US3798915A

US3798915A - Offshore apparatus and installation technique

Info

Publication number: US3798915A
Application number: US00232158A
Authority: US
Inventors: J Burkhardt; J Ortloff; W Loth; R Hickox
Original assignee: Exxon Production Research Co
Current assignee: ExxonMobil Upstream Research Co
Priority date: 1972-03-06
Filing date: 1972-03-06
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26

Abstract

Method and apparatus are disclosed for imparting stability to a storage vessel or similar structure as it is moved vertically between the surface and floor of a body of water. The method comprises disposing a quantity of a lighter-than-water fluid within the structure in an orientation about its center of gravity such that whenever the structure is situated at or beneath the water surface and is displaced from a vertical orientation, the fluid will generate a righting moment which tends to restore the structure to its original position. The fluid disposed within the structure is constrained against migration and the structure may then, for example, be lowered towards the floor of the body of water. The method is particularly applicable to the underwater installation of storage vessels of the type having at least three separate storage chambers and provided with means for regulating the quantity of fluid in each of the chambers without permitting migration between storage chambers.

Description

United States Patent [191 Burkhardt et al.

[ Mar. 26, 1974 OFFSHORE APPARATUS AND INSTALLATION TECHNIQUE [75] Inventors: Joseph A. Burkhardt, Chatsworth,

Califi; John E. Ortloff, Houston, Tex.; William D. Loth, Houston, Tex.; Robert E. Hickox, Kingsville, Tex.

[73] Assignee: Esso Production Research Company,

Houston, Tex.

22 Filed: Mar. 6, 1972 21 Appl. No.: 232,158

Related U.S. Application Data [63] Continuation-impart of Ser. No. 72,291, Sept. 15,

1970, abandoned.

Resos 220/1 B X Courbon 61/46 X [5 7] ABSTRACT Method and apparatus are disclosed for imparting stability to a storage vessel or similar structure as it is moved vertically between thesurface and floor of a body of water. The method comprises disposing a quantity of a lighter-than-water fluid within the structure in an orientation about its center of gravity such that whenever the structure is situated at or beneath the water surface and is displaced from a vertical orientation, the fluid will generate a righting moment which tends to restore the structure to its original position. The fluid disposed within the structure is constrained against migration and the structure may then, for example, be lowered towards the floor of the body of water. The method is particularly applicable to the underwater installation of storage vessels of the type having at least three separate storage chambers and provided with means for regulating the quantity of fluid in each of the chambers without permitting migration between storage chambers.

19 Claims, 12 Drawing Figures PATENTED R26 I974 SHEET t [If 6 PATENIEB MAR 26 I974 SHEET 5 [IF 6 sum s or 5 w UE OFFSHORE APPARATUS AND INSTALLATION TECHNIQUE CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 72,291, filed Sept. 15, 1970 and now abandoned BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to stabilizing a structure as it is moved between the surface of a body of water and the floor thereof and is particularly concerned with method and apparatus for imparting stability to a vessel for storage of crude oil offshore during its installation on the bottom of a body of water.

2. Description of the Prior Art Underwater storage of crude oil can offer significant advantages over surface storage facilities. This is particularly true where extreme water depths, distances, and sea conditions require tremendous pipeline investments. A number of storage vessels for use underwater and techniques for their installation have been disclosed in the past. Most such tanks are constructed with an open bottom to reduce differential pressure between the hydrostatic head and the interior of the tank. Installation of these storage tanks presents a number of serious problems, particularly where the installation is to be made in deep water.

The tanks are constructed onshore, filled with air, and floated to location. At the installation site the vessel is normally transferred to bottom with its interior either filled with water or both air and water. Both approaches have serious disadvantages. If a massive tank is completely filled with water before lowering, the force required to restrain its descent is tremendous, requiring a large number of derrick barges or similar expensive hoisting equipment at the offshore installation site. On the other hand, if air is maintained within the vessel to reduce its negative buoyancy, as the vessel descends, the hydrostatic head of the surrounding water compresses the air and thus continuously increases the negative buoyancy of the vessel. The result is a vessel that varies in effective weight as it descends and requires continual change in the amount of force applied by the surface support vessel. Such a varying load is both undesirable and hazardous. It has also been suggested that the tank be filled with an incompressible fluid such as oil to reduce negative buoyancy during descent. Such a fluid provides a vessel having a substantially constant weight and thus overcomes some of the problems inherent in prior art installation techniques; however, this approach leaves other problems unsolved.

