South Korean ferry MV Sewol flips, 1/2 sunk in shallows, people trapped.....

Discussion in 'Stability' started by Squidly-Diddly, Apr 17, 2014.

  1. jehardiman
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    jehardiman Senior Member



    I'm waiting for a good report on what condition changed or happened before the casualty. While it is possible through tank mismanagement to reduce the initial stability of a vessel to nil (c.f. the COUGAR ACE), I agree that an intact vessel should have enough ultimate stability to survive that. Also, we know that small amounts of damage flooding, left unchecked to cause a large free surface, can cause precipitous loss of stability (c.f. the ROCKNES, but not the ESTONIA and HERALD FREE ENTERPISE which both suffered large flooding openings to the car deck).

    Perhaps this accident can best be compared to the EXPLORER sinking in 2007. In a thread at the time, I pointed out that the idea of commerical damage control and stability is only to provide time for the vessel to be safely abandoned. This is similar to what happened to S/V CONCORDIA, she stayed afloat long enough to be safely abandoned.

    As to the cause, we need to wait for an investigation of the hull. However, as IEWinkle pointed out, she capsized on an even keel, which most likely rules out a swung prop or shaft, or a rudder loss. I can't recall a photo that shows the prop in place, so it is possible she may have lost the tail shaft and flooded up through the stern tube. I did notice, however, that her stbd active stablizer was deployed. It is possible that a failure in the port stablizer caused enough damage for a slow flood up. In that case, this is like COSTA CONCORDIA, where the vessel had enough initial damage stability, but command indecision led to the loss. In a damage situation, if you put off a decision long enough, it WILL be made for you.
     
  2. morkisthatu
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    morkisthatu Junior Member

    Sewol

    I noted that at 8:48- 8:49 the ship made an abrupt turn, and the captain returned to the bridge and attempted to re-balance the ship. Could that be why the stabilizers were deployed?
     
  3. morkisthatu
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    morkisthatu Junior Member

    [And I'm still confused by conflicting reports of the abrupt turn and which direction it took. Would a hard turn to port cause the cargo to shift left and create the capsize, or would centrifugal force shift the cargo to the starboard side? I am uneducated in these matters and assume it was a left turn, and the cause of the turn is the piece which solves this terrible puzzle.
     
  4. Sailor Alan
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    Sailor Alan Senior Member

    Ferry

    Thank you for the analysis. I am impressed we can see a serious analysis of this incident from public domain information, just as i have seen for aircraft incidents.

    Here is a photo that fascinates me. It shows an obviously official fairly large craft alongside the ferry long before it rolled to its side proper. No or few life rafts deployed yet. I can only assume someone in authority still thought they could evacuate the PAX over the deck, i.e. without them getting cold/wet.

    I did hear a recording from the shore control giving conflicting orders (suggestions?) to the captain between the emergency call, and his departure from the boat. These cannot have helped much.

    Was he in 'controlled' waters where shore based people had some authority, but i assume he always has the last word. i.e. pilots suggestions, captains orders.
     

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  5. Sailor Alan
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    Sailor Alan Senior Member

    This photo shows the remarkable evenness, or symmetry, with which the ferry took on water, at least initially. I assume this is the kind of photo IEWinkle was referring to when he remarked on this phenomenon. I assume the stern sinking faster later on may have something to do with larger machinery spaces aft filling quicker, but thats speculation.

    I traveled across the North Sea from Bergen to Newcastle. As a sailor i slept soundly, but in the morning there were ambulances carting PAX off with various broken bones, perhaps worse. I was impressed that some windows (i cant bring myself to call them ports, they were 4’ square, with rounded corners, with a through bolted flange) were stove in 20’ or more above LWL.

    I don't know what the weather is like off the Korean NW coast, but most northern hemisphere NW coast seem to be pretty rough. I remember some pretty bad crossings from Liverpool to Belfast as well. Note; at the time the catamarans had a slightly higher weather allowance than the then monohull ferry.
     

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  6. powerabout
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    powerabout Senior Member

    how come it appears that not a single liferaft floated away after hydrostatic release?
     
  7. jehardiman
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    jehardiman Senior Member

    There are 2 in one photo I saw, but IIRC a capsule will not release if the ship totally inverted when the hydo release activates, it assumes that the canister will be deployed before that. The buoyant raft has to pull the lanyard to inflate, which means the canister has to open and let the raft float free. If the buoyant raft cannot pull the lanyard, i.e. the canister does not open and the raft is trapped inside, or the canister is not deployed, then the raft does not inflate. I know that some rafts require ~50 yards of tether be pulled before they will deploy.
     
