Patent Application
20250164073
The present invention relates to a thermally insulated membrane tank placed inside a surrounding supporting structure for storage and transport of cold, very cold or hot fluids. More specifically, the invention provides a membrane tank particularly feasible for storage and transport of cryogenic fluids and a method for fabricating the tank.
Storage and transport of cooled and cryogenic fluids are challenging, particularly at cryogenic temperature ranges. There exists a broad variety of gases or liquefied gases that are stored and transported at very low temperatures and are typically kept in insulated containments. One important example is Liquefied Natural Gas (LNG) which typically liquefies at −163° C. which is the “boiling point” at atmospheric pressure. Regardless type of fluid, all fluids that are gaseous at normal temperature and atmospheric pressure can be efficiently stored and transported as in liquid and gaseous form in containments during cooled and pressurized condition. Whereas it for some gases may be preferable to apply a combination of pressure and cooling, there are also cases where the gas is liquefied by cooling at atmospheric pressure and thereafter transferred and stored at such condition in the containments. LNG is an example of a liquified gas normally stored without applied pressure.
What happens with the stored, liquified gas principally follows two main paths: Any substance kept at temperature lower than the surroundings will receive some heat, thermally insulated or not. As a consequence, the liquified gas will start boiling with gas boil-off being released from the containment. Alternatively, the fluid transformed to gas is not allowed to escape from the containment while pressure and temperature will increase as a consequence. The first case is normally referred to as a “unpressurized” containment since the storage tank is not designed for substantial pressure build-up. Since climate damaging gases like LNG are very harmful when released to the atmosphere, and release may be prohibited by regulations, the gas boil-off may have to be burned, used in engines, or returned to the storage after being re-liquified. The second case requires the containment must be designed as a proper pressure vessel with strength capable of sustaining the pressure build-up during the anticipated maximum holding time for the gas inside.
The containment strategy chosen for a specific case depends on many factors such as type of gas, performance of thermal insulation and, the actual size of the tank. Small tanks have a larger surface-to-volume ratio than larger tanks; this means that the heat ingress problem will be much more severe for small tanks than for large tanks with the same type of insulation. Consequently, relatively small tanks, such as LNG fuel tanks on small ships, are likely to be built as pressure vessels capable of sustaining pressure build-up. On the other hand, very large tanks, such as land storage tanks for LNG and LNG transport ships, are normally designed for keeping the gas at atmospheric or near atmospheric pressure. The present invention primarily concerns unpressurized or low-pressure tanks.
There are essentially two main types of storage tanks for liquified gas under atmospheric pressure; those are shell type tanks and membrane tanks. Shell tanks are normally prismatic or spherical in shape with thermal insulation on the outside. These tanks are independent from the outer structure and often called the independent tanks because they are designed to withstand all the loads from the stored fluids, which can be vapor pressure, static hydraulic pressure, and ship motion-induced dynamic pressure. This is why these tanks are much heavier than the membrane tanks. The other insulated type of tank, membrane tanks, depend on being supported by a load-bearing, outer structure, such as a cargo room in a ship, or a silo type structure on land. The reason for this is that the barrier against the cold, liquified gas is only a thin and rather weak membrane with thermal insulation that depend on being supported by the outer loadbearing structure via an intermediate layer of insulation. Clearly, the membrane will take on the same temperature as the cold fluid inside the tank and, consequently, contract significantly from the original state at room temperature because of cooling. Thus, since the surrounding support structure is not cooled and it is much stiffer and stronger than the membrane, the membrane alone will have to deal with the thermal contraction problem. This could potentially overstress and break the membrane with leakage as consequence. The way of dealing with the thermal contraction of a membrane is to provide deformational flexibility in the plane of the membrane. This can be done by shaping the membrane plate with geometric corrugations patterns in two perpendicular directions such that in-plane thermal strains can be compensated by bending and stretching of the corrugations while planar parts of the membrane can contract and remain flat without being severely stressed. As recognized by many, development of effective corrugation geometries that secure acceptable stresses in the membrane is no simple matter. Corrugation geometries employed by industry so far have to a large extent been based on experience and testing while the evidence of securing low stress levels has been less clear. In all cases it is a particular challenge to obtain sufficient deformability and acceptable stress levels at the locations where corrugation channels in different directions intersect.
