Structural integrity of pipe-joint entirety under multi-axial loading: Comparative assessment of butt-joints and mid-joints

• A modular modeling strategy was developed to evaluate the mechanical response of pipe-joint systems under multi-axial loads. • Failure mechanisms under combined torsion, tension, and internal pressure were systematically examined. • Sealing failure was identified as the dominant failure mode under internal pressure, rather than material yielding. • The performance of butt-joint and mid-joint was compared, and critical differences in load sensitivity were observed. • A failure envelope was established to define safe and critical operating domains for the structural pipe-joint system. Flexible pipes are critical infrastructure in offshore energy systems, yet their structural performance under combined loading conditions remains insufficiently explored. This study investigates the mechanical behaviour and failure mechanisms of steel strip reinforced thermoplastic pipes (SSRTPs), focusing on both butt-joints and mid-joints, which are commonly used for structural continuity and system integration. A comprehensive finite element model, developed in ABAQUS, is used to simulate the effects of combined torsion, axial tension, and internal pressure, allowing the identification of failure patterns, load interaction effects, and the corresponding failure thresholds. Experimental tests are conducted under specific load combinations to verify joint integrity and support the numerical analysis. Furthermore, a failure envelope is constructed to map the safe and critical domains under multi-axial loading, providing a visual and quantitative reference for evaluating joint reliability. The results reveal distinct differences between the two joint types: butt-joints maintain structural and sealing stability across a wider and more balanced range of combined loads, whereas mid-joints exhibit earlier sealing degradation and greater sensitivity to multi-axial interactions.