US3791152A

US3791152A - Offshore storage system

Info

Publication number: US3791152A
Application number: US00226775A
Authority: US
Inventors: W Davis; G Burns
Original assignee: Chicago Bridge and Iron Co
Current assignee: Chicago Bridge and Iron Co
Priority date: 1972-02-16
Filing date: 1972-02-16
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12
Also published as: JPS4894008A; FR2172651A5; DE2262304A1; IT975893B; NL7215888A; JPS5247574B2; CA962608A; GB1372903A; AU473518B2; ES409225A1; AU4944272A

Abstract

An offshore storage tank having a circular cylindrical vertical shell, a continuous horizontal rigid bottom containing solid ballast joined to the shell and adapted to rest on a sea floor, said vertical shell being tall enough to project above sea level, when the bottom is on the sea floor, for a height adequate for the shell to hold, when full, a liquid to a liquid level above sea level to provide a pressure, together with the pressure of the tank weight, adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned and said vertical shell, only when supplemented by said internal liquid pressure, being able to withstand storm conditions without collapsing when the tank rests on a sea floor.

Description

United States Patent [191 Davis et a1.

[ OFFSHORE STORAGE SYSTEM [75] Inventors: William Allen Davis, Glen Ellyn;

Gerald Edward Burns, Villa Park, both of 111.

[73] Assignee: Chicago Bridge & Iron Company, Oak Brook, Ill.

22 Filed: Feb. 16, 1972 211 Appl. No.: 226,775

[451 Feb. 12,1974

Primary Examiner-Jacob Shapiro Attorney, Agent, or Firm-Merriam, Marshall, Shapiro & Klose [5 7] ABSTRACT An offshore storage tank having a circular cylindrical vertical shell, a continuous horizontal rigid bottom containing solid ballast joined to the shell and adapted to rest on a sea floor, said vertical shell being tall enough to project above sea level, when the bottom is on the sea floor, for a height adequate for the shell to hold, when full, a liquid to a liquid level above sea level to provide a pressure, together with the pressure of the tank weight, adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned and said vertical shell, only when supplemented by said internal liquid pressure, being able to withstand storm conditions without collapsing when the tank rests on a sea floor.

10 Claims, 4 Drawing Figures PATENTEU FEB 1 2 I974 SHEET 2 or 3 OFFSHORE STORAGE SYSTEM This invention relates to tanks for storing liquids. More particularly, this invention is concerned with an offshore storage tank which can be floated to a site and submerged to rest on the sea floor and which can be used for storing liquid materials including liquid hydrocarbons which have a specific gravity less than the specific gravity of fresh water and sea water, as well as the storage of heavier liquids.

Large amounts of oil are obtained from offshore wells, many of which are many miles from the nearest land. While the oil from such wells can be transported by pipeline to shore for storage, it is generally considered more suitable and economical for the oil to be stored close to the producing well until the amount accumulated is adequate for a tanker to remove the oil and transport it to a suitable destination.

Many types of structures have been proposed, and a considerable number have actually been developed and used, for the storage of liquid hydrocarbons, such as oil, offshore. Many of the offshore storage tanks, however, are expensive to construct and to transport to an offshore site for submergence to the sea floor. Also, after submergence the offshore tank must be sufficiently strong to withstand the maximum storm conditions expected while the tank is in use at the site. In general, such tanks must be calculated to have adequate structural integrity to withstand what is generally referred to as a 100-year storm condition at the site where the tank is positioned on the sea floor. In addition to meeting such requirements for safety purposes,

' an offshore tank should be capable of holding and storing the oil at the lowest possible cost per gallon or barrel consistent with safety and good ecological practice.

