US3777497A

US3777497A - Storage tank for offshore storage of liquid and method of constructing and installing same

Info

Publication number: US3777497A
Application number: US00250732A
Authority: US
Inventors: N Edwards
Original assignee: Pittsburgh Des Moines Steel Co
Current assignee: PITTSBURGH-DES MOINES Corp
Priority date: 1972-05-05
Filing date: 1972-05-05
Publication date: 1973-12-11
Anticipated expiration: 1990-12-11
Also published as: DK138594C; GB1384360A; JPS5247575B2; DK138594B; JPS4954915A

Abstract

A storage tank for offshore storage of liquid such as oil and the like and the method of constructing and installing same, wherein the storage tank has a side wall with the shape of a frustum of a hollow right circular cone and an enlarged diameter base or lower end and a reduced diameter top or upper end having a roof thereon, the frusto-conical configuration of the side wall of the tank substantially reducing the forces acting on the tank from a surrounding body of water or from wind or the like, and further resulting in a low center of gravity of the tank and a large water plane area so that the tank is inherently stable for towing in a body of water.

Description

United States Patent 1 Edwards 1 Dec. 11, 1973 [75] Inventor: Norman W. Edwards, Sewickley,

[73] Assignee: Pittsburgh-Des Moines Steel Company, Pittsburgh, Pa.

[22] Filed: May 5, 1972 [21] App]. No.: 250,732

[52] US. Cl 6l/46.5, 52/426, 220/10,

3,472,033 10/1969 Brown 61/46 3,708,987 1/1973 Roulet 3,339,512 9/1967 Siege] 114/.5 T

Primary ExaminerJacob Shapiro Att0rneyLuke A. Mattare et al.

[57] ABSTRACT A storage tank for offshore storage of liquid such as .oil and the like and the method of constructing and installing same, wherein the storage tank has a side wall with the shape of a frustum of a hollow right circular cone and an enlarged diameter base or lower end and a reduced diameter top or upper end having a roof thereon, the frusto-conical configuration of the side wall of the tank substantially reducing the forces acting on the tank from a surrounding body of water or from wind or the like, and further resulting in a low center of gravity of the tank and a large water plane area so that the tank is inherently stable for towing in a body of water.

17 Claims, 12 Drawing Figures PMENTEH DEC} 1 1975 wwaw "1:: WAT E R:

WWW/W STORAGE TANK FOR OFFSHORE STORAGE OF LIQUID AND METHOD OF CONSTRUCTING AND INSTALLING SAME BACKGROUND OF THE INVENTION In recent years there has been an increased development of offshore oil fields and consequently, there has been an increased demand for offshore oil storage facilities. This demand has resulted in several designs for offshore oil storage facilities and many of these designs involve the concept of an open bottom tank anchored to a foundation in the sea floor. Most of these proposed tanks have generally a cylindrical configuration with some sort of roof such as a dome roof or the like thereon. Some of these prior art storage tanks are designed to be installed completely under the surface of the sea and the cylindrical side walls thereof are on the order of 35 to 50 feet high, with some sort of dome roof construction on the top thereof, said roof being typically constructed of steel plates stiffened with common structural shapes.

A further type of prior art offshore oil storage tank has an open bottom and a cylindrical side wall and is commonly referred to as a shallow water tank. These tanks are often designed as a double wallled cylinder extending from the bottom of the body of water to the surface thereof and several feet above the surface of the body of water. Such storage tanks may or may not be equipped with a stiffened dome roof, as desired.

The cylindrical configuration of the side walls of such prior art storage tanks results in large stress resultants in the tanks due to the hydrodynamic forces imparted thereto by the action of waves and current and the like in the body of water in which the tanks are positioned.

Further, due to the cylindrical configuration of such prior art tanks, the hydrodynamic drag on the tank and the overturning effect of hydrodynamic pressures acting on the tank are relatively large, and various mooring means or large and expensive foundations must frequently be provided to prevent overturning or tipping of the storage tank.

