2.15 Impact Loads on the Fore Ship above the Waterline
In 1971 the writer worked for Lloyd's Register of Shipping and was asked to investigate a number of damages to bow structure above waterline on tankers and bulk carriers - plastic deformation of plates and stiffeners. It was found that transient and random impact forces on structure above waterline, similar to slamming forces on the bottom of the ship, were to blame. These impact forces increased in number and amplitude, when the angle between the bow shell and waterline (flare angle) was reduced and when the shell plate was flatter (less rounded). The impact pressure, the impact was produced by compressed air that was trapped between the water and the hull, could be ten times bigger than a periodic and hydrodynamic external wave load but of much shorter duration and over a much smaller area. The high pressure could cause local plastic deformation of shell plates and stiffeners. When 'Estonia' listed >34 degrees to starboard the effective flair angle was almost zero degree and therefore an impact load could develop sideways, when that side hit the water surface. The Commission has never considered that the visor was struck off sideways in such way.
Actually - in retrospect - the following could have happened. The visor on the 'Estonia' was not well maintained. The bottom lock was probably not locked 3.7 - it may have been damaged earlier. Only the side locks were in use.
After the sudden listing at 01.02 hrs the visor was still attached, but when the angle of list increased, the side locks and the port deck hinge were damaged, when the visor was subject to an impact load - high pressure - sideways at say 01.16 hrs.
However, the starboard deck hinge held and the visor was still attached to the ship by it, when it sank. The visor was apparently later removed from the wreck under water using explosives!
2.16 The Stability of the Estonia' and how the Listing developed
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Chapter 3.7.3 of the Final Report (5) states that a new trim and stability booklet was developed and approved by the Bureau Veritas in connection with the change of flag (January 93) (it is not included in the supplement of the Final Report). However chapter 3.6.2 of the Final report states that only temporary certificates PSSC and LL were issued in June and September 1994, as another (sic) new trim and stability booklet was developed. No correct stability book is included in the Final report. Nevertheless - to load a ferry is not difficult and the stability is seldom critical in part loaded conditions (and the ship was not fully loaded on September 27, 1994). Assume that there were 500 tons of fuel aboard in various tanks of the hull, a couple of hundreds tons of fresh water, 1 000 tons of cargo (cars, lorries, trailers) on deck 2 (the car deck) inside the superstructure and 100 tons of passengers and luggage and port trim tank full, 185 tonnes, to balance (?) heavy cargo on starboard side. Then the deadweight (dwt) is about 2 200 tonnes and the draft (d) is about 5.1-5.2 meter. Deck 1 below the car deck (deck 2) is then below the waterline. The car deck, the bottom of the superstructure, is 2,5 meters above the waterline: 2.17 for a detailed loading condition. There was also a stern trim. Full deadweight was >3 000 tons, so you could have loaded another 800 tons (of fixed cargo) on the ship (e.g. on the car deck) without overloading it. Extra fixed cargo on the car deck would in fact be loaded below the ship's centre of gravity, G, and would have increased the stability. Let's assume that the bow ramp of the superstructure is open and that water flows into the superstructure due to forward speed of the ship and pitching up/down of the bow and that the water does not flow out through the opening. Evidently as soon the ship stops all water flows out again when the ship pitches and trims on the bow. Note that the hull is undamaged and that the ship floats normally. The inflowing water is only extra weight being 'loaded' on the ferry. The vessel heels about 10 degrees with 600 000 litres of water loaded on deck 2 - figure 2.16.1B. The water does not flow down to deck 1 as the door openings are at the centreline and fitted with 20 cms high sills. The water is always trapped on the side of the sloping deck 2 - or with stern trim it ends up at the stern, increasing the trim. With its large beam (B) 'Estonia' had always good, built in stability. You need about 1 200 tons of water 'loaded' on the car deck, (deck 2) 7.62 meter above the keel to list the vessel about 20 degrees to starboard. This free water, 1 200 tons, forms a 2,8 meter high wedge with its base against the starboard side and with a lever about 7,22 meter from centreline, which lists the ship (a fair number of trucks and trailers were parked on the starboard side - water filled the space below and beside the trucks and the centre of gravity of the water wedge was pushed inboard) - see figure 2.16.1C above. The top of the wedge is many metres from the ship's centreline and almost a meter below the sills of the fire doors, when the ship