ClassNK Completes Forensic Study of MOL Comfort Structural Failure

MOL Comfort breaks in two
MOL Comfort suffers a crack amidships while underway in the Indian Ocean, June 17, 2013.

With known weather and sea conditions at the time of the breakup, finite element modeling conducted by Japanese classification society ClassNK showed that the MOL Comfort’s hull should have had no problem handling the stress.  Nevertheless, the 8,000 TEU-class MOL Comfort suffered a hull girder fracture amidships on June 17, 2013, eventually split into two sections, and slowly drifted apart in the middle of the Indian Ocean.

Four days later the stern and hundreds (if not thousands) of containers sank in waters 4,000 meters deep.

Nearly three weeks later, the fore section of the ship caught fire in a blaze of glory and sank, finally putting an end to the MOL Comfort saga.

The loss of the MOL Comfort is considered the single worst containership loss in history.

The Investigative Panel on Large Container Ship Safety was established in light of the findings from an interim report released in December 2013 by the Committee on Large Container Ship Safety, which found that the hull fracture originated from the buckling collapse of the bottom shell plates underneath the No. 6 Cargo Hold.

ClassNK has calculated the total bending moment (torque) at the time of the incident was 13.8 billion N-m or approximately 10.1 billion foot-pounds.   This calculation was derived from the bending moment of the ship while in still water, a calculation that takes into account the cargo loading and other loads that are internal to the ship.  This calculation also factored-in the external, wave-induced loads and the loads generated by the whipping action of the ship’s structure as it goes from a hogging to a sagging condition while in a wave train.

Based on further calculations that factor in the probability of what the total vertical bending moment was and how strong the ship’s structure was at the time of the incident, ClassNK shows that it is physically possible, although unlikely, that the loads placed on the ship exceeded the strength of the ship’s structure.

load vs strength mol comfort
Relationship between strength of the ship’s structure and the loads place on the ship, via ClassNK

The following image would support such as theory:

mol-comfort-1

Looking deeper into the issue, the stresses on the ship in the fore and aft direction are only one part of the issue.  Lateral forces are also always in play inside the structure of a ship due to Poisson’s Effect which says that when a material is compressed, it tends to expand in the opposite direction, and vice versa.  While sailing into an oblique wave pattern, or with uneven cargo loading, these lateral stresses are greatly compounded inside the hull structure.

ClassNK modeled this situation in the following graphic:

buckling collapse strength mol comfort
Via ClassNK

Screen Shot 2014-09-30 at 3.38.07 PM

In the above graphic, the longitudinal (fore and aft) stress on a stiffened bottom panel is indicated on the x-axis and transverse (side-to-side) stress is indicated on the y-axis.  The redline shows progressively higher stress on a hull structure when under a one-bay empty condition – which is a worst case situation and one that containerships are designed to handle.  In this one-bay loading scenario, there are no containers to balance out the upwards hydrostatic force acting perpendicular to the hull.

For Post-Panamax-type vessels such at the MOL Comfort, ClassNK notes this is a particularly tricky situation as the hull form has greater beam and thus higher hydrostatic pressure acting on the hull than Panamax vessels.   In addition, with an engine room located mostly aft, the ship is in a perpetual hogging condition its entire life.

With this interaction between longitudinal and transverse stresses factored in, ClassNK concluded that the bottom shell plates experienced plastic deformation in the transverse direction just before the ship reached the maximum load of the longitudinal hull girders.

via ClassNK
via ClassNK

This led to a chain reaction of deformations in the steel structure leading to the final fracture of the hull girder. ClassNK explains this in more technical jargon here:

Screen Shot 2014-09-30 at 3.50.22 PM

In conclusion, ClassNK notes that to prevent future incidents such as the sinking of the MOL Comfort, “it is necessary to assess hull girder ultimate strength in proper consideration of the effects of lateral loads and to assess the buckling collapse strength of stiffened bottom panels in the middle parts of the holds.”

Also, ClassNK points out that although the design of Post-Panamax containerships provides a more stable platform than Panamax ships, the subseqently minimzation of ballast water results in a situation where the still-water bending moment reaches close to the allowable value.

The entire 123 page report can be found HERE