Load on Top
Load on Top (LOT) is a pollution-prevention procedure on crude-oil tankers. Instead of pumping oily ballast and tank washings into the sea, the oily mixtures are collected in a slop tank and allowed to settle; the separated water is decanted off within limits, and the retained oil (the slops) is kept on board. The next cargo is then loaded on top of these retained slops.
LOT and the slop-tank retention system were among the first major operational measures under MARPOL to cut deliberate oil pollution from tankers, and they work alongside segregated ballast and crude oil washing to keep oil out of the sea.
Load on top was a landmark in cutting operational oil pollution: by retaining and reusing the oily residues instead of discharging them, it turned a major routine pollution source into a closed, compliant cycle. It remains part of how modern tankers stay within the MARPOL discharge limits.
How does load on top prevent pollution?
Oily washings are retained and settled in a slop tank rather than discharged; only water within the oil-content limits is released, and the next cargo is loaded on top of the retained oil, keeping it out of the sea.