lists. Some water flows out from the car deck via the existing scuppers. All water should of course flow out through the bow opening, when the ship stops! But we assume that the water does not flow out! The more water that is loaded on the car deck, the more the 'Estonia' lists as all water is in the side, and at a certain angle of heel, with a certain amount of water on the car deck, she tips upside down - CAPSIZE! Then she floats upside down. The reason for capsize is that the righting arm, GZ, becomes 0 at about 34 degrees heel, figure 2.16.1D, and the vessel then is unstable. The vessel cannot float on the undamaged hull with a list 90 degrees, see figure 2.16.1E above, which is an unstable position. The vessel then capsizes - turns up side down ... and floats upside down. However, if the ship stops prior CAPSIZE, all water flows out again by itself = NO CAPSIZE. Let's assume capsize takes place. |
Why do new passenger ships suddenly lose stability and roll over? (This article is published here 2006 as many Internet users searching about the subject ends up in chapter 2.16 of my book about the 'Estonia' stability left). Recently (2006) a newly built passenger ship suddenly - in a few seconds without warning - rolled over >20° in fine weather and many passengers were hurt losing balance and being thrown into walls and on decks. All water in the deck swimming pool flowed out. There was panic. Then the vessel stabilized itself at abt 15° angle of list … and slowly the vessel became upright again. Why did this happen? It has of course happened before! A couple of years ago (1999) it happened to another newly built passenger ship at breakfast time. The ship suddenly rolled over, the whole breakfast buffet was thrown into the wall and on the deck and passengers lost balance and were thrown around. The ship owners quickly blamed the sudden loll on the rudders (sic)! They had turned too quickly … and the vessel listed. Do you believe that? In the latter case the vessel remained at >15° list even after the rudders were put back straight. And slowly the vessel became upright. What actually happened? The answer why many newly built passenger ships suddenly rolls over is that they lack regularly inherent stability or GM. What is GM? It is very simple. Stability explained G is the centre of gravity of all weights of the vessel and M is a point vertically above G, through which the buoyancy force of the vessel underwater hull acts, when the ship is upright or rolling or listing. Evidently the total weight of the vessel acting down is the same as the buoyancy force acting upward (remember Archimedes 328 BC) and the ship floats upright. GM is thus the positive distance between G and M and a measure of its intact stability. M is always above G - otherwise the vessel is not upright. When the vessel rolls a certain angle, the buoyancy is shifted a little sideways and a positive righting lever GZ as a function of GM and the angle of list develops. It ensures that the buoyancy force of the intact vessel always uprights the ship at angles of list up to 50°! The intact stability represented by GM must be very good in order for the vessel to survive damage and flooding of the hull, i.e. two watertight compartments of the vessel are assumed flooded. Then the vessel loses buoyancy but still floats. There are lose liquids inside the hull and point M for various reasons comes down closer to G, i.e. GM (and also GZ) are reduced but are still positive, so that the there is enough stability also in damaged condition. Evidently there must also be enough buoyancy, so that the vessel does not sink. Why newly built passenger ships lose stability! The reason is simply that the actual intact GM is too small! And the reason for that is commercial! The ship owner has simply added extra cabins and facilities high up in the new vessel to earn more money. G becomes situated higher up in the vessel closer to M and GM is reduced. But GM is still positive and the vessel is stable … and on the outside all appears OK. Evidently the vessel will not survive the damage condition (GM then becomes <0) but this is such an unlikely event so the risk is taken by the immoral ship owner. Evidently the ship owner must demonstrate to its maritime administration that the vessel complies with the damage stability criteria but it is very simple to fool the administration with a false report of G (very few people need, e.g. be bribed). So on paper all looks OK - vessel's official G is falsified so that the official GM provides regular damage and intact stability, even if the actual GM may only be half the official one! So what goes wrong then? G is evidently not constant - it varies as bunkers and water are consumed or transferred and when ballast is taken on, etc. Simply speaking the consumption and transfer of liquids aboard (often done early in the morning) is similar to a small damage to the vessel. Liquids are moving around inside the hull and when this happens M is affected - it moves down during the transfer of liquids - and GM and GZ are temporarily reduced. If the officers in charge of the transfer of liquids are not