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Damage to Cargo Spaces on Ships

Common damage/ defects that may occur on watertight transverse bulkheads situated at the ends of dry cargo holds of a bulk carrier.    

The following are examples of the more common damage/defects that may occur:

  1. Fractures at the boundaries of corrugations and bulkhead stools, particularly in way of shelf plates, shedder plates, deck, inner bottom, etc..
  2. Buckling of the plating/corrugations, leading to the failure and collapse of the bulkhead under water pressure in an emergency situation. .
  3. Excessive wastage/corrosion, in particular at the mid-height and bottom of bulkheads, which may look in deceptively good condition. This is created by the corrosive effect of cargo and environment, in particular when the structure is not coated. In this respect special attention should be given to the following areas:
    • Bulkhead plating adjacent to the shell plating;
    • Bulkhead trunks which form part of the venting, filling and discharging arrangements between the topside tanks and the hopper tanks;
    • Bulkhead plating and weld connections to the lower/upper stool shelf plates;
    • Weld connections of stool plating to the lower/upper stool shelf plates and inner bottom;
    • In way of weld connections to topside tanks and hopper tanks;
    • Any areas where coatings have broken down and there is evidence of corrosion or wastage. It is recommended that random thickness determination be taken to establish the level of diminution; and
    • Other structures, e.g., diaphragms inside the stools, particularly at their upper and lower weld connections.

Actions you would take as a Chief Officer to avoid the detrimental effects on bulk carriers due to corrosion, fatigue and improper cargo handling:

Fracturing of the Structural members of a transverse bulkhead of a Bulk carrier are:-

  1. Side Shell Plating
  2. Connection of Bulkhead plating to side shell.
  3. Connection of side shell frame & end brackets to the shell plating and hopper side tank plating by close-up inspection.
  4. Connection of side shell frame & end brackets to the shell plating and topside tank plating.

Actions to control structural stresses and fatigue:-

  1. Routing Planned Maintenance & Inspection of Structural members of cargo holds & hatch covers.
  2. Identification of susceptible areas prone to Fatigue & Stresses.
  3. Ensure accessibility of the area to be surveyed with due regards to the area under inspection shall be clean and will lit.
  4. Careful planning of the surveys to be undergone with all personnel involved.
  5. Where a fracture which not been caused by contact damage is found in the main hull structure on one side of a ship, corresponding structure on the opposite side should be examined to see if a similar fatigue has occurred. Fractures of this nature are of concern, especially where corrosion is associated with failure & may have been contributing factor.

Structural Surveys include:-

  1. Routine Inspections.
  2. Planning Surveys
  3. Surveys of known defects.
  4. Classification surveys.
  5. Annual surveys
  6. Intermediate Surveys
  7. Special surveys
  8. Condition Surveys
  9. CAP Surveys
  10. Life extension surveys, etc.

Fatigue cracking of steel structures:-

  • Fatigue cracking occurs at points such as hatch covers where stresses are locally high.
  • Such cracking is the result of cumulative damage caused by cyclic loading of structure & invariable starts at welded joints.
  • The fatigue life is the time required in service for the structure to experience enough stress cycles for a crack to occur. A bulker is so designed that with proper maintenance cracking should not occur until a fatigue life span of 20 years.
  • Fatigue life span once used up cannot be regained except by complete replacement of welded joints.
  • Therefore, it is very essential that the welded joints of an overstressed structure are replaced, before proceeding with life exterior surveys/ programmes.
  • Fatigue cracking have been most frequently observed in the bracket toes at the connection of the main frames to the hopper and topside tanks and in the boundaries of the vertically corrugated transverse bulkheads with upper stools, lower stools and topside tanks.

Corrosion / Structural Deterioration:-

  • The major barrier preventing structural deterioration on ships, especially in very corrosive environment of Water Ballast Tanks and to a slightly lesser extent cargo holds is the tank/ hold coating. Since the establishment of ESP water ballast tanks and CH of all new ships must be fully coated. Such coatings should be hard coating & preferably light in colour.
  • To maintain a good and efficient standard of coating is an important part of structural condition management. If coatings are kept in good condition the structure will be effectively protected and the original scantlings will be available to resists “service loads” & “stresses”.
  • Sea staff therefore should meaningfully report the condition of tank/ hold coatings during routines inspections and same to be documented in the company PMS system.
  • The most effective method of protecting a structure of a bulk carrier is to maintain coatings and carry out repairs of coating breakdown in good time.
  • Installation of Anodes, subject to the tank being regularly ballasted is an effective back-up that will prevent accelerated corrosion in the way of local coating breakdown.
  • The most effective time to carry out coating repairs in the tanks i.e. freshwater washing, surface preparation and coating application, can be done during a routine repair period of reasonable duration.

