US3889477A

US3889477A - Crude oil offshore storage vessel

Info

Publication number: US3889477A
Application number: US436905A
Authority: US
Inventors: William A Tam
Original assignee: Chicago Bridge and Iron Co
Current assignee: Chicago Bridge and Iron Co
Priority date: 1974-01-28
Filing date: 1974-01-28
Publication date: 1975-06-17
Anticipated expiration: 1992-06-17

Abstract

A self-ballasting offshore oil storage tank comprises a vertical enclosure having a middle section with a cross-sectional area smaller than those of upper and lower sections joined thereto. The configuration of the upper and lower sections are made such that, with the middle section located at the water surface, oil stored within the vessel automatically apportions itself between the upper and lower sections in such a way that the weight of the oil stored in the upper section balances the buoyant effect of the oil stored in lower section.

Description

United States Patent 91 Tam 1 CRUDE OIL OFFSHORE STORAGE VESSEL [75] Inventor: William A. Tam, Warrenville, 111.

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

[22] Filed: Jan. 28, 1974 [21] Appl. No.: 436,905

[52] U.S. Cl 61/46.5; 114/.5 T; 220/85 A [51] Int. Cl. B65G 5/00; B65D 89/10 [58] Field of Search 61/46.5, 46; 114/.5 T; 220/85 A [56] References Cited UNITED STATES PATENTS 3,552,131 1/1971 Mott et al. 61/46 3,599,590 8/1971 Rego 114/.5 T

3,798,919 3/1974 Hershner, Sr 61/46.5 X

[ June 17, 1975 Primary Examiner 1ac0b Shapiro Attorney, Agent, or FirmMerriam, Marshall, Shapiro & Klose [5 7 ABSTRACT A self-ballasting offshore oil storage tank comprises a vertical enclosure having a middle section with a cross-sectional area smaller than those of upper and lower sections joined thereto. The configuration of the upper and lower sections are made such that, with the middle section located at the water surface, oil stored within the vessel automatically apportions itself between the upper and lower sections in such a way that the weight of the oil stored in the upper section balances the buoyant effect of the oil stored in lower section.

10 Claims, 6 Drawing Figures Ila SHEET PATENTEDJUN 17 ms FIGJ FIG. 2

PATENTEDJUN 17 ms SHEET FIG. 4

CRUDE OIL OFFSHORE STORAGE VESSEL This invention relates to a novel self-ballasting offshore oil storage tank, which automatically compensates for the buoyant effect of the oil contained therein, regardless of the quantity thereof.

The use of offshore storage tanks for the storage of water-immiscible liquids such as crude oil has become common. Most of these offshore tanks operate on the water-displacement principle, in accordance with which water is taken into or displaced from the storage tank as the volume of stored oil varies. The design and operation of such tanks is complicated by the fact that the stored liquid, being lighter than water, exerts a substantial buoyant effect which in the case ofa large vessel can be of the order of millions of pounds. In order to maintain a constant draft in the case of conventional floating storage tanks, extensive mechanical pumping systems are used to compensate for variations in the buoyant effect of the stored oil. The operation and maintenance of such pumping systems present substan tial operating problems. Not only are the systems complicated and therefore expensive but there also exists the possibility that a hazardous condition will be created through an operational error or malfunction.

In the case of storage tanks which are fixed to the bottom of the body of water in which they are situated, an anchoring system, typically including piles and/r ballast weights, must be used to accommodate the enormous upward buoyant effect caused by the stored liquid. Since the design must be based on the extreme case of the storage tank full of oil, the anchoring system is excessively large for periods of operations in which the tank is less than completely filled.

