Evaluating frequency domain methods for fatigue analysis of fixed jacket type offshore platform

Fatigue damage in offshore platforms is a major issue since they are exposed to periodic wave loads. Frequency domain methods of fatigue damage calculation are more efficient than time domain method, but they usually provide inaccurate and conservative results. Several alternatives have been developed to modify the results of frequency domain methods. In this paper, accuracy of these methods for application to fatigue analysis of tubular joints of fixed offshore platforms is evaluated by studying a typical platform in the Persian Gulf, considering various sea states and structural dynamic characteristics. The results are compared with those of time domain analysis using rainflow count method. They show that the frequency domain methods generally present conservative results, when the bandwidth parameter of stress spectrum is greater than 0.7. It is also found that the slope of S-N curve has a significant impact on the results. Furthermore, among the existing methods, Dirlik and Tovo-Benasciutti ones while being conservative, provide relatively most accurate results for fatigue life of tubular joints.