![]() ![]() I) A gravity fed self-unloader: the cargo is dropped onto a conveyor belt running in a duct keel under the cargo holds that carries the cargo towards the bow or stern of the vessel where another conveyor lifts it for discharge ashore. Similar in hull structure to other bulk carriers, these vessels are fitted with one of two systems for discharging cargo: bilge lines (for use with dry cargoes).pipeline system (ballast, bilge and cargo) fitted through the duct keel.tank/hold cleaning system similar to the crude oil washing system on oil tankers. ![]() ![]() pumproom for load/discharge of liquid cargoes.Oiltight hatch covers to allow the carriage of liquid cargoes.Many OBOs have void spaces formed by fitting transverse bulkheads between two cargo holds. OBO construction is similar to that of a bulk carrier except that they have larger wing tanks and their DB tanks are deeper to improve stability when carrying ore cargoes. Most remaining OBOs are limited to one cargo type. Owing to the number of losses, notably the Berg Istra (1976), Berg Vanga (1979) and the Derbyshire (1980), combined with the high maintenance costs, there has been a reduction in the number of these ships, with few new OBOs built. Major problems on these ships included gas freeing to load dry cargo after an oil cargo and the high maintenance costs caused by heavy wear and tear. For this dual purpose operation, OBOs were fitted with pipelines, pumps and other oil tanker equipment. However, the hatch covers were designed to be `oiltight' so the same ship could be loaded with oil cargoes with strengthened holds for ore cargoes. They were designed with large hatches to facilitate loading and discharging. ![]()
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