A floating offshore plant for oil and gas production is excited by various types of dynamic environmental loads. The stress response from dynamic environmental loads are usually described by spectrum in frequency domain. At the initial design stage of the offshore plant, relevant fatigue damage models are useful in estimating the fatigue damage by dynamic environmental loads quickly. During the development process of fatigue damage model, idealized response spectra of offshore plant are commonly used, and a proper fatigue damage model is derived with regression analysis using data obtained from Rain-Flow Counting(RFC) method. This study suggests an optimized process for calculating a reasonable time history preserving various statistical characteristics and estimating fatigue damage quickly using the RFC method. Ten idealized spectra for describing real response spectra are used for a validation of proposed optimized process. Three main parameters for generating time history, length of time history, frequency spacing size, and time step size are selected for a parametric study. These parameters variously affect calculation time and reliability of fatigue damage estimated by the RFC method. As a result, optimized three parameters are suggested in terms of various statistical characteristics and fatigue damage using the RFC method.

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