Theoretical and Experimental Analysis of the Global Response of a Flexible Pipe Under Combined Axisymmetric and Bending Loads

Abstract

The bending stiffness of flexible pipes is highly dependent on curvature, driven by the interaction between their structural layers—a behavior often misrepresented by traditional numerical models. To overcome this limitation, a finite-difference-based model was developed, integrating previously proposed formulations for monotonic bending and axisymmetric responses into a return-mapping algorithm to capture hysteretic behavior under cyclic loading. The model was calibrated against pure bending and pressurized tests, accounting for interlayer adhesion and friction, which govern stiffness variation, force levels, and energy dissipation. Results showed excellent agreement with experimental data across different load combinations, confirming the model’s predictive capability. Parametric analyses revealed that higher adhesion and friction coefficients increase imposed forces until a no-slip condition is achieved, while energy dissipation follows a nonlinear dependence on interlayer friction, peaking at intermediate values and vanishing under no-slip conditions. Cyclic bending tests performed on degraded samples demonstrated that, despite wire deterioration, the global bending response remains essentially unchanged, reinforcing the stability of riser behavior over time. However, fatigue resistance must still be reassessed through updated S–N curves to account for material degradation. These findings underscore the crucial role of interlayer mechanics in determining the overall performance of flexible pipes and offer a validated framework for assessing fatigue and integrity.

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