There is provided by the subject invention an offshore storage tank which can be built essentially completely onshore, or in a suitable area offshore, and then tow-floated to a site offshore for submergence to the sea floor. The offshore tank provided by the invention can provide a large storage capacity at a comparatively low cost per unit of storage volume because of its relative simplicity of design and construction. The tank has a circular cylindrical vertical shell which can be made of any suitable material but advisably is made of metal plate. The shell however can also be made of concrete. The tank has a continuous horizontal rigid bottom containing solid ballast, joined to the shell. The bottom is essentially flat and is adapted to rest on top of a sea floor at a site where the tank is to be submerged for use. The vertical shell is made sufficiently tall or high enough so as to extend from the sea floor to a height above sea level adequate to store fresh water, sea water or some other liquid at a liquid level above the surrounding sea level so as to provide a net downward force against the bottom of the tank. The combined weight of the tank plus the pressure applied by the liquid head in the tank above sea level is to be adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned. Furthermore, the vertical shell, only when supplemented by saidd internal liquid pressure, is able to withstand storm conditions without buckling and collapsing when the tank rests on the sea floor. Without such a liquid level in the tank, the shell is insufficiently strong to withstand storm conditions. It should be understood however that the shell is sufficiently strong and still adequately ballasted to withstand sea pressure during nonstorm con ditions when the internal liquid level is at about sea level.

When the offshore storage tank is to be used for storing a liquid hydrocarbon, the tank is provided with means for supplying a liquid hydrocarbon thereto and for withdrawing it from the tank, and means for supplying water to the tank and for withdrawing it therefrom. Both of such means however are operable to effect the desired interchange of liquids in the tank while simultaneously keeping the tank interior full of liquid at least to a level above sea level so as to provide a substantial downward pressure on the tank bottom adequate to hold the tank in place during storm conditions. The vertical shell of the tank, only when supplemented by said internal liquid pressure, is able to withstand storm conditions without buckling and collapsing when the tank rests on the sea floor.

In one embodiment, the bottom of the tank advisably includes a horizontal metal plate which is joined to the circular vertical cylindrical metal shell. A layer of solid ballast material, such as concrete, is located on top of the metal plate comprising part of the bottom. It is furthermore advisable for a metal plate to be located on top of the ballast layer and to have such top metal plate joined to the circular vertical cylindrical shell. The ballast layer is furthermore advisably reinforced to provide added strength. In addition, tying means advisably join the ballast layer to the metal plates above and below the ballast layer, as well as to the circular vertical cylindrical shell.

Because the tank of the subject invention is intended to be made of large size, such as of at least l00 feet in diameter, the vertical metal shell is advisably made of a series of horizontal metal courses or rings fabricated from metal plate. The thickness of each ring may be of uniform thickness except for plates used around openings or the like where heavier plates are sometimes desirable for added strength. However, the rings of the shell which are subject to wave action are advisably made thicker and thereby stronger than the other rings so that the tank can more readily resist the forces applied during storms. By using thinner metal plates for the lower rings, less expensive material and fabrication costs are entailed.

To aid in preventing excessive bending or ovaling of a circular cylindrical vertical metal shell, horizontally positioned spaced-apart stiffener rings can be joined to the internal surface of the shell. As distinguished from ovaling, which does not affect the integrity of the tank, the liquid head maintained in the tank is necessary to keep the tank shell from buckling and collapsing even though it permits ovaling. Ovaling is, of course, less of a problem with a concrete shell since it is far less flexible than a metal shell.

A skirt can be vertically positioned to extend downwardly from the lower part of the tank into the sea floor to prevent sea bed material from eroding from beneath the tank bottom. The skirt more specifically is positioned to project downwardly from the lower outer edge of the vertical shell. The skirt is forced into the sea floor when the tank is submerged to rest on the sea floor.

The tank can be left open at the top or it can'be covered with a fixed position roof of a domed or conical shape. In addition, a substantially flat fixed position roof can be employed. Such fixed position roof can be mounted on the upper part of the circular cylindrical vertical shell. However, a floating roof can be employed inside of the vertical cylindrical shell with or without a fixed outer roof. However, since the tank is to be kept full of a liquid above the level of the surrounding sea, the vertical displacement of the floating roof advisably is limited so as not to go significantly below sea level when the tank is in operation. To provide for free vertical displacement of a floating roof on the liquid in the tank when the vertical shell has interior spaced-apart stiffening rings, it is advisable to position a cylindrical roof guiding metal shell over the stiffeners in the upper portion of the tank vertical shell.

