ClassNK R&D paves way for thicker plates in mega box ships

JULY 28, 2016 — In what it says is a breakthrough finding for ultra-large containerships, ClassNK has concluded that the minimum required brittle crack arrest toughness for extremely thick brittle crack arrest steel plates of 100 mm used in ultra-large containerships is 8,000 N/mm3/2.

The conclusion is based on verification tests carried out as part of a joint R&D project in collaboration with Japan Welding Engineering Society (JWES).

To enhance the structural reliability of large container vessels, IACS (the International Association of Classification Societies) released its Unified Requirements on brittle crack arrest design for the use of extremely thick steel plates in January 2013, outlining the necessary measures to be taken for the prevention of brittle crack propagation in the event of occurrence of a crack.

The requirements state that brittle crack arrest steel plates with a thickness of 80 mm or less must have a brittle crack arrest toughness value (Kca) of 6,000 N/mm3/2 or more at -10°C (the minimum design temperature for commercial vessels). However, there are no specific rules for cases where the thickness of brittle crack arrest steel exceeds 80 mm. The required Kca value for brittle crack arrest steel plate with a thickness exceeding 80 mm is to be specifically agreed with each classification society.

In order to clarify the required brittle crack arrest toughness value (Kca) for extremely thick steel plates exceeding 80 mm, ClassNK carried out investigative research through a joint R&D project with JWES. As part of the joint project, verification tests were carried out by ultra-large-scale test specimens simulating the actual construction of the hatch side coaming and upper deck of ultra-large containerships .

The results revealed for the first time that brittle crack arrest steel plates with a thickness of 100 mm must have a brittle crack arrest toughness value (Kca) of 8,000 N/mm3/2 or more at -10°C.

Based on the results of these verification tests, and as part of its response to the ever-increasing size of the design and construction of container vessels, ClassNK will, in principle, require brittle crack arrest steel plates with a thickness of more than 80 mm and not exceeding 100 mm to have a brittle crack arrest toughness value (Kca) of 8,000 N/mm3/2 or more at -10°C, as the severest conditions.

ClassNK says that, to enable facilitate the use of extremely thick steel plates in the shipbuilding industry, it will endeavor have the new finding reflected the IACS Unified Requirements.

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