Along with sustained thermal stressing, variable pressure loading from the internal fluid can also be a problem for the integrity and sustainability of the membrane. For liquefied gas membrane tanks onboard ocean going ships the ship motion will generate internal liquid surface waves within the tank which can result in severe cyclic loading on the membrane, cracking, and leakage failure.
This problem is particularly associated with high stress concentrations in the beforementioned corrugations. Moreover, severe surface motion, often referred to as sloshing, can lead to very high, local dynamic pressures when such waves hit walls and internal corners of the tank. These problems are recognized as a specific challenge for membrane tanks, and through the years, there have been cases requiring long off-time and costly repair of damage.
Thermally insulated membrane tanks for LNG have been used in the gas and shipping industries for more than 60 years and much experience has been gained during this time. There has also been a remarkable growth in the size of membrane tanks used onboard LNG carrying ships. Single tank capacities for ships and floating terminals may be more than 50000 m3 (several hundred thousand m3 for a series of tanks onboard) whereas single land tanks may have the size of several hundred thousand m3.
However, a demand still exists for providing membrane tanks for storage of cryogenic fluids such as LNG and even LH2 (liquid hydrogen), cold fluids or warm fluids, relative to ambient temperature, with a combination of increased safety and reduced cost. Cryogenic fluid is usually defined as fluid with boiling point at −90° C. or lower at atmospheric pressure. The objective of the invention is to provide a membrane tank and a method with beneficial effects on safety, versatility and/or cost.
Searching has not revealed any publication with description or illustrations of a membrane tank as provided by the present invention. The nearest publications, describing and illustrating the current state of the art in the technical field, apparently are: US4,149,652A, DE2251688B2, KR20210152835A, US4,119,241A, EP 1732828, KR100213686B1, WO 2021037483 A1, US 20,202,56514 A1, US2018073678 A1 and KR 20160087652 A. None of said publications includes description or illustrations of a membrane tank with fully scalable vacuum insulation. And none of said publications includes description or illustrations of corrugations having shape, as seen in cross section, of a cosine function or a natural buckling function, or why such shape is beneficial.
The invention provides a membrane tank for containment of fluids at temperature that can differ significantly from ambient temperature, for example for containing a cryogenic fluid, wherein the membrane tank comprises, in direction from an inner containment volume:
wherein the primary membrane comprises areas of flat, curved or double curved shape and a corrugation in between said areas, wherein said areas are fastened to the underlaying insulation and the corrugations are taking up thermally induced strain and to some extent deformations caused by hydrostatic and dynamic loads.
The membrane tank is distinguished in that it further comprises a coupling part for connecting a vacuum pump operatively to the whole insulation layer or the inner insulation layer, for enabling vacuum in the whole insulation layer or the inner insulation layer, during loading, containment and unloading of cryogenic fluid.
For clarity, cryogenic fluid is used as an example of how the tank membrane behaves, specifically how the corrugations are stretched, upon filling the cryogenic fluid. A most preferable corrugation shape, as seen in cross section, of a cosine function or a natural buckling function, refers to said shape at ambient temperature. The colder the fluid compared to ambient temperature, the more thermal contraction of the areas of flat, curved, or doubly curved areas, and the more stretching of the corrugations. If a warmer fluid than ambient temperature is filled into the tank, the deformations will be opposite, in that the flat, curved and double curved areas will expand, and the corrugations will be compressed. The present invention provides membrane tank embodiments for cold or cryogenic fluid storage, with membrane stretching as described, and membrane tank embodiments for warm fluid storage, with opposite deformation, with compression of the corrugations. Some membrane tank embodiments of the invention are for containing cold or warm fluid. In each embodiment of the invention, with cold, cryogenic, or warm fluid containment relative to ambient temperature, the corrugations have a shape, as seen in cross section, of a cosine function or a buckling function, wherein a minimum or near minimum of elastic energy is stored in the corrugations by said stretching, or compressing, and a minimum or near minimum of stress is resulting in the corrugations by said stretching or compressing. This will be clearly described below and illustrated in figures.