Further, such cylindrical tanks are typically constructed on shore and then towed to the offshore site, and lowered in the body of water until they rest on the floor of the body of water. With many prior art storage tanks, the steps of towing the tank to the site and lowering it to the floor of the body of water are quite complicated and difficult to accomplish due to the geometry of the tanks which in some cases necessitates that the tanks be towed on their side and then righted at the site prior to lowering the tanks to the floor of the body of water. Such designs and methods require complicated and expensive equipment, and accordingly, the cost of such offshore oil storage facilities is quite large.

The offshore storage tank of the present invention has a configuration which economically and effectively overcomes these problems inherent in prior art storage tank constructions.

In accordance with the present invention, the tank has essentially the shape of a frustum of a right circular cone with a roof thereon. Such a tank construction is better able to withstand the hydrodynamic pressure exerted thereon from ocean waves, since the stress resultants at a given location on any tank are proportional to the radius of curvature of the tank at that point and the pressure at that point. Large pressures and large radii of curvature accordingly result in large stresses.

The pressures acting on the tank due to waves and the like are obviously largest near the surface of the body of water, and it is in this vicinity that the tank of the present invention has the smallest radius of curvature and thus presents the least projected area. Thus, the detrimental effects of large pressures near the surface of the body of water are substantially reduced in the present invention. At the same time, the tank of the present invention has a large radius of curvature near the base of the tank. Such a design reduces the requirements of the foundation of the tank since the foundation design is largely controlled by the hydrodynamic drag on the tank and the overturning effect of hydrodynamic pressures acting on the tank. Since the tank of the present invention has the shape of the frustum of a hollow right circular cone, or stated differently, is frusto-conical in shape, the tank has a large diameter at its base, and this large diameter enables the tank to resist overturning or tipping much more readily than prior art cylindrical tank constructions, since hydrodynamic loads applied to a tank near the surface of a body of water tend to act normal to the wall of the tank and, accordingly, in the present invention, wherein the wall of the tank is frusto conically shaped, these loads or forces tend to act in the direction of the foundation centroid and therefore have a smaller moment arm than those which, for example, act on the walls of a right circular cylinder as in the prior art.

Further, the tank of the present invention is much more stable during towing than most other tank designs which have been proposed in the prior art. This is due to the larger diameter of the tank of the present invention at the towing water plane than conventional tanks of cylindrical configuration and similar capacities. This enlarged diameter at the towing water plane enhances stability of the tank. Also, the center of gravity of the tank is much lower than the center of gravity of conventional tanks. In fact, preliminary studies indicate that while towing a tank constructed in accordance with the present invention at a 30 foot draft, the center of gravity of the tank is actually below the center of buoyancy, and the tank is accordingly inherently stable in that configuration and all subsequent configurations.

Even further, the side wall of the tank in accordance with the present invention is a double wall construction and thus several other advantages. accrue to the tank of the present invention. First, the width of the wall contributes to the towing stability, and from the standpoint of structural integrity, the double walled construction with concrete between the inner and outer walls eliminates both local and overall buckling of the tank walls due to large external hydrodynamic forces. Conventional tank constructions have to be heavily stiffened with standard structural members to assure against this type of structural failure. Further, the dead weight of the concrete between the two walls of the tank decreases, and in some cases completely eliminates, the uplift on the tank due to the lighter weight of the oil stored therein, and depending upon the foundation conditions and requirements, this can be a very important consideration in the design of the piles used to anchor the tank to the floor of the body of water.

Still further, work platforms can be very easily constructed on the tank thus resulting in a substantial savings in the total cost of developing the oil fields.

OBJECTS OF THE INVENTION It is an object of this invention to provide a storage tank for offshore storage of oil and the like and to the method of constructing and installing same wherein the tank presents a reduced radius of curvature at the surface of the body of water so that the tank is subjected to smaller loads and a reduced tendency to overturn or tip due to hydrodynamic and other forces acting thereon.

