Pre-COW practice of letting tank washings settle, decanting the water, and loading new cargo on top of the oil residue.
Regulatory detail & full definition
Replaced largely by Crude Oil Washing under MARPOL Annex I but still permitted for some product tankers and in certain circumstances. The retained slop is mixed into the next cargo. Discharge of the separated water is governed by the 1/15,000 rule and Oil Discharge Monitoring Equipment (ODME).