We in Finland have large-scale commercial experience in storing crude oils and oil products in unlined underground caverns. The operating and maintenance costs of underground caverns are lower than those of aboveground steel tanks. The risk of fire and explosion is small, and the eventual damage can be limited with proper design. There have been problems as to sludge and emulsions but these problems have been solved.
The first underground storage facility in Finland was built during the second world war. The tank itself was of steel construction, located in a cavity excavated in the rock. Incidentally, Neste"s Naantali refinery was later built at the same location. The unlined oil storage facility was introduced in Finland during the sixties, first in safety storage use. In 1967 Neste constructed three crude storage caverns for the Porvoo refinery feed. Since then a total of nineteen underground caverns, with a total volume of 4.3 million m3 have been built in this area. At the moment there are three more caverns under construction, which will increase the total volume of underground storage to over 5 million m3 (more than 32 million bbl). These caverns have been in intensive use, forming a part of the operational storage space for the 300,000 bbl/day refinery. The total volume of underground storage facilities in Finland is about 10 million m3 (63 million bb1).
Underground storage caverns have been built for four different purposes: -strategic stockpiling -storing of crude as well as oil and gas products for refinery operations _ commercial storage caverns of traders and consumers _ balancing of peak loads in long transfer lines The need for storing oil today is greater than it was fifteen years ago, when the first large-scale underground caverns were built. The uncertain supply of crude oil has created a national need for stockpiling. Many governments are planning or arranging strategic storing of crude and/or oil and gas products. The refineries today need more tank capacity than earlier, because the price differences between different products are large. Therefore, the optimum blending of components is a commercial necessity which means more tank volume. In the present market situation the refineries are compelled to use several different qualities of crude, and this further increases the need for extra storage volume. As for traders and consumers, larger storage units are feasible, because the increase in the oil price will involve expected gains. Seasonal variations in prices also justify the storage capacity when the oil market is steady.
An underground oil storage can be an exhausted mine, an especially excavated cavern, or a space leached in the rock salt. An excavated rock cavern is located below the ground-water level so that the pressure of ground-water prevents all leakages of oil and gas to the surrounding rock. In case of an exhausted mine, there should be a positive ground-water pressure in all areas where oil will be stored so that ground-water will seal the cavity.