These difficulties are eliminated by the present invention, with substantial savings in cost and improved safety. In accordance with the invention, there is provided an offshore storage tank which is automatically self-ballasting. The tank comprises a vertically elongated enclosure which is open at its bottom for ingress and egress of water as the volume of stored liquid varies, in accordance with conventional practice. The enclosure has an upper section which extends above the liquid level of the water body and a lower section which is always submerged. The enclosure further has a crosssection of minimum area at a location between the upper and lower sections. In use, the tank is so positioned that its minimum cross-section is located at the level of the surface of the body of water in which the enclosure is situated. By suitably proportioning the upper and lower sections of the enclosure, the oil stored in the vessel automatically distributes itself between the upper and lower sections in such a way that the buoyant force due to the stored oil below the water surface is balanced by the weight of the oil in the upper section above the water line. The self-ballasting feature of the invention assures that a floating storage tank made in accordance therewith will always maintain the same draft without the necessity for a mechanical pumping system, regardless of the quantity of stored liquid, and that the force which must be resisted by the supporting structure of a fixed facility remains is greatly reduced compared to conventional fixed storage tanks of the same capacity.

The invention will be better understood from the following detailed description thereof, taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a portion of one embodiment of the invention in the vicinity of the water level;

FIG. 2 shows a different embodiment of the invention, of the floating type;

FIG. 3 shows another embodiment of the invention which is fixed on the floor of the body of water;

FIG. 4 shows another embodiment of the fixed type, having a different configuration in the vicinity of the water level;

FIG. 5 shows a composite structure consisting of several embodiments similar to that of FIG. 3 interconnected to form a large capacity storage unit; and

FIG. 6 represents still another embodiment of the invention of the fixed type.

Referring to FIG. 1, there is depicted a portion of one embodiment of the invention, in the vicinity of the level of the water in which the enclosure is situated. As shown, the enclosure 10 comprises a cylindrical middle section 13, an upper section 11 having a cylindrical portion 11a and a curved transition portion 11b joining section 11a to the top of section 13, a lower section 12 having a cylindrical portion 12a (equal in diameter to 11a) and a curved transition portion 12b joining cylindrical portion 12a to the bottom of section 13. The line L represents the surface level of the water 14 in which the enclosure is situated. The middle section 13 of the vessel is full ofa charge of oil 15, the top surface 16 of which extends a distance Y above L and lower surface 17 of which extends a distance X below L. In order to satisfy the equilibrium pressure condition which exist and in order to balance the forces of buoyancy and gravity, the distance Y and X are related by the formula where d is the density of the water and d is the density of the oil. Under the condition depicted, the net buoyant effect of the portion of the oil below the water surface, which extends to the bottom of middle section 13, is exactly balanced by the weight of the column of oil in the portion of section 13 which extends above the surface of the water. A

If there is now added to the interior of container 10 an additional charge of oil, the newly added oil will distribute itself under the force of gravity between the upper and lower sections of the enclosure, causing the upper surface 16 of the oil to rise an additional distance AI and the lower surface 17' of the oil to descend an additional distance AX below the water level L. Under the new conditions the level of the oil above the surface of the water L, i.e., the distance (Y AI), will automatically be sufficient to achieve a pressure equilibrium throughout the column of oil such that the pressure at the bottom of the oil phase, i.e., at a distance (X AX) below the surface of the water, will be equal to the hydrostatic water pressure at that depth. The net buoyant effect of the submerged oil phase, however, will not necessarily remain constant under the conditions described, unless the weight of the added oil incorporated in the structure above the water level is equal to the additional buoyant force produced by the added oil enclosed in the structure below the surface of the water.

In accordance with the invention, the tank is made self-ballasting by suitably proportioning the upper and lower sections 11 and 12 of the enclosure 10. This result is achieved by making the configuration of enclosure such that the cross-sectional area A of the upper surface of the oil phase contained therein is at all times equal to the cross-sectional area A of the lower surface of the oil.

In the structure depicted in FIG. 1, section 13 is cylindrical, while sections 11 and 12 are also cylindrical above and below the transition portions 11,, and 1212 respectively. Accordingly, when sufficient oil is charged within the vessel so that the oil extends into the cylindrical portions of sections 11 and 12, the conditions for self-ballasting will be present. In the transition portions between the extremities of section 13 and the cylindrical portions of sections 11 and 12, the shape of the enclosure must be such that the interfacial areas at the top and bottom of the oil phase, i.e., the area of the interface between the oil and the air at the top of the oil phase, and that between the oil and the water at the bottom of the oil phase, remain equal at all times.