It is another object of this invention to provide a storage tank for offshore storage of liquids wherein the tank side wall has the shape of a frustum of a hollow right circular cone, thus presenting less area to hydrodynamic pressures at the surface of the body of water where such forces are largest, and at the same time providing a large storage capacity as compared with prior art devices.

A further object of the invention is to provide a storage tank for offshore storage of liquids wherein the tank has a generally frusto conical configuration with a dome roof thereon which can be easily adapted to receive a work platform or the like over the roof.

An even further object of the invention is to provide a storage tank for offshore storage of liquid and to the method of constructing and installing same wherein the tank has a frusto-conically shaped side wall and a dome roof, said frusto-conical side wall resulting in the tank having a low center of gravity and a large water plane area, thus enabling the tank to be towed with inherent stability from a construction site to an installation site.

A still further object of the invention is to provide a tank having a frusto-conically shaped side wall and a dome roof and wherein lines drawn perpendicular to the surface of the side wall near the upper end thereof extend through the area bounded by the lower end of the side wall.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top perspective view with a portion thereof broken away of a storage tank in accordance with the present invention in position in a body of water.

FIG. 2 is an enlarged plan view of the tank of FIG. 1.

FIG. 3 is an enlarged sectional view taken along line 33 in FIG. 2.

FIG. 4 is a fragmentary enlarged sectional view of a portion of the tank side wall in FIG. 3.

FIG. 5 is a fragmentary enlarged sectional view with a portion broken away of the tank side wall and foot flange thereon in FIG. 3.

FIG. 6 is a schematic sectional view in elevation of a tank in accordance with the present invention being constructed in a graving dock immediately prior to floating the tank.

FIG. 7 is a view similar to FIG. 6 of the tank floating on a body of water.

FIG. 8 is a view similar to FIG. 7 of the tank being towed in deep water to the site of an offshore facility.

FIG. 9 is a view similar to FIG. 8 with the tank at the site of the offshore facility and being lowered toward the bottom of the body of water.

FIG. 10 is a schematic sectional view in elevation of a tank in accordance with the present invention installed on the floor of a body of water at an offshore facility.

FIG. 11 is an enlarged view in elevation with a part shown in section of a modified and enlarged frustoconical oil storage tank, and showing a dome roof in full lines and a conical roof in phantom lines.

FIG. 12 is a schematic sectional view in elevation of a further modified tank in accordance with the present invention with a work platform constructed thereon, and with internal stiffening walls and a bottom.

DETAILED DESCRIPTION OF THE INVENTION In the drawings, wherein like reference numerals indicate like parts throughout the several views, a tank is indicated generally at T in FIG. 1 and comprises a side wall W having the shape of a frustum of a right circular cone with an annular foot flange F on the base thereof and a dome roof R on the top thereof.

The tank T is shown in position on the floor or bottom'B of a body of water. A plurality of piles P are extended through the foot flange F into the floor of the body of water to anchor the tank T in position. The upper end of the side wall W and the roof R extend above the surface of the body of water in this particular installation, although the tank could equally as well be completely submerged beneath the surface of a body of water, if desired.

In FIGS. 3, 4 and 5, further details of construction of the tank T can be readily seen, and the side wall W of the tank is of double wall construction and includes an inner wall 10 and an outer wall 12, each comprised of a plurality of steel plates welded or otherwise suitably joined together. The roof R similarly comprises a plurality of steel plates welded or otherwise suitably secured together and extending upwardly from the upper edge of the inner wall 10. A suitable vent V is provided in the roof R for venting air from the interior of the tank. The inner and

outer walls

10 and 12 are in radially spaced relationship to one another and concrete 13 fills the space between the

walls

10 and 12. Suitable reinforcing means, such as a plurality of reinforcing rods 14 or the like, bent into a zig-zag shape, are welded to the respective inner and

outer walls

10 and 12 and are arranged in a criss-crossing pattern as seen in FIG. 1 to reinforce the concrete between the inner and

outer walls

10 and 12. A plurality of spaced apart channel members or studs 15 are also welded or otherwise suitably secured to the inside facing surfaces of the inner and

outer walls

10 and 12 to prevent shearing action between the concrete 13 and adjacent inner and

outer walls

10 and 12.

