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2.10 TANK ATMOSPHERES AND STATIC ELECTRICITY
One of the pillars of tanker safety is the separation of hydrocarbon
vapours from possible sources of ignition. The first line of defence
in this separation is the integrity of the cargo tanks. With the vapours
contained in a cargo tank, protected by intact bulkheads, decks and
gasketed hatches, there is little opportunity for an accidental source
of ignition to ignite the vapours in the tank. Unfortunately, the process
of tank washing provides opportunities for a source of ignition to occur
inside the cargo tank through accumulation and discharge of static electricity.
To maintain the safest possible tank washing operation, the procedures
must:
Establish and maintain a safe tank atmosphere and
Eliminate accumulation or discharge of static electricity.
With these precautions, a double layer of safety protection is maintained.
If one preventive measure fails, the second will protect the ship against
explosion or fire. If only one preventive measure is used, its failure
could result in a disaster.
2.10.1 Controlling tank atmospheres
After unloading, the empty cargo tanks can contain a range of gas mixtures.
The tank may be stratified, with a too-lean mixture at the top, an explosive
mixture in the middle levels and a too-rich atmosphere at the bottom.
The methods used to eliminate any explosive mixtures from cargo tanks
are:
Reduction of the oxygen level to below the minimum necessary to support
combustion (11% oxygen), ie. creating an oxygen deficient atmosphere,
or,
Reduction of the hydrocarbon concentration to below the lower explosive
limit (LEL), ie. creating a too lean atmosphere.
To monitor the tank atmospheres the ship must be equipped with tank
sampling/testing equipment capable of accurately measuring the concentrations
of oxygen and hydrocarbons. Spare equipment must be carried, along with
suitable calibration kits. Tanker officers must be properly trained
in the calibration and use of tank testing equipment.
Owners must provide detailed operating procedures for tank atmosphere
control, directing how ventilation equipment will be operated, how tank
conditions will be monitored and the tank atmosphere conditions under
which tank washing may proceed.
2.10.2 Maintaining too-lean conditions
To maintain a tank in too-lean condition during tank washing, it must
be thoroughly ventilated to well below the LEL before washing commences.
Washing should not begin until the hydrocarbon vapour concentrations
are below 15% LEL. During the initial ventilation period, the tank atmosphere
will pass through the explosive zone.
It is imperative that no sources of ignition be present near the tank
or the vent exhaust during that interval. While washing with too-lean
atmosphere, a number of precautions must be observed. These precautions
are important for all tank washing using a too-lean atmosphere condition
and are absolutely essential when washing tanks of more than 10,000
cubic meters capacity:
Flush the tank bottom with water and strip dry before washing begins.
Wash only one tank at a time.
Concentrate the vessel's tank ventilation in the tank being washed. If
tank ventilation machinery stops, secure the washing operation.
Ensure tank vapour concentration is below 15% LEL before starting the
washing operation.
Use portable tank washing machines; do not use permanent, high-pressure
tank washing machines. Observe all precautions regarding tank washing
hoses and machines.
Ensure that the stripping system keeps up with the egress of wash water
into the tank.
Wash with clean, cold sea water. Do not recirculate water; do not heat
the wash water; do not add detergents; do not steam the tank.
Measure tank atmosphere frequently during washing (measuring equipment
must have suitable intake filters to prevent internal moisture damage).
• Suspend tank washing if the hydrocarbon vapour concentration exceeds
40% LEL; resume when levels throughout the tank are again below 15%
LEL.
Vent tanks only through the tank venting system. If tanks are vented to
a common header, isolate the tank being washed to prevent entry of gas
from other tanks.
Verify that the expelled vapours are not accumulating dangerously elsewhere
on the ship.
As long as relative winds exceed 10 knots, there is little possibility
of dangerous accumulation of vapours. If necessary, change course to
obtain a relative wind speed of 10 knots. During ventilation and washing,
the same precautions used for loading must be in place, including:
Remove all possible sources of ignition.
Keep accommodation and machinery doors closed.
Minimise the number of tank openings which are open at any one time.
Have the fire fighting system in readiness.
The initial washing operation can be expected to stir up cargo residues
and increase the LEL reading. After one hour of washing, enough residues
will have been removed that there should be little possibility of again
exceeding the 45% level. In any event, tank monitoring should continue
until the washing program is complete.
2.10.3 Inert gas ventilation
If fixed, high pressure tank washing machines are to be used, regular
ventilation techniques cannot maintain a safe atmosphere; inert gas
must be injected into the tank to reduce the oxygen level below that
necessary to sustain combustion. With the tank inerted, it can be safely
washed with high pressure tank washing machines.
