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

• Published in: Journal of occupational and environmental hygiene Vol. 20; no. 8; pp. 1 - 328
• Main Author: Hedlund, Frank Huess
• Format: Journal Article
• Language: English
• Published: England Taylor & Francis LLC 03.08.2023
• Subjects: Accidents; Bacteria; Cargo; Confined spaces; Cooking oils; Fatalities; Hydrogen sulfide; Inspection; Nutrients; Seawater; Sulfate reduction; Sulfate-reducing bacteria; Sulfates; Sulfur; Tankers; Technicians; Toxic hazards; Vegetable oils; Vegetables; Wash water; enclosed space; sulfate; hydrogen sulfide; safety; confined space entry; Bacteria; sulfate-reducing bacteria
• ISSN: 1545-9624; 1545-9632
• DOI: 10.1080/15459624.2023.2211638

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 m of plain seawater is sufficient to create an immediately fatal concentration of H S gas in the 4,500 m 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.