Class societies have reacted to current challenges and requirements within cargo stowage with updated rules that boost optimisation and reliability
As vessels have increased in size, so cargo-securing techniques have evolved to cope with the added scale. External lashing has now become almost standard, especially on larger vessels, because it allows operators to transport greater weights.
To address such changes, DNV GL launched its Modern Deck Container Stowage (MDCS) solution, which led to the development of new StowLash software. This was published in Q3 this year.
DNV GL inspector of cargo stowage and securing Daniel Abt says this is the first time the three-dimensional behaviour of a container stack has been considered in stowage software.
He comments “This model is more physically correct than any model used before. The 3D approach also takes into account the torsional strength of the containers correctly. If you can calculate more accurately, you can optimise stowage, and this helps large vessels with external lashing systems. Our previous assumption was quite conservative and now our calculations are more precise.”
This has allowed ships to load more containers. Mr Abt says it has made a “huge difference in how much cargo they can handle.” Of course, he notes there are a few cases where a more accurate calculation model may lead to slightly more restrictive container stowage than under the old model.
DNV GL created the model based on tests at the Pella Sietas shipyard in Hamburg.
Mr Abt says “This is the first time a full container model has been based on real, full-scale tests and allows the new calculation system to better know the distribution of forces in the container lashing system. The full-scale testing of different container stack configurations has been used as a basis for dynamic transient simulation, which is reflected in the 3D model.”
Another major development is that DNV GL has optimised its software to enable much faster calculation times. Mr Abt says “The dynamic transient simulation software used before meant these calculations usually took eight hours per stack. In the stowage software we have developed, we now perform the same calculations in under half a second. That is quite incredible. Improving the calculations to achieve these results has been a very complex undertaking.”
Introducing dynamic link library
Fellow class society ABS has been actively supporting several initiatives related to container securing.
In particular, ABS has developed a dynamic link library (DLL) within its ABS C-LASH software programme. ABS container ship market sector lead Jan de Kat says “The DLL is a fast, executable code that can be easily linked to existing loading computer software systems which has been distributed to loading instrument providers and allows them to incorporate the ABS C-LASH approach into their software.”
2019 updates to the ABS Container Securing Guide include an update to the tables of allowable stack weight below deck. In addition, ABS has introduced an approach to weather-specific lashing for vessels on short voyages.
Highlighting the challenges container ship operators face when it comes to container stowage, Mr de Kat says verifying loading plans throughout a voyage can be challenging.
He explains “As a vessel is unloaded, its center of gravity may be lowered and with the resulting increase in metacentric height, the natural roll period may decrease with the vessel becoming more susceptible to the encountered wave periods, which in turn may lead to higher accelerations.
Securing containers safely has become very complex given the challenges introduced by larger ships, higher container stacks, flexible bridges and increased stack weights.
The advent of very large and ultra large container ships has led to an increase in the number of containers stacked above deck. Mr de Kat says “Semi and fully automatic twistlocks have been introduced to speed up the container loading and unloading process, also with the benefit of safer stevedoring.”
“At the same time, however, higher stack heights and twistlock gaps result in effects not covered by traditional linear analysis methods. This may lead to an underestimation of loads acting on the containers and lashing system. Hence, the ABS cargo loading software solution C-LASH is designed to capture these effects and alert operators to risks.”
ABS technology can also be applied to assess the impact of ‘non-linearities’ on container stacks for feeder size container ships. “Smaller container ships commonly use a linear lashing analysis in view of the limited stack heights,” says Mr van Kat.
However, a recent study carried out for several ships with capacities between 3,000 and 4,000 TEU suggests that non-linear effects such as twistlock gaps can lead to significantly higher lashing rod loads and corner compression forces than predicted by linear analysis.
Therefore, says Mr de Kat, in cases of five-tier high container stacks, where internal lashing and fully automatic locks are used, it is recommended to apply a proper non-linear lashing analysis using ABS C-LASH software.
Snapshot CV: Jan de Kat (ABS)
Jan Otto de Kat has worked as a director at ABS since 2013 in Copenhagen. He is responsible for co-ordinating activities related to sustainable shipping in northern Europe and is the market sector lead for container ships. His previous areas of responsibility include energy efficiency, operational and environmental performance. He worked as senior director and head of innovation at AP Moller-Maersk between 2007 and 2012.
From 1989 through 2006 he worked at the Maritime Research Institute Netherlands, Wageningen, the last seven years as director of research and development.