As seen in FIG. 1, shear members or channels 15 may extend circumferentially around the tank, if desired.

In FIG. 5, details of construction of the foot flange F are shown, and the foot flange F comprises upper and lower

annular walls

16 and 17 and inner and outer vertically extending

annular walls

18 and 19. A plurality of cylindrical pile sleeves 20 are welded in vertical relationship between the top and

bottom walls

16 and 17 for receiving the piles P therethrough.

The upper ends of the

side walls

10 and 12 are connected together by an annular, slightly inclined frustoconically shaped ring or wall 21 welded or otherwise suitably secured at its upper and lower edges to the respective adjacent upper edges of the

walls

10 and 12 to provide a substantially unitary side wall construction having a pleasing appearance.

Suitable vent means, not shown, such as pipes or the like may be provided at the base or lower end of the side wall W for establishing communication between the surrounding bodyof water and the interior of the tank T, if desired.

As indicated in FIG. 3, hydrodynamic forces acting on the tank due to waves and the like in the surround ing body of water tend to act in a direction normal to the surface of the tank as indicated by the arrow f, and these forces accordingly act along a line L extending generally in the direction of the centroid C of the base of the tank T, thus substantially reducing the overturning or tipping effect on the tank due to the hydrodynamic forces acting on the tank from the surrounding body of water. Also, as seen in this Figure, oil is stored in the tank and has in interface I,,, with water therebeneath and an interface 1,, with air thereabove.

Further, the roof R has a radius r selected so that the roof continues smoothly upwardly from the upper edge of the inner wall 10.

In a typical installation, the side wall W has a height H of approximately 100 feet for use in a body of water having a depth h of approximately 90 feet. The tank has a diameter D of approximately 315 feet at the base thereof and a diameter d of approximately 115 feet at the top of the side wall W. The roof R has a radius of curvature r of approximately 81.3 feet, and the plates in the roof have a thickness of about 1. inch, whereas the plates in the outer side wall 12 and the plates in the inner wall have a thickness of approximately onehalf inch. The side wall W has an overall thickness of approximately 40.0 inches, and the foot flange is approximately 9 feet wide and 6 feet high. The piles P are spaced around the foot flange on approximately 4 feet centers.

The shear connectors or channel members are spaced approximately 4 feet apart and the reinforcing rods or bars 14 are spaced 3 feet apart. A tank thus constructed can readily store 500,000 barrels of oil or other liquid.

In FIGS. 6 through 10, the manner or method in which the tank T is constructed and towed to the site of an offshore facility is schematically illustrated, and in FIG. 6, the tank T is shown in a graving dock GD. In the graving dock, concrete 13 is poured between the

walls

10 and 12 to a level of approximately 40 feet to provide ballast, and the graving dock is then flooded. Air is then pumped through a suitable means 22 to the interior of the tank to a pressure of approximately 3.0 psig, and the tank is then floated'and towed with a draft of approximately 18 feet into shallow water. When the tank reaches deep water, air is vented from the tank T through vent V until the tank assumes a draft of approximately 30 feet and the pressure inside the tank is about 3.0 psig. At this draft, the air-water interface in the tank is spaced approximately 25 feet upwardly from the bottom edge of the side wall W. In this position, as seen in FIG. 8, preliminary studies have indicated that the center of gravity CG of the tank is actually below the center of buoyancy of the tank, and the tank accordingly is inherently stable and will be inherently stable at all subsequent lower levels in the body of water.