As a tank is lowered, it can be subjected to external forces that displace it from its normal vertical orientation. If such a force is not opposed, the tank will list and can ultimately capsize. This may result in loss of the vessel and pollution of the surrounding body of water, as well as any adjacent shoreline, by the oil or other fluid used to provide buoyancy. Heretofore, stability of the tank during its descent has been provided by means of a rigid tether pipe or similar external structure that extends upwardly from the tank to the water surface. While in relatively shallow water these elongated members will satisfactorily oppose external forces and prevent the tank from capsizing, where substantial water depths are encountered, the stresses on these elongated external structures become excessive. Moreover, once the tank is installed on bottom, anchor piles are normally drilled in around the periphery of the tank to hold it down and counteract the buoyant forces that arise when it is subsequently filled with oil. The floating vessels used to drill in the anchor piles must be positioned directly above the tank and it will be apparent that such surface-extending structures are hazardous to their operation. It will therefore be apparent that there exists a need for a method of installing underwater storage tanks which can be used in deep water and does not require a structural member that extends from the tank to the water surface.

SUMMARY OF THE INVENTION The method and apparatus of the invention generally alleviate the problems of prior art installation techniques by imparting stability to a storage tank or similar structure during its descent to the submerged bottom without requiring a tether pipe or similar external structure. The improved method of moving a structure vertically between the surface of a body of water and the floor thereof comprises disposing a quantity of a lighter-than-water fluid within the structure in an orientation about its center of gravity such that when the structure is in a submerged state the material will generate a righting moment in response to the structure being tilted from its normal vertical orientation. The fluid constrained against migration and the structure is then, for example, lowered toward the floor of the body of water.

The method of the invention is particularly applicable to storage tanks provided with at least three separate storage chambers for constraining the buoyant material against migration and provided with valve means for introducing or withdrawing fluid selectively from any chamber while preventing migration of fluid between chambers. Stabilization of the tank in accordance with the method of the invention involves disposing a quantity of a lighter-than-water fluid within storage chambers arranged about the center of gravity of the tank in a configuration such that when the tank is displaced from its normal vertical orientation the fluid constrained therein will generate a righting moment which tends to restore the tank to its original position. In a more general sense the method may be carried out by distributing a lighter-than-water fluid within a storage vessel or similar structural enclosure in at least three bodies constrained in laterally separate relation and situated in a substantially symmetrical orientation about the center of gravity of the structure, the position of the bodies being fixed such that when the tank is submerged its center of buoyancy will be above its center of gravity and will undergo no appreciable migration. Thereafter, the quantity of fluid in each such body or compartment is regulated to control the buoyancy of the structure without effecting its stability. Fluid communication is normally maintained between a lower portion of each body of fluid and the body of water.

The present method is preferably employed with a cylindrical tank of the type having a top and an open bottom and including a plurality of vertical circular and radial partitions that depend from the top and divide the tank into a plurality of separate concentric cylindrical storage chambers which are radially subdivided. The tank is filled with air or other flotation material and towed to the installation site. A substantially incompressible, lighter-than-water material, for example oil, is then introduced into the tank in a quantity sufficient to give the tank no more than a neutral buoyancy when the original flotation material is withdrawn. The oil is disposed in at least three compartments oriented in relation to the center of gravity such that when all the air is withdrawn and the tank is submerged its center of buoyancy will be above its center of gravity and will be situated on the vertical axis extending therethrough. Air is then gradually displaced by water. Air withdrawals are normally continued until only the incompressible fluid and water remain. The tank which is then preferably slightly negatively buoyant will re main stable as it is lowered to the ocean floor. Once the tank is on bottom, it is frequently desirable to displace the remainder of the buoyant fluid with water to further increase the negative buoyancy of the vessel. This causes the tank to rest more securely on the bottom until it can be permanently anchored in place.

