Parametric Study on the Influence of Aging on the Stiffness of Thermoplastic Composite Pipes

This research investigates the effects of material aging and stacking sequence on the structural stiffness of thermoplastic composite pipes (TCPs) used in offshore oil and gas applications. The research aims to quantify the degradation of axial, bending, and torsional stiffness in TCPs exposed to supercritical CO2 environments, comparing polyetheretherketone (PEEK) and polyvinylidene fluoride (PVDF) matrices. Additionally, it evaluates the impact of ply orientation, laminate thickness, and combined loading conditions (internal/external pressure, tension, and thermal variations) on TCP performance. A combined analytical, numerical, and experimental methodology was employed. Classical Laminate Plate Theory (CLPT) provided baseline stiffness calculations, while finite element analysis with S4R shell elements captured nonlinear effects. Aging simulations involved exposing samples to 90°C (PVDF) and 130°C (PEEK) under 1,380 bar of CO2 for up to 28 days. Mechanical testing included tensile, flexural, and torsional evaluations post-aging. Parametric studies assessed 15 stacking sequences (e.g., unidirectional, helical, and combined angles) under operational loads. Aging reduced PVDF/CF stiffness by up to 50.9% (axial) and 49.8% (flexural), while PEEK/CF showed <10% degradation, highlighting its superior stability. Analytical models exhibited 8.7% average error for axial stiffness but underestimated torsional stiffness by 67% due to interlaminar shear complexities. Numerical results aligned better with experimental data, particularly for bending (20.3% error). Smaller ply angles (e.g., ±25°) maximized axial and flexural stiffness, while intermediate angles (±55°) optimized torsional resistance. Internal pressure increased stiffness by 15–20% via fiber pre-tension, whereas external pressure and elevated temperatures reduced performance, especially in PVDF. This work provides a validated framework for designing TCPs in harsh environments, emphasizing material selection (PEEK over PVDF) and optimal ply configurations (balanced helical angles) to mitigate aging effects. The findings support API and DNV standards for composite pipe reliability in ultra-deepwater applications.