Cleaning crude oil tanks with their own cargo as the wash medium, replacing earlier seawater-flush methods.
Regulatory detail & full definition
Mandatory for crude tankers > 20,000 DWT under MARPOL Annex I Reg. 33. Tanks are washed during discharge using high-pressure crude oil through fixed machines. Reduces clingage and slop generation, and lowers oil pollution risk vs the older Load-On-Top method. Inert gas (< 8% O2) must be present throughout.