As the MOL Comfort disaster clearly showed, there is a point at which the load on a ship’s structure exceeds the breaking strength.
One might say the vessel was brought “beyond the environment” or perhaps “outside the environment”… but those would be an incorrect assessments.
The steel structure of a vessel is made up of a complex arrangement of transverse (sideways) and longitudinal (lengthwise) plates and beams with precisely measured cross sections that contribute to the overall “section modulus” of the vessel, a measure of the overall bending strength of a given structure. In the case of the MOL Comfort, the vessel fractured in a transverse fashion because the stress on the structure of the vessel eventually exceeded the fracture point of the sum of the individual steel components.
The steel doesn’t simply snap in pieces like a glass might if you dropped it on a concrete floor. Because the steel itself has elastic properties, the ship itself is elastic. When put under load, the steel flexes and moves elastically, returning to its original shape every time.
The following video shows the elastic flexing of the containership MOL Excellence underway:
In the case of the MOL Comfort, the load put on the steel components exceeded the elastic range of the steel. That is, The stress put on the steel caused it to flex (strain) so much that it did not return to its original shape after the load was removed. At that point, the steel exceeded its yield point.
Plastic deformation was noted by ClassNK when they conducted their evaluations of the sister vessels of the MOL Comfort. They note that “buckling type deformations measuring approximately 20mm in height were observed on the bottom shell plates in the vicinity of center line of midship area.”
Just because the steel exceeded the yield point, does not mean however that the steel began to break. Steel, like many other materials actually becomes stronger as it goes through “plastic deformation.” This is called strain hardening, a process used by metalworkers to make things like sword blades or steel tools harder.
As the stress on the vessel continued to grow, the steel could not deform any further and it fractured.
Hull of the MOL Comfort begins to crack on June 17, 2013.
MOL Comfort sister ships had ‘deformed’ hulls, reveals probe into boxship disaster
The possible causes of the disaster that befell the 8,100teu containership MOL Comfort, which dramatically broke up and sank in the Indian Ocean earlier this year, have begun to emerge from Japan’s ship classification society, ClassNK.
ClassNK set up an investigation team in the wake of the disaster, while the Japanese ministry of transport established a large containership safety committee in August.
The society’s investigation confirmed that water entered the vessel through the bottom of the hull amidships, via a fracture “considered to have originated from the bottom part of the vessel”.
After the two halves of the vessel sank, between mid-June and early July, MOL pulled its six sisterships from their deployment on the G6 Alliance’s Asia-Europe strings, and had their hulls examined.
ClassNK said: “During the inspection of the sister vessels, buckling-type deformation[s], measuring approximately 20mm in height, were observed on the bottom shell plates in the vicinity of the centre line of the midship area.
“However, it remains unclear at this stage as to whether this type of deformation could have served as a trigger for the casualty.”
As a result, the sister vessels have had their hulls reinforced and theMOL Celebration, MOL Competence, MOL Courage, MOL Creation andMOL Commitment have returned from dry-dock and re-entered operations. Work is still under way on the MOL Charisma.
Inspectors have also been dispatched to check other large containerships, of different design to the MOL Comfort series, to see if similar “deformations” are prevalent.
However, sources close to the investigation have admitted that pinpointing the exact cause of the casualty will remain as much as matter of guesswork as science, given that both sections of the vessel are several kilometres below the surface and unlikely ever to be raised.
A source said: “So the investigation is basically speculation about possible causes based on the limited information available, and the actual exact cause might never be known.”
Meanwhile, the financial fall-out from the casualty has yet to be assessed, with sources in the insurance community suggesting that insurers could be facing claims totalling $400m.
MOL holds a $66m hull and machinery policy on the Mol Comfort – although according to vesselsvalue.com, the sister ships are now each worth between $46.1m and $49.6m.
The MOL Comfort was carrying 4,382 containers when it sank, with its cargo insured at sales price rather than cost of goods. Estimates have put the average value of the cargo per container at $50,000-$75,000 – which is not unrealistic given that those containers carrying hi-tech consumer goods will have been insured for up to $1m – thus leaving insurers facing $200m-$400m in possible claims.
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