Frequency Domain Fatigue Analysis for a Unbonded Flexible Riser: Damage Induced by Dynamic Tension

Evaluation of fatigue damage of offshore flexible risers is critical in flexible riser system design. For deepwater application, irregular wave time domain approach is often adopted as the state of practice to avoid excessive conservatism due to its better representation of the stochastic offshore environment. The approach can indeed fully capture the non-linear behaviors of the system at a significant cost of computational time. For example, computational time typically takes over 3∼4 weeks for a deep water free hanging riser system with thousands of fatigue load-cases and the full 3-hour simulations. On the other hand, the same scope of simulation can be completed in frequency domain within day(s), which will enable the designer to accelerate the optimization of riser system design. This paper presents an analysis method in frequency domain for assessing the fatigue damage of tensile armour wires inside the top end fitting (EF), which is induced by dynamic tension variation and often governs the riser service life in deep water applications. A validation measurement is also implemented to ensure the accuracy and practicability of this frequency domain approach in riser system design.

Fatigue Damage Study of Helical Wires in Catenary Unbonded Flexible Riser Near Touchdown Point

This study presents an analytical model of flexible riser and implements it into finite-element software abaqus to investigate the fatigue damage of helical wires near touchdown point (TDP). In the analytical model, the interlayer contact pressure is simulated by setting up springs between adjacent interlayers. The spring stiffness is iteratively updated based on the interlayer penetration and separation conditions in the axisymmetric analysis. During the bending behavior, the axial stress of helical wire along the circumferential direction is traced to determine whether the axial force overcomes the interlayer friction force and thus lead to sliding. Based on the experimental data in the literature, the model is verified. The present study implements this model into abaqus to carry out the global analysis of the catenary flexible riser. In the global analysis, the riser–seabed interaction is simulated by using a hysteretic seabed model in the literature. The effect of the seabed stiffness and interlayer friction on the fatigue damage of helical wire near touchdown point is parametrically studied, and the results indicate that these two aspects significantly affect the helical wire fatigue damage, and the sliding of helical wires should be taken into account in the global analysis for accurate prediction of fatigue damage. Meanwhile, different from the steel catenary riser, high seabed stiffness may not correspond to high fatigue damage of helical wires.