The technical effects of the invention are many, as will be described below. The two most significant technical effects are that the novel membrane geometry provides corrugations and intersections between corrugations which are formed by less plastic straining and result in significantly lower thermally induced stresses and better long-term structural integrity. This offers numerous technical benefits, wherein one is to enable larger distance between corrugations and thereby attain more efficient fabrication and installation, as well as lower cost. For example, while a typical distance between corrugations is 0.2-0.5 m between corrugations in tanks for LNG, the distance between corrugations for the tanks of the invention, using similar or identical membrane material and for LNG, easily can exceed 1 m, which is significantly longer distance between corrugations compared to state of the art. This results in fewer corrugations, fewer crossings of corrugations, less welding or other joining, fewer sources of failure, facilitated production and reduced cost. A major achievement with the invention is that many membrane tank embodiments are feasible for liquid hydrogen containment, LH2, or even liquid helium containment, which so far apparently is not existing or possible with current membrane tank technology. Even for membrane tanks for LH2 containment, the distance between corrugations may exceed 1 m.
Preferably, the corrugations, as seen in cross section at ambient temperature without thermal induced strain, have shape in accordance with geometric functions consistent with buckling of the membrane over the corrugation spans when subjected to uniform compression in two directions within the plane of the membrane or/and shape as defined by cosine functions, wherein the shape is exact or within an acceptable deviation. The technical effect of the invention is clear from the description above and below. Geometric deviations from exact cosine function shape or buckling function shape of corrugations as seen in cross-section, are allowable up to the extent the technical effect over state-of the art membrane tanks still is present. Preferably, the shape is exact or within an acceptable geometric deviation of less than 5%, 3%, 2% or 1% from a perfect cosine function or buckling function amplitude at any point.
Preferably, the shape of crossing corrugations complies with a superimposed shape of the corrugations crossing.
The membrane tank preferably is further comprising an intermediate membrane that is fluid tight, dividing the insulation into two insulation layers, each of which can be circulated by inert gas for leak detection or evacuated to avoid air condensation and/or solidification for liquid hydrogen or helium storage.
The membrane tank preferably is comprising membrane sections with corrugations, formed by die pressing or otherwise, with section sides at maximum distance from corrugation crossings, preferably with sides perpendicular to corrugations extending out through the sides, possibly the membrane is stress relief treated before welding into a complete fluid tight membrane, wherein the stress level of the membrane is minimized.
The membrane tank preferably is comprising blocks of insulation in the insulation layer, wherein the blocks match the dimensions and shape of the flat, curved or double curved primary membrane areas inside and preferably is covered by primary membrane sections with corrugations and crossing corrugations over or inside where insulation blocks are joined, with insulation block joints crossings under corrugation crossings at or near centre of primary membrane sections, wherein the blocks are arranged side by side, wherein the blocks as arranged at ambient temperature preferably are contacting for tanks for cryogenic or cold fluid containment, preferably with tongue and notch for reducing thermal radiation when cooled down but preferably with a gap Δe or Δf in between the blocks for warm fluid containment. Thereby, the strain and stress are reduced while the building is facilitated.
The invention also provides a method of building a membrane tank according to the invention, comprising the steps:
According to the method, the tank is built layer by layer and/or is built block by block with blocks comprising all or some of the insulation and membrane structures required, whereby the blocks are finalized if required, arranged and joined on the site in the membrane tank.
Preferably, the primary membrane is formed as plate sections that are joined by welding or otherwise methods to complete the continuous membrane, wherein crossing corrugations are at the centre and/or within the sides of the plate sections, such that plate sections are joined preferably at maximum distance from crossing corrugations, and the corrugations are preferably formed by plastic die pressing or similar forming operations, preferably with sides perpendicular to corrugations extending out through the sides, preferably the stress in the membrane is relieved by post heat treatment such as annealing before welding or joining otherwise into a complete fluid tight membrane, wherein the stress level of the membrane is minimized.
The invention also provides use of the membrane tank of the invention, for storage and/or transport of fluid having temperature different from ambient temperature, such as cryogenic fluid such as LH2, LHe, LO2, LN2, LNG, LPG, ammonia, carbon dioxide, other cold fluid, or warm fluid.
