Membrane tanks suitable for cryogenic service

Abstract

A membrane tank for the containment of fluids at temperatures that may differ significantly from ambient temperature, for example for containing cryogenic fluids, said membrane tank comprising, in a sequential direction from an internal containment volume, a primary membrane which is fluid-tight and faces the contained fluid during operation and acts as a primary fluid barrier, an insulating layer externally surrounding the primary membrane, and an outer structure, such as a ship's hull or bulkhead or other structure, which supports the insulating layer and the primary membrane internally and thereby carries the resulting forces, at least one opening for loading and unloading of fluids, and an optional secondary membrane, if the outer structure is a steel structure or other structure that becomes brittle at cryogenic temperatures such as the outer structure of a ship hull, which divides the insulation layer into an inner insulation between the primary membrane and the secondary membrane and an outer insulation between the secondary membrane and the outer structure, the primary membrane comprising areas of flat, curved or doubly curved shape and corrugations between said areas, said areas being secured to the underlying insulation, the corrugations absorbing thermally induced strains and, to some extent, deformations caused by hydrostatic pressure and dynamic loading, the membrane tank further comprising a coupling for operatively connecting a vacuum pump to the entire insulation layer or the inner insulation layer to enable a vacuum within the entire insulation layer or the inner insulation layer during loading, containment and removal of the cryogenic fluid.

Description

[Technical field]

[0001] The present invention relates to an insulated membrane tank disposed within a surrounding support structure for the storage and transport of low, very low or high temperature fluids. More specifically, the present invention provides a membrane tank that is particularly viable for the storage and transport of cryogenic fluids, as well as a method for manufacturing the membrane tank. [Background technology]

[0002] Storage and transportation of refrigerated and cryogenic fluids is challenging, especially in the cryogenic temperature range. There are a wide variety of gases or liquefied gases that are stored and transported at very low temperatures, typically kept in insulated containment vessels. One important example is liquefied natural gas (LNG), which typically liquefies at its "boiling point" of -163°C at atmospheric pressure. Regardless of the type of fluid, all fluids that are gases at normal temperature and atmospheric pressure can be efficiently stored and transported in liquid and gaseous form in containment vessels while in a cooled and pressurized state. While it may be preferable to apply a combination of pressure and refrigeration for some gases, there are also cases where the gas is liquefied by cooling at atmospheric pressure and then transferred and stored in such a state in a vessel. LNG is an example of a liquefied gas that is typically stored without pressurization.

[0003] What happens with stored liquefied gas primarily follows two main paths, either any material kept at a temperature lower than the surroundings receives some heat, is insulated, or is not. As a result, the liquefied gas begins to boil with the gas boil-off being released from the containment. Or, the fluid converted to gas is not allowed to escape from the containment, resulting in an increase in pressure and temperature. The first case is usually referred to as an "unpressurized" containment, since the storage tank is not designed for a substantial pressure increase. Since climate-damaging gases like LNG can be very harmful if released into the atmosphere, and releases may be prohibited by regulations, the gas boil-off may have to be flared, used in engines, or returned to storage after re-liquefaction. The second case requires that the containment must be designed as a suitable pressure vessel with the strength to be able to maintain the pressure increase during the maximum expected retention time of the gas inside.

[0004] The containment strategy selected in a particular case depends on many factors, such as the type of gas, the performance of the insulation, and the actual size of the tank. Small tanks have a larger surface area to volume ratio than larger tanks, which means that the problem of heat intrusion is much more severe in small tanks than in larger tanks with the same type of insulation. As a result, relatively small tanks, such as LNG fuel tanks in small ships, are more likely to be constructed as pressure vessels capable of sustaining increased pressure. On the other hand, very large tanks, such as onshore storage tanks for LNG and LNG carriers, are usually designed to keep the gas at atmospheric or near atmospheric pressure. The present invention is primarily concerned with unpressurized or low pressure tanks.

[0005] There are essentially two main types of storage tanks for liquefied gases at atmospheric pressure: shell-type tanks and membrane tanks. Shell tanks are usually prismatic or spherical with an insulated exterior. These tanks are independent of the outer structure and are often called independent tanks, as they are designed to withstand all loads from the stored fluid, which may be vapor pressure, hydrostatic pressure, and ship motion induced forces. For this reason, these tanks are much heavier than membrane tanks. The other insulated type of tank, the membrane tank, depends on being supported by a load-bearing outer structure, e.g. a cargo hold on board a ship, or a silo-type structure on land. The reason for this is that the barrier against the cryogenic liquefied gas is only a thin, rather weak membrane, whose insulation depends on being supported by an outer load-bearing support structure through an intermediate layer of insulation. Obviously, the membrane takes on the same temperature as the cryogenic fluid in the tank, and as a result, it shrinks significantly from its original state at room temperature due to cooling. Therefore, only the membrane has to deal with the thermal shrinkage problem, as the surrounding support structure, which is not cooled and is much stiffer and stronger than the membrane. This can potentially overstress and destroy the membrane with resulting leaks. The way to deal with the thermal shrinkage of the membrane is to provide deformation flexibility in the plane of the membrane. This can be done by molding the membrane plate with geometric corrugation patterns in two perpendicular directions, so that the in-plane thermal distortion can be compensated by bending and stretching of the corrugations, while the planar part of the membrane can shrink and remain flat without being severely stressed. As recognized by many, the development of an effective corrugation shape that ensures an acceptable stress in the membrane is not simple. The corrugation geometries adopted in the industry so far have been based largely on experience and testing, but the evidence for ensuring low stress levels has been less clear. In all cases, obtaining sufficient deformability and acceptable stress levels at the locations where corrugated channels of different directions intersect is a particular challenge.

[0006] In addition to sustained thermal stresses, variable pressure loads from the internal fluid can also be an issue for the integrity and sustainability of the membrane. In the case of liquefied gas membrane tanks installed on ships, the ship's motions generate internal liquid surface waves within the tank, which can result in severe cyclic loading on the membrane, cracking, and leakage failure. This issue is particularly relevant for the high stress concentrations in the aforementioned waveforms. Furthermore, severe surface motions, often referred to as sloshing, can result in very high local dynamic pressures when such waves strike the tank walls and internal corners. These issues have been recognized as specific challenges for membrane tanks, and there have been cases over the years that have required long periods of downtime and expensive repairs of damage.

[0007] Insulated membrane tanks for LNG have been used in the gas and shipping industries for over 60 years and a lot of experience has been gained during this time. The size of membrane tanks installed on LNG carriers has also increased significantly. Single tank capacities for ships and floating terminals are up to 50,000 m 3 (Hundreds of thousands of m for a series of tanks on a ship 3 ), whereas a single land tank can hold hundreds of thousands of m 3 The size of the sigma-based ... Summary of the Invention [Problem to be solved by the invention]

[0008] However, there remains a need to provide a membrane tank for storing cryogenic, cold or warm fluids, such as LNG and even LH2 (liquid hydrogen), relative to ambient temperature, in a combination of improved safety and reduced cost. Cryogenic fluids are usually defined as fluids that have a boiling point below -90°C at atmospheric pressure. It is an object of the present invention to provide a membrane tank and method that have beneficial effects on safety, versatility and / or cost.

[0009] The search does not reveal any publications with a description or illustration of the membrane tank provided by the present invention. The closest publications that describe and illustrate the current state of the art in the art are clearly US4149652A, DE2251688B2, KR20210152835A, US4119241A, EP1732828, KR100213686B1, WO2021037483A1, US2020256514A1, US2018073678A1 and KR20160087652A. None of the above publications contain a description or illustration of a membrane tank with fully expandable vacuum insulation. Also, none of the above publications contain a description or illustration of a corrugation with a shape as seen in the cross section of a cosine function or natural buckling function, or the reason why such a shape is beneficial. [Means for solving the problem]

[0010] The present invention provides a membrane tank for the containment of a fluid at a temperature which may differ significantly from the ambient temperature, for example for the containment of a cryogenic fluid, the membrane tank comprising, in a sequential direction from an internal containment volume: a primary membrane that is fluid tight and faces the contained fluid during operation and acts as a primary fluid barrier; an insulating layer surrounding the primary membrane on the outside; an outer structure, such as a hull or bulkhead or other structure, which supports the insulation layer and the primary membrane internally and thereby carries the resulting forces; at least one opening for loading and unloading fluids; an optional secondary membrane, if the outer structure is a steel structure or other structure that becomes brittle at cryogenic temperatures, such as the outer structure of a ship hull, which divides the insulation layer into inner insulation between the primary membrane and the secondary membrane and outer insulation between the secondary membrane and the outer structure; The primary membrane has areas of flat, curved or doubly curved shape and corrugations between said areas, which are secured to the underlying substrate insulation, and the corrugations absorb thermally induced strains and, to some extent, deformations caused by hydrostatic pressure and dynamic loading.

