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Ship Types and Hazardous Gases
The gas hazards on board will vary depending on the ship type and the likely gases that will be present. Some toxic gases such as hydrogen sulphide (H2S) will be potentially present on all ships as they may be created by the sewage plant on board. It is important for crew and officers on all ship types to be aware of the dangers of gases and crucially know how to detect them.
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Conventional Tankers
Oil tankers have obvious potential gas hazards from the cargo in the form of petroleum fumes as well as other toxic mixtures present within the cargo. This makes gas detection critical and the regulations from SOLAS for construction do necessitate robust built-in detection methods.
The detection of hydrocarbons, flammable gases, and in many cases the O2 content is required by OCIMF, Oil Companies International Marine Forum as well as SOLAS. Hydrogen sulphide detection is often also requested by charterers and terminals. The areas covered by built-in detectors are normally adjacent to, above, and below the cargo pumps and the ventilation outlet airshafts where hydrocarbon gases are likely to accumulate. The cargo and ballast pump rooms on tankers then require fixed gas detection systems.
Classification rules state that on oil-carrying tankers the double-bottom ballast tanks surrounding the cargo tanks must be fitted with fixed gas sampling measuring flammable gases and in some cases oxygen levels. According to new rules, two suction points for gas testing in each ballast tank that is adjacent to a cargo tank are required. Many existing vessels feature only one point, in the bottom part of the tank. Local rules and oil terminals may also require the hydrogen sulphide levels to be monitored inside the tanks.
As a rule, fixed gas detectors will be present on these vessels in the following areas:
- Compressor rooms, cofferdams, pipe tunnels, and other void spaces adjacent to tanks carrying dangerous cargo,
- Cargo tank insulation areas,
- Accommodation areas, ship entrances, ventilation inlets, and engine room and boiler areas.
On LNG vessels specifically, various methods of detection are normally utilised to handle the different areas in which gas monitoring is required. A combination of sampling and pipe extraction systems and single-point detection is used to ensure that the gases can be detected in the case of a spill. This is due to the chill nature of the cargo that will cause any leaked gas to sink to the bottom of spaces, causing a very dangerous, explosive atmosphere that may not be readily apparent to personal detection methods.
Enclosed Spaces on All Ship Types
IMO dictates that gas levels are to be monitored before entry into any enclosed space. The limits are:
- 21% oxygen level.
- Not more than 1% of the Lower Flammable Limit, LFL, is indicated.
- Not more than 50% of OEL of any toxic vapours and gases.
- OEL: Occupational Exposure Limits, or Long Term Exposure Limits. The OEL limit for toxic gas like H2S is often 10 ppm.
- Persons entering the space should be provided with calibrated and tested multi-gas detectors. Ventilation should be continuous during such work.
- The attending crew should not enter the space in case of an emergency until help has arrived and the safety of the rescue crew has been evaluated.
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