Why salt?

Some locations in the UK have salt deposits deep underground that were created millions of years ago by oceans evaporating. The salt and other minerals left behind were covered by layers of mud, silt and rock and are now hundreds or thousands of metres underground.

Salt is suitable for storing gas as it is effectively impervious (meaning gas cannot pass through the rocks). The salt properties also mean that it 'creeps' under the pressure of all the layers of rock above it so any geological faults or cracks in the salt 'seal up', reducing the available pathways for gas to escape. Storing gas in salt caverns is a proven technique used safely for over 30 years in Europe and the UK.

Salt cavern creation - the basics

Where the salt is thick enough, underground caverns can be created through a process known as solution mining. Water is pumped down a pipe inside a well into the salt. The water dissolves the salt, creating brine which flows back up the well.

The development of the individual caverns can be controlled by adjusting the position of the pipes in the well, while sonar surveys ensure that the final size and shape is accurate.

Once thorough testing has been completed, the brine is slowly pushed out by injecting gas from the National Transmission System (NTS) until the cavern is full and ready for operation.

Operation

A processing plant is used to monitor and control the flow and quality of the gas being injected into the caverns and returned to the National Transmission System (NTS).

The plant consists of compressors to push the gas in and/or out of the caverns and a drying plant to remove water from the gas as it leaves the caverns.

Some gas is always left in the cavern to maintain the minimum pressure required to sustain the cavern structure. The maximum pressure is carefully controlled whenever gas is injected into the cavern.

A focus on safety

Our failsafe design philosophy means we develop our facilities with a range of core safety features including:

• Sonar surveys of caverns to confirm shape conforms to design criteria;
• Pressure tests of caverns and wells before the gas is introduced;
• Steel well casings cemented to rocks to ensure a gas-tight seal;
• Production tubing sealed inside casings to provide double containment;
• Monitoring of space between casings and production tubes to allow for immediate detection of leaks;
• Sub-surface safety valve 50m underground which shuts off the gas in event of any surface incident, and;
• All critical safety control and monitoring systems are duplicated to ensure safety.

Furthermore, our plants are designed, built and operated to comply with all codes and standards relating to good engineering practice.

We work with regulatory authorities to ensure the plants meet strict rules which govern sites storing gas including the:

• COMAH (Control of Major Accident Hazards) regulations;
• Planning (Hazardous Substances) Act, and;
• Pipeline Safety Regulations.