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2019 August
Applying OR Methodology to a News Item: The Search for a More Effective Approach to Fighting Fires in Cathedrals

Applying OR Methodology to a News Item: The Search for a More Effective Approach to Fighting Fires in Cathedrals

Part 1: Initial Thoughts

Along with much of the world, I was shocked on 15 April to see the entire roof of Notre-Dame de Paris engulfed in fire. In particular I was shocked by the apparent feeble effect of the fire-fighting effort. (That statement is not intended as a criticism of the fire- fighters who could be seen to be doing their utmost; in particular special mention to the firefighters who fought bravely in the tower to prevent the fire talking a hold there.)

I did, therefore, begin to wonder whether it would be possible to apply OR-type methodology to consider whether there is an alternative approach. This does seem to be the sort of thing where OR people excelled in the early days – coming up with radical solutions to major problems, in the days before OR had silos, before OR became a silo.

The ideas presented here may be nonsense, but I have an idealistic notion that the great advances of OR, in the early days, often started with an idea that was nonsense, but which was then developed into something both workable and effective.

Ideally this would be considered by a multidisciplinary team, but I have had to consider this on my own – although I have identified the expertise that would be needed to consider this more fully (if this were to be taken forward).

The Problem (or Why the Current Approach Doesn’t Seem to Work)

The problem, as observed by me from a few minutes of coverage on the news, is that it requires a great deal of effort to get water to the height of the fire; it then falls to the ground – thereby spending a minimal time at the location of the fire. It was also reported that, once the water has fallen through the fire, it tends to cause much damage to the fabric, art work and general artefacts within the building. Particular reference was made to this consequence of the 1984 fire at York Minister.

Exploring Possible Alternatives

Since the issue (at least at first sight) seems to be the use of water, the obvious alternative is to use something else to fight the fire. My first thought was to pump carbon dioxide into the building – to remove the supply of oxygen (apparently this is termed ‘hypoxic air fire prevention’) – however, I have seen carbon dioxide described as being inappropriate for fighting wood fires (water is recommended).

One issue with carbon dioxide is that it is heavier than air and therefore may tend to settle at ground level and leak out of the building at ground level. (For a roof fire, the objective would be that the substance will leak through the roof – i.e. through the fire.) For this reason I turned my attention to nitrogen – nitrogen is 78% of air, and has much the same density.

This option (pumping an inert gas into the building) utilises the cavernous nature of a cathedral – since this allows the free flow of any gas being pumped in. Also the cavernous nature enables rapid evacuation of the building, and enables it to be confirmed that the building has been evacuated. (It would not be appropriate to fight a fire by removing the oxygen in
circumstances where there is life to be preserved. Regulations for hypoxic air fire prevention require breathing apparatus if the oxygen level is below 19.5% – which is not much less than the normal 20.9%). Thus the solutions discussed in these articles utilise the cavernous nature of the building, and are constrained to apply to this simple and special case.

Basic Feasibility Assessment

It is possible to perform a basic assessment of the feasibility of using nitrogen in the Notre Dame case – albeit restricted by the limited information available (and my limited expertise). The following paragraphs describe such an assessment:

The first step in assessing the feasibility is to assess the quantity of liquid gas required – this is relatively straightforward. The volume of Notre Dame has been estimated as 282,624 cubic metres. Using this, it is simple to calculate that it would take 13 trucks of liquid nitrogen to fill Notre Dame (each truck carrying 27.9 tonnes). This is an upper limit – i.e. it is the quantity to fill Notre-Dame. It may be possible to use less, through two different mechanisms:

Hypoxia air for fire prevention aims to reduce
the percentage of oxygen from 20.9% (in normal
air) to 10%-15%. To achieve this would require
approximately 28% of the quantity above.
It may be possible to direct the liquid gas at the
fire – so that the first delivery has an immediate
impact, and delivery of the full amount is
unnecessary.

The logistics of transporting this amount to the site do not seem insurmountable. However, whether this quantity could be sourced in the required timescale requires expert (industry) input.

The second step is to assess the energy required. A considerable amount of energy will be required to evaporate and heat the gas. It is not necessary for this energy to be supplied (i.e. fuelled) – the gas will take it from the environment. (In the same way that the refrigerant in your refrigerator will take heat out of your groceries.) Thus it is not a question of supplying this energy, but rather ensuring that there are no negative consequences of this quantity of energy (heat) from being taken out of the environment. To evaporate the liquid nitrogen would require 19.5 thousand kilowatt-hours of energy. To bring the gas to ambient temperature would require a further 20 thousand kilowatt-hours.

These estimates of heat required have been calculated on the assumption that the gas is stored at 1 atmosphere of pressure – in practice such gases are stored under pressure (at a higher
temperature, albeit still cold). Therefore the energy required may be less than quoted above.

Finally, further deliveries may be required to replace any leakage (of the gas). It is likely that there will be leakage – indeed leakage through the roof (i.e. the fire) is the mechanism through which the objective (of bringing the fire under control) will be achieved. An expert view on leakage rate should be sought. Prior to expert opinion – which might be specific to an individual case. A leakage of 10% per hour, would require an additional delivery (one truck) every 47 minutes.

Next Step: Developing General Requirements

Pumping inert gas into the building may be unrealistic, but this basic idea can be used to derive generic requirements (for any solution), determine operational requirements and identify the members of a multi-disciplinary team needed to take this forward. These points will be covered in the second part next month.

bit.ly/NotreDameWarning

bit.ly/Hypoxic