US20080223859A1

US20080223859A1 - Storage tank with inventory reduction

Info

Publication number: US20080223859A1
Application number: US11/717,888
Authority: US
Inventors: Larry C. Hiner
Original assignee: Chicago Bridge and Iron Co
Current assignee: Chicago Bridge and Iron Co
Priority date: 2007-03-14
Filing date: 2007-03-14
Publication date: 2008-09-18
Also published as: CA2625124A1

Abstract

The disclosed floating-roof storage tank has an articulated floor with an annular outer portion, an intermediate transition portion, and a raised inner portion. Pipe elements or equipment that may hang from an outer, annular section on the roof or extend from the tank wall fit around the outside of the transition portion. Liquid-tight seams connect the transition portion of the floor to the outer portion and to the raised inner portion of the floor, sealing off an inventory-reduction compartment beneath the raised portion of the floor. The inventory-reduction compartment is supported by a solid or liquid fill or by a metal support structure. The raised inner portion of the floor may support the floating roof in a low position, and may support columns or the like that are used to support a fixed roof above the floating roof.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates generally to floating-roof storage tanks used to store liquid natural resources such as crude oil, gasoline, or the like. The tanks may have an open top or have a fixed roof that provides essentially a weather shield for the floating roof.
Floating-roof storage tanks are particularly useful in the oil and gas industry. Oil refineries and storage terminals use floating roof tanks to store liquid hydrocarbon products that have a relatively high vapor pressure, such as gasoline, napthas, and crude oil. The roof on such tanks floats on the surface of the stored liquid, minimizing the vapor space and thus limiting undesired vaporization of the liquid.
An ordinary 50-foot tall, 150-foot diameter tank can hold approximately 148,000 barrels of liquid. However, some of the effective capacity of a traditional floating-roof tank is wasted in what is called a “heel.”
Floating-roof tanks are commonly filled and emptied in cycles. As liquid is removed from the tank, the liquid level in the tank descends. The roof, floating on the surface of the liquid, also descends, maintaining a low-vaporization condition. However, tank appurtenances such as a mixer, interior piping, or nozzles within the tank generally prevent the roof of the tank from descending all the way to the floor of the tank. Continuing to empty the tank after the roof has descended to its lowermost operating position would create a vapor space between the roof and the surface of the liquid, significantly increasing vaporization of the liquid and creating a potentially combustible atmosphere. Consequently, emptying operations are generally stopped when the roof reaches its lowermost operating position, and a significant volume of liquid remains in the tank. This volume of liquid is the heel.
In an ordinary 150-foot diameter tank, the lowermost operating position of the roof may be four or five feet above the floor of the tank, resulting in a heel of 10,000-15,000 barrels of liquid. Because that volume of the liquid is not emptied during normal operations, that heel represents a significant loss in effective working volume of the tank, and also represents a significant inventory of product not used in normal operations.
There have been efforts to address this problem. Some builders have sloped the floor of the tank. However, sloping the floor generally eliminated only 10-30% of the heel. U.S. Pat. No. 4,957,214 proposed mounting a volume-occupying container on the bottom of the floating roof or placing a layer of ballast or relatively-dense liquid in a dam on the floor of the tank. While this dam arrangement could provide significant heel reduction (eliminating more than 50% of the heel), it made maintenance of the tank more difficult. For example, in order to check for leaks beneath the ballast, the ballast would have to be removed.

BRIEF SUMMARY

The applicant has found a solution that can provide significantly more heel reduction than a sloped floor and can be significantly easier to maintain than the kind of dam arrangement suggested in the '214 patent.
Like prior known devices, the new storage tank has cylindrical walls and a roof that floats on the surface of liquid stored in the tank. Unlike prior known tanks, the new tank has an articulated floor with an annular outer portion, a raised inner portion, and an intermediate transition portion that is narrower in width than the raised inner portion. Liquid-tight seams (for example, welded seams) connect the intermediate transition portion of the floor to the outer portion and to the inner portion.
Generally, the raised inner portion of the floor will be at least 6″ and no more than 60″ above the outer portion of the floor. The raised inner portion will generally be made primarily of steel that is at least ⅛ of an inch thick and no more than ¾ of an inch thick. In some embodiments of the invention, the raised inner portion of the floor may support the roof in a low position. The inner portion of the floor may also be sloped toward the intermediate transition portion of the floor.
The inclusion of the new transition portion of the floor helps to maximize the heel reduction. In floating roof tanks, the roof may have an outer, annular section that has a bottom surface with depending pipe elements. In the new arrangement, these elements may be positioned so that they fit around the outside of the intermediate transition portion of the floor.
Because the raised inner portion and the intermediate transition portion of the floor are sealed together, liquid stored in the tank does not enter the space beneath the raised inner portion of the floor. Consequently, when the tank is filled, the raised inner portion and the intermediate transition portion of the floor may be subjected to significant differential pressures. To handle these pressures, the volume under the raised portion of the floor may be provided with a solid fill (such as a structural granular fill or concrete), with a metal support structure, or, if the outer portion of the floor is part of a continuous floor that extends beneath the raised portion of the floor, with a liquid fill.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by referring to the accompanying drawings, in which:

FIGS. 1 a and 1 b are schematic side views of two floating roof tanks that utilize the new arrangement. The tank in FIG. 1 a is an open-top tank, and the tank in FIG. 1B is a fixed-roof tank.

