A hidden chemical interaction in a vessel’s bilge system can turn routine cargo and cleaning operations into a serious toxic-gas hazard.
A hydrogen sulphide (H₂S) was detected by portable gas detectors in an open area of the main deck during a vessel’s transit back to port. The gas was traced to certain bilge vents, with the suspected source linked to mud loaded for a rig. No personnel were injured or suffered ill health, but the incident exposed gaps in bilge-system cleaning, chemical compatibility assessment and H₂S monitoring.
What happened
During the vessel’s return to port, portable gas detectors detected H₂S in an open area on the main deck, specifically around certain bilge vents.
The affected area was isolated and continuously monitored, while personnel working on the main deck and in the engine, room were equipped with portable gas detectors.
Sludge and bilge disposal services were arranged for the vessel’s arrival in port.
There were no injuries or reported health effects.
The suspected source of the H₂S was the mud that had been loaded for the rig. However, the incident was not attributed simply to the presence of the mud. Several residues and conditions within the bilge system had created an environment in which H₂S could form during the voyage.
What went wrong
The bilge system had not been flushed following previous chemical handling or before the mud was loaded. This left reactive residues in the system.
The investigation identified a procedural gap: the vessel’s cleaning procedures did not require mandatory bilge flushing following tank cleaning or chemical transfers, even where multiple chemicals had been handled.
Several substances were present in the system, including brine, polymer and black water. At the same time, anaerobic conditions in bilge tanks can support processes that generate sulphides.
The risk assessment also lacked an integrated assessment of chemical compatibility and the possible interactions between residues from different operations.
Another important point was that the fixed gas detection system did not provide an early warning. The contamination developed later during the transit, meaning the fixed system did not alarm when the initial operations were taking place.
How can H₂S form?
KCl polymer mud and CaCl₂ brine do not, by themselves, produce H₂S. The risk arises when other conditions and substances are present.
Bilge water can contain organic matter and anaerobic bacteria capable of generating sulphides. Drilling mud residues may also contain sulphide minerals, such as iron sulphide or sodium sulphide, depending on the formations encountered.
If sulphides are present and the pH subsequently falls into acidic conditions, H₂S gas can be released.
Several factors can therefore contribute to the hazard, including:
- sulphides in drilling mud or bilge water;
- stagnant bilge water and anaerobic bacterial activity;
- acidic contamination, including CO₂ ingress or acid cleaning fluids; and
- higher temperatures, which can accelerate chemical reactions.
This means that a substance that is not inherently an H₂S generator can still become part of a chain of conditions leading to H₂S formation.
The big picture
The incident illustrates why chemical risk assessments need to look beyond individual substances and consider what happens when residues from different operations remain in the same system.
Bilge systems can contain a mixture of water, organic material, cleaning products and chemical residues. When the vessel subsequently loads or handles another substance, the interaction between these residues may not be immediately obvious.
The absence of an initial gas alarm should also not be interpreted as confirmation that the system is safe. In this case, the contamination developed during transit, after the relevant operations had already taken place.
The IMCA lessons therefore focus not only on detecting H₂S once it appears, but on preventing the conditions that allow it to develop.
What could have reduced the risk?
The vessel updated its standard operating procedures to require bilge flushing after chemical handling and before mud loading.
Chemical compatibility should also form part of the toolbox talk, with a specific checklist used to identify risks from reactive residues before work begins.
Cleaning arrangements were expanded so that the bilge system is included when tanks are pressure washed, rather than treating the bilge system as separate from the wider cleaning operation.
The vessel also introduced periodic H₂S checks in bilge tanks during transit and reinforced crew training on chemical interactions and the risks associated with anaerobic conditions.
For operators, the incident highlights the importance of asking not only whether a chemical is hazardous on its own, but what residues may already be present in the system and what conditions could develop later.
Ask your team
- Is bilge flushing mandatory after chemical handling and before loading a different product or mud?
- Does the risk assessment consider the combined effects of residues from previous operations?
- Could stagnant bilge water create conditions for H₂S generation during a voyage?
- Are H₂S checks carried out when contamination could develop after departure?
- Do crew understand how changes in pH, temperature and anaerobic conditions can affect chemical residues?
Key takeaways
- Ensured vessel standard operating procedures include mandatory bilge flushing after chemical handling and before mud loading.
- Chemical compatibility review: Implemented a checklist for reactive residue risks, to be done during the t toolbox talk.
- Extended cleaning scope to bilge system when tanks are pressure washed.
- Added periodic H₂S checks in bilge tanks during transit.
- Crew training: Emphasise chemical interaction risks and anaerobic conditions in bilge systems.