US3319430A - Liquid supported cryogenic container - Google Patents

Description

May 15, 1967 A. B. SMALL.

LIQUID SUPPORTED CRYOGENIC CONTAINER Filed Jan. 25, 1965 FIG. I

INVENTOR. AUGUSTUS B. SMALL wfbaaw,

FIC-3.2

ATTORNEY United States Patent iice 3,319,430 Patented VMay 16, 1967 3,319,430 LIQUID SUPPORTED CRYOGENIC CONTAINER Augustus B. Small, Westfield, NJ., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed Jan. 25, 1965, Ser. No. 427,712 9 Claims. (Cl. 62--45) The present invention 4relates in general to insulated containers for use in cargo ships adapted to transport cryogenic cargoes, such as liquefied natural gas at atmospheric pressure. More particularly, the present invention relates to an improved structure and method of supporting the inner or primary tank from a surrounding insulated secondary tank aixed to the shi-ps hull.

In accordance with the invention, a thin walled selfsupporting primary tank is supported by a small volume of non-freezing liquid of relatively high density compared to the density of the cryogenic material to be placed in the primary container. The higher density supporting liquid is maintained in the space between the primary and secondary tanks. The novel fluid suspension system of the invention balances the principle hydrostatic forces upon the primary tank to permit it to be made of substantially thinner material than would otherwise be required.

In orderv to transfer liquefied gas in a practical and economical manner in relatively large volumes, it iS necessary to store the liqueiied gas at approximately atmospheric pressure, since large pressure vessels would be impractical, if not impossible, to construct for use on seagoing tankers or the like. However, liquefied gases maintained at atmospheric pressures have extremely low boilpoints, ranging from about 320 F. for liquefied nitrogen to -28 F. for liquefied ammonia, and these unusually low temperatures of the liquids present certain problems inthe design and production of insulated cargo containers. Specifically, the containers must be properly thermally insulated to prevent heat losses which would lead to excessive volatilization of the stored liquefied gas and of suicient strength to withstand the internal stresses that may be induced therein by large temperature gradients through the walls of the container. In addition, the ship must be safeguarded against uncontrolled flow of the low temperature liquid into contact with parts of the structure which could be damaged thereby. Accordingly, for the purposes of increased safety and reliability and in accordance with accepted regulatory Coast Guard codes, it has been established practice to provide a primary liquid-tight barrier and a secondary liquid-tight barrier in containers used for the storage of liquefied natural gases at cryogenic temperatures, each of which barriers is independently capable of withstanding the thermaland hydrostatic stresses imposed by the liquiiied gases.

In meeting the requirements briefly outlined above it has been suggested that a double walled self-supporting container be placed in an insulated space within the'hold of the cargo ship. Alternatively, the regulations vhave been proposed to be met by a container wherein the insulation system is backed by the ships structure and the face of the insulation system is the containing surface for the liquid cargo. The rst of these suggested systems is expensive in nature in that the structural strength thereof must be provided by means independent of the ships structure, which strength must be adequate to withstand the complete hydraulic load of the container contents. The second of the insulation techniques outlined above raises numerous problems associated with the thermal stress between the primary barrier attached to the insulation and the insulation per se, as Well as problems associated with maintenance during such periods of thermal stress. v l

In contrast, the present invention due to its use of an automatic counterbalancing external supporting liquid may employ a relatively thin walled and inexpensive primary barrier which need not be constructed to the rigid structural specications normally required. Furthermore, because of the uid support of the primary tank, differential thermal stresses between the primary tank and the ships supporting structure are compensated for automatically without recourse to elaborate mechanical techniques. In other words, the invention produced an improved cryogenic container system wherein the primary barrier may be constructed with considerable economy because it need not be designed to take the complete hydrostatic iilled load of the primary container but need only have sufficient structural rigidity to be self-supporting when in the imloaded state.

Accordingly, it is a principle of the present invention to provide an improved fluid-supported primary tank for a double walled cryogenic container used in a cargo ship.

