Combination carrier. Fundamentally, this is a vessel designed to carry either liquid or dry bulk cargoes. If a vessel is a specialist vessel, for example, only carrying crude oil, there is a disadvantage to the shipowner because the vessel would have to be ballasted on one leg of each passage.
The vessel cannot earn money for the shipowner while it is in ballast, unless, of course, it is on a period time charter or on a demise charter. The combined carrier was developed mainly as bulk carrier capable of carrying oil in wing tanks. The type was originally developed as an ore/oil carrier (0/0) but further design developments led to a vessel which is flexible enough with the cargoes that it can carry to be called an OBO, or Ore/Bulk/Oil.
The O/O carrier is similar in construction to an ore carrier which has strengthened structural components for the heavy ore cargoes and wing tanks for the oil. The ore compartments are usually high up in the vessel so that the centre of gravity of the high-density ore cargo is raised. If this were not done, the vessel would have too large a “metacentric height” and be too stiff, moving very violently in a seaway.
The OBO is similar in construction to a conventional bulk carrier but can carry lighter dry bulk cargoes or ores and oil in wing tanks.
The combination carrier is more expensive to build but gives the owner some flexibility.