In FIG. 9, the tank T is shown at the site of installation of the tank and is illustrated in the process of being lowered in the body of water. Air is vented from the interior of the tank in a controlled manner through the vent V as necessary to effect lowering of the tank.

In FIG. 10, the tank T is shown in position on the floor of the body of water. Once the tank has been lowered to the floor of the body of water as seen in this Figure, concrete is poured to the final depth between the

walls

10 and 12 and the piles P are installed through the foot flange F into the floor of the body of water. Oil or other liquid to be stored is then pumped into the tank by suitable means, not shown. As also seen in this fig ure, the hydrodynamic force f on the tank T at the surface S of the body of water is directed along a line ex tending generally toward the centroid C of the foundation of the tank and the tipping or overturning moment applied to the tank by waves or the like S is accordingly substantially reduced since the moment arm is relatively short because of the direction of the force f toward the centroid C.

Alternatively, the tank T could be completely submerged in deper water, if desired, rather than positioned in relatively shallow water with the upper end thereof projecting above the surface of the body of water as illustrated in FIGS. 1 through 10.

If the tank is to be installed in deeper water, it would be constructed and towed to the installation site as previously described, but upon arriving at the site, air would be bled from beneath the tank until it became neutrally buoyant and at this stage suitable means would be attached to the tank to lower it in the body of water to the floor of the body of water. One example of a suitable means for this purpose is illustrated and described in U.S. Pat. No. 3,621,802.

In FIG. 11, a modified tank T in accordance with the invention is illustrated, and is substantially identical in construction with the tank illustrated in FIGS. 1 through 10. The only difference in this tank is that it is designed for installation in deeper water and has substantially twice the storage capacity of tank T. For example, tank T has a diameter at the base thereof of approximately 365 feet and a diameter at the upper end of the frusto-conical wall W of approximately 50 feet. The frusto-conical wall W is approximately 180 feet high, and the tank is intended to be used in water having a depth of approximately feet. Also, the steel plates comprising the outer side wall 12 are approximately /z inch thick, and the plates comprising the inner wall 10 are approximately of an inch thick. The reinforcing rods 14 are spaced 3 6 inches apart. The foot flange F is approximately 11 feet wide and 8 feet high and the shear connectors or channels 15 are spaced 3 feet apart. The most noticable difference in this modification is that the roof R has a radius of curvature of about 33 feet as compared with a radius of curvature of approximately 81 feet in the first described embodiment. In all other respects, the construction and principles of operation of this tank are the same as that of tank T. Also, due to the difference in size, there are slight differences in the amount of draft of the tank as it is being towed. For example, the tank T is constructed in a graving dock GD as previously described, and concrete is poured between the inner and

outer side walls

10 and 12 to a of about 40 feet to provide ballast. The graving dock is then flooded and the tank pressurized with air to a pressure of approximately 4.0 psig, and the tank towed into shallow water. With this air pressure in the tank, the tank floats with a draft of approximately 20 feet and the bottom edge of the tank is spaced approximately 10.0 feet downwardly below the air-water interface within the tank. When the tank reaches deep water, part of the air is vented from the vessel until the vessel assumes a draft of approximately 35 feet, at which point the bottom edge of the tank is spaced approximately 25 feet below the air-water interface in the tank. The pressure of the air in the tank at this point is then about 5.0 psig. The tank is towed to the installation site and air is then vented from the tank in a controlled manner until the tank rests on the sea floor. Concrete is then poured to the final depth between the

wall

10 and 12 and the piles P are installed.

The tank T in FIG. 1 1 could be provided with a conical roof R", illustrated in phantom lines, rather than the dome roof R illustrated in full lines, if desired. The roofR would simply comprise an extension of the side wall.

