What caused Sewol sinking?
Human errors, which account for 85% of all maritime accidents; will not reduce accidents waiting to happen; but the number of accidents could decrease in a climate of improved safety in the maritime sector. All of us are responsible for undertaking this challenging task. Article by Capt. Cahit Istikbal
Article by Capt. Cahit Istikbal
The 6,825-ton Sewol was carrying 476 people, including 325 students from a high school in Ansan, just south of Seoul, when it sent out a distress signal at 8:58 a.m. in waters 20 kilometers off the island of Byeongpoong.
MS Sewol, previously Ferry Naminoue or Naminoue-Maru, was built by the Japanese company Hayashikane in 1994. She had 146 m (479 ft) in length and 22 m (72 ft) in width, it could carry 921 passengers, 956 including the crew.
She was operated by Cheonghaejin Marine Company, Incheon.
Sewol had been reported to have space for 180 or 220 cars and could carry 152 20-foot shipping containers. Her maximum speed was 22 knots (41 km/h; 25 mph)
Sewol operated in Japan for 18 years (from 1994 to 2012) and brought to Korea from Japan on October 2012.
Since then extra passenger cabins were added on the third, fourth and fifth decks, increasing the passenger capacity by 181, and increasing the weight of the ship by 239 tons. The construction was legal and passed regulatory tests.
After regulatory safety checks by the government of South Korea, the ship began its operation in South Korea on 15 March 2013.
The ship then made two-to-three round-trips every week from Incheon to Jeju.
It was reported that Sewol again passed a vessel safety inspection by the South Korean Cost Guard on 19 February 2014.
This inspection was called an “Intermediate Survey” which according to International Association of Classification Societies (IACS), “include examinations and checks as specified in the Rules to determine whether the ship remains in a general condition which satisfies the Rule requirements.”
How the accident happened?
On 16 April 2014; about 30 kilometres (19 mil) off the southwest coast, the ferry began to list badly as it headed for Jeju. There were reports of the ferry having veered off course but the co-ordinates of the accident provided by port authorities indicated it was not far out of the regular shipping lane.
The ship, reportedly altered the course with a very big rudder angle (a sharp turn) and during that turn (to starboard side?), ship very heavily listed on her port side. Right at that moment, rescued passengers reported hearing a loud noise and the ferry coming to a shuddering halt – indicating it may have run aground although the water was reportedly 37 metres (121 ft) deep at the place where the ship capsized. Rescued passengers also reported that they were told “don’t move” by an announcement over the ship’s intercom system, whilst the ship was sinking.
Inside the ferry, chaos unfolded, survivors said, as the walls and floor seemed to exchange positions. Bottles and dishes fell. The ship’s twisting stairways became almost impossible to negotiate. Passengers were tossed to one side. Trays and soup bowls overturned, said Song Ji-cheol, a college student who worked part-time in the cafeteria.
The ferry was reported to be sinking at 8:58 am Korean Time.
At 9:30 am, the ferry was reported to have tilted 20 degrees to the Port side..
By around 11:18 am, the bow of the ship was submerged, with about 2 metres of height and 20 to 30 metres in length showing. As of 1:03 pm, the ship was completely submerged.
During the capsizing, it was first believed that passengers trapped in the vessel were able to send text messages to friends and family as the vessel sank. However, subsequent investigations by the Cyber Terror Response Center reported that survivors had not used their phones from noon on the 16th to 10 am on the 17th and determined that all reported text messages were fake.
Ocean temperatures in the area where the ship capsized were around 12 °C (54 °F), and the length of time before signs of hypothermia are exhibited at that temperature is approximately 90 minutes.
As of 19 April 2014; 28 of the ship’s 476 passengers and crew members confirmed dead and about 270 missing, compared to 179 survivors.
Captain accused of leaving the bridge
Initial investigations showed that at the time of accident, the third officer- an inexperienced officer who started to work onboard just 6 months ago- was on duty and the Ship’s captain Lee Joon-seok- a 68-years old professional- arrived into the bridge right after the accident. According the report of third officer, the Captain was calm and asked the angle of ship’s list.
In my experience, the Captain’s actions under the circumstances were as per standard practice. The Captain cannot be expected to be on the bridge at all times and the duty officer is qualified to navigate at open seas as long as the ship is not navigating in high risk areas or pilotage waters. So what happened?
We can only speculate.
