Fatigue design of armor layers of flexible risers in ultra-deep water depth fields is challenging. Very high internal pressure and tension impose extreme contact pressure between steel armor layers. The paper outlines how these challenges were handled focusing on the physical effects that needed to be included in the structural mechanics model and the procedure used to calculate the lifetime. Residual stresses due to manufacturing plays an important role in reducing the fatigue performance of the pressure layer by affecting the mean stress level. However, this effect can be improved by selecting appropriate factory acceptance test (FAT). A tailor-made finite element program (BFLEX) has been designed to make this sort of analyses possible and ensure the safety of the flexible riser design for ultra-deep water depth. The study is also extended to discuss the fatigue damage owing to friction in the armor layer for the ultra-deep water flexible because of higher contact pressure on the armor layer. Finally, the fatigue behavior of the pressure amour is a typical multi-axial fatigue problem. Various models are proposed to address how different stress components are exposed for the fatigue calculation.

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