informed about the actual G and bases their calculations on the official, false G (shown in all official papers), actual GM and GZ may become <0 during such routine transfers. And then the lose liquids flow to one side, the ship loses its upright stability condition and rolls over … to a new equilibrium at a certain angle of list. This is what happened to the newly built passenger ships quoted above. When you stop or finish the transfer of liquids, M moves up again … and the vessel becomes more stable and uprights. It evidently only happens to newly built passenger vessels, where all officers are not informed about the false G. After the first incident routines are changed to avoid mishaps, etc, but the false G remains. The only serious solution is to remove the extra top weight to bring the vessel into compliance with the rules … but this never happens - the owner will lose plenty money and the officers aboard will lose their jobs. Easier to bribe a few civil servants (and some officers aboard). Many new passenger ships do not comply with all the rules anyway. If you are a passenger on such ships, have a look yourself in the lower decks. You may find watertight doors there … and they are open. But they should not be there at all. The passenger ferry 'Estonia' is another sad example. She didn't comply with any essential safety rules at all ... and sank like a stone due to a small hull leakage. Read the full story Disasterinvestigation! But there are many other examples! |
(Translation of Swedish text in figure 2.16.1 above - Water on car deck.
1A. Initial position.
1B. 600 tons of water on the car deck above the heeled waterline. No water flows down to deck 1.
1C. 1 200 tons of water on the car deck, which is still above the heeled waterline. 20 degrees list.
1D. 2 000 tons of water on the car deck, now below the heeled waterline. 34 degrees list. Water on decks 4 and 5. Righting lever GZ=0. The ship turns upside down.
1E. 90 degrees list at instable capsize. The ship turns upside down in seconds. Centre of gravity G outside centre of buoyancy B!
1F. Final condition. Ship floats upside down on the undamaged, tight hull).
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Then the vessel is on its way of turning turtle with the whole deckhouse flooded, figure 2.16.1F. When the 'Estonia' was turning upside down, she should have floated with the centreline and the openings on the car deck down to deck 1 three, four metres above the waterline, figures 2.16.1E and 2.16.3C. Very little water could during that time flow down to hull spaces below the garage. One minute later she floats upside down - figures 2.16.1F above and 2.16.3D right below. Strathclyde University has simulated the above and produced some video pictures shown right. Figure 2.16.3A right shows what the water inside the superstructure would have looked like seen from the bow. |
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There are >2 000 tons of water on the car deck, the list is >40 degrees and the water reaches the underside of deck 4. Figure 2.16.3B shows the outside situation. Nobody can walk on any decks and the condition is completely unstable. A few moments later the list is 90 degrees - figure 2.16.3C - and soon after - when the deck house is flooded - the ship floats upside down - figure 2.16.3D. The whole sequence would take a few minutes. But it never happened to the 'Estonia'. The 'Estonia' floats on the Deck House and the Water does not flow out through the Bow Opening |
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The Swedish NMA (director general Jan-Olof Selén and director of safety at sea Johan Franson) has commented upon the above in a letter dated 2000-12-15 reference 0799-0036172 to the Swedish ministry of Economy (and Transport) - minister Ms Mona Sahlin: ''The (Swedish) NMA will underline that, when calculating damage stability, you are not permitted to allow for the buoyancy in a deckhouse unless it is watertight. On ferries the deckhouse is not watertight because there are doors which are easy to open and windows that cannot resist water pressure. The situation that you from safety point of view are not permitted to assume and to calculate with the buoyancy of a deckhouse, does not exclude that such a bouyancy actually exists. It exists and therefore the sequence of events as described by the Commission is very likely'. |
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The 'Estonia' floats upright or upside down on the Hull
The volume of the hull below the car deck is about 18 000 m3 in 14 water/airtight compartments and that air cannot leak out, when the ship is upside down. As the lightship was only 9 000 tonnes and the dead-weight 2 200 tonnes, there was plenty of buoyancy left inside the ship (about 13 000 m3 of slightly compressed air and >4 000 m3 of buoyant material in hull, superstructure and deckhouse), so that the 'Estonia' should in the end have floated upside down, if she had capsized with water in the superstructure on deck 2 - figures 2.16.1F and 2.16.3D, the latter made by University of Strathclyde. But she did not do that. She sank at once!