A further object of the invention is to provide a novel method for supporting a relatively thin walled primary cargo container whereby a minimum of thermal stress is experienced and wherein dilierential movement between the primary and secondary. walls of the cryogenic container may readily occur due to an intermediate liquid lilm therebetween.

Another object of the invention is to provide a novel cryogenic liquid container system wherein the hydraulic loading across the primary tank is essentially zero due to the fluid suspension thereof. y Another object of the invention is to provide a relatively inexpensive cryogenic container system which is simple in design, rugged in construction and economical to manufacture with a minimum number of `parts while being trouble-free in operation.

For a better understanding of the nature of the invention, reference should be had to the accompanying drawings in which: i

FIG. l is a cross-sectional view of the invention in an unloaded condition as it would appear in a tanker vessel; and

FIG. 2 is a view similar to FIG. l wherein the primary tank ofthe invention is shown in its loaded condition. It will be understood that the drawings illustrate merely a preferred embodiment of the invention and that other embodiments are contemplated Within the scope of the claims hereafter set forth. y

Referring to the drawings in particular, a representative cross-sectional view of a tanker hull is shown. The conventional tanker hull includes an outer skin designated 10 of generally U shaped coniiguration which connects to an outer deck portion 12. Located within the hull is a double walled insulated cryogenic container 14. The container 14 is held in spaced relationship to the hull 10 by a plurality of suppont members 16 axed at one end to the interior ofthe hull and at the other end to the exterior of the container. The support members 16 are shown in a representative manner but those skilled in the art will readily understand that numerous types of support members may be substituted without departing from the inventive concept herein described. The double walled container 14 includes an outer tankor secondary shell 18, the interior of which is lined with a suitable cryogenic insulation system designated generally at 20. The insulation 20 is preferably of the type impervious to liquid so that a small quantity of suspension liquid designated 24 is not absorbed into the interstices of the insulation 20. Disposed within the interior of the void defined by the insulation 20 and buoyantly supported in the suspension liquid 24 is a thin walled inner or primary tank structure 22. The primary tank 22 is preferably made of aluminum, stainless steel or other similar material ranging in thickness from 1/16 to 1/8 of an inch. This thickness of material will be suicient to render the tanks structurally rigid enough to be self-supporting when empty, yet will keep the cost of the primary tank within acceptable limits. As seen in the empty condition of the tank shown in FIG. 1, the suspension liquid 24 is of suflcient quantity to support the inner tak 22 in a buoyant fashion. A relatively large amount of air space designated 26 is shown above the level of the liquid 24 to provide space into which the liquid 24 may rise as the tank 22 is filled with the cryogenic liquid to be transported.

The suspension liquid 24 is preferably nonflammable and of a density higher than the density of the cryogenic liquid to be stored within the tank 22. As envisioned, a density of two times is preferable so that as the tank 22 is filled the upper level of the liquid 24 when in hydraulic equilibrium with the weight of the filled tank 22 is approximately half way up the vertical height of the tank. The liquid 24 is also selected to have a freezing point lower than the temperature of the liquid to be transported within the container 22 so that the suspension liquid will remain in a liquid state at all times when the tank is loaded and thereby permit relative movement between the walls of the container without introducing mechanical stress considerations. The bottom edges of the tank 22 are rounded as shown to distribute the load and stresses which will arise when the container is filled and initially chilled by the introduction of the cryogenic liquid therein. As the tank 22 is loaded, the liquid 24 will rise in an annulus about the exterior walls of the tank 22 so that the exterior hydraulic loading across the container 22 is equal to the interior hydraulic loading. Under this condition, the pressure difference across the thin primary tank is zero or essentially zero.

