Technology
corner

From Floating Tops to Flat Bottoms

Audio version
Posted / Last update: 01-03-1999
Publication: Petroleum Equipment & Technology Archive
Issued: March 1999
Author: Myers Philip E.

Large Aboveground Storage Tanks:

Chevron’s Phil Myers discusses why an atmospheric AST isn’t kept at atmospheric pressure, how tanks change from the refinery to the retail service station and other essential facts about large ASTs for the petroleum industry.

Large crude oil aboveground storage tanks, Photo courtesy of Chevron.

First, the basics. The most fundamental classification of storage tanks is based upon whether they are above or below ground. Aboveground tanks have most of their structure aboveground. The bottom of the tank is usually placed directly on an earthen or concrete foundation for containment purposes. Sometimes these tanks are placed on grillage, structures or heavy screen so that the bottoms of the tanks can be inspected on the underside and leaks can be more easily detected. The aboveground tank is usually easier to construct, costs less and can be built in far larger capacities than underground storage tanks (USTs).

While there are many types of ASTs (including, of course, those used for retail fueling), this article will focus on flat bottom aboveground petroleum storage tanks. This type of tank is most commonly used for ASTs with volumes of from 10,000 gallons to 25 million gallons or more.

Because there is no uniform regulation requiring registration of ASTs in the petroleum industry, the exact number in existence is unknown. However, approximately 10 years ago, the American Petroleum Institute (API) conducted a survey on the subject.

Table 1 shows that when the ASTs in all sectors of the petroleum industry are counted, the total comes to about 700,000. (Table 1 also provides the breakdown of numbers and capacities of tanks by sector.) However, the smallest capacity of tank surveyed was around 1,100 gallons; thus, the survey excluded very small tanks.

By way of comparison, EPA has estimated that there are 1.3 million regulated underground storage tanks, with an unknown number of exempt underground tanks used for home heating oil and farm fuel.

 

Tank Capacity (Bbl)

Number

Est. Median Diameter (Ft)

Total Shell Capacity, MBbl

Marketing

26          to 500
500        to 1,000
1,000     to 10,000
10,000   to 100,000
100,000 or More
                Tatal >

64,793
4,417
7,434
11,469
416
88,529

10
15
25
70
220

486,925

Refining

26          to 500
500        to 1,000
1,000     to 10,000
10,000   to 100,000
100,000 or More
                Tatal >

3,913
2,460
9,665
11,625
2,060
29,727

12
15
30
90
220

945,092

Transportation

26          to 500
500        to 1,000
1,000     to 10,000
10,000   to 100,000
100,000 or More
                Tatal >

694
307
1,468
5,048
1,680
9,197

10
15
35
120
220

556,183

Production

26          to 500
500        to 1,000
1,000     to 10,000
10,000   to 100,000
100,000 or More
                Tatal >

510,045
37,628
23,946
974
27
572,620

10
16
30
70
200

280,595

All Sectors

26          to 500
500        to 1,000
1,000     to 10,000
10,000   to 100,000
100,000 or More
                Tatal >

579,445
44,812
42,513
29,120
4,183
700,073

--
--
--
--
--

2,268,795

Table1: Summary of Petroleum Industry AST Survey
Source: Entropy Limited, “Aboveground Storage Tank Survey,” April 1989; and Gruy Engineering Corporation, “Assessment of the Economic Impact of Certain Anticipated SPCC Regulations Pertaining to Aboveground Storage Tanks,” September 1990.

The distribution chain
Although exceptions exist, certain generalizations can be made concerning the sizes, distribution and the relative uses of USTs and ASTs in the petroleum distribution chain. For the most part, the size of the tank will diminish from the beginning of the chain (large tanks) to the point of end use (smaller tanks).

Again, while there are a number of notable exceptions, the following quotes describe the process. The quotes (italicized) are from the Aboveground Storage Tank Guide published by Thompson Publishing Group, Washington, D.C. 1994.

Summary of the beginning process—
“The economics of ASTs and USTs is best understood by visualizing the distribution chain involving most liquid products. Beginning at the production end, very large quantities of liquids are produced, and must be handled, stored, and transported to intermediate destinations in the manufacturing process and further downstream to the ultimate consumer. Depending on the liquid material involved, and the processes in which it is used, product storage can involve simple or complex operations. It is safe to say, however, that, as the liquid moves closer to the ultimate consumer, there is less need to store large quantities for long periods.”

Summary of intermediate movements—
“Economics dictate that most intermediate movements in the distribution chain be of the largest volume that can be moved at one time. As the ultimate consumer usually requires very low volumes, the storage and movement of liquids at the wholesale and retail end can be in smaller quantities, occurring more frequently. Use of underground storage, with its smaller capacities, thus, becomes practical and feasible at this point. For example, let us follow the path of petroleum products from the oil well to the motorist.”

More ASTs are being used for retail fueling. Photo courtesy of Chevron

Description of pathway

  • “Crude oil at the wellhead is produced in very large quantities, measured in thousands of gallons-or barrels—per day. Many wells are connected to pipelines that transport the production output directly to either a refinery or distribution terminal point, from which it can be transhipped to a refinery for processing. Systems for storing this output awaiting transport must be of large volume to accommodate the flow arriving daily. The tank size needed, therefore, precludes use of UST systems, and the storage terminals normally have adequate land area to accommodate the size of AST systems required. Moving the accumulated quantities efficiently and at lowest cost requires either pipeline or very large tankers.”

  • “Receiving the cargoes from these vessels at the refinery, where the crude oil is processed into useful products, also requires large storage capacities. The finished products, in turn, must be stored temporarily until moved to the marketplace. Depending upon the methods and timing of transshipment—that is, by tanker, barge, railcar, pipeline, or truck—various types of ASTs are used. Generally, up to this point, the volumes handled are of such quantities as to preclude the use of UST facilities, due to their size and configuration limitations. Instead, AST systems are used almost universally.”

Post a comment

Please login or register to post comments.

Subscribe for weekly newsletter