It does not matter if there are errors in the weight assumptions, i.e. if the ship and the cargo, etc. were lighter or heavier, or if the stability was better or worse or the heeling/righting levers were longer or shorter, because the principal result is always the same. You need substantial amounts of water on the car deck in the superstructure to heel the ship 18 degrees, and you need about 2 000 tonnes of water on the car deck to heel the ship about 34 degrees, where it turns turtle in a very short time and floats up side down. There are many examples of this. Of course, if there is not sufficient air left inside the hull and the remaining buoyancy is less than the weight, it sinks at once with pockets of air inside the hull.
Why didn't the 'Estonia' trim?
Water in the superstructure does not only heel the ferry. The water also trims the vessel either on the bow or on the stern. 1 200 - 2000 tons of water inside a superstructure is a very big, moving weight.
The water always collects at the lowest point on the car deck, which shifts position, when the ship heels and trims due to the water.
With 1 200 tonnes of water on deck 2 in the superstructure (a very large moving weight - as stated!) the ship trims about one meter either on the stern (1 200 tonnes water aft - the opening in the superstructure bow moves up several metres above the waterline and makes further water entry more difficult - or on the bow (1 200 tonnes forward) - which means that the deck 2 is almost below the waterline forward and facilitates water entry with speed forward - BUT - all water flows out, when the ship stops!
In both cases (assuming no water flows out) you would expect that the 'Estonia' had turned turtle in a very short time - as 'Herald of Free Enterprise' outside Zeebrügge 1987.
The 'Herald of Free Enterprise' however only ended up on the side, as the water depth was 12-13 metres where she capsized, i.e. she never sank below the water surface but rested on the bottom with the side above water. It went very fast - all passengers inside cabins of the port side drowned immediately, all passengers in cabins on the starboard side - above water - were thrown into the wall, that became a floor and where they could await beings rescued. Passengers in the full breadth saloons ended up in water between the floor and ceiling, where the former starboard side bulkhead with windows and doors became a new roof high above. They were caught in a 'swimming pool' with 10 meters high sides! In the garage all vehicles were pushed to port and smashed to pieces. There was no - zero - time for evacuation. Evidently the 'Estonia' did not capsize like the 'Herald of Free Enterprise'.
According to the Final Report (5) it took about 20 minutes for the 'Estonia' to list 90 degrees (at 01.35 hrs) after a sudden list 15 degrees at 01.15 hrs 1.9. Then it took another 19 minutes to sink at 01.54 hrs. We know that this scenario is a falsification based on manipulated calculations of (a) inflow of water through the bow opening, (b) stability, angles of heel and amount of trim and (c) that the water does not flow out when the ship stops. We also know that the Commission has never explained how the hull below deck 2 was water filled, so that the ship would have sunk. The Swedish government desperately asked its Board of Psychological Defence, SPF, on 19 April 2001 to prepare one example how the 'Estonia' could have sunk, without capsizing 1.49 due to 2 000 tons of water inside the superstructure but the SPF has not been able to do it. The task is of course impossible. As soon as the ship stops and has not capsized, all water flows out and the ship is safe!
We know that if the inner ramp was completely open and if there was 15 knots speed forward, that the vessel would have turned turtle in a few minutes due to a very large inflow of water Appendix 4. We know that that scenario of the Commission is 100% false, because it never happened. So let's study how a ship normally sinks.
Normal Sinking
If a watertight compartment below deck 1 below the car deck of the 'Estonia' is flooded (figure 2.16.2) with water, i.e. the ship hull is leaking, the stability, the metacentric height GoM, is reduced due to free water surfaces (loss of inertia to prevent the vessel to list). If two compartments are flooded (figure 2.16.2B), the metacentric height is further reduced and there remains only minimal inherent stability. It means that the ship is still stable, but that she rolls slower. This is the rule requirement. Passenger ships, but not cargo ships, shall survive with two flooded compartments due to leakage. Passenger ships shall not sink due to leakage!
