Compared to smaller vessels, the largest container ships afloat are likelier to pick up vibrations caused by waves. These vibrations can cause structural damage, potentially endangering lives. This difference must be taken into account when new ultra-large container ships (ULCSs) are built. EU-funded research has charted a course towards safer ship design.
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Gone are the days when a ship carrying a 5 000 20-foot (about 6 metre) containers was considered huge. Some of today's ULCSs can handle up to 18 000 containers and are close to 400 metres long. Several shipyards are already designing even bigger vessels.
Due to their sheer size, these behemoths of the sea are more prone to springing and whipping, two types of vibration-related phenomena that can damage the structure. The EU-funded TULCS project has analysed these risks, improved modelling tools and contributed to the development of guidelines to boost safety at sea.
A new twist
Compared to smaller ships, ULCSs lack one important structural element: a deck to close off the midship section and strengthen the hull structure. As a result, ULCSs are more vulnerable to structural bending and twisting.
Their structure also has lower natural frequencies, which are more likely to resonate with sea waves. Resonance can cause a continuous vibration in the ship's structure, explains Dr Sime Malenica of Bureau Véritas, who led the TULCS project. This phenomenon is known as 'springing'. It is quite subtle and crews are unlikely to notice it, but it can damage the structure over time.
Springing is due to short and relatively mild waves resonating with the natural frequency of the ship. Whipping, in contrast, is caused by heavy seas – big waves, by which part of the ship is lifted clear and then slams back on to the water. This impact puts a lot of stress on the ship’s structure and causes the short-lived but massive vibrations called whipping.
“These things are serious problems,” says Malenica, pointing to shipwrecks in 2007 and 2013 that are likely to have been caused by such phenomena. “The MSC Napoli was broken in two, most probably due to these effects,” he says, “and recently there was another container ship, the MOL Comfort.”
The problems are not new, he explains, and the safety margins built into international ship design rules take them into account for smaller vessels. But, says Malenica, for ULCSs, the rules should be more explicit.
Not as tough as they look
The TULCS project, which included leading members of the shipbuilding community, set out to develop and validate simulation tools and guidelines that would address the design specific to ULCSs, with a particular emphasis on springing and whipping. This task involved experimentation in the lab and measurements at sea to collect data and crosscheck results.
By the time the project ended in November 2012, it had generated a wealth of new knowledge. All partners had improved their design software and shared results with each other. The project’s findings are feeding into the development of new rules and design guidelines for ULCSs, helping to keep crews and containers safe at sea.
But there is still a lot to learn, says Malenica. A number of new research projects have taken up the work initiated by TULCS and are focusing on specific aspects of wave-induced vibration.
And the partners are continuing to exploit the project’s results. “The full-scale measurements from instruments we put on an actual container ship are still ongoing,” Malenica notes. “We continue to collect this data. This means that we will have more real-time feedback and more data for the validation of our models.”