Confined space hazards: Plain seawater, an insidious source of hydrogen sulfide

Abstract

In 2022, a confined space entry accident occurred on a Danish product tanker in which two technicians died from hydrogen sulfide poisoning during the inspection of an empty cargo tank that had contained vegetable cooking oil. The source of the hydrogen sulfide was enigmatic. About three weeks before the accident, the cargo tank was prewashed with seawater. The wash water did not seem likely to present a toxic hazard and was left in the tank. However, the seawater’s natural content of dissolved sulfate was converted to sulfide by sulfate-reducing bacteria, and the low-sulfur vegetable oil residue provided the nutrients necessary for bacterial growth. Calculations confirm that sulfate in just 10 m³ of plain seawater is sufficient to create an immediately fatal concentration of H₂S gas in the 4,500 m³ cargo tank of the product tanker. Accident statistics show that fatal accidents within enclosed spaces are a serious and stubborn problem. Strict adherence to routine forced ventilation and extensive gas testing of cargo tanks before entry would offer simple and effective preventive measures.

Keywords:

• Bacteria
• confined space entry
• enclosed space
• hydrogen sulfide
• safety
• sulfate
• sulfate-reducing bacteria

Acknowledgements

The author wishes to thank Mr. Øssur Jarleivson Hilduberg, head of the Danish Maritime Accident Investigation Board, for helpful discussion.

Data availability statement

The case investigation report is available in the public domain.

Disclosure statement

The author declares that he has no conflicts of interest.

Additional information

Funding

The article was produced as voluntary work and has not received any funding. Opinions and/or conclusions expressed are solely those of the author, not of any of his organizations.