COMPARATIVE FATIGUE ANALYSIS OF OFFSHORE STRUCTURES USING SPECTRAL AND DETERMINISTIC METHODS

Abstract

The jacket of bottom fixed type offshore structures has to be designed its tubular connections to sustain the cyclic loading induced by the ocean waves throughout their operation life. In this study, two primary methods for assessing fatigue life are applied to evaluate three different structures characterized by their distinct natural periods. These structural models that varied in mass and stiffness are prepared for sensitivity study to evaluate consistency in the results with respect to the effect of structure configurations on fatigue life using deterministic fatigue and spectral fatigue. The deterministic method solely relies on linear wave theory in the selection of wave parameters, while the spectral method accounts for the structural responds to determine wave parameters. In this study, the wave probability distribution is tabulated using a single wave scatter data (presented in  and ), in which the Longuet-Higgins' method is applied to obtain the individual wave distribution (presented in H and T) to be used in the deterministic fatigue analysis. In addition, this study also evaluates the application of Longuet-Higgins’ wave conversion theory for three structural models with varying natural periods (  = 1.6 s, 3.1 s, and 4.6 s). In order to minimize the discrepancies between deterministic fatigue and spectral fatigue, the structural normalized stresses are calculated dynamically to obtain the dynamic response, and identical fatigue parameters were applied: the API X’ S-N curve and Efthymiou’s stress concentration factor (SCF) theory. The results show the deterministic fatigue has comparable patterns to spectral fatigue as it shows consistent patterns. This study concludes that dynamic analysis is crucial for assessing fatigue life and highlights the importance of selecting appropriate wave height (H) and period (T) ranges for Longuet-Higgins’ wave conversion.