<img style=”margin-left: 5px; margin-right: 5px;” title=”safespace” src=”http://maritimeaccident.org/wp-content/uploads/2011/04/safespace.png” alt=”” />Sarah Knapton of the Telegraph could have been writing about a maritime tragedy of the 21th century, so familiar are the details: A ship’s officer falls into a confined space, those who attempt to rescue him are overcome by fumes, or more probably oxygen deficiency, and themselves become victims. The ship was HMS Lennox in the 17th century.
Knapton’s article concerns a theory by engineer and naval historian Richard Ender that another vessel, HMS London, exploded and sank in 1685 following the ignition of methane in the bilges. The methane was, he suggests, generated by the fermentation of seafarers’ faeces in the bilge. This suggestion was inspired by a report of the HMS Lennox incident :”a lieutenant accidentally fell into the bottom of the hold and when crew members climbed down to rescue him ‘they were rendered in a manner dead by the stench’”. Enders believes the culprit was methane, a gas familiar as the major component of liquid natural gas, LNG, possibly ignited by a seafarer carrying a lamp down into the bilge to relieve himself.
Other historians believe that the HMS London incident was the result of poor practices when storing gunpowder: Another lesson in the danger of sloppy practices brought on by doing things on the cheap. Let’s accept for the moment that the gas concerned was methane.
In order to explode, Methane requires oxygen. If there’s too much oxygen, methane cannot ignite, if there’s too much methane, then, again it won’t ignite. There must at least 5 per cent methane by volume mixed with air for it to ignite, known as the lower flammable limit or LFL. There is also an upper flammable limit, the highest percentage at which methane will ignite – don’t be lazy, do a search for “methane” and “upper flammable limit”*.
Methane is heavier than air so tends to ‘pool’ in the bottom of bilges and other spaces.
The other gas we have to worry about is oxygen. Normal atmosphere is around 21 per cent of the air we breathe, much less than that and you will die. Technically speaking, the level of oxygen can be as low as 18 per cent, and a minimum of 19.6 per cent is sometimes assumed to be safe, but these are not good working minima: if there’s less than 21 per cent then something else might be making up the balance, and that something might be deadly. Never enter an enclosed space where the atmosphere is less than 21 per cent. When taking measurements, test the bottom, middle and top of the space and make sure you check either side of the access point, measurements taken too close to the access point may be unreliable.
What level of oxygen was present in the hold of HMS Lennox? Let’s look at what happens at various stages of oxygen deficiency.
Between 11 and 18 per cent oxygen physical and mental performance is reduced, usually without the suffer being aware. From 8 per cent to 11 percent fainting can occur within a short period while levels of less than 11 per cent carry a high risk of death. Suffers who fall unconscious at levels of 6 per cent to 8 per cent can usually be revived if rescued in time while below 6 per cent unconsciousness is almost immediate and brain damage may occur in those who are revived.
It’s a fair assumption that the oxygen level that the seafarers were exposed to in the hold of HMS Lennox was somewhere from around 6 per cent to 8 per cent for them to become unconscious.
Since the lieutenant was probably at the bottom of the hold the seafarers attempting the rescue almost certainly moved from the top of the hold, where there may have been an acceptable level of oxygen, to the lower part of the hold where oxygen had reduced to a level that was potentially deadly, at least capable of causing unconsciousness.
Much the same occurred with tragic consequences aboard Viking Islay in 2008.
Now we can make a rough guess at the level of oxygen, 6 per cent to 8 per cent, we can work out how much methane might have been present. The reference to a ‘stench’ indicates biological decomposition which suggests methane. In other cases the degradation of molasses has produced carbon dioxide while warmed coconut oil has produced carbon monoxide, leading to deaths and injuries in both cases, neither gas is explosive.
If the oxygen level was 6 per cent to 8 per cent then the absolute upper level for the amount of methane present would have been 92 per cent to 94 per cent, but the methane would have been diluted by other gases somewhat but possibly producing a relatively thick ‘band’ of air in which the mix fell between the upper and lower flammable limits for methane. Anyone entering that band with a source of ignition, like a lantern or candle could have ignited the methane.
If conditions aboard HMS Lennox were typical of the period, and there is reason to suppose they were, then Ender’s theory regarding the fate of HMS London is not out of bounds.
Although the 17th century was, shall we say, rather robust, the lessons of the HMS Lennox incident remain as relevant today: Ensure that a confined or enclosed space is fully ventilated and tested before entry; keep safety and rescue equipment close to the point of entry, including SCBA; maintain a safety watch with communications with the bridge; follow the procedures laid down in the SMS; adhere to the permit to work system.
Tempting and natural though it is, if a seafarer is seen to be in trouble in a confined or enclosed space, consider your own safety first. Alert the bridge. If alone, wait for someone else to come before attempting a rescue. Do not enter the space unless wearing SCBA, do not attempt to enter unless you know how to wear SCBA and use it correctly.
Ensure that drills are carried out to rehearse confined/enclosed space rescue and do not use the same confined or enclosed space for every drill, pulling someone from, say, a ship’s laundry room is likely to be very different to pulling someone from an anchor cable locker.
Your ship’s Safety Management System will identify places that your company nominates as confined or enclosed spaces and precautions to be taken. Do not regard that list as complete. Look around your own vessel and the places you work: Is it difficult to move around? Then it’s a confined space. If there is little or no natural ventilation then it is an enclosed space. If in doubt, regard it as an enclosed or confined space regardless of what is in the SMS.
Think about how you would rescue someone with minimum risk to your own life.
Death in confined spaces remains a class of tragedy that seems to defy resolution. Even more sadly, they too often involve multiple deaths or injuries as crewmates try to rescue someone without taking appropriate precautions to preserve their own lives.
Your life is important to your family, don’t waste it.
*Okay, it’s 15 per cent.