2.15 TANK ENTRY, CLEANING, AND INSPECTION
The job of entering, inspecting and cleaning ship's cargo, ballast,
and fuel tanks, void spaces and cofferdams is essentially one of confined
space entry. The five basic requirements for safe confined space entry
are: The space has adequate oxygen.
The space of free of toxic vapours or substances.
The tank entry personnel are properly trained.
A written procedure is being followed (checklist used).
Means of rescue and trained rescue personnel are immediately available.
The following procedures refer primarily to cargo tank entry, but
the precautions apply to all confined space entry activities
on board a tanker. All spaces which are considered 'confined spaces'
for manned entry must be prominently labelled as such at each point
2.15.1 Tank ventilation
When tank washing is completed, the cargo tank will contain an atmosphere
which is not safe for men. If the tank has been ventilated during washing,
to maintain a too-lean atmosphere, it may still contain more hydrocarbon
vapours than are permitted for 'bare-face' entry. If the tank has been
inerted during washing, it will have both insufficient oxygen and excessive
hydrocarbon vapours for entry.
Tanks which have been ventilated during washing are easier to prepare
for entry. Ventilation is continued after washing is completed. The
ventilation should be arranged so that the an efficient exchange of
tank atmosphere is achieved.
Ventilation by displacement
The most effective method of ventilating a tank is by extracting the
heavier hydrocarbon vapours from the bottom of the tank, while allowing
fresh air to enter from the top, displacing the tank atmosphere.
This method can be used with either fixed or portable equipment as indicated.
The advantage of the displacement method is that only a little more
than one air exchange is required to ventilate the tank, saving both
time and energy. Because it achieves the quickest results, the displacement
method is also the safest, since the tank atmosphere will be in the
explosive range for the shortest amount of time.
Ventilation by displacement:
heavier tank atmosphere is extracted from the bottom of the tank -
allowing fresh air to flow in from the top. A single air exchange
usually clears the tank of inert gas or hydrocarbon vapour.
Ventilation by dilution
The alternative method of ventilation (and the one most commonly used
on smaller tankers), involves blowing air into the top of the tank at
one end and forcing the tank atmosphere out a deck opening at the other
end of the tank. To be effective, an extension trunk must be used so
that the air enters the tank at the bottom. The old (hazardous) atmosphere
is removed by dilution. The corners of the tank most distant from the
ventilation fan will take a considerable amount of time to reach a safe
condition. If the tank is large, with deep bottom framing, the framing
bays distant from the fan may retain dangerous pockets of hydrocarbon
vapour after the remainder of the tank passes the test for safe tank
The least efficient method of tank ventilation involves blowing air
into the top of the tank without extension trunking. Dilution is very
slow with this method and dangerous gas pockets are likely to remain
in the distant corners of the tank. This method should be used for small
Ventilation by dilution:
air flow in the tank requires several air changes before the tank
atmosphere is diluted to a safe level.
The atmosphere leaving the tank must exit as an unimpeded vertical flow.
If air is being forced out of a tanklid, the tanklid should be fully
open. Tank atmospheres should not be exhausted under a catwalk or other
structure which would promote accumulations of dangerous vapour at deck
level. Generally, a relative wind of more than ten knots will prevent
any accumulation of vapours, but care must be taken when ventilating
tanks near the superstructure if a relative wind from astern may produce
a lee where the ventilation is taking place. Sections 16.4 and 16.5
of the International safety guide for oil tankers and terminals
(ISGOTT), provides excellent guidance for tank venting.
IGS system ventilation
Inerted tanks can be ventilated in the same manner as non-inerted tanks,
but doing so will at some point produce an explosive atmosphere within
the tank. This situation can be avoided if, after washing, the tank
is first purged with inert gas to remove hydrocarbon vapours. With the
hydrocarbon vapour level below 2% (by volume), the tank may then be
ventilated with outside air without the tank atmosphere passing through
the explosive range. (Note that a conventional Combustible Gas Indicator
cannot be used to measure hydrocarbon levels in an inerted tank.)
