Why and how did the MOL Comfort break in half? The obvious answer is that no one knows, yet. We will learn more following an investigation, which is many months in the future. In the mean time, what are the likely causes of such a casualty? How and why could an only five year old ship, delivered in 2008, built at good quality shipyard, Nagasaki Shipyard & Engine Works, operated by a well known and established ship owner, Mitsui O.S.K. Lines, simply break in half?
The how is easier to answer right now than the why. From the photographs immediately after the ship began to crack, it appears that the hull failed due to excessive longitudinal bending stress. Container ships typically operate with a certain amount of bending stress due to the difference between the downward load of the weight of the cargo and the upward thrust of the buoyancy of the water. The downward load is distributed more evenly along the length of the container ship while the upward buoyancy is more concentrated toward the middle of the ship. Most container ships sail with a moderate amount of hog, which is to say with a slight downward droop in the bow and stern due to the imbalanced load. This is only noticeable when taking the draft of the ship at her marks forward, amidships and aft. Otherwise it is effectively invisible. The bending stress only increases as a ship works in a seaway. Ships, of course, are designed to handle these stresses, which is what makes the break up of the MOL Comfort such a shock.
So what could cause such a catastrophic failure? The two most obvious answers are a structural flaw in the ship’s hull girder or improper loading of container cargo. The first alternative, a structural flaw caused by improper design or fabrication, is not unheard but is highly unusual in a new ship. It has happened before with older ships.
MSC Carla
MSC CarlaIn 1997, for example the MSC Carla, a 25 year old ship sailed from La Havre bound for Boston and broke in half roughly amidships, off the Azores. The ship had been lengthen in 1984 and the failure was at the forward end of the new midbody. The design and installation of the new structure by the shipyard, Hyundai, was found to have been faulty.
While a structural flaw could be explanation for the MOL Comfort breaking in half, it seems to me much more likely that the ship was improperly loaded. Traditionally, the first concern in loading a container ship was stability, for obvious reasons. One glance at containers stacked 6 and 7 high on deck of a typical large container ship makes it clear why it is important to keep the heavier containers low so the ship does not roll over. Stability is generally well monitored and planned for. It is also obvious if you get it wrong. In addition to using loading programs to perform stability calculations, a captain can judge the stability of his ship by simply timing the roll period in a swell. If a mistake has been made, it should be evident.
Longitudinal strength is not quite as easy. For many years, longitudinal strength wasn’t a concern in most general cargo ships, including most early container ships. As ships grew longer and larger, however, mates an stevedores had to consider the ship’s longitudinal strength alongside the ship’s stability. Now, just as they have had to be very careful in the cargo’s center of gravity for stability, they also have to take care that they do not load too much weight in the ends of the ship, particularly on deck, so that the bending stresses will remain with within allowable limits. Fortunately, loading programs now allow the mates and stevedores to calculate longitudinal strength.
But how could have things gone so wrong on the MOL Comfort? If the cargo was loaded improperly how did it happen? One of the best and worst aspects of container ship operation is that the ships do not spend much time in port. Cargo handling is quick and efficient and a ship can be back to sea in hours rather than days or even weeks as was the case in the days of breakbulk cargo ships. If a ship in port for eight hours for example that doesn’t leave much time to for the Chief Mate to properly study the loading plan as he attends to other duties. Properly loading an 8,000 TEU ship like the MOL Comfort is immensely complicated. Multiple port calls also adds to the complexity. Container ships do not simply load in one port, cross an ocean and discharge in another. When the MOL Comfort sailed from Jeddah she had been calling multiple ports, loading and discharging containers along the way. If the containers loaded at Jeddah happened to be loaded toward the ends of the ship, and the ship had enough stability, it is possible that she sailed with a higher than acceptable bending stresses.
This is all speculation on my part. Nevertheless, improper loading seems a potentially cause for the catastrophic hull failure on the MOL Comfort. The failure could indeed have been caused by a structural flaw, or even by the combination of a structural flaw and improper loading. That being said, I wouldn’t be at all surprised if the failure was due to improper loading alone. Given the constraints and the operating tempo of container ship operation,it is not a particularly surprising mistake to have seen made. If this does turn out to be the case, we will see far more attention paid to calculation longitudinal strength when loading container ships in the future.
