Buckling assessment procedure for large diameter vessel with multiple local thin areas subjected to external moment

The procedures described in Ibaraki FFS rule (2006) based on the p–M (internal pressure ratio and external bending moment ratio) method (Konosu and Mukaimachi, 2006) were used to predict the plastic initiation condition and the collapse condition for cylinders with a local thin area (LTA) subjected to combined internal pressure and bending moment. The effective wall thickness, t*, for the buckling assessment on a vessel with an LTA was proposed for use instead of wall thickness based on approximation which simply took into account only the effect of section modulus reduction (Konosu and Mukaimachi, 2006). The buckling evaluation on a vessel with an LTA was presented by Mukaimachi and Konosu (2009) and that was shown to be easily and adequately conducted by applying t* to the well-known Donnellʹs or Millerʹs equation. In this paper, the attention was focused on the procedure for assessing the buckling of large D/t vessels with multiple aligned or non-aligned local thin areas. From the comparison with the results of non-linear FEA, a combination rule and a buckling assessment procedure for multiple LTAs (flaws) subjected to external moment are proposed. The interaction-affected area for multiple cracks can be applied as the combination rule to evaluate the buckling of a large diameter vessel with multiple flaws subjected to external moment. From the comparison between the buckling loads of FEA results and the conventional buckling equations using the measured yield stress, an additional safety factor ‘ks’ is proposed so as to maintain the margin of 1.5. When the specified minimum yield stress is applied to Donnellʹs or Millerʹs equations, the margin for the buckling load of a cylinder with multiple flaws is more than 2.08 and 2.29, respectively.