Sea cargo reference
Crude Carriers
The heaviest tankers in service. The notes below cover the cargo, the way the hull is arranged, and the limit that puts a ship in this class.
- Ship's gear
- Self-discharging by cargo pump
- How capacity is counted
- Deadweight tonnes (DWT); cargo in barrels
- Usual size
- From about 80,000 DWT to over 320,000 DWT
- Cargo type
- Crude oil
Crude carriers
These tankers move crude from producing regions to refineries. By cargo weight they are the largest merchant ships in service.
The market names the sizes: Aframax, Suezmax, VLCC (Very Large Crude Carrier, around 300,000 DWT) and ULCC above that. The names follow routes and canal limits, not a written standard.
Crude does not need the coatings that refined products do, so these ships are built for volume rather than for switching cargo. The largest often load and discharge offshore, because few ports are deep enough to take them alongside.
Cargo on board
How the hull is arranged
- VLCCs and ULCCs are the sizes most used on long crude routes, carrying roughly 200,000 to 400,000 tonnes.
- Aframax and Suezmax sit below them. They are chosen when the route runs through a narrow waterway, or when the terminal cannot take a larger ship.
- A double hull keeps cargo tanks off the sea, with ballast space on the sides and bottom. MARPOL requires this on oil tankers.
- The cargo sits in several tanks. Longitudinal and transverse bulkheads limit how far the liquid can slosh.
- An inert gas system holds the tank atmosphere below the oxygen level at which the vapour could catch fire.
- Cargo pumps and a fixed pipeline meet a manifold amidships, so the ship pumps its own cargo ashore.
- Tanks for cargo that must stay warm enough to pump carry heating coils.
Why this trade uses it
- Large volumes of liquid move with no packaging.
- The ship pumps itself out, so a terminal needs pipes and tanks rather than heavy cranes.
- Separate tanks let several grades travel on one voyage without mixing.
Settle these points before you fix
- Washing tanks between cargoes costs time and money. What the ship carried last decides what it may load next.
- Oil majors vet ships on top of class and flag rules, and that vetting decides who will take the ship.
- Pollution liability is heavy. Ship and terminal both follow strict transfer rules.