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Rise Of OBO's:
We have been studying different types of sailing ships and now we will look at an OBO. Oil-Bulk-Ore carriers are also called as combination carriers as they are designed to carry cargoes of different nature. Sometimes, these ships are also called as PROBO's. PROBO-Product-Oil-bulk-ore carriers. These ships are designed to carry dry & wet cargoes at the same time & same voyage. These ships came into existence of having the idea of reducing the number of ballast voyages. Earlier Oil Tankers & Bulk carriers had cargoes only one way & returned empty. This resulted in loss of revenue. Thus Owners thought of building a ship with greater flexibility,thus capable of handling cargoes on both voyages. The ship can be identified on seeing the deck area bacause these ships have both the hatches & pipelines on the deck area.
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The History of OBO's: Ore-Oil carriers..!
The rise of O/O carriers: Initially oil-ore carriers were designed. These ships were designed to carry full deadweight when trading as tankers and also for ores. Heavy ores can be can be carried only in the center holds. But oil can be carried either in center holds & also in cargo wing tanks. The centre hold tank-tops are normally arranged sloping inwards, facilitating a self-stowing factor when ores are loaded. The wing tanks are conventionally designed which can carry either cargo oil or ballast water in them.
The center compartment is generally free from structural stiffeners, so that loading/discharging of the cargoes will not be hindered. The cargo holds were constructed upto 1/2 of the breadth of the ship. Cargo pipelines where usually located in the wing tanks, where ballast pipelines pass through double bottom tanks.
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From O/O to OBO....!
These type of ships are exclusively designed to carry high-density ores, and oil similar to Ore-Oil carriers. In addition to that they are capable of carrying other cargoes of dry-bulk in nature. i.e grains, fertilizers etc...The holds are arranged to extend to almost full breadth of the ship, enabling full access to the cargo hold when handling dry cargoes. These types of ships have upper & lower hopper tanks with double bottom tanks. These are visible in the pictures, which are self-explanatory. Mostly the cargo holds carry Oil/ Dry ore as cargo. The upper hoppers carry oil as cargo and water in ballast voyage. Certain designs of these ships also has wing tanks. If wing tanks are present, they may be used as slop tanks too. These wings tanks may be located at aft of the cargo holds. The hatches may be similar of bulk carriers, with side rolling arrangements, with special type of sealing might be fitted.
When oil is carried in center cargo holds, then check should be kept on the size of the holds. If size of the holds are too big, then oil cargo may lead to vapour formation and it cumulates for the process of "pyrophoric ignition" of iron sulphides, which are formed as part of rusting process. The formation pyrophores may lead to a spark and finally explosion of the cargo tanks. When oil is carried as main cargo, then pollution prevention plays a main role. Because the hatches are big in size, and the sealing is not very effective when compared to oil tankers, there is a question with regards to weather tightness & gas tightness. Also the slops has to be handled properly taking precautions not to get mixed with the ballast water.
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OBO's Dawn To Dusk...!
Even though the OBO's were famous during 1960's to 1980's, they were unable to substantiate their position in the market due to many reasons. Many of the OBO's decided to carry dry bulk as cargo, because of the problems associated with carrying oil as cargo in them. Due to uneven load distribution throughout the ship, uneven thermal variation & rough weather wave pattern affects the structure of the ship. This leads to bending or deflection of different parts of the ship. Thus hatch coamings get deflected and sealing of holds becomes a problem. When oil is carried as cargo, inert gas is used up to press up the tanks so that the oil vapour accumulation is overcome by the inert gas pressure thus eliminating the possibility of static discharge. This inert gas tend to leak through the hatch seals and creates the possibility of explosion. Also these kind of ships are difficult to buid and the design of these ships are so complicated, which involves cost. Thus these features made the ship owners to jump back to the conventional bulk carriers and oil tankers. Now in existence there are only very few ships of this nature.
image & content credits: personal.