Storage vessel for ship transport of liquefied gas

Abstract

A ship having a free-standing or self-supporting tank for transporting a cryogenic liquefied gas, the tank comprising an enclosed metal shell which is circular in horizontal section and which has a cylindrical or conical upwardly projecting lower walled portion joined at its bottom edge to the ship hold bottom, a metal secondary bottom in the tank spaced above the ship hold bottom and extending to the cylindrical or conical lower walled portion, a circular weld supporting the secondary bottom in position above the ship hold bottom, insulation between the secondary bottom and the ship hold bottom, a metal primary bottom in the tank spaced above the secondary bottom and extending to the cylindrical or conical lower walled portion and a circular weld which serves to support the metal primary bottom in position above the secondary bottom.

Description

This invention relates to ships used for transporting cryogenic liquefied gases. More particularly, this invention is concerned with improvements in ship tanks for transporting cryogenic liquefied gases and which tanks are separate, independent, structures supported by the ship bottom and not necessarily otherwise dependent on the ship hull or hold structure for support.

Many useful gases are available or are produced at geographical locations far removed from the locations where they are used or needed. Although some such gases can be economically transported under pressure in the form of a gas, it is generally more desirable to liquefy the gas and transport it in that state because of the increased volume of gas which can be transported in liquid state rather than in the gaseous state.

Some gases can be liquefied at moderate pressures and shipped in tanks capable of maintaining the gas under such pressure to keep it in the liquefied state. Propane and butane are representative of such gases. Because the pressure needed to liquefy such gases at atmospheric temperature is not unduly great, the pressure vessel required for storage of the so liquefied gas can be built economically and of a relatively large size. Other gases, however, cannot be readily liquefied even at fairly high pressures unless the temperature of the gas is also reduced substantially below atmospheric temperature. Because it is difficult and expensive to construct a pressure vessel capable of storing a cryogenic liquefied gas at high pressure in large volume, it has been found more practical and less expensive to cool the liquefied gas to a temperature at which it can be stored in a tank designed to withstand a minimum internal pressure plus dynamic loads due to ship motions. For example, it has been found convenient to store liquefied natural gas, which is essentially methane, at about -260.degree.F. and at about 15 psia or just slightly above atmospheric pressure. Other cryogenic liquefied gases such as hydrogen, helium and ethylene can similarly be stored at about atmospheric pressure following their refrigeration to a temperature below the boiling point of the gas at such pressure.

Tanks for transporting cryogenic liquefied gases at about atmospheric pressure in a ship are of two main types. The first type of tank is one in which the tank walls and bottom are substantially continuously supported by the ship hold. Such a tank relies upon the structure of the ship for the strength and support needed to contain the liquid being stored. The second type of tank is a structurally self-supporting or free-standing tank which is spaced or separated from the ship hold wall. Such a tank does not rely on the strength of the hold walls for necessary reinforcement because it is structurally independent of the ship walls insofar as its ability to effectively contain the stored liquid is concerned.

One of the structurally independent type of tanks is spherical. Spherical tanks have been mounted in the ship hold on a metal cylindrical skirt or on a plurality of columns. Such mounting means, while adequate, are expensive and tend to undesirably concentrate the load in a small area of the ship hull instead of spreading it over a wider space. These mounting means however permit relatively easy insulation of the tank, or the ship hold, for the purposes of retarding boil-off of the stored liquefied gas and preventing the refrigerated contents of the tank from cooling the ship structure to a temperature low enough to cause structural failure of ship parts not made of metal suitable for low temperature use. There are nevertheless needed tank structures which are largely free standing, and substantially independent of the ship structure, for ship transport of cryogenic liquids which are relatively economical, simple to construct and which distribute the load over a large area of the ship hull or hold but which still permit the ready insulative separation of the tank and its contents from the ship hull.

