A method is proposed to convert the fatigue damage of each sea state to a constant stress range value. Firstly, the FE model of the semi-submersible platform is established. Secondly, the procedures of the spectral-based fatigue analysis method are clarified. The element with Maximum fatigue damage is screened out. Then, the fatigue damage of the element for each sea state can be obtained. Finally, the fatigue damage of the sea state is equivalent to a constant stress range, which can be a guidance for the design of stress range for fatigue experiments of marine structures.
The Fatigue analysis and failure of marine structures are important issues. The method of treating fatigue behavior of tubular joints in offshore platforms was investigated (Graff et al., 1983). The prediction methods and steps of fatigue analysis were proposed for offshore platforms (Fang et al., 2000). Engineering calculation procedures of structural fatigue strength assessment for aging platform structures were summarized based on the theory of spectral fatigue analysis (Feng et al., 2010). The finite element simplified fatigue analysis method was proposed (Bao et al., 2011) to be used to effectively evaluate the fatigue properties of composite non-tubular joint structures. The fatigue life of semi-submersible was calculated to study the influence of different fatigue life assessment methods (Yang et al., 2011; Yang et al., 2012). The mooring chain fatigue damage was developed by coupled dynamic analyses (Du et al., 2016) under different conditions of semi-submersible platforms. Fatigue analysis methods and procedures (Liu et al., 2016) were established for analyzing fatigue damage in deepwater test strings subjected to wave action. The evaluation of the remaining life for offshore platforms in its in-place condition was conducted by dynamic spectral fatigue analysis (Saadian et al., 2018). The FE analysis model was built to simulate the fatigue life of spudcan for offshore platforms on the basis of reasonably simplification (Yang et al., 2020). The hydrodynamic-structure interaction analysis (Lee et al., 2022) was used to calculate the fatigue damage of semi-submersible platforms. The theory and methods of fatigue analysis for semi-submersible platforms are mostly focus on fatigue life assessment. However there few methods give the relationship between the FE results with the design of relevant experiments. According to the theory of spectral fatigue, the stress range of any sea state is not a specific constant value but conforms to a certain probability distribution. Therefore, it is necessary to assume a virtual constant stress range S0 to equate the fatigue damage generated by a specific sea state, which is meaningful to fatigue experiments to design the force and cycle number for structures.
