MV Sewol - What Possibly Happened?

The case of the sinking of the South Korean Ferry MV Sewol surely has a lot of untold mysteries hidden underneath. She started heeling rapidly to her port side sometime before 08:58 AM (Korea Standard Time), then distress calls were made to Jindo Vessel Traffic Services Center (VTS). Not less than two hours later, the first ship had approached for rescue operations. But the first question that must be pinging your mind now is "Where were the lifeboats? Liferafts? Why didn't the Captain of the ship give an evacuation order with ample time in hand? "

According to the possibilities that have been taken into consideration by our team of Naval Architecture students, we have discussed possible scenarios that may have led to the tragedy which has been reported to take away 185 lives as to today (25 / 04 / 2014)

Rather than directly answering to the questions (which would be vague at this stage of the investigation), lets make this much easier with a little bit of background. The picture below shows the standard route followed by Sweol in its voyages from Incheon to Cheju do island. 

Route of MV Sewol from Incheon to Cheju do and the accident site (Courtsey: Google Earth)

If you notice carefully in and around the area of the accident site, the colour of the sea water shown changes to blackish. It is because the region is full of underwater rocks. So you must be thinking she hit a rock and got grounded. As reported by a state broadcaster, she was off her usual course on the day of accident. But possibility of grounding can still be easily ruled out. Wondering why? Understand the figures below:

Weight, Buoyancy and Vertical Reaction acting on a ship's hull when grounded.

Clear from the above diagram, the side of the hull hitting the rock will eventually tend to emerge from the water and the other side will tend to sink below. That is, the ship heels by the side which has not faced grounding impact. If one considers that Sewol's hull hit an underwater rock, then going by the capsizing pattern (she capsized by the Port Side), she must have hit the rock by her starboard side. At higher angles of heel as shown below, the opening created by the rock on her steel hull would be clearly visible. Her starboard side was intact. Grounding ruled out!

Starboard side of MV Sewol's hull didnot have any marks of grounding impact.

MV Sewol was "Ferry Naminoue" before it was renovated in 2013. What happened during the renovations is actually a question raising issue. In 2012, extra passenger cabins were added to her third, fourth and fifth decks, increasing the passanger capacity by 181. This caused a rise in the center of gravity of the ship by 0.51 meters. Passanger ships generally operate in moderate GM (metacentric height). A high GM would make the ship too stiff and uncomfortable for passengers. On the other side, an unsafely low GM is also not preferred so as to maintain the required stability margin. But the rise in center of gravity of the ship eventually caused a decrease in its GM after the renovation. It is even unknown if the owner had made any other changes on the ship after regulatory approvals were completed. 

Not only this, the ship carried more than three times of cargo weight on the day of accident. She was designed to carry cargo load of 987 tons but she had 3608 tons of cargo that day. Obviously, another cause supporting the rise of center of gravity and even decreased metacentric height.  If a ship with marginal metacentric height turns at high speed, it is likely to heel by significant angle, and in worst cases even capsize.

This scenario matches pretty well with MV Sewol's case. The Captain was possibly aware of the marginal metacentric height and this is why he may not have ordered the passangers to move to the upper decks during the first one hour, expecting that the ship's heel could be controlled, if weight of the passangers (64.2 tons approximately, if an average passanger has weight of 65 kg) were limited to the lower decks thus preventing further rise in center of gravity.

The third mate Park Han-Geyol reportedly ordered the helmsman to make a 5 degree turn, which was a part of the ship's course. But tracking data show that she made a turn of 45 degrees at a speed of around 18 knots (4 knots below its design speed). Whether this was a careless move, or a failure of the steering mechanism, is unknown as of yet. But two things can be easily inferred:

• If it was not a failure of the steering mechanism, the human error involving carelessness of the helmsman and the navigation officers was the prime reason behind such a steep turn.
• But a ship is not designed to heel over and capsize even when it turns steeply at its design speed. In this case, the speed was 4 knots lower. There comes in the problem of stability which Sewol surely had. She was operating at a marginal metacentric height (already explained 2 minutes ago!). The more the metacentric height of the ship, greater is the uprighting moment of a ship when it heels to either side. A reduced GM must have caused the righting lever GZ to fall below what it should have been for safe operation of the ship. And as the ship took a steep turn of 45 degrees at 18 knots (which is not much below its maximum design speed of 22 knots), the centripetal force acting on the ship caused it to heel to an angle at which the righting lever reduced to zero, causing her to capsize. Sewol obviously made this turn to her starboard, as she capsized by heeling towards port side (apply Laws of Physics and understand the figure below or watch the video below).

Forces on a ship when it turns to Starboard Side.

Sewol was not only designed to carry passengers. It had additional capacity to carry cars and containers. Reportedly, many passangers heard loud explosions after the sudden heeling of the ship. That was possibly due to shift of unproperly lashed containers and cars in the ship's hull. This might have not only gave rise to explosions, but caused the ship to heel further to the port side due to a shift in transverse center of gravity.

Also, unlike other ships, ROPAX vessels like MV Sewol are not designed with bulkheads that divide the ship into watertight compartments. This possibly caused the entire car decks to get flooded once the ship started taking in water due to large angles of heel. The problem with ships taking in water is actually something that results in exponential rise in risk of capsizing. Why? If a ship starts taking in water, the free surface generated by the water in the ship creates a free surface effect which raises the center of gravity thus rendering the ship more unstable and prone to taking in more water. Since ROPAX ship's donot have bulkheads, this problem cannot be limited to a compartment, and the entire ship floods eventually.

What can be seen from all these evidences by the help of Naval Architectural Principles is that, the prime cause of the ship's capsizing must have been the reduced stability, coupled with the human error of the crew involved in not carrying out effective evacuation procedures at the correct time.

Official investigations are on the way and actions have already been taken against the Captain and officers. Not only this, the authenticity of Korean Register of Shipping is also under investigation as there can be every possibility of illegal approval of many such similar ship designs that are already sailing even now with hundreds of lives in danger!

Article By: Soumya Chakraborty

Author's Note: This was a report of an investigation done by the team of students who own the LSD blog. Conclusions in this article have been drawn on the basis of informations obtained from News and by application of Naval Architectural principles to estimate what might have actually caused MV Sewol to sink the way it had, on 16th April, 2014. The video used in this doesnot belong to LSD and full credit goes to the owner. Thank You for reading. In case of any queries and doubts please comment or write to learnshipdesign@gmail.com