The stripping of cargo tanks is carried out when the vessel needs to discharge the entire cargo parcel from its cargo tanks. To achieve this, the vessel uses its spray pumps and runs them until the desired level is reached, ensuring the maximum liquid volume is discharged from the tank.
The use of spray pumps is preferred because they are lower-capacity pumps, suitable for handling lower liquid levels inside the cargo tanks. The stripping operation is also performed in preparation for an upcoming dry dock.
The purpose of stripping cargo tanks is the removal of the dead heel, leaving the cargo tanks as empty of LNG as possible. This can be achieved by using the stripping/spray pumps to strip out all the remaining LNG after the cargo pumps have been used to empty the cargo tanks as far as possible.
The stripping/spray pumps would generally be started before the cargo pumps are stopped and at a cargo tank level above the minimum stripping/spray pump start level. The stripping/spray pumps discharge to the spray line, to the stripping crossover, and then to the manifold to the terminal.
On some occasions, it may prove more effective to set the stripping/spray pumps to discharge into one cargo tank to consolidate the remaining LNG. The remaining cargo tank would continue using the cargo pump to discharge all the LNG to the terminal before this cargo tank stripping/spray pump continues to empty the final cargo tank.
The operation can be conducted either with or without vapour return and would normally be conducted in a similar manner to the main unloading operation with regard to vapour handling.
