Like tankers, the general arrangement of cargo spaces is dictated by the facts that the cargo is in the form of homogeneous particles of
more or less uniform size, and can be transferred by blowers, conveyors, or grab buckets.
Fig :Profile of a typical bulk carrier
Cargo spaces are divided into holds to meet structural
and subdivision requirements, to restrain cargo movements and resulting upsetting moments, to permit the carrying of different cargoes
simultaneously, and to provide for ballasting. Machinery is invariably aft, and the nonperishable nature of the cargoes leads to speeds in the
12- to 16-knot range, with attendant full hull forms.
Fig : A Bulk carrier on sea passage
Relatively small volumes of dense ores and similar cargoes will settle a ship to her summer load line. Holds on ore carriers are therefore quite small,
bounded by broad wing tanks and deep double bottoms. The double bottom and longitudinal bulkheads are of heavy
construction to carry the heavy ore load.
The narrow hold breadth limits transverse weight shifts and the depth of the double bottom is sufficient
to keep the center of gravity of the ore high enough to prevent stiff rolling in a seaway. Large volume wing tanks are used for ballast.
Fig : Cargo hold construction of a typical bulk carrier
Designed for low-density cargoes, dry bulk carriers require much greater hold volume than ore carriers, and therefore have much shallower inner
In some designs the topside tanks are omitted or fitted with bolted plates in the sloping plating facing the
hold. When very light cargoes are carried, the plates are removed and the tanks are filled along with the hold; the cargo in the tanks feeds
into the hold by gravity when discharging. Larger carriers are sometimes built with an inner side shell, which eases hold cleaning and provides
additional ballast space.
Shallow double-bottom bulk carriers are sometimes designed to carry high-density cargo, by arranging them with alternate long and short holds.
High-density cargo is loaded only in alternate holds to keep the center of gravity high enough to prevent excessive metacentric height.
double-bottom structure under the holds intended for heavy cargo is augmented. The alternating cargo distribution causes high vertical shear
near the bounding bulkheads, which may require increased shell scantlings.
With the increase in industrial demand for raw materials paralleling that for petroleum, the design of bulk carriers, like tankers, also evolved
to include larger hulls. Bulk carrier deadweights range from quite small to over 200,000 tons.
In order to increase the proportion of payload
operation above the 50-percent level typical of most straight bulk carriers (for tankers or dry bulk carriers operating between specific ports, cargo
is often carried on only one leg of the journey), a trend toward combination carriers began about 1950. At first, these were dual purpose ships
(ore/oil, bulk/oil) which carried different cargos on separate legs of a voyage cycle consisting of two or more legs. This development has
evolved into combination carriers known as ore/bulk/oil ships (OBO). Despite differences, bulk carriers of all types have certain features in
i) Single cargo deck, without ’tween decks.
ii) Machinery aft of cargo spaces so shaft tunnel does not interfere with discharging gear.
iii) Large ballast capacity.
iv) Double bottoms under bulk cargo holds.
To facilitate rapid cargo discharge and minimize cleaning requirements, holds are designed with a minimum of internal obstructions that might
catch and hold cargo. Bulkhead stiffening is attained by the use of corrugated plate rather than welded stiffeners. Hold cross section, is arranged so that cargo is self-trimming and self-loading:
i) Cargo will flow outwards from the point of discharge of bucket grabs or gravity chutes to fill the entire cargo space with a
minimum of hand trimming.
ii) The narrowing width at the top of the hold limits transverse cargo shifts when the hold is not completely filled.
iii) During discharge, remaining cargo will flow to a fairly small area where it can be picked up by the discharging equipment.
Holds of different lengths may be distributed throughout the length of the ship for flexibility in cargo distribution; cargoes of varying densities
can be distributed so as to keep the longitudinal bending moment within acceptable limits.
Except for equipment to open or remove hatch
covers, most bulk carriers are without cargo gear. Cargo is loaded by gravity chutes or derrick grabs and discharged by grabs, conveyor systems,
or in the case of grain and similar light cargo, by suction.
Some bulk carriers are built as self unloaders, either by the provision of derrick grabs,
or by trimming the cargo spaces to belt conveyers running under the holds to a bucket conveyer which transfers the cargo to another belt
conveyor on a long unloading boom. Conveyor type self-unloaders are fairly common on the Great Lakes . Combination
carriers are fitted with cargo pumps and piping systems for discharging oil cargoes.
Related information on ships structural arrangements:
Mechanical steel hatch covers arrangement and watertight integrity onboard a bulk carrier
Ships structural problem because of corrosion & metal fatigues - related countermeasures
Classification Society Establishes Design Standards For Bulk Carriers
High rate of corrosion at sea and preventing methods
Care and survey of hatch covers of dry
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"Sea going Bulk carriers are ship types intended primarily to carry dry cargo
in bulk, including such types as ore carriers and
Operation of sea going bulk carriers involved numerous hazards . Careful planning and exercising due caution for all critical shipboard matters are important . This site is a quick reference to international shipping community with guidance and information on the loading and discharging of modern bulk carriers so as to remain within the limitations as specified by the classification society.
It is vital to reduce the likelihood of over-stressing the ship's structure and also complying with all essential safety measures for a safe passage at sea.
Our detail pages contain various bulk carrier related topics that might be useful for people working on board and those who working ashore in the terminal.
- Bulk carrier types - Ore carriers, OBO ships, forest product carrier , self unloader and more
- Definitions of available Bulk carrier sizes
- Reason of bulk carrier failure in the past
- Tank construction & required strength for ships carrying bulk cargo
- Defining a bulk carrier transverse section
- Causes of structural damage and countermeasures
- Deterioration of ships hull and consequences of hull damage /forward flooding
- How to avoid damage during cargo operation
- How to arrange repair of damage during cargo loading/unloading