Patent Application
20080223859
Not applicable.
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.
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.
The invention may be better understood by referring to the accompanying drawings, in which:
a and 1b are schematic side views of two floating roof tanks that utilize the new arrangement. The tank in
The tank 10 seen in
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
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
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
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.
