Safety Assessment of Container Ships

Container ships have been increasing in size noticeably in recent years, and very large container ships with load capacity exceeding 10,000 TEU have been completed worldwide. Very large 8,000 TEU class container ships have already been built to NK class. With the increasing size of the hull, very thick steel plates exceeding 70mm in thickness are being used in the hull structure around the upper deck. Laboratory studies in recent years, however, have reported that brittle cracks that occur in the welds of very thick steel plates in fact propagate in straight lines without swerving and may not stop even after penetrating the parent material, which is contrary to conventional wisdom.

This suggests that if a brittle crack does in fact occur in a hull structure using very thick steel plates, there is a risk that this brittle crack could propagate at high speed and cause a major or catastrophic accident such as failure or collapse of the hull structure. Thus, the proper study of the brittle crack propagation behavior of very thick steel plates and the establishment of suitable technical standards to prevent the occurrence of brittle fracture accidents can be considered as being critical matters of concern.

The present research and development work undertaken by ClassNK is aimed at studying the brittle crack propagation behavior of very thick steel plates used in very large container ships, and formulating rational technical standards for suitable design methodologies for arresting brittle cracks from the viewpoint of ensuring structural safety. The R&D project is to be carried out during fiscal 2007 and 2008 and its scope includes:

• Large-scale model tests as necessary to establish technical standards related to the brittle crack propagation behavior of very thick steel plates based on the latest knowledge.
• Study design methods that are effective in arresting the propagation of brittle crack based on the knowledge obtained from the above tests.
• Develop technical standards for designs incorporating suitable brittle crack arrest techniques, and at the same time study the establishment of related rule requirements.