Stability at Shipboard Fires
Land-Based Firefighter talking to a ship’s Master…
“Well, the fire went out”….
“Yeah but you sank my ship!”….
“I know that, but the fire did go out!”
There have been many tragic stories of fire departments valiantly fighting a fire aboard a large ship for hours, and even days, only to have the ship capsize or sink at the dock because of the weight of the firefighting water that was applied. The results are embarrassing, frustrating, expensive and sometimes fatal.
There are only three ways to prevent this disaster. One way would be to remove all of the firefighting water as quickly as it is being applied. The second way would be to use less water, or no water. The third way would be to move the water to a part of the ship where it will be less destabilising.
Removing all the water with dewatering pumps is the best. Using less water will be covered in a future article dealing with the use of fog streams and using no water will be discussed in future articles dealing with fixed firefighting appliances. So that sort of narrows down my choices for this article to relocating the water. But before we can discuss how to move this water we must first learn why it is important to do anything at all about this water.
Whenever you are presented with a ship or boat fire you have to take some basic laws of physics into account. A vessel stays afloat by offsetting two forces of nature. Gravity is attempting to push the ship down into the water (sinking), while the buoyancy of the water will be pushing up from the bottom. If either force wins, you’re in trouble.
If you are not a Chief Officer, and never hope to be one, then all you need to know about ship stability is that any weight added to a ship, beyond that which it is designed for, may cause the ship to sink. The higher up in the ship that this weight is added, the worse the problem. OK, anyone who might ever be in command of a ship fire, (or any really smart Company Officers, Firefighters, or mariners), must have an understanding of the principles of stability. So only they should read on!
I’m in no way an engineer or a physicist so I probably will be able to simply explain just what stability is.
Well here goes…
The Centre of Buoyancy is the point at which all vertical upward forces (buoyant forces) are said to act. It is the centre of the volume of the immersed portion of the vessel.
For mathematical equations this point is simply identified as “B” – a lot of deep thinking went into that choice!
The opposite force is Gravity – I wonder what letter the math wizards will use for the center of that!
The Center of Gravity is the centre of the total weight of the loaded vessel and yes; the letter “G” identifies it. Shipbuilders know where this point should be even at the design stage of a ship’s construction.
It is the point where the entire weight of the ship, and its contents, is concentrated. All the underlined stuff is going to be important later!
If additional, excess, weight is added to the ship then this point “G” will be located higher and the ship will become less stable, but more about that later as I’m getting ahead of myself.
When both of these forces press equally against each other from directly opposite directions then the ship is said to be “At Rest” or upright, however this is rarely the case.
External forces such as wind and waves will usually change this.
If the ship was built and loaded correctly, these outside forces may cause the vessel to temporally tilt from side to side (Heel1); however the vessel should return to the upright by itself. Let’s say that the Exterior Force of a wave caused this ship to “Heel” over to the side.
If the weight of the ship and its contents has not changed, or shifted (there’s that underline again!), then the centre of gravity (“G”) will remain in place.
But look at the “B”. It moved in the direction of the lower side. THAT point, “B1” is now the centre of Buoyancy and the upward forces of Buoyancy will seem to push harder on this lower side. The result will be a return to the upright.
Remember, this is on a ship whose weight has not been changed or shifted.
Not too bad so far. That’s just two letters to remember. But you know that “they” can’t leave that alone. You know “they’re” going to throw in another letter to confuse us.
The letter “M” will stand for the Metacentre.
Don’t give up! I don’t like this either but it will all come together soon (I hope). The Metacentre is a point near the centreline of the ship. It is a point that will normally be stationary and directly over the point of buoyancy regardless of which side “B” moves.
If you consider “B” as the ball at the bottom of a pendulum then “M” is the connecting point from which that pendulum swings.
The Metacentric Height then is the distance between the center of gravity “G” and the metacenter “M” and is simply called “GM”. The “GM” is crucial to stability.
The further apart that “G” and “M” become so the more stable the ship and the quicker it will right itself. This long “GM” also causes an uncomfortable whipping motion that anyone ever assigned to a Navy destroyer or frigate in the North Atlantic can attest to. Destroyers and frigates have a notoriously long “GM” and therefore a terrible ride!
Hey; wake up! I’m nearly getting to the end..
To provide a more comfortable ride, most passenger ships are built with a shorter Metacentric Height. This will allow the vessel to slowly recover to the upright and not throw the passengers against a bulkhead.
I mentioned before that as long as the weight of a vessel, and the location of that weight, remains constant, that the center of gravity would not move.