The membrane tank preferably is comprising that:
The thickness of the membrane plate depends on the material from which it is made, pressure on static and dynamic pressure exerted from the fluid contained in the tank, thermal stress and strain, and the effect of plate thickness on the deformability and plastic straining during the initial forming of the corrugations. Typical plate thickness in consideration of this may be from 0.2 to 4 mm. The membrane plate material must be of type suited for the operation temperatures to which it is exposed; for instance, cryogenic temperatures require special steel or aluminum alloys that remain ductile during very low temperatures. Examples of liquified gases are natural gas with storage temperature of about −163° C. and liquid hydrogen with −253° C. at atmospheric pressure. Moreover, the membrane material must be resistant to degradation caused by chemical or electrochemical interaction with the fluid inside the tank. In addition to membranes made of metal plates, corrugated membranes may be made of non-metallic materials such as plastic or composites. In such case the membrane may be cast directly with corrugations of the type defined for the invention rather making the corrugations by plastic forming.
A distinguishing feature of the membrane tanks of the invention, is a corrugation geometry that allows for significant thermal contraction or expansion of the membrane without resulting in overstressing of the membrane. A further distinguishing feature of the membrane tanks of the invention is to be able to achieve this goal even if the distance between corrugations is longer than what is commonly used or possible for membrane tanks provided by the industry today.
A main target by corrugations is to provide flexibility in the plane of the membrane to compensate for in-plane thermal contraction/expansion and, since thermal straining of an isotropic material is the same in all directions, the required flexibility can be achieved by a regular, checkered pattern of corrugations. The corrugations can rather simply be made with some form of channel-type geometry; however, the main problem arises at the intersections or crossing of corrugation channels since straight, intersecting continuations of the channel geometry through the intersections imply that the deformation will be fully “locked” at the point of crossing.
locking against flexibility is achieved by significant bending and folding of the channels at their intersections. Although such corrugation geometries are widely in use there are some significant concerns regarding their performance. The highly folded plate geometry implies very severe plastic straining during the initial, plastic forming. Moreover, the sharp intersection folds also represent very stiff local zones and stress concentrations, such zones are often referred to as “hot spots”. Along with concerns regarding high thermal stresses at such points, an additional problem can be that such points are particularly prone to fatigue during cyclic stressing as may occur both from thermal variations and from variable pressure caused dynamic fluid motion within the tank. The current invention provides a way of achieving high in-plane flexibility without high, local stress concentrations within the corrugation channels and, most importantly, at the intersection zones between corrugations. The invention is basing the channel and intersection geometry on utilizing the concept of minimum potential energy to derive a natural “buckling pattern” that a thin plate naturally will adopt when pressed together across a checkered pattern of free spans. Notably the natural, elastic buckling pattern represents the geometric shape that results in the least stored elastic energy during a forced, uniform contraction in the two directions of the membrane. This also ensures lowest possible stresses and smooth distribution of stresses. Further details about how geometric shapes according to the invention can be generated will be explained in the following.
As explained, the membrane consists of a crossing pattern of corrugations as well as membrane sections in between the distinct corrugations. In the simplest case, flat corrugated membranes can fit flat floor, wall, and ceiling zones of a tank. Tanks with prismatic shape will inevitably have corner lines and pointed corners. This problem can be dealt with by the present invention by shaping the membrane sections between the corrugations with consistently cylindrical or spherical geometry. Likewise, when the outer supporting structure has cylindrical shape, as itself being cylindrical, the primary corrugated membrane can be made with sections between corrugations that are correspondingly cylindrical. The joining areas between a cylindrical tank wall and the floor or roof sections can similarly be shaped such that the sections between corrugations are made with curved or doubly curved shape. Circular floor or ceiling zones can be fit with crossing patterns of corrugations.
The smooth shape of the membrane corrugations of the invention provides significant advantages in production as compared with corrugated membranes most frequently used today. In particular, the fact that there are no doubly folding geometry and no very high plastic straining simplifies production of the corrugations very much and reduces requirements to ductility of the membrane material. The manufacturing of the corrugation may thus largely be based on stamping out the corrugation pattern while the flat parts between corrugations are restrained out-of-plane to remain flat while allowing for in-plane motion to avoid unwanted stretching and thinning of the membrane during forming. The press forming or stamping out of corrugations and crossing corrugations is facilitated by the cosine or buckling shape, since no double folding exists and no sharp corners. Accordingly, the corrugations and crossing corrugations are conical and slip easily out of the forming machines and the shapes minimize strain also during forming, not only during operation for containing cryogenic fluid.
