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Bulk carriers hold loading conditions and distribution of cargo along ships length
Bulk carriers are designed and approved to carry a variety of cargoes. The distribution of cargo along the ship's length has a direct influence on both the global bending and shearing of the hull girder and on the stress in the localised hull structure.
Improper distribution of cargo may cause a ship to be legally unseaworthy in two ways:
In considering how to avoid unseaworthiness resulting from improper stowage, reasonable care or due diligence must be employed, particularly in relation to cargo distribution, point loading, cargo securing, effect of cargo, machinery and equipment and practice of good seamanship.
- it may make her unfit for the voyage contemplated, e.g., if cargo is inadequately secured and liable to break loose, the ship may become unstable;
- it may make her unfit to receive further cargo, e.g., if cargo already on board is inadequately secured and liable to break loose, subsequently loaded cargo will be at risk of damage.
Various cargo distribution methods are available :
- Homogeneous hold loading condition.
- ii) Alternate hold loading condition.
- iii) Block hold loading condition.
- iv) Part hold loading condition.
Homogeneous Hold Loading Conditions (Fully Loaded)
A homogeneous hold loading condition refers to the carriage of cargo, evenly distributed in all cargo holds . This loaded distribution, in general, is permitted for all bulk carriers and is usually adopted for the carriage of light (low density) cargoes, such as coal and grain. However, heavy (high density) cargoes such as iron ore may be carried homogeneously.
Alternate Hold Loading Conditions (Fully Loaded)
Heavy cargo, such as iron ore, is often carried in alternate cargo holds on bulk carriers . It is common for large bulk carriers to stow high density cargo in odd numbered holds with the remaining holds empty. This type of cargo distribution will raise the ship's centre of gravity, which eases the ship's rolling motion. When high density cargo is stowed in alternate holds, the weight of cargo carried in each hold is approximately double that carried in a homogeneous load distribution.
To support the loading of the heavy cargo in the holds, the local structure needs to be specially designed and reinforced. It is important to note that the holds which remain empty, with this type of cargo distribution, have not been reinforced for the carriage of heavy cargoes with a non-homogeneous distribution.
Ships not approved for the carriage of heavy cargoes in alternate holds by their classification society must not adopt this cargo load distribution.
Fig : Deck cranes of a typical bulk carrier
Block Hold Loading and Part Loaded Conditions
A block hold loading condition refers to the stowage of cargo in a block of two or more adjoining cargo holds with the cargo holds adjacent to the block of loaded cargo holds empty, . In many cases, block hold loading is adopted when the ship is partly loaded. Part loaded and block hold loading conditions are not usually described in the ship's loading manual unless they are specially requested to be considered in the design of the ship. When adopting a part loaded condition, to avoid over-stressing of the hull structure, careful consideration needs to be given to the amount of cargo carried in each cargo hold and the anticipated sailing draught.
When a ship is partly loaded, the cargo transported is less than the full cargo carrying capacity of the ship. Hence, the sailing draught of the ship is likely to be less than its maximum design draught. The weight of cargo in each hold must be adequately supported by the buoyancy upthrust acting on the bottom shell. A reduction in the ship's draught causes a reduction in the buoyancy upthrust on the bottom shell to counteract the downward force exerted by the cargo in the hold. Therefore, when a ship is partly loaded with a reduced draught, it may be necessary to reduce the amount of cargo carried in any hold.
To enable cargoes to be carried in blocks, the cross deck and double bottom structure needs to be specially designed and reinforced. Block loading results in higher stresses in the localised structure in way of the cross deck and double bottom structures and higher shear stress in the transverse bulkheads between the block loaded holds. The weight of cargo that can be carried in the block of cargo holds needs to be specially considered against the ship's sailing draught and the capability of the structure. In general, the cargo load that can be carried in blocks is much less than the sum of the full cargo capacity of the individual holds at the maximum draught condition.
Part loaded and block hold loading conditions should only be adopted in either of the following situations:
i) The loading distributions are described in the ship's loading manual. In this case, the ship's structure has been approved for the carriage of cargo in the specified loading condition and the loading conditions described in the ship's loading manual should be adhered to, or,
ii) The ship is provided with a set of approved local loading criteria which define the maximum cargo weight limit as a function of ship's mean draught for each cargo hold and block of cargo hold(s). In this case, it is necessary to ensure that the amount of cargo carried in each hold satisfies the cargo weight and draught limits specified by the local loading criteria and the hull girder SWSF and SWBM values are within their permissible limits.
Fig : Cargo hold construction of a typical bulk carrier
Our detail pages illustrated many safety aspects of Bulk carrier
- Ship duties and responsibilities while unloading cargo
- Risk of Heavy cargoes & Monitoring the Ship's Loading limits
- Inadequate cargo weight measurement during loading - How to avoid shortfall
- How to avoid risk of vessel being overloaded
- Local loading criteria defining maximum allowable cargo weight in each cargo hold
- High loading rates by shore terminal and potential problems for bulk carriers
- Requirement for ballast exchange ar sea
- Risk of partially filled ballast tanks
- Bulk carrier design limitations - Over stressing on ships structure & countermeasures
- Causes of structural damage and countermeasures
- Deterioration of ships hull and consequences of hull damage /forward flooding
- Bulk carrier hull damage - causes and preventive measures
- How to avoid damage during cargo operation
- How to arrange repair of damage during cargo loading/unloading
- Causes of bulk carrier hull damage and failure in operation
- Bulk carrier water ingress problem
- Preparation for ships carrying bulk cargo & standard loading condition
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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. For any remarks please Contact us
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