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In case of marine structures, fatigue cracks may initiate and propagatedue to repeated wave exciting pressure. Propagation of fatigue crack causes not onlyinstantaneous fracture but also loss of serviceability such as the oil and/or watertightness of a compartment. Fatigue cracks could be detected by periodic inspections,but it is sometimes difficult to find a crack due to bad accessibility or visibility. In orderto manage the fatigue crack propagation, it is desirable to develop design method thatrenders cracks detectable and non-hazardous even though they may initiate. Such adesign concept is studied by analyses and experiments in the present paper. Asimulation system is also used as a tool to obtain the paths and growth-behavior of afatigue crack. This system may also be used to determine the timing of repair of fatiguecracks found during in-service inspection.