Damage to ship structure by improper cargo handling:-

  • Ships can be and are frequently damaged during discharging discharging over this especially if the operation is carried out by grabs or payloaders making heavy contact with the ship’s structure.
  • Local overloading, when bulk corners are loaded in ways not for seen by their classification society or shown in their loading manual, resulting in cracking of deck plating at the hatch covers & backing of plating between the hatchways.
  • A faulty distribution of weight can occur when:-
    • A ship jump loads to her tropical marks and corner less than full bunkers. In that situation some or all of the strengthened holds are subjected to greater tonnage of cargo than they were designed to carry.
    • Particular holds are loaded with greater tonnage then the tonnage for which the hold was designed.
    • The vessel is block loaded – i.e., where two or more adjoining holds are heavily loaded with adjacent holds empty. Although it is imperative that the longitudinal stresses are negligible but still the vessel suffers structural damage of the cross deck structures which separates adjacent cargo hatchways at the upper deck level.
  • Failure to Trim cargo reasonably:-
    • Cargoes with a low angle of repose are particularly liable to dry surface movement aboard ship.
    • To overcome this problem the code states that such cargoes should be trimmed as reasonably leveled & spaces completely filled without causing any excessive stresses on adjoining structure.

Importance of assessing defects and damage to cargo spaces:-

  1. It is important that the protective coatings in cargo holds and water ballast tanks are maintained. Therefore, it is imperative that the cargo holds and deck areas should be inspected by ship’s deck officers upon completion of cargo operations to identify any signs of Physical Damage, corrosion or coating damage to the ship’s structure.
  2. Where hull damage is identified, which may affect the integrity of the hull structure and sea-worthiness of the ship, it should be reported accordingly to the classification society.
  3. The internal hold structure and protective coatings in the cargo hold and adjacent double bottom spaces are vulnerable to damage when the cargo is discharged by using grabs. Grabs are made from toughened steel material and when carelessly used can cause considerable damage to the ship’s structure.
  4. Chipping (sharp indentations) and the local buckling or detachment of side frames and end brackets at lower connections could lead to cracking of the side shell plating which would allow the ingress of water into the cargo spaces.
  5. The protective coating which may be required to be applied in the cargo hold are also subject to deterioration caused by the corrosive nature of the cargo, high temperature cargoes, cargo settlement during the voyage and abrasive action of the cargo.
  6. Where no protective coating has been applied or the applied protective coatings have broken down, the rate of corrosion in that area will greatly increase, especially when carrying corrosive cargoes such as coal.
  7. Corrosion will weaken the ship’s structure and may eventually seriously affect the ship’s structural integrity. The severity of the corrosion caused by a structural member may not be easily detected, without close-up inspection or until the corrosion causes serious structural problems such as the collapse or detachment of hold frames, resulting in cracks propagating in the side shell.

It is therefore extremely important for a close-up inspection of the cargo spaces, after discharge operations when such corrosive and high temperature cargoes are being carried.

  • When the main grab discharge is ended, front end loaders (Pay Loaders) are usually lowered into the hold to gather the cargo from the ends and wings and pile it in the square of the hatch, to facilitate easy discharge by the grab.

Also, trimmers men are employed to shovel up the last of cargo from the position which front end Payloader, cannot reach, i.e. all the inaccessible places, elsewhere by the Payloader, in order to assist in discharge of final remaining cargo.

  • Standards of trimming vary considerably and ship’s officers are therefore encouraged to inspect the holds, thoroughly, whilst the trimmers are working in the hold, in order to remove as much residual cargo as possible. Thereby, utilizing the maximum efficiency and preparing hold for hold cleaning preparations.
  • The final inspection, after each cargo operation should include that no bilge gratings or manhole cover plates are missing that the securing bolts have not been damaged, that all the sounding pipes, airpipes and ballast lines and their pipeguards are intact, that no new indents can be seen in the plating of the tanktop, lower or upper hopper sides, or athwart ship bulkheads, that the side frames are regular and undamaged with brackets undamaged and that the hold ladders platforms, rails are complete and undamaged.
  • Any damage to the tank top plating, hopper sides, shell plating and framing, hatch coamings, hatch covers, bulkheads, stools and upper deck plating, air and sounding pipes, which could affect the sea-worthiness of the vessel, should be directly reported to the classification society.
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