formed of a material which is elastic under cryogenic conditions and allows relative movement between adjacent layers to compensate for thermal expansion and contraction in the container 10. For instance, a resilient layer can be provided between first polymeric liquid barrier layer 50 and first thermally insulating layer 60, or between second liquid barrier layer 100 and second thermally insulating layer 110. If such a resilient layer is provided between the first polymeric liquid barrier layer 50 and the annulus 200, it must allow fluid communication between the first polymeric liquid barrier layer 50 the annulus 200. The resilient layer may be, for instance, fluid permeable or provided with fluid channels.

The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention as defined by the appended claims.

Claims

C L A I M S

1. A cryogenic container (10), for storing a cryogenic liquefied gas, the container comprising one or more walls (20) and a base (30), together capable of holding a load of the cryogenic liquefied gas, said one or more walls (20) comprising at least: a first liquid barrier layer (50); a second liquid barrier layer (100); one or more spacer elements (150), disposed between the first and second liquid barrier layers (50, 100), to separate the first and second liquid barrier layers (50, 100), thereby providing an annulus (200); and one or more fluid vents (250), in direct fluid communication with the annulus (200) for venting fluid from the annulus (200) .

2. The cryogenic container (10) of claim 1, wherein at least one of the one or more vents (250) is connected to a pressure differential device (320), the container further comprising: - sealing means (260) to contain the annulus (200); a pressure differential device (320) for pressurising the annulus (200) to a pressure above atmospheric pressure by pumping gas into the annulus; a pressure measuring device connected to the annulus for monitoring the pressure in the annulus over time; and a comparator for comparing at least two pressure measurements from the pressure measuring device.

3. The cryogenic container (10) of claim 2, comprising a sound detection device for detecting the location of a leak.

4. The cryogenic container (10) of claim 2, comprising:

- a predetermined substance that is introduced in the annulus; and

- a substance detection device for detecting the presence of the predetermined substance outside of the annulus.

5. The cryogenic container according to one or more of claims 2-4, wherein the pressure differential device (320) is a vacuum pump to remove any fluid from the annulus (200) .

6. The cryogenic container (10) of claim 5 wherein the pressure differential device (320) passes an inert gas to at least one of the one or more vents (250) of the annulus (200), thereby flushing the annulus (200) with inert gas .

7. The cryogenic container (10) according to one or more of the preceding claims further comprising a load bearing structural outer shell (120) .

8. The cryogenic container (10) according to one or more of the preceding claims further comprising one or more thermally insulating layers (60, 110) .

9. The cryogenic container (10) according to claim 8 wherein the one or more thermally insulating layers (60, 110) are positioned between one or both of (i) first and second liquid barrier layers (50, 100) and (ii) the second liquid barrier layer (100) and the outer shell (120) .

10. The cryogenic container (10) according to any of the preceding claims wherein the one or more spacer elements

(150) comprise a shaped element selected from one or both of a corrugated support and a mesh support.

11. The cryogenic container (10) according to any one of claims 1-9 wherein the one or more spacer elements (150) are selected from the group comprising: a polymeric material, a layer of porous material, a fibrous material and a granular material.

12. The cryogenic container (10) according to claim 11, wherein the layer of porous material is an open cell foam, the fibrous material comprises glass fibre and/or the granular material is selected from the group comprising barite and alumina.

13. The cryogenic container (10) according to any one of the preceding claims, wherein one or both of the first and second liquid barriers (50, 100), preferably at least the first liquid barrier (50), is made of a polymeric material .

14. The cryogenic container (10) according to claim 13 wherein the one or more spacer elements (150) are made from a polymeric material that comprises the polymeric material of one or both of the first and second polymeric liquid barrier layers (50, 100) .

15. The cryogenic container (10) according to claim 13 or 14, wherein one or both of the first and second polymeric liquid barrier layers (50, 100) is a composite material having :

(a) a tensile Young's modulus of less than 50 GPa at ambient conditions; and

(b) a tensile strain at break of at least 5% at ambient conditions.

16. The cryogenic container (10) according to claim 15 wherein the composite material comprises a mono-material composite or a plastic matrix material reinforced by a reinforcer .

17. The cryogenic container (10) according to claim 16 wherein the mono-material composite is a thermoplastic polymer, said mono-material composite comprising an oriented thermoplastic polymer and a matrix between the oriented thermoplastic polymer, all comprising the same thermoplastic polymer.

18. The cryogenic container (10) according to any one of claims 15 to 17 wherein the composite material further comprises a structural support layer.

19. Use of a cryogenic container (10) as defined in any one of claims 1 to 18.

20. A method of leak testing a cryogenic container, the method comprising the steps of: (a) pressurising an annulus (200) to a pressure above atmospheric pressure by pumping gas into the annulus with a pressure differential device (320) ;