Comparative analysis of the long-term extreme VWBM calculated according to IACS Rec. No 34 rev 1 and rev 2

Abstract

After facing some criticism regarding assumptions contained in IACS Rec. No. 34 Rev 1 (2001), efforts have been put in by IACS members resulting in a new revision of this document, which provides a procedure for computation of the long-term extreme wave loads on ocean-going ships. The main changes include a new scatter diagram, modification of the wave spectral model and energy spreading function, as well as probability level of design load. Preliminary, the reduction of extreme loads from 10% to 30%, depending on the vessel and response type, is estimated. As this reduction could have serious implications for ship structural design, the present research aims to further investigate consequences on the most important load component, i.e., vertical wave bending moment (VWBM) at midship. Transfer functions of VWBM are calculated using closed-form expressions, which are convenient for comparative studies where many computations are required. Ships of different types and sizes are subjected to the long-term VWBM analysis comparing extreme values based on the procedures proposed in Rev 1 and Rev 2. Although this study is not intended to provide extreme VWBM in absolute terms, the comparison with IACS UR 11 is nevertheless presented as a reference. It is found that Rev 2 long-term VWBMs never exceed corresponding IACS Rule values, which can’t be stated for Rev 1. Results also indicate that differences in scatter diagrams between Rev 1 and Rev 2 could explain a large decrease in the extreme VWBM.

Consequences of the Improved Wave Statistics on a Hull Girder Reliability of Double Hull Oil Tankers

“…2 provided lower lifetime VWBM compared to Rev. 1, as already reported in [12,14]. 5, while more details regarding the regression analysis can be found in [26].…”

“…The finding was confirmed by the analysis of extreme vertical wave bending moments (VWBM), showing that the extreme values calculated by Rev. 2 were reduced by 13% to 19% for tankers and 16% to 23% for container ships, with a larger reduction being obtained for smaller ships [14]. Differences in wave scatter diagrams were found to be the most important for these reductions, supported by the fact that the extreme wave amplitude for a 25-year return period was 2.2 m higher for Rev.…”

“…1 compared to Rev. 2 [14]. The analysis of significant wave height resulting in the same extreme VWBM as from the long-term distribution was performed in [15].…”