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

FIG. 1 is a vertical cross-sectional view of an offshore storage tank, provided according to the invention, havng an open top and a floating roof;

FIG. 2 is a enlarged view of the bottom structure of the offshore storage tank of FIG. 1;

FIG. 3 is another embodiment of an offshore storage tank provided by the invention shown in vertical sectional view resting on a sea floor and having a stationary roof; and

FIG. 4 is a vertical sectional view of'a tank such as shown in FIG. 1 in which the vertical metal shell is thicker at the area of wave action than the areas below and above.

So far as is practical, the same parts or elements which are identical and appear in the different figures of the drawings will be identified by the same numbers.

With reference to FIGS. 1 and 2, the offshore storage tank has a vertical cylindrical metal shell or wall ll made of a plurality of horizontal courses or rings of metal plate. Each ring is circular in horizontal section and can be of any suitable height but generally will be from about six to 10 feet tall. Any number of such rings can be employed to make the vertical cylindrical shell as high as desired, but at least high enough to extend substantially above sea level at a site where the tank is to besubmerged. The bottom of tank 10 has a lower horizontal metal plate 12 and an upper horizontal metal plate 13, both of which extend over the area circumscribed by vertical shell 11. Concrete is positioned between bottom plate 12 and upper plate 13 and constitutes ballast material. Sufficient ballast is employed to firmly anchor the tank on the sea floor when it is submerged and contains a liquid to a level above sea level adequate to prevent the tank shell from collapsing during storm conditions.

A plurality of metal stiffener rings 15 can be positioned on the inside surface of vertical shell 11 in spaced-apart horizontal relationship with respect to one another. The stiffener rings however can be placed on the outside of the shell. Skirt 16, made of metal plate, is positioned around the lower bottom portion of the vertical metal shell. It extends below bottom plate 12 for a significant distance so that it can penetrate into the sea floor as the tank is submerged to rest thereon. Grout I7 is placed between skirt l6 and vertical shell 11 to hold the skirt firmly in place before the tank is submerged.

Floating roof 19 is displaceable vertically with rise and fall of liquid in the tank. Vertical metal plate ring 18 is located in the top inner portion of vertical shell 11 to provide a cylindrical wall having a smooth surface against which circular floating roof 19 may rub without obstruction during vertical displacement.

Standpipe 20 communicates at its lower end 21 with the internal bottom space of tank 10. The upper outlet end 22 of standpipe 20 is located at the maximum water level which tank 10 is designed to hold to provide extra downward pressure to seat the tank firmly on the sea floor so that it can safely withstand storm action. Conduit 23 is used to feed oil into storage tank 10 and to remove oil therefrom. Conduit 23 is located at the same height as the water level when the tank is full of water and, therefore, it is at the same height as the overflow end 22 of standpipe 20. Conduit 24 communicates with the lower internal portion of tank 10 and with pump 25 for pumping water into the tank when oil is to be removed therefrom.

FIG. 2 illustrates the construction of the bottom of the tank in more detail than is shown in FIG. 1. A plurality of spaced-apart metal reinforcing members 31 and 33 are positioned horizontally to form a criss-cross pattern above metal plate 12 and are held there by shear connectors 32, which are also attached to plate 12, until the concrete solidifies. Similarly, reinforcing member 34 are positioned spaced beneath metal plate 13 and reinforcing members 35 are positioned spaced apart from one another lateral to reinforcing members 34. Shear connectors 36 maintain the reinforcing bars in position relative to one another and plate 13. The perimeter of concrete ballast 14 has reinforcing bars 37 and 38 crisscrossing one another and held in fixed spaced-away position from vertical shell 11 by shear connectors 39, which are joined to the shell 11, until the concrete hardens.

The offshore storage tank 10 shown in FIGS. 1 and 2 can be built onshore in a graving dock and thereafter floated to a site for submergence. Submergence of the tank can be achieved by pumping sea water into the tank. The tank may tilt to 10 or 15 in the early stages of descent but will become stable in the upright position as it submerges to deeper depths.

