MOL Comfort and Container Weights

MOL Comfort and Container Weights

Many have heard about the loss of the five-year-old MOL Comfort, a container ship classed by NKK.  After breaking in half at sea, the two hull sections drifted independently until sinking.  The aft section sank on June 27, 2013.  The forward section, which was consumed by fire in the interim, followed it thirteen days later, on July 10th.  Since comparable vessels in similar circumstances have sunk in much less time, one cannot blame the crew of 26 for abandoning ship so rapidly. 

Photo shows two life raft and the port lifeboat being readied to abandon ship. 

First, some facts about this incident which should aid in the discussion of the wider issues of ship design and cargo loading:

The MOL Comfort was carrying 4,382 containers on her last departure, barely 54% of her stated 8,110 TEU capacity.  Two factors must be kept in mind:

  1. The extended time interval of nearly two weeks for both ship sections to sink is astounding, though each was breached and open to the sea.  The weather was severe, measured as a Beaufort Force 7, with waves of 5 to 6 meters.  This demonstrates the large size of these ships, where consecutive flooding of adjoining compartments takes appreciable time to accomplish, in some instances even in heavy weather.
  2. The ship was only at half cargo capacity, thus not at maximum load draft.  Many cells remained empty, even though most units would have been stowed in the holds below the hatch covers.  As maximum weight and hull stress would have been achieved with a fully laden ship, this raises the question how a half full vessel could have broken her back, one whose scantlings were fairly new and not yet appreciably affected by incipient corrosion.

The salvors initially hoped to tow the forward section to a port-of-refuge where the structure could be studied to determine the cause of failure.  Though fire damage would have been superimposed on the structural aspect, the physical evidence would have been invaluable.  Ultimately, the tow cables parted and the two hull sections now rest at 4,000 meters of water depth.

NKK, the classification society, in an apparent bind, was left with no recourse but to address the perceived need to enhance hull strength – more specifically, the longitudinal hull strength.  It is in this plane the initial fracture occurred as evidenced by an extreme hogging force and transverse cracks in the weather deck.  This approach may not address the actual cause of failure, but can be regarded as a reasonable precaution until more evidence comes to light.

Since the loss of the MOL Comfort, the line announced that six of her sister vessels will be retrofitted with hull strengthening members, allegedly increasing hull strength to “twice as much” as the safety standard.  The classification society and shipyard have been very generic about how they intend to achieve this hull strengthening.  They do not specifically mention the customary means, such as external hull doubler plates, inserting interstitial shell plate longitudinals, laying straps on the weather deck, doubling and stiffening internals, or increasing overall scantlings in select locations.

What could have caused this loss, and how should the industry move forward to learn from it?

Lately a few container ships have been constructed with twice the carrying capacity of the MOL Comfort.  Principles of naval architecture are so refined that model ships are tested in virtual 3D environments before the keel is ever laid.  This virtual ship is repeatedly cycled through extreme conditions of sag and hog, so the design element leaves open two questions:

  • Are principles of naval architecture sufficient to address these larger modulus hulls?  Are shipyards, naval architects, and class societies inured from commercial pressures to maximize the carrying capacity at a lower cost per ton of newbuilding?

or

  • Does the answer lie elsewhere, such as the apparently well-stowed ship actually harboring unequal or even unknown weights about the midbody?  What were the departure drafts fore and aft … and midships for the MOL Comfort?  Does the shipboard loading computer act as a safety check or only as a rubber stamp to loading decisions made by shoreside staff?

Traditionally, masters have authorization to act if they believe their ships are not being loaded correctly.  In fact, they could amend any stow plan or countermand any loading operation if the ship or the cargo is being placed in jeopardy.  With the large scale of cargo evolutions and the rapid turnaround of ships in port, such belated actions are certainly not encouraged and may appear to the uninitiated as an alarmist afterthought.  In this vein, even if a vessel (such as the MOL Comfort) departed her load port with a pronounced sag, such a condition would have conceivably been duly noted in the log book and perhaps reflected in an e-mail to company headquarters.  Typically, the make-do response is to shift fuel or ballast on board to counteract any perceived weight imbalance.

At this point, we should reflect on the complex nature of loading a container ship and the extent of planning that goes into such an effort.  A proper stow requires an accurate stow plan, taking into consideration a complex interaction of requirements for various containers:  20-foot versus 40-foot cells, the number of available connections for reefer containers, hazardous cargo which must be segregated, boxes mandating below deck or above deck carriage, oversized and the occasional un-containerized special bookings.

In addition, there is the burden of meeting the customer’s challenges of trying to beat the cutoff with late bookings, misdeclared goods and, most notably, low-ball figures on actual weights.  One container looks much like any other.  The routine of loading a ship is so automated and uniform and the processing of bookings so labor intensive that suspecting the shipper’s declaration of outright falsehood is a mental leap most individuals and organizations do not attempt.  Customs has enough to do just catching a small portion of the contraband that smugglers attempt to ship.

Under-reported weights have long been recognized as an endemic problem.  The cumulative effect of under-declared container weights can lead to capsize, hull failure, or individual stack weight limits being exceeded.  On occasion, containers have surpassed safe working loads of cranes and toppled handling equipment.  The suggestion of weighing containers by means of the spreader assembly on the container gantry cranes is too late in the planning process, hardly the time to reconfigure the stow.

Calculating adequate hull strength with maximum bending moments about midships through a theoretical wave motion does not appear to be sufficient.  The least favorable distribution of relatively unknown container weights would need to be considered as well and added to the maximum sag or hog condition.  If ships are to leave port under the present system, then all container ships would need to be retrofitted with these “hull strengthening” measures to counteract the potential effect of overweight containers.  That is an expensive and not altogether ideal solution.

A stow plan is only as good as the data placed into the system, whether in the terminal or on board ship.  To completely safeguard ships and their crews, as well as valuable cargo, an efficient method must be devised whereby the terminal is capable of weighing each container at drop-off.  This is a true solution with correct data, which would allow the necessary lead time for an effective stow plan to be devised.  Retrofitting hulls to double strength is not an effective way to address unanticipated bending forces.  With that logic, we may as well strengthen individual containers, for fear that excessive stack weight will lead to crushed containers (as has occurred) rather than verifying individual weights in the first instance.

RTI’s marine experts have decades of experience in the handling, securing, and transport of many kinds of cargoes throughout the world, including the inspection and damage survey for the ocean and river vessels that carry them.  RTI is well placed to provide such services and can respond to requests for warranty work and more detailed investigations into causes of damage that may arise.

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