The use of mechanically mined caverns for crude oil storage raises the possibility of explosions of hydrocarbon vapor/air ratios during filling, emptying, and subsequent refilling if air is in the cavern. Potential ignition sources are examined along with vapor diffusion conditions which could result in an explosive mixture. Pressure, time, and time temperature conditions of explosions in the form of deflagrations or detonations are discussed relative to loading conditions on facility structures and equipment. The impact of these loadings are reviewed, leading to the conclusion that mechanical equipment such as pipes is the most likely to be damaged by an explosion, but design measures can mitigate such damage. It is also concluded that if the cavern is not purged of air, design conditions for the facility should include explosive loading.
As world oil suppliers become scarcer and subject to decreased reliability in traditional sources, the need to provide strategic storage reserves of crude oil increases. One means of providing the vast storage volumes required at potentially minimum cost is to use existing mechanically mined caverns for storage. Depending upon the type of cavern system being used, there exists a possibility that explosive hydrocarbon vapor/air mixtures will form. These explosive mixtures pose a threat of a potential explosion within the cavern if ignited.
This threat raises several questions regarding cavern design, operation, and safety:
What is the probability of an ignition?
What are the characteristics of an explosion if an ignition takes place?
What would be the consequences of an explosion on the cavern itself and any civil or mechanical structures located in the mine or part of the facility design?
The remainder of this paper will examine each of these questions in light of currently available information. However, the examination will be general so that only the concepts involved will be discussed. Specific quantitative aspects of the threat of explosions are highly site-specific and beyond the scope of this paper.
Mechanically mined caverns used for crude oil storage may be broadly grouped into one of two classifications depending on the host rock media. The first is caverns located in host rocks which are sufficiently permeable that containment of the stored crude depends on local groundwater conditions. Caverns in granite or limestone are examples of such caverns. The second classification is caverns in host rocks which are water soluble and sufficiently impermeable to provide containment of the stored crude. Caverns in salt are examples of this second classification. Caverns located in host rocks such as granite or limestone may further be grouped into the following two classifications depending upon the nature of cavern operations relative to filling and withdrawal of the crude:
moving waterbed systems, and
fixed waterbed systems.
Moving waterbed systems are those in which the cavern is never empty of liquid. Oil is stored over water because of its lesser specific gravity. When the cavern is completely empty of oil, it is filled with water.