Friday, February 20, 2015

Container Ship Disaster Accelerates Stress Failure Investigation as Freight Vessels Continue to Grow

New Steel Standard Failed the Only Real Test - Now Classification Society Studies the Problem
Shipping News Feature

JAPAN – WORLDWIDE – Classification society ClassNK is to carry out a joint research project aimed at improving safety standards for ultra-large container ships, by working towards establishing safer brittle crack arrest design in the construction of the large vessels. The increasing size of containerships has naturally led to the use of ever larger and thicker steel plates and the preventative measures for brittle fracture has become even more important, especially in areas where higher stress occurs such as the upper deck and hatch side coaming of the cargo holds.

Such thick steel plates are used in the construction of containerships in order to satisfy the requirements for longitudinal hull girder strength, especially in light of the open deck design of these ships. The open deck design means that longitudinal strength must be ensured with limited structural members. The application of such extremely thick steel plates in hull structures, however, raises a few concerns as to the brittleness of the plates, and considering that a failure in the hull girder strength is believed to have led to the MOL Comfort disaster, these concerns are valid.

Used especially in the construction of strength deck structures, such as hatch side coamings, strength deck, sheer strakes, and longitudinal bulkheads in order to accommodate greater loads these plates can exceed 80 mm in thickness, with some plates reaching thicknesses of up to 100 mm, sizes previously uncommon in shipbuilding.

In order to improve the safety standards of large container vessels, IACS released its Unified Requirements for Use of Extremely Thick Steel Plates in January 2013, which outlined the 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 any large scale fracture of the hull girder.

One of the preventative measures IACS outlined is the use of brittle crack arrest steel, defined as steel with measured crack arrest properties. However, the IACS requirements apply only to brittle crack arrest steel plates with a thickness of up to 80 mm and with the introduction of 20,000 TEU ultra-large container ships, the increased use of brittle crack arrest steel exceeding 80 mm in thickness is expected. Where the thickness of these steel plates exceeds 80 mm, the crack arrest parameter must be specifically agreed with each classification society.

To ensure smooth adoption of thicker steel in the industry, ClassNK has embarked on a joint research project to develop technical standards in order to clarify the crack arrest parameter for steel plates exceeding 80 mm in thickness. Through this joint research, ClassNK aims to establish clearer evaluation methods to form the proposal for unified IACS requirements for brittle crack arrest steel exceeding 80 mm for the benefit of the entire maritime industry.

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 is scheduled to be completed in mid-2016.

ClassNK has looked into this project before and its outcome was the development and the practical application of a new grade of higher tensile steel plate with a specified yield point of 47 kgf/mm2, known as YP47 steel plate. The MOL Comfort and her six sister ships were the first series of ships to use this YP47 plate and following the Comfort’s disastrous outcome and ClassNK's subsequent report, the other six vessels needed to have their hulls reinforced last year, to twice the standard required by ClassNK.

Photo: The bow section of the ill-fated MOL Comfort catches fire after the stern had drifted away only for both to sink later.