[0011] The membrane tank is distinguished in that it further comprises a coupling for operably connecting a vacuum pump to the entire insulation layer or the inner insulation layer to enable a vacuum within the entire insulation layer or the inner insulation layer during loading, containment, and removal of the cryogenic fluid.

[0012] For clarity, cryogenic fluid is used as an example of how the tank membrane behaves, specifically how the corrugations are stretched, when filled with a cryogenic fluid. The most favorable corrugation shape seen in the cross section of the cosine function or natural buckling function refers to said shape at ambient temperature. The colder the fluid is compared to ambient temperature, the greater the thermal contraction of the areas of flat, curved or doubly curved regions and the greater the stretching of the corrugations. When the tank is filled with a fluid warmer than ambient temperature, the deformation is reversed in that the flat and doubly curved regions expand and the corrugations are compressed. The invention provides embodiments of membrane tanks for cold or cryogenic fluid storage with membrane stretching as described above, and embodiments of membrane tanks for warm fluid storage with the opposite deformation with compression of the corrugations. Some membrane tank embodiments of the invention are for containing cold or warm fluids. In embodiments of the invention, when having a cold, cryogenic, or warm fluid containment relative to the ambient temperature, the corrugations, as viewed in cross section, have the shape of a cosine function or buckling function, with minimum or near minimum elastic energy stored in the corrugations due to said stretching or compression, and minimum or near minimum stress resulting from said stretching or compression, as will be clearly explained and illustrated below.

[0013] The technical effects of the present invention are numerous, as described below. The two most important technical effects are that the novel membrane geometry provides for corrugations and inter-corrugation intersections formed with less plastic strain, resulting in significantly lower thermally induced stresses and better long-term structural integrity. This provides many technical advantages, one of which is that it allows for greater distances between corrugations, thereby achieving more efficient manufacturing and installation, as well as lower costs. For example, while the typical distance between corrugations is 0.2-0.5m between corrugations in tanks for LNG, the distance between corrugations in the tanks of the present invention, using similar or identical membrane materials, can easily exceed 1m for LNG, which is a significantly longer distance between corrugations compared to the state of the art. This results in fewer corrugations, fewer corrugation intersections, fewer welds or other joints, fewer sources of failure, facilitated manufacturing, and reduced costs. The main outcome of the present invention is that many membrane tank embodiments are feasible for liquid hydrogen containment, LH2, or liquid helium containment, which obviously does not exist or is not possible with current membrane tank technology. Even in membrane tanks for LH2 containment, the distance between corrugations can exceed 1m.

[0014] Preferably, the corrugations, as viewed in cross section at ambient temperature without thermally induced distortion, have a shape that follows a geometric function consistent with buckling of the membrane across the corrugation span when subjected to uniform compression in two directions within the plane of the membrane and / or shape as defined by a cosine function, the shape being exact or within an acceptable deviation. The technical effect of the invention is clear from the above and following description. Geometric deviations from the exact cosine function or buckling function shape of the corrugations as viewed in cross section are acceptable to the extent that there is still a technical effect over state of the art membrane tanks. Preferably, the shape is exact or within an acceptable geometric deviation of less than 5%, 3%, 2% or 1% from the perfect cosine function or buckling function amplitude at any point.

[0015] Preferably, the shape of the intersecting corrugations matches the superimposed shape of the intersecting corrugations.

[0016] The membrane tank further comprises an intermediate membrane, preferably fluid-tight, dividing the insulation into two insulating layers, each of which can be cycled with inert gas for leak detection or evacuated to avoid air condensation and / or solidification for liquid hydrogen or helium storage.

[0017] The membrane tank comprises a membrane section having corrugations, preferably formed by die pressing or other method, with section sides preferably at a maximum distance from the corrugation intersections, preferably with sides perpendicular to the corrugations extending therethrough, and the membrane is stress relieved prior to welding into an integral fluid-tight membrane, so that stress levels in the membrane are minimized.

[0018] The membrane tank preferably includes insulating blocks within the insulating layer, the blocks conforming to the size and shape of the inner flat, curved or doubly curved primary membrane area, preferably the insulating blocks are covered by a primary membrane section with joined inner or inner corrugations, with the insulating block joints under the corrugations intersections in or near the center of the primary membrane section, the blocks are arranged side by side, the blocks arranged at ambient temperature preferably contact the tank for cryogenic or cold fluid containment, preferably with tongues and notches to reduce thermal radiation when cooled, but preferably with a Δe or Δf gap between the blocks for warm fluid containment. This reduces distortions and stresses while facilitating building.

[0019] The present invention also provides a method for constructing a membrane tank according to the present invention, comprising the steps of: - constructing an insulation layer, the insulation layer being placed inside a hull or other load-bearing structure on land or offshore, surrounding the insulation on the outside; constructing or arranging at least one opening for loading and unloading of fluids; and constructing and placing a fluid-tight primary membrane on an insulating surface, supporting the insulation and primary membrane inside the insulation and carrying the resulting forces, the membrane containing, for example, a cryogenic fluid, the membrane including areas of flat, curved or double-curved shape, the areas being secured to the underlying insulation, the membrane further including corrugations between the areas to absorb thermally induced strains and any deformations caused by hydrodynamic loading. This method is distinguished by further including placing bonds for operatively connecting a vacuum pump to the insulation layer to allow for a vacuum within the insulation layer, between the primary membrane and the outer structure, or between the primary membrane and any secondary membrane during loading, containment and removal of the cryogenic or other fluid.

[0020] According to the method, the tank is constructed layer by layer and / or block by block with the blocks containing all or part of the required insulation and membrane structures, whereby the blocks are finalized, if necessary, placed on site within the membrane tank and joined.

[0021] Preferably the primary membrane is formed as plate sections which are welded or otherwise joined to complete a continuous membrane, the cross corrugations are in the centre and / or sides of the plate sections, the plate sections are preferably joined at a maximum distance from the cross corrugations, the corrugations are preferably formed by plastic die pressing or similar forming operation, preferably with sides perpendicular to the corrugations extending through the sides, and preferably stresses in the membrane are relieved by a post heat treatment such as annealing before welding or otherwise joining into a complete fluid-tight membrane, so that stress levels in the membrane are minimised. The present invention also provides the use of the membrane tank of the present invention for the storage and / or transport of fluids having a temperature different from the ambient temperature, such as cryogenic fluids such as LH2, LHe, LO2, LN2, LNG, LPG, ammonia, carbon dioxide, other cold fluids, or warm fluids.

[0022] The membrane tank preferably includes the following components: - the membrane is made of a thin sheet material having mechanical and chemical properties suitable for the purpose taking into account the properties of the fluid and the pressure exerted by the fluid in the tank; - the membrane has a geometric corrugation of a particular geometry suitable for sustaining significant thermal deformations, as further described below; - The plate sections between the corrugations may be flat, single curved, or double curved, as may be required to provide a tight, leak-free containment solution that conforms to the geometry of the external support structure; the membrane has a geometric shape which allows a simple manufacturing method by plastic forming or punching of a flat plate or by casting, - The membrane is attached to the surrounding insulation by suitable mechanical or adhesive means; - the insulation layer comprises insulation layers (several), insulation block layers and insulation boxes, which allow good insulation and significant deformation differences between the membrane and the outer support and have sufficient mechanical strength to support the actual pressure loads directly or by means of reinforcement such as a fiber reinforcement or plate load-bearing box, - the insulation layer may comprise sub-layers with fluid-tight membranes between them to ensure extra leak-tightness in compliance with applicable code requirements; - The outer support structure has sufficient rigidity and strength, - capable of carrying all loads from the inner membrane tank and can, in principle, be of any shape and form suitable for a particular purpose; - Unlike many other membrane tank designs, it significantly reduces or even completely reduces cargo losses due to boil-off - In most cases, improved insulation reduces the need to cool and condense boiling cargo for recirculating cargo, - Capable of storing liquid gases such as liquid hydrogen and helium at temperatures below the condensation temperature of air.

[0023] The thickness of the membrane plate depends on the material from which it is made, the static and dynamic pressures applied from the fluid contained in the tank, the thermal stresses and strains, and the effect of the plate thickness on the deformability and plastic strain during the initial formation of the corrugations. A typical plate thickness taking this into account may be 0.2-4 mm. The membrane plate material must be of a type suitable for the operating temperatures to which it will be exposed, for example, cryogenic temperatures require special steel or aluminum alloys that maintain their ductility at cryogenic temperatures. Examples of liquefied gases are natural gas, which has a storage temperature of about -163°C, and liquid hydrogen, which has -253°C at atmospheric pressure. Furthermore, the membrane material must be resistant to degradation caused by chemical or electrochemical interactions with the fluid in the tank. In addition to membranes made of metal plates, corrugated membranes may be made of non-metallic materials such as plastics or composites. In such cases, the membrane may be directly cast with the type of corrugation defined for the present invention, or rather the corrugations may be created by plastic forming.