It must be remembered that while purging with IGS, an explosive
atmosphere can accumulate on the open deck of the ship. In addition,
IGS may contain nitrogen dioxide, sulphur dioxide and carbon monoxide,
all of which are toxic. It is important that the precautions regarding
relative wind speed and direction are applied to the IGS purging.
If the tank atmosphere measurements show 9% hydrocarbon
gas and 6% oxygen (point X) and the inert gas has 3% oxygen and (of
course) 0% hydrocarbon gas (point Y), then the tank atmosphere will
change along line XY as purging progresses.
Consequently we can estimate the change of the tank atmosphere while
purging and can stop the purging when the atmosphere is below the critical
dilution line and then gas freeing can start. In the example if
the purging was stopped at point Z (3% hydrocarbon gas and 4% oxygen)
then during gas freeing of the tank the atmosphere will change along
line ZE to 21% oxygen and 0% hydrocarbon gas and always be outside of
the explosive range.
When ventilating a cargo tank, tank openings of adjacent tanks should
be closed to prevent entry by hydrocarbon vapours from the tank being
ventilated. All accommodation openings should be closed and windward
side only used for access.
2.15.2 Testing tank atmospheres The test equipment is properly calibrated.
When the tank has been ventilated sufficiently to provide a safe atmosphere
for safe bare face entry, it must be tested using a combustible gas
indicator (CGI) and oxygen meter. The results of such a test will be
valid only if:
The operator is properly trained.
Correct test procedures are used.
The gas indicator is suitable for testing the atmosphere which is being
The Combustible Gas Indicator (or 'Explosimeter'), should be calibrated
before each ballast voyage. To do this efficiently, the ship must have
a calibration/test kit suitable for the cargoes it carries
and an officer properly trained in the calibration procedure. The test
equipment must have the appropriate scales to indicate safe entry levels.
Two indicators should be carried, so that a spare is available in the
event one is damaged. If a calibration kit is not provided, then the
ship will need three indicators, one for immediate use, one as a spare
and one ashore being professionally calibrated. If a calibration kit
is not provided, the ship must be provided with a gas detector (chemical
absorption) tube tank testing set which can be used to verify the
results being obtained by the electronic combustible gas indicator.
The important concept is that the ship is provided with some means of
verifying that the gas indicator in use is functioning correctly. A gas reading of 1% of the lower explosive limit (LEL), or less is obtained
by gas sensor measurement at all areas where gas concentrations are
The author once served as chief officer of a product tanker on which
he found four combustible gas detectors and no calibration kit. Two
of the indicators were broken, one was out of calibration giving low
readings and one functioned properly but had a contaminated filter!
None of the indicators were marked or tagged to indicate their condition!
Three indicators were sent ashore for repair and calibration and a calibration
kit and spare sampling filters were ordered. The purchasing department
responded by asking why the author was requesting services and equipment
which no other ship had previously requested!
For tank testing to be effective, the chief officer must be fully trained
in the operation of the test equipment and in the techniques for tank
testing. Tank testing equipment should only be purchased if instruction
materials and training videos are available in the language of the officer
who will be using the equipment. Chief officers should be trained in
the same tank testing procedures as that used by chemists certified
to issue gas-free certificates.
Chief officers must be aware that 'catalytic filament' type combustible
gas indicators are not suitable for measuring inerted atmospheres because
the indicator relies on the presence of oxygen in the sample to accurately
measure hydrocarbon levels. Inerted tanks do not have enough oxygen
to permit proper operation of filament type CGI's.
When the chief officer believes that a tank is safe for entry, the complete
kit of rescue equipment must be moved to the tank access hatch and set
up. No personnel should enter the tank until the rescue equipment and
personnel are ready in all respects and the preparation checklist is
complete. The tank is first tested using the long sampling hose for
the combustible gas indicator. In using the long sampling hose, make
sure that it is lowered close to, but not touching, the tank bottom.
Coloured tape or paint on the hose should be used to indicate the appropriate
length. The chief officer must also know how many times to squeeze the
aspirator bulb, or how long to run an indicator with a powered fan,
to draw a sample of the tank bottom atmosphere into the testing chamber.
