Member Advisory - SKULD
Applicability: Shipowners and Charterers
Members will be aware of the issues involving self-heating and methane emitting coal cargoes,
which have recently been originating from Indonesia. The logistics of dealing with coal cargoes
apply to all regardless of origin.
The Association is grateful to Messrs. Brookes Bell for assistance with this update.
"Problems during the voyage and at discharge"
Once coal is loaded and assuming it complied with the carriage requirements of the IMSBC
Code (the “Code”), i.e.,
"This cargo shall only be accepted for loading when the temperature of the cargo is
not higher than 55°C"
Then the monitoring and management of the cargo during the voyage becomes important. Even
if the cargo was loaded with temperatures below 55°C, this does not mean that problems will
not start during the voyage.
As temperature sounding pipes are located within a hold, but at the periphery, any temperature
readings can only provide a general indication of the temperature of the cargo near the pipe and
will provide no information concerning what the cargo temperatures are elsewhere in the hold.
Consequently, gas measurements are the preferred and most reliable method for cargo
monitoring during a voyage.
Access to the headspaces via the gas sampling ports enables assessment of the gas levels
without opening the hatch covers so that oxygen (O2), methane (CH4) and carbon monoxide
(CO) levels can be reliably monitored. It is also extremely important to know which gases are
being checked and why:
Oxygen (O2) –a self-heating coal should show decreasing O2 levels. Without O2 self-heating
cannot occur.
Carbon monoxide (CO) – this gas is produced as a result of self-heating/combustion occurring
at low oxygen levels. This gas has no smell and is a ‘silent killer’ because it binds to
haemoglobin in the blood leading effectively to suffocation.
Methane (CH4) – is a gas released by some coals, so an increasing level indicates the coal is
emitting CH4 and needs to be closely monitored. If the concentration increases to reach 20% of
the lower explosive limit (LEL), ventilation is required.
It should be noted that most of the commonly used multi gas meters encountered onboard ships
will not be reliable for hydrocarbons or other flammable gases at low oxygen levels - in fact they
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will not work properly below about 12% O2 values. This is because most are actually
combustion sensors and need oxygen to function to produce reliable hydrocarbon/flammable
gas readings.
Gas monitoring during the voyage:
Changes in the gas concentrations will indicate whether self-heating/combustion, or methane
emission, is taking place. All vessels intending to carry coal are required to have appropriate
gas monitoring equipment for the duration of the voyage and it is important that this equipment
be in good working order with an up-to-date calibration certificate prior to the commencement of
loading. Since gas monitoring is a requirement of the Code for coal and some related cargoes, it
is very important to know how the ship’s gas meter works BEFORE being faced with an
emergency situation. Please see first advisory for further details on gas meters.
During the voyage
Having loaded a Code-compliant coal, the shipment should be monitored for gas variations
during the voyage. The Code indicates that if there is no declaration of any hazards such as
self-heating or methane emission, then the holds should be ventilated for the first 24 hours.
During this time, the gas in the headspace of the holds should be monitored and this will require
the ventilation to be stopped for a suitable period (the Code recommends this is not less than
four hours) prior to the gas readings being taken. Assuming no methane is detected or if it is, it
remains at very low levels, then the holds should be sealed and gas monitoring continued.
Self-heating coals
Self-heating/combustion will be evident if high (and increasing) CO levels are detected in
combination with decreasing oxygen levels. If the CO increases but the oxygen does not
decrease, then this indicates that the holds are not sealed effectively. If necessary, Ramneck
tape and expanding foam should be used to assist with the sealing of the holds, but these can
only be used as a last resort because cargo holds are not designed to be gas tight.
The Code indicates that CO levels above 50 ppm are an indication that the cargo may be selfheating. Even when sealed and oxygen supply is restricted, the CO levels in the headspace
above a self-heating coal could exceed 500 ppm (some gas meters cannot measure levels
above this, so the actual value may be higher). The CO levels should 'level out' as the oxygen is
used up and as long as no methane is detected (or if the methane levels remain below the limits
in the Code, see next section), then the holds should remain sealed until each hold is ready to
discharge.
