Two types of underground storage are commonly used for hydrocarbons – refrigerated and pressurised. The latter is easier to install and operate and more common. Both are unlined and rely on the presence of groundwater to seal them, in refrigerated stores to create a frozen zone around the cavern, and in a pressurised cavern using the containing effect of the groundwater in rock fractures. This works too for compressed air and natural gas.
Early pressurised versions were developed in the US in the 1950s; some used leached out cavities beneath salt domes and others were "room and pillar" chambers located fairly deep under water bearing rock. Access was via a lined shaft. European development used traditional tunnel access to enlarged "tunnel" extended caverns.
To ensure enough pressure to hold the LPG in liquid form means siting the caverns fairly deep, 65m for butane and 100m for the more volatile propane in northern areas. In tropical areas, where rock temperatures can be much higher, the LPG tends to boil and needs higher pressures and even greater depth.
Modern caverns have an "umbrella" of pressurised water filled drill holes above to ensure the whole is within groundwater at all times: the shaft and access tunnels are plugged and filled with water to natural groundwater level after construction. Gas pipes, water pump lines and instruments pass through the plug.
In operation there is a continuous leakage of water into the chamber which is pounded by a collar around the sump to form a water bed surface on which the LPG floats. Surplus water is pumped from the sump with submersibles and just above the sump surface is also the point where the gas filling line enters. The LPG typically arrives aboard refrigerated ocean going tankers and must, ironically, be heated before it can pass into the chamber. Lower temperatures would freeze both cavern equipment and groundwater, creating problems.
At refrigerated temperatures there is little vapour pressure in LPG though it is not completely eliminated because temperatures are held as close to liquefaction point as possible. Caverns which hold the gas in a refrigerated state can therefore use lower groundwater pressures and be sited higher in the ground. But they still require a complete groundwater presence.
The design is similar to that for pressurised caverns; there is, for example, a pump sump, though there is no water bed and, once running, the water becomes frozen. The shaft and tunnels are plugged and water filled after construction.
To begin operations LPG is sprayed into the chamber through roof nozzles, cooling it by vaporisation. Groundwater seals the cavern and is pumped out, though it needs an antifreeze in it.
The resulting gas is re-compressed and liquefied at the surface and then pumped back down. After some time the cavern walls begin to freeze, slowing and stopping the groundwater entry. Because thermal conductivity of rock is low and heat capacity high this process takes time. But once the steady state is achieved most of the compressor capacity is no longer required, and even for emergency conditions a relatively small compressor/refrigeration input is needed to cope.
Only a few fairly recent examples of the refrigerated chambers are in existence because they are more complex to engineer.