(Translation of Swedish text in figure 2.16.2 above - Water below car deck.
2A. Initial position.
2B. Underwater hull damaged. 300 tons of water in one or two compartments. GoM reduced. 0 degree heel.
2C. 600 tons of water in the ship. Water spreads through open watertight doors. GoM almost 0. Other compartments on deck 0 flooded.
2D. Ship has heeled (GoM<0) 15 degrees to a new equilibrium (GoM>0), where free water surface effects are smaller. No water on the car deck.
2E. 45 degrees list. Water spreads through open watertight doors and fills compartments on deck 0. Water on deck 4 and 5. Stable condition!
2F. 90 degrees list. The ship does not capsize, when water enters both below and above the car deck. STABLE CONDITION!).
Sudden Listing
If three compartments are partly flooded on deck 0, the initial stability becomes negative and the passenger ship may suddenly list 50 degrees assisted by rolling. Because it is only a certain, small amount of water on deck 0, the ship will become stable again, when it has listed a certain angle - figure 2.16.2D, because the free water surfaces are reduced by the heeling, when the water is pushed up against e.g. a watertight car deck (deck 2) from below - the case of the 'Estonia. Open watertight doors are temporarily 'on the dry' and no water spreads. Also the righting lever (GZ) is positive at larger angles of heel: 2.17 for detailed calculations. When more water flows in, she sinks.
That three or more spaces could be flooded on 'Estonia' during the night of the accident is clear. The watertight doors between all six watertight compartments on deck 1 forward of the engine room were open. The following probably happened.
First (at about 00.55 hrs) one or two compartments (including maybe the swimmingpool (11)) on deck 0 were flooded due to a shell damage caused by, e.g. a collision, and the vessel was still stable - figure 2.16.2B. Sillaste was called to assist unless he was already in the engine room - start the bilge pumps 2.1.
When the water reached deck 1 (at about 00.57 hrs), it spilled out there (figure 2.16.2C) on the starboard side (the ship had a small permanent starboard list), which was observed by many passengers on deck 1, who had just heard the big crashes due to the collision. The bridge was alerted. Silver Linde was sent to check! While a large number of passengers on deck 1 started to evacuate their cabins and climb to deck 7, some watertight doors on deck 0 were opened by the crew checking what was going on and the water spread on deck 0. The engine crew was standing in water to the knees.
The result was that the initial stability (GoM and GZ) became zero and that the ship listed suddenly to starboard at 01.02 hrs (figure 2.16.2D). This was noted by all survivors onboard
Then the ship again became temporarily stable (GZ>0), when the water could not spread through the watertight doors temporarily in a dry position at the centreline and when the free surfaces were reduced.
But water continued to flow in - figure 2.16.2E. This water made the ship temporarily more stable, it uprighted a little, but the water could soon after spread through the open watertight doors at centre line and the angle of list increased.
Then the deck house was flooded, so that the ship heeled more and more - 70 degrees at 01.25 hrs, when the car deck started to flood aft from above via the ventilators on deck 4 - and sank 01.32-01.36 hrs. There is a possibility that the starboard pilot door of the superstructure 1.16 was open and that water started to flood the superstructure (decks 2-3) already at 15 degrees list - see figure 2.16.2D above through this opening in the side of the superstructure that then was below water.
The Cause of Sinking - normal Hull Leakage
That the ship finally sank (01.36 hrs) and did not, e.g. tip over up side down, was due to the fact that there was a damage of the hull below waterline - figure 2.16.2F - and plenty of water (weight) in the hull below the car deck, which stabilised the ship during the 30 minutes of sinking. All air in the ship below the car deck and forward of the engine room escaped through the ventilation system and open watertight doors, while the angle of heel was less than 90 degrees and the buoyancy was reduced to <12 000 tons. The engine room was also flooded, so 'Estonia' could not float on that. Thus she sank with the stern first.