It is imperative that the membrane provides a leak-tight barrier which fully surrounds the stored fluid inside 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 come loose from the layer of insulation since dynamic pressures, gravitational forces and deformational stresses could cause detachment and damage to the membrane. Chosen means of attachment depends largely on the type of materials used for the membrane and the insulation layer. In some cases, it may be sufficient to employ some suitable means of adhesive gluing. However, mechanical means of connecting the membrane with the insulation may also be preferable. Such means may include weld attachments, locking devices, anchoring bolts or anchor pins and the like depending on the outer structural layer of the insulation and the actual consistency of the thermal insulation. The membrane will normally have openings for various types of piping, instrumentation and possibly also manhole access. Such connections may be located within a tank dome. In any case, all such openings in the membrane must be leak tight and designed in a way that accounts for thermal deformations.
It is an objective of the invention to be applicable for many types of thermal insulation on the outside of the membrane. Although use of thermal insulation is a part of the invention it is not limited to specific designs of such insulation.
Much used solutions with reinforced insulation systems including porous particles such as perlite, fiber and foam type insulation materials, with or without strengthening fibers, are thus applicable with the invention. This also includes systems where the insulation systems comprise box elements with reinforcing plating such as plywood type structures or glass fiber reinforced plating.
The membrane that is fluid tight and containing the fluid stored is made with material that is suited for the range of temperatures to which it is exposed and have sufficient deformability, examples are austenitic stainless steel such as AISI 304 or −304L, other austenitic stainless steel, or nickel austenitic steel alloy. Good proven weldability combined with lower cost can be preferred over low thermal coefficient of expansion, for example stainless steel AISI 304L or −304.
The international gas code requires secondary leakage barriers to be included for membrane tanks to protect the external, load bearing structure, such as a cargo room in a ship, against leakage of cryogenic fluid in case of leakage of 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 percent Ni), or single- or double-layer flexible membranes made of composite material with woven fiber fabric such as for “Triplex”. A further alternative will be to use corrugated membranes formed according to the geometry defined by the current invention. As the secondary membrane is normally subjected to less loading and straining the amplitude of corrugations may be less and the thickness of the membrane may be thinner than for the primary membrane.
It is recognized that vacuum that is essentially void of air or other gases provides excellent thermal insulation. The main embodiment of the invention provides use of vacuum insulation in connection with membrane tank by using leak tight membranes of the current type both for the primary and secondary membranes and evacuation of air within the layer in between. By vacuum is here meant that internal gas, typically air, is pumped out to an extent practically feasible implying extremely low internal pressure and near vacuum condition, for example 0,01, 0.001 or 0,0001 atmosphere or lower. In such case the space between the two membrane layers shall be filled with a supporting, porous or fibrous layer that has sufficient structural strength to carry the pressure from the fluid, including atmospheric overpressure from outside the vacuum layer. This layer should be suitable for vacuuming by pumping out the air trapped within the layer. Additional insulation may be used external to the secondary membrane. An interesting application by this solution is that the membrane tank may be used for storing fluids at extremely low temperatures, such as liquid hydrogen and liquid helium. For membrane tanks of the invention for storage of liquid hydrogen or liquid helium, the temperature difference created by the vacuum layer must be larger than the temperature difference between that of the stored fluid and the condensation temperature of air. The insulation on the outside of the secondary membrane may be of conventional, air-filled type and have sufficient strength as well as thickness to ensure adequate insulation against the outside air or supporting structure or could possibly or preferably also be vacuum insulation.
An objective of the invention is to provide membrane tank solutions for the widest possible range of applications, including fuel and cargo tanks for cryogenic fluids placed inside ships, platforms, for land storage, and underground storage. The design of actual supporting structure must be such that it can carry the weight of the fluid stored in the tank, the weight of the insulation system and static and dynamic pressures transferred from the fluid inside the tank. Versatility of the design of the supporting structure is an integral part of the invention, and the invention can be used with most types of structural external tank solutions.
