Fatigue damage assessment at the flexible riser hang-off location, where the pipe frequently endures maximum tension and curvature variations, is key to verify design integrity for service life.

Traditionally, the fatigue analysis is performed in a separate local structure model, as the commercial global analysis software lacks the capability of handling the local behavior of the riser structural component, which is dependent on materials and manufacturing processes. During global fatigue analysis, the riser configuration is built with pinned connected at the hang-off point. The resulting tension and angle responses at the hang-off location, are then input to a local model to perform the stress and fatigue analysis, where the detailed pipe layer structure and bend stiffener are modeled. This traditional approach is conservative, time costly and is often limited to regular wave approach.

Wellstream developed an external function to work with specialized commercial riser dynamic analysis software. The external function simulates the detailed behaviour of flexible pipe structure components and the resulting bending hysteresis during dynamic simulation in the time domain. Therefore, the stress time history of the tensile armour becomes available at the end of global simulation in the time domain and is ready for fatigue damage assessment by rain flow counting.

This paper presents a study case where the fatigue assessment is performed directly at the hang-off region within the riser global dynamic simulation. The riser hang-off is situated at the top of the I-tube and a bend stiffener is fixed at the bottom of the I-tube. I-tube and pipe section are precisely modeled as pipe-in-pipe facility, where the interaction of riser/I-tube can be captured.

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