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Requirement of stability, hull strength, draft, trim, suitability of cargo for a bulk carrier

The ability of a vessel to return to an upright position when heeled by some external force, such as the action of waves, is a measure of her stability. The force of gravity acting downwards and the buoyancy force acting in opposition cause a righting lever which returns the ship to the upright when heeled.

The magnitude of this lever is determined by the position of the Centre of Gravity within the ship which is itself affected by the disposition of cargo, fuel, ballast, fresh water, etc. Broadly speaking, the lower the weights in the ship, the lower the Centre of Gravity; the lower the Centre of Gravity, the larger will be the righting lever at successive angles of heel (i.e., the greater the ship's ability to return to the upright).

A Bulk carrier encountering head seas
Encountering rough sea conditions

To an extent this suggests that heavier goods be stowed in the lower part of the ship with lighter goods on top. However, it should be borne in mind that very large righting levers (an excess of stability by virtue of a low Centre of Gravity) may give rise to excessive violent rolling and potential damage to both ship and cargo. Some Classification Societies restrict the maximum permissible GM. This may be known as "super stability" and restrict the deadweight intake of closeweight cargoes. Careful loading will ensure adequate but not excessive stability, i.e., the ship will be neither "tender" nor "stiff.

When performing stability calculations the centres of gravity of various parcels of cargo can often only be arrived at by approximation. It is better to err on the side of safety, assuming the centre of gravity to be higher than it probably is.

Apart from stability considerations, distribution of cargo (and to a lesser extent bunkers, ballast and fresh water) can induce unacceptable bending moments, sheer forces and torque­particularly in larger vessels. Care should be exercised to ensure that any limits established by her designers are not exceeded. Instances are on record of vessels having broken in two during cargo operations; continually subjecting larger vessels to excessive loads throughout the ship's life can give rise to structural failure in a seaway. Loading a vessel with excessive weights at each end also tends to make her hog and sluggish in rising to a head sea and thus liable to undue strain in heavy weather.

The conditions of stability, hull strength, draft and trim of bulk carriers at sea and on arrival / departure at / from port and during loading / unloading cargo, bunkering and water ballast exchange, should be worked out, ensuring safety of the vessel. Safety of the cargo vessel depends on proper GM, stress calculation and other factors as being within appropriate Limits.

During stowage the first consideration must be given to safety, i.e. the cargo must be stowed so that the ship will be stable and seaworthy, and it must be secured in such a manner that it cannot shift if the vessel encounters bad weather. The type of vessel, the cubic capacity of her compartments destined for the cargo and the appliances on board or on shore for loading or discharging, as well as the nature of the cargo, affect the question of how to stow the cargo in the best possible manner.

The ship must be made neither stiff nor too tender. The next consideration is for the safety of the cargo itself: it must not be damaged by shifting; certain commodities become easily tainted by others, water might find its way into the hold and condensation or sweating must be prevented. Valuable cargo may be stolen or broached.

Finally, the Chief Officer must bear in mind the various destinations of the goods the ship carries, and arrange things, as far as he can, to see that the cargo for a certain place can be lifted out without disturbing the other cargo. The Chief Officer must watch closely the ship's stability (i.e. what the ship's trim is or how she is sitting).

Since a ship is supported by fluid pressure she will incline in any direction according to the position of the weights placed on her. The trim, therefore, is the angle that a ship is making, fore and aft, with the water.

The levels are read by numbers painted on the ship's stem and stem. These are called draught marks. Another word is heel. This means a list or inclination from one side to another, caused by loading. The Chief Officer must watch the load lines. They are welded or punched on and then painted.

Following are the check item confirming stability and hull strength of cargo ship:
  1. The GM value within acceptable limits as specified in the loading manual and in compliance with IMO rules upto arrival next port?

  2. GZ curves of the vessel to be fully understood, and their characteristics confirmed

  3. Expected weather and sea conditions, to be taken into consideration when confirming stability & hull strength

  4. Free surface effects and any sloshing effects for the planned passage to be taken into consideration.

  5. Any restrictions specified in the loading manual to be taken into consideration.

  6. Values of bending moments, shearing forces and torsional stresses at sea to be within acceptable limits upto arrival next port.

  7. The sailing draft to be within applicable loadline or port/passage limits/restrictions.

  8. Air draft limitations due to bridges, cargo handling equipment or other obstructions to be assessed as necessary.

  9. The cargo density in accordance with maximum permissible values and precautions as per the loading manual to be followed.

  10. Forward draft limit (per loading manual) to prevent slamming to be confirmed.

  11. Propeller immersion ratio to be assured.

  12. Trim and draft changes during voyage in fresh or brackish water such as rivers, canals and lakes, to be taken into consideration.

  13. Squat due to shallow water effect to be taken into consideration.

  14. Safe under keel clearance to be assured .

  15. Fuel oil and fresh water consumptions to be taken into consideration.

Check items

Following are the guideline to check suitability of loading/ unloading solid bulk cargo

i) Cargo holds and hatch openings are suitable for cargo operations

ii) Holds are clearly numbered on hatch covers/ coamings

iii) Hatch covers, hatch operating systems and safety devices are in good operational condition

iv) List indication lights, if fitted, have been tested prior to arrival and are operational

v) If applicable, loading instrument is certified and operational

vi) Propulsion/auxiliary machinery is in good operational order

vii) Mooring equipment is in good functional order

A Bulk carrier encountering head seas

Fig: Bulk carrier encountering rough sea conditions

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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
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