[0024] A feature of the membrane tank of the present invention is the corrugated shape that allows for significant thermal contraction or expansion of the membrane without causing undue stress in the membrane. A further feature of the membrane tank of the present invention is that this goal can be achieved even if the distance between the corrugations is longer than is commonly used or possible for membrane tanks offered by industry today.

[0025] The main goal with corrugations is to provide flexibility in the plane of the membrane to compensate for in-plane thermal contraction / expansion, and since the thermal strain of an isotropic material is the same in all directions, the required flexibility can be achieved by regular checkerboard corrugations. Corrugations can rather simply be made with some form of channel-type geometry; however, the main problems arise at the intersections or crossings of the corrugated channels, since a straight intersecting succession of the channel geometry through the intersections means that the deformation is completely "locked" at the crossings. Figure 1 shows an example of a membrane tank corrugation commonly used by industry today, especially in connection with large membrane tanks for storing liquefied natural gas (LNG). In this case, the channel heights in the two directions are different, and the locking against flexibility is achieved by significant bending and folding of the channels at the channel intersections. Such corrugations are widely used, but there are some significant concerns regarding their performance. The highly folded plate geometry means very severe plastic strains during the initial plastic forming. Moreover, sharp cross folds also represent very stiff local zones and stress concentrations, such zones are often referred to as "hot spots". In addition to concerns regarding high thermal stresses at such points, an additional problem may be that such points are particularly prone to fatigue during cyclic stress applications, which may result from both thermal fluctuations in the tank and dynamic fluid motions caused by variable pressures. The present invention provides a method to achieve high in-plane flexibility without high local stress concentrations within the corrugated channels, and most importantly at the crossing zones between the corrugations. The present invention utilizes the concept of minimum potential energy to base channel and crossing geometries that derive the natural "buckling pattern" that lamellae naturally adopt when pressed together across the checkerboard pattern of their free spans. In particular, the natural elastic buckling pattern represents a geometry that results in the least stored elastic energy during a forced, uniform contraction of the membrane in two directions. This also ensures the lowest possible stresses and a smooth distribution of stresses. Further details on how the geometry according to the present invention can be generated are explained below.

[0026] As described, the membrane consists of a cross pattern of corrugations as well as membrane sections between the distinct corrugations. In the simplest case, a flat corrugated membrane can be fitted to the flat floor, wall and ceiling zones of the tank. A tank with a prismatic shape will necessarily have corner lines and sharp corners. This problem can be addressed by the present invention by shaping the membrane sections between the corrugations with a consistently cylindrical or spherical geometric shape. Similarly, when the outer support structure has a cylindrical shape, the primary corrugated membrane can be made with sections between the corrugations that are correspondingly cylindrical, so that they are themselves cylindrical. The joining area between the cylindrical tank wall and the floor or roof section can be similarly shaped so that the sections between the corrugations are made with a curved or double curved shape. A circular floor or ceiling zone can be fitted to the cross pattern of corrugations.

[0027] The smooth shape of the membrane corrugations of the present invention offers significant advantages in manufacturing compared to the corrugated membranes most frequently used today. In particular, the fact that there is no double-folding geometry and no very high plastic strains makes the manufacturing of the corrugations very simple and reduces the requirements for the ductility of the membrane material. The manufacturing of the corrugations may therefore be mainly based on punching the corrugation pattern, while the flat parts between the corrugations are restrained out of plane to avoid undesired stretching and thinning of the membrane during formation, while allowing in-plane movement. The cosine or buckled shape makes it easier to press or punch out of the corrugations and cross corrugations, since there are no double folds and no sharp corners. The corrugations and cross corrugations are therefore conical, slide easily out of the forming machine, and the shape minimizes distortion during forming, as well as during operation to contain cryogenic fluids.

[0028] It is essential that the membrane provides a leak-tight barrier that completely surrounds the stored fluid in the tank. This means that the membrane must provide a complete enclosure with bottom, side walls, and roof. It is important that the membrane does not detach from the insulation layer, since dynamic pressure, gravity, and deformation stresses can cause delamination and damage to the membrane. The attachment means vary greatly depending on the material of the membrane and the insulation. In some cases, it may be sufficient to use the adhesion of some suitable adhesive means. However, mechanical means of connecting the membrane to the insulation may also be preferred. Such means may include welded attachments, locking devices, anchor bolts or anchor pins, etc., depending on the outer structural layer of insulation and the actual consistency of the insulation. The membrane usually has openings for various types of piping, instrumentation, and possibly manhole access. Such connections may be located in the tank dome. In any case, all such openings in the membrane must be leak-tight and must be designed taking into account thermal deformations.

[0029] The aim of the present invention is to be applicable to many types of insulation outside the membrane. The use of insulation is part of the present invention, but is not limited to a specific design of such insulation. Thus, many used solutions are applicable to the present invention, with reinforced insulation systems including porous particles such as perlite, fiber and foam type insulation materials, with or without reinforcing fibers. This also includes that the insulation system is a system with a box element with a plywood type structure or a reinforcing plating such as a fiberglass reinforcing plating.

[0030] The membrane, which is fluid tight and contains the fluid to be stored, is made of a material suitable for the temperature range to which it will be exposed and has sufficient deformability, examples being austenitic stainless steels such as AISI 304 or -304L, other austenitic stainless steels, or nickel-austenitic steel alloys. Good demonstrated weldability combined with low cost is preferred over a low coefficient of thermal expansion, for example stainless steels AISI 304L or -304.

[0031] The International Gas Code requires that a secondary leakage barrier be included for the membrane tank to protect external load-bearing structures, such as cargo holds inside a ship, from the leakage of the cryogenic fluid in case of leakage of the cryogenic fluid through the primary membrane. Various solutions are available for the secondary membrane, such as corrugated membranes, flat membranes made of temperature-insensitive Invar (34% Ni), or single- or double-layer flexible membranes made of composite materials with woven fabrics, such as "Triplex". A further alternative is to use corrugated membranes formed according to the geometry defined by the present invention. Since the secondary membrane is usually subjected to less load and strain, the amplitude of the corrugations can be smaller than the primary membrane, and the thickness of the membrane can be thinner than the primary membrane.

[0032] It is recognized that a vacuum, essentially free of air or other gases, provides excellent insulation. The main embodiment of the invention provides for the use of vacuum insulation in conjunction with membrane tanks, by using current types of leak-proof membranes for both the primary and secondary membranes, and by evacuating the air in the layer between them. By vacuum we mean here that the internal gas, typically air, is evacuated to the extent practically feasible, meaning a very low internal pressure and near-vacuum state, e.g., below 0.01, 0.001, or 0.0001 atmospheres. In such a case, the space between the two membrane layers must be filled with a support layer, a porous layer, or a fibrous layer, having sufficient structural strength to carry the pressure from the fluid, including the atmospheric overpressure from outside the vacuum layer. This layer should be suitable for evacuating by pumping out the air trapped in the layer. An additional insulation can be used outside the secondary membrane. An interesting application with this solution is that the membrane tank can be used to store fluids at cryogenic temperatures, such as liquid hydrogen and liquid helium. In the case of the membrane tank of the present invention for the storage of liquid hydrogen or liquid helium, the temperature difference created by the vacuum layer must be greater than the temperature difference between the temperature difference of the stored fluid and the condensation temperature of the air. The insulation on the outside of the secondary membrane may be of the conventional air-filled type, of sufficient strength and thickness to ensure adequate insulation against the outside air or the supporting structure, or, optionally or preferably, may be vacuum insulation.

[0033] The objective of the present invention is to provide a membrane tank solution for the widest possible range of applications, including fuel and cargo tanks for cryogenic fluids deployed on ships, platforms, onshore storage, and for underground storage. The design of the actual support structure must be such that it can carry the weight of the fluid stored in the tank, the weight of the insulation system, and the static and dynamic pressures transmitted from the fluid in the tank. The versatility of the support structure design is an integral part of the invention, and the invention can be used with most types of structural external tank solutions.