For purposes of the invention it is necessary that the enclosure have a configuration such that there it has a minimum cross-sectional area at a location between the ends of the enclosure, the location of minimum area being situated to correspond with the water level of the body in which the enclosure is positioned. With the enclosure thus positioned, the enclosure will be selfballasting if the rate of change in cross-sectional area with increasing distance above the water line is a fraction, equal to d /d d of the rate of change of crosssectional area with increasing distance below the water level. As a result of this condition, the change in cross sectional area above the water line per unit of vertical distance is greater than, but proportional to, the change in cross-sectional area which occurs per unit distance below the water line.

In a container having the configuration shown in FIG. 1, in which the cross-sectional shape is circular at all points, it will be seen that the vertical configuration of the vessel above the water line is similar to that below the water line, but more accentuated, a given change in cross-section area occurring in less vertical distance.

A more generalized embodiment of the invention is illustrated in FIG. 6. As shown, container 60 has an upper section which tapers upwardly and inwardly, meeting at a location which has a minimum crosssectional area positioned at the level L of the surrounding body of water. The cross-sectional areas of the enclosure above and below the water line are so designed that the top and bottom interfacial areas of a charge of oil introduced to the enclosure will always be equal. Accordingly, no net change in the buoyancy produced by the oil contained within the vessel will be produced, regardless of the quantity of oil stored.

It should be noted in connection with FIG. 6 that although for practical reasons it will be generally found more convenient to construct an enclosure which is symmetrical about a vertical axis, the invention is not limited to such structures. The only requirement is that the interfacial area of the top of the oil charge be at all times equal to the interfacial area of the bottom of the oil charge. The relative shapes of the structures above and below the water line are unimportant. Further, although the structure of FIG. 6 illustrates an operative embodiment of the invention, the complex curves of the upper and lower section of the enclosure may present practical difficulties in construction. For this reason, it will be generally preferred to make the enclosures of the invention with individual portions that are cylindrical, spherical or conical, rather than continuously curving, as in the case of FIG. 6. Examples of such structures are shown in FIGS. 2, 3, 4, and 5.

Illustrated in FIGS. 3 and 4 are two embodiments of fixed structures made in accordance with the invention. In each case the enclosures (30, 40) are supported on the floor (31, 41 of the body of water and are provided with suitable ballast means (32, 42) for holding the structures in position. In these embodiments, the middle section (33, 43) of the enclosure, in the vicinity of the water line L, is cylindrical. Joined to the tops and bottoms of the cylindrical sections are transition por tions (34, 36, 44, 46) which lead from the cylindrical middle section to cylindrical upper and lower sections (37, 38, 47, 48) of the same diameter. In FIG. 3, the transition zones (34, 36) are conical whereas in FIG. 4 the transition zones (44, 46) are more-or-less spherical. In each case, however, the vertical rate (measured from the water line L) of change of cross-sectional area in the top section is equal to the same fraction (d /d -d,,) of the rate of change of cross-sectional area in the bottom section, so that the enclosures are selfballasting.

In FIG. 5, there is depicted a multiple-tank storage facility comprising a number of individual enclosures similar to that of FIG. 3 which are interconnected by pipes 39 to form an installation of increased storage capacity.

FIG. 2 depicts a floating storage tank made in accordance with the invention. As shown, the enclosure 20 comprises a middle cylindrical section 21 and an upper section comprising a conical transition portion 22 joining a cylindrical portion 23. The lower section of the enclosure also comprises a conical transition portion 24 joining a cylindrical portion 26 of the same diameter as portion 23.

Attached to the lower section of the enclosure 20 is a ring 27 of ballast material sufficient to stabilize the enclosure in an upright position. In the vicinity of the water line L the middle section 21 is surrounded by a cylindrical jacket 28 joined at its upper and lower extremities to the top and bottom sections of the enclosure to define a sealed buoyancy chamber 29 which is sufficient to support the weight of the enclosure when it is full of water (i.e., no stored oil). The buoyant volume is adjusted such that the structure will float with the central cylindrical section 21 partly above the water level and partly below, the ratio of the portion of section 21 protruding above the water level to that extending below the level being equal to d,,. d /d as previously described. With the

conical transition zones

22, 24 being so proportioned that a charge of oil added to the vessel maintains upper and lower interfacial areas which are always equal, there is no net buoyant effect produced by the contained oil, regardless of the quantity thereof. Accordingly, the tank will always float with the same draft and the anchoring chains 31 attached to anchors 32 embedded in the floor 34 of the body of water will always encounter the same magnitude of forces.

