Uncovering the polymer composite mysteries from decommissioned subsea unbonded flexible flowline through advanced characterisation testing

Reinforced thermoplastic composite (RTC) and fibre-reinforced polymer (FRP) are commonly employed in subsea flexible flowlines due to their exceptional chemical inertness, high strength-to-weight ratio, and excellent electrical and thermal properties. However, current waste management practices for composite materials are limited due to the inadequate availability of recycling technologies and infrastructure. Furthermore, a major challenge is the absence of data on how composite materials degrade in long-term, harsh subsea environments. Despite the industry raising concerns about heavy contamination in the decommissioning, there is a limited understanding of appropriate characterisation methodologies. Thus, there is a need to conduct a thorough characterisation to create a foundational key for planning safer decommissioning, selecting suitable recycling routes, decarbonising waste management, and promoting resource efficiency. To comprehensively characterise waste composites extracted from a real-world decommissioning environment, multi-analytical techniques were employed to provide insight into the changes in chemical, physical, structural and morphological properties of composites from ageing and prolonged subsea exposure. The findings revealed that both waste and new flowline samples are composed of identical material, polyethylene terephthalate (PET). Additives, N-butylbenzensulphonamide (NBBS) plasticiser (23 %), contaminants (36–97 % Fe rust) and trace inorganic fillers (Cr, Mn, and Ti) were found in the waste materials. The chemical composition profiles demonstrated that the waste materials retain their primary chains with minimal structural degradation and high thermal stabilities (442–444°C). This indicates their potential reusability in new structural applications and opportunities for waste valorisation treatment. • Multi-analytical techniques were used to study subsea flowline PET composites. • PET chemical structure remained intact after prolonged service exposure. • Findings inform safe decommissioning and material reuse/recycling strategies.