Testing must continue until the reading stabilises. If the readings
are low, the expanded scale must be used. When the tank has been found
to contain safe hydrocarbon vapour levels, it must then be tested for
oxygen content. If the last cargo was a crude with high hydrogen sulphide
content, the tank must also be tested for H2S
To determine the safety of a tanks atmosphere, the chief officer must
also know the Threshold Limit Value (TLV) of the cargo last carried
in the tank. The guidance provided by ISGOTT should be followed in evaluating
the tank atmosphere.
Entry into tanks should not be permitted until testing has demonstrated
that the following limitations are met:
A maximum benzene concentration of 10 ppm as measured by chemical absorption
tube indicator (Draeger or equivalent).
A maximum hydrogen sulphide concentration of 10 ppm (subject to ongoing
regulatory review and reduction) as measured by a chemical adsorption
tube indicator (Draeger or equivalent).
An oxygen concentration of 21% or more by volume as measured by an oxygen
Maximum hydrocarbon vapour concentration of less than the threshold limit
value (TLV), - Time weighted average for the actual mixtures to which
personnel will be exposed, as measured by a chemical absorption tube indicator.
(Draeger or equivalent).
Any other limitation indicated in the material hazard data sheet for the
product last carried in the tank.
Tanks which were inerted before ventilation may contain traces of toxic
gasses. Vigorous ventilation should remove all such gasses, or reduce
their concentrations to below their TLV concentrations. If ventilation
is questionable, then the tank should be tested for:Maximum carbon monoxide of less than 50 ppm as measured by chemical
absorption tube indicator (Draeger or equivalent).
Maximum nitrogen dioxide of less than 3 ppm as measured by chemical absorption
tube indicator (Draeger or equivalent).
Maximum nitric oxide of less than 25 ppm as measured by chemical absorption
tube indicator (Draeger or equivalent).
Maximum sulphur dioxide of less than 2 ppm as measured by chemical absorption
tube indicator (Draeger or equivalent).
The tank should be tested for gas every two or three hours. An alternate
member of the entry team should conduct the tests to confirm the readings
obtained by the first inspector.
Portable hydrogen sulphide and oxygen alarms, with audible alarm, should
be worn by team members in tanks to provide the earliest warning of
any change in conditions.
2.15.3 Tank entry precautions Space(s) to be entered.
When the tank to be entered has been tested and found to have a safe
atmosphere at the places tested, the tank entry team is assembled
and checked off. Tank entry training should have been conducted for
each member of the team and each crew member must be familiar with his
duties. Each member of the tank entry team is assigned a particular
duty in checking the equipment and in entering the tank. The tank entry
and rescue equipment is checked off using a checklist, which is dated
and signed by the chief officer and the rescue team leader. The tank
entry checklist should include (but may not be limited to):
Names of personnel entering the confined space.
Expected duration of in-tank work.
Protective clothing and equipment of the in-tank workmen, especially if
leaded gasoline tanks are to be cleaned.
Condition and operability of the equipment/bucket hoist.
Condition of the rescue breathing apparatus (2 sets minimum).
Condition of any Surface Air Breathing Apparatus (SABA), unit if used,
especially the masks, hoses and hose connectors.
Closure/securing of inert gas line valve(s) to the tank(s).
Closure/securing/isolation of cargo line valve(s) and controls to the
Condition/readiness of the resuscitation equipment.
Condition of the tank atmosphere testing instruments.
Continuous operation of tank ventilation equipment; method of warning
in-tank personnel if failure occurs.
Closure, labelling and lashing/sealing of all valves to the tank being
Tank cleaning buckets, scoops and brooms.
Operability of any pumps to be used in the tanks and their associated
air and liquid discharge lines.
Condition/operability of battery lights or in-tank, air-driven lighting.
Correct function of portable radios to be used for communications between
the tank entry crew and the deck/bridge watch.
Condition of all lines and hardware to be used for lowering/ hoisting
equipment into/from the tank.
Complete set up and readiness of rescue tripod and equipment.
Proper fit of entry team safety harnesses.