In FIG. 12 a further modified tank T" is shown and a work platform or the like 23 is shown constructed on top of the tank T. The platform 23 is supported by means of a plurality of vertically extending supports 24 connected to the tank T at the upper end of the side wall W. A floor or bottom 25 of reinforced concrete or the like is in the tank T" to provide weight to the tank and to provide a mat type foundation and is secured to the lower edge of the side wall W of the tank. A plurality of vents V extend through the wall W for establishing communication between the interior of the tank and the surrounding body of water. A plurality of radially extending internal stiffening walls 26 of reinforced concrete or the like are secured to the inner surface of the wall W of the tank T and to the floor or bottom 25. The frusto-conical configuration of the tank and the double wall construction thereof renders it capable of readily supporting such a work platform and thus the total cost of developing the oil field is substantially reduced. In this figure, the platform 23 comprises or forms the roof for the tank, but a dome roof R, shown in phantom lines, or any other suitable roof could be provided, if desired. The work platform could be constructed on top of the tank irregardless of whether the tank extends above the still water level or not.

Each of the tanks described herein could be provided with a bottom and/or internal stiffening walls, and a work platform and/or vents, if desired.

Although the tanks as illustrated and described herein have been described as comprising a plurality of steel plates welded together, and as having particular dimensional relationships, it is to be understood that any suitable material could be used in constructing the tank and that the specific dimensions of the tank may be varied so long as the inventive concept of the invention is realized.

Further, the pile anchoring of the tank to the sea floor is not necessary in all situations, and as an alternate, the total weight of the tank could be adjusted such that the weight of the tank maintains the tank on the sea floor and resists all wind, waves and currents. This weight could comprise, for example, the floor or bottom 25 seen in FIG. 12.

As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the metes and bounds of the claims or that form their functional as well as conjointly cooperative equivalents, are therefore intended to be embraced by those claims.

I claim:

1. A storage tank for storage of liquids in a body'of water, said tank comprising a unique side wall having substantially the shape of a frustum of a right circular cone and with an upper end thereof above the surface of the body of water and a lower end thereof fixed to the floor of the body of water, an annular, outwardly extending foot flange integral with the lower end of the side wall, a plurality of circumferentially spaced piles extending through the foot flange into the floor of the body of water fixing the tank to the floor of the body of water, said side wall comprising a pair of spaced apart, substantially coextensive, parallel inner and outer walls, reinforcing means fixed to the opposed, confronting interior surfaces of the parallel inner and outer walls and joining the inner and outer walls together, suitable ballast means in the space between the parallel inner and outer walls and substantially filling same, and a rigid, concavo-convex roof fixed on the upper end of said side wall above the surface of the body of water and closing the upper end of said tank, said roof comprising a continuation of the side wall, the height of said roof being considerably less than the height of the side wall, and the relationship of the diameter of the side wall at its upper end to its diameter at its lower end and to the height of the side wall being such that a line drawn perpendicular to the side wall at its upper end extends through the space bounded by the lower end thereof, the tank thus having a small radius of curvature and a small area at the upper end thereof subject to wave action and the like and thereby effectively resisting stresses and forces on the tank from the surrounding body of water tending to overturn the tank.

2. A storage tank as in claim 1, wherein the diameter of the lower end of the side wall is at least twice as great as the diameter of the upper end of the side wall.

3. A storage tank as in claim 1, wherein the diameter of the lower end of the side wall is greater than the height of the side wall.

4. A storage tank as in claim 1, wherein the annular space between the inner and outer side walls is filled with concrete.

5. A storage tank as in claim 1, wherein an annular,

frusto-conically shaped wall is connected between the adjacent upper ends of the inner and outer side walls.

6. A storage tank as in claim 1, wherein each of said inner and outer side walls and said roof comprises a plurality of steel plates welded or otherwise suitably secured together in edge-to-edge relationship.

7. A storage tank as in claim 6, wherein the roof is welded or otherwise suitably fixed to the upper end of the side wall.

