Back to Main Page ----- Back to Chapter 2


Case study
Following a trans-Atlantic passage, a chemical tanker with 0.72 meters freeboard arrived at a UK port with one tank of acetic acid contaminated by sea water. A careful examination determined the probable cause to be entry of sea water through a broken temperature probe fitting in a deck penetration. A large part of the following investigations concentrated on when the probe was last known to be operational, when the damage was noted and whether the master had physically conducted a survey of the main deck himself prior to departure from the loading port.

Case analysis
This case demonstrates the importance of checking every opening into the cargo tank to ensure that the tanks are 'tight and staunch'. ('Staunch' is a word derived from the middle English word 'stanch' which meant 'water tight'.) The chief officer or master must make a careful, detailed and diligent inspection of the main deck, examining every possible means of sea water entry or cargo escape, and using appropriate tests and checks to confirm that the tanks are oil tight from within and water tight from without.

When all tank entry personnel and equipment have been removed from the cargo tank and the chief officer has completed his inspection, the tank must be securely closed up. Each of the tank openings in the deck must be closed oil, water and vapour tight before the next cargo is loaded. This is part of the duty of the owner and master to ensure proper care of the cargo.
The following should be checked for tightness before the conclusion of the ballast voyage:

  • Covers to tank cleaning openings.

  • Cargo tank access hatch covers.

  • Sampling and gauging port covers/caps.

  • Pressure/vacuum valves.

  • Inert gas line valves or blinds.

  • Inert gas line check valve.

  • Cargo hatches (on bulk carriers).

  • Covers to any tank instrumentation opened in the course of repair/inspection.

  • Covers to automatic tank gauging installations.

  • Valves and blanks on direct loading lines.

  • Blanks in COW lines.

  • Temperature or pressure probe fittings.
  • If any of these tank openings are not tightly closed, they offer a means of cargo escape, or sea water entry and cargo contamination. Any liquid or vapour cargo leakage will cause a loss to the cargo owner, and liquid leakage may cause a pollution incident with heavy fines and adverse publicity for the ship owner.
    Each tank should be checked for proper closure using a checklist, which can be dated, signed and retained to demonstrate due diligence. The most positive way to verify tightness of the tank fittings is by applying pressure to the tank with the IGS system and then brush liquid soap on the edges of the openings. Any leaks will be indicated by escaping air forming soap bubbles.
    Tank cleaning openings should be carefully cleaned on the gasket face before replacing the cover. Rubber 'O'-ring gaskets should be carefully inspected. Fibre gaskets should be replaced with new gaskets. A thin application of a proprietary non-seizing compound aids removal of both gaskets and the securing nuts. Cap nuts are best for this application since they prevent water from entering the threads. The nuts should be hand tightened to the recommended torque. The chief officer should spot check nuts for tightness. On a parcel tanker, a careful chief officer will check every nut for tightness himself, due to the high values of the cargoes and the ease with which they can be contaminated by salt water leakage.
    Cargo tank hatch lid gaskets should be examined before closing. Do not attempt to seal a hatch lid with a defective gasket by applying extra torque. Excessive torque may permanently deform the hatch lid. Replace defective gaskets. Leaking tank hatches are one of the two most significant causes of voyage losses in high RVP cargoes.
    Pressure/vacuum valves should be check every three months. Record the date and time of the inspection, the condition found and detailed actions taken if a valve is found defective. Poor P/V valve tightness is the second greatest cause of significant voyage vapour losses.
    Gauging/sampling port covers receive heavy use and frequently are found in poor condition. Defective gaskets should be replaced by the crew. The ship should carry a couple of spare covers so that the defective one can be removed to the workshop to do the job properly. If the cover itself is defective, replace it and order another spare.
    Covers to any tank gauging devices or tank instrumentation cabinets opened for inspection or repair should receive new fibre gaskets when being closed up. Rubber gaskets should be carefully examined and replaced if necessary.

    page top
    Hosted by uCoz