Investigative committee reaches inconclusive results after examining hull of Sewol ferry

Posted on : 2018-08-07 16:06 KST Modified on : 2019-10-19 20:29 KST
Still several conflicting theories on what caused the tragic sinking
The Sewol at its temporary preservation site at Mokpo’s New Port in South Jeolla Province.
The Sewol at its temporary preservation site at Mokpo’s New Port in South Jeolla Province.

A committee examining the hull of the Sewol ferry wrapped up 13 months of activity on Aug. 6 without reaching a unified conclusion on the cause of the ship’s sinking in Apr. 2014. It also failed to reach a decision on where to preserve the Sewol’s hull. A summary report submitted by the committee to President Moon Jae-in that day included two conclusions: an “internal hypothesis,” which states that the Sewol sank due to machinery defects or other such causes, and an “open hypothesis,” which proposes additional investigations of possible causes such as a collision or other outside force.

The cause of the Sewol’s sinking has long been a subject of debate. Initially, prosecutors presented an official conclusion on the sinking’s cause. In Oct. 2014, the Supreme Prosecutors’ Office (SPO) declared that the sinking took place because cargo that had not been properly secured shifted to one side while the vessel was listing due to steering error at a time when poorly considered structural changes and overloading had weakened stability.

But in Apr. 2015, Gwangju High Court suggested the Sewol may have sunk because of an engine failure rather than a steering error. It also indicated that the vessel would need to be raised to determine the exact cause of the accident.

Investigative committee’s on-and-off activity

A special committee to investigate the Sewol tragedy was subsequently launched in Mar. 2015. But the investigation was hampered by obstruction tactics by the Park Geun-hye administration, and the committee was forcibly disbanded on Sept. 30, 2016. At the time, the Sewol had not yet been raised, making a full-scale investigation of the sinking’s cause impossible. The hull examination committee took over from there, launched on Mar. 28 of last year – five days after the Sewol was raised on Mar. 23.

Now that committee has concluded its 13 months of activities without reaching an agreed-upon conclusion. The task of determining the Sewol sinking’s cause now falls on another special committee launched in March of this year to investigate the Sewol tragedy and a series of deaths and health conditions linked to the use of humidifier disinfectant.

The internal cause and open hypotheses suggested by the hull examination committee on Aug. 6 differ in their determinations of the GoM value, which represents the vessel’s stability. Stability in this case refers to a vessel’s ability to return to its original position when it is listing due to difference between its center of gravity (G) and center of flotation; the GoM value is a quantification of this. Typically, a higher GoM value is seen as safer and a lower one as more dangerous. The internal cause hypothesis includes respective GoM values of 0.406m when the Sewol first embarked and 0.306m when the accident occurred. The conclusion was the vessel’s stability was compromised.

“The Sewol was a dangerous ship that should never have left its port,” hull examination committee member and internal cause hypothesis proponent Kim Cheol-seung said in a press conference the same day.

Open hypothesis versus closed hypothesis

Members who support the open hypothesis estimated GoM values of 0.71m when the Sewol left port and 0.59m at the time of the accident – suggesting the vessel was relatively safe for travel when it initially departed. A GoM value of 0.59m was also given for the Sewol in an expert institution study commissioned by prosecutors at the time of their investigation.

The difference in presumed conditions has given rise to differing conclusions. According to the internal cause hypothesis, the low GoM value means the ship was always dangerous to operate, with the sinking occurring due to a failure of the solenoid valve used to operate the rudder. The solenoid valve is an important piece of equipment that uses electrical singles and hydraulic pressure to adjust a vessel’s rear rudder as the steering gear turns. If that valve fails, the rudder is left free to move on its own regardless of how the steering gear is operated in the wheelhouse.

The internal cause hypothesis was supported by committee chairperson Kim Chang-joon, vice chairperson Kim Young-mo, and Kim Cheol-seung, who concluded that the solenoid valve malfunctioned at around 8:48:57 am just before the Sewol’s sinking on Apr. 16, 2014. It was this malfunctioning, they argued, that resulted in the rudder continuing to return to the right side even as the helmsman turned the steering gear by five degrees.

As a result, the vessel made a sharp 34-degree turn from its forward direction over a period of 26 seconds between 8:49:13 and 8:49:39 am, entering a list of close to 20 degrees to its port side. As the vessel listed, improperly secured cargo shifted and the ship’s tilt worsened until it quickly capsized, they concluded.

In contrast, first subcommittee chairman Kwon Young-bin and committee members Lee Dong-gwon and Jang Beom-seon advocated an open hypothesis that includes the possibility of outside forces operating. Proponents of the open hypothesis agreed on the matter of the solenoid valve malfunctioning, but claimed the conclusion that the ship’s sharp turn occurred because of hardover (the rudder returned to one direction at maximum power after jamming) had been inadequately verified.

Analysis of dash cam footage from vehicles on board the Sewol showed the boat turning rapidly to starboard at a rate of over three degrees a second with a grinding sound at around 8:49 am on the day of the tragedy. The committee members’ conclusion is that there was some sudden change that could not be confirmed in the hundreds of experiments conducted with a model of the ship by the Maritime Research Institute Netherlands (MARIN). Since the sinking cannot be explained entirely in terms of mechanical failure or inadequate stability, external forces or other factors should also be considered, they suggested.

Possibility of external forces causing capsize

The differing GoM calculations from the internal cause and open hypothesis proponents stem from different determinations about the loads for the fourth and fifth ballast water tanks. Ballast water is added to tanks to improve a vessel’s stability. For the international cause hypothesis, the loads for the fourth and fifth tanks were calculated at 95 percent; for the open hypothesis, they were put at 98 percent.

The former saw the failure to maintain the ballast water tanks at full capacity as having added to the risks of the Sewol’s operation; the latter concluded that because the tanks were close to full and stability was not severely compromised, another force or factor would have been needed to cause the Sewol to capsize.

Determinations on the possibility of an external force were also mixed. Internal cause hypothesis proponents argued that the lack of any evidence of a submarine or signs of collision on the hull suggest no outside force was applied. In a visiting survey of the navy, the committee confirmed that of the three out of 14 submarines that were engaged in operations at the time of the Sewol incident, none came within 100 nautical miles of the scene. Open hypothesis proponents said the inability to identify the source of an external force does not prove that none occurred. They also called for additional example of “tearing” within the hull that was found around the fin stabilizer room after the hull was mounted.

“Major damage found within the Sewol’s hull and elsewhere on Aug. 1 could be interpreted as evidence of an external force,” Kwon Young-bin said in a talk with reporters the same day.

In response, internal cause hypothesis proponent Kim Cheol-seung said, “For there to have been an external force, major damage would have to be confirmed on the surface, and there is no evidence that can be seen as such.”

“It makes no sense to conclude that there was an external force based on severe damage internally when there was little damage to the outer panel,” Kim said.

By Jung Hwan-bong and Jung Eun-ju, staff reporters

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