It should be noted that the selection of the density of the suspension liquid 24 greater than the density of the liquid materials of the primary tank 22 results in rendering the upper portions of the loaded tank unsupported by exterior liquid. These unsupported upper side wall portions of the tank are free to expand horizontally under the force of the interior load to contact the inner walls of the insulation 20 as shown in FIG. 2. This increased diameter of the upper portion of the tank 22 is effective to locate and stabilize the lled tank within the insulation system. During initial tank loading the void area 26 is quickly cooled and assumes a partial vacuum to thereby expand the inner container. Thereafter, as the internal load of the tank increases, the hydrostatic interior load insures tank stabilization.

Referring again to FIG. 1, the upper portion of the primary tank 22 includes a neck 28 which is capped by a suitable removable closure plate schematically designated at 30. The exterior of the neck 28 is provided with an annular flange 32 to which is secured a flexible sealing collar designated 34. The outer periphery of the collar 34 is secured to the deck of the cargo ship. It should be noted that the height of the flange 32 in FIG. 1 is considerably above the level of the deck 12 in the unloaded condition. As the container is loaded with cryogenic material the container sinks Within the suspension liquid 24 as well as contracts due to the chilling of the tank to the position indicated in full lines in FIG. 2. In FIG. 2 the prior position of the tank in the unloaded state is designated with dotted lines. As the tank 22 becomes filled to the level indicated, its sinking and contraction will gradually lower the flange 32 to the relationship shown in FIG. 2 taking so-me of the stress oft of the centering collar 34 which assumes a plane essentially level with the ship deck.

Preferably the tank 22 is proportioned and the density of the supporting liquid 24 selected so that the net interior Volume of the space between the outside of the tank 22 and the inside of the insulation film remains constant between the loaded and unloaded condition. This balancing of the expansion of the tank versus the contraction of the tank is desirable so that venting of the void area 26 to atmosphere is unnecessary during the loading operation. Furthermore, as previously pointed out, the unvented void area 26 during the initial loading and cooling cycle will be subject to a partial vacuum to expand the tank into stabilizing Contact with the insulated walls. Ideally, in the loaded condition the increased volume due to the expansion of the upper wall portions of the tank 22 counterbalance and compensate `for the thermal shrinking of the tank as it is cooled.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. In combi-nation, a tanker having a cargo hold of predetermined shape and an insulated container for liquefied gas at atmospheric pressure and cryogenic temperature mounted within said hold; said container comprising, an uninsulated primary tank, a secondary tank surrounding said primary tank and spaced therefrom, liquid means at atmospheric pressure between said primary and secondary tanks for supporting said primary tank, said liquid means constituting substantially the sole means for supporting said primary tank and having a lower freezing point :than the temperature of the liqueed gas to be transported within said container, said primary tank being formed of low temperature resistant metal of suicient thickness `and rigidity to be self-supporting when empty and being suliciently flexible to expand in width when loaded into direct Contact with interior portions of said `secondary tank, and insulation means for maintaining the container at cryogenic temperatures.

2. The combination of claim 1 wherein said insulation means is secured to the interior surface of the secondary tank and is impervious to the liquid means container therein whereby dimensional changes in said primary tank may occur without imparting stresses to said secondary tank.

3. The combination in accordance with claim 1 wherein the specific density of said liquid means is substantiallil greater than the combined density of the primary tank and the liquefied gas to be placed within the primary tank whereby when said tank is fully loaded the supporting liquid means upper level rises to an `approximate midpor- -tion of the primary tank leaving the upper portion of the primary tank unsupported by liquid and in direct Contact with the `interior surface of said insulation means.

4. Apparatus in `accordance with claim 3 wherein the volume of air space exterior of the primary tank and interior of the secondary tank when the container is empty and at normal atmospheric temperature is adjusted .to be substantially equal to the volume above .the -top of the primary tank after the primary tank has been loaded and has reached its designed normal cryogenic service temperature.