FIG. 2 is an enlarged, fragmentary view of the lower outer portions of the tank seen in FIG. 1 a.

FIG. 3 is a top plan view of the tank, with the floating roof removed.

DETAILED DESCRIPTION

The tank 10 seen in FIG. 1 a has a cylindrical wall 12, a floating roof 14, and a floor 16. The tank can be built on any conventional foundation, such as a foundation ringwall 18 (FIG. 2), slab, berm, etc. The tank seen in FIG. 1 b is similar, but also has a fixed roof 19.
The wall 12 of the tank can vary from 10 feet to 90 feet in height, and can be made of any suitable material. In many cases, the wall can be made of 3/16″ to 1¾″-thick steel plates.
The floating roof 14 is positioned within the tank wall 12, and floats on the surface of liquid stored in the tank 10. The illustrated floating roof is a pontoon roof, and the invention can also be used with other conventional floating roofs, including pan roofs. The roof can be made of a wide range of materials, including steel, aluminum, composite material, or other nonmetallic material.
The illustrated roof 14 has an outer, annular section 20 that has a bottom surface 22. Elements such as a drain or a vent may hang below this illustrated annular section. The drain or vent can be used to drain liquid off the top of the floating roof.
As best seen in FIG. 2, the floor 16 is articulated, having an outer portion 30, an intermediate transition portion 32, and a raised inner portion 34 that are joined by liquid-tight seams 38. The various portions of the floor can be made of conventional materials, such as steel plates or structural members.
The outer portion 30 of the illustrated floor 16 can be part of a continuous floor that extends beneath the raised inner portion 34 of the floor. Alternatively, the outer portion of the floor can terminate at or just inside the lower edge of the intermediate transition portion 32 of the floor, as seen in FIG. 2 and explained in more detail below. When desired, the outer portion of the floor can incorporate a double-bottom configuration or an underbottom liner.
The illustrated raised inner portion 34 of the floor 16 is made of ¼″ steel plate and is positioned about 36″ above the outer portion 30 of the floor, creating an inventory-reduction compartment 40 beneath the raised inner portion. Because the liquid-tight floor keeps stored liquid out of the inventory-reduction compartment, maintenance is simplified.
The raised inner portion 34 of the floor 16 can support the roof 14 in the low position seen in FIG. 1. In fixed-roof tanks, such as the one seen in FIG. 1 b, the raised inner portion can also serve as a base for a column 41 used to support the fixed roof. The illustrated inner portion of the floor slopes outwardly toward the outer portion 30 of the floor, rising approximately 1″ every 10 feet of run. Other configurations can be used. For example, the inner portion of the floor can sometimes be as thin as ⅛ of an inch thick or as thick as ¾ of an inch thick. It can sometimes be positioned as little as 6″ or as much as 5 feet above the outer portion of the floor. It can sometimes be flat or can sometimes slope as much as 6 inches for every 10 feet of run.
The intermediate transition portion 32 of the floor 16 connects the outer portion 30 and the inner portion 34 of the floor, forming the lateral boundary of the inventory-reduction compartment 40. The illustrated transition portion of the floor is vertical, and is spaced relatively close to the wall 12 of the tank. Preferably, the transition portion of the floor may be within 10 feet of the tank wall, but far enough from the tank wall to enable pipe elements, etc., that may hang down from the annular section 20 of the roof 14 to fit around the transition portion 32. Although other arrangements could be used, this vertical arrangement of the transition portion of the floor optimizes the heel-reducing volume of the inventory-reduction compartment.
The heel is the volume of the tank below the low operating level of the roof. The heel reduction provided by the invention can be measured by comparing the heel of the product to the heel that would exist if the inventory-reduction compartment 40 were omitted, as in a conventional tank. Above the horizontal plane 50 that extends through the raised inner portion of the floor 20, the volume of the illustrated tank is the same as a conventional tank. Below that plane, however, the volumes differ. While a conventional tank would have a heel that encompasses the entire volume below the plane 50, a tank utilizing the inventory reduction chamber has a reduced heel volume 52 that is equal to the difference between the corresponding volume of a conventional tank and the volume of the inventory reduction chamber. Preferably, the volume of the heel reduction compartment is at least twice the volume of the reduced heel volume. This provides significant advantages over previously-known designs.
The transition portion 32 of the illustrated floor 16 takes the form of a step wall that is made of steel plate or a structural member that is welded to a step baseplate 42 on the outer portion 30 of the floor. When the outer portion of the floor is part of a continuous floor, this step baseplate may not be necessary.
Although the articulated floor 16 offers significant advantages in simplified maintenance and heel reduction, the arrangement can present a special design challenge. Because the volume beneath the raised inner portion 34 of the floor is not in fluid communication with the liquid stored in the tank, the intermediate transition portion 32 of the floor and the raised inner portion 34 of the floor become subject to differential hydraulic pressures when the tank is filled. Thus, special support for the inventory-reduction compartment 40 may be needed.
The support can be provided in a variety of ways. Structural granular fill or concrete can be provided beneath the inner portion 34 of the floor 16, as seen in the figures. A metal support structure including, for example, conventional structural members such as I-beams, can also be used to support the floor. Alternatively, if the outer portion 30 of the floor is part of a continuous floor that seals the bottom of the inventory-reduction compartment 40, the inventory-reduction compartment can be filled with liquid, such as water, to provide support.
This description of various embodiments of the invention has been provided for illustrative purposes. Revisions or modifications may be apparent to those of ordinary skill in the art without departing from the invention. The full scope of the invention is set forth in the following claims.