According to the present invention there is provided, in combination, a ship having a hold with a bottom and walls and a tank, for transporting a cryogenic liquefied gas, with a structurally self-supporting or shape retaining wall and roof which is spaced or separated from the ship hold walls and which has a bottom structure which spreads the weight of the tank and any load therein over a substantial area of the ship hold bottom. The tank structure permits the ready installation of insulation to effect envelopment of the tank by an insulating environment which prevents the refrigerated contents of the tank from cooling the ship structure to a temperature low enough to cause structural failure of parts of the ship made of metal unsuitable for cryogenic uses.

The shape of the tank is not narrowly critical since tanks of many shapes can be successfully employed in practicing the invention. It is important however that the lower part of the tank be of such shape as to facilitate the utilization of separated primary and secondary bottoms positioned in spaced apart relationship above the bottom of the ship hold. The tank however will generally comprise a metal shell with a walled portion and a roof portion. The roof portion can be domed, ellipsoidal or spherical in shape. In addition, the roof portion can be primarily conical or conical with a lower peripheral curved portion.

The tank will generally comprise an enclosed metal shell which is essentially circular in horizontal section throughout its height. Also, the tank shell advisably will have an upwardly projecting lower walled portion, which is cylindrical or conical or a combination of these, which is joined at its bottom edge, such as by a continuous fusion weld or other suitable leak-proof joint, to the ship hold bottom. The tank sidewall is thus anchored to the bottom plating of the hold bottom and it depends on this anchorage to develop the uplift and downward forces resulting from the liquid product, ship motions and internal vapor pressure. The vertical force of the liquid product is, of course, supported by the hold bottom and not the tank sidewall.

Most of the tanks suitable for use in the invention will have a walled portion which has a horizontal circular bottom edge and a surface of revolution generated by the movement of a generatrix line which moves in a path touching the horizontal circular bottom edge and is constantly in the same spaced relationship to a straight vertical line perpendicular and central to the horizontal circular bottom edge. The walled portion as defined thus includes within its definition cylindrical shaped walled portions, as well as inverted conical walled portions in which the circular bottom edge has a smaller diameter than a conical section of the walled portion taken thereabove parallel to the said bottom edge.

Within the lower walled portion of the tank shell there is positioned a metal secondary bottom above the ship hold bottom and a metal primary bottom positioned above the secondary bottom. Support means maintains the secondary bottom in position above the ship hold bottom and support means maintains the primary bottom in position above the secondary bottom. Insulation is positioned between the secondary bottom and the ship hold bottom and in this regard when the insulation is of the load bearing type it can also function as the support means to keep the secondary bottom in position above the ship hold bottom. The support means maintaining the primary bottom in position above the secondary bottom can also be, although not essential, load bearing insulation which occupies most if not all of the space between the primary bottom and the secondary bottom.

The secondary bottom as well as the primary bottom can each be positioned horizontally and be essentially planar. They can however each be dished or one or the other can be planar while the other is dished. Furthermore, one or both of the secondary and primary bottoms can be dome shaped. The secondary bottom, however, must remain spaced above the ship hold bottom.

Each of the tank bottoms is usually made of thin metal and functions as a leak tight liner. The bottoms thus rely, or depend, on the ship hold bottom structure for ultimate support of the tank product load. Care is thus taken to avoid creation of a vacuum inside of the tank when empty since this could lead to an uplift of one or both of the tank bottoms because of a higher pressure between the tank bottoms, or between the secondary bottom and the ship hold bottom.

The tank shell as well as the secondary bottom and primary bottom are constructed of metal which is suitable for use at cryogenic temperatures. The novel tank structure however permits use of conventional carbon steel for the ship hull and the ship hold bottom and walls. The height of the tank walled portion from the ship hold bottom to the secondary bottom is made great enough or high enough to accommodate dimensional changes from the differential temperature gradient and, also, to prevent cooling of the ship hold bottom by a cryogenic liquid in the tank to a low enough temperature to endanger the structural integrity of the hold bottom.

