Bulk Carrier construction
The design of a bulk carrier depends largely on whether it is
intended to carry a particular cargo or whether any type of bulk
cargo is to be catered for. A vessel required to carry iron ore has
a small hold capacity since the ore is heavy. A deep double
bottom is fitted, together with longitudinal bulkheads which
restrict the ore and maintain a high center of gravity consistent
with comfortable rolling.
A ship designed to carry bauxite,
however, requires twice the volume of space for cargo and will
therefore have a normal height of double bottom although
longitudinal bulkheads may be used to restrict the ore space.
A bulk ‘tramp’ if one can coin a term, i.e., one which maybe
required to carry any type of bulk cargo, must have restricted
volume for an iron ore cargo and at the same time must have
sufficient cargo capacity to carry its full deadweight of light
grain which requires three times the volume of the ore.
One
method of overcoming this difficulty is to design the ship to load
ore in alternate holds.
It may readily be seen, therefore, that the design of bulk
carriers will vary considerably.
The structure is having
longitudinal framing at the deck, bottom and side shell,
longitudinal bulkhead and tank top. These longitudinals are
supported by transverse webs 2.5 m apart. The supporting
members are, as far as possible, fitted in the tanks rather than
the ore space, to facilitate discharge of cargo using grabs. For
the same reason it is common practice to increase the thickness
of the tank top beyond that required by the Classification
Societies.
In this design
the double bottom, lower hopper and upper hopper spaces are
available for water ballast, the upper tanks raising the center of
gravity of the ship and hence reducing the stiffness of roll.
The Bulk carriers greater than 198m in length are normally framed longitudinally because of their long length, the flexibility and strength are provided by longitudinals.
The longitudinal are usually offset bulb plates or T-sections. The bottom longitudinals, are much heavier than deck longitudinals, while the side londitudinals are of increased dimensions towards the bottom shell of the ship.
The longitudinals are supported by deep transverse webs forming a ring around the ship at an interval of 4 to 5 m to which they are attached by flat bars or brackets. Side transverses are fitted in line with the bottom transverse to support the longitudinals against compressive loading.
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