The Fatigue Assessment Methods For Ship Structures

Fatigue and fracture have been an important issue for ship and offshore structures for a long time. Growing attention has been paid to it from researchers and institutes in this field. Until now, the major classification societies have already released their fatigue assessments notes. But due to the complexity of factors influencing fatigue performance, such as wave load and pressure from cargo, the combination of different stress components, stress concentration of local structure details, mean stress and the corrosive environments, etc, there are different specifications with varying classification societies, so it will lead to the diversity of results from different fatigue assessment methods. The prime goal of this paper is to conduct the fatigue analysis for certain type of structural details of 12300T Ro/Ro vessel by means of three different approaches stated in DNV classification notes "the fatigue assessment for ship structures" and contrast the results from the three different methods.Firstly, the key points of DNV fatigue assessment are addressed, and then the fatigue strength of some nodes concerned is conducted using the simplified analysis provided by DNV fatigue assessment rules.. And the key the influence of mean stress for fatigue strength is computed and discussed when the mean stress is accounted for or not.Secondly, the molded lines and structural alignment should be referenced rationally in the process of the finite element modeling, but structural idealization should be based on the stiffness and anticipated response of the ship structure. And an approach is given to seek the response amplitude needed by spectral analysis method aiming at simplified the response amplitude seeking procedure.Thirdly, imposing pressure load on the wetted surface is a hard job because the pressure value is various and there is no regularity to fellow especially when the model is large. So it's very important to find a feasible automated loading approach that can be used in the FEM analysis of ship structure. This paper proposes an automated loading approach based on Patran command language that will enhance the efficiency of loading.Finally, the spectral fatigue analysis has been conducted under linear wave load and ship motion. In this analysis procedure, the mean stress is accounted for by obtaining the equivalent stress derived from Goodman formula or not and the difference between the two cases are discussed. At the same time, the contribution of each sea state to the fatigue damage is dissertated.