Keeping the world's super-ships safer

More and more ships are sailing the world’s oceans as trade becomes increasingly global. This has led to larger and larger ships being built to cope better with this increased demand for cargo capacity – but with bigger ships come bigger structural problems that can lead to disasters like the sinking of MSC Napoli in 2007. TULCS is a project that has spent three years investigating these structural challenges and now has the models available to avoid them at the design stage.

The unstoppable increase in global trade of recent years has led to huge growth in the amount of sea traffic. A consequence of this has been the building of larger and larger ships, culminating in a new class of ship called Ultra Large Container Ships (ULCS).

These vessels have capacities of up to 14,000 TEU and can be as long as 360m, and generally travel at a speed of around 27 knots. However, their prodigious size has led to some novel structural problems not seen in smaller ships, as Dr Sime Malenica of Bureau Veritas explains:

“Due to their large dimensions, ULCS are much ‘softer’ than other ships meaning that their structural natural frequencies, especially in torsion, are much lower,” he says. “This means that they tend to suffer from hydroelastic structural problems caused by the waves exciting these frequencies. This has become a critical issue in ship design and needs to be properly modelled.

“There are also problems with the particular structural design of the ships; the open midship sections result in an increased sensitivity to torsional and horizontal bending.”

Global hydroelastic structural responses in ships can be divided into two groups: springing and whipping. Springing describes resonant hull girder vibrations due to wave loads, whereas whipping describes transient hull girder vibrations induced by impulsive types of loading such as slamming (when the bottom of the hull ‘slams’ against the surface of the water). This whipping is strongly suspected to have played a role in the sinking of the MSC Napoli off the coast of Devon in 2007.

With the realisation that these problems were becoming more and more pronounced within larger scale ships, MAIB (the UK institution that deals with maritime problems) made a call for more investigations to be done in the field, as they believed it had previously not been addressed adequately. Thus TULCS was born, a project that aims to gain a better insight into avoiding these difficulties at the design stage.

Dr Sime explains how the project has utilised a number of approaches to try to better understand the problems facing these ships.

“We have investigated the issues via three different methods: one involves numerical modelling, the second is an experimental method using wave basins and the final one is taking full scale measurements on a real ship. By combining all of this data, we hope in the future to be able to give reliable advice to shipyards advising them how to avoid structural failures at the design stage.”

On the numerical side of things, one of the main objectives has been the development of efficient numerical hydrodynamic tools for seakeeping problems with forward speed, as well as the development of tools and methodologies for the inclusion of hydroelastic effects. This has been tackled in slightly different but complementary ways by the different partners involved, and these approaches now represent the state of the art of the subject.

Ecole Centrale de Marseille in France has successfully performed the first model test campaign on the simplified elastic model, and the planning for the second campaign is ready. At the same time, experimental plans for the sophisticated model tests on 9,200 TEU Container Ships have been agreed upon, and thanks to cooperation with another EU project, Extreme Seas, a much more detailed investigation on hydroelastic phenomena will be taking place. These tests are currently ongoing in the model basin of CEHIPAR in Spain.

As far as the full-scale measurements are concerned, one of the main objectives has been the installation of the ‘sea state measurement system’ onboard the 9,200 TEU Container Ship “Rigoletto” from CMA-CGM shipping company, who are also part of the project. Sea state is the general condition of the free surface on the ocean, and the correct estimation of the encountered sea state is critical for these full-scale measurements. Plans have also been made to use additional strain sensors in order to better cover the ship’s structural response.

One of the strongest points of the projects as a whole is the level of international collaboration that has been maintained throughout.

The team is composed of universities, research institutes and small companies as well as Hyundai Heavy Industries, the world’s largest shipbuilding company, and it is utilising different types and levels of knowledge from throughout the supply chain that has allowed the project to be a success. The first international workshop on springing and whipping was organised within the project, and this allowed the meeting of minds between all the key players as well as demonstrating how much interest there is for the subject presently.

Dr Sime is optimistic and excited about the future of the project: “TULCS is now in its third year, and there is a lot going on. We have a meeting in July that will be a critical point for the project, in which we will be looking at some of our more complex modelling of hydroelasticity. This is an area that we believe has pretty much been neglected until now, and so hopefully through our work we will be able to provide a much-improved level of information.

“What we have discovered so far is more or less in line with what we outlined in our original objectives,” continues Sime, “but it is important to note that even though this project is specifically oriented towards ULCS, the results could be used for other types of ships too. The methodologies and tools we have developed can be applied, with small modifications, to any other ship type and even floating off-shore platforms.”

In a world in which increasing demand for shipping capacity is a cold hard reality, ensuring the safety and integrity of these larger ships is of paramount importance. The consequences of the structural failure of one of these vessels could be far reaching, and as well as jeopardising the safety of their crew could have massive economic and environmental impacts. Hopefully, this project will help to ensure that disasters such as the MSC Napoli will be a thing of the past.

Click here to access the project website.

Published: Wednesday, 18th January 2012