This is where our problem comes in as Firefighters. Weight is always added to a ship in the form of cargo, fuel, provisions, and passengers. These items have all been previously calculated in the ships stability plan and should therefore not cause a problem.
However, as the unexpected, excess weight of firefighting water is added, the ships center of gravity will rise and the vessel will become less stable. If the center of gravity rises above the metacenter the ship will capsize. Also, as the ship develops a list2, its center of gravity may now move in the same direction as the point of buoyancy if there is also a corresponding shift in weight as in the diagram.(Here’s where we get to the importance of the shifting that was underlined before.)
This new location of the downward force of gravity will now fight against the righting tendency of the new point of buoyancy that had shifted previously. The result is that the ship will no longer return to the upright. This shifting of water can also have some drastic consequences.
If an area flooded with the firefighting water is only partially filled, and there is no restriction to the side-to-side movement of the water, the result can be devastating (See fig. below).3 A defective bow door of this European ferry allowed water onto the undivided car deck. As the ship listed, the water was unrestricted, and rushed to the low side causing the vessel to capsize with a large loss of life.
This free movement or sudden shifting of water is called “Free Surface Effect” and is a major problem in ship stability during fire operations.
A defective bow door of this European ferry allowed water onto the undivided car deck. As the ship listed, the water was unrestricted, and rushed to the low side causing the vessel to capsize with a large loss of life.
If the area is completely flooded or if there are restricting boundaries to act as baffles then the water is no longer subject to “Free Surface Effect” and there will either be no movement of the water or it will be reduced.
The location of this weight is important also. Here, I finally get to the original topic of this article, “moving the water to a location where it will be less destabilising.” Firefighting water that accumulates on the upper levels of a vessel will be more likely to decrease stability than water accumulating in lower areas due to the raising of the center of gravity which leads to listing and eventual capsizing.
If all of the water went directly to the lowest part of the ship it could actually help to make a ship more stable by lowering the center of gravity – up to a limited amount of course. This is just the same as when water is added to an empty ship to provide ballast.4 Ideally you would first want to remove this water from the ship, but that can’t always be done. Some reasons that might prevent you from removing the water overboard are; lack of pumps, lack of power or pollutants mixed with water.
In these cases your next best solution is to transfer this water as low in the ship as possible. If you have pumps but are unable to pump overboard due to the height, then you can pump the water down into the ships bilge where it can then either be pumped overboard by the ships own bilge pumps or it can remain at this safer, lower area although you will still have to be monitoring the ships draft!
One trick that has been used successfully at many ship fires to send water to lower levels, and which doesn’t require the use of pumps, is to break apart toilets in any flooded upper lever accommodation spaces. This will allow the water to drain into the ships sanitary tanks which are located in the lower levels of the ship. If the Coast Guard permits, these tanks can then be pumped overboard. However, it’s not likely that the Coast Guard will approve due to environmental reasons. However, at least the water will now be at the lowest level, which is less destabilising.
To sum up, anytime you are going to be putting firefighting water on a ship you must simultaneously begin removing or relocating that water. When starting any dewatering operation you must realise that the priority must be given to upper areas especially those that are subject to the “Free Surface Effect”. Here is the only math formula I want you to learn. “1 litre in” minus “1 litre out” equals “no problem from your firefighting water.”
One trick that has been used successfully at many ship fires to send water to lower levels, and which doesn’t require the use of pumps, is to break apart toilets in any flooded upper lever accommodation spaces. This will allow the water to drain into the ships sanitary tanks which are located in the lower levels of the ship.
One of my first calls at a ship fire would be to the Coast Guard to send someone to monitor stability as they know what they are doing and can relieve you of this task. Although until they arrive it will be up to you but it’s worth noting that the ship’s licensed crew members should also have a good knowledge of vessel stability.
I hope that this article was written clearly enough so that you now understand the basics of this topic.
1Heel – The temporary tilting of a vessel from side to side. If this tilt becomes permanent then it is called a List
2List – A permanent tilting of a vessel to one side or the other
3Investigations into several recent ferry disaster investigations in Europe, such as the vehicle ferry MV Estonia, have listed the “Free Surface Effect” of water on the open vehicle deck as a cause for the sudden capsizing of the vessels. Photo credit – The report of the Joint Accident Investigation Commission
4Under normal circumstances, after a ship is unloaded, the Ballast Tanks in the ships lowest levels are filled with sea water to increase the ship’s stability. The ship is then said to be “In Ballast”.
For more information, go to www.marinefirefighting.com
Stability graphics courtesy of the Icelandic Maritime Administration.
Main photograph courtesy of the US Navy