Jack-up is typically an exploration and production platform in shallow water operation. The possibility of moving this type of offshore structure from one location to other place and lifting up the deck above sea level made it attractive for offshore industry to use in deeper water and harsher environments. The jack-up structure consists of various components and behaves in a nonlinear and dynamic manner. Degradation phenomena such as fatigue may influence the structural performance and reduce strength of platform during its lifetime. Appropriate estimation of fatigue degradation strongly related to the suitable assessment of stress fluctuation, however, there are some uncertainties both in fatigue characteristics and environmental loads. These uncertainties can be taken into account in fatigue reliability framework. In this paper, several nonlinear models proposed by Wirsching and Light (W&L), Zhao and Baker (Z&L), Winterstein (Hermite model), in addition to the typical narrow banded model are utilized to specify nonlinearity in the stress fluctuation. The fatigue reliability is calculated based on these models and are compared with more advanced technique of rain flow counting method. The result shows a dependency between nonlinear behaviour of stress response and the selected model.

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