The LNG Process
There is a five-step ‘process’ to get natural gas into the UK Natural Gas Transmission System and into homes and businesses in the UK from gas fields in remote locations.
1. Exploration and Production
Natural gas is pumped to the surface through techniques commonly used world-wide whether in the North Sea or overseas.
2. Liquefaction process
This activity takes place in the producing country when all impurities are removed from the gas, which in the industry is known as ‘sweetening’, prior to cooling. Cooling of natural gas to -160ºC allows it to be transported economically by reducing the volume by 600:1. This process is called liquefaction and produces a stable liquid ready for shipping.
3. LNG (liquefied natural gas) transportation
LNG is transported in special double-hulled ships built using two different technologies known as Moss Rosenberg (spheres) and membrane (using material with an expansion coefficient of practically nil). The LNG is off-loaded as a liquid and pumped from the jetty to storage tanks at the terminal. The LNG remains at -160ºC for the duration of the process. Off-loading takes approximately 24 hours and is managed using tried and tested procedures common to all international facilities.
4. Storing LNG (liquefied natural gas)
LNG is stored in full containment design storage tanks. In a full containment system two tanks are employed, an inner tank which contains the stored liquid, and an outer tank which provides security in the event of any loss of containment or leak from the inner tank. The inner shell is made of a special nickel alloy, designed to resist the low temperature. The outer shell is of pre-stressed concrete with a reinforced concrete base slab and roof. Sophisticated automatic protection systems are employed to monitor the tank levels, pressures, temperatures and any potential leakage from the inner tank. The tanks are well insulated to keep the natural gas at -160°C (in its liquid form).
5. Re-gasification process
This is the operational work of the terminals in the receiving country. Re-gasification is simpler than liquification, it is purely physical and not chemical. The LNG is pumped out of the tanks and warmed so that it returns to its natural gaseous state. The natural gas is then pumped in to the UK’s natural gas transmission system, owned and operated by National Grid (formerly Transco), as required.