The early indication from the press pictures was clear. The Napoli had broken in half just forward of the engine room, giving clear evidence of a major structural breakdown.
This is unusual for a container ship, which is designed with a double hull and bottom and in which every alternate bulkhead is watertight. This is in fact what we are striving for with other ship types, such as bulk carriers and tankers.
Our best resources were put on the job from day one. Our ambition was to understand the reason for the structural failure. The collapse of the hull girder was caused by excessive loading. The container load condition was inside permissible limits but the sea state was heavy. The total loading exceeded the capacity of the hull girder, causing the ship to break in half just forward of the engine room. The crack on both sides could be observed from the many pictures available as the incident developed.
If we could reproduce or simulate the accident on a computer, could we then deduce something about the root cause of the structural failure?
This was the hypothesis put forward by our researchers. So that meant making a reliable model of the ship structure itself, with bunkers, ballast and cargo correctly distributed. Then as good a picture as possible of the environmental conditions had to be established.
This is what designers and class normally do when new designs are to be built. Global FEM analyses and hydrodynamic models of the sea loading are made and it is more or less an everyday sort of happening. Such “newbuild type” analyses were the first to be conducted in our investigation.
But with the Napoli, the challenge went way beyond that point. Our ambition was to simulate a structural breakdown, involving the collapse of plating and stiffeners and the consequent deformation of major structures. A traditional linear analysis does not suffice for this purpose. As the structure is deformed beyond the steel’s yield point, non-linear tools had to be put to work. This was a major undertaking as regards modelling and computing time, and is not normally done. I do not recall us ever having created a similar model.
We were able to simulate the structural failure and reproduce the crack in the ship side. The results can be verified by anyone comparing the picture from the model and the helicopter picture of the Napoli at sea. All of this can be seen in the short film we made to illustrate our point.
The buckling strength in the forward part of the engine room was insufficient. Where longitudinal stiffening of the fore and mid ship ends and transverse stiffening of the engine room starts is where the critical area lies. The longitudinally stiffened plate field has about twice the buckling strength of a similar transversely stiffened plate field.
The remedy is to fit buckling stiffeners in way. This is a minor structural modification which may be done afloat and involves a small amount of steel. Alternatively, a reduction in the allowable still water bending moment could be introduced.
Were other container ships at risk? A screening programme was devised to identify candidates for further investigation. The screening revealed some designs that needed to be looked at more closely. That is being done now. DNV has also shared this screening procedure with other IACS societies, and I believe all container ships classed by IACS societies are now being looked at. The screening procedure and results have also been shared with the MAIB.
Newbuilding rules and procedures are being examined. DNV has invited all IACS societies to cooperate on a unified approach to dealing with both ships in operation and newbuilds.
Have we done all we can? As far as we can see now, yes, we have. But we should always be open for input from other investigations and from the industry itself to find good ways of rectifying such problems and paving the way for good quality ships in the future.