Oil & Gas

Marine LNG facilities

LNG is a cleaner fuel than other fuel types used to power vessels, e.g. diesel. Some new-built ferries are designed to be fuelled on LNG. LNG to be used as marine fuel is stored in large onshore tanks in the marine LNG facilities and distributed on small volumes to the ferries by barges. The LNG storage tanks constitute the main elements of the marine LNG facilities.

LNG is stored fully refrigerated at a temperature of approximately −162 °C and atmospheric pressure. The storage of LNG involves evaporation and removal of vapours 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 vapours 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.

Liquefied natural gas takes up about 1/600th the volume of natural gas in its gaseous state which makes it possible to transport it from remote locations to markets worldwide

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 LNG tank roll-over issues

To avoid the issue of composition change, the boil-off vapours can be re-liquefied into LNG that is re-circulated into the LNG storage tanks. With this, all the boil-off vapours are recovered and the composition of the gas in the storage tanks remains unchanged.

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.