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Inspections in cargo tanks are part of the ship's preventive maintenance program. The structure and all of the equipment each tank contains are vital to the work of the tanker. Information on condition of the tanks and cargo equipment is essential to preparation of requirements for shipyard or underway repairs. For tank inspection information to be useful to the crew and owners, the vessel must have a program for keeping tank inspection records and for filing tank inspection reports.

2.16.1 Tank structural inspection

Port wing tank looking forward.

Because of the flexing and stresses imposed on the hull structure during every voyage, the structure must be routinely inspected for early detection of defects. Defects can be impact damage, corrosion, cracks, buckling or distortion. Reporting defects requires a clear description of the size and type of defect and its exact location in the tank. Figure 2.16.1 is a diagram of a typical wing tank. Owners should provide a book of such diagrams (laminated), including one diagram of each tank, to provide the officer responsible for tank inspections with a convenient way of accurately indicating defects. Examples of recorded defects might be:

1 'No.3 port cargo oil tank, frame 67, inboard, aft side, bracket stiffener weld to longitudinal is cracked for 9 cm, 7th longitudinal from bottom.'

2 'No.5 port cargo oil tank, frame 60 to 61, bottom, in way of main longitudinal, bottom set in about 6 cm, for length of 2 meters and breadth of 1.3 meters; longitudinal buckled and weld cracked at connection to frame 60.'

3 'No.5 port ballast tank, Frame 60, deckhead, coating deteriorated and severe corrosion to longitudinal and transverse framing from main longitudinal inboard to longitudinal bulkhead.'

Each of these defects is described with enough detail to enable the owner's repair engineer or a shipyard foreman to locate the defect. Each defect could be recorded more quickly by use of a copy of the diagrams suggested above. Using the diagram it is only necessary to circle the defective area or location and make a note such as '9 cm crack in weld'. The defect can be marked with erasable pen on the laminated copy (which constitutes the ship's running record of tank defects) and a photo copy of the diagram sent to the owner's office as the defect report.
Observation No.3 above is an example of a defect which must be reported so that owners are fully informed regarding the condition of their ship. Steel wastage, combined with structural fatigue is an increasing concern, particularly in larger vessels. Unless close attention is given to corrosion throughout a ship's life, the risk of structural failure will steadily increase.

2.16.2 Tank inspection checklist
Other equipment in the cargo tank which requires inspection at each opportunity includes:

  • Drain plugs.

  • Ladders.

  • Valve operation reach rods, hydraulic/pneumatic actuator lines.

  • Tank coating.

  • Sacrificial anodes in ballast tanks.

  • Pipelines, including welds, flanges, supports, clamps and coatings.

  • Valves, including actuators, position indicators and glands.

  • Valve suction bell-mouths (for foreign objects/rags).

  • Accumulation of sediment, scale, or sludge.

  • Fixed tank gauging equipment including standpipes, wires, floats and tapes.

  • Fixed tank washing machines.

  • Temperature probes.

  • High level alarms.

  • Deepwell pumps.

  • Heating coils.

  • Bulkheads; examine bulkheads adjoining ballast tanks for any signs of leakage.

  • Rivets; examine all rivetted seams for leaks.
  • Cargo tanks are not frequently gas free, and the chief officer should make the best of every opportunity to verify that a cargo tank and its equipment are in good order. A chief officer who returns frequently to the same vessel can take the opportunity of a shipyard period to photograph the critical equipment in each cargo tank. An album of these photographs is an invaluable reference when problems with in-tank equipment occur.

    2.16.3 Testing heating coils
    Heating coils are prone to leakage and can cause significant contamination of cargo in heated tanks if such leaks are not immediately detected.
    Heating coils should be blown through with air and tested prior to:

  • Loading heated cargo.

  • Changing from low flash to high flash cargo.

  • Changing from dirty to clean product.

  • Carrying out tank repairs.

  • Gas freeing for repairs or dry docking.
  • Whenever the heating coils are placed in service, the vent should be opened and the water which is ejected for the coils (as the steam begins to fill them) tested for oil content. The test can be as simple as the feel and odour of the water. Or a sample can be captured in a glass container, allowed to settle and examined for oil contamination on the surface. If a coiled cargo tank is entered for inspection, the heating coils should be filled with water and enough pressure placed on the coil to provide an effective leak test. Every decimetre of the heating coils should be visually examined. Any leaks detected should be repaired by clamping or isolated until permanently repaired at the earliest opportunity. A tank defect report must be prepared and submitted to the owner's port engineer.
    Heating coil piping above the main deck must also be carefully inspected for condition. Give particular attention to the securing of supply and return lines on deck. Lines must be tightly anchored at each fastening to prevent damage by boarding seas. Steam line deck penetrations must be carefully checked for signs of leaks. Inspections of deck penetrations should include soap testing while the tank is under moderate IGS pressure.

    2.16.4 Cargo and ballast piping tests
    On ships where ballast piping runs through a cargo tank, the ballast piping should be filled with water and placed under pressure during the in-tank inspection. While under pressure, the ballast piping should be carefully inspected for any leaks, especially at couplings.
    If a ballast tank is being inspected and the tank contains cargo pipelines, the cargo lines should be placed under a similar pressure test and closely examined during the in-tank inspection.
    Deck and pumproom cargo lines must be thoroughly washed and isolated before repair work to them is begun. The lines and/or pumps to be repaired must be opened, ventilated, and checked for hydrocarbon and H2S vapours before repairs. Regular checks for hydrocarbons and H2S must be made during the progress of the repairs.

    2.16.5 Tank gauge repairs
    Automatic tank gauge housings may be opened for maintenance/repair and cold repair work carried out without gas freeing the cargo tank. The tank must be isolated from the IGS main and vented to reduce the tank pressure to atmospheric pressure before the housing is opened. Check for H2S before beginning work and regularly until work is complete.

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