Positioning the buoyant material in laterally separate chambers, as for example in a radially and circularly compartmented tank, creates a distribution of buoyant material which, because it is constrained by the compartmentation, cannot migrate appreciably when the tank is tilted. This unique constrained distribution of fluid buoyant material within the vessel assures that the center of buoyancy will not shift and thus assures the generation of a righting moment that tends to restore the vessel to a vertical alignment whenever it is displaced. Thus, the method of the present invention imparts stability to the tank as it is moved vertically through a body of water without requiring a tether pipe or similar external structure and will be seen to have significant advantages over prior art methods.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. IA1D schematically compare the forces that act on an uncompartmented storage vessel, both when floating upright and when tilted, with those that act on a similarly oriented compartmented vessel.

FIGS. 2A and 2B schematically compare the forces acting on two compartmented storage vessels situated beneath the water surface and displaced from their normal vertical orientations when the center of gravity is positioned above the center of buoyancy (a) and below the center of buoyancy (b).

FIG. 3 is an elevation of a storage tank, useful with the method of the invention, that has a plurality of radially subdivided, concentric cylindrical storage chambers.

FIG. 4 is a cross-sectional plan view of the storage tank of FIG. 3.

FIG. 5 is a schematic elevation view, partially in section, of the storage tank of FIGS. 3 and 4, filled with air and floating on a water surface, together with associated support equipment.

FIG. 6 is a schematic elevation view, partially in section, of the storage tank of FIGS. 3 and 4 after the air has been displaced with oil.

FIG. 7 is a schematic elevation view, partially in section, of the storage tank of FIGS. 3 and 4, with oil positioned in the compartments, during its descent to the ocean floor.

FIG. 8 depicts the storage tank of FIGS. 3 and 4 on the ocean floor after substantially all of the remaining oil has been removed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, as a body floating on a body of water is forced downward, increasing its displacement, its buoyancy increases correspondingly, tending to restore the floating body to its original position. If the vertical axis of a solid floating body in a condition of stable equilibrium is tilted, the additional fluid displaced by the downward side of the floating body normally creates a buoyant restoring force that increases in proportion to the magnitude the vessel is tilted and ultimately causes the vessel to return to its original position. In the case of a storage tank floating on a body of water and containing an unconstrained buoyant fluid, displacement will not necessarily result in the generation ofa righting moment. As the tank is tilted, the buoyant fluid migrates within the vessel, tending to maintain an interface parallel to the free air-water interface. This migration of buoyant fluid substantially decreases the volume of water displaced by the downward tilted side of the tank, preventing the creation of a righting moment.

Referring to FIG. 1A, for the open bottom cylindrical tank shown floating on a body of water, the center of gravity (CG) is above the center of buoyancy (CB). The latter is defined as the centroid of the volume of the displaced water. Buoyant forces act as a single vertical force directed upward through the center of buoyancy; the sum of the gravitational forces act as a single vertical force directed downward through the center of gravity. When the tank is tipped, FIG. 1B, the buoyant medium migrates, altering the shape of the volume of water displaced by the tank and thereby causing the center of buoyancy to migrate. FIG. 1B shows that the buoyant and gravitational forces create a capsizing moment and continued rotation results in an increasingly greater force in the direction of rotation. The tank will therefore continue to rotate, ultimately capsizing. The tank shown in FIGS. 1C and 1D is compartmented to constrain the buoyant medium and thereby prevent its migration. Because the buoyant medium is constrained, as the tank is tilted, FIG. ID, a greater displacement of buoyant fluid occurs on the downward tilted side of the tank. This causes the center of buoyancy to be displaced from the center of gravity in the direction the tank is tilted, resulting in the generation of a righting moment. As the degree of compartmentation is increased, the fluid becomes more confined, approaching the behavior of a solid, and the magnitude of the righting moment increases correspondingly, approaching a maximum. To assure the creation of a righting moment when the floating tank is tipped, the tank should be sufficiently compartmented to restrain migration of the buoyant medium to the extent required to assure that the center of buoyancy is located on the downward tilted side of the tank and is displaced outwardly from the center of gravity.