  8. powerabout
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    powerabout Senior Member

    the liferaft is buoyant in the cannister so when underwater the hydostatic will open and the lanyard is supposed to be tied off so they will self inflate as long as the cannister can get away from the vessel and pull it.
    You dont need human intervention if they are set up correctly.
    Yes true the size of the raft will determine the length of the lanyard.
    They say they vessel is 65' underwater I would expect at least half to escape and pop up.
     
  9. IEWinkle
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    IEWinkle Retired Naval Architect

    Unused Liferafts!

    This is the telling photo in which you see an attempt to launch the only two liferafts from what appears to be the equivalent of the 'boat deck' on any other ferry. This is where I would have expected to see the passengers assembled for disembarkation into the liferafts (to both port and starboard) - but nothing (except the Captain and possibly a few crew!). At this point the 3 tiers of superstructure were beginning to flood and the final (inevitable) capsize was starting to accelerate!
     
  10. IEWinkle
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    IEWinkle Retired Naval Architect

    Cargo Shift

    A hard turn to Starboard might cause the cargo to shift to port under the action of Centrifugal force, which causes the vessel to heel away from its direction of turn. This should not be sufficient to cause the vessel to capsize in calm conditions as a heel of at least 35 degrees would be required to put the upper deck (at 14 m above base) underwater, unless there were faulty vehicle deck scuppers whose non-return valves were jammed open. If this were the case then a heel of only 9 degrees would be required to start vehicle deck flooding (albeit at at a relatively slow rate) which would allow a slow capsize to initiate. The starboard scuppers were obviously open as you can see them venting air in the final stages of capsize as the vehicle deck fills rapidly (probably through the fore and aft ventilators and other access points near the centreline). Whether the non-return valves on the port side were working after 20 years of operation is an interesting question? We will only know for sure when the vessel is raised and we can see the port side for any signs of hull damage - a more likely initiator!
     
    Last edited: Apr 23, 2014
  11. IEWinkle
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    IEWinkle Retired Naval Architect

    Stabilisers

    Stabilisers cannot be used to rebalance a ship - only large quantities of water ballast in wing tanks might do that! They are normally deployed to smooth out the minor motions that occur in anything but a flat calm on most Ro-Pax vessels to keep passengers happy. However, if the port stabiliser had struck a submerged object at speed, the resulting impact might have torn a hole in the ship's side and caused penetration of the vehicle deck above.
     
  12. jehardiman
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    jehardiman Senior Member

    Correct, the canister is supposed to float, but as I pointed out before, it is supposed to float UP out of the foundation. If the ship is inverted, the hydrostatic release has operated and the lashing strap is free, but the cannister is trying to float up, which is now towards the deck, so it is just being pressed back into its foundation or is actually up against the deck, neither of which will cause the lanyard to be pulled. And those cradles are fairly stout, being designed to prevent the canister from being swept away.

    Yes, it is possible that some of the hydrostatic releases failed or that they had been circumvented, or that the rafts failed to function, but in a capsize it is not a given that the raft will float free. Note the photo in post #19, only the two in the manual ready launch have been deployed. The others are well contained and by the time they reach activation depth (15-20 feet), the ship will have rolled to 90 so they would be trapped by the railing. Likewise, the ship will have turtled before the starboard canisters are immersed.

    The really sad thing about that photo is that that deck should be covered with people and the crew should be freeing the rafts. As I said earlier, the difference between this unnecessary loss of life in the calm near shore and the complete success of the EXPLORER abandonment in one of the most inhospitable places in the world was the immediate preperation by the crew to have the passangers abandon.
     
  13. morkisthatu
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    morkisthatu Junior Member

    I'm sure most of you have seen the updated news that the ship did not make an abrupt turn as originally reported. The ship made a gentle 180 degree turn taking nearly 3 minutes. I'll be interested to see how that fits into the mystery of this disaster.
     
  14. IEWinkle
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    IEWinkle Retired Naval Architect

    Thanks for this. It seem my original information that it hit a reef at 9:00KST may be correct after all.
     

  15. morkisthatu
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    morkisthatu Junior Member

    Sewol

    I don't know about the reef. All the reports I've seen say it was a 2 mile wide channel, 120+ feet deep, no rocks or reef in the area of the ship, cloudy but no fog, 1/2 meter swells. Why would a ship make a 180 degree turn?
     
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