The tank entry leader (usually the chief officer) then enters the
tank with the combustible gas indicator and the oxygen analyser (and
H2S detector if required), stopping to test
the tank atmosphere at each level as he descends into the tank. The
tank entry leader wears a tended lifeline until his initial survey is
The tank must remain under continuous ventilation during tank entry
operations and until all work in the tank is completed and all personnel
When the tank atmosphere has been proven safe to the bottom of the tank,
each framing bay must be sampled and proven safe before the tank can
be declared gas-free. Any framing bay with heavy sludge or scale accumulation
may contain a higher concentration of hydrocarbon vapour, so every bay
must be tested before work in the tank can begin.
Enclosed (confined), space entry safety checklist
After the tank work has begun, the tank atmosphere must be retested,
both for hydrocarbon and oxygen concentrations, at least hourly and
after every smoke or coffee break. If the tank is not absolutely clear
of hydrocarbons, (if the hydrocarbon level is 100 to 300 ppm), change
the ill-tank crew and deck support/rescue team crew members after each
break. In addition to this precaution, at least two of the in-tank workers
should carry belt-mounted hydrocarbon and oxygen deficiency sensors
which alarm when unsafe conditions are detected.
Tank entry is one of the shipboard activities which should be conducted
according to a 'Work Permit System' and a shipboard gas-free 'certificate'
issued just as if the ship were in port. Indeed, it would seem more
urgent to issue a gas free 'certificate' at sea, for the seamen are
entitled to the same safety precautions as shore workers and are a lot
further from skilled medical help in the event of an accident!
2.15.4 Work in tanks
At sea, only 'cold work' should be performed in cargo or bunker tanks.
The tank must be gas freed to below the TLV or to 1% LFL, whichever
is lower and care must be taken to monitor the gas level in the area
of the work.
Cold work may include: Lifting sludge or scale from tanks,
Chipping or hammering,
Cutting (with hacksaw or file), or drilling, Use of air driven power
Hot work is not permitted at sea and any work involving the following
is prohibited:Welding, soldering or flame cutting.
Grinding or grit blasting.
Fire of any kind.
Use of electrical equipment without an explosion proof or intrinsically
Personnel entering tanks should be equipped with: Hard hat.
Hearing protection (if needed).
Safety boots with non-slip soles.
Full length clothing or coveralls.
Flashlight or hand lantern.
Activated 12-hour light stick fastened to the shoulder.
30-minute light stick in a pocket (not activated).
10-minute escape air packs.
Belt mounted gas-oxygen sensor/alarms (on two men).
A full set of breathing apparatus, a fire extinguisher and an intrinsically
safe UHF radio should be maintained in the tank with the work party.
2.15.5 Tank cleaning/de-scaling Scooping scale and filling buckets.
The most common reason for tank entry is to permit a work party to remove
loose scale and sediment from the tank bottom. This operation requires
sufficient men to perform the functions of:
Passing buckets to the hoisting point.
Hooking on/off full/empty buckets.
Operating the bucket hoist.
Dumping full buckets overboard or into containers and returning them to
A team of seven or eight men will be required to do this work efficiently,
particularly if the tank is large. If the ship has a regular descaling
program, the accumulation in any tank should not be significant and
the cleaning of a single tank can normally be completed in one day.
Tanks to be de-scaled must be thoroughly bottom washed with hot water
before ventilation and entry. This is done for two reasons. First, the
scale tends to hold hydrocarbons, particularly light product and will
release these hydrocarbons when disturbed. If any gas is given off by
the de-scaling (or 'mucking') operation, the tank may quickly become
unsafe for men and the work will have to be suspended until the tank
is again washed and ventilated. Secondly, it is illegal for scale or
sediment lifted from cargo tanks to be discharged at sea if this material
contains oil. Any sediment or scale containing oil must be drummed and
retained on board for disposal ashore.
Scale/sediment should be dumped overboard on the side opposite the engine
room intakes to prevent any of the disposed material being drawn into
the ship's machinery.
Efficient safe set-up for tank cleaning to remove
sediment and scale. Equipment set up for the de-rusting operation.
