From Floating Tops to Flat Bottoms

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.

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.”