Phillips 66 and Calor Gas Ltd jointly own two vast underground caverns for storing propane and butane liquefied petroleum gas (LPG). The caverns, mined in chalk 180 metres underground, can each store up to 60,000 tonnes of LPG, and are designed to ensure that peak winter demand for LPG can be met. One cavern contains propane, the other holds butane. The gas is contained as a liquid in the caverns by the hydrostatic pressure of water in the surrounding chalk.
Why is such a large storage required?
LPG is produced at Humber refinery at a fairly constant rate all year round. Due to the widespread use of LPG for heating, the demand rises dramatically when the weather is cold, then as the weather gets warmer demand falls off again.
As it is impossible to produce LPG at such variable rates it is necessary to store summer production for use in the winter enabling us to meet peak demands.
Why store LPG underground?
The most economical surface storage of LPG requires very large refrigerated tanks. These tanks and their associated plant are costly to build and maintain. The refrigerated tanks also have a finite life of possibly less than 30 years. Underground storage on the other hand is less expensive to build and requires little or no maintenance. The plant associated with the underground storage caverns is relatively simple, easy to maintain and can be easily replaced when worn out.
Another big advantage of underground storage is that it does not intrude at all on the surface environment.
What stops the gas from entering the ground and coming up to the surface?
The South Killingholme caverns have been mined into rock that is virtually a solid amorphous mass and are at a depth of almost twice that required to balance the pressure of propane. The static head of the ground water (water table) is greater than that exerted by the stored product thereby preventing outward movement of LPG into the rock.