Advanced Integrated Design Method for Fatigue Assessment of Large Elastic Ship Structures

Abstract

This paper is focused on an advanced method for ship structures fatigue assessment. The ships classification societies standard rules for fatigue analysis are based on simplified procedures, with wave induced loads obtained by linear oscillation analysis (low frequency, around 0.1 Hz), or equivalent statistical wave loads. In the case of large elastic ship structures, with hull length over 150 m, the global wave induced vibration response (high frequency, around 1 Hz) becomes significant. The developed integrated method for large ships fatigue assessment includes three interlinked analyses, as follows: the hot-spot stresses evaluation by 3D finite element models, wave induced loads by short term linear and non-linear hydroelastic dynamic analysis, ship service life and fatigue assessment by damage cumulative ratio method. As testing ship, it is considered a double hull LPG Liquefied Petroleum Gas carrier, with total length 239 m, for a set of structural details with stress hot-spots. Based on the non-linear hydroelastic wave loads, the integrated method of fatigue assessment becomes more accurate, predicting for the amidships structure 14 years of ship service life, instead of over 20 years according to the rules standard approach, so that the confidence on ship structure fatigue evaluation can be increased in the design process.