[0034] The invention also provides a membrane tank for the containment of a fluid at a temperature which may differ significantly from the ambient temperature, for example for containing a cryogenic fluid, the membrane tank comprising, in sequential direction from an internal containment volume: a primary membrane that is fluid tight and faces the contained fluid during operation and acts as a primary fluid barrier; an insulating layer surrounding the outside of the primary membrane; an outer structure, such as a ship's hull or bulkhead or other structure, which supports the insulation layer and the primary membrane on its inside and carries the resulting forces; at least one opening for loading and unloading fluids; and an optional secondary membrane dividing the insulation layer into an inner insulation between the primary and secondary membranes and an outer insulation between the secondary membrane and the outer structure, if the outer structure is a steel structure that becomes brittle at cryogenic temperatures, such as the outer structure of a ship hull; the primary membrane has regions of flat, curved or doubly curved shape and corrugations between said regions, said regions being fixed to an underlying insulation, the corrugations undergo thermally induced distortion, and the membrane tank has a shape as seen in cross section of a cosine function or natural buckling function, characterized in that when the tank is cooled under a cryogenic fluid load, minimal elastic energy is stored in the corrugations due to thermally induced contraction, and only elastic stresses are produced in the corrugations due to thermal contraction; and preferably the shape of the intersecting corrugations is free of sharp bends or corners and free of double folds, allowing simple die forming, and preferably the shape is accurate or within an acceptable geometric deviation of less than 5%, 3%, 2% or 1% from a perfect cosine or buckling function amplitude at any point, and preferably the actual stretching Δe and Δf upon cooling of the film by ΔT spans e and f of the initial corrugations at ambient temperature, where c and d are the dimensions between each corrugation, as follows: Δe=e T -e=cc T =-cαΔT, and Δf=f T -f=-dαΔT (3) Further, α is the secant coefficient (coefficient) of thermal expansion of the membrane, preferably the membrane tank includes insulating blocks within an insulating layer, the blocks conform to the size and shape of an inner flat, curved, or doubly curved primary membrane region, and are preferably covered by a primary membrane region having inner or inner corrugations and cross corrugations to which the insulating blocks are joined, with the insulating block joints below the corrugation intersections at or near the center of the primary membrane region, the blocks being arranged side by side, and the blocks arranged at ambient temperature preferably have tongues and notches to reduce thermal radiation when cooled, but preferably have gaps Δe or Δf between the blocks for containment of warm fluids.

[0035] The present invention also relates to a method for constructing a membrane tank according to the present invention, comprising the following steps: - constructing an insulation layer, said insulation layer being placed inside a hull or bulkhead or other load-bearing structure of a vessel and surrounding an exterior of the insulation; constructing or arranging at least one opening for loading and unloading of fluids; a first membrane that is fluid tight on said insulating surface, said external structure supporting the insulation and the first membrane inside said insulation and carrying the resulting forces, said membrane containing, for example, a cryogenic fluid, said membrane including areas of flat, curved or doubly curved shape, said areas being secured to the underlying insulation, said membrane further including corrugations between said areas for capturing thermally induced strains, said membrane being shaped to have corrugations between areas of flat, curved or doubly curved shape, said areas being secured to the underlying insulation, said corrugations having, in cross section, the shape of a cosine or buckling function; an insulation layer, wherein elastic energy is stored in the corrugations during formation and by thermally induced stretching of the tank as it cools when it is loaded with the cryogenic fluid, so that the stretching creates minimal stress in the corrugations; Preferably, the membrane tank is constructed to include an insulating block within the insulating layer, the block conforming to the dimensions and shape of the inner flat, curved or doubly curved primary membrane area, preferably covered by a primary membrane area with inner or inner corrugations and cross corrugations to which the insulating block is joined, with an insulating block joint below the corrugation intersection at or near the center of the primary membrane area, the block preferably located at ambient temperature, preferably in contact with the tank for cryogenic or cold fluid storage, preferably with tongues and notches to reduce thermal radiation when cooled, but preferably with gaps Δe or Δf between the blocks for warm fluid storage, resulting in reduced stresses and strains and easier construction.

[0036] A typical membrane tank of the present invention is a tank for transporting LH2, LNG and other cryogenic or low temperature liquids in a ship, where the hull structure and possibly further internal structures such as bulkheads are the outer load-bearing structure. Other typical membrane tanks of the present invention are stationary storage tanks such as concrete tanks or metal tanks standing on the ground or buried in the ground or located underground such as caverns, where the concrete, metal, other structures or surrounding cavern structures are or are part of the outer load-bearing structure. In many preferred embodiments of the membrane tank, the insulation comprises a foam with open pores such as PU-polyurethane, which is strong and at the same time good to vacuum, preferably molded as a block, preferably with grooves facing the primary membrane to facilitate vacuuming when installed. [Brief description of the drawings]

[0037] [Figure 1] FIG. 1 shows a corrugation pattern not according to the invention that is used in many current insulating membrane tanks. [Diagram 2] Figure 2 shows a membrane cargo tank inside the hull of a ship. [Figure 3a] FIG. 3a shows a membrane tank according to the invention with outward and inward corrugations and two insulating layers. [Figure 3b] FIG. 3b shows an alternative embodiment of a membrane tank according to the invention. [Figure 4] FIG. 4 shows a membrane tank according to the invention having one layer of vacuum insulation and one layer of conventional insulation. [Diagram 5] FIG. 5 shows the thermal contraction of a membrane layer where the deformation is mainly handled by the corrugated grid. [Figure 6] FIG. 6 illustrates the principle of producing a low energy corrugated shape without large stress concentrations. [Figure 7] FIG. 7 shows a further extension of the low energy membrane geometry for corrugation intersections. [Figure 8] FIG. 8 shows how the geometry of the membrane at the intersection of the corrugations can be derived by use of computer simulation. [Figure 9] FIG. 9 shows an example of a membrane corrugation shape generated by computer simulation and computer graphics. [Figure 10] FIG. 10 shows some examples of the adaptation of the present invention to different tank shapes. [Figure 11] FIG. 11 shows how a membrane section having multiple corrugations can be manufactured. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The problem of thermal contraction or expansion in multi-barrier insulation systems is often addressed using some form of geometric corrugation, whereby the flexible membrane barrier allows for deformation resulting from temperature changes on different sides of the insulation. A typical example of dealing with different thermal conditions and deformations is the design currently used for insulated tanks for storing refrigerated or cryogenic fluids such as liquefied natural gas (LNG). In the case of membrane-type cargo tanks for carrying LNG on board ships, it is the ship structure itself that provides the load-bearing support structure, while the cryogenic fluid is kept insulated and separated from this structure by an insulating layer with sufficient insulating capacity and strength, and a leak-tight membrane to the internal fluid. Regulations may also require a secondary leak-tight barrier inside the insulating layer for safety reasons. The basic problem arises when the membrane barrier to the cryogenic fluid thermally contracts significantly, but the tank structure as part of the ship does not. With extensive thermal contraction, the flat membrane apparently breaks or becomes loose due to thermal contraction and distortion. This problem is typically addressed by providing geometric corrugations in an initially flat membrane such that the corrugated zones compensate for shrinkage due to bending and stretching of the corrugations. Complicating this problem is that thermal shrinkage naturally occurs in both directions of the membrane, which requires that the corrugations must also be oriented in two directions that are typically perpendicular to each other. Thus, a grid of corrugations includes corrugation intersections. This requires that the corrugations cannot continue uninterrupted through the intersections, but must be "broken" at these intersections to fully allow two-dimensional shrinkage.

[0039] Figure 1 shows a typical example of how the corrugation intersection problem is solved by current technology. In addition to the basic membrane surface 10, there is a corrugated channel 11 in one direction and a somewhat larger corrugated channel 12 in the perpendicular direction. The "breaking" of the intersecting corrugations is performed by forming folds or "knots" 13 and 14 perpendicular to the length of the two corrugations, so that each corrugation can contract in its own length direction. It can be seen that the corrugations have multiple rather sharp bends, which imply significant local plastic distortions during the geometric formation of the corrugation pattern. Obviously, additional heat-generated stresses arise during operation caused by the actual thermal contraction of the membrane. The double channel shape with the double folded knots implies very stiff structural zones that typically give rise to strong stress concentrations or "hot spots", and these stress concentrations can cause very high stresses that exceed the stress levels normally tolerated for the particular material used for the membrane. The invention is based on the principle of defining significantly different types of corrugations and cross shapes that minimize the deformation energy stored during thermal contraction, thereby strongly reducing the elastic and any plastic distortions during cooling or heating, and / or the invention is based on a fully expandable vacuum insulation, which also has beneficial consequences with regard to fatigue damage during thermal or mechanical cycles.

