Cargo stowage reacts to container ship challenges

Larger vessels, bigger problems

As vessels have increased in size, so cargo-securing techniques have evolved to cope with the added scale. For example, external lashing has become almost standard, especially on larger vessels, because it allows operators to transport greater weights.

DNV GL’s Modern Deck Container Stowage (MDCS) solution led to the development of new StowLash software, published in Q3 last year.

DNV GL inspector of cargo stowage and securing Daniel Abt says this is the first time that the three-dimensional behaviour of a container stack has been considered in stowage software.

He comments: “This model is more physically correct than any other model. The 3D approach is completely different to previous analytical approaches as it also takes into account the torsional strength of the containers. If you can calculate more accurately, you can optimise stowage, and this helps large vessels with external lashings systems. Our previous assumption was quite conservative and now our calculations are more precise.”

This has led to ships being able to load more containers; indeed, Mr Abt says it has made a “huge difference in how much cargo they can handle.” He does however note that in a few cases, more accurate calculation models may lead to slightly more restrictive container stowage.

DNV GL created the model based on tests at the Pella Sietas shipyard in Hamburg.

Mr Abt says: “This is the first time that a full container model has been based on real, full-scale tests and allows the new calculation system to better know the distributions 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.”

DNV GL has also optimised its software to enable faster calculation times. Mr Abt says: “The dynamic transient simulation software used before meant that these calculations usually took eight hours per stack. In the stowage software we have developed, we can 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.”

Fellow class society ABS has been actively supporting several initiatives related to container securing, including investigating the impact of stack dynamics on container securing systems. The aim is to provide better insight into the dynamic loads and response of container stacks and lashing systems as regards ship motions and hull response in waves.

ABS has developed a Dynamic Link Library (DLL) within its ABS C-LASH software programme. ABS containership 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. It 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 challenges that container ship operators face when it comes to container stowage, Mr de Kat explains: “As a vessel is unloaded, its center of gravity may be lowered and with the resulting increase in metacentric height (GM), 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.

“One driver for the weather-specific approach is to allow operators to take advantage of lower forces expected with benign weather when transiting on a short voyage, an approach which could lead to a reduction of necessary re-stowage of containers.”

A complex challenge

Securing containers safely has been made more difficult as larger ships have been introduced, necessitating higher container stacks, flexible bridges and increased stack weights.

The advent of very large and ultra large containerships has led to an increase in the number of containers stacked above deck. Mr de Kat says says: “Semi- and fully-automatic twist locks have been introduced to speed up the container loading and unloading process, with the benefit of safer stevedoring.

“At the same time, however, higher stack heights and twistlock gaps result in effects that are 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.”

As noted, fire safety has been another significant challenge for containerships and ABS has updated its Guide for Fire-Fighting Systems for Cargo Areas of Container Carriers. This guide outlines optional notations that provide crew protection, means of detection, firefighting systems and equipment that exceed minimum SOLAS requirements. Mr de Kat explains that the guide was developed in close co-operation with shipping companies and based on lessons learned from recent containership fires.

ABS technology can also be applied to assessing 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 nonlinear 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 for example internal lashing and fully automatic locks are used, it is recommended to apply a proper nonlinear lashing analysis using ABS C-LASH software.

Cell guide design development
The growing number of 45 ft containers, in addition to the larger size of ships, has had an impact on cell guide development.

Cell guides are used under deck, on deck, or in hatch coverless vessels, encompassing tank top to uppermost containers.

MacGregor senior naval architect Kari Tirkkonen comments: “Traditional cell guide design was based on fixed locations of standard and high cube containers, but we recommend calculating cell guides for any random stowage of different heights of containers.”

He adds that the cell guide design needs to cater for possible different widths of containers, by adjustable cells or by solutions with loose lashings.

Highlighting the impact of recent developments, Mr Tirkkonen says: “For Europe trades, the growing number of 45-ft containers creates challenges for cell guide design, because use of movable cell guides or other solutions need to be considered.”

In the case of too-high stacks and considering container strength, he says that the MacGregor stack splitter can be used to cut the stack in cells.

Speaking about the impact of larger container ships, Mr Tirkkonen says: “Basically, when the cell guides are higher, the complexity of random stowage has to be considered and the strength of cell guides has to be ensured, with consideration of connection to the ship’s structures.”

Other developments relate to cargo loading software. Mr Tirkkonen says that previously the stack weights of containers in cell guides were based on tables provided by classification societies. He says: “Today, on deck and in hold, MacGregor software can take care of the correct and safe planning of the loading in cells with or without stack splitters and also leave a clear documentation.”

Jan de Kat (ABS) – snapshot bio

2013 – present: ABS, director responsible for co-ordination of activities related to sustainable shipping in northern Europe and market sector lead for containerships. Previous areas of responsibility included energy efficiency, operational and environmental performance

2007 – 2012: A.P. Moller-Maersk; senior director and head of innovation

1989 – 2006: Maritime Research Institute Netherlands (MARIN), Wageningen, the last seven years of which as director of research and development.