Gradual cooling of LNG/LPG cargo tanks with cargo vapour before loading to prevent thermal shock to membrane / pressure vessel walls.
Regulatory detail & full definition
On membrane LNG carriers cool-down typically takes 8-12 hours, achieved by spraying cargo through ring-mains and bleeding vapour to the GCU. Aim is to bring the tank wall to a few degrees above cargo temperature before bulk loading at full rate. Skipping or rushing cool-down risks cracking the membrane and triggering ESD-2.