According to the UK Club
The Club strongly advises all members to familiarise themselves with and follow the detailed requirement in the schedule for COAL, in particular as they relate to gas monitoring, ventilation and pH monitoring of the bilges.
In addition, the Club issued recommendations for vessels loading Australian coal. Accordingly:
Gas monitoring (self-heating/methane emission hazards)
- The main hazards of the shipment of COAL are that it may self-heat and it may (also) emit methane. Monitoring of the levels of oxygen, carbon monoxide and flammable gases (methane) via the gas ports is the method for assessing these hazards. During the lengthy delays, we would suggest frequent gas monitoring is conducted and the results recorded and reported.
- Compared to other countries of origin (e.g. Indonesia), Australian coal is not generally associated with a significant risk in either of these respects. However, coal from each mining location may differ.
- During a long delay, slow oxygen depletion is likely in unventilated cargo spaces for all coal cargoes. This is not on its own an indication of a significant self-heating hazard, unless accompanied by high and/or rapidly increasing carbon monoxide.
- The most common type of gas meter employed on ships uses catalytic sensors to detect flammable gases (methane). This type of sensor does not function correctly in holds with depleted oxygen, and this may give rise to spurious high methane readings. Methane readings in holds with less than about 10% oxygen should be conducted either with a gas meter equipped with an infrared sensor for flammable gas, or using a “splitter”-type attachment that augments the oxygen levels in the gas supply to the sensor. If in doubt, seek expert advice.
pH monitoring (corrosion hazard)
It is stated that the water draining out of some coals can be highly acidic, and may cause corrosion to uncoated metal surfaces including tanktops, sounding pipes and/or bilge systems. The risk of acidic drainage may vary significantly between coal mines. It is not known at this stage if any specific Australian mines present a higher risk in this respect than others.
Concerning monitoring, the IMSBC Code requires all vessels carrying coal to be equipped to measure and record the pH value of cargo space bilge samples. Regular bilge testing must be systematically carried out during the voyage and during any waiting periods or delays. If pH monitoring indicates that a corrosion risk exists, bilges should be frequently pumped out during the voyage to avoid possible accumulation of acids on tanktops and in the bilge system.
The Club reminds that in previous long-delayed coal cargoes, significant localised corrosion in the form of flat peaks and deep valleys over the tanktop surface was observed after discharge, most likely through pools of acid water under stagnant and stationary conditions rather than as a uniform attack on the metal over the surface. The bare tanktops appeared to be most affected in areas where water was able to settle (aft and hold sides), and this may vary depending on the trim and the stationary nature of being at anchorage. Under normal voyage conditions the motion of the sea may inhibit the formation of these stagnant pools of acid.
A further consideration for vessels at anchorage is that depending on the distance from shore, they may not be permitted to the pump the bilges,
... UK Club adds.
They concluded that reported cases of significant corrosion are relatively rare, and at this stage it is not known if there is an increased risk of corrosion for the vessels currently waiting off China with Australian coal.