[0040] Figure 2 shows the general principle of a membrane tank. Since the membrane has little structural strength by itself, it must be supported by a load-carrying containment structure that carries all the static and dynamic pressures from the liquid phase fluid and the gas phase fluid transferred from the fluid inside the tank. In figure 2 this is shown for a cargo tank in a ship, where the ship's hull 20 with the cargo hold 21 provides a complete and sufficiently strong tank enclosure to withstand the pressures from the fluids in the form of liquid 22 and gas 23. The tank must be able to hold a fluid at a completely different temperature than the surrounding structure, which may be a very hot or highly refrigerated fluid, so it is necessary to separate the fluid from the containing structure with a suitable insulation layer 24. Obviously, the fluid must be contained within a leak-proof barrier, here a relatively thin and flexible membrane 25. The membrane 25 is therefore the primary barrier to the contained fluid and acquires the temperature of the contained fluid. Further references to the types and properties of insulation of the membrane are given later. The figure also shows openings with piping 26 and 27 extending to the outside, for the purpose of filling and emptying the tank. The tanks usually include openings for human access for inspection, as well as for various forms of cables for monitoring, instrumentation, and internal pumps. The tanks in the figures reflect the typical shape of membrane tanks on ships, such as LNG cargo tanks. However, the support structures for the membrane tanks of the invention may have very different shapes, such as cylindrical or box-like shapes, and may be located on marine structures or on land. The membrane tanks of the invention are equally well applied in such cases.

[0041] Figure 3a shows the primary membrane layer of the invention with insulation in more detail. Here, the membrane 25 is fully connected with the underlying insulation and is therefore characterized by a smooth section 30 that is flat, cylindrical or doubly curved with rounded corners depending on the geometry of the outer tank 21. Assuming the fluid is highly cooled, the membrane section will shrink according to the differential temperature between the manufacturing temperature of the installed membrane and the temperature of the stored fluid during operation, such shrinkage is indicated by the double arrow in the figure. A continuously smooth membrane can easily break because the rigid outer load-bearing structure completely resists the shrinkage and the thermal distortion of the restrained membrane must be handled by the membrane itself without breaking the membrane, possibly resulting in other forms of damage such as delamination and separation. A way to address this problem is to mold the membrane with distinct curved corrugations, as indicated by the protruding sections 31. Thus, the shrinkage of the smooth section 30 can be addressed by the stretching and bending of the corrugated sections 31. The membrane is preferably thin to keep the bending stresses of the corrugations at an acceptably low level. Nonetheless, the membrane thickness must be sufficient to maintain fluid pressure against the free-floating corrugations.

[0042] The figure also shows that the insulation may consist of two layers: a primary insulation layer 32 and a secondary insulation layer 34 separated by a secondary leak-tight membrane 33. The main insulation material used for the insulation layer is usually an insulating foam, such as polyurethane foam, with or without additional fiber reinforcement. Fiber insulation and weaker types of insulation, such as perlite pebbles, can also be used. It may therefore be necessary to reinforce the insulation layer with box-shaped load-bearing elements made of plywood or other suitable reinforcement materials to transfer pressure from the fluid inside the tank to the support structures 20, 21. Technical solutions for the insulation layer are known from existing industrial practice and are readily available.

[0043] The secondary membrane 33 is a safety measure to protect the surrounding structure 20 from exposure to very cold fluids in the event of a leak through the primary membrane 30. Having a secondary membrane is a safety measure typically required by codes for LNG membrane tanks. The figure shows a secondary membrane without corrugations, which is insensitive to thermal contraction and therefore can be made from a material that does not shrink, such as the steel-nickel alloy Invar. Alternatively, it is made from some form of woven material, such as a Triplex membrane, where the membrane itself is leak-proof and at the same time sufficiently elastic. As will be shown later, the secondary membrane can also be made with corrugations like the primary membrane. The primary membrane must be attached to the insulating layer to maintain its position. There are various ways to do this, for example by various forms of mechanical attachments that can also include gluing. http: / / www.ivt.ntnu.no / ept / fag / tep4215 / innhold / LNG%20Conferences / 2007 / fscommand / PO_11_Y_Lee_s.pdf

[0044] Figure 3b shows an alternative in which the corrugations 35 of the invention are directed into the insulation rather than into the tank. The corrugations of this embodiment can equally well accommodate thermal contraction due to bending and stretching of the corrugations, the difference being that the fluid pressure acts on a "suspension" type shape rather than an "arch" type shape. It can also be seen from the figure that the primary insulation layer has grooves 36 to accommodate the corrugations entering the insulation layer. When choosing between the options shown in Figures 3a and 3b, it can be considered that it is easier to weld together the sections and corrugations of type 3a compared to corrugations that are bent into the insulation layer.

[0045] The object of the present invention is to be applicable to different types of insulation systems. One alternative type of insulation is vacuum insulation, which is based on the principle that the air or gas in the vacuum insulation layer is evacuated to a very large extent, such as a small fraction of atmospheric pressure, thereby effectively reducing heat transfer by conduction and convection through the layer. It is also important to note that vacuum insulation is the only practical type of insulation for storing liquid hydrogen at -253°C (20K), since porous or fibrous insulation filled with air or other gases would rapidly liquefy and even solidify at such temperatures, thereby causing the insulation layer to lose its insulating capacity.

[0046] FIG. 4 shows how a vacuum insulation layer can be implemented for the present invention. The insulation system consists of two layers, a vacuum insulation layer 40 and an insulation layer with regular porous or fibrous type insulation 41. These layers are separated by a leak-proof membrane 42 that ensures that air does not leak from the outside into the vacuum layer. When storing liquid hydrogen in the tank, the thickness of these layers should be balanced so that the temperature in the second layer 41 adjacent to the secondary membrane 42 is kept above the liquefaction temperature of the air. In all applications, the use of vacuum insulation can be an effective and space-saving way to achieve the desired insulation properties. It is clear that in all cases the secondary membrane 42 also undergoes substantial cooling and thereby significant shrinkage. With respect to the primary membrane 30, it is important to avoid excessive stress of the secondary membrane due to thermal shrinkage, which is easily addressed by applying corrugations 43 that bend and stretch as the membrane 42 undergoes cooling. The corrugation shape 31 provided by the present invention, used for the primary membrane, can be applied equally well to the secondary membrane. The fact that the cooling and thermal contraction is somewhat smaller than that of the primary membrane allows to use a somewhat smaller amplitude of the corrugations 43 of the secondary membrane. The space between the two membrane layers is subjected to the pressure from the stored fluid, as well as an additional compression (suction) from the vacuum (an additional compression of about 1 atmosphere). The space between the two membrane layers must therefore safely carry the resulting compression, which takes place in a porous or fibrous material 44 with sufficient compressive stiffness and strength. This spacing material may include a reinforcement system with plate reinforcement, as previously mentioned in relation to figures 3a-3b, and figure 4 also shows that there may be an open space 45 between the zones of the supporting material, as indicated by the openings between the two layers of corrugations 31 and 43. These openings 45 provide a system of "tubes" that extend throughout the vacuum enclosure around the inner tanks 24, 25. This tube network is very useful for the process of efficient evacuation of the space between the two membranes. Obviously, the supporting layer 44 must also be air evacuated through these tubes 45.

[0047] As will be readily appreciated, it is an alternative to using insulating corrugations oriented as shown in FIG. 3b for the primary membrane rather than corrugations oriented as shown in FIG. 3a and FIG. 4 for the vacuum insulation layer.

[0048] The present type of vacuum insulation can also be used as an effective means of insulation for storage of many types of liquids other than liquid hydrogen in the membrane tanks of the present invention.

[0049] Figure 5 shows in more detail how the invention works. Figure 5a shows a cross section with a 3x3 pattern of membrane corrugations installed before the tank is filled with cold fluid. In particular, the membrane tank of the invention works equally well for hot liquids, but this will not be repeated here, as it is understood that this simply means that the thermal deformation has the opposite sign to that described below. Line 50 shows the pattern of system lines for the corrugations on the surface of the primary membrane before cooling takes place. The distance between the system lines is "a" in one direction and "b" in the other, but in most cases the two distances are the same. The figure also shows a smooth membrane area 51 between the corrugations and a shaded corrugation pattern 52 (inward or outward) located between the blocks before cooling. The smooth part of the membrane 51 corresponds to 30, while the shaded area 52 corresponds to the corrugations 31 in Figure 3a, with the difference that 51 also includes the intersections between the intersecting corrugations. The size of the contact area between the membrane and the underlying insulating support is c in one direction and d in the other. Thus the corrugation span, before cooling, is e=ac in one direction and f=bd in the other direction.

[0050] Figure 5b shows the state after thermal cooling has been performed, with a uniform inner surface and the thermal contraction strain εT being the same in all directions of the membrane. This contraction is indicated by the arrows in the diagram, which show how the flat parts of the membrane contract. Furthermore, the distance between the system lines 50 on the primary side is still in the same place, since the membrane tank is completely locked by the fact that it is restrained by the outer support structure 20, 21, which does not undergo the cooling caused by the cold fluid in the tank. Since the membrane contracts and the system lines do not move, the thermal contraction must be compensated by bending and stretching of the flexible corrugations 52, while the smooth parts 51 contract according to the temperature change and the thermal properties of the applied membrane material. The thermal strain εT and the actual contraction of the smooth parts of the membrane depend on the temperature change ΔT after the water bath cooling and the secant thermal expansion coefficient α of the membrane.