A tank according to the embodiment of FIGS. 1 and 2 should provide an adequate downward net pressure by the weight of the material used in the tank plus concrete ballast 14 to hold the tank securely against the sea floor when the tank has at least no more liquid therein than is equal to the pressure exerted by a water content in the tank up to sea level. However, to minimize the amount of ballast which need be employed, it is advisable to always have a liquid content in the tank at a level substantially above sea level so that additional downward pressure is provided to stabilize the tank. The tank of FIGS. 1 and 2 is so designed as to have such a liquid head above sea level to provide the added downward pressure. Such liquid head is however required for the tank shell to withstand storm conditions without collapsing. Therefore, such liquid head is advisably always maintained in the tank once it is submerged.

The operation of the tank of FIGS. 1 and 2 in storing oil is not complicated. When the tank 10 is positioned in place on a sea floor, the water level therein is raised to the level of the overflow end 22 of standpipe 20. To fill the tank with oil, oil is pumped into the tank through conduit 23. Because the oil naturally has a specific gravity less than sea water, floating roof 19 will rise and, at the same time, the interface between the oil and the water will move downwardly. The pressure head thereby developed inside of tank will cause water to flow from the tank through standpipe and out outlet end 22. As pumping of oil into the tank continues, the oil-water interface will continue dropping in tank 10 and is permitted to drop until it reaches the lower end 21 of the standpipe. At that point the tank is considered full of oil so the pumping of oil is terminated. Floating roof 19 is displaced upwardly when the tank is filled with oil until it is essentially located at the top edge of vertical cylindrical shell 11. When the tank is full of oil, floating roof 19 will be in its most upward position shown in phantom as 19A. When the tank is full of oil, the net pressure of the oil on the tank bottom will be essentially identical to the net pressure obtained when the tank is full of water to the level of the outlet 22 of standpipe 20 so that as a result there is a substantially uniform pressure exerted on the tank bottom whether the liquid in the tank is oil, water or two separate layers thereof. Such uniform pressure is calculated to keep the tank shell from collapsing during storm conditions.

To remove oil from the tank of FIG. 1, a valve (not shown) in conduit 23 can be opened and, as a result, oil will flow out therefrom. Water can be pumped in by conduit 24 by means of pump 25 to force the oil level up sufficiently high so that oil will flow out conduit 23 by gravity.

With reference to FIG. 3, the tank 40 shown therein is intended for offshore use for the storage of oil. Tank 40 has a vertical circular cylindrical metal shell 41 which is attached or connected at its lower bottom portion to concrete ballast slab 42 having no metal top or bottom, although either or both can be used if desired. Concrete ballast slab 42 has a peripheral edge 50 which extends beyond shell 41 to restrict the tank against tipping during storm conditions. Skirt 51 extends from the bottom of ballast slab 42 into the sea floor to keep sea bed material from being eroded from beneath ballast slab 42.

Stiffener rings 43 are positioned horizontally in spaced-apart relationship inside of vertical shell 41 to reinforce it against bending through the pressure of the sea and forces developed through wave and of the sea and forces developed through wave and storm action. If desired, the stiffener rings can be positioned on the outside of the shell. Metal semielliptically domed roof 44 is supported by vertical shell 41. Vent 45 in domed roof 44 permits flow of air in both directions to prevent development of an internal pressure different from atmospheric pressure.

The offshore storage tank of FIG. 3 can be constructed onshore in a graving dock and then floated to a suitable site for submergence. It can be submerged by pumping water inside of the tank with care being taken in the tank design so that the weight distribution prevents uncontrolled tilting and capsizing. Once the tank is partially submerged, and the center of gravity is below the center of buoyancy, it will be in a stable condition and no further means need be employed to stabilize the tank during further submergence.