In the event the primary bottom of the tank fails, cryogenic liquid or vapor thereabove will flow into the space below the primary bottom where it will be restrained against further flow by the secondary bottom which maintains its structural integrity at the low temperatures to which it is subjected. Furthermore, even though the secondary bottom is subjected to low cryogenic temperatures, no adverse cooling of the ship hold bottom is effected because of the presence of the insulation between the ship hold bottom and the tank secondary bottom. Not only does the tank provide the described safety features but in addition the tank at all times effects wide distribution of the load over the ship hold bottom thereby minimizing the concentration of stresses in the ship hull.

The invention will be described further in conjunction with the attached drawings, in which:

FIG. 1 is an isometric view of a ship having cryogenic storage tanks of the type herein provided;

FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1 through the ship hull and a storage tank positioned in the ship;

FIGS. 3A to 3E are sectional views of joint structures which can be used to connect or join the primary bottom and/or the secondary bottom to the lower walled portion of a tank shell;

FIG. 4 is a vertical sectional view similar to FIG. 2 but of a tank which has a dished primary bottom;

FIGS. 5A to 5C are sectional views of joint structures which can be used to join the periphery of a dished tank bottom to a tank lower walled portion;

FIG. 6 is a vertical sectional view similar to FIG. 2 but of a tank which has a transition joint in the lower walled portion;

FIGS. 7A and 7B illustrate transition joint structures for joining a tank to a ship hold bottom;

FIG. 8 is a vertical sectional view through a ship and a cylindrical circular tank positioned in the ship hold;

FIG. 9 is a vertical sectional view through a ship hull and a tank having a conical lower walled portion and a spherical upper or roof portion; and

FIG. 10 is a view similar to FIG. 9 but with insulation applied against the ship hold walls rather than on the surface of the tank below the ship main deck as in the tank of FIG. 9.

So far as is practical, the same identifying numbers will be used for the same or similar parts or elements which appear in the different views of the drawings.

With reference to FIG. 1 of the drawings, the ship 10 is provided with five tanks 11 for the sea transportation of cryogenic liquids. Each of the tanks 11 is supported by ship hold bottom 12 as shown in FIG. 2. The tank 11 comprises an enclosed metal shell having a metal ellipsoidal roof portion 13 which is joined to and is supported by a metal lower walled portion 14 which is substantially cylindrical and circular in horizontal section. The bottom edge 15 of the lower walled portion 14 is joined to the ship hold bottom 12, such as by fusion welding or other suitable means. The tank lower walled portion 14 is spaced inwardly from the ship hold walls 16 to provide access space for inspection of the tank and the ship for safety purposes.

Secondary bottom 18 is joined at its circular periphery to the inside surface of tank wall 14 such as by welding. Load bearing insulation 17 is placed between the ship hold bottom 12 and tank secondary bottom 18. Primary bottom 20 is joined at its circular periphery, such as by welding, to the inside surface of the tank lower walled portion 14 and it is supported in position by supporting means 19 which can be a load bearing material which is insulating or non-insulating. Instead of using a load bearing insulation material 17, the secondary bottom 18 can be supported in position by use of a plurality of suitable posts and insulation placed therebetween. Similarly, a series of posts can be positioned between secondary bottom 18 and primary bottom 20 to support the primary bottom in position. When posts are used as described, insulating means should be employed to prevent development of cold spots in the ship bottom by heat transfer through the posts.

The tank 11 shown in FIG. 2 can be suitably insulated to prevent heat leak into the tank and to also prevent the cryogenic liquid contents of the tank from adversely cooling the ship hull. Thus, insulation board 21 can be secured to the outer surface of the major portion of the tank wall 14 and have a splash shield 27 covering its outer surface. The upper domed portion of the tank, the tank roof, and the cupola 28 which holds service pipes and controls, can be suitably insulated by placing a resilient glass fiber blanket 22 in contact with the tank surface and a granular free flowing insulating material 23 between the resilient blanket and a tank protective metal cover 24. Settling of the granular insulating material 23 is prevented by expansion and contraction of the resilient blanket with temperature induced dimensional changes of the tank metal shell. A conventional expansion joint 25 is employed to keep the granular insulation 23 from flowing downwardly into the ship hold with temperature induced lateral displacement of the tank shell. Joint 25, can have appropriate holes, not shown, through which any escaped liquid from the tank above the joint can flow downwardly to collect in the drip pan 29.