The how is easier to answer right now than the why. From the photographs immediately after the ship began to crack, it appears that the hull failed due to excessive longitudinal bending stress. Container ships typically operate with a certain amount of bending stress due to the difference between the downward load of the weight of the cargo and the upward thrust of the buoyancy of the water. The downward load is distributed more evenly along the length of the container ship while the upward buoyancy is more concentrated toward the middle of the ship. Most container ships sail with a moderate amount of hog, which is to say with a slight downward droop in the bow and stern due to the imbalanced load. This is only noticeable when taking the draft of the ship at her marks forward, amidships and aft. Otherwise it is effectively invisible. The bending stress only increases as a ship works in a seaway. Ships, of course, are designed to handle these stresses, which is what makes the break up of the MOL Comfort such a shock.
So what could cause such a catastrophic failure? The two most obvious answers are a structural flaw in the ship’s hull girder or improper loading of container cargo. The first alternative, a structural flaw caused by improper design or fabrication, is not unheard but is highly unusual in a new ship. It has happened before with older ships.
MSC Carla
MSC CarlaIn 1997, for example the MSC Carla, a 25 year old ship sailed from La Havre bound for Boston and broke in half roughly amidships, off the Azores. The ship had been lengthen in 1984 and the failure was at the forward end of the new midbody. The design and installation of the new structure by the shipyard, Hyundai, was found to have been faulty.While a structural flaw could be explanation for the MOL Comfort breaking in half, it seems to me much more likely that the ship was improperly loaded. Traditionally, the first concern in loading a container ship was stability, for obvious reasons. One glance at containers stacked 6 and 7 high on deck of a typical large container ship makes it clear why it is important to keep the heavier containers low so the ship does not roll over. Stability is generally well monitored and planned for. It is also obvious if you get it wrong. In addition to using loading programs to perform stability calculations, a captain can judge the stability of his ship by simply timing the roll period in a swell. If a mistake has been made, it should be evident.
Longitudinal strength is not quite as easy. For many years, longitudinal strength wasn’t a concern in most general cargo ships, including most early container ships. As ships grew longer and larger, however, mates an stevedores had to consider the ship’s longitudinal strength alongside the ship’s stability. Now, just as they have had to be very careful in the cargo’s center of gravity for stability, they also have to take care that they do not load too much weight in the ends of the ship, particularly on deck, so that the bending stresses will remain with within allowable limits. Fortunately, loading programs now allow the mates and stevedores to calculate longitudinal strength.
But how could have things gone so wrong on the MOL Comfort? If the cargo was loaded improperly how did it happen? One of the best and worst aspects of container ship operation is that the ships do not spend much time in port. Cargo handling is quick and efficient and a ship can be back to sea in hours rather than days or even weeks as was the case in the days of breakbulk cargo ships. If a ship in port for eight hours for example that doesn’t leave much time to for the Chief Mate to properly study the loading plan as he attends to other duties. Properly loading an 8,000 TEU ship like the MOL Comfort is immensely complicated. Multiple port calls also adds to the complexity. Container ships do not simply load in one port, cross an ocean and discharge in another. When the MOL Comfort sailed from Jeddah she had been calling multiple ports, loading and discharging containers along the way. If the containers loaded at Jeddah happened to be loaded toward the ends of the ship, and the ship had enough stability, it is possible that she sailed with a higher than acceptable bending stresses.
This is all speculation on my part. Nevertheless, improper loading seems a potentially cause for the catastrophic hull failure on the MOL Comfort. The failure could indeed have been caused by a structural flaw, or even by the combination of a structural flaw and improper loading. That being said, I wouldn’t be at all surprised if the failure was due to improper loading alone. Given the constraints and the operating tempo of container ship operation,it is not a particularly surprising mistake to have seen made. If this does turn out to be the case, we will see far more attention paid to calculation longitudinal strength when loading container ships in the future.