The invention also provides a membrane tank for containment of fluids at temperature that can differ significantly from ambient temperature, for example for containing a cryogenic fluid, wherein the membrane tank comprises, in direction from an inner containment volume:
wherein the primary membrane comprises areas of flat, curved or double curved shape and a corrugation in between said areas, wherein said areas are fastened to the underlaying insulation and the corrugations are taking up thermally induced strain, wherein the membrane tank is distinguished in that the corrugations have a shape, as seen in cross section, of a cosine function or a natural buckling function, resulting in that a minimum of elastic energy is stored in the corrugations by thermally induced contraction when cooling down the tank upon loading cryogenic fluid, resulting in only elastic stresses in the corrugations by said thermal contraction,
wherein α is the secant modulus (coefficient) of thermal expansion for the membrane,
The invention also provides a method of building a membrane tank according to the invention, comprising the steps:
Typical membrane tanks of the invention are tanks for transport of LH2, LNG and other cryogenic or cold liquids, in ships, wherein the ship hull structure and possibly further internal structure such as bulkheads are the outer loadbearing structure. Other typical membrane tanks of the invention are stationary storage tanks, such as concrete tanks or metal tanks standing on ground or buried in ground or arranged underground such as in caves, wherein the concrete, metal, other structure or the surrounding cave structure is or is part of the outer loadbearing structure. For many preferable embodiments of the membrane tank, the insulation comprises foam with open pores, such as PU-polyurethane, that is strong and good for vacuuming at the same time, preferably shaped as blocks, preferably with grooves facing the primary membranes forming channels for easier vacuuming as installed.
The problem with thermal contraction or expansion within multi-barrier insulation systems is often dealt with using some form of geometric corrugation by which a flexible membrane barrier allows for the deformations arising from temperatures changes at the different sides of the insulation. A typical example of dealing with different thermal conditions and deformations is the design currently used for thermally insulated tanks for storage of cooled or cryogenic fluids such as liquefied natural gas (LNG). In the case of membrane type cargo tanks for carrying LNG onboard ships it is the ship structure itself that provides the load bearing support structure whereas the cryogenic fluid is kept insulated and separated from this structure by a layer of thermal insulation with sufficient thermal insulation capacity and strength and a leak-tight membrane against the internal fluid. Regulations may also require secondary, leak-tight barriers inside the insulation layer for safety reasons. A basic problem arises when the membrane barrier against the cold fluid thermally contracts significantly whereas the tank structure, as part of the ship, does not contract. With major thermal contraction a flat membrane would clearly break or come loose due to thermal contraction and straining. This problem is normally dealt with by providing the initially flat membrane with geometric corrugations in order that the corrugation zones compensate for the contraction through bending and stretching of the corrugations. What makes this problem difficult is that the thermal contraction naturally takes place in both directions of the membrane which requires that the corrugations also must be oriented in two directions that usually will be normal to each other. Consequently, the grid of corrugations includes crossing of corrugations. This necessitates that the corrugations cannot continue uninterrupted through the crossings, but have to be “broken” at these intersections to fully enable two-dimensional contraction.
However, the supporting structure for a membrane tank of the invention may also have a very different shape, such as a cylindrical or box-like form, and be positioned on offshore structure or on land. The membrane tank of the invention applies equally well for such cases.
The figure also shows that thermal 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 materials used for the insulation layers are normally insulation foam, such as polyurethane foam, with or without additional fiber reinforcement. Weaker types of insulation materials, such as fibrous insulation and perlite pebbles, may also be used. It may thus be necessary to strengthen the insulation layers with box-like load carrying elements made of plywood or other suitable strengthening materials for transferring the pressure from the fluid inside the tank to the supporting structure 20, 21. Technical solutions for the insulation layers are known from existing industrial practice and readily available.
The secondary membrane 33 is a safety measure to protect the surrounding structure 20 against being exposed to very cold fluid in case there is a leakage through the primary membrane 30. Having a secondary membrane is a safety measure which is typically required by the codes for LNG membrane tanks. The figure shows a secondary membrane without corrugations; this can be made of a material that is insensitive to thermal contraction and thus does not contract, such as the steel-nickel alloy invar. Alternatively, the membrane itself is made of some form of woven material, such as Triplex membrane, that has sufficient elasticity as the same time as being leak-proof. 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 insulation layers to keep it position. There are various ways of doing this such as by various forms of mechanical attachments which can also include gluing. http://www.ivt.ninu.no/epl/fag/lep4215/innhold/LNG%20Conferences/2007/fsco mmand/PO11YLee s.pdf
It is an objective by the present innovation to be applicable with different types of thermal 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 significant that vacuum insulation is the only practical type of insulation for storing liquid hydrogen at −253° C. (20 degrees K) since porous or fibrous insulation filled with air or other gas will quickly liquefy and even solidify at such temperatures, thereby making the insulation layer lose its thermal insulation capability.
