Years ago if you wanted to import bananas you had two choices. You could pick the bananas when they were still green and un-ripened so they would arrive a week later by ship in a semi-yellowed state but still tasted un-ripened. Your second choice would be to pick them at the peak of ripeness and have them delivered soft and near rotting. Refrigeration helped but you still could not get your product to the consumer at its peak flavor.
Someone discovered that the ripening process needed oxygen for it to take place. If you somehow removed the oxygen the bananas could be picked when ripe and shipped in a compartment where the oxygen was removed. The bananas would not ripen much more and would be delivered at their best flavor.
This is done today on container ships and break-bulk carriers that carry produce. The usual manner the oxygen is removed from the cargo hold is by the introduction of nitrogen or other inerting gas. An inerting gas simply pushes the oxygen out of an area and then takes its place. Actually the inerting gasses are usually non-toxic in themselves however, the lack of oxygen will kill anyone who enters the area without an SCBA.
Aboard ship there are many areas that have similar hidden dangers of oxygen deficiency which some crew members may be unaware. I think that we all would have serious doubts about entering a newly emptied cargo tank aboard an eight hundred foot gasoline tanker. However, would you also be concerned about entering the chain locker in the bow of this same vessel where only the anchor chain is stored? After all, what kind of a danger could all that nonflammable chain present?
What happens to the outside of the metal chain as it sits in the damp chain locker?
The metal will start rusting. We all remember our training from your Basic and Advanced Firefighting that rusting is a form of oxidation. And oxidation is a burning which consumes oxygen. The oxygen acts with the wet iron or steel and a pyrolysis reaction takes place. This “slow burning” gives off some heat but the metals mass and the surrounding atmosphere dissipate most of that. Our problem arises in the fact that this oxidation is using up the available oxygen that we need to survive. If the compartment in question has no inlet and outlet to allow a circulation of fresh air, we can have an oxygen deficient atmosphere.
Other areas aboard ship which may contain dangerous levels of gasses or reduced oxygen are the ballast tanks (photo left) and other areas within the hull. These areas are generally sealed and not normally occupied. Accumulations of water, sewerage, and cargo leakage may cause a hazardous atmosphere of either toxic gasses or a lack of oxygen.
Normal air contains twenty one percent (21%) oxygen. As this level is decreased, the danger to anyone entering that space increases. It becomes a hazard when the level drops below 19.5%. The progression is as follows:
|19.5%||Minimum acceptable oxygen level.|
|15 – 19%||Respiration increases. Poor judgment.|
|12-14%||Decreased ability to work strenuously. Impair coordination.|
|10-12%||Respiration increases. Lips blue|
|8-10%||Mental failure. Fainting. Nausea Unconsciousness. Vomiting.|
|6-8%||8 minutes – fatal, 6 minutes – 50% fatal, 4-5 minutes – possible recovery.|
|4-6%||Coma in 40 seconds. Death|
A response to a fire or emergency aboard ship should mandate the inclusion of oxygen meters as well as gas meters in your tool list. Any area aboard ship that is not normally occupied or entered should be considered suspicious.
Most vessels are made of steel. All of these metal surfaces are subject to the oxidation of rusting that we previously discussed. (Just ask the ships deck hands who are constantly kept busy chipping and painting the rusted areas.) Air samples must be taken at all levels of the compartment prior to entry. If you do not have the proper sampling equipment ask a ships officer if they have these meters.
Generally all ships, but especially tankers, will have these meters on board. Regardless of whether the meters are available or not, ONLY QUALIFIED, CONFINED SPACE CERTIFIED personnel should ever enter any space considered a confined space. If you are in doubt as to whether an area is a confined space then…..
“IT IS A CONFINED SPACE!”
My training programs address all of the dangers, which can be encountered at a ship fire or an emergency during your watch. Gas inerting, Oxygen Deficient Atmospheres (in addition to the one discussed here), explosive atmospheres, entrapment dangers, electrical dangers (440 – 880v), and the list goes on. Placing your crew into fire and emergency situations without proper indoctrination and training is unsafe.
Join Tom at the International Work Boat Show in New Orleans on October 10, 2013. Tom will make his presentation from 11 AM to 12 Noon on work boats and Liquefied Natural Gas (LNG). Or you can just stop by to say hello.
He is currently a participating member of the Society of Naval Architects and Marine Engineers (SNAME) Fishing Vessel Operations and Safety panel and also their Small Working Vessel Operations and Safety panel. Tom is also a Principal Member of the NFPA Technical Committee on Merchant Vessels.
His articles on Marine Firefighting have been published both nationally and internationally.
Tomâ€™s company Marine Firefighting Inc. is involved in consulting and training mariners and land-based firefighters in all aspects of marine firefighting. Visit his web-site at MarineFirefighting.com or E-mail Tom at MarineFires@aol.com