Although the center of buoyancy of the tank when floating is normally below its center of gravity, after the tank passes beneath the water surface the center of buoyancy must be positioned above the center of gravity. FIG. 2 illustrates the forces acting on a submerged tank containing a buoyant fluid which is constrained against migration. FIG. 2A depicts the forces which act on the tank when it is displaced from a vertical orientation if the center of gravity is situated above the center of buoyancy. As will be noted, the gravitational forces and buoyant forces act in opposition along lines of action which create a capsizing moment. FIG. 2B shows a similar tank which is also tilted but which has its center of buoyancy positioned above its center of gravity. The result is a righting moment. From the two cases shown in FIG. 2 it will be apparent that even if the fluid is confined against migration, the center of buoyancy of the tank when submerged must still be situated above the center of gravity to assure stability.

The magnitude of the righting moment generated when a tank or similar structure is tilted, whether in a floating or submerged configuration, is directly proportional to the volume of water displaced by the buoyant medium and to the horizontal distance between the center of buoyancy and the center of gravity of the structure in its tilted position. Moreover, it will be apparent that for a submerged vessel, the higher the center of buoyancy is situated relative to the center of gravity, the greater will be the magnitude of the restoring moment generated when the tank is tilted. It will further be apparent that to assure stability it is desirable to determine, for the particular tank geometry and fluid distribution, the position of both the center of gravity and the center of buoyancy when the tank is tilted in a submerged state as well as the magnitude and direction of the moment generated.

It will also be clear that to the extent the structure, when submerged, is free to orient itself, it will rotate until the center of gravity is situated vertically beneath the center of buoyancy. In this connection it will be noted that most configurations of storage tanks that are commercially feasible are symmetrical about a vertical axis, c.g., cylindrical, spherical or conical structures. It will further be noted that such structures will have a center of gravity that is situated at a point on the vertical axis. In order to assure that the tank remains in a vertical orientation when submerged it is further necessary to dispose the buoyant fluid symmetrically about the center of gravity and thus about the vertical axis of the tank. To prevent migration of the fluid and thus the center of buoyancy, it is further desirable that the fluid be disposed in at least three laterally separate bodies oriented symmetrically about the center of gravity of the tank.

FIGS. 3 and 4 depict a storage vessel, particularly suited for use with the method of the invention, in elevation and cross-sectional plan views respectively. Tank is shown as a cylindrical vessel having a conical top 12 and an open bottom. Radial partitions l4 and circular partitions l6 divide the tank into two radially subdivided concentric cylindrical storage chambers. While two concentric rings of storage chambers are shown, it will be apparent that a larger or smaller number could be used. The tank shown is preferably constructed of steel, but other materials such as reinforced concrete may also be used. It is also preferred that the configuration of the tank be substantially that of a right circular cylinder and that the tank be closed at the top and open at the bottom; however, as mentioned earlier, the method of the invention is not limited to vessels of this configuration. The present method may be applied to any vessel in which separate bodies of buoyant fluid can be disposed and constrained in an orientation about the center of gravity of the tank such that the fluid will create a righting moment tending to restore the tank whenever it is tilted.

