Be aware of liquid domes on liquefied gas carriers

IACS warns that in case there is overflow of cargo liquid into the vapour line or into the tank pressure relief valves PRVs, emergency procedures should be followed to mitigate consequences.

Under normal conditions, the vapour space is continuous and in communication with the vapour/liquid domes where the vapour line and cargo tank pressure relief valves (PRVs) are located. However, there may be times when a vapour pocket can be formed in a cargo tank on a liquefied gas carrier which is not in communication with the vapour/liquid domes.

The vast majority of these conditions occur in a dynamic condition and are dissipated by the motion of the ship. However, there can be situations where the pocket exists in a static condition, for instance, due to damage to the ship caused by an accident such as grounding or collision.

Even though the IGC Code states that the PRVs should be in the vapour phase under conditions of 15° list and 0.015L trim and presumes that no isolated vapour pockets are formed within this range in principle, this scenario can occur at other trim and list values based upon the filling level of the tank since the ship is designed to survive a damage condition up to 30° of list.

In this condition, there is the potential for liquid build-up in the vapour/liquid domes caused by a pressure differential between the isolated vapour pocket and the vapour/liquid domes resulting in a possible overflow of cargo liquid into the vapour line or into the tank PRVs.

IACS highlights that there is little likelihood that this situation will ever occur, however, emergency procedures should be in place to mitigate the risks to the vessel caused by isolated vapour pockets. These procedures should identify the condition when isolated vapour pockets can be present and contain measures to reduce or eliminate them and/or mitigate their consequences such as cargo jettisoning, transfer of cargo between tanks, and cargo vapourization/utilization based upon different scenarios following the accident, including, but not limited to, loss of power, limited ability to reduce angle of heel or trim.