The tank of FIG. 3 is provided with sufficient weight by ballast 42, vertical shell 41 and roof 44 to provide an adequate downward pressure when full of water to sea level sufficient to maintain the tank in a stable position on a sea floor site. However, to provide additional downward force for added stability, tank 40 is designed to normally hold water up to the line WP, and to hold oil up to the line OP, shown in FIG. 3. The oil level line OP and the water level line WP are so positioned as to provide essentially the same pressure on the tank bottom when the tank is full with oil or water to the designated level. The oil level line OP is located above the water level line WP because oil has a specific gravity (about 0.85) lower than water and therefore a larger volume of oil must be stored in the tank to obtain a pressure equal to the pressure exerted by a volume of water in the tank having a level at the water level line WP. Both the oil level line OP and the water level line WP are located above sea level so that a suitable pressure head is maintained under most circumstances to provide the desired downward pressure on the tank bottom.

Inlet-outlet conduit 46 is located at water level line WP. Furthermore, outlet 48 of standpipe 47 is also positioned at the water level line WP. The lower end 49 of standpipe 47 communicates with the lower internal space of the tank. Pump 52 is used to pump water through conduit 53 into the lower space of tank 40.

When tank 40 is not employed for storing oil, it is maintained full of water up to the water level line WP. When it is to be used for oil storage, oil is pumped in through conduit 46. As the volume of oil supplied to tank 40 increases, the interface between the oil layer on top of the water continually descends and water is forced through standpipe 47 and out outlet 48. Once the oil-water interface descends to the lower end 49 of the standpipe, pumping of oil to the tank is terminated. At such point, the level of oil in the tank will be at the oil level line OP.

When the tank 40 is full of oil, oil may be removed through conduit 46. When oil is removed through conduit 46, water is pumped into the tank by pump 52 through conduit 53. Pumping of water is continued until the water level in the tank reaches the line WP so that there is an added downward pressure against the tank bottom produced by the height of the water in the tank above the external sea level.

Operation and use of the tank, the means for supplying oil to the tank, and the means for withdrawing water from the tank, are intended to maintain a total liquid content in the tank which exerts a hydrostatic pressure on the tank bottom per unit of area above the external sea pressure. The result of this is that the weight of the tank, including the ballast, always applies a downward pressure which holds the tank stable under normal sea conditions. By maintaining the internal water or oil level above sea level, the downward pressure on the bottom provides stability adequate for the tank shell wall to withstand design storm conditions.

EXAMPLE 1 An offshore storage tank for oil is constructed for placing on a sea floor at a site having a mean sealevel of feet above the sea floor. The tank is circular in horizontal cross-section and has a vertical cylindrical metal shell 176 feet high and is of the general shape shown in FIG. 1. A six-foot thick horizontal concrete bottom slab having a metal cover on each side provides ballast. A skirt 16 is prepositioned to penetrate into the sea floor for five feet. The tank has a floating roof which is vertically displaceable within a cylindrical ring 18 having a 208-foot diameter and a 32-foot height. The tank is designed to have a maximum internal water level of feet from the bottom of the sea and a maximum internal oil level of 174 feet from the sea bottom.

The vertical cylindrical shell 11 is constructed of 22 horizontal rings eight feet high as shown in FIG. 4. Each ring is made of steel plate. The thickness of the rings starting at the bottom of the shell and progressing upwardly is shown in FIG. 4 as follows: four rings inch thick (A,A four rings inch thick (B -B three rings 1 inch thick (C -C one ring 1 inches thick (D one ring 1% inches thick (15,); one ring 1% inches thick (F two rings 1% inches thick (G G one ring 1 inches thick (H one ring 1% inches thick one ring 1 inch thick (J one ring inch thick (K and two rings )4; inch thick (L,L The vertical shell 11 of the tank is therefore thicker at the area effected by wave action than the areas below and above.

When full of water to the intended height, the tank will have a positive pressure of 2000 lbs/sq. ft. and the same pressure is intended to be applied when the tank is full of oil. The capacity of the tank is slightly over 1,000,000 barrels of oil.

