VLCC recently reported a near miss
One of our VLCCs recently reported a near miss. The Master, on his way to the wheelhouse to conduct an emergency drill, became aware of a strong smell of hydrogen sulphide (H2S) gas.
On arriving on the bridge, he was told that due to a sudden rise in the pressure in one cargo tank, the chief officer had ordered the mast riser to be opened briefly. In violation of SMS procedures, the mast riser had been opened without Master’s permission or issuing any warnings to the crew.
At the time, the true wind was blowing from astern at a velocity of just under the ship’s speed, so a very light relative wind was blowing from right ahead. The toxic gas left the riser and settled on the deck, entering the open intakes of the engine room fans and the accommodation heating, ventilation and air-conditioning (HVAC) system.
Despite plenty of sea room, no seamanlike precaution was taken to alter course to ensure that the vented gas would be blown clear of living spaces while permitting A/C fresh air intakes to be adjusted to full recirculation mode. Fortunately, as the vessel was passing through a known piracy threat area at the time, only one accommodation door was open to the deck, thus limiting the intake of H2S to the living quarters.
Root cause
Non-compliance with risk assessment procedure for venting cargo tanks during navigation.
Lessons learnt
- The human element can never be discounted. Despite being aware of the dangers, frequent onboard training sessions and the adoption of best industry practices by the company, a temporary lapse of concentration/omission of procedures by a senior officer seriously compromised the safety of all the crew;
- Post-incident investigation also revealed poor apprecia-tion of hazards associated with venting of H2S-rich vapours by some of the ship’s crew, despite a recent company circular relating to a previous similar incident on another company vessel, which resulted in the hospitalisation of two crew members during the venting of a cargo tank containing high sulphur crude.
Corrective/preventative action
A further circular was transmitted to the fleet on the latest incident, highlighting the dangers and the importance of following correct
Hydrogen sulphide has a very low odour threshold, with its smell being easily perceptible at concentrations well below 1 part per million (ppm) in air. The odour increases as the gas becomes more concentrated, with a strong smell of rotten eggs recognisable up to 30 ppm. Above this level, the gas is reported to have a sickeningly sweet odour up to around 100 ppm. However, at concentrations above 100 ppm, a person’s ability to detect the gas is affected by rapid temporary paralysis of the olfactory nerves in the nose, leading to a loss of the sense of smell. This means that the gas can be present at dangerously high concentrations with no perceptible odour. This unusual property of hydrogen sulphide makes it extremely dangerous to rely totally on the sense of smell to warn of the presence of the gas.
H2S concentration (ppm) | Potential effect |
10 to 20 | Eye irritation, especially in hyper-susceptible workers. |
20 to 100 | Inflammation, corneal blistering and the capacity of the eye, loss of the sense of smell, headache, cough, nausea. |
100 to 300 | Respiratory difficulty, pulmonary edema, respiratory depression and irritation (30 min-8 hrs). |
300 to 600 | Central and peripheral nervous system effects, eg tremors, weakness, numbness of extremities, unconsciousness and convulsions (several minutes – hrs). |
600 to 1000 | Rapid breaths, unconsciousness resulting in death if emergency aid is not promptly administered. |
1000 and greater | Cessation of breathing (instantaneous) and death. |
Table of potential effects on health of H2S at various concentrations
Source: Mars/Nautical Institute