2.17 CLOSING AND SECURING CARGO TANKS
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
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. Covers to tank cleaning openings.
The following should be checked for tightness before the conclusion
of the ballast voyage:
Cargo tank access hatch covers.
Sampling and gauging port covers/caps.
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
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