Taking the Heat Out of Ship Fires
On-board a commercial ship, fire can swiftly lead to catastrophic consequences for the lives aboard, the ship, its cargo and the environment surrounding it. Fire is a destructive force that requires a broad range of knowledge and skill to overcome. Accurately recognising the type and extent of a problem is the first step in formulating a sound plan to deal with it. Making the best use of resources and techniques to limit damage and efficiently extinguish a fire is the ultimate goal.
In recent times, a number of fire agencies within Australia have taken a proactive role in refreshing their skills and knowledge in marine firefighting due to the associated risks associated with an ever increasing number of vessel movements through our ports.
While there are many parallels between responding to large structure fires and ship fires, fighting fires on-board commercial vessels introduces some challenges that require different techniques and principles to be employed for the best outcome to be achieved. One of those techniques is ‘boundary cooling’ and it seems that every mariner and firefighter has a slightly different view on the concept.
Almost every marine firefighting reference book will briefly explain one of two possible types of boundary cooling, for example: Robert j Brady Co. Marine Fire Prevention, Firefighting & Fire Safety book, endorses the use of water fog to prevent the spread of conducted heat while the Navel Handbook for Ship Firefighters (2006) in 0414, point f. prescribes “Applying the minimum amount of water to keep the boundary damp, removing bulkhead linings if required.” In addition, trolling through YouTube footage of actual ship fires and you will find plenty of examples of fire attack parties deluging huge quantities of water onto fire boundaries. So, why is there such a discrepancy?
I believe part of the problem is that the term ‘boundary cooling’ is a generic term for applying water to a ships bulkheads containing the fire but, that boundary cooling can be used to achieve two opposite results.
To help clarify the difference I have termed the two styles as defensive and offensive boundary cooling which are explained below.
Defensive Boundary cooling (Heavy application of water).
Traditional boundary cooling typically requires copious quantities of water to be applied onto bulkheads or hull plate boundaries of a fire. This is to prevent or reduce heat from escaping the space on fire by protecting exposures and halting the spread of fire.
Ensuring that surrounding bulkheads around, above and below the compartment on fire are kept cold so that little or no heat can escape. This method effectively insulates the boundaries of the fire by keeping the heat energy contained within the affected compartment thereby stopping the possibility of heat causing fire in adjacent compartments. This is very easy to achieve if plenty of water is available. The choice of nozzle or water pattern has little impact on the ability to successfully use this technique.
On the face of it, this is a sound strategy as there is no argument to discount the fact that that if heat cannot escape then fire cannot exist beyond the boundary of decks and bulkheads containing it but, there are some side issues to manage when this method is employed.
The first consideration is the weight of water being introduced to the vessel and stability. With anywhere between 200 and 700 kilos of water delivered every minute from each line of hose, stability of the ship will need to be addressed within a very short time due to sheer weight or the free surface destabilising effect of water, especially above the waterline. Is the boundary cooling water being drained or pumped off the vessel quickly enough? Are firefighting activities currently or likely to affect the stability of the vessel? Is the amount of water being used really necessary to achieve the required result?
The next consideration is that, by keeping the bulkhead material cold any heat inside the fire affected space will increase significantly making direct attack techniques riskier for fire attack teams.
By the same token however, if there is no intention of a direct fire attack then this method is ideal to help contain the fire as much as possible.
Offensive boundary cooling (Light application of water).
Although the origin of this method of boundary cooling is unclear it has been successfully used for many years with great effect. I have called this variation “offensive boundary cooling” as it assists in cooling the inside temperature of a compartment on fire thereby allowing entry by a direct fire attack team more quickly and more safely than would be the case otherwise. Just like adding another iron to the golf bag, this is a different technique that we can call upon when the need or opportunity requires it.
Smaller quantities of water are applied as a light spray onto hot bulk heads which will boil or steam off where enough heat is present. Turning water into steam requires a tremendous amount of heat energy to change from its liquid state into vapour or steam.
A common high school science equation states that to raise the temperature of 1 gram of water by 1˚ Fahrenheit, requires 1 thermal calorie of energy but, to change water into steam requires an additional spike of 540 thermal calories for every single gram of water! The only source of energy is heat and the source of heat is by large coming from the fire itself.
In other words steam on the outside equals less heat energy on the inside.
With this principle in mind, the aim is to produce as much steam as possible and to limit the amount of water reaching the deck as much as is practicable. The result will show a relatively rapid reduction of heat energy within the fire affected space as heat is continually drawn through the bulkhead and dissipated into the outside atmosphere as steam.
During the initial stages of a fires growth there may be enough energy available for heavy sprays of water to be effective but, remember that the point of this tactic is to create steam. If water is flowing off and onto the deck simply reduce the flow of water or completely stop applying water until the bulkhead metal dries! Only then should another light covering of water be reapplied. More water will only serve to insulate the bulkhead material as in the defensive boundary cooling method discussed earlier.
For this method, a nozzle capable of delivering smaller quantities of water is best suited and a simple jet/spray nozzle will facilitate this very easily whereby lever action nozzles or branches will need tighter controls on technique to achieve the same result. Short bursts or pulses of water mist are the most appropriate way to apply water with lever action nozzles.
From a safety perspective it is sensible to acknowledge that anyone undertaking this method of boundary cooling will be exposed to a lot of steam, especially for the first few minutes of activity. Above deck this will have a negligent impact on firefighters wearing minimum PPE but crews should remain vigilant to avoid steam burns. The same operation below deck may require a much more cautious approach and higher levels of protection. Tactical ventilation around the fire affected compartment should be considered to dissipate the steam produced and SCBA may be required.
Having introduced the difference between both styles of boundary cooling it is important to consider the surfaces to be cooled. Metals are generally good conductors of heat and it is this quality that introduces the ability to boundary cool in the first place. Some bulkhead linings will limit conduction by design in the case of fire resistant divisions or by accident where decorative linings have been installed. Unless heat is coming through a bulkhead or deck there is no advantage in boundary cooling at all. Remove external bulkhead and deck linings to gain access to the metal underneath if possible then monitor for heat and manage it using defensive or offensive boundary cooling as the need requires.
Defensive boundary cooling: More water, less steam = cold bulkheads and insulated fire boundaries, stops the spread of fire. Temperature within the fire affected space is not reduced.
Offensive boundary cooling: Less water, more steam = cooler temperatures within the fire affected space, facilitates entry by a hose attack team sooner by reducing the inside temperature.
Use one or the other, or a combination to achieve the best tactical and safest outcome.
For more information, go to www.amc.edu.au