The analysis report by the University of Hamburg stated that the container loss of the ZOE happened in beam seas where the dominant roll mode is rolling due to the excitation of the direct wave moment. The ship is rolling with the encounter period, which is approx. 13s. As the vessel travels (more or less exactly) in beam waves, the encounter period becomes (nearly) independent from the ship speed. The roll period was abt. 16 s, which is sufficiently far away from the encounter period. Consequently, resonance effects do not play a role for the container loss.
It further said:
It becomes immediately obvious that this roll angle is only possible if the ship is hit by a group of larger waves. When the ship was hit by a higher wave group, the rolling increased and the accelerations were severe enough that the lashing failed and the cargo fell overboard. It becomes immediately obvious that this roll angle is only possible if the ship is hit by a group of larger waves. Otherwise, the roll motion declines and the roll angle is abt. 10 degree.
Also:
Most probably, the crew was not aware of the fact that their hull did not produce sufficient roll damping and that the selection of the low ship speed would favour roll angles which lead to the cargo loss.
So not synchronous rolling in the case of the Zoe,
That U of Hamburg paper is interesting. Looks like those big ships will throw off containers with only a 15 degree roll due to the high accelerations. A 15 degree roll would be considered a bad roll on a car ship and corrective action would be taken if possible but just one or two 15 degree rolls would not likely be enough to part lashing gear.
A graph of the roll angle versus the acceleration. For the radius of gyration, which influences the value of H, I calculated with the usual formula 0.39B and used that in the graph. However, the report of the University of Hamburg mentions a more accurate formula which gives a radius of 0.3B. Will look into it later.
We learned from the report the importance of roll damping which is depending on the speed of the ship. It is a bit contradictory to what we usually do in bad weather, we are more in the habit of slowing down and now we have to increase the speed for better roll damping. Insufficient roll damping leads to cargo loss!
The types which you mention are not economically viable solutions, as are the fin stabilizers. Using the rudder for simultaneous steering and roll damping has been investigated for several decades. Early results demonstrated the importance of available rudder rate to achieve desired roll damping. Especially in the seventies and eighties a lot of research was going on into these much cheaper Rudder Roll Stabilization (RRS) systems which work with an adapted autopilot.
Two prototype vessels from the USCG Hamilton class already went for trials with a RRS in 1979. These trials went further with regards to the evaluation of ship performance under rudder roll damping; they assessed not only the roll reduction and heading interference, but also the increase in ship operability with different controllers. The roll reductions reported were between 31% and 49% for beam seas, about 22% in bow seas and 28% in quartering seas.
I wonder, seen the number of incidents with ever increasing numbers of lost containers, why the shipping world is not using this simple system with a proven track record on the vulnerable container ships. Or are they?
Perhaps this is too simple, stay the fuck out of bad weather, don’t worry about ETA’s. If you can’t, which happens sometimes, All the math and formulas aren’t gonna save your ass or your crew and cargo.
Passive Anti-Roll tanks are not economically viable?? What is the operational cost involved?
Unless you need optimal DW capacity, or take up cargo space, there is no revenue loss either.
Container ships are seldom (if ever) loaded to max DW. If DB or Wing tanks are used there are no loss of cargo space.
Passive Anti-roll Tanks has been used extensively for many decades and works even when slow steaming, heaved to, or operating on DP (stationary)
There is always an exception to the rule. Container vessels loaded out of Australasia, and South Africa with a cargo of frozen meat and dairy means that DW does become a concern.
The incident will be investigated by the Underwriters’ surveyors that will check any broken lashings and the logbook about speed and weather, when the incident took place, etc.
In my opinion the proximate cause is probably crew negligence. The ship was going too fast in the weather from way point to way point and the lashings were not correct. The crew was sleeping at the switches in their sound insulated cabins relying on the automation. Nobody thought of slowing down. We will see.
As Klaveness said, in that case the Anti-Roll tanks can be left empty, not to “steal” DW.
(How many times does that happen??)
BTW; If all containers are heavy for whatever reason, the need to stack 8-9 high on deck is probably not there. Thus less acceleration on top stow = less strain on lashings/support structure.
PS> Leaving some Fuel and FW tanks slack may be necessary to avoid a too stiff ship.
They don’t appear to have started cargo work in that picture, the spring and bow lines are just on but there is no stern line and the hatch has not been removed. I think the angle the picture was taken at is fooling you.
As both, the ship and the depression’s center far in the North, proceeded eastwards, there should have been big swell waves coming in from slightly different northerly directions. Ingredients for an isolated “monster wave”.
Within the totality of the discussion the lowly container itself needs to be looked at. How much can the box withstand during the conditions of dynamic loading before it begins to buckle/fail. During a roll period there are force vectors upwards, downwards as well as to the side.