[0051] c T =c(1+ε T )=c(1+αΔT), and d T =d(1+αΔT)(1) In addition, ΔT is negative considering that the initial temperature before cooling is 20°C, while liquefied natural gas is 163°C and liquefied hydrogen is 253°C. The contraction of the smooth part between the corrugations means that the width of the corrugations is stretched, and therefore, e T =ac T , and f T =bd T (2) where the actual mechanical stretch of the corrugation is Δe=e T -e=cc T =-cαΔT, and Δf=f T -f=-dαΔT (3) and negative ΔT indicates positive values ​​for elongation of Δe and Δf. This is shown as an increase in the size of the corrugated zone in Figure 5b. The membrane thickness and geometric parameters such as a, b, c, d and the size and shape of the corrugations are selected according to the practical conditions of the particular application and the requirements regarding the allowable flow stress and strain. The large spacing a and b between the corrugations means less corrugations, less welding and a cheaper solution. Numerical simulations show that distances between corrugations of more than 1 m are feasible with the present invention, which is significantly larger than common current membrane tank designs such as the corrugations shown in Figure 1, up to about 0.2 m being typical.

[0052] As shown in Fig. 4, the secondary membranes 42, 43 can also be implemented as corrugated membranes according to the present invention. The primary insulation layer 44 can be a conventional insulation or a vacuum insulation depending on the embodiment of the present invention. The temperature change ΔT of the secondary membrane will be smaller than that of the primary membrane and therefore will undergo less thermal contraction / expansion. Therefore, the corrugation amplitude of the secondary membrane can be somewhat smaller than that of the primary membrane.

[0053] It is mentioned herein that the thermal shrinkage is addressed by the flexibility of the corrugation. The objective is therefore to establish a corrugation shape with the best possible performance, taking into account both the plastic strain during manufacture and the combination of flexibility and minimum stress that arises when it is stretched by cooling (or compressed by heating). A basic principle of mechanics states that a linear elastic body under load always deforms such that the sum (integral) of the elastic energy stored during deformation is minimal (principle of minimum potential energy). Thus, the first step is to establish an initial shape of the corrugated channel with spans e and f (see FIG. 5a) that represents the minimum potential energy shape when the corrugated channel is changed by thermal deformation.

[0054] As is known, a clamped beam or plate strip subjected to an axial load, or equivalently, a forced end shortening, buckles in a geometric shape precisely defined by a mathematical "cosine" function. This solution can be derived from the beam equations using the principle of minimum potential energy for stability problems. The cosine function therefore represents the shape that gives the smallest possible accumulation of stresses in the clamped beam or plate strip during buckling. In particular, the cosine function is the preferred geometric shape for membrane corrugations, since it also represents the minimum energy condition for buckling or compression of a thin plate strip with widths e and f. Figure 6a shows a thin elastic plate 60 with a clamped side 61 subjected to an end load 62. The clamped end state corresponds to the transition to a flat membrane zone supported by a thermal insulation layer.

[0055] FIG. 6b correspondingly further illustrates the elastic buckling shape 63 as a result of a finite displacement Δ at the end 64. Assuming small displacements, the buckling shapes for both the force load 62 and the forced displacement 64 are exact cosine functions. Although the cosine function applies only to infinitesimal deformations, it can be easily scaled to any corrugation span 65 and amplitude 66 of the corrugation defined by its apex, the “apex” 67. Numerical simulation studies have confirmed that the scaled-up cosine function works very well with the initial corrugation shape definition. In particular, the cosine shape provides very smooth curvature changes and moderate or minimal plastic distortions during the plastic forming of the corrugation at the time of manufacture. Going a step further, the principle of using buckling shapes for corrugations can be easily extended by utilizing more advanced buckling shapes, taking into account nonlinear and large displacement effects. Thus, rather than using a simple cosine function, a preferred shape can be generated by computer simulation of plate buckling that takes into account large displacement effects. In particular, such shapes have been shown to work even better than small modified cosine functions. The large displacement buckling shape can also be scaled according to the desired span 65 and amplitude 66 for the corrugation. Thus, Figure 6b shows a shape that may be said to represent the nonlinear theoretical buckling shape for the initial corrugation shape.

[0056] Figure 6c shows that generating a corrugated shape can also account for the change in geometric shape caused by the lateral pressure q (see 68). In linear elastic theory, the deformed shape of a clamped beam subjected to a uniform lateral pressure is given by a fourth-order polynomial with a maximum amplitude 69. For pure pressure in the tank without thermal deformation, this would be a good choice for the corrugated geometry. However, for a membrane tank dominated by thermal distortion, the pressure effect on the corrugation is secondary and can be neglected. Depending on the actual thermal versus pressure deformation, it is also possible to augment the cosine buckled shape with a pressure polynomial depending on the actual pressure. Similarly, by using a large displacement computer-simulated shape that considers both the large displacement effect and the precisely applied pressure load, it is possible to generate a highly deformed shape 63 with the desired amplitude 66 according to the method described herein.

[0057] A particular challenge arises in that the simple extension of the geometric shapes defined for the straight sections of the corrugations ("channels") cannot be extended and applied without modification to the intersection of the corrugated channels shown in FIG. 5. The reason for this is that the continuous straight apex line 67 through the intersecting corrugations completely "locks" against the flexibility required to compensate for the thermal deformation of the intersecting corrugations at the intersection of the apex lines (see line 50 in FIG. 5). This problem is solved here by applying an extension of the method using the buckled shapes generated as previously defined by the present invention. FIG. 7 shows the intersection area between two intersecting corrugations 70 and 71. Specifically, FIG. 7a shows two intersecting corrugations with a system line 50, as also shown in FIG. 5. For simplicity, as will be the case for most applications, the intersecting corrugations in the figure have the same shape as the corrugation span e1. The key to generating the lowest possible stresses is to avoid stress concentrations and hard points, therefore the border between the intersecting corrugations and the flat zone should be rounded, as shown by 72, here shown as a circle with a radius of curvature R.

[0058] The geometric lines of the buckled shape of the intersecting cosine-type corrugations are shown in Figures 7b, 7c, and 7d. Figure 7b shows the initial cosine or buckled cross-sectional shape 73 of the corrugated membrane, which represents the straight sections of the corrugations all the way up to their transition to the intersection zone marked by lines AA and BB in Figure 7a. The figure shows the corrugation span e1, marked 74, the initial corrugation height h1, marked 75, and the underlying support from the primary insulation 76. The figure shows that there may be gaps 77 between the insulation blocks below the corrugations, the size of which is not significant provided that it is smaller than e1. 78 shows the apex lines of the straight corrugations positioned according to the system lines 50.

[0059] Figures 7c and 7d further show an intersection shape according to the invention that prevents locking of the intersecting corrugations. Instead of directly continuing a straight corrugation with apex 78 according to a line 79 through the intersection, the intersecting corrugation is augmented with an additional buckle with apex 80, with amplitude h2, marked 81. The span of this additional buckling function is also e2, marked 82, between the transition lines AA and BB in Figure 7a. The flexibility provided by this superimposed buckle with length e2 allows the intersection to deform along the system line 50 as well as in the diagonal direction, as shown as a prerequisite in Figure 5b.

[0060] Further definition of the shape at the intersection is given in FIG. 7d, showing the corrugation shape 83 along the diagonal sections EE and FF. The span of the diagonal portion is e3 marked 85, and the corrugation height 84 is the sum of h1 and h2, where e2 and e3 are direct functions of the magnitude of e1 and R. The transition between the corrugation intersection and the flat portion of the membrane by the edge function 72 can alternatively be fitted to a geometric shape other than a circular portion. Examples of this are parabolic functions, bi-ellipses, B-splines, or other geometric functions that provide a smooth, continuous corner function marking the boundary to the corrugation.

[0061] The choice of parameters e1, R, h1 and h2 implicitly defines the corrugation shape along cross sections AA, BB, boundary line 72 with radius R, and cross sections CC, DD, EE, FF. The buckling geometry can be chosen to be a cosine function depending on span and amplitude according to the selected parameters. The smooth edge function 72 has an outward normal slope equal to zero, which coincides with flat or smooth parts of the membrane. Based on these defined characterizing buckling lines and boundary conditions, a perfect and smooth buckling shape can be defined using a geometric surface fit. Such smoothing techniques between certain geometric curves are widely used in numerical computer graphics. For example, various B-spline type techniques including "Nerve" (non-uniform rational basis spline) are much used in computer generated surfaces and animated movies.

