In order to estimate/calculate the magnitude of this pulling force, the excess tilting moment is to be divided by the total lever arm length.


Pulling force = (MT - MR ) / widthSTACK = 1,871.3kNm / 2.44m = 766.9kN


Thus, even in such a simple example, it can be seen that there are already approximately 78mt pulling vertically upwards on the twist locks.


Using the same principle and performing a calculation using different values, namely 12mt per container and with an average transverse acceleration as stated in the MSC Zoe investigation report together with the additional vertical acceleration, this would yield the following result:


MT MR Pulling force = (MT


= 5.665 x 7 x 12 x 7.89m = 3,754.5kNm = 4.2 x 7 x 12 x 1.22m = 430.4kNm


- MS ) / stack width = 3,324.1kNm / 2.44m = 1,362.3kN


It is at this point, at the very latest, the following issues should be considered:


- Even if the twist locks do not unhook due to vibration, transverse impacts or other factors, a maximum of two twist locks are counteracting the aforementioned pulling force. However, the twist locks of a well-known and certified manufacturer only have a rated tensile strength of 500kN. This means that even allowing for two twist locks, they are still both overloaded.


- Based on the documents of a classification society, the maximum vertical tensile strength of the corner castings is assumed as being 250kN. Accordingly, on the basis of the above calculation, these would also be overloaded.


- Is the use of fully-automatic twist locks sensible?


Of course, the use of fully-automatic twist locks saves time as compared to the semi-automatic twist locks which are otherwise used.


Arising questions


Nevertheless, the following questions arise:


- Can these savings in favour of a few companies justify the losses to the affected shippers, consignees and the entire insurance sector?


- Could these savings be perhaps used to mitigate the environmental damage caused? Is it at all possible to “mitigate” environmental damage?


- Possible solutions would be the use of fully automatic twist locks in bays where the above- mentioned tipping problems do not occur. Or one would have to counteract the tipping problem


with bridge fitting and thus, so to speak, create a large block from several individual stacks. Ultimately, it begs the question: which effort is ultimately greater?


- What will be the developments in future international guidelines?


In December 2020, a revised version of Annex 13 in the CSS Code was published by the IMO.


Based on the new Annex 13, acceleration values are reduced by reduction factors for short voyages and/or voyages with easily foreseeable weather phenomena. This is not actually relevant, especially with regard to large container ships.


The Report • June 2021 • Issue 96 | 59


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