Fatigue Strength Analysis Of A Large-scale SWATH

Compared with conventional mono-hull ship, small water-plane area twin hull ship(SWATH) contains complex structure form and special wave loads. Fatigue problem of SWATH was more serious. In recent years, because of large-scale design and extensive use of high-strength steels, fatigue strength problems of typical details of SWATH became much more difficult to meet the requirements. At present, relevant norms and studies on large-scale SWATH is very limited both in the domestic and foreign. Corresponding anti-fatigue design basis is rare. In this paper, the evaluation methods of structure fatigue strength for SWATH were researched with a large-scale SWATH as an example. The main contents were as follows:1) Fatigue load simplified calculation and its stress response analyzed by FEM in SWATH rules of CCS was studied. The differences among fatigue strength simplified evaluation process presented by CCS, DNV and ABS fatigue rules were compared. And the applicability of each rules were discussed.2) Focus on the problem that the number of fatigue details of SWATH that needs to be checked is too large, a selection method based on hotspot stress concentration factor and nominal stress FEM calculation was proposed. Spectrum analysis method, which is based on wave load direct calculation, was used to evaluate the fatigue strength of the large-scale SWATH. Computing processes and key parameters of this method was discussed.3) Wet deck slamming is a unique slamming mode of SWATH. Its impact of fatigue strength cannot be ignored. Based on wave spectrum, time history of relative motion between wave and hull was analyzed. And slamming imformation when occurred conditions were satisfied was recorded and statistics. Referencing others' works, the assumption about time and space distribution and peak value of slamming pressure was used to analyze the slamming load of SWATH, and its dynamic stress reponse was calculated by FEM. And then, stress history under both wave load and slamming load was calculated. Combined with probability of sea state occurrence, the accumulated damage of ship life was finally provided.4) To solve serious fatigue problem of SWATH connecting bridge structure, this paper presented a typical details optimization method based on parameters sub-model. Through the whole ship finite element analysis, Firstly the entire ship FEM analysis was done. Typical detail cumulative damage obtained by design wave and parameters sub-model was regarded as the objective function. And then anti-fatigue design was done. Optimization results offered reference for structure design.