In addition to the self-ballasting feature of the invention, there are several advantages which stem from the requirement that the cross-sectional area is a minimum in the vicinity of the water line. The narrowed section at the surface of the water encounters less resistance from wave motion and thus lowers the forces which must be resisted by the structure. In addition, the configuration of a floating vessel such as that of FIG. 2, with the buoyancy chamber inside the cylinder, permits the anchor system to be so designed as to minimize the tendency of the floating vessel to heave. Similarly, the dimensions of a structure made in accordance with the invention are well adapted for the design of advantageous surge and pitch properties, in accordance with known design principals.

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:

1. A selfballasting offshore storage tank of the water-displacement type for the storage of a liquid lighter than and immiscible with water comprising:

a vertically elongaged continuous enclosure open at the bottom for ingress and egress of water;

means for positioning said tank at a substantially fixed level relative to the surface ofa body of water in which said tank is situated, when said tank contains only water filling the lower portion of said enclosure up to the level of the surrounding body of water;

said continuous enclosure having an upper section which extends above the surface of said body of water and a lower section which is always submerged;

said enclosure having a cross-sectional area which has a minimum value at the junction between said upper and lower sections, each of said sections having a respective maximum cross-sectional area larger than said minimum value, said area varying above and below said junction in a manner such that the upper and lower interfacial surfaces of a body of said liquid stored in said enclosure remain substantially equal regardless of the quantity of said liquid.

2. An off-shore storage tank in accordance with claim 1 in which said enclosure has a substantially circular cross-section throughout.

3. An off-shore storage tank in accordance with claim 2 in which said enclosure has a vertical substantially cylindrical middle section in the vicinity of the surface of said body of water, said cylindrical section extending a first distance above said surface and a second distance below said surface, the ratio of said first distance to said second distance being substantially equal to d,,- d ld where d is the density of said water and d,, is the density of said liquid.

4. An off-shore storage tank in accordance with claim 3 in which said enclosure comprises a substantially cylindrical upper section joined to the top of said middle section and a substantially cylindrical lower section joined to the bottom of said middle section, said upper and lower sections having substantially the same diameter.

5. A storage tank in accordance with claim 3 in which said enclosure comprises a first, upwardly opening substantially conical section joined to the upper end of said cylindrical section and a second, downwardly opening substantially conical section joined to the lower end of said cylindrical section, the ratio of the rate of increase of cross-sectional area in the upper conical section with increasing distance above the surface of said body of water to the rate of increase of cross-sectional area in the lower conical section with increasing distance below said surface being substantially equal to (l /d d where (1,, is the density of said water and d,, is the density of said liquid.

6. An off-shore storage tank in accordance with claim 5 in which said means for positioning said enclosure at said fixed level relative to the surface of said body of water comprises a substantially cylindrical l0 jacket encircling said cylindrical section and joined to the enclosure at the upper and lower ends thereof to form a sealed flotation chamber.

7. An off-shore storage tank in accordance with claim 1 in which said means for positioning said tank at a constant level comprises flotation means attached thereto.

8. An off-shore structure in accordance with claim 1 in which said means for positioning said tank at a constant level comprises means connecting said structure to the floor of said body of water.

9. An off-shore structure in accordance with claim 8 in which the lower end of said enclosure rests on the floor of said body of water.

10. A self-ballasting off-shore storage tank of the water-displacement type for the storage of a liquid lighter than and immiscible with water comprising:

a vertically elongated continuous enclosure open at the bottom, said enclosure having a top section all of which extends above the surface of a body of water in which the storage tank is positioned, a bottom section which is always submerged and a partially submerged cylindrical middle section joining said top and bottom sections;

a cylindrical jacket encircling said middle cylindrical section and joined to said top and bottom sections to form a sealed flotation chamber, said flotation chamber having sufficient buoyancy to float said enclosure in said body of water at a constant level relative to the surface thereof in a position such that a portion of said central cylindrical section extends a first distance above the surface of said body and a second distance below the surface thereof, the ratio of said first distance to said second distance being equal to d d,,/d where d is the density of said water and d is the density of said liquid;