To effect withdrawal of fluid from the bodies of buoyant fluid situated in laterally separate compartments without disturbing the stability of the vessel, care must be taken to prevent migration of fluid between storage chambers. Otherwise the fluid contained therein will act as an unconstrained fluid and any tilting of the tank will lead to a shifting of its center of buoyancy with attendant instability. Shown in FIG. 4 are a plurality of valves 17 each associated with one of the compartments. The valves situated in the chambers included in each ring may conveniently be connected by means of conduits shown as 18 to multiport routing valves 19. These routing valves have one port which is always open to conduits 20 and another that may be positioned selectively to communicate with any one of the conduits leading to individual storage chambers. Conduits 20 lead from the multiport routing valves 19 for each of storage chambers to multiport routing valve 15. Routing valve 15 has one port that is always open to a conduit, as for example, a riser, used to introduce or withdraw fluid from the tank. The remaining ports correspond to either conduits 20 or a closed position. Valve 15 can be positioned to divert flow to or from any one of the multiport routing valves 19 which control flow to and from specific rings of compartments. Thus fluid may be introduced into or withdrawn from individual compartments without distrubing the contents of the remaining storage compartments. The manifolding and valving shown is exemplary and it will be understood that the tank must merely be provided with a means for selectively controlling flow of fluid to or from each compartment while preventing migration of fluid between compartments.

Although tanks which have bottoms can be used with the method of the invention, tanks which are openbottomed are particularly advantageous in that water can migrate into and out of the tank to make up for changes in the volume of stored oil. Bottomless tanks are preferred since they have the additional advantage that significant savings in material can be realized. In that regard, it will be understood that tanks which have bottoms will normally have one or more ports through the bottom or have valving which controls migration of water between the tank and the surrounding body 0 water.

Cylindrical pile guides 21, shown as an integral part of the storage vessel, serve to guide the pilings that are drilled or driven to anchor the vessel to the submerged bottom once it is properly positioned. The pile guides extend somewhat below the vessel itself and have support pads 22 mounted thereon between the end of each guide and the tank. While the configuration shown is preferred, it will be apparent that other means of attaching the piles to the tank could also be employed.

The storage vessel will normally be constructed onshore and, because of its size, floated to the installation site. The buoyancy for flotation may conveniently be provided by filling the vessel with air. Another alternative is to fill the vessel with a liquid having a lower specific gravity than that ofwater. The use ofa liquid does, however, result in a deeper draft and the resulting increase in resistance to movement may make this less attractive where the tank is to be towed a substantial distance. It is contemplated that one or more seagoing tugs or similar vessels will be required to tow the float ing tank to the offshore location for installation.

Surface support equipment is shown provided at the installation site in FIGS. S8 and includes a small utility barge 23 and an oil storage vessel 24. Mounted on the oil storage vessel are power supply 26, pump 28, and a suitable length of oil hose 30 mounted on hose reel 32. Also mounted on the oil storage vessel are winch 34, pulley 36, and support line 38, used to support the storage tank and lower it to bottom. Mounted on the utility barge are a similar winch 40, pulley 42, and support line 44, which are also used to support the storage tank.

Once the tank, filled with air 50, has been towed to location, HO. 5, the utility barge and oil storage vessel will generally be positioned on opposite sides of the tank. Rigging operations are then undertaken. The oil hose 30 is connected to routing valve or a similar manifold near the top of the storage tank.

Support lines

38 and 44 are connected to pad eyes 48 or similar adapters attached to the storage tank. For convenience of description only, two support lines are shown; however, it will be apparent that three or more lines would be preferable and would be helpful in preventing the tank from listing. Similarly, three or more surface support vessels may be used.

With the support vessels rigged to the storage tank, submergence operations are initiated. The tank may be first filled with a substantially incompressible liquid having a specific gravity less than that of water, e.g., oil. This may be carried out prior to transporting the vessel to the location, or, if air has been used to supply the buoyancy, it is displaced by oil which can be pumped into the storage tank from oil storage vessel 24. It should be noted, however, that what must be accomplished is the conversion of the tank from a vessel having a substantially positive buoyancy to one having a slightly negative buoyancy. This is facilitated by disposing a substantially incompressible fluid in selected buoyancy chambers arranged so that the fluid will generate a righting moment tending to restore the tank to its original position whenever it is tilted while in the submerged state. The preferred substantially incompressible fluid for use during submergence operations is crude oil. A number of other liquids, as for example kerosene or gasoline, would, however, also be appropriate. In addition the fluid may be comprised ofa liquid containing a solid material, as for example, hollow glass or metal spheres.

