Coal Cargo Heating: Rising CO in No. 2 Hold
Your ship is a 32,000 dwt bulk carrier loaded with 26,000 t of Indonesian steam coal in five holds. Loading finished 30 hours ago at Taboneo anchorage in intermittent rain; hold 2 was trimmed with coal that had been lying in the stockpile exposed to rain for two days. All hatches are closed and secured. You are the deck rating on watch and take the daily readings: hold 2 carbon monoxide 130 ppm and rising (it was 40 ppm 12 hours ago), hold 2 temperature 36°C against an outside ambient of 27°C, hold 4 CO 15 ppm and 26°C. Relative humidity is 85% and the forecast is for more rain. Methane readings are 2% LEL at the hold 2 sample point and 0% elsewhere. The chief officer is ashore at the agent's office and the master asks you for the readings.
1. What do the hold 2 readings tell you, and what are the two separate hazards you must consider?
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Two separate things are happening in No. 2 hold, and they need different responses.
Self-heating. CO has gone from 40 to 130 ppm in 12 hours and the hold is at 36°C against 27°C ambient. Coal oxidises, consuming oxygen and giving off CO and heat; a hold temperature above ambient together with a rising CO trend is the standard early warning. Wet coal from the rainy stockpile makes it worse, because moisture promotes heating.
Methane. 2% LEL is about 0.1% methane by volume: low, but not zero. Coal gives off methane continuously and it collects at the top of the stow and in voids. The explosive range is 5–15% by volume, so 2% LEL is roughly one-fiftieth of the lower explosive limit - a monitoring number, not an emergency.
No. 4 hold (15 ppm, 26°C, only 1°C above ambient) looks normal. That pair is your baseline: measure the same points, the same way, at the same time each day, and record it.
2. Should No. 2 hold be ventilated? Explain the reasoning for each gas and for the weather forecast.
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No. 2: do not ventilate, and keep the hatches closed. Surface ventilation is the textbook answer for methane, but once self-heating is suspected, every cubic metre of air pushed in supplies oxygen that accelerates the oxidation and the heating. With the CO trend rising and the hold hotter than ambient, the priority is to starve it, not feed it. The rain forecast reinforces this: damp air adds moisture to the coal.
No. 4: methane at 0% and a normal temperature - no action beyond routine monitoring.
The general rule is that you cannot ventilate a coal hold to remove methane and simultaneously stop self-heating; you must decide which hazard is dominant and record why you decided it. Note that heating and methane can both be present in the same hold, so the decision has to be reviewed at every set of readings, not made once. Inform the master, who informs the company and, if the trend continues, arranges diversion or shore assistance. Do not open the hatch to 'check'.
3. What are the dangers of opening No. 2 hatch, and what should be done next?
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Opening No. 2 would:
- let in air and speed up the heating, possibly to a hot spot or a fire deep in the stow;
- release methane around the hatch, where a hot-work spark, a cigarette or a non-intrinsically-safe torch could ignite it;
- release CO, which is odourless and lethal at a few hundred ppm;
- put a person on top of a coal stow that can collapse or form a cavity.
What should happen next: sealed sampling through the hold's gas sampling points, not the hatch; readings at fixed intervals recorded in the log; all hot work, smoking and naked lights prohibited on deck; fire hoses and fixed firefighting arrangements ready; master to assess the situation under the IMSBC Code coal schedule and the ship's emergency plan; if CO keeps climbing and the hold keeps heating, prepare to head for the nearest port. Treat the readings as a developing emergency, not a daily chore.