A slight positive pressure (not less than 100 mm water gauge), must
be maintained on the tank being washed, to prevent entry of oxygen-rich
atmosphere into the tank. Tank washing should not commence until the
oxygen level is below 5% by volume. The tank must be regularly checked
to ensure that an atmosphere containing less than 8% oxygen is present
at all times. If the oxygen concentration rises above 8%, tank washing
must be suspended and the tank purged with IGS until the oxygen content
is reduced to below 8%.
Some tank atmosphere testing equipment is incapable of accurately measuring
hydrocarbon concentration in an oxygen deficient atmosphere. If hydrocarbon
concentrations need to be measured in inerted tanks, the chief officer
must ensure that his equipment is capable of giving accurate readings
under such conditions.
If there is any reason to doubt that the tank is properly inerted, it
should be considered a non-inerted tank and all necessary precautions
observed.
2.10.4 Tank washing with uncontrolled atmospheres
When it is not possible to control the tank atmosphere in large cargo
tanks, the following precautions must be observed:
Use only low-capacity, portable tank washing machines.
Use a maximum of four machines each having a flow rate less than 35
cubic meters per hour, or not more than three machines in one tank each
having a flow rate of 35 to 60 cubic meters per hour.
Verify tank washing hose bonding wire continuity before hoses are used.
Flush washing hoses with clean sea water before using.
Use only cold, clean sea water for washing; do not heat or recirculate
wash water. If hot water must be used, then tanks should be washed with
cold water first and then ventilated before hot water washing; do not
add chemicals.
If cargo pumps are used to provide wash water, they must be thoroughly
flushed beforehand.
Strip tanks being washed continuously to avoid any build-up of wash water.
Flush and drain all lines before ventilating tanks with fixed tank ventilation
systems after washing.
When draining tank washing hoses do not uncouple the hoses from the washing
main! Hoses must remain connected to maintain the electrical grounding
of the machine and hose while it is in the tank. To drain the hose, loosen
the coupling only enough to let the hose drain; then re-tighten the coupling
while pulling the hose and machine out of the tank.
Remove portable tank washing machines before ventilating tanks.
Whenever tanks are being washed in an uncontrolled atmosphere, the
precautions regarding sources of ignition and static electricity must
be scrupulously observed! All chipping and scaling and any machinery
repairs are prohibited on the main deck while tank washing or ventilation
are in progress. All door and port openings onto the main deck must
be closed.
2.10.5 Static electricity while tank washing
During a three week period in 1969, a series of three supertanker explosions
sank one ship and took several lives. All three explosions occurred
while centre cargo tanks were being washed. The two vessels which did
not sink required extensive, costly repairs. The exact cause of the
explosions was never determined, but the most probable cause was accumulation
and discharge of electrostatic charge while tank washing. The investigations
which followed found that large electrostatic charges could develop
while tank washing due to:
Commencing washing with large accumulations of oil remaining in the
tank.
Adding chemical agents to the tank washing water.
Recirculating tank washing water.
Washing large cargo tanks (greater that 10,000 cubic meters).
Static electricity, or more properly 'electrostatic charging' can be
produced during tank washing operations by:
Accumulation of tank washings in the cargo tank. The oil/water mixture
will be subject to charge separation and may produce an accumulated
electrostatic charge.
Spraying or splashing of oil. Crude oil washing will produce electrostatic
charging in the cargo tank. Water washing of a tank which has not been
adequately drained may also cause charge accumulation.
Use of conductive ullage tapes to measure tank bottoms during tank washing.
Permanently installed ullaging equipment should be used to verify correct
stripping during tank washing. Wait one hour after tank washing (if artificial
ventilation is used), before sounding a washed tank, five hours if only
natural ventilation is used. Tanks may be sounded in less time if a natural
fibre line and wooden sounding rod are used. No precautions are necessary
if sounding inside a metal sounding pipe.
Floating conductors in the tank. Dropped or lost sample cans which remain
afloat in the tank do not accumulate charge, but can produce a path for
the short circuit of an accumulated charge from the surface of accumulated
tank washings to the tank structure.
Steaming of cargo tanks. If cargo tanks are steamed using hoses which
are not properly bonded to the ship's piping, an electrostatic charge
will accumulate on the hose. The water droplets from the steam will accumulate
electrostatic charge and may cause an incendiary spark if the tank is
steamed too vigorously. Therefore tanks should be steamed only with fixed
apparatus, only at low velocities and only when the tank is free of any
grounded probes (including properly bonded tank washing machines).
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