FIG. 6 depicts the storage tank after the air has been completely withdrawn and only oil 52 remains in the interior. The tank still has a positive buoyancy but floats with a greater incompressible, The substantially incompessible, lighter-than-water fluid is then gradually withdrawn from the storage chambers and water is permitted to enter the tank. Alternatively, the desired quantity of incompressible fluid may be distributed within the tank while displacing only a portion of the air. Thereafter air is gradually withdrawn until the tank is in a submerged state and contains only the incompressible fluid. lneither case, liquid withdrawals may readily be accomplished by connecting the oil hose back to the oil storage vessel, positioning the valves on the manifold and allowing the hydrostatic head of the surrounding water to displace oil from selected compartments in the tank. Care must, however, be taken in scheduling withdrawals to assure the distribution is substantially uniform about the center of gravity. Otherwise the tank may begin to list.

Thereafter, the storage tank is lowered (see FIG. 7) by

support lines

38 and 44 connected to

winches

34 and 40. Oil hose 30 is reeled out by hose reel 32 as the storage vessel is lowered. The rate of movement of the vessel can be closely controlled at any time by introducing or withdrawing oil from the tank. These operations are continued until the storage vessel contacts the ocean floor as shown in FIG. 8. With the tank resting on bottom and having a slightly negative buoyancy, most of the remaining oil within the tank will normally be withdrawn. This may conveniently be carried out by using the hydrostatic head of the sea water to displace the oil upwardly through oil hose 30 into storage aboard oil storage vessel 24. The displacement of the lower specific gravity liquid by water significantly increases the negative buoyancy of the storage tank, causing the pile guides 21 to penetrate the submerged bottom until support pads 22 come to rest on the surface of the bottom.

With the storage vessel in place on the floor of the body of water, divers or a submersible craft with manipulators may be employed to unhook the support lines and the oil hose from the storage'vessel. These are then reeled back aboard the surface support vessels. To complete the installation, a floating drilling vessel or similar work platform is employed to drill in and cement the piles through pile guides 21 to anchor the storage vessel to the ocean floor. Thereafter the storage vessel may be hooked up to a submerged gathering system, a flexible riser, or other conduits necessary to supply oil to the storage vessel and to remove oil therefrom.

While the method of the invention has been described in relation to the installation of a storage vessel on the floor beneath a body of water it is not limited to such applications. It will be clear that the present invention also has application in the raising of such tanks to the water surface and to moving other structures between the surface of a body of water and the floor thereof.

What is claimed is:

1. In a method of installing a structure on the floor of a body of water, the method of moving the structure between the surface of said body of water and the floor thereof comprising:

a. disposing a quantity of a lighter-than-water, incompressible fluid within the structure, in an orientation about the center of gravity of the structure such that whenever the structure is totally immersed and is tilted from a vertical orientation, the fluid will generate a righting moment tending to restore the structure to the vertical orientation;

1;. constraining said fluid against lateral migration within said structure while permitting pressure communication between said fluid and the water situated therebelow;

c. adjusting the quantity of said lighter-than-water fluid within said structure without permitting lateral migration thereof as required to render said structure no more than neutrally buoyant when immersed; and

d. moving said structure vertically between the surface of said body of water and the floor thereof.

2. The method of claim 1 wherein said structure is a storage vessel and said fluid is constrained against migration by a plurality of partitions disposed within said vessel.

3. A method of claim 2 wherein said incompressible fluid is a liquid.

4. The method of claim 3 wherein said liquid is an oil.

5. The method of claim 2 wherein said incompressible fluid includes a solid material.

6. In a method of moving a storage tank between the surface of a body of water and the floor thereof, wherein said tank is a cylindrical vessel having a top and a plurality of circular and radial partitions dividing the same into a plurality of laterally separate compartments, the improvement comprising disposing a quantity of a lighter-than-water fluid within said tank in laterally separate compartments oriented about the center of gravity such that when the tank is totally immersed and is tilted from a vertical orientation, the fluid will generate a righting moment.

