Jul 262009
 

A vessel loaded slops from a North Sea installation, believing it to be “grey-black muddy water”, as it was so described on the analysis report, being free from H2S and explosive gas. However, upon arrival for discharge ashore, it was discovered that the slops were contaminated and had a ‘moderate’ H2S content of 25ppm. Further, it subsequently emerged that this was the
fourth such instance in ten days. The health risks to the crew and contractors are obvious.

Safety Flash 09/07 was issued by the Marine Safety Forum on the 1st April on this very issue. Specifically it states that all installations/rigs must complete the agreed analysis and hazard documentation to the vessel before offloading to the vessel commences. Vessels are expected to refuse to take slops until the documentation has been supplied.

Key Lessons:
H2S is an extremely hazardous, toxic compound. It is a colourless, flammable gas that can be identified even in relatively low concentrations (<1ppm) by an aroma of rotten eggs. However, above 30ppm the gas is reported to have a sickeningly sweet odour and at concentrations above 100ppm a person’s ability to detect the gas is affected by a rapid loss of the sense of smell. This unusual property of H2S makes it extremely dangerous to rely totally on the sense of smell to warn of the presence of gas.

Chemical analysis was conducted ten days before the vessel was able to return to back load the cargo ashore. The combination of hot weather and the substantial period of time the cargo spent in offshore storage tanks was undoubtedly the reason for the unexpectedly high concentration of H2S. H2S is a chemical asphyxiant and inhibits cellular respiration and uptake of oxygen (similar to carbon monoxide).

Typical exposure symptoms include:

At low concentrations (0 – 10ppm) irritation of the eyes, nose and throat;

Moderate concentration (10 – 50ppm) headache, dizziness, nausea and vomiting, coughing and difficulty breathing;

High concentration (50 – 200ppm) severe respiratory tract irritation, eye irritation, shock, convulsions, coma, and, in severe cases, death.

Actions Taken:
• Contact Enviroco and establish the best means of neutralising H2S in the contaminated tanks.
• Raise the issue and vessel’s concerns with owners and operators.

The Marine Safety Forum would like to remind ALL ships and our members to refer to the guidelines for oily slops in the NWEA Guidelines (http://www.nwea.info/). Cargoes may be round tripped through lack of time and the importance of pretreating slops to prevent H2S is to be considered.

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