8. A storage tank as in claim 4, wherein channel members are welded or otherwise suitably fixed to the confronting surfaces of the inner and outer side walls to prohibit shearing of the concrete relative to the walls.

9. A storage tank as in claim 8, wherein the upper diameter of the side wall is about 1 15 feet, the lower diameter of the side wall is about 315 feet, the height of the side wall is about feet, the roof is dome shaped and has a radius of curvature of about 81 feet, and the annular space between the inner and outer side walls is about 40.0 inches, said tank having a capacity of about 500,000 barrels of liquid.

10. A storage tank as in claim 9, wherein the roof and side wall of the tank comprise a plurality of steel plates welded together, said steel plates comprising the side wall having a thickness of about one-half of an inch, and the steel plates comprising the dome roof having a thickness of about 1 inch.

11. A storage tank as in claim 8, wherein the upper diameter of the side wall is about 50 feet, the lower diameter of the side wall is about 365 feet, the height of the side wall is about 180 feet, the roof is dome shaped and has a radius of curvature of about 33 feet, and the annular space between the inner and outer side walls is about 54 inches, said tank having a capacity of about 1 million barrels of liquid.

12. A storage tank as in claim 11, wherein the roof and side wall comprise a plurality of welded together steel plates, said steel plates comprising the side wall 16. A storage tank as in claim ll, wherein a substan tially flat bottom is attached to the: lower end of the side wall.

17. A storage tank as in claim 16, wherein the bottom provides a mat foundation, and the weight of the tank is sufficient to resist movement of the tank.

Claims

1. A storage tank for storage of liquids in a body of water, said tank comprising a unique side wall having substantially the shape of a frustum of a right circular cone and with an upper end thereof above the surface of the body of water and a lower end thereof fixed to the floor of the body of water, an annular, outwardly extending foot flange integral with the lower end of the side wall, a plurality of circumferentially spaced piles extending through the foot flange into the floor of the body of water fixing the tank to the floor of the body of water, said side wall comprising a pair of spaced apart, substantially coextensive, parallel inner and outer walls, reinforcing means fixed to the opposed, confronting interior surfaces of the parallel inner and outer walls and joining the inner and outer walls together, suitable ballast means in the space between the parallel inner and outer walls and substantially filling same, and a rigid, concavo-convex roof fixed on the upper end of said side wall above the surface of the body of water and closing the upper end of said tank, said roof comprising a continuation of the side wall, the height of said roof being considerably less than the height of the side wall, and the relationship of the diameter of the side wall at its upper end to its diameter at its lower end and to the height of the side wall being such that a line drawn perpendicular to the side wall at its upper end extends through the space bounded by the lower end thereof, the tank thus having a small radius of curvature and a small area at the upper end thereof subject to wave action and the like and thereby effectively resisting stresses and forces on the tank from the surrounding body of water tending to overturn the tank.

5. A storage tank as in claim 1, wherein an annular, frusto-conically shaped wall is connected between the adjacent upper ends of the inner and outer side walls.

9. A storage tank as in claim 8, wherein the upper diameter of the side wall is about 115 feet, the lower diameter of the side wall is about 315 feet, the height of the side wall is about 100 feet, the roof is dome shaped and has a radius of curvature of about 81 feet, and the annular space between the inner and outer side walls is about 40.0 inches, said tank having a capacity of about 500,000 barrels of liquid.

12. A storage tank as in claim 11, wherein the roof and side wall comprise a plurality of welded together steel plates, said steel plates comprising the side wall being about 5/8 of an inch thick, and the steel plates comprising the roof being about 1 inch thick.

13. A storage tank as in claim 1, wherein a work platform is on top of said tank.

14. A storage tank as in claim 1, wherein the means which maintains the position of the tank relative to the floor of the body of water comprises the weight of the tank.

15. A storage tank as in claim 13, wherein the work platform forms a part or all of the roof.

16. A storage tank as in claim 1, wherein a substantially flat bottom is attached to the lower end of the side wall.