Modern container ship designs extensively employ high tensile (HT) steel. This is necessary to keep the plate and stiffener dimensions at an acceptable level.
The growth in container ship size is leading to even higher tensile steels in order to reduce the required dimensions. The higher strength steels give rise to concern about the fatigue life which needs to be properly addressed during the design and approval stages.
The use of HT steels leads to higher stress levels. Although the base material HT steel has a higher fatigue capacity than mild steel, most of this is lost when the HT material is welded. So, for welded structures, the fatigue capacity of HT steel is much the same as that of mild steel. It therefore becomes even more important, as HT strength increases, to pay closer attention to the design of local details to improve the stress concentration factor, and thus the fatigue life prediction.
The issue of reduced fatigue life for HT steel and the need for improved stress concentration factors have been known to the industry for many years. In order to address this issue, IACS adopted a Unified Requirement for material factors for HT32 (yield stress minimum 315 N/mm2) and HT36 (yield stress minimum 355 N/mm2) steel in 1973 (IACS UR S4).
The material factor (k-factor) is used to establish the allowable stresses for strength calculations. (Note that the material factor used by IACS and other class societies is the inverse of the material factor as defined by DNV.)
Due to the implied reduction in fatigue life, the unified material factors are not proportionally related to the yield strength of the material. The factor implies a lower degree of utilisation compared to the yield strength as the steel strength increases. This is illustrated in the figure, where the straight line illustrates a material factor proportionally related to the yield strength, while the curved line with associated points illustrates the unified material factor as adopted by IACS.
HT40 steel (yield stress minimum 390 N/mm2) is typically applied in the longitudinal and continuous hatch coamings of post Panmax container ships. The material factor for HT40 steel was, however, not unified for IACS members until IACS adopted the requirement in April 2007. Prior to implementation, each classification society applies different material factors for HT40 steel, ranging from 1.43 to 1.52. The material factor is directly linked to the IACS minimum section modulus requirement. Without a unified material factor for HT steel, the minimum section modulus requirement is in practice not unified. Hence, although IACS had in place a unified strength requirement for the midship region, this was in practice not unified for ships with HT40 steel due to the lack of a unified material factor for this steel quality. This issue was addressed with the adoption of rev. 2 of IACS UR S4 of April 2007, which specifies a material factor of 1.47 for HT40 steel. The normal IACS implementation schedule is such that all member societies should implement the revised Unified Requirement in their rules not later than one year after adoption, namely April 2008.
Ship builders and designers are today designing ultra large container ships using high tensile HT47 steel with a minimum yield strength of 460 N/mm2 in the continuous longitudinal hatch coamings. One reason for this is obviously to reduce the scantlings and lightship weight. Another argument for using HT47 steel is that the reduction in the plate thickness of the hatch coaming enables easier welding with less risk of introducing weld defects.
Although IACS has yet to agree on a common material factor for HT47 steel and higher tensile steel, it is likely that IACS will form a group to decide on a unified material factor for HT47 steel too. Owners ordering ships where HT47 is used should be aware that the IACS unified material factor is not yet in place and that classification societies may adopt different values. This may result in different stress levels and utilisation of the material depending on the class society.