Global and Detailed Local Fatigue Assessment of a Container Vessel - A comparison between linear and nonlinear FE-analyses
Abstract: Container ships operating in the North Atlantic have a challenging structural design withrespect to fatigue. The container vessels are long and slender and they have large openings inthe deck. The structural design, in a combination of high wave loads, makes the ship sensitiveto fatigue cracks. The estimated fatigue life according to class rule procedures is too short tobe realistic in several fatigue-critical locations of container vessels. The class rules mainlybase their assumptions on stress-based approach, where high stress-concentration factors arecommonly accepted. This could be one of the reasons why plastic deformation occurs locallyin ship structures and fatigue cracks appear at an early stage.The objective with this study is to simulate numerically and assess the structural response of acontainer ship by assuming either linear material, or nonlinear, material response. In order toenable such a study, a hydrodynamic analysis is adopted to simulate the wave-inducedstructural loads, followed by a linear and nonlinear FE-analysis in order to obtain thestructural strength. A long-term fatigue analysis is performed to obtain the critical locations inthe ship. A script is created that makes it possible to change between a linear and nonlinearFE-solver.The results from the long-term fatigue analysis indicate that the critical locations exist in thehatch corners, forward engine room bulkhead and in the bilge region. However, the side shellon the port side was chosen for further study. The results from the nonlinear structural FEanalysisindicate that almost no plastic deformation will occur in the critical region of the sideshell. Hence, the conclusion was made that a strain-based approach to fatigue is not necessaryfor this location. A linear structural FE-analysis with the effect from internal ballast water wasstudied. The results from the linear structural FE-analysis with the effect from internal ballastpressure showed only a small impact on the stress-levels.For future work it is recommended to study a local sub-model that is located in a region wherehigher stress concentrations are expected such as, for example, in the bilge region on thestarboard side of the ship. The presence from welds and redistribution
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