It replaced the load on top and seawater washing systems, both of which involved discharging oil-contaminated water into the sea.
MARPOL 73/78 made this mandatory equipment for oil tankers of 20,000 tons or greater deadweight.
The discharges from seawater washing were still considered a problem, and during the 1960s the load-on-top approach began to be adopted.
This was pumped into the main tanks and the new cargo loaded on top of it, recovering as much as 800 tons of oil which was formerly discarded.
Crude oil washing equipment became mandatory for new tankers of 20,000 tons or more deadweight with the 1978 Protocol to the 1973 MARPOL Convention.