ClassNK to improve steel safety standards

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19 February 2015
Brittle crack arrest design steel for use on the upper deck and hatch side coaming. Photo: ClassNK

Classification society ClassNK has announced it will begin a joint research project aimed at improving safety standards for ultra-large container ships.

To ensure the smooth adoption of thicker steel in the industry, ClassNK has embarked on developing technical standards in order to clarify the crack arrest parameter for brittle crack arrest steel plates exceeding 80mm in thickness. It aims to create clear evaluation methods in a proposal it will submit for the unified IACS requirements.

The project will be carried out as part of the ClassNK Joint R&D for Industry Program in collaboration with the Japanese Welding Engineering Society (JWES), steel manufacturers, shipbuilders, and neutral research institutes, and will be completed in mid-2016.

In steel, brittle fracture is a break or cracking of the material into separate parts, but there is little or no plastic deformation. It appears as a clean break. It is also identified by rapid crack propagation with low energy release, again without any significant plastic deformation.

In January 2013, in light of this problem and to improve the safety standards of large container vessels, IACS published its 'Unified Requirements for Use of Extremely Thick Steel Plates'. Outlined in the document are necessary measures to be taken for the prevention of brittle crack propagation in the block-to-block butt joints of the hatch side coaming and upper deck to prevent large-scale fracture of the hull girder.

The MOL Comfort which broke in half and sank was thought to have broken apart due to a failure of the hull girder.

One remedy is to use brittle crack arrest steel, a material with measured crack arrest properties. However, the IACS requirements apply only to this special steel with a thickness of up to 80mm and vessels are now being constructed with plates that are 100mm thick. Due to a lack of guidance in the requirements when the thickness exceeds 80mm, the crack arrest parameter must be specifically agreed with each classification society.

ClassNK said, "With the introduction of 20,000teu ultra-large container ships, the increased use of brittle crack arrest steel exceeding 80mm in thickness is expected."