When the described tank is placed on a sea floor and filled with liquid, no piles are needed to maintain it stable even with 40 foot waves, such as might develop during a storm condition. The tank has sufficient dead weight to provide stability for storms involving maximum wave heights of 15 feet. Therefore, the tank could fill with water during tanker loading of oil until the internal water level is at the same level as the external sea level, after which additional water can be added to the tank to the maximum water level line 19 feet above sea level. A 48-foot head of oil (specific gravity 0.85) in the tank gives a positive pressure equivalent to a head of 19 feet of water.

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.

What is claimed is:

1. In combination, an offshore storage tank supported by a sea floor comprising:

a circular cylindrical vertical metal shell;

a continuous horizontal rigid bottom containing solid ballast, joined to the shell and resting on the sea floor;

said vertical metal shell extending from the rigid bottom upwardly to above sea level for a liquid to a height adequate for the shell to hold liquid to a liquid level above the level of the sea surrounding the tank to provide a pressure together with the pressure of the tank weight, which is adequate to hold the tank in place on the sea floor during storm conditions;

said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without buckling and collapsing with the tank resting on the sea floor; and

liquid in the shell to a level above the surrounding sea which is adequate to hold the tank in place on the sea floor during storm conditions without buckling and collapsing.

2. A combination according to claim 1 in which the tank is covered by a fixed-position roof supported above sea level by the vertical metal shell.

3. In combination, an offshore storage tank supported by a sea floor comprising:

a circular cylindrical vertical metal shell;

said vertical shell extending from the rigid bottom upwardly to above sea level for a height adequate for the shell to hold a liquid to a liquid level above the level of the sea surrounding the tank to provide a pressure together with the pressure of the tank weight, which is adequate to hold the tank in place on the sea floor during storm conditions;

said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without buckling and collapsing with the tank resting on the sea floor;

means for supplying a liquid hydrocarbon to, and for withdrawing it from, the tank, and means for supplying water to the tank and withdrawing it therefrom;

said means for supplying a liquid hydrocarbon to, and withdrawing it from, the tank and said means for supplying water to, and withdrawing water from, the tank being operable to maintain a total liquid content in the tank which exerts a pressure on the tank bottom per unit area greater than the external sea pressure; and

4. A combination according to claim 3 in which the bottom comprises a metal plate on top of which is a substantially uniform layer of concrete.

5. A combination according to claim 4 in which a metal plate is located on top of the concrete layer and is joined to the vertical shell.

6. A combination according to claim 3 in which a metal skirt extends vertically downwardly from the tank bottom.

7. A combination according to claim 3 in which the meansfor withdrawing water from the tank includes a conduit inlet communicating with the bottom interior space of the tank and an outlet located at the height of the maximum water level which the tank is calculated to hold.

8. A combination according to claim 3 in which the means for withdrawing liquid hydrocarbon from the tank includes a conduit having an inlet in the tank at the height of the maximum water level which the tank is calculated to hold.

9. An offshore storage tank comprising:

a circular cylindrical vertical metal shell made of horizontal metal rings with the rings of the shell subject to wave action thicker than the other rings in the vertical metal shell;

a continuous horizontal rigid bottom comprising solid ballast, joined to the shell and adapted to rest on a sea floor;

said vertical shell being tall enough to project above sea level, when the bottom is on the sea floor, for a height adequate for the shell to hold, when full, a liquid to a liquid level above sea level to provide a pressure together with the pressure of the tank weight, adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned; and

said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without collapsing when the tank rests on the sea floor.

10. An offshore storage tank comprising:

a circular cylindrical vertical metal shell having a plurality of horizontal spaced-apart stiffener rings joined to the internal surface thereof;

said vertical metal shell having a cylindrical metal roof-guiding shell inside the upper portion thereof ballast, joined to the shell and adapted to rest on a sea floor;

, said vertical shell being tall enough to project above sea level, when the bottom is on the sea floor, for a height adequate for the shell to hold, when full,

A a liquid to a liquid level above sea level to provide a pressure together with the pressure of the tank weight, adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned; and said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without collapsing when the tank rests on the sea floor.