Can a sharp turn cause sinking?
Answer is quite simple: Yes, it can. It can and for various reasons this can take place. But, there should be other assisting factors for this to occur.
First, if the ship has got a very small GM (Meta centric Height), or negative GM this can occur.
Such ships are called “tender ship" (Contrary to a “stiff ship”) and they usually have difficulty in correcting themselves if listed to one side.
The metacentric height (GM) is a measurement of the initial static stability of a floating body.
It is calculated as the distance between the centre of gravity of a ship and its metacentre.
A larger metacentric height implies greater initial stability against overturning.
Metacentric height also has implication on the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers.
Hence, a sufficiently high but not excessively high metacentric height is considered ideal for passenger ships.
An excessively low or negative GM increases the risk of a ship capsizing in rough weather, for example HMS Captain or the Vasa.
It also puts the vessel at risk of potential for large angles of heel if the cargo or ballast shifts, such as with the Cougar Ace.
A ship with low GM is less safe if damaged and partially flooded because the lower metacentric height leaves less safety margin.
For this reason, maritime regulatory agencies such as the International Maritime Organization specify minimum safety margins for seagoing vessels.
A larger metacentric height on the other hand can cause a vessel to be too “stiff”; excessive stability is uncomfortable for passengers and crew. As greater the metacentrik height goes, righting lever increases accordingly. It corrects the ship to come upright again.
If a ship floods, the loss of stability is caused by the increase in KB, the centre of buoyancy, and the loss of waterplane area - thus a loss of the waterplane moment of inertia - which decreases the metacentric height.
This additional mass will also reduce freeboard (distance from water to the deck) and the ship’s angle of down flooding (minimum angle of heel at which water will be able to flow into the hull).
The range of positive stability will be reduced to the angle of down flooding resulting in a reduced righting lever.
When the vessel is inclined, the fluid in the flooded volume will move to the lower side, shifting its centre of gravity toward the list, further extending the heeling force. This is known as the free surface effect.
The island group on the Port side of the ship may indicate that the ship altered course to Starboard and not to Port. There is a technical explanation on this issue which I will point out later.The island group on the Port side of the ship let us think that ship altered the course to Starboard, not to Port.
Which could be the case in Sewol accident?
If Sewol was not in a collision-with a submerged rock or any other unknown object- then, most probably, the ship had a very narrow metacenter height and large heel periods. Using the helm with a great angle- due to the inexperience of the officer on watch or a technical failure- could result in a big outwards heel.
When turning, especially in a sharp turn for which a greater angle of rudder has been used, the initial heel when the wheel is put over is inwards, because the rudder force is acting at a point below the centre of gravity of the ship.
As the ship begins to turn, the centripetal force on the hull (which is greater than the rudder force), acting through water pressure at a point below the centre of gravity, overcomes the tendency to heel inwards and causes her to heel outwards.
This outward heel is very noticeable when turning at good speed. If the wheel is eased quickly the angle of outward heel will increase, because the counteractive rudder force is removed while the centripetal force remains, until the rate of turning decreases. Should an alarming heel develop, speed should be reduced instantly.
Returning to the Sewol case; this is a question which remains to be answered: why was the ship's rudder put at a very large angle-most probably too hard to starboard (less likely to port) which resulted in the sharp turn and heel? Answers may vary: there might have been another ship, a sailing or fishing boat, or the island was too close on the port bow due to a drift (remembering the strong currents in the area, drifting should be expected) and the inexperience of the officer might have played a role here and caused the officer to panic forcing her to put the helm hard to either side. The inexperience of the watch officer could have played a role here because if she was aware of the metacenter height of the ship, she would not have done so.
On the other hand, the officer on duty, who appears to be a female officer, said to reporters that, she did not make a sharp turn, but "the steering turned much more than usual."
What is the meaning of this? To my interpretation; she means that she did not give a rudder command with a big angle; but, however, due to an error by the helmsman or a technical failure, the rudder went all the way to the side. Well, hard to port or hard to starboard is an exceptional order which is commonly used in harbor operations but never –except in case of an emergency- on a ship with little metacentric height at full speed ahead!Then, was the helm under manual or auto-pilot command? Here is another difficult question. Most probably it was on aoutopilot The officer just turned the knob for course alteration and autopilot pushed the rudder all the way to the side Because in autopilot use you can have no control on the rudder angle.