As is easily understood, it is an alternative for the vacuum insulation layer to use insulation corrugation oriented as shown in
The present type of corrugated layer of vacuum insulation can also be used as effective means of thermal insulation for many more types of fluid storage than liquid hydrogen in a membrane tank of the invention.
Note that ΔT is negative for cooling considering −163° C. for liquefied natural gas and −253° C. for liquid hydrogen in relation to an initial temperature of 20° C. before cooling. The shrinking of the smooth sections between the corrugations means that the span of the corrugations will be stretched, thus
where the actual mechanical stretching of the corrugations is
Note that negative ΔT gives positive values for the stretching Δe and Δf. This is shown as increased size of the corrugation zones in
As shown in
The description hereto has referred to that thermal contraction will be dealt with by the flexibility of the corrugations. The objective is thus to establish a corrugation geometry with the best possible performance considering both plastic straining during fabrication and the combination of flexibility and minimum stresses being generated when it is stretched by cooling (or compressed by heating). A fundamental principle of mechanics states that a loaded, linearly elastic body will always deform in such way that the total (integral of) elastic energy accumulated during deformations will be at a minimum (the principle of minimum potential energy). Thus, a first step is to establish an initial geometry of the corrugation channels that represents a minimum potential energy geometry when the corrugation channels with span e and f, see of
As will be known, a clamped beam or plate strip that is subjected to axial loading, or equivalently, to a forced end shortening, will buckle into a geometric shape that is defined exactly by a mathematical “cosine” function. This solution may be derived from beam equations using the principle of minimum potential energy for the stability problem. Hence, the cosine function represents the shape giving the smallest possible accumulation of stresses within a clamped beam or plate strip during buckling. Notably, the cosine function is also a preferred geometry for membrane corrugations since it also represents minimum energy condition for buckling or compression of a thin plate strip with width e and f.
A particular challenge arises in that a simple extension of the geometries defined for the straight sections of the corrugation (“channels”) cannot be extended and applied without modifications to the crossing of corrugation channels shown in
Specifically,
Buckling shape geometry lines for the intersecting cosine-type corrugations are shown
A further definition of geometry of the intersection geometry is given in
The choice of parameters e1, R, h1 and h2 implicitly defines the corrugation geometry along sections A-A, B-B, border line 72 with radius R, and sections C-C, D-D, E-E, F-F. The geometry of the buckle along can be chosen to be cosine function with span and amplitude in accordance with the chosen parameters. The smooth edge function 72 has outward normal slope equal to zero consistent with the flat or smooth part of the membrane. Based on these defined, characterizing buckling lines and boundary conditions, a complete, smooth buckle geometry can be defined using geometric surface fitting. Such smoothing techniques between specific geometric curves are widely used in numerical computer graphics. For instance, various B-spline type techniques including “nurbs” (non-uniform rational basis spline) are much used in computer generated surfaces and in animation movies.
The concept of using a buckling function to define the corrugation geometry suggests an alternative approach whereby the buckle geometry is generated more directly by simulation of buckling of the corrugation intersection by use of nonlinear computational mechanics with large displacement effects. The basis for this is the membrane thermal contraction mechanism illustrated in
A thermally insulated membrane tank normally comprises a full enclosure with bottom, side walls, and top ceiling. This is illustrated for a typical ship cargo tank in
The membrane system lends itself easily to fabrication. Flat metal plates as shown in
Secondary membranes may also be fabricated using the corrugation system of the invention and by following a similar procedure as described for the primary membrane. The thermal deformations of secondary membranes will normally be smaller than for the primary membranes because of less thermal contraction due to the primary insulation layer. Considering also that there is no direct fluid pressure on the secondary membrane these factors may allow for smaller corrugation amplitudes than for the primary membrane. A special case is when the primary insulation layer is vacuumed to achieve better insulation properties. Vacuum within the primary insulation layer may enable the current membrane tank system to be used with fluids with temperatures far 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 as well as the secondary insulation layer such tanks may also be capable of sustaining a moderate pressure from inside the membrane tank.
