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Korean ferry Sewol
- Thread starter Savannah
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A possible cause?
It would seem that the vessel may have suffered raking damage underwater similar to that sustained by Costa Concordia. If this was the case it is likely to have opened up a number of empty wing compartments on the port side up to 4.5 m wide by about 8.0m deep. These should be cross-connected to the starboard spaces, but if sufficient inflow occurred to more than 2 of these spaces it is possible that the vessel's list would become larger than that which would allow the cross-connections to function, effectively holding the vessel down on its port side, an effect compounded by cargo movement on the vehicle deck. The lack of trim in the later photos suggests that such damage would probably have been around the mid section of the vessel where these compartments would have the largest volume. Although the vessel seems to have stabilised with a large list, progressive flooding within the superstructure would have gradually increased the list until capsize.
Such a scenario may be unique in the Ro-Ro world and might suggest the need for a review of the practice of using large void wing tanks in such vessels (there are several examples of large Ro-Pax vessels where these spaces are filled with empty plastic drums to reduce permeability).
I have just seen photographic evidence that this vessel has also had an extra deck added to the aft end of the superstructure aft of the funnel within the last two years (when she transferred from Japanese ownership) which would have compromised her stability to some extent as there were no compensating blisters added.
Much will no doubt be said about the apparent lack of crew comprehension of the critical state the vessel was in and the lack of any organised evacuation of passengers who had been told to stay where they were inside the vessel - a probable death sentence in a vessel listing to more than 45 degrees! This is further reinforced by the fact that only two of the many life rafts were released for use just before the capsize from their storage racks to port and starboard just behind the bridge. There do not appear to have been any other form of lifesaving appliances on board other than life jackets!
It would seem that the vessel may have suffered raking damage underwater similar to that sustained by Costa Concordia. If this was the case it is likely to have opened up a number of empty wing compartments on the port side up to 4.5 m wide by about 8.0m deep. These should be cross-connected to the starboard spaces, but if sufficient inflow occurred to more than 2 of these spaces it is possible that the vessel's list would become larger than that which would allow the cross-connections to function, effectively holding the vessel down on its port side, an effect compounded by cargo movement on the vehicle deck. The lack of trim in the later photos suggests that such damage would probably have been around the mid section of the vessel where these compartments would have the largest volume. Although the vessel seems to have stabilised with a large list, progressive flooding within the superstructure would have gradually increased the list until capsize.
Such a scenario may be unique in the Ro-Ro world and might suggest the need for a review of the practice of using large void wing tanks in such vessels (there are several examples of large Ro-Pax vessels where these spaces are filled with empty plastic drums to reduce permeability).
I have just seen photographic evidence that this vessel has also had an extra deck added to the aft end of the superstructure aft of the funnel within the last two years (when she transferred from Japanese ownership) which would have compromised her stability to some extent as there were no compensating blisters added.
Much will no doubt be said about the apparent lack of crew comprehension of the critical state the vessel was in and the lack of any organised evacuation of passengers who had been told to stay where they were inside the vessel - a probable death sentence in a vessel listing to more than 45 degrees! This is further reinforced by the fact that only two of the many life rafts were released for use just before the capsize from their storage racks to port and starboard just behind the bridge. There do not appear to have been any other form of lifesaving appliances on board other than life jackets!
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Some further thoughts and analysis
Further to my Post 2 above, I have managed to estimate the effect of flooding from 5 to 15 % of the vessel's displacement (about 520 to 1550 tonnes) in its port wing tanks. Despite considering initial GM values from 0.15m (IMO minimum) to 2.0m I can only obtain angles of list up to 28 degrees (with the largest flooding value and lowest GM) and about 10 degrees for the lowest flooding and largest GM. This is not sufficient to flood the superstructure above the vehicle spaces which should have been watertight at least to 14m depth (about 35 degrees).
At angles of 25 degrees plus there will be added contributions from some movement of cargo but in calm conditions this will not become significant until about 35 degrees or more depending on cargo type. In order to get the larger list clearly indicated in the the many photographs it is necessary to consider flooding of the vehicle deck, but there seems to be no obvious entry point other than the possibility of faulty scuppers which should have non-return valves fitted. If these were jammed open, then progressive flooding of the vehicle space could take place which would provide a slow mechanism to achieve angles of 45 degrees or so within about half an hour to an hour. This would require about 1000-1500 tonne of water on the vehicle deck as well as the flooded wing tanks below. Shortly after this the superstructure decks reach the water level and will start to accelerate the capsize as water enters through open doors etc. It is clear from the immediate pre-capsize photos and video when the vessel is at about 85 degrees that more than half the vessel is out of the water indicating that she still had a great deal of further flooding to take place before she could roll right over and sink. In the final capsize video sequences there are clearly visible openings venting air which would appear to be from some of the vehicle deck scuppers as water flooded in.
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Further to my Post 2 above, I have managed to estimate the effect of flooding from 5 to 15 % of the vessel's displacement (about 520 to 1550 tonnes) in its port wing tanks. Despite considering initial GM values from 0.15m (IMO minimum) to 2.0m I can only obtain angles of list up to 28 degrees (with the largest flooding value and lowest GM) and about 10 degrees for the lowest flooding and largest GM. This is not sufficient to flood the superstructure above the vehicle spaces which should have been watertight at least to 14m depth (about 35 degrees).
At angles of 25 degrees plus there will be added contributions from some movement of cargo but in calm conditions this will not become significant until about 35 degrees or more depending on cargo type. In order to get the larger list clearly indicated in the the many photographs it is necessary to consider flooding of the vehicle deck, but there seems to be no obvious entry point other than the possibility of faulty scuppers which should have non-return valves fitted. If these were jammed open, then progressive flooding of the vehicle space could take place which would provide a slow mechanism to achieve angles of 45 degrees or so within about half an hour to an hour. This would require about 1000-1500 tonne of water on the vehicle deck as well as the flooded wing tanks below. Shortly after this the superstructure decks reach the water level and will start to accelerate the capsize as water enters through open doors etc. It is clear from the immediate pre-capsize photos and video when the vessel is at about 85 degrees that more than half the vessel is out of the water indicating that she still had a great deal of further flooding to take place before she could roll right over and sink. In the final capsize video sequences there are clearly visible openings venting air which would appear to be from some of the vehicle deck scuppers as water flooded in.
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There is another thread started on this same subject which has more posts so let's continue the discussion there:
http://www.boatdesign.net/forums/st...s-1-2-sunk-shallows-people-trapped-50198.html
http://www.boatdesign.net/forums/st...s-1-2-sunk-shallows-people-trapped-50198.html
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