How is a cargo tank cleaned on a tanker?

Cleaning a cargo tank is a major planned operation on tankers and some bulk carriers, carried out when the ship must load a cargo that would be contaminated by residues of the previous one, before personnel enter the tank for inspection or repair, and before dry-docking or hot work. The cleaning itself is done by tank-washing machines, either fixed nozzles built into the tank or portable machines lowered through openings on hoses, which rotate and drive high-pressure jets of washing medium across every surface in a programmed pattern so the whole tank is covered.

The washing medium depends on the trade. Product and chemical tankers normally wash with seawater, cold or heated depending on the previous cargo, sometimes with cleaning agents for stubborn residues. Crude carriers use crude oil washing (COW), spraying the cargo itself against the surfaces during discharge so the oil dissolves and carries clinging residues to the pumps; MARPOL requires COW capability on modern crude tankers because it sharply reduces the slops generated. During any washing of a tank that has carried oil, the atmosphere must be kept inert, generally below 8 percent oxygen by volume, because washing generates static electricity and oil mist that could otherwise ignite.

All washings drain to the slop tank, where oil and water separate. The water may be decanted overboard only under the strict MARPOL Annex I conditions monitored by the oil discharge monitoring equipment, and the oily residue is retained for discharge ashore. Never pump slops directly overboard: this is the classic magic-pipe offence and it ends careers and companies.

After washing, the tank is drained and mopped or squeegeed dry, then gas-freed by ventilation with fresh air. Before entry, the atmosphere is tested at several levels with calibrated instruments: 20.9 percent oxygen, hydrocarbons below 1 percent of the lower explosive limit if hot work is planned, and toxic vapours such as benzene or hydrogen sulphide below their occupational limits. Entry then proceeds only under a full enclosed-space entry permit with an attendant, rescue equipment and continuous monitoring, because a tank that looks clean can still hold pockets of gas in pipework, pump stacks and under sediment. Treat every stage as safety-critical: most tanker fatalities during cleaning come from entering too early or trusting a single atmosphere reading.