If gas levels indicate self-heating is taking place the only thing which can be done is to limit the
amount of oxygen available in the holds. Since self-heating/combustion requires oxygen, if the
holds are sealed then the oxygen level should fall to the point where further combustion is
prevented from occurring.
Methane emitting coals
Methane and other flammable gases are often emitted from coals. The reaction of coal with
oxygen results in self-heating/combustion and can also release flammable gases from the coal
possibly generating an explosive atmosphere, although in our experience this is quite rare.
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In the reaction between methane and oxygen, twice as much oxygen as methane is required for
the “perfect match”, and since the oxygen content of air is about 21% by volume, then the
amount of methane required for the most violent reaction is around 10%. Mixtures of between 5
to 15% methane in air are explosive mixtures. While sealing the hatches and stopping
ventilation should control or restrict self-heating/combustion, if the coal starts emitting methane
or other inflammable gases, then these may require removal in order to prevent development of
a potentially explosive atmosphere. In this situation, the only option would be to open the vents
but we would strongly recommend seeking expert advice in this situation.
To assist with safety assessments, it is important to know what the lowest explosive limit or LEL
is for a given gas mixture. For methane, the LEL is 5%, i.e., the lowest amount of methane
mixed with air that can be explosive. However, for safety reasons, the IMSBC Code COAL entry
states that the amount of methane in a hold, which is considered as the trigger point for action
by the master, is 20% of the LEL - therefore this is actually equivalent to only 1% methane in air
(volume by volume). The gas concentrations should be monitored and a record kept, since their
levels will determine the advice given by an expert. If oxygen levels drop and stay low, even if
methane levels are high, then there is unlikely to be an explosion. However, if methane is above
20% of LEL, then ventilation may be advised.
After Coward H.F. and Jones, G.W. (1952). Limits of flammability of gases and vapours; US
Bureau of Mines, Bulletin 503: 1-155.
Hazard management – which to choose?
Explosion – stop methane build up - Ventilate
Combustion – restrict oxygen access - Don’t ventilate
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COAL fires on ships
Usually fires in coal are small and rarely become 'raging infernos'. However, they do tend to
produce lots of smoke and toxic gases. If the coal or related cargo appears to be on fire adding
lots of water to a hold fire is not a practical option. The added weight of the water may damage
the ship by over stressing the hold plating (stresses and bending moments will need to be
considered). Equally impractical is that to discharge a hold full of dirty, contaminated, and
potentially acidic water is going to be difficult and expensive due to environmental concerns.
Unless all the cargo is fully submerged, it will not usually put the fire out.
However, water can be used to cool the grabs used for the discharge and help control the
smoke by dowsing the smouldering/burning areas so that once the majority of the cargo is
removed, flooding the remaining amount to extinguish the fire might be an option. Bulldozers
and their drivers cannot be put into a hold to move the cargo to allow access for the grab if the
air or the cargo is too hot, the air in the hold is lacking oxygen, and/or has unsafe carbon
monoxide levels. It is likely to be very impractical if not impossible to find stevedores who are
trained to operate and discharge cargo in full breathing apparatus.
Coal and related cargoes may produce acidic liquids which may corrode metal structures. It is
therefore necessary to check the pH of any water in bilge wells.
If the ship is still at sea, the main logistical challenge will be to stop oxygen entering the holds.
An alternative option may be to use inerting gas such as carbon dioxide or nitrogen but these
may not be available or may not be available in sufficient quantity. Seek expert advice if
considering the use of inerting gases. Boundary cooling of the external parts of the hold will at
least cool the coamings and deck surface and so assist with access. If sealing the holds to
starve the fire of oxygen does not work, the only option will be to discharge the cargo.
Discharging a 'problem' Coal cargo.
