The International Association of Classifications Societies (IACS) has adopted new Unified Requirements that are aimed at further improving the safety of large container ships by enhancing consistency between pre-existing Class Society provisions in this area.
IACS established at the beginning of 2014 an expert group on structural safety of container ships which carried out a post “MOL Comfort” review of available information which also took into account a number of past casualties. This work has resulted in the development of Unified Requirement (UR S11A) which is a longitudinal strength standard for Containerships that explicitly addresses the three issues requested of both individual classification societies and IACS by the authors of the MLIT investigation report.
These cover:
Bi-axial stresses which would be induced by lateral loading, i.e. external pressure on the bottom shell: IACS Members have for many years addressed these bi-axial stresses in their individual rules and procedures. The effect of the lateral loads which induce bi-axial stresses of bottom shell plates should be considered in the requirements of the hull girder ultimate strength and this will now be recognised in the new IACS Longitudinal Strength Standard for Container Ships, known as Unified Requirement S11A which will enter into force on 1 July 2016.
The whipping effect on container ships: Although this phenomenon continues to be the subject of research, the effects are becoming better understood and some individual IACS Members have developed specific rule requirements in this regard. The development of an IACS Unified Requirements for the whipping component of hull girder loading will take time, however in the interim IACS has introduced a functional requirement into the new Unified Requirement S11A which requires IACS Members to take into account whipping in accordance with their individual procedures. Entry into force is again 1 July 2016.
A revised wave bending magnitude and longitudinal distribution has been included in the development of the new Unified Requirement S11A full details of which will be made available on the IACS website shortly.
IACS expects UR S34 to set consistent requirements among IACS members by defining the unified minimum load cases used while performing strength assessment of container ships by Finite Element (FE) analysis.
“This fulfills two principal aims. Firstly, by prescribing high-level “functional requirements” on loads, the bottom line of structural strength becomes unified and, secondly, by developing a minimum set of common loading conditions for Cargo Hold Analysis in the midship region, a baseline for structural strength at cargo hold in the midship region is achieved, ” IACS said.
New S34 is applicable to container ships only and will apply from 1 July 2016 and requires a Global (full ship) analysis for ships with length ≥ 290m and a Cargo hold analysis for ships with length ≥ 150m.