5. The method of mounting a relatively thin walled uninsulated primary cryogenic liquid container within a secondary container mounted in .the hold of a ship comprising the steps of, floating the primary container within the secondary container by placing a limited predetermined quantity of liquid into the secondary container, selecting the density of said liquid to be substantially -greater than the density. of the cryogenic liquid to be stored within the primary container, said liquid having a freezing temperature lower than the temperature of the cryogenic liquid .to be stored in said primary container, maintaining the pressure of the liquid interior and exterior of said primary conrtainer at `atmospheric pressure, and insulating said secondary container to prevent transfer of the cryogenic temperature to the structure of Ithe ship.

6. The method in accordance with claim 5 including `the funther step of selecting the density of the supporting liquid to be approximately two times the density of the liquid to be stored in the primary container so that when the primary tank is fully loaded the upper level ofthe supporting liquid thereabout is approximately half the height of said primary tank.

7. In combination, a tanker and an insulated container for liquefied gas a-t atmospheric pressure and cryogenic temperature; said container comprising, a relatively thin walled uninsulated primary tank formed of low temperature resistant metal suiciently rigid ,to be self-supporting When empty but requiring additional external support when full, a secondary tank connected to said tanker and spaced from and sur-rounding said primary tank, and liquid means at atmospheric pressure within said secondary tank for supporting said primary tank within said secondary tank, said liquid means constituting substantially the sole means for supporting said primary tank and having a freezing temperature less than the cryogenic temperature of the liqueed gas to be stored within the primary tank, said primary tank having vertical sides free to move relative to said secondary tank as it is loaded whereby said liquid means is displaced in .a rising annulus about the sides of said primary tank, the upper pontions of said sides expanding into direct contact with said secondary tank when said primary tank is filled.

8. The combination of claim 7 wherein said liquid means has a density of approximately twice the density of the liqueed gas to be stored within said primary container.

9. The combination of claim 8 wherein said secondary tank includes insulation means fastened upon an interior surface thereof, said insulation means being impervious to said liquid means.

References Cited by the Examiner UNITED STATES PATENTS '2,269,994 1/1-942 Sperry 220-154 2,318,414 5/ 1943 Palmer 220-15 2,933,902 4/1960 Howard 62-45 2,994,452 `8/'1961 .Morrison 114-74 3,011,321 12/1961 Clauson 62-45 3,039,418 l6/-1962 Verslius 62-45 3,052,203 9/1962 Henry 114--74 3,097,084 7/ 1963 Putman 62-45 3,272,373 9/1-966 Alleaume et al. 220--9 LLOYID L. KING, Primary Examiner.

Claims (1)

1. IN COMBINATION, A TANKER HAVING A CARGO HOLD OF PREDETERMINED SHAPE AND AN INSULATED CONTAINER FOR LIQUEFIED GAS AT ATMOSPHERIC PRESSURE AND CRYOGENIC TEMPERATURE MOUNTED WITH SAID HOLD; SAID CONTAINER COMPRISING, AN UNINSULATED PRIMARY TANK, A SECONDARY TANK SURROUNDING SAID PRIMARY TANK AND SPACED THEREFROM, LIQUID MEANS AT ATMOSPHERIC PRESSURE BETWEEN SAID PRIMARY AND SECONDARY TANKS FOR SUPPORTING SAID PRIMARY TANK, SAID LIQUID MEANS CONSTITUTING SUBSTANTIALLY THE SOLE MEANS FOR SUPPORTING SAID PRIMARY TANK AND HAVING A LOWER FREEZING POINT THAN THE TEMPERATURE OF THE LIQUEFIED GAS TO BE TRANSPORTED WITHIN SAID CONTAINER, SAID PRIMARY TANK BEING FORMED OF LOW TEMPERATURE RESISTANT METAL OF SUFFICIENT THICKNESS AND RIGIDITY TO BE SELF-SUPPORTING WHEN EMPTY AND BEING SUFFICIENTLY FLEXIBLE TO EXPAND IN WIDTH WHEN LOADED INTO DIRECT CONTACT WITH INTERIOR PORTIONS OF SAID SECONDARY TANK, AND INSULATION MEANS FOR MAINTAINING THE CONTAINER AT CRYOGTENIC TEMPERATURES.

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