[0062] The concept of using buckling functions to define the corrugation shape suggests an alternative approach in which the buckled shape is generated more directly by simulating the buckling of the corrugation intersections by the use of nonlinear computational mechanics with large displacement effects. Its basis is the thermal contraction mechanism of the membrane shown in Figure 5b, but the buckling phenomenon occurs by the inverse problem of thermal contraction, which is thermal expansion. Figure 8 shows a membrane intersection with an undeformed free span zone 90. The arrows 91 along the edges of the free span zones show how these boundaries move when the membrane is heated and the boundary line 92 moves accordingly. Note that these arrows are opposite to those shown in Figure 5. When the movement along the boundary line 92 is large enough, buckling of the free span zone occurs. This process can be simulated using computational mechanics, for example, by using the finite element method. The figure shows a discretized mesh 93 for one-quarter of the buckling field. For simplicity, only a fairly coarse mesh is shown in the figure. The smooth part of the membrane 94 with the corrugated edge line 92 is constrained against out-of-plane buckling. Further details on the calculations are outside the present scope, and it should suffice to state that the computational model can include the entire field shown in the figure, or it can include smaller 1 / 4 or 8 models by introducing appropriate boundary conditions along the lines of symmetry. The minimum energy buckling shape can be established by numerical simulation by imposing appropriate displacement conditions around the edges of the zones included in the computational model. The buckling calculation can be linearized as an eigenvalue problem, or based on a fully nonlinear, large displacement formulation. The resulting buckling shape can be scaled in plane and in amplitude to arrive at a suitable corrugation shape that matches the main geometric parameters of the corrugated membrane. This geometry can then be applied to plastically form the corrugated membrane during manufacturing. Although it may seem paradoxical, the buckling shape produced by compression also represents the minimum energy form for shrinkage. The reason for this is that the stresses from small deformations of the corrugation shape are essentially the same, but with opposite sign, upon shrinkage (cooling) rather than when compression is applied.

[0063] FIG. 9 shows a computer generated plot of the corrugated shape produced in accordance with the present invention. Note that there are no sharp bends or folds, only smooth transition shapes. The geometry shown offers significant advantages over the current membrane tank geometry shown in FIG. 1, and this improved performance relates to both the magnitude of plastic strain generated during the formation of the corrugations, as well as the stresses and strains induced by thermal strains from primary warming or cooling of the membrane. Computer simulations of the membrane geometry confirmed the highly advantageous performance of the corrugated shapes produced in accordance with the present invention.

[0064] Insulated membrane tanks usually comprise a complete enclosure with a bottom, side walls, and a top roof. This is illustrated for a typical ship cargo tank in FIG. 2, but many other tank geometries can also be applied to ships, platforms, and land. The membrane geometry of the present invention is easily extended to different tank geometries, for example, considering the transition between different tank wall segments. FIG. 10 shows some examples of how the present invention can be implemented for membrane tanks of different geometries. The figure is greatly simplified to focus on the corrugations for the primary membrane, while other parts such as the primary and secondary insulation layers and the secondary insulation layer are not shown. FIG. 10a shows an example with a planar joint for a support tank structure 100 with a prismatic shape. The corrugations 101 on the primary membrane 102 can be placed at the corners between the joint surfaces. The insulation layer with the secondary membrane is shown in a simplified manner by 103. This approach is suitable for inclination angles 104 (greater than 90 degrees). Obviously, there can be multiple corrugations on the plane 102 as well, although not shown here. FIG. 10b shows the case where the support structure 100 has a 90 degree corner 105, where corner corrugations are less suitable. This problem can be addressed by rounding the sharp corners, preferably by giving the support structure an enhanced rounded geometry 106, or alternatively by filling the corner area with insulation. Depending on the radius of curvature and the appropriate distance between the corrugations, there may be several corrugations 107 in the curved corner section 108 of the primary membrane. FIG. 10c shows the adaptation of the current corrugation system to a cylindrical tank. There are multiple corrugations 109 of the primary membrane around the entire cylindrical surface. Cylindrical tanks for onshore LNG are often made of a combination of reinforced concrete and a steel layer 109, which is not shown in detail here. FIG. 10d is a three-dimensional perspective view of a sharp prismatic tank corner 110, corresponding to the prismatic tank of FIG. 10b, where the associated membrane tank corner 111 is rounded like the insulation layer below. The shaded strip 112 shows the overall corrugation system. A spherical membrane corner zone 113 is surrounded by a triangular pattern of corrugations 114 .Notably, adjacent corner corrugations still cross each other at 90 degrees, as indicated by 115.

[0065] Membrane systems easily lend themselves to manufacturing. A flat metal plate, as shown in FIG. 11, can be pressed into the desired corrugated shape by using a press die with a selected and defined corrugation shape. A prefabricated membrane plate section 120 can contain one or several corrugation units 121, depending on the size of the plate and the distance between the corrugation system lines 122. FIG. 11 shows an example with a 3×2 corrugation intersection. The outer part of the plate 123 has a width equal to half the system line distance. In this way, the corrugated plate section of the membrane can be joined along the rim 124, usually by welding, at a maximum distance away from the corrugation. Depending on the plate size and the distance between the corrugations, there are many alternatives for the pattern of corrugations in one fabricated plate, such as 1×1, 1×2, 2×2, 2×3, etc. The choice of the size of the plate section can also depend on the insulation system below the primary membrane, such as the specific insulation unit size, for example, whether insulation blocks or insulation boxes are present. The system for the attachment of the primary membrane to the insulation layer below also depends on the properties of the insulation layer, and such attachment can include adhesive and mechanical attachment.

[0066] Secondary membranes can also be manufactured using the corrugation system of the present invention following a similar procedure as described for the primary membrane. The thermal deformation of the secondary membrane will usually be less than that of the primary membrane due to less thermal shrinkage caused by the primary insulation layer. Taking into account also the absence of direct fluid pressure on the secondary membrane, these factors may allow for smaller corrugation amplitudes than the primary membrane. A special case is when the primary insulation layer is evacuated to achieve better insulation properties. The vacuum in the primary insulation layer may allow the current membrane tank system to be used with fluids having temperatures well below the condensation temperature of dry air. One important such application may be for large volume, low pressure, liquid hydrogen tanks. Depending on the stiffness and strength of the primary and secondary insulation layers, such tanks may also be able to sustain moderate pressure from inside the membrane tank.

[0067] Further explanation of the design, principles and implementation of the present invention The present invention relates to insulated membrane tanks that hold fluids at low and cryogenic temperatures or that hold very hot fluids. In principle, membrane tanks require that the tank system includes an external support structure that can carry static and dynamic pressures from the fluid inside the tank. The present invention focuses on a new type of membrane tank that can handle large thermal deformations due to thermally significant temperature differences between the fluid inside the tank and the temperature of the external support structure. This ability is achieved by the use of prescribed shapes for the membrane corrugations based on the utilization of shapes generated by the use of buckling shapes related to in-plane loads (or kinematic constraints) of beams and lamellae, and / or by having a vacuum at least in the inner insulation layer. The geometry of the corrugations, including their intersections, can be based on specifically defined buckling functions known from classical mechanics literature. Alternatively, and often better, the corrugation shapes can be derived by using nonlinear computational mechanics, following specified procedures.

[0068] Furthermore, the invention relates to insulation between the primary membrane and the external support structure of the tank. This insulation may consist of several layers, such as a primary and secondary layer of insulation, possibly with a secondary membrane for leak protection. In one main embodiment, the invention is not limited to a particular type of insulation and can be used with most types of insulation systems available today, including pebble, fiber, and porous type insulation, with or without a reinforcement system or internal load-bearing elements. In another main embodiment of the invention, at least the inner insulation layer is in a vacuum state when the tank so insulated is in operation during loading, containing, transporting, and unloading of a fluid such as LH2. For demanding operations such as LH2, preferably LNG, the two main embodiments are preferably combined.

[0069] While most current types of membrane tanks have a straight secondary membrane without corrugations, the present invention uses a corrugated membrane of the type defined by the present invention also for the secondary membrane. This allows certain applications where the primary insulation layer, also called the internal insulation layer or insulation, can be evacuated at very low internal pressure. This provides a very efficient type of insulation with better insulating properties than most insulation without vacuum. Another main advantage, which is open for certain embodiments of the present invention, is that the tank can be used for liquefied gases with very low condensation temperatures, such as liquid hydrogen and liquid helium.

[0070] The present invention can be used for a wide variety of storage, cargo, and fuel membrane tanks, both in terms of geometry and size. If the external support structure has sufficient strength, the current modular membrane corrugation system is fully scalable in sizes including tens of thousands of cubic meters. The membrane layer can be designed for significant internal tank pressures, but in most cases the tank will be limited by the ability of the insulation layer to carry the pressure.