said upper section of said enclosure comprising a first, upwardly opening substantially conical portion joined to the upper end of said cylindrical section;

said lower section of said enclosure comprising a second, downwardly opening substantially conical portion joined to the lower end of said cylindrical section;

the ratio of the rate of increase of cross-sectional area in said first conical portion with increasing distance above the surface of said body of water of the rate of increase of cross-sectional area in said second conical portion with increasing distance below said surface being substantially equal to d /d d where d is the density of said water and d is the density of said liquid; and

conduit means communicating from the exterior of said enclosure to the interior thereof for supplying said liquid to and removing said liquid from the interior of said enclosure.

Claims

1. A self-ballasting offshore storage tank of the waterdisplacement type for the storage of a liquid lighter than and immiscible with water comprising: a vertically elongaged continuous enclosure open at the bottom for ingress and egress of water; means for positioning said tank at a substantially fixed level relative to the surface of a body of water in which said tank is situated, when said tank contains only water filling the lower portion of said enclosure up to the level of the surrounding body of water; said continuous enclosure having an upper section which extends above the surface of said body of water and a lower section which is always submerged; said enclosure having a cross-sectional area which has a minimum value at the junction between said upper and lower sections, each of said sections having a respective maximum crosssectional area larger than said minimum value, said area varying above and below said junction in a manner such that the upper and lower interfacial surfaces of a body of said liquid stored in said enclosure remain substantially equal regardless of the quantity of said liquid.

3. An off-shore storage tank in accordance with claim 2 in which said enclosure has a vertical substantially cylindrical middle section in the vicinity of the surface of said body of water, said cylindrical section extending a first distance above said surface and a second distance below said surface, the ratio of said first distance to said second distance being substantially equal to dw - do/do where dw is the density of said water and do is the density of said liquid.

5. A storage tank in accordance with claim 3 in which said enclosure comprises a first, upwardly opening substantially conical section joined to the upper end of said cylindrical section and a second, downwardly opening substantially conical section joined to the lower end of said cylindrical section, the ratio of the rate of increase of cross-sectional area in the upper conical section with increasing distance above the surface of said body of water to the rate of increase of cross-sectional area in the lower conical section with increasing distance below said surface being substantially equal to do/dw - do, where dw is the density of said water and do is the density of said liquid.

6. An off-shore storage tank in accordance with claim 5 in which said means for positioning said enclosure at said fixed level relative to the surface of said body of water comprises a substantially cylindrical jacket encircling said cylindrical section and joined to the enclosure at the upper and lower ends thereof to form a sealed flotation chamber.

10. A self-ballasting off-shore storage tank of the water-displacement type for the storage of a liquid lighter than and immiscible with water comprising: a vertically elongated continuous enclosure open at the bottom, said enclosure having a top section all of which extends above the surface of a body of water in which the storage tank is positioned, a bottom section which is always submerged and a partially submerged cylindrical middle section joining said top and bottom sections; a cylindrical jacket encircling said middle cylindrical section and joined to said top and bottom sections to form a sealed flotation chamber, said flotation chamber having sufficient buoyancy to float said enclosure in said body of water at a constant level relative to the surface thereof in a position such that a portion of said central cylindrical section extends a first distance above the surface of said body and a second distance below the surface thereof, the ratio of said first distance to said second distance being equal to dw - do/do where dw is the density of said water and do is the density of said liquid; said upper section of said enclosure comprising a first, upwardly opening substantially conical portion joined to the upper end of said cylindrical section; said lower section of said enclosure comprising a second, downwardly opening substantially conical portion joined to the lower end of said cylindrical section; the ratio of the rate of increase of cross-sectional area in said first conical portion with increasing distance above the surface of said body of water of the rate of increase of cross-sectional area in said second conical portion with increasing distance below said surface being substantially equal to do/dw - do, where dw is the density of said water and do is the density of said liquid; and conduit means communicating from the exterior of said enclosure to the interior thereof for supplying said liquid to and removing said liquid from the interior of said enclosure.