The main problems associated with discharging a self-heating cargo of coal are associated with
the fact that even for a small localised area of smouldering, burning coal, a considerable amount
of smoke will be generated. This smoke accumulates in the headspace of the hold and will be
released en-masse when the hatch covers are opened thereby appearing to make the situation
worse than it really is. In our experience this frequently causes undue concern with the
Receivers and has resulted in some coal terminals refusing to discharge the cargo despite the
fact that it was the hatches being opened which introduced fresh supplies of oxygen into the
cargo holds to support continued self-heating/combustion. We recommend therefore that the
hatch covers are only opened when there is agreement to discharge the cargo directly. Until
that point, they should remain firmly sealed such that exposure to oxygen is restricted.
The best way to deal with such a situation is to have the area of smouldering or burning cargo
removed but obviously this requires the area to be accessible and for it to be safe, in terms of
carbon monoxide levels, to enter the hold to remove it. Localised spraying of fresh water to
dampen down such areas is recognised as a temporary solution but excessive use of water
should be avoided since any significant increase in the moisture content of the cargo will reduce
the calorific value of the coal somewhat. In addition, if the hot area isn’t removed but remains
exposed to oxygen then it is likely to re-ignite within a matter of hours or days. Any water used
for firefighting should be fresh water rather than seawater if at all possible since Receivers rarely
tolerate chloride contamination as it can potentially cause corrosion to the power station boilers
when the coal is ultimately burnt.
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In terminals where the coal is discharged to conveyor lines, Receivers and Terminal Operators
may be reluctant to put hot coal onto their belts for fear of damaging them i.e., melting the
rubber, or worse, setting fire to them. Since coal fired power stations are dependent upon a
continuous supply of coal, damage which renders the conveyor system inoperable for any length
of time will be of real concern to the Receivers, despite the likelihood of such damage occurring
being remote. In addition, the use of water in firefighting can also affect the cargo handling
properties of some coals and when wetted, they may become 'sticky' and difficult to discharge
onto conveyor lines.
Once discharge of a self-heating coal cargo commences it is important that the holds are
discharged completely and in a timely manner since part discharged holds have a larger
reservoir of oxygen in them with which to feed the self-heating reactions in any remaining cargo
stow. This can cause ignition or re-ignition of 'hot spots' within the stow. Obviously whether or
not the Terminal suspends discharge for any reason is likely to be beyond the control of the
Master and crew but there have been a number of cases where part discharged cargo holds
ignited whilst the vessel waited at the anchorage for completion of discharge. If such a situation
arises, all that can be done is to attempt to restrict exposure of the cargo to oxygen as much as
possible by closing all the ventilators etc., and for the monitoring of the headspace gases to
continue. Providing the vessel has done all that it can to prevent self-heating within the stows by
restricting exposure of the cargo to oxygen throughout the voyage, then ignition of the cargo
during discharge, if it does occur, is beyond the control of the vessel and simply due to the
inherent nature of the coal. Unfortunately though, this will not always prevent claims against the
vessel.
Finally, Receivers may also take issue with the smoke produced from burning coal, partly as this
may restrict stevedore access to the holds, and partly due to the environmental impact. Care
should be taken to ensure that once discharge starts (in a problem coal) that it is completed, and
that partly discharged holds are not left open.
When discharging hot, methane emitting coal and related products, additional care will be
required because the opening of the hatches will allow oxygen to enter the holds and when
mixed with the methane could potentially result in an explosion. Ensuring there are NO ignition
sources around the holds and the hatch cover wheels are greased should reduce the possibly of
sparks igniting the methane and oxygen mixture. Seeking expert advice will be critical prior to
starting the discharge process.
Initially, it may be suggested that the gases are vented off via the hatch cover vents however this
allow oxygen to enter the holds. Equally, it may be advised that an inerting gas is pumped into
the hatches just prior to opening. This will reduce the risk of an explosion occurring by flushing
out the flammable gases via a second open vent. This should only be considered with expert
advice.
Having fire fighting personnel on standby may be required to both assist in the event of an
incident as well as to spray the cargo with water once it is onshore. Spraying a small amount of
fresh water onto the area of concern may enable this patch to be removed by the grabs.
However, once the discharge has reached the limits of the grabs, then the gas levels may
restrict personnel e.g., the bull dozer driver, from entering the holds.
Dave Myers, Fire investigator,
Penelope Cooke, Cargo scientist and
Nick Crouch, Cargo scientist
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