4 2 UNTTEE STATES PATENT OFFICE- CERTIFICATE 0F CORREC O .Patent No. 3,791,152 Dated February 12, 1974 Inventor) William Allen Davis and Gerald Edward Burns .It is certified that error appears in the above-identified patent .and that said Letters Patent are hereby corrected as shown below:

Column 1, line 63, change "saidd" to said column 7, line 11, change '1 inches" to -l-3/8 inches-; line 47,

delete "liquid to a"; column 8, line 55, change "comprising" to --containing-.

Signedj'a-nd sealed this 3rd day of September 1974.

csEAL Attestt FMcCOY M. GIBSON, JR. a c. .MARSHALL EANN Attesting Officer Commissioner of Patents

Claims

1. In combination, an offshore storage tank supported by a sea floor comprising: a circular cylindrical vertical metal shell; a continuous horizontal rigid bottom containing solid ballast, joined to the shell and resting on the sea floor; said vertical metal shell extending from the rigid bottom upwardly to above sea level for a height adequate for the shell to hold liquid to a liquid level above the level of the sea surrounding the tank to provide a pressure together with the pressure of the tank weight, which is adequate to hold the tank in place on the sea floor during storm conditions; said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without buckling and collapsing with the tank resting on the sea floor; and liquid in the shell to a level above the surrounding sea which is adequate to hold the tank in place on the sea floor during storm conditions without buckling and collapsing.

3. In combination, an offshore storage tank supported by a sea floor comprising: a circular cylindrical vertical metal shell; a continuous horizontal rigid bottom containing solid ballast, joined to the shell and resting on the sea floor; said vertical shell extending from the rigid bottom upwardly to above sea level for a height adequate for the shell to hold a liquid to a liquid level above the level of the sea surrounding the tank to provide a pressure together with the pressure of the tank weight, which is adequate to hold the tank in place on the sea floor during storm conditions; said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without buckling and collapsing with the tank resting on the sea floor; means for supplying a liquid hydrocarbon to, and for withdrawing it from, the tank, and means for supplying water to the tank and withdrawing it therefrom; said means for supplying a liquid hydrocarbon to, and withdrawing it from, the tank and said means for supplying water to, and withdrawing water from, the tank being operable to maintain a total liquid content in the tank which exerts a pressure on the tank bottom per unit area greater than the external sea pressure; and liquid in the shell to a level above the surrounding sea which is adequate to hold the tank in place on the sea floor during storm conditions without buckling and collapsing.

7. A combination according to claim 3 in which the means for withdrawing water from the tank includes a conduit inlet communicating with the bottom interior space of the tank and an outlet located at the height of the maximum water level wHich the tank is calculated to hold.

9. An offshore storage tank comprising: a circular cylindrical vertical metal shell made of horizontal metal rings with the rings of the shell subject to wave action thicker than the other rings in the vertical metal shell; a continuous horizontal rigid bottom comprising solid ballast, joined to the shell and adapted to rest on a sea floor; said vertical shell being tall enough to project above sea level, when the bottom is on the sea floor, for a height adequate for the shell to hold, when full, a liquid to a liquid level above sea level to provide a pressure together with the pressure of the tank weight, adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned; and said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without collapsing when the tank rests on the sea floor.

10. An offshore storage tank comprising: a circular cylindrical vertical metal shell having a plurality of horizontal spaced-apart stiffener rings joined to the internal surface thereof; said vertical metal shell having a cylindrical metal roof-guiding shell inside the upper portion thereof over the stiffeners adjacent thereto and the cylindrical metal roof-guiding shell has a height from maximum water level to maximum oil level in the tank; a floating roof having a periphery in close proximity to the cylindrical metal roof-guiding shell internal surface; a continuous horizontal rigid bottom containing solid ballast, joined to the shell and adapted to rest on a sea floor; said vertical shell being tall enough to project above sea level, when the bottom is on the sea floor, for a height adequate for the shell to hold, when full, a liquid to a liquid level above sea level to provide a pressure together with the pressure of the tank weight, adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned; and said vertical shell only when supplemented by said internal liquid pressure being able to withstand storm conditions without collapsing when the tank rests on the sea floor.