KR Webzine Vol.161

1. Introduction

IACS Common Structure Rule (CSR) for Bulk Carriers and Oil Tankers – ‘Rule Change Notice 1 to 01 JAN 2020 version’ was released in early January 2021, and these rules will enter into force on 1st July 2021 for bulk carriers of 90m or more and oil tanker of 150m or more, built on or after July 1, 2021. Many changes are included in this version of the rule, in particular, the stiffener buckling formulae, in order to improve the inaccuracy of the current beam-based formulae for the estimation of the global buckling capacity of stiffened panels under combined stresses.

CSR requires compliance with a number of possible failure modes, which are yield, buckling and fatigue failure for the ship structure.  Buckling failure mode is the instability phenomena of structural members following a sudden change in shape (deformation) of a structural component under load, such as the bowing of a column under compression stress (the force that cause a material to deform to occupy a smaller volume). It causes permanent displacement or a collapse of the hull structure as a result of smaller compressive stress rather than the material’s yield stress (the stress corresponding to the yield point at which the material begins to deform plastically).

If the allowable stress is designed based on only the yield strength, the structure may fail against the compressive load. Therefore, the design should be completed in order to have sufficient buckling strength against the compressive load.

2.Rule Revision

In ‘Rule Change Notice 1 to 01 JAN 2020 version’, the buckling assessment method for a stiffener has changed from ‘beam theory’ to ‘orthotropic plate theory’. For some stiffeners, such as the structural members subjected to the large transverse load, it has been found that their calculated buckling strength according to the previous CSR might decrease when either increasing the plate thickness or reducing the stiffener spacing. In general, if the plate thickness is increased, the strength capacity should be increased because of the larger strength or reducing the applied stress, but the strength is decreased on the contrary. The reason for this phenomenon might be the inaccuracy of the current beam-based formulae in previous CSR, for the estimation of the global buckling capacity of stiffener panels under bi-axial loads.

Beam theory is a simplification of the linear theory of elasticity which provides a means of calculating the load-carrying and deflection characteristics of beams and is not available to calculate imperfection against the transverse load. Because of the limitation of beam-theory, orthotropic plate theory is adapted to calculate the behavior of the structure subjected to axial and transverse load.

The orthotropic plate theory-based buckling assessment might be conducted on the deflection of the structure subjected to the bi-axial load more accurately than the current CSR. CSR has been revised to improve design capacity according to increasing the plate thickness. The buckling assessment rules of overall stiffened panel capacity and ultimate buckling capacity have been revised and it’s expected that the buckling utilization factor will be increased under the current CSR in some structural members, such as the stiffened panel with flat-bar (or small size) stiffener and the position applied the large transverse load.

3.Conclusion

KR released SeaTrust-HullScan incorporating the new buckling assessment based on the ‘Rule Change Notice 1 to 01 JAN 2020 version’ preemptively on 15 March, 2021. SeaTrust-HullScan developed by KR can be easily and effectively applied to the structural analysis of all ship types covering not only CSR for oil tankers and bulk carriers, but also container ships, LNG carriers and FPSOs.  It has a broad base of support with a variety of shipyards and design companies thanks to its variety of features which support ship modeling and strength assessment.

KR hopes to contribute to the improvement of design quality and productivity of ship design engineering by providing timely technical services to customers through the release of SeaTrust-HullScan incorporating the new CSR. Designers will be able to evaluate the buckling strength of stiffeners subjected to bi-axial loads more accurately using the new CSR.