Flexible splash shield 27A can be located between the blanket 22 and the granular insulation 23. Splash shield 27A works in conjunction with splash shield 27 to direct any escaped liquid to drip pan 29 which is made of metal which can withstand cryogenic temperatures. Insulation 32 is placed between the drip pan 29 and the ship hold wall to prevent the ship hull from being cooled excessively if the tank cracks and the drip pan accumulates liquefied gas.

The lower end portion 26 of the tank wall extending from the secondary bottom 18 to the ship hold bottom 12 is made sufficiently high so that a temperature transition zone can develop therein from the cryogenic temperatures to which the tank shell would be subjected in the event the primary bottom 20 fails and liquid flows into contact with the secondary bottom 18 to the temperature of the ship hold bottom 12, which would normally be at a temperature high enough such that the carbon steel of which it normally is constructed would not be adversely affected by the temperature to which it is subjected in usage.

It will be seen readily that the novel structure illustrated by FIG. 2 permits wide distribution of the tank weight, and any liquid load therein, over nearly the full ship hold area to thereby substantially reduce the creation of localized stresses in the ship hull. The tank furthermore has inherent safety characteristics because if the primary bottom 20 of the tank were to fail the secondary bottom 18 would prevent the cryogenic liquid from flowing into contact with the ship hold structure. The insulation 17 would prevent the ship hold bottom from being cooled to a temperature low enough to severely affect the strength of the metal used in constructing the ship.

FIGS. 3A and 3B illustrate two related joint structures which can be used for welding the secondary and primary bottoms to the tank shell wall. Tee member 30 can be welded integrally in the tank shell wall 14 and the secondary bottom 18 can be peripherally butt welded to the horizontal flange 31 of the tee member 30 as shown in FIG. 3A or the secondary bottom 18 can be lap welded to the horizontal member 31 as shown in FIG. 3B. It should be understood that, instead of secondary bottom 18 being joined as shown in FIGS. 3A and 3B, the primary bottom 20 can be joined in the same way.

FIG. 3C shows an alternative joint in which a round metal bar 33 is welded horizontally between the ends of plates used in forming the tank shell wall 14. The peripheral edge of secondary bottom 18 is butt welded to rod 33 to complete the joint. A similar joint may be used to fasten primary bottom 20 to the tank shell wall 14.

The joint structure of FIG. 3D has a machined member 34 integrally welded into the tank shell wall 14. The member 34 is provided with a flange 35 on which the secondary bottom 18 can be lap welded at its periphery and an upper flange 36 to which primary bottom 20 can be lap welded at its periphery.

FIG. 3E illustrates another means to join either the secondary bottom 18, or the primary bottom 20, to the tank shell wall 14. As shown in this figure, the lower edge of tank shell wall 14 is flanged inwardly to a horizontal position and to the end thereof is welded, in horizontal abutting position, the peripheral edge of primary bottom 20. A vertically positioned metal ring 144 projects downwardly from wall 14 and constitutes a continuation of the tank shell wall.

The tank structure shown in FIG. 4 is similar to that shown in FIG. 2 and differs therefrom only in that the primary bottom 40 has a dished profile. The dished primary bottom 40 of metal plate is welded at its peripheral edge to a flange 41 which extends around the circular inside surface of tank wall 14.

FIGS. 5A to 5C illustrate joint structures which can be used to join a dished or curved primary bottom and/or secondary bottom to the tank shell wall 14. As shown in FIGS. 5A and 5B, the Y-shaped member 43 is integrally welded into the tank shell wall 14 and to the downwardly and inwardly extending leg 44 is joined dished primary bottom 40, by butt welding as shown in FIG. 5A, or by a lap weld joint as shown in FIG. 5B. FIG. 5C shows another joint with round bar 45 used to join portions 14 of the tank wall together and with the edge of dished primary bottom 40 welded to the bar 45.

