Tanker shipping: liquids carried in bulk

Tanks, coatings and segregation

A tanker is divided into separate tanks served by separate pipework, which lets one vessel carry several grades at once without them meeting. What each tank may hold depends on its lining: epoxy and zinc coatings suit some products, while aggressive chemicals require stainless steel. Crude carriers, clean product tankers, chemical tankers and gas carriers are effectively different ships, and cross-trading between those roles is limited by construction rather than by preference.

Loading and discharge as a pumping operation

Cargo passes through a manifold on deck, connected to shore by hoses or articulated loading arms. Before anything flows, ship and terminal complete a joint safety check covering connections, communications, emergency stop arrangements and the condition of moorings. Oil tankers maintain an inert atmosphere in cargo tanks so the vapour space cannot support combustion, and monitoring that atmosphere is continuous throughout the operation.

Cleaning between cargoes and the prior-cargo question

Residue matters enormously. Tanks are washed, stripped, dried and inspected between grades, and for sensitive cargoes such as edible oils the previous cargoes carried in that tank are restricted, with documentary evidence of what went before. Wash water is itself a regulated waste that cannot simply be discharged. Cleaning takes real time and is a scheduling constraint in its own right, not an afterthought between voyages.

Temperature, viscosity and heated cargoes

Many cargoes will not flow at ambient temperature. Heavy fuel oils, bitumen, waxes and some chemicals are carried in tanks fitted with heating coils and held within a specified range for the whole voyage. Too cold and the cargo cannot be pumped out; too warm and it may degrade or, in the case of some products, become a hazard. The heating record is part of what the receiver examines on arrival.

The pollution-prevention frame around everything

Because a release goes straight into the sea, tanker operation sits inside a dense international regime covering construction, tank arrangement, discharge of washings, record keeping and equipment. The international maritime regulator publishes the conventions and codes that apply, and port states enforce them through inspection. Operators plan around those rules as fixed constraints rather than variables.

Frequently asked questions

Why can one tanker not carry any liquid cargo?

Because the tank lining, pipework and equipment are built for particular product families. A coating suitable for refined products may be attacked by an aggressive chemical, and gas cargoes need pressure or refrigeration systems that ordinary tanks do not have.

What is the prior-cargo restriction about?

For sensitive cargoes, notably edible oils, the acceptability of a tank depends on what it carried previously, since traces can remain after washing. The vessel provides documented evidence of the preceding cargoes so the receiver can judge suitability.