With thanks to Dr Penelope Cooke and Dr Martin Jonas of Brookes Bell for their contributions.
Prolonged waiting periods of vessels carrying Australian coal to China: cargo hazards and precautions while waiting to discharge coal.
The Club has learned that a number of ships loaded with Australian coal are experiencing significant delays in receiving authorisation to discharge in Chinese ports. Commercial and charterparty implications of this have been covered in a recent issue of the UK Defence Club INSIGHTS newsletter of November, 2020 - FAQs: legal issues arising from delays in discharging Australian coal in China.
As coal is a hazardous cargo within Group B of the IMSBC Code, members may be concerned about the effect of such significant delays on the safety of the vessel.
The provisions of the IMSBC Code are mandatory, and 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.
The recommended precautions are:
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)
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.
To monitor potential corrosion damage to the vessel, 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.
Based on past cases, the pH values may be acidic from the outset, or may decrease from neutral to acidic over time. In this event, expert advice should be obtained. If there is a concern about potential corrosion, a thorough inspection of the metal surfaces which have been in contact with the coal cargo or bilge water should be carried out after discharge.
In some 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.
However, 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.