In the past fifty years, container ships have been getting bigger and bigger. The ever increasing container-carrying capacity comes at a risk with large numbers of containers being lost overboard. This review explores the causes of some of the major incidents in the past few decades.
While at sea, a ship, the containers and the lashing devices can be subject to significant forces. Shipboard lashing arrangements, based on the strength of the containers and lashing devices, must be able to withstand these forces. The container securing arrangement must ensure that the maximum permissible forces on the container structures are not exceeded. The maximum permissible racking force for a standard ISO container is, for instance, 15 metric tons, while compressive forces should not exceed 85 metric tons. The safe working load of lashing gear, such as twistlocks and lashing rods is typically about 25 metric tons. Larger ships, carrying container stacks of up to 10 tiers on deck, use lashing bridges to further strengthen the vertical securing arrangement.
In addition to the containers’ standard design values and the strength of lashing devices, other important factors include
- severe weather conditions
- type and dimensions of the ship
- exposure to wind force (outermost stacks)
- the stability arm of the ship (GM value *)
- the stowage plan
*The GM value determines the ship’s initial static stability and righting lever. GM is variable, depending on the loading conditions. GM also influences the natural rolling of the ship: a higher GM results in larger rolling angles and shorter periods of roll, increasing the forces on cargo and lashing devices.
Dedicated software for container planning
The stowage plan limits stack weights and determines vertical distribution of weights. Suffice to say that heavy weights in the stack’s top tiers may result in exceeding the maximum permissible forces. Heavier containers must be relegated to the lower tiers of the stack.
The Cargo Securing Manual (CSM) calculations are based on the maximum permissible values allowing for GM, container weights, vertical weight division and the lashing configuration. Exceeding the permissible forces may result in the collapse of a container stack and even the collapse of an entire bay.
CSMs are typically configured for just one or two stability conditions, which does not suffice for larger ships. Dedicated loading software brings relief, allowing greater flexibility in container planning. The software quickly calculates and highlights areas where forces exceed the tolerance level, requiring remedial action.
Resonance rolling
Despite proper stowage and securing, substantial numbers of containers are lost at sea. Looking at the major incidents in the past few decades, we can conclude that many were caused by the ship’s rolling motion in severe weather conditions.
A review of these incidents shows that large roll angles and rolling acceleration resulted in exceeding the maximum permissible forces and consequently the loss of containers. This extreme rolling is caused by resonance events such as synchronous rolling and pitch-induced, or ‘parametric’ rolling.
The dangers of resonance rolling were described by IMO as early as 2007 in Circular MSC. /Circ. 1228 Revised Guidance to the Master for avoiding dangerous situations in adverse weather and sea conditions.
As said, an example of resonance rolling is pitch-induced or parametric rolling. Parametric roll motions result in large and dangerous roll amplitudes due to the variation of stability between the position on the wave crest and the position in the wave trough. Ships with fine bows and wide sterns, such as container ships, are prone to parametric roll, particularly when
- the ship is proceeding in longitudinal seas, i.e. head seas or following seas
- the wave encounter period – which is associated with the pitching period – is equal to or half the ship’s natural roll period
- roll damping is low (roll damping is related to forward speed, the shape of the ship’s hull and bilge keels; lower speeds reduce roll damping)
- the wave height is above a certain threshold level
Parametric roll being a resonance phenomenon, the combination of parameters may result in sudden, extreme rolling.
Forecasting and prevention
The various cases show that high waves combined with strong winds induced sudden extreme rolls, resulting in the collapse of container stacks and overboard loss of containers.
To assess a potential risk of parametric roll resonance, the Master must establish the wave encounter angle and period. A daunting task, considering the size of these ships, the severe weather conditions which are at the heart of resonance roll and often limited visibility, particularly at night. This is why ship motion monitoring, motion forecasting and decision support tools have been developed.
The software combines the ship’s navigation and stability data with weather forecast data and ship motion measurements of on-board sensors. The results are visualised in different ways. A polar diagram, showing the ship in the middle of a circle and its direction as a vector, clearly visualises the areas to be avoided. Alternatively, time trace information shows the historical probability of parametric roll, expressing the probability in a percentage. Whatever the form of the output, it helps to decide on what remedial actions to take, to steer the ship out of potential trouble.
Motion monitoring and forecasting software – ABB Octopus.
No matter how good the tools and the decisions taken, adverse weather and seas may still cause resonance roll. At this point it is vital that the resonance is eliminated, for which there are only two courses of action available:
- a drastic change in speed
- a drastic change of course
Minimising risks
Our review of over ten major cases shows that dedicated forecasting software could have prevented the loss of hundreds of containers. Proper lashing & securing, combined with excellent resonance forecasting and decision making is vital to the safety of the ship, its crew, its cargo and the environment.
Container stack on deck.