FIG. 6 illustrates a tank-ship combination which in most respects is similar to the structure shown in FIGS. 2 and 4. The tank shown in FIG. 6, however, is made of a metal which is not directly weldable to a carbon steel ship hold bottom. Thus, the lower wall portion 14A, as well as the roof portion 13A of the tank, are made of aluminum plate. Since aluminum cannot be welded directly to carbon steel plate, an intermediate or transition joint is provided to effect secure mounting of the tank in place on the ship hold bottom. As shown in FIGS. 6 and 7A, a vertically positioned carbon steel ring 52 is welded at its lower edge to the ship hold bottom 12 of carbon steel plate and the upper edge of the ring 52 is welded to a horizontally positioned plate 53. Plate 53 has a top sheet of aluminum clad to a sheet of carbon steel or stainless steel. The bottom edge 54 of the tank wall portion 14A is welded to the upper surface of plate 53 to thereby provide a suitable transition joint from the tank aluminum wall 14A to a ship hold bottom 12 of carbon steel. Drip pan 55 is supported by wall portion 14A above plate 53. Insulation 51 is placed between drip pan 55 and the ship hold wall to prevent the ship hull from becoming excessively cold if a cryogenic liquefied gas accumulates in the drip pan through a crack failure in the tank.

FIG. 7B illustrates another type of transition joint which can be used when it is not feasible or possible to directly weld the lower edge of the tank wall to the ship hold bottom because of the incompatability of the metal of which each is composed. Projecting upright from ship hold bottom 12 is a vertically positioned carbon steel ring 56 which is welded at its lower or bottom edge to the ship hold bottom 12 of carbon steel. A carbon steel horizontally positioned plate 57 is welded to the top edge of ring 56. Horizontally positioned aluminum plate 58 is welded to the bottom edge of aluminum tank wall 14A. Stainless steel bolts 59 secure the two plates 57 and 58 together to thereby firmly secure the tank in position to the ship hold bottom.

FIG. 8 illustrates a tank positioned in a ship hold which in most respects is similar to the structure shown in FIG. 2 with the main difference being in the insulating system used in conjunction with the tank shown in FIG. 8. The tank shown in FIG. 8 has a cylindrical circular walled portion 114 and a ellipsoidal roof portion 113. The tank also has a secondary bottom 118 and a primary bottom 120 with load supporting insulation 117 between the ship hold bottom 112 and secondary bottom 118 and supporting material 119 between the secondary bottom 118 and the primary bottom 120. A resilient glass fiber blanket 61 is positioned over the outside surface of the ellipsoidal roof portion 113 and the cylindrical walled portion 114 of the tank. Aluminum sheet splash shield 62 is shown placed over the outer surface of resilient blanket 61. It can, however, be placed between the blanket and the tank shell. Granular free flowing insulation 63 is placed over the aluminum sheet spray shield 62 and over the granular insulation there is positioned a weather cover 64 which is joined at its bottom edge to drip pan 65. Drip pan 65 is joined at its bottom edge to the outer surface of the tank walled portion 114. Insulation 66 is placed outside drip pan 65 to prevent the ship hull from becoming excessively cold if liquefied gas accumulates in the drip pan from a tank leak.

The granular insulation 63 is maintained in position by the pressure exerted against it by the resilient blanket 61. As the metal tank contracts with a reduction in temperature, such as when the tank is filled with a cryogenic liquid, the resilient blanket 61 expands to accommodate the reduction in size of the tank. When the tank shell expands, such as when the tank is emptied of cryogenic liquid and is brought to a prevailing atmospheric temperature, tank enlargement results and applies pressure to the resilient blanket 61 to thereby compress it. Sufficient pressure is always applied by means of the resilient blanket 61, whether the tank shell is at atmospheric temperature or at the cryogenic temperature of a liquefied gas stored in the tank, to maintain the granular insulation 63 in position and to prevent it from settling along the walls of the tank with the creation of a noninsulated void in the vicinity of the tank roof.

