Research On Fatigue Strength Assessment Method Of Ship Structures

Fatigue damage is one of the main failure modes for naval architecture and ocean structures and has been a vital research field for a long time in ship building and ocean engineering. The major purpose of this thesis is to make the research on the method for the fatigue assessment of ship structures. The main contents are as follows:1) Research on the simplified procedures by classification societies. In this paper, the simplified method presented by DNV, ABS, CCS and the common structural rules for double hull oil tankers by IACS(CSR) are mainly investigated. According to the different specifications in DNV, CCS, CSR and ABS rules, the discussion concentrates on the wave induced hull girder bending moment, the external dynamic sea pressure, the calculation of ship accelerations and motions, the internal cargo pressure due to ship motions, the hull girder stress, the local secondary bending stress, the local bending stress of longitudinal, reference stress, the regulation on corrosion addition, the regulation on mean stress, S-N curves as well as the criterion of fatigue assessment. The differences between the four rules are compared in details and the existing problems related to each rule are analyzed.2) Research on the spectral-based fatigue analysis. First, the basic theory and procedures of the spectral-based fatigue analysis of ship structures are dissertated and the regulations on the finite element modeling of ship structures in the direct fatigue strength assessment are given, herein the model extension, the meshing technique, and the selection of element type, etc. are included .Then, the application of fatigue loads is studied and a PCL-based automatic loading approach for wave pressures and inertial cargo pressures is presented. Finally, the other problems in FE analysis of fatigue stress, such as the calculation of stress transfer function, the boundary condition application and the stress extraction are studied. An automatic extraction of stresses and the cumulative fatigue damage calculation programs are written. Consequently, a solution scheme on the direct calculation of fatigue assessment is formed based on FE modeling, the automatic loading, the stress calculation, automatic stress extraction and the computation of cumulative fatigue damage. Based on the solution scheme, the fatigue assessment of three tankers and one FPSO are conducted at the connection of the longitudinal and web frames.3) Research on the design wave approach. Under the linear wave loads, the dominant load parameter and the selection of design wave in terms of dominant load parameters are dissertated. Then the characteristics of the design wave approach for fatigue assessment are discussed. Based on concept of the design wave, a new method is presented to select a design wave and the fatigue assessment of three tankers and one FPSO are conducted at the connection of the longitudinal and web frames. The fatigue assessment results based on the new design wave is compared with the results based on the spectral fatigue analysis.4) Under the deterministic fatigue assessment method, the fatigue reliability is considered. In accordance with the lognormal scheme and Weibull scheme, the fatigue reliability analysis of three tankers and one FPSO are conducted at the connection of the longitudinal and web frames, and the degree of the reliability of the fatigue life are given.5) With the APDL language, a parametric FE model of the structures detail at the connection of the longitudinal and web frame is built. In order to decrease the stress concentration and enhance the fatigue life, the shape optimization of the structural detail is carried out with the first order optimization method.