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Crude oil and product tanker cargo work is built around three linked concerns: keeping the tank atmosphere out of the flammable range at every stage, moving very large volumes of flammable liquid quickly and accurately, and preventing oil discharge to the sea. The operational rulebook is ISGOTT, published jointly by OCIMF, ICS and IAPH; the underlying mandatory instruments are SOLAS Chapter II-2 (fire safety) and MARPOL Annex I (prevention of pollution by oil). This page covers the cargo cycle from loading to discharge.
A tanker voyage runs through a repeating sequence: tank preparation (inspection, inerting or gas-freeing depending on the next cargo), loading, laden passage with the cargo kept inert and stable, discharge and stripping, and — where the trade requires it — tank cleaning before the next loading. Each transition is governed by a ship-shore safety conference and checklist before hoses are connected.
SOLAS Chapter II-2 requires cargo tanks on most oil tankers to be kept in an inert condition throughout the voyage, other than during gas-freeing for inspection or dry-docking. Inert gas is either flue gas drawn from the ship’s boiler uptakes and scrubbed of soot and sulphur, or nitrogen from a dedicated generator on ships without a suitable boiler source.
The IGS keeps the oxygen content of the tank atmosphere below the level that can support combustion — the long-standing operational standard is to maintain oxygen content below 8% by volume, well under the roughly 11% minimum generally needed to sustain a hydrocarbon fire. Continuous oxygen monitoring at the IGS deck main and at individual tanks is required, and the system will alarm on high-oxygen or low-pressure conditions.
Flue-gas IGS is the traditional arrangement on larger tankers with oil-fired boilers: uptake gas is scrubbed, cooled and delivered to the tanks via a deck water seal and non-return valve to prevent any flashback. Nitrogen-generator IGS produces inert gas independent of the boiler and is common on smaller tankers, gas carriers and chemical tankers where a cleaner inert medium is wanted; see also the inerting arrangements described for gas carriers on the LPG carrier operations page.
ISGOTT sets out the flammability diagram concept used to plan every atmosphere-changing operation (gas-freeing, purging, inerting). A hydrocarbon/air/inert-gas mixture is only flammable within a band bounded by its lower and upper flammable limits and by the oxygen content needed to sustain combustion. Any transition from an inerted tank to a gas-free tank (or the reverse) must be planned so the mixture does not pass slowly through the flammable zone — either the oxygen is kept low throughout (inerting first, then purging with fresh air) or the hydrocarbon content is driven down before oxygen is introduced. Getting the sequence wrong is a recognised cause of tank explosions.
Crude oil washing is required under MARPOL Annex I, Regulation 33, for most crude oil tankers of 20,000 DWT and above. Instead of washing cargo tanks with water, high-pressure jets of the crude cargo itself are used during discharge to strip wax and sludge deposits from the tank internals, dissolving them back into the cargo being pumped ashore. This reduces the volume of oily residue that would otherwise need separate treatment, cuts the quantity of oil retained on board, and reduces the sludge that would otherwise require slop-tank processing.
COW is carried out tank-by-tank in a planned sequence during discharge, with the tank kept inert throughout — COW machines operate inside an oxygen-depleted atmosphere as a further precaution against static ignition. A COW operations and equipment manual, approved by the flag administration, is carried on board and followed for every washing sequence.
Discharge follows a planned pumping sequence to maintain trim and stress within limits as tanks empty. As each tank nears empty, cargo pumps are backed up by stripping pumps or eductors to remove the last practicable quantity of liquid from the tank bottom — the residue left behind after stripping is a key commercial figure (on-hire/off-hire and outturn disputes often turn on stripping performance).
Under the load-on-top procedure, oily tank washings and residues from a ballast voyage are collected in a dedicated slop tank and allowed to settle so the oil separates and rises above the water layer. The clean water layer beneath is decanted overboard only through the oil discharge monitoring equipment, and the next cargo is then loaded on top of the retained oily residue rather than discharging it separately.
MARPOL Annex I requires an approved ODME on tankers for any overboard discharge of oily mixtures from cargo areas. The ODME continuously records oil content and discharge rate and automatically stops the discharge if the applicable MARPOL limits (instantaneous rate, total quantity, and distance from the nearest land) would be exceeded. The record is retained and cross-checked against the Oil Record Book.
Every cargo transfer — loading or discharge — is preceded by the ISGOTT Ship-Shore Safety Checklist, completed jointly by the ship and terminal and covering moorings, hoses, communications, the state of the inert gas system, and emergency shutdown arrangements. It is re-verified at intervals throughout a long cargo operation and retained as part of the cargo record.
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