A further embodiment of the invention is shown in FIG. 9 in which the tank mounted in the ship 10 has a spheroidal shell upper portion 70 which is joined to a lower conical shell portion 71 fusion welded at its bottom edge to the ship hold bottom 12. This type of tank permits the use of a haunch 85 of substantial size to reinforce the juncture of the ship bottom with the hull sides without greatly reducing the liquid carrying capacity of the tank through excessive void space between the ship hold wall and the tank surface. Also, this tank shape takes advantage of the ability of the spherical shape to resist lateral force (dynamic) yet still take vertical gravity loads directly into the hull. The tank has a secondary bottom 18 and a primary bottom 20 as previously described with reference to FIG. 2. Insulation 17 is of the load bearing type and supports the secondary bottom 18. Material 19 can also be of the load bearing type, although it need not be a material with insulating properties, to support primary bottom 20. Drip gutter 72, made of a metal which can withstand cryogenic temperatures, is mounted around the lower portion of the tank, and advisably it is positioned at the location of secondary bottom 18 or slightly therebelow. Insulation board 73 is positioned over that part of the outer surface of the tank which is below the insulation expansion joint 74. Spray shield 75 of aluminum sheeting is attached to the outer surface of insulation 73 for the purpose of directing any liquid or vapor, escaping from the tank, downwardly to the drip gutter or pan 72, having insulation 81 on the outside thereof. Resilient glass fiber blanket 76 is mounted over that part of the upper surface of the tank of FIG. 9 and it is covered with a spray shield 77, such as one made of aluminum sheeting or some other material which maintains its strength at the low temperatures to which it will be subjected. Granular free flowing insulation 78, such as perlite, is positioned over the spray shield 77 and over the granular insulation 78 there is located a metal weather cover 79 which terminates at its lower edge at the ship deck.

FIG. 10 illustrates another embodiment of the invention, but an embodiment which is very similar to the one illustrated by FIG. 9. Thus, the tank of FIG. 10, which is essentially identical to the tank shown in FIG. 9, has its lower portion below the insulation expansion joint 74 left free of insulation. Instead, insulation 80 comprising insulation board stock is applied to the ship hold walls and hold bottom to the lower peripheral edge of the tank. The result is to have the tank surrounded by an insulated environment to thereby retard heat leak into the tank. Splash shield 82 is placed over insulation 80, and also between blanket 76 and granular insulation 78, to direct any liquid which escapes from the tank to drip pan 83.

Because the lower edge of the tank structures can be joined completely leak tight to the ship hold bottom, neither the primary nor the secondary bottom need be designed to resist external pressure, such as if the hold floods and the ship hold bottom retains its structural integrity. The ship hold bottom is designed to resist such external pressure and therefore such external pressure is not applied to the tank secondary bottom or primary bottom if the hold floods. However, the space between the ship hold bottom and the secondary bottom, as well as the space between the secondary bottom and the primary bottom, should be monitored when the tank is empty to prevent a pressure build-up in such spaces by a leak.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

Claims

What is claimed is:

1. In combination:

a ship having a hold with a bottom and walls; and

a tank, for transporting a cryogenic liquefied gas, having a structurally self-supporting wall and roof and which is spaced or separated from the ship hold walls;

said tank comprising a metal shell with a walled portion and a roof portion;

the walled portion having a horizontal circular bottom edge and a surface of revolution generated by the movement of a generatrix line which moves in a path touching the horizontal circular bottom edge and is constantly in the same spaced relationship to a straight vertical line perpendicular and central to the horizontal circular bottom edge;

the bottom edge of the walled portion being in direct contact with, and continuously joined to, the ship hold bottom;

a metal secondary bottom in the tank spaced above the ship hold bottom and extending to the shell walled portion;

means supporting the secondary bottom in position above the ship hold bottom;

insulation between the secondary bottom and the ship hold bottom;

a metal primary bottom in the tank spaced above the secondary bottom and extending to the shell walled portion; and

storage load support means supporting the metal primary bottom in position above the secondary bottom.

