Oil/Bulk/Ore carrier (OBO)

An oil-bulk-ore carrier (OBO) is a combination carrier that can load crude oil, dry bulk such as coal and grain, or iron ore, switching between wet and dry cargoes to cut ballast legs. The design peaked in the 1970s and 1980s, when carrying oil one way and ore the other promised higher utilization than a single-trade ship. The fleet has shrunk to a handful because the dual role is costly: oil-cargo piping runs through the holds, hold cleaning and gas-freeing between wet and dry cargoes is slow and expensive, and the combined structure carries heavy maintenance. Petroleum and dry bulk are never carried at the same time.