7. The method of claim 6 wherein said lighter-thanwater fluid is substantially incompressible.

8. The method of claim 7 wherein said incompressible fluid is an oil.

9. The method of claim 7 wherein said incompressible fluid includes a solid material.

10. A method of transferring a storage vessel situated at the surface of a body of water to the floor thereof, comprising:

a. disposing a lighter-than-water fluid within said vessel in a quantity sufficient to impart a positive buoyancy to said vessel and constrained therewithin in at least three laterally separate bodies in an orientation about the center of gravity of the vessel such that said constrained bodies of said fluid will generate a righting moment when the vessel is tilted from a vertical orientation;

b. attaching a flexible lowering means to said storage vessel;

c. withdrawing quantities of fluid from said bodies of fluid until said vessel is no more than neutrally buoyant and the center of gravity of said vessel is situated below the center of buoyancy thereof; and

d. lowering said vessel to the floor of said body of water with said flexible lowering means while supporting same from the water surface.

11. The method of claim 10 wherein said fluid is substantially incompressible.

12. The method of claim 11 wherein said incompressible fluid is an oil.

13. The method of claim 11 wherein said incompressible fluid includes a solid material.

14. The method of claim 10 including the step of regulating the quantity of said fluid within said vessel to control the buoyancy of said vessel during its descent.

15. A method of installing a storage vessel on the floor of a body of water comprising:

a. disposing a lighter-than-water compressible fluid within said vessel in a quantity sufficient to impart a positive buoyancy to said vessel in at least three laterally separate bodies, arranged symmetrically about a vertical axis through the center of gravity of said vessel, each said body containing a substantially equal volume of said compressible fluid;

b. constraining the compressible fluid contained in said bodies against movement in relation to said vessel when said vessel is tilted from a vertical orientation;

c. floating said vessel containing said compressible fluid constrained therein to an offshore installation site;

d. disposing a substantially incompressible, lighterthan-water-fluid within said vessel in at least three laterally separate bodies arranged symmetrically about a vertical axis through the center of gravity of said vessel each said body containing a substantially equal volume of said incompressible fluid;

e. attaching a flexible lowering means to said storage vessel;

withdrawing quantities of said compressible fluid from said vessel until said vessel contains only said incompressible fluid, is no more than neutrally buoyant, and has its center of buoyancy situated above its center of gravity;

g. lowering said vessel to bottom with said flexible lowering means while simultaneously constraining said bodies of incompressible fluid against movement in relation to said vessel when said vessel is tilted from its vertical orientation; and thereafter,

[1. withdrawing additional quantities of said incompressible fluid from said vessel with said vessel situated on said submerged bottom.

16. The method of claim 15 wherein said compressible fluid is air and said incompressible fluid is an oil.

17. Apparatus comprising:

a. a cylindrical storage vessel having an open bottom and a closed top;

b. a plurality of radial and circular vertical partitions situated in said vessel that divide the vessel into a plurality of separate, radially subdivided, concentric storage chambers, said partitions preventing fluid communication therethrough; and

0. means for regulating the quantity of a buoyant fluid in each of said storage chambers without permitting migration of said fluid between storage chambers.

18. The apparatus defined by claim 17 wherein said vessel includes a plurality of vertical pile guides extending therethrough.

19. Apparatus comprising:

a. a circular cylindrical vessel having a closed top;

b. a plurality of circular partitions disposed within said vessel and depending vertically from the top thereof;

0. a plurality of radial partitions depending vertically from the top of said vessel and extending between said circular partitions to divide said tank into a plurality oflaterally separate storage chambers;

migration of fluid between said storage chambers.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION a PATENT NO. 3,798,915

DATED 1 March 26, 1974 |NVENTOR(S) Joseph A. Burkhardt, et al It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below: a

At column 2, line 61, delete "effecting" insert affecting--. At column 6, line 21, between "each" and "of" insert -ring-. At column 7, line 53, delete "incompressible," insert draft..