1 Men engaged in lifting scale
and sediment. 2 Bucket into which scale is being shovelled.
3 Full bucket being hauled to deck. 4
Single tail block with gantline. 5 Man to handle buckets
(sometimes two required). 6 Depositing contents of
full buckets overside. 7 Wood chute (metal lined).
8 Pipeline. 9 Suction valve. 10
Suction valve control wheel (lashed or sealed closed). 11
Drums for oily scale.
The condition of the lift wire, hook and air hoist machinery should
be inspected hourly by the chief officer to ensure that it remains free
of defects. Each bucket should be inspected before use to verify that
the handle and attaching points are in good condition. The man hooking
on buckets at the bottom of the tank should not remain under the buckets
while they are being hoisted. One of the responsibilities of the bucket-man
is to verify the condition of each of his comrades between bucket lifts.
This is facilitated by the light-stick fastened to each man's shoulder,
which makes it easy to count the men in the darkness of the tank.
As the work proceeds across the bottom of the tank, the hoist and ventilation
must be moved to remain above the work party. Movement of the ventilation
should be by first setting up and starting a second blower before the
first one is stopped. Two blowers should be in use at all times, one
where the men are working and one covering their next work area. The
movements of the hoist and blower are coordinated by use of the UHF
While working in the tank, each member must remain alert for signs of
hydrocarbon exposure, either in his behaviour or in the behaviour of
others. Excessive hilarity, loss of coordination, slurred speech, or
loss of sense of responsibility are signals that the tank cleaning team
should evacuate the space. A gas-free tank is only gas free at the time
it is tested, the operation of removing accumulated scale can release
fresh hydrocarbon vapours which quickly make the tank atmosphere unsafe.
Constant ventilation and alertness are essential.
At the end of the tank cleaning operation the equipment is recovered
to the deck using the hoist. The chief officer makes a final inspection
with one attendant watching him from the lowest platform of the ladder.
When the chief officer is satisfied that the tank is suitably cleaned
he returns to the deck and verifies that all the equipment which went
into the tank has been accounted for by using the entry checklist.
Tanks which have carried numerous cargoes of leaded gasoline are a special
problem. The tetra-ethyl-lead compound which is added to leaded gasoline
is extremely toxic. Men assigned to de-scale such tanks must wear full
disposable coveralls, organic vapour respirators and rubber gloves.
TEL is readily absorbed through the skin and all contact with scale
or sediment from leaded gasoline tanks must be avoided. After tank cleaning
the coveralls and gloves must be disposed of.
2.15.6 Repairs in tanks
In-tank repairs usually involve the pipelines, valves or valve operators,
or the fixed tank gauging equipment. Men working above the tank bottom
must work from adequate staging to provide secure footing. If repairs
to the tank bulkheads, sounding pipes, or valve actuator lines are required
above the tank bottom, adequate ladders must be provided (fibreglass
rather than aluminum) and properly secured. Men working on ladders or
staging must wear safety harnesses with safety lines properly secured.
2.15.7 Entry to non-gas free or low oxygen tanks
In most cases, tanks can be ventilated sufficiently to provide for 'bare
face' entry, where a safe atmosphere in the tank permits entry by personnel
without breathing apparatus. In some cases, it is not possible to gas
free the tank or space for entry. Such cases should be exceptional and
men should not be asked to enter unsafe tanks unless no reasonable alternative
exists. The work to be completed should be essential to the safety of
The requirements for entry to tanks containing a dangerous atmosphere
must be strictly observed and only fully trained personnel should be
permitted to make the entry.
The basic requirements for entry are: All entry personnel wear breathing apparatus and a lifeline.
Each lifeline is tended by an individual outside the space.
For each man in the tank there is a man standing by outside the tank wearing
a self contained breathing apparatus.
A complete check of all equipment is made prior to tank entry and recorded
on a checklist.
A meeting is held with all tank entry and support team members to discuss
the duties of each man and the exact procedure to be followed. Use a tank
Each tank entry person must wear an activated light stick and carry a
Escape pack breathing apparatus sets should be used.