Of course, a human error of helmsman, or a mechanical failure of the rudder, should not be omitted. What the third officer said could be the truth. But the result remains unchanged: A rudder command was given with a great angle while the ship was proceeding at full speed.
As a counter action, the ship’s speed should be dropped immediately and in this case, according to wittness reports, this was also done.
Whatever happened; had happened at the initial heel, the very first one. Witnesses say that there was a very big noise. If not crashing to a submerged rock, or obstacle, what could be the cause of this, if not the clushing of cargo, either with each other or with ship’s side plates?
And, in this case, as the cargo of Sewol were nothing else than cars and vehicles in the garage and some containers on the fore deck, shifting-sliding-overturning of the vehicles at ship’s garage during the heavy listing could be the case. This could push the center of gravity towards the listed side and crashing of vehicles could damage the ship’s shell plates which could lead sea water inside- further worsening the scenario. The result is an immediate and inevitable capsize.
Sewol Captain: Did he know the ship was tender?
And how was this emergency situation was handled by the ship’s crew? Reports say that the crew- and obviously the Captain- underestimated the situation at the very first instant. A good risk evaluation would have resulted in notifying all crew and passengers to gather at muster stations. Ship’s emergency alarm should be sounded. However, this was not the case. The passengers were asked to stay in their cabins, assuming this would be safer. A wrong assumption!
Many people are trying to find an answer to the question: why the Sewol’s captain was so late to give the order for evacuation? The Captain's answer to this question is clear: "I did not want to risk the passengers in cold waters and strong currents until assistance had arrived”. But he could have ordered passengers to vacate the inner cabins and come to the upper decks, where they would have had a greater chance of survival -- without telling them to abandon ship. So why did he not do that?
In my opinion: the Captain knew the small metacenteric height that the ship had. If all passengers rushed up to upper deck, this would have worsened the situation by changing the center of gravity further up. That's why he did not call all passengers to the upper deck, hoping to balance the ship by then. Big risk, at the cost of the life of passengers!Lessons to take
This is another maritime accident to take lessons from but it has come at the cost of so many young lives. Maritime business requires seriousness. Accidents have no mercy once they occur. Human errors, which account for 85% of all maritime accidents; will not reduce accidents waiting to happen; but the number of accidents could decrease in a climate of improved safety in the maritime sector. All of us are responsible for undertaking this challenging task.
Some compare this accident with the Costa Concordia accident. In some ways, Captain Schettino handled the emergency situation much better where many more passengers had been evacuated and with much lesser victims in comparison to the Sewol. But, he was luckier. He had much more time before the ship capsized. But, we must appreciate that except his very early escape out of the ship, and the wrong maneuvering right before hitting the rocks (see my article Costa Concordia and Titanic: Same errors, same fate! ) Capt. Schettino did the right things in an emergency situation. But every situation has its exceptions.(NOTE: I wrote this article just 2 days after the accident, when the details were not yet very clear. But afterwards, all the facts proved my insights were all correct. Below are some of them.)
“You couldn’t turn the wheel very sharply on that boat. You just couldn’t. It was dangerous because the dynamic stability was not very good.” This is the statement one of the ex-crewmembers of Sewol ferry which was released after I wrote my article, concurring with my early remarks about the accident that the ship had a stability problem with marginal metacenter height.
Ex-workers also said that “the vertical extension and renovations to the cabin deck raised the ship’s center of gravity without taking into proper account the water levels in the ballast tanks” which further justifies my insight.
UPDATE-2The Korean Coast Guard has concluded that an "unreasonably sudden turn" to starboard made between 8:48 and 8:49 a.m. was the cause of the capsizing. According to the Coast Guard, the sudden turn caused the cargo to shift to the left, causing the ship to experience an incline and to eventually become unmanageable for the crew. The existence of the sudden turn has been confirmed by the analysis of the ship's Automatic Identification System data. The crew of the ferry has agreed that the main cause was the sudden turn. These facts, all concurs with my insights. Note that I found out that the turn should have been done to “starboard” just by analyzing the ship’s capsized photo.
* (Capt. Cahit Istikbal is President of Turkish Maritime Safety Association, Past-Vice President of International Maritime Pilot's Association, Past-President of Turkish Maritime Pilots' Association, and currently Chief Pilot at the Port of Istanbul.)
-----------------*Wikipedia, the online encyclopedia was used for some technical explanations in this article.