Coplanar capacitive sensing technique for defect detection on nylon liners of composite pipes: array design and applicability

The nylon liners of multilayer composite pipelines used in subsea mineral resource transportation are prone to internal wear and thinning caused by abrasive slurry flow, posing serious safety and reliability concerns. To enable the efficient and cost-effective, non-destructive evaluation of such nonconductive liners, this study investigates the applicability of coplanar capacitive sensing (CCS) technology for defect detection and develops an optimised flexible CCS array for internal pipe inspection. Numerical simulations were conducted to analyse the influence of an underlying conductive layer on the fringing electric field and the resulting detection sensitivity. The results reveal that the conductive layer compresses the electric field, leading to reduced sensitivity and the emergence of a negative sensitivity region with increasing defect depth. The parametric optimisation of electrode geometry demonstrates that longer and wider detecting electrodes, together with smaller detecting electrode space, significantly enhance detectability. A flexible CCS array fabricated using flexible printed circuit board technology was constructed and experimentally validated. The results confirm that capacitance variation exhibits a nonlinear relationship with defect depth and inflection behaviour in the presence of a conductive layer. The proposed CCS array successfully detected defects on the nylon liner, demonstrating its feasibility for internal inspection applications.