Strategic LNG storage facilities

An LNG terminal dedicated to strategic storage of LNG will operate with low or no gas export from the terminal for long periods of time leading to long term storage of LNG.

The main issue to be considered due to long storage periods with limited export relates to changes in composition of the gas in the LNG storage tanks.

Melkøya, the world's most northerly export facility for LNG (Liquefied Natural Gas).

LNG is stored fully refrigerated at a temperature of approximately −162 °C and atmospheric pressure. The storage of LNG involves evaporation and removal of vapors from the storage tanks in order to maintain the temperature in the storage tanks. Boil-off gas is taken off the top of the LNG storage tanks and has increased concentration of the light components of the LNG.

Export gas quality variations

In the evaporation process at the storage tanks, the lighter components of the gas are the first ones to evaporate and, as a result, the gas remaining in the storage tanks becomes heavier with time as the vapors are continuously removed.

Continued removal of boil-off gas from the storage tanks for a long period will change the specification of the product in the storage tank, possibly with the consequence that the LNG in the storage tank becomes off-spec product.

LNG (Liquefied Natural Gas) is stored fully refrigerated at a temperature of appoximately -162 degrees Celcius

Density – light or heavy LNG

The density of the LNG in the top of the storage tanks increases over time as the light components are continuously removed, as boil-off gas, from the top part of the LNG storage tank. However, the density of the LNG at the bottom of the tank increases at a much slower rate. This creates an unstable situation with light LNG product at the bottom of the tank and heavy LNG product in the top of the tank.

At some stage, heavy LNG will start to sink to the bottom of the storage tank and light LNG will float to the top of the storage tank. Once this process has started, the operation is difficult to control and the shift of location of the two products with different density is typically very rapid, with pressure fluctuations and venting of large quantities of gas through the vent system as a consequence. This process is called LNG tank roll-over and should be avoided.

Avoiding gas quality and roll-over issues

To avoid the issue of composition change, the boil-off vapors can be re-liquefied into LNG that is re-circulated into the LNG storage tanks. With this, all the boil-off vapors are recovered and the composition of the gas in the storage tanks remains unchanged during the long storage periods with limited or no gas export.

Re-liquefaction process

Boil-off vapors from the LNG Storage Tank are taken off from the top of the LNG storage tank, compressed by boil-off compressors and cooled by a nitrogen refrigerant cycle where they are liquefied in heat exchangers and routed back to the storage tank. The re-liquefaction system is shown below.