An Inert Gas System (IGS) is mandatory safety equipment on tankers, delivering low-oxygen gas into cargo tanks to keep the atmosphere below the oxygen threshold needed to support combustion.
Why Tankers Specifically Need This
Crude and product tankers carry cargo that produces flammable vapor — as tanks are discharged, loaded, or cleaned, the risk of an explosive atmosphere forming is genuine and has caused real historical tanker disasters. IGS exists specifically to keep tank atmospheres inert (below roughly 8% oxygen, well under the ~11% typically needed to sustain combustion) throughout these operations.
How It Works
Inert gas is commonly generated from the ship’s boiler exhaust (flue gas, naturally low in oxygen after combustion) or from a dedicated inert gas generator, then scrubbed, cooled, and delivered into cargo tanks via a dedicated piping system — maintaining slight positive pressure to prevent air (and its oxygen) from entering.
When IGS Is Used
- During cargo discharge — replacing the volume vacated by departing cargo with inert gas rather than allowing air to enter
- During tank cleaning — including Crude Oil Washing, keeping the atmosphere inert throughout
- During voyage — maintaining positive inert pressure in tanks to prevent air ingress
Oxygen Monitoring
Continuous oxygen content monitoring is a core, mandatory part of IGS operation — crew must verify the system is actually maintaining safe oxygen levels, not simply assume it’s functioning correctly, since IGS failure during cargo operations creates a genuine explosion risk.
Exam Relevance
IGS operation, oxygen level requirements, and its role in preventing tank explosions are core, heavily-tested Tanker Operations and Safety topics in deck and engineering officer competency exams.