Fatigue Assessment Of Very Large Floating Structures Subjected To Complex Environmental Loads

Very large floating structure(VLFS) is a huge multi-modular platform that moored at specific waters to serve as airports or supply bases, which plays an important role in maritime security and resource exploitation. However, due to the structure detail and the large scale which can be more than one thousand meters, the multiaxial stress fields of the key joints produced by the complex environmental loads may lead to severe fatigue problem.The S-N curve method has been widely used to predict the structure’s fatigue performance in the field of ship and ocean engineering. However, there hasn’t been a benchmark for multiaxial fatigue problem yet. A multiaxial fatigue assessment of the VLFS’ welded strut joints has been taken here, which could be a reference for the design and configuration of VLFS and other benchmark study of multiaxial fatigue assessment in ship and ocean engineering.Hydrodynamic loads of the VLFS are closely related with its structure form. The paper presents the vertical moment transfer functions of of the VLFS and analyzes its relation with the distance between the underwater pontoons. Then the stress response of the strut joints and distribution of their amplitude and phase are given for further investigation on the multiaxial stress of the joints.The amplitude and phase of the stress response determines the variation of its principal stress. The paper presents the variation of the principal stress of the strut joints in regular waves and short-term sea state, which provides a basis for multiaxial fatigue assessment.Based on the multiaxial fatigue test data, several common multiaxial criteria are employed to analyze the effect of proportional and non-proportional stress on the material fatigue limit.At last, the paper establishes multiaxial spectral analysis method to assess the multiaxial fatigue damage of the VLFS’ strut joints, which introduces the transformation of multiaxial stress spectrum. Traditional spectral analysis given in ship societies’ rules, criterion recommended by International Institute of Welding and multiaxial spectral analysis are employed to assess the fatigue damage of the welded joints. Comparison of the yielded results indicates that multiaxial effect is little in regular waves or short-term sea state except several heading waves, but can not be ignored during the whole service life, in which case waves of different directions may occure successively.