This invention deals with insulated membrane tanks holding fluids at low and cryogenic temperatures or, alternatively, holding fluid of very high temperatures. In principle, a membrane tank requires that the tank system includes an external, supporting structure capable of carrying the static and dynamic pressures from the fluid inside the tank. The invention focuses on a new type of membrane tank capable of dealing with major thermal deformations due to thermal significant difference in temperature between the fluid in the tank and the temperature of the external, supporting structure. This capability is achieved by use of a defined geometry for membrane corrugations based on utilization of geometries generated by use of buckling geometries associated with in-plane loading (or kinematic constraints) of beams and thin plates, and/or is achieved by having vacuum in at least the inner insulation layer. The geometry of the corrugations including their crossing can be based on specifically defined buckling functions known from the classical mechanics literature. Alternatively, and in many cases even better, the corrugation geometry can be derived following a specified procedure and by use of nonlinear computational mechanics.
Further, the invention concerns thermal insulation between the primary membrane of the tank and the external supporting structure. This insulation may consist of several layers, such as a primary and secondary layer, of insulation, as well as having a secondary membrane for leak prevention. For one main embodiment, the invention is not restricted to a particular type of thermal insulation but can be used along with most types of thermal insulation systems available today, including pebbles, fibers, and porous types of insulation, with or without strengthening systems or internal load carrying elements. For another main embodiment of the invention, at least the inner insulation layer is at vacuum when the so insulated tank is in operation during loading, containing, transporting and unloading of fluid, such as LH2. For demanding operations, such as for LH2 and preferably also LNG, the two main embodiments are preferably combined.
Whereas most current types of membrane tanks have secondary membranes that are straight without corrugations the invention opens for use of corrugated membrane of the type defined by the invention also for the secondary membrane. This enables a particular application whereby the primary insulation layer, also termed inner insulation layer or insulation, may be vacuumed with extremely low internal pressure. This provides a very efficient type of insulation with better insulation properties than most insulations that do not have vacuum. Another major advantage is that this opens for a particular embodiment of the invention whereby the tank can be used for liquefied gases with extremely low condensation temperature, such as liquid hydrogen and liquid helium.
The invention can be used for a huge variety of storage, cargo, and fuel membrane tanks, both with respect to geometry and size. Provided the external, supporting structure has sufficient strength the current modular, membrane corrugation system is fully scalable in size including tens of thousands of cubic meters. The membrane layer can be designed for significant internal tank pressures whereas, in most cases, the tank may be limited by the ability of the insulation layers to carry pressure.
The corrugated membranes of the invention can be made by use of plastic forming of metal plates, or by casting of a material suited for the purpose.
Fabricated plate units may contain a pattern of multiple corrugations. Plate units of the primary or secondary membranes may be joined together by welding or other types of joining techniques. The membranes may also be shaped to fit cylindrical tank surfaces and transitions between different tank planes such as for box-like or prismatic tank geometries.
The invention provides a membrane tank capable of storing fluids with temperature very much different from the external supporting structure. A particular feature is that the specific geometry of the corrugated membrane can sustain very large deformations caused by thermal contraction or thermal expansion while stresses are kept within acceptable level. This corrugation geometry has also major advantages with respect to minimizing plastic strains during forming of the corrugations. Some specific features of the invention can be summarized as follows:
1 The continuously smooth corrugations of the present invention results in larger distance between the crossing corrugations than the state-of-the-art corrugation. This means fewer corrugations per area, lowering the risk of failure and cost of fabrication. For example, assuming 1 m2 membrane area, with one corrugation crossing in the center position, in a membrane of a membrane tank of the invention, then there is 1 crossing per m2 membrane. For comparison, a state-of-the-art membrane with 0.5 m between corrugations will contain 4 corrugation crossings per m2 membrane.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20220270 | Mar 2022 | NO | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NO2023/050049 | 3/3/2023 | WO |