Signals to be used for tending lines must be understood and rehearsed.
The established tank entry permit procedure has been completed and a valid
Ventilation is provided to the maximum extent possible.
The types of tank entry equipment available include fresh air breathing
apparatus, self contained breathing apparatus, and surface air breathing
Fresh air breathing apparatus is the oldest and simplest type, using
a hand powered pump to provide air by hose to a full-face face piece.
They are easy to use, but require the same care and maintenance as any
safety equipment. On one occasion, the author was at the bottom of a
gasoline tank, standing over one meter of gasoline, when the inside
liner of his air hose began to peel off in flakes and accumulate in
the mask. On another occasion, (this time in a heating oil tank), the
air hose connection to the face piece snapped off at the bottom of the
Self contained breathing apparatus provides a completely portable supply
of air and is excellent where mobility is relatively unimpeded and minimum
set up time is important. A pressure gauge and alarm on the unit indicate
the amount of air remaining.
SABA, or air line breathing equipment permits extended in-tank work
and provides better mobility in confined spaces. The air supply is provided
by a battery of compressed air bottles ('cascade system'), or by an
air compressor with compressed air bottles as backup. Escape packs should
be worn with air line units. The maximum length of hose is 90 meters.
The minimum number of personnel should be sent into the space. If the
entry route is complex, it may be necessary to station one or two additional
personnel at points inside the tank so that the workers can be kept
under continuous observation.
2.15.8 Tank rescue operations
The crew attending a tank entry team must be competently trained in
confined space rescue procedures and have all the necessary rescue equipment
prepared at the tank site before tank entry operations begin. If the
tank entry team in a gas-free tank is attended by a single crew member,
he must be strictly instructed not to enter the tank, but to pass the
word (by radio) and wait for assistance from the rescue team if the
in-tank workers encounter difficulties or appear to be incapacitated.
No crew member should ever enter a tank to assist another crew member
apparently overcome by vapours unless the rescuer is equipped with a
tended safety line and breathing apparatus.
The most dangerous tanks are those with deficient oxygen. Such spaces
are never entered bare-faced and the provision of a tended rescue line
permits rapid rescue if the man in the tank is injured. This is important
because a man without breathing apparatus in an oxygen deficient tank
will die in two to three minutes.
In a tank with an adequate supply of oxygen, personnel incapacitated
by hydrocarbon vapour inhalation can remain for some time without harm
as long as they are breathing. The following times to death are indications
| Time until death in a tank with
adequate oxygen and:
High concentration of petroleum vapour
Medium concentration of petroleum vapour
Low concentration of petroleum vapour
If the rescue equipment is ready, there is plenty of time to rescue
men overcome in a tank where cleaning operations have generated a low
concentration of hydrocarbon vapour. At least three men are needed for
a rescue. The rescue team must organise itself, correctly don its equipment
and then descend into the tank with the necessary rescue equipment.
They must learn from the tank attendant how many men are in the tank
and where they were last working. The chemical lights worn by the in-tank
workers will aid greatly in locating them.
If the victims are breathing
If the vapour concentrations are low, the rescue team may be able to
rouse the men in the tank. These men should be fitted with tended lifelines
to assist them in climbing out of the tank.
If the vapour concentrations are medium or high, rescue equipment will
be needed and the men will be removed by rescue harness or by stretcher.
If the affected personnel have not activated their escape sets (ELSA
sets), they should be activated and placed over their heads. Unconscious
victims may react violently and will probably vomit when recovering
consciousness. A close watch must be maintained as men revive, especially
if a hood or face mask has been fitted.
If the victims are not breathing
Any men found not breathing should be removed first if resuscitation
cannot be given in the tank. Any of the victims who have fallen should
be removed next by stretcher. The remaining unconscious personnel can
then be removed by stretcher or harness.
The necessary first aid measures following removal of affected personnel
are not within the scope of this text.
It goes without saying that rescue of personnel from the distant areas
of a cargo tank will only be successful if it has been diligently practiced
during emergency drills. Without such practice, what might have been
a successful rescue could become only an effort ending in futility and
Crew positions and lifting arrangements for a