2. A combination according to claim 1 in which the walled portion has a cylindrical surface.

3. A combination according to claim 2 in which the roof portion is domed.

4. A combination according to claim 1 in which the walled portion includes one or more conical surfaces having a circular bottom edge and with sections of the walled portion taken thereabove parallel to the said bottom edge also circular.

5. A combination according to claim 1 in which the insulation between the secondary bottom and the ship hold bottom is load bearing insulation which also constitutes the means supporting the secondary bottom in position above the ship hold bottom.

6. A combination according to claim 1 in which insulation is located between the secondary bottom and the primary bottom.

7. A combination according to claim 6 in which the insulation also constitutes the storage load support means supporting the metal primary bottom in position above the secondary bottom.

8. A combination according to claim 1 in which the tank shell, secondary bottom and primary bottom are of metal which is suitable for use at cryogenic temperatures.

9. A combination according to claim 8 in which the ship hold bottom is of carbon steel and the height of the tank walled portion from the secondary bottom to the ship hold bottom is great enough to prevent cooling by a cryogenic liquid in the tank of the ship hold bottom to a low enough temperature to endanger the structural integrity of the hold bottom.

10. A combination according to claim 1 in which at least the roof portion of the tank extends above the top deck of the ship and is insulated.

11. A combination according to claim 10 in which the inside surfaces of the hold are insulated.

12. A combination according to claim 10 in which the walled portion of the tank shell is externally insulated.

13. A combination according to claim 1 in which a drip gutter is joined to the outer surface of the walled portion below the secondary bottom and above the ship hold bottom.

14. In combination:

a ship having a hold with a bottom and walls; and

a tank, for transporting a cryogenic liquefied gas, having a structurally self-supporting wall and roof and which is spaced or separated from the ship hold walls;

said tank comprising a metal shell with a walled portion and a roof portion;

the walled portion having a horizontal circular bottom edge and a surface of revolution generated by the movement of a generatrix line which moves in a path touching the horizontal circular bottom edge and is constantly in the same spaced relationship to a straight vertical line perpendicular and central to the horizontal circular bottom edge;

the bottom edge of the walled portion being in contact with and joined to the ship hold bottom;

a metal secondary bottom in the tank spaced above the ship hold bottom and extending to the shell walled portion;

means supporting the secondary bottom in position above the ship hold bottom;

insulation filling the space between the secondary bottom and the ship hold bottom;

a metal primary bottom in the tank spaced above the secondary bottom and extending to the shell walled portion;

storage load support means supporting the metal primary bottom in position above the secondary bottom; and

insulation filling the space between the primary bottom and the secondary bottom.

15. A combination according to claim 14 in which the bottom edge of the walled portion is continuously joined to the ship hold bottom.

16. In combination:

a ship having a hold with a bottom and walls; and

a tank, for transporting a cryogenic liquefied gas, having a structurally self-supporting wall and roof and which is spaced or separated from the ship hold walls;

said tank comprising a metal shell with a walled portion and a roof portion;

the walled portion having a horizontal circular bottom edge and a surface of revolution generated by the movement of a generatrix line which moves in a path touching the horizontal circular bottom edge and is constantly in the same spaced relationship to a straight vertical line perpendicular and central to the horizontal circular bottom edge;

the bottom edge of the walled portion being in contact with and joined to the ship hold bottom;

a thin metal secondary bottom in the tank spaced above the ship hold bottom and extending to the shell walled portion;

means supporting the secondary bottom in position above the ship hold bottom;

insulation between the secondary bottom and the ship hold bottom;

a thin metal primary bottom in the tank spaced above the secondary bottom and extending to the shell walled portion; and

storage load support means supporting the metal primary bottom in position above the secondary bottom,

whereby the thin metal primary and secondary bottoms depend on the ship hold bottom structure for ultimate support of the tank product load.