9 Signed and Sealed this sixteenth D 3) Of September 1975 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Parenrs and Trademarks

Claims

1. In a method of installing a structure on the floor of a body of water, the method of moving the structure between the surface of said body of water and the floor thereof comprising: a. disposing a quantity of a lighter-than-water, incompressible fluid within the structure, in an orientation about the center of gravity of the structure such that whenever the structure is totally immersed and is tilted from a vertical orientation, the fluid will generate a righting moment tending to restore the structure to the vertical orientation; b. constraining said fluid against lateral migration within said structure while permitting pressure communication between said fluid and the water situated therebelow; c. adjusting the quantity of said lighter-than-water fluid within said structure without permitting lateral migration thereof as required to render said structure no more than neutrally buoyant when immersed; and d. moving said structure vertically between the surface of said body of water and the floor thereof.

3. A method of claim 2 wherein said incompressible fluid is a liquid.

4. The method of claim 3 wherein said liquid is an oil.

7. The method of claim 6 wherein said lighter-than-water fluid is substantially incompressible.

8. The method of claim 7 wherein said incompressible fluid is an oil.

10. A method of transferring a storage vessel situated at the surface of a body of water to the floor thereof, comprising: a. disposing a lighter-than-water fluid within said vessel in a quantity sufficient to impart a positive buoyancy to said vessel and constrained therewithin in at least three laterally separate bodies in an orientation about the center of gravity of the vessel such that said constrained bodies of said fluid will generate a righting moment when the vessel is tilted from a vertical orientation; b. attaching a flexible lowering means to said storage vessel; c. withdrawing quantities of fluid from said bodies of fluid until said vessel is no more than neutrally buoyant and the center of gravity of said vessel is situated below the center of buoyancy thereof; and d. lowering said vessel to the floor of said body of water with said flexible lowering means while supporting same from the water surface.

11. The method of claim 10 wherein said fluid is substantially incompressible.

12. The method of claim 11 wherein said incompressible fluid is an oil.

15. A method of installing a storage vessel on the floor of a body of water comprising: a. disposing a lighter-than-water compressible fluid within said vessel in a quantity sufficient to impart a positive buoyancy to said vessel in at least three laterally separate bodies, arranged symmetrically abouT a vertical axis through the center of gravity of said vessel, each said body containing a substantially equal volume of said compressible fluid; b. constraining the compressible fluid contained in said bodies against movement in relation to said vessel when said vessel is tilted from a vertical orientation; c. floating said vessel containing said compressible fluid constrained therein to an offshore installation site; d. disposing a substantially incompressible, lighter-than-water fluid within said vessel in at least three laterally separate bodies arranged symmetrically about a vertical axis through the center of gravity of said vessel each said body containing a substantially equal volume of said incompressible fluid; e. attaching a flexible lowering means to said storage vessel; f. withdrawing quantities of said compressible fluid from said vessel until said vessel contains only said incompressible fluid, is no more than neutrally buoyant, and has its center of buoyancy situated above its center of gravity; g. lowering said vessel to bottom with said flexible lowering means while simultaneously constraining said bodies of incompressible fluid against movement in relation to said vessel when said vessel is tilted from its vertical orientation; and thereafter, h. withdrawing additional quantities of said incompressible fluid from said vessel with said vessel situated on said submerged bottom.

17. Apparatus comprising: a. a cylindrical storage vessel having an open bottom and a closed top; b. a plurality of radial and circular vertical partitions situated in said vessel that divide the vessel into a plurality of separate, radially subdivided, concentric storage chambers, said partitions preventing fluid communication therethrough; and c. means for regulating the quantity of a buoyant fluid in each of said storage chambers without permitting migration of said fluid between storage chambers.

19. Apparatus comprising: a. a circular cylindrical vessel having a closed top; b. a plurality of circular partitions disposed within said vessel and depending vertically from the top thereof; c. a plurality of radial partitions depending vertically from the top of said vessel and extending between said circular partitions to divide said tank into a plurality of laterally separate storage chambers; d. valve means positioned on each of said storage chambers to control the introduction and withdrawal of fluid therefrom; and e. manifold means mounted on said vessel for permitting introduction and withdrawal of fluid from each chamber from a single conduit while preventing migration of fluid between said storage chambers.