Flexible riser tensile armour stress assessment in the bend stiffener region

The flexible riser within the bend stiffener in the top connection is a structurally vulnerable region for extreme and fatigue loads as it is subjected to large dynamic tensile forces and curvatures. This paper presents a nonlinear finite element model (FEM) for the flexible riser with bend stiffener taking into account interlayer friction and riser-bend stiffener contact interaction subjected to combined axisymmetric and bending loads. A case study with typical extreme and fatigue load cases is carried out to assess the curvature variation and bend stiffener contact pressure influence in the riser tensile armours stresses. For the extreme load case, detailed finite element results such as contact pressures and friction, normal and transverse bending stresses are compared with a uniform curvature FEM and available analytical models. A fatigue load case is also performed, where the stress amplitudes and cumulative fatigue damages for the internal tensile armour wires are compared with a uniform curvature FEM and analytical models. From the case study results under extreme loading, it can be observed that the model with bend stiffener interaction predicts larger stresses in comparison to the uniform curvature FEM and analytical models. Under typical fatigue loading conditions, significant differences in the cumulative damage are observed between the models, highlighting the importance of an accurate modeling for lifetime integrity assessment in a region of highly variable riser curvature.

Automated summary

This paper presents a nonlinear finite element model for the flexible riser with bend stiffener, taking into account interlayer friction and riser-bend stiffener contact interaction. A case study is carried out to assess the influence of curvature variation and bend stiffener contact pressure on the riser tensile armours stresses under extreme and fatigue load cases. The results show that the model with bend stiffener interaction predicts larger stresses compared to the uniform curvature FEM and analytical models under extreme loading, and significant differences in cumulative damage are observed between the models under typical fatigue loading conditions.