Combination carrier (OBO, OO, COMBO)

A combination carrier is a ship able to carry both liquid and dry bulk cargoes, switching trades to reduce empty ballast voyages. The two main types are the OBO (oil-bulk-ore), which takes crude oil, dry bulk, or ore, and the OO (ore-oil), which takes crude oil or ore but not general dry bulk. The economic case rested on loading oil one way and ore or bulk the other. The fleet has nearly disappeared since its 1970s and 1980s peak: piping inside the cargo holds complicates the cleaning and gas-freeing needed when switching between wet and dry cargo, and the complex structure carries high maintenance. Wet and dry cargoes are never carried together.