[0071] The corrugated membrane of the present invention can be made by using plastic forming of metal plates or by casting a material suitable for the purpose. The fabricated plate units can include multiple corrugation patterns. The plate units of the primary or secondary membrane can be joined together by welding or other types of joining techniques. The membrane can also be shaped to conform to cylindrical tank surfaces and transitions between different tank planes, such as box-like or prismatic tank shapes.

[0072] The present invention provides a membrane tank capable of storing fluids at temperatures very different from the external support structure. A particular feature is that the particular geometry of the corrugated membrane is able to withstand very large deformations caused by thermal contraction or expansion while the stresses are kept within acceptable levels. This corrugated shape also has a major advantage in terms of minimizing plastic strains during the formation of the corrugations. Some specific features of the present invention can be summarized as follows: - The continuously smooth corrugations of the present invention result in a greater distance between crossing corrugations than state-of-the-art corrugations. This means fewer corrugations per area, lowering the risk of defects and manufacturing costs. For example, 2 Assuming a membrane area of ​​1 m, one corrugation intersects at the center position, and the membrane of the membrane tank of the present invention has a surface area of ​​1 m 2 In contrast, a state-of-the-art membrane with 0.5m corrugations has 1m 2 Each membrane contains four corrugated intersections. - The membrane portion of the membrane tank of the present invention can be easily manufactured by press die forming techniques, because the wavy intersections are monotonically tapered and conical in shape, and therefore do not lock into a configuration with folds or "knots" 13 and 14, as exemplified for state of the art wavy intersections, otherwise the slope from the flat, curved or doubly curved area to the crest of the intersecting corrugations is always less than 90 degrees. - On the other hand, the primary membrane will contract (or expand) significantly due to the large difference in temperature between the fluid and the external structure supporting the membrane, but the bending and stretching of the corrugations allows such deformations to be accommodated without any part of the membrane being overstressed. - The plastic forming distortion is significantly reduced. - Wave shape including intersections. - Any insulation system. - Connection between the membrane and the insulation layers of the primary and secondary insulation layers. - Secondary membrane. - Vacuum insulation. - Tanks of any shape. - Scalability. - Ease of manufacture. - Installation inside external structures such as cavities, underground tanks, free-standing tanks, tanks integrated into the hull and bulkheads. - The wave shape of the cosine or buckling function follows known formulas from textbooks, such as known polynomials or Taylor series, with accuracy within the next series part and / or is determined by numerical simulation. - widths e, f, where e is the same or different from f, for cold or cryogenic use in filling with cold or cryogenic fluids and cooling the membrane, the corrugations must be high and wide enough to be stretched by Δe or Δf. For operation with warm fluids, the corrugations of widths e and f must be high and wide enough to be elastically compressed by Δe or Δf. For membrane tanks of the present invention for containing either cold / cryogenic or warm fluids, the corrugations of widths e and f must be high and wide enough to be elastically stretched or compressed by ±Δe or ±Δf. - It can be used with various types of insulation, including vacuum insulation, and insulation suitable for cryogenic or cryogenic fluids. The membrane tank of the present invention for storing LH2 or helium has a primary vacuum insulation layer and a secondary insulation layer of sufficient thickness to avoid condensation of air. - The plastic forming process of corrugations usually leaves significant residual stresses which can be reduced by the application of stress relief methods and treatments.

Claims

1. A membrane tank for containing a fluid at a temperature that may differ significantly from the ambient temperature, for example for containing a cryogenic fluid, said membrane tank comprising, in a sequential direction from an internal containment volume: a primary membrane that is fluid tight and faces the contained fluid during operation and acts as a primary fluid barrier; an insulating layer surrounding the primary membrane on the outside; an outer structure, such as a hull or bulkhead or other structure, which internally supports the insulation layer and the primary membrane, thereby carrying the resulting forces; at least one opening for loading and unloading fluids; an optional secondary membrane for additional safety and prevention of leaks in outer structures, such as steel structures that may become brittle at cryogenic temperatures and thereby fracture, such as the outer structure of a ship's hull, the secondary membrane dividing the insulation layer into inner insulation between the primary membrane and the secondary membrane and outer insulation between the secondary membrane and the outer structure; the primary membrane has flat, curved or doubly curved regions and corrugations between said regions, said regions being secured to an underlying insulation, the corrugations absorbing thermally induced strain; The membrane tank further comprises a coupling for operably connecting a vacuum pump to the entire insulation layer or the inner insulation layer to enable a vacuum within the entire insulation layer or the inner insulation layer during loading, containment, and removal of the cryogenic fluid.

2. 2. The membrane tank of claim 1, wherein the corrugations, when viewed in cross-section, have a shape of a cosine function or a natural buckling function, and wherein when the tank cools during loading of a cryogenic fluid, minimal elastic energy is stored in the corrugations due to thermally induced contraction, and the thermally induced contraction produces only elastic stresses in the corrugations.

3. 3. The membrane tank according to claim 1 or 2, wherein the actual stretching Δe and Δf upon cooling of the membrane by ΔT results in the initial corrugation spans e and f at ambient temperature, where c and d are the dimensions between the respective corrugations: Δe=e Ｔ eccc Ｔ [cαΔ], Ｔ ____________________________ where α is the secant coefficient (coefficient) of thermal expansion of the film.

4. 4. The membrane tank according to any one of claims 1 to 3, wherein the shape of the intersecting corrugations matches the overlapping shape of the corrugations, has no sharp bends or corners, and has no double folds, allowing simple die forming.

5. 5. The membrane tank according to claim 1, further comprising a fluid-tight intermediate or secondary membrane dividing the insulation into two insulation layers, an inner insulation layer and an outer insulation layer, said intermediate or secondary membranes being the same or different.

6. 6. The membrane tank of claim 5, wherein the coupling for connecting the vacuum pump and the vacuum pump are arranged to provide a vacuum within the inner primary insulation layer and, if necessary, also within the outer secondary insulation layer or entirely within a single insulation layer between the primary membrane and the outer structure for improved insulation capacity and / or reduced insulation thickness.

7. 7. The membrane tank of any one of claims 1 to 6, comprising a membrane section having corrugations formed by die pressing or other method, the sides of the membrane section being at a maximum distance from a corrugation intersection, such as in or near the center of the section area, the sides of the membrane section being perpendicular to the corrugations extending through the sides.

8. A method for constructing a membrane tank according to any one of claims 1 to 7, comprising the steps of: - constructing an insulation layer, the insulation layer being placed inside a hull or other load-bearing structure on land or offshore, surrounding the insulation on the outside; constructing or arranging at least one opening for loading and unloading of fluids; constructing and placing a fluid-tight primary membrane on an insulating surface, supporting the insulation and primary membrane inside the insulation and carrying the resulting forces, the membrane containing, for example, a cryogenic fluid, the membrane including regions of flat, curved or doubly curved shape, the regions being secured to the underlying insulation, the membrane further including corrugations between the regions to absorb thermally induced strains and any deformations caused by hydrodynamic loads; The method further includes disposing a coupling for operably connecting a vacuum pump to the insulation layer to enable a vacuum within the insulation layer, between the primary membrane and the outer structure, or between the primary membrane and any secondary membrane during loading, containment and removal of the cryogenic or other fluid.

9. 9. The method of claim 8, wherein the membrane is shaped to have corrugations between flat, curved, or doubly curved shaped regions, the regions being secured to the underlying insulation, the corrugations, when viewed in cross section, have a cosine or buckling function shape, and minimal elastic energy is stored in the corrugations during formation and thermally induced stretching of the corrugations as the tank cools when loaded with cryogenic fluid, and minimal stress is created in the corrugations by the thermally induced stretching.

10. 10. A method according to claim 8 or 9, wherein the primary membrane is formed as plate sections which are welded or otherwise joined to complete the membrane, the intersecting corrugations are within the centre and / or sides of the plate sections, the plate sections are joined a maximum distance away from the intersecting corrugations, the corrugations are preferably formed by a plastic die pressing or similar pressing operation, preferably with sides perpendicular to the corrugations extending through the sides, and residual stresses from the formation of the corrugations may be reduced by suitable heat and stress relief treatment methods and, if necessary, by geometric corrections before welding or otherwise joining to a complete fluid-tight membrane.

11. Use of a tank according to any one of claims 1 to 7 for the loading, storage, transport and / or removal of fluids having a temperature different from the ambient temperature, such as cryogenic fluids like LH2, LHe, LO2, LN2, LNG, LPG, ammonia, carbon dioxide, other cold fluids or warm fluids where the thermal distortion is reversed and the corrugation is compressed instead of stretched.

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