of swell. A ship with high stability will right itself again quickly, and a ship with less stability will right itself more slowly. Too much or too little stability is bad, because high stability results in high acceleration, whereas inadequate stability could cause the ship to capsize.


The securing systems of a container ship are designed according to longitudinal, lateral and vertical acceleration values, which are calculated using empirical formulas of the classification societies. These formulas are based on swell statistics and transfer functions for ship behavior. Because container ships are


still evolving in terms of size, speed and container stowage height, these formulas are also constantly being revised. However, they never show the worst case scenario, and instead represent assumptions where there is only a very low, but generally accepted level of probability that they will be exceeded.


“ On average,


between 450 and 650 containers are lost on the world’s oceans every year, which is only a millionth of the total


number of units transported.


” 54 | The Report • June 2019 • Issue 88


The actual amount of acceleration caused by swell will depend on the characteristics of the ship and the swell itself. As a rule, the larger the ship, the less likely it is to be caught in seas that will cause it to move violently. Unfortunately, there is an important exception to this rule in respect of rolling movements. If small stimuli, even from relatively mild seas, strike the ship time and again at something even approaching a frequency that is in sync with the ship’s own roll oscillation, which is a function of its stability, the oscillation will be reinforced. This is known as resonance and works in the same way as children working a swing ever higher. This results in large roll angles and large lateral acceleration.


It is easy to see that such resonance can occur in particular when the swell strikes the ship transversely or at an angle. But there is also a special form of rolling motion, which can occur due to the periodic fluctuations of the ship’s stability in a longitudinal swell, i.e. when the waves come from the front. Due to the stability parameter, this form has been called parametric excitation. It is considered by practitioners to be particularly insidious, since it had previously been believed that rolling


oscillations could be minimized by steering into the swell.


Technical aids for the early detection of resonance of any kind are under development, but a decisive breakthrough does not yet seem to have been achieved.


On average, between 450 and 650 containers are lost on the world’s oceans every year, which is only a millionth of the total number of units transported. But if they are lost close to the coast, as was the case with the MSC Zoe, the cargo being washed ashore can cause major damage and attract unwelcome publicity. Hazardous goods can also be a threat to ecosystems and people. The reality is that such accidents are to be avoided if at all possible, and a great deal of effort is therefore being invested in exploring and clearing up the last of the unknowns. But container ships are getting bigger all the time, which brings along new problems. The container itself is part of the problem, as it experiences loads that test it to its limits. The onboard lashing systems, on the other hand, will generally have a 100 percent safety margin. But it only takes one link in the securing chain to fail and loss of cargo could well be the consequence. The way in which the cargo is secured in the containers would have to be monitored more closely, as would the weight of the containers themselves, although the verified gross mass (VGM) should long ago have ensured that this was correct. So there will always be a residual uncertainty, and this needs to be further reduced.


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