A Simplified Formulation of the Linear Design VWBM Based on IACS Rec. No. 34 Rev.2

A simplified formulation for the linear part of the design vertical wave bending moment (VWBM) at midship is proposed. The formulation is based on the computation of the long-term VWBM for 25 ships of diverse types, using assumptions embedded in the new IACS Rec. No. 34 Rev.2. Transfer functions of VWBM at midship are calculated using semi-analytical Closed-Form Expressions. Long-term probability distributions of VWBM are fitted by the two-parameter Weibull probability function. Weibull scale and shape parameters are then approximated by linear multiple regression analysis, where the main ship particulars are taken as the regression parameters. As the most probable extreme value according to the IACS Rec. No. 34 Rev.2 is determined for the return period of 25 years, the average zero crossing response period in the ship design lifetime is also subjected to the linear regression analysis. Design VWBM is then calculated as the most probable extreme value by the proposed formulation and compared to the results of the long-term distribution. The results indicate that the regression equations can accurately approximate the results of the long-term distribution. Although numerical comparison with the existing Rule formulation is not the aim of the study, results are nevertheless compared with the VWBM from IACS UR S11 and UR S11A. It was found that the design VWBM from this study is lower compared with the existing IACS rule formulations. The proposed approach can be considered as the rule formulation of design VWBM and as a surrogate model of extreme VWBM for ship structural reliability analysis.