PSI - Issue 77

Sunny O. Uguzo et al. / Procedia Structural Integrity 77 (2026) 521–528

522

allows for tailoring the fibre layup to meet varying load bearing requirements. Despite benefits, TCPs experience temperature limits, environment sensitivity, low transverse stiffness and complex interface behaviours. Furthermore, deep water risers made from TCPs would be subjected to multiaxial thermomechanical loads including internal pressure, external hydrostatic pressure, significant sag bend/overbend curvature, axial tension/compression and through thickness thermal gradients etc. Subsequently, failure response could become highly sensitive to temperature dependent material properties and laminate configuration, thereby prompting extensive studies. FE analyses by Yu et al. (2015) highlighted the importance of D/t and initial ovality as well as the dominance of buckling, noting the effects of [±65/±75] layups in enhancing buckling capacity across different load paths. Wang et al. (2023), showed the high sensitivity of failure coefficients to ply angle, with optimal angles found between 27 o and 63 o under torsion loading. The study also combined the closed form solution with FEA to map ply angle safety zones for composite pipes under torsion, which peaked near 45 o . It was found in further studies that low to moderate laminate ply angles favour torsion resistance and pressure loading, while more hoop aligned plies enhance bending resistance (Li et al., 2023, Wang et al., 2025, Zheng et al., 2025). The stability and ultimate strength of compressed thin-walled carbon-epoxy composite columns with square cross section were also studied, showing that [45/ – 45/90/0] s and [0/90/0/90] s configurations achieved the highest and lowest critical loads respectively, with the latter exhibiting a 73% post buckling reserve (Rozylo and Debski, 2024). Under combined bending and external pressure, laminates with [±50/±60] and [±54.7] 2 orientations handled loads marginally better than [±40/±50] and [±60/±70] laminates (Yao et al., 2018). Another study noted the enhancement of burst pressure with increased number of reinforcement plies, whilst optimal winding angle relating to failure pressure varied with increasing laminate layer thickness (Xia et al., 2021). Wang et al. (2024) also showed that safe stacking for sequences ([± ϕ₁/±ϕ₂], [+ϕ₁/−ϕ₂/−ϕ₁/+ϕ₂], [+ϕ₁/−ϕ₂/+ϕ₂/−ϕ₁]) concentrate d at fibre angles near the material principal direction, which improved axial strength and higher angles reduced axial load resistance under bending. They further found that for a [+ ϕ₁/−ϕ₂/+ϕ₂/−ϕ₁] sequence, low angles near the outside improved stiffness and for [± ϕ₁/±ϕ₂] and [+ϕ₁/−ϕ₂/−ϕ₁/+ϕ₂], making ϕ₂ the outer, low -angle layer prevented failure regardless of ϕ₁, implying low-angle plies are better placed near the outer laminate to resist axial stresses. Furthermore, Hastie et al. (2021) found that [±55]₄ ply arrangement, often optimal for biaxial pressure, was susceptible to failure under pure bending due to transverse tensile stress at the pipe top , whereas [±30]₄ failed at the intrados by fibre compression and was unsuitable beyond spooling and in deep water. A [±42.5]₄ layup delivered the best resistance to failure, dominated by in-plane shear while avoiding excessive transverse tensile and fibre-compression stresses, and showed superior, temperature insensitive spooling. Hastie et al. (2020) also recommended low angles for high-tension/low pressure differential cases and pairing ±55° with lower-angle plies for balanced hoop – axial performance under higher pressure ratios. Using numerical optimization, Amaechi et al. (2019) further identified [0₄/±63.5₅/90₄ ] arrangement as an effective deep water riser layup, balancing fibre, transverse, and shear stresses. Collectively, these studies demonstrate that optimal winding angles are load-specific, and tailoring lay-ups, rather than defaulting to ±55°, yields superior multi-axial performance. Furthermore, neglecting temperature-dependent properties leads to non-conservative designs because strength and stiffness exhibit thermal decay with elevated temperatures, potentially shifting the critical failure mode, reducing design margins and service life predictions in high-temperature environments. Therefore, this study examines how laminate architecture and pipe geometry interact with temperature dependent material degradation under a combination of thermal and mechanical loads. 2. Problem formulation and method 2.1. Problem formulation This study models the sag bend of deep-water composite risers, where contact with the seabed could induce bending and ovalization, hydrostatic depth imposes external pressure, hot pressurised fluids cause internal pressure and through thickness thermal gradients and off design events (collision or accidents) introduce axial compression loadings. Figure 1 depicts a riser system, bringing the sag bend to focus and depicting the combined operational load environment.