Use of Vertical Wave Bending Moments From Hydrodynamic Analysis In Design of Oil Tankers

Abstract

SUMMARY The paper describes calculation of design vertical wave bending moments for unconventional oil product tankers of new generation that are characterized by low length-to-beam ratio. The linear strip theory is employed for the calculation of transfer functions, while standard IACS procedure is used for long-term prediction of extreme values. Since the calculated design vertical wave bending moments at midship largely overestimate the values from IACS UR S11, additional analyses are performed to explain these differences. For that purpose, the same analysis is performed for conventional oil tanker which fully complies with contemporary IACS rules. Large overestimations of rule vertical wave bending moments are obtained also for that ship. The inaccuracies of the linear strip theory may partially explain these overestimations, as discussed in the paper. The wave scatter diagram proposed by IACS is analysed and found to be too severe for strength analysis of ship structures. However, the severity of the IACS scatter diagram is compensated by some other unconservative assumptions in the standard long-term prediction procedure, which is also elaborated. After all these aspects are put together, the obvious conclusion is that the IACS rule vertical wave bending moment should be substantially increased to enable safe design of oil tankers navigating in the North Atlantic wave climate. Finally, some suggestions regarding incorporation of hydrodynamic analysis for the calculation of extreme wave loads in ship design are given.