The LNG ‘Process’

There is a four-step ‘process’ to get natural gas into the UK Natural Gas transmission system and on to 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 North Africa.

2. Liquefaction process

This activity takes place in the producing country:

• All impurities are removed from the gas, which in the industry is known as ‘sweetening’, prior to cooling.

• The cooling of natural gas to -160º allows it to be transported. This process is called liquefaction and produces a stable liquid ready for shipping.

• The LNG is stored at -160º to await loading onto LNG carrier ships.

3. LNG transportation

• LNG is transported in special double-hulled ships built using two different technologies Moss Rosenberg (spheres) and membrane (material with an expansion coefficient of practically nil) with current carrier capacities varying between 20-50,000 m3 LNG (15 ships), 50-100,000 m3 LNG (15 ships) and 100-140,000 m3 LNG.

• Larger carriers of some 165,000 - 200,000 m3 LNG are now under design.

• Marine tugging contracts will be in place to assist ships enter the Haven waterway, dock and depart.

• Off-loading takes up to 24 hours and is managed using tried and tested procedures common to all international facilities.

• The LNG is off-loaded as a liquid and pumped from the jetty to storage tanks at the terminal. The LNG remains at -160º for the duration of the process.

• Once off-loading is complete the ship departs the Haven.

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4. Re-gasification process

This is the operational work of the terminals in Milford Haven:

• Re-gasification is simpler compared to liquefaction, it is purely physical and not chemical.

• The LNG tanks will be roughly 45m high and 80m in diameter. They are well insulated to keep the LNG at -161º in stable liquid form.

• The tanks being built are two-wall tanks, with the first wall made out of nickel steel that prevents low temperature failures. The outer wall is made out of concrete.

• The LNG is pumped out of the tanks pressurised and warmed so that it returns to natural gas.

•The natural gas is then pumped in to the UK’s natural gas transmission system owned and operated by National Grid (formerly Transco).

• During the storage of LNG there is a slight warming of the liquid resulting in a small amount of gas at the top of the tank. This gas is then moved in to the natural gas transmission system.

5. How do you store LNG?

• LNG is stored in storage tanks of around 160,000 m3 capacity and stored just above atmospheric pressure. The LNG tanks are of a full containment design. 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 temperatures. 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.

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