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Loading of high density cargo and water ballast distribution for bulk carriers

Bulk carriers are usually discharged by grabs or by suction pipes. Pouring the cargo through a shooter or via a conveyor belt does the loading. Bulk carriers have large upper and lower ballast tanks to give the empty vessel enough draught and a better behaviour whilst in transit. The safe operation of bulk carriers is dependant on not exceeding allowable stresses in the cycle of loading, discharging, ballasting and de-ballasting.

It is recommended that high density cargo be stowed uniformly over the cargo space and trimming be applied to level the cargo, as far as practicable, to minimise the risk of damage to the hull structure and cargo shift in heavy weather.

The distribution of cargo in a hold, and water ballast distribution, have an important influence on the resultant stress in the hull structure. The double bottom and the cross deck structure are designed based upon a trimmed cargo distributed symmetrically in a hold space.

Typical bulk carrier transverse section

Fig : Transverse section of a typical bulk carrier

Still water shear forces and bending moments given in the ship's loading manual and the corresponding calculations from onboard loading instruments are based on an even distribution of cargo in a hold space, unless otherwise indicated.

Still water shear force and bending moments calculated with an onboard loading instrument do not consider the torsional loads acting on the hull girder resulting from asymmetrical cargo or ballast loading

Grab discharging

Fig :Grab discharging of a typical bulk carrier

When heavy cargo is poured into a cargo space at one end of the cargo hold, the lateral cargo pressure acting on the transverse bulkhead, as a result of the cargo piling up at one end of the cargo space , will increase the loads carried by the transverse bulkhead structure and the magnitude of transverse compressive stresses in the cross deck.

When the same loading pattern is also adopted for the adjacent cargo hold , the lateral cargo pressure acting on the transverse bulkhead will be largely cancelled out. However, in this situation, a large proportion of the vertical forces on the double bottom is transferred to the bulkhead between the two loaded holds which could lead to shear buckling of the transverse bulkhead structure, compression buckling of the cross deck and increased SWBM in way of the transverse bulkhead. Cargo should always be stowed symmetrically in the longitudinal direction, and trimmed, as far as practical.

Stowing cargo asymmetrically about the ship's centre line in a cargo space induces torsional loads into the structure which causes twisting of the hull girder. When the hull girder is subjected to torsion, warping of the hull section occurs which gives rise to shearing and bending of the cross deck structure. Water ballast should always be carried symmetrically in port and starboard tanks with equal levels of filling. The final fill level of all water ballast tanks and holds must satisfy the requirements specified in the ship's approved loading manual to avoid damage to the internal structure due to sloshing effects.

For the Ballast Water Management Plan to be effective the Master and chief officer must ensure that it is:
The ballasting and deballasting of port and starboard ballast tanks should be carried out simultaneously so that the amount of water ballast in each corresponding pair of port and starboard ballast tanks remains the same through out ballasting or deballasting operations . Asymmetrical distribution of water ballast induces torsional loads, causing twisting of the hull girder.

Torsional loading of the hull girder is considered to be an important contributory factor to recurring cracking at the hatch corners and to problems associated with hatch cover alignment and fittings. In extreme cases, this can lead to extensive buckling of the cross deck structure between the hatch openings.

Exceeding the permissible limits specified in the ship's approved loading manual will lead to over-stressing of the ship's structure and may result in catastrophic failure of the hull structure.

Related guideline

  1. Ballast exchange procedure at sea

  2. Practical method for the control of transportation of harmful marine organisms

  3. Safety precautions during ballast operation

  4. Regulation of pumping system of bulk carriers

  5. Risk of partially filled ballast tanks

  6. Handling water ingress problems in bulk carrier, investigation and countermeasures

Reference publications

  1. MARPOL 73/78
  2. IMO Resolution A.774 (18) – “Guidelines for Preventing the Introduction of Unwanted Aquatic Organisms and Pathogens from Ship’s Ballast Water and Sediment Discharged”
  3. Ship’s “Procedure and Arrangements manual” (Approved by Class)
  4. Guide to Port Entry
  5. US NPDES Vessel General Permit Compliance Manual

Related Information

  1. Bulk carrier types - Ore carriers, OBO ships, forest product carrier , self unloader and more

  2. Risk of partially filled ballast tanks

  3. Bulk carrier design limitations - Over stressing on ships structure & countermeasures

  4. Causes of structural damage and countermeasures

  5. Deterioration of ships hull and consequences of hull damage /forward flooding

  6. Bulk carrier hull damage - causes and preventive measures

  7. Ventilation requirement for bulk cargo loaded

  8. How to avoid damage during cargo operation

  9. How to arrange repair of damage during cargo loading/unloading

  10. Limitations of overloading of cargo holds & countermeasures

  11. Preparation for ships carrying bulk cargo & standard loading condition

  12. Various categories of garbage and management onboard

Our detail pages illustrated many safety aspects of Bulk carrier

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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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