Claims

1. A storage tank for liquid resources such as crude oil, gasoline, or the like that has a cylindrical outer wall and a floor that has:

an annular outer portion;

a raised inner portion;

an intermediate transition portion that is narrower in width than the raised inner portion and connects the outer portion of the floor and the inner portion of the floor; and

liquid-tight seams that connect the transition portion to the outer portion of the floor and to the inner portion of the floor.

2. A storage tank as recited in claim 1, in which the tank also has a floating roof.

3. A storage tank as recited in claim 2, in which the roof has a peripheral, annular section that has a bottom surface that fits around the transition portion.

4. A storage tank as recited in claim 2, in which the tank has pipe elements or equipment that extends inwardly from the outer wall toward the intermediate portion of the floor.

5. A storage tank as recited in claim 2, in which the roof has pipe elements that extend beneath a peripheral, annular section of the roof that has a bottom surface that fits around the transition portion of the floor.

6. A storage tank as recited in claim 2, in which the raised inner portion of the floor supports the roof in a low position.

7. A storage tank as recited in claim 1, in which structural granular fill or concrete is provided beneath the raised inner portion of the floor.

8. A storage tank as recited in claim 1, in which a metal support structure is provided beneath the raised inner portion of the floor.

9. A storage tank as recited in claim 1, in which the outer portion of the floor is part of a continuous floor that extends beneath the raised inner portion of the floor.

10. A storage tank as recited in claim 1, in which the outer portion of the floor is part of a continuous floor that extends beneath the raised inner portion of the floor, and the tank includes a sealed, liquid-filled inventory-reduction compartment beneath the raised inner portion.

11. A storage tank as recited in claim 1, in which the raised inner portion of the floor is made primarily of steel that is at least ⅛ of an inch thick and no more than ¾ of an inch thick.

12. A storage tank as recited in claim 1, in which the transition portion is welded to the outer portion of the floor.

13. A storage tank as recited in claim 1, in which the raised inner portion of the floor is at least 6″ and no more than 60″ above the outer portion of the floor.

14. A storage tank as recited in claim 1, in which the raised inner portion of the floor is sloped toward the transition portion of the floor.

15. A storage tank as recited in claim 1, in which the intermediate transition portion is approximately vertical.

16. A storage tank as recited in claim 1, in which the storage tank has a diameter of at least 60 feet and the intermediate transition portion of the floor begins to rise upwardly within 20 feet of the cylindrical outer wall of the tank.

17. A storage tank as recited in claim 1, in which the storage tank has a diameter of at least 60 feet and the intermediate transition portion of the floor is approximately vertical and is positioned within 20 feet of the cylindrical outer wall of the tank.

18. A storage tank for liquid resources such as crude oil, gasoline, or the like that has a cylindrical wall, the tank comprising a floor that has:

an annular outer portion;

a raised inner portion;

an approximately vertical intermediate transition portion that connects the outer portion of the floor and the inner portion of the floor;

liquid-tight seams connecting the transition portion to the outer portion and to the inner portion.

19. A storage tank for liquid resources such as crude oil, gasoline, or the like that has a cylindrical outer wall and a floor that has:

an annular outer portion that is adjacent a reduced storage volume;

a raised inner portion;

an intermediate transition portion that connects the outer portion of the floor and the inner portion of the floor;

an inventory-reduction compartment beneath the raised inner portion of the floor, the volume of the inventory-reduction compartment being at least double a reduced heel volume; and

20. A storage tank as recited in claim 19, in which the intermediate transition portion is narrower in width than the raised inner portion.