Hydrogen Sulphide Without Entering the Space: 29 of 56 Deaths Happened Outside the Tank

The Mistake This Corrects

H₂S is normally taught as a hazard of entering a space. Gard’s message is that it should not be seen only as an entry hazard. It is a release hazard. Crew can be exposed during ordinary work whenever a contaminated system is opened, disturbed or vented. In the IMO review, victims were opening tank manholes, dismantling pumps and pipelines, working near tank vents, and trying rescues. Many had no intention of entering anything.

A Case From The Record

Gard cites the Marshall Islands report on the drillship Valaris DS-17 (2023). A second engineer and a motorman opened a bilge water tank access hatch to ventilate it before cleaning. The second engineer collapsed as he climbed away from the hatch, and both men fell into the tank. Both died despite rescue attempts. The investigation found oily water and residue that could generate H₂S, and concluded that opening the hatch probably released gas into the surrounding pump room before anyone entered.

Why It Hits So Fast

• H₂S is colourless, highly toxic, flammable and heavier than air. It collects in tank bottoms, bilges, pits and poorly ventilated spaces.
• Around 700 to 1,000 ppm can cause collapse after only a few breaths, followed by respiratory paralysis and death within minutes.
• The rotten-egg smell is not a warning: the sense of smell can be lost very quickly.
• It forms when sulphate-reducing bacteria act on organic or oily residues without oxygen. Seawater mixed with sludge, sewage, grey water, fuel residues, food waste, cargo residues or contaminated ballast can all supply the conditions. Gas can stay dissolved in a liquid and be released suddenly when a tank is opened, agitated, pumped, washed or its pressure changes.

What Gard Recommends

1. Treat opening as a release risk. Assess tank openings, pump and pipe openings and the disturbance of residues as potential toxic gas releases, including connected spaces.
2. Assess before opening. Look at the space’s history, contents, storage time, temperature and agitation. Sample through a closed-system point where possible. Do not open or vent until the assessment is done and the controls are in place.
3. Plan the release path. Keep people away from likely release routes, low points and restricted escape routes. If an opening must be cracked for testing, do it a little at a time, from a safe position, with monitoring and emergency arrangements ready.
4. Test continuously with a calibrated multi-gas detector covering H₂S, oxygen and flammables, and retest after washing, pumping, agitation or ventilation changes. Gas testing alone is not enough, because a sudden release can exceed a detector’s range or act faster than it can warn.
5. Control tank contents. Minimise stored oily water, keep galley and sanitary waste out of oily-water systems, and avoid stagnant, oxygen-free conditions.
6. Prepare the rescue without creating more casualties. Do not rush into an untested area. Use suitable breathing apparatus, trained people and retrieval equipment, and treat connected spaces as hazardous.

The Reference List

Gard points to IMO MSC.581(110), the Bahamas Maritime Authority’s Safety Alert 26-01, UK MGN 659 (M+F) Amendment 3, and the AMSA guidance on hazardous gases on vessels. Our own guide to the MSC.581(110) update covers the entry side of the rule.

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Related Reading

• Entering enclosed spaces: the IMO MSC.581(110) update
• The Battery on the Pellets: SELO
• Sunbeam: an RSW tank fatality