Designing an optimised hinged Magnetic Flux Leakage (MFL) system for remote localised defect monitoring in tank floors and pipelines

Abstract

A significant demand exists for magnetic flux leakage (MFL) systems designed for permanent installation in difficult-to-access areas, such as pipe elbows, where localised corrosion occurrences are prevalent. These systems enable continuous and long-term remote monitoring. Designing a magnetic circuit is a critical stage in MFL development, ensuring that induced magnetisation intensity and uniform flux distribution are maintained to facilitate effective defect detection. This work aims to develop a novel optimal-performance and compact MFL system that can be used to inspect a wide range of inspection components, such as flat plates and pipes, including pipe elbows. This research was based on a simulation-led design of an MFL circuit using finite element analysis (FEA) and optimisation algorithms, and an experimental programme to verify and validate the developed designs. Analyses and optimisation of the magnetic circuit were performed by parametric study of a reference configuration, followed by optimisation of multiple magnetic circuit parameters, resulting in the identification of alternative solutions. Three magnetic circuit types were developed: a reference (Ref) circuit with improved design parameters, a novel hinged (Hin) based circuit and a novel optimised (Opt) circuit. Using a comprehensive simulation program, the MFL reference configuration underwent a detailed parametric study based on sensitivity analysis of the circuit components. This led to an improved Ref circuit, considering the specified sample magnetisation level, recommended operation points on the B-H curve, and improved uniformity of magnetic flux density within the sample. The next step in the design process was the development of a new magnetic circuit with a hinged magnet bridge, which allowed for improved MFL application on non-flat inspection surfaces, e.g. pipes and elbows. Out of the four hinged configurations, the most suitable was adopted for further parameter and topology optimisations using Bound Optimisation by Quadratic Approximation (BOBYQA) and Method of Moving Asymptotes (MMA) algorithms. The simulation-based design was validated experimentally for all three (Ref, Hin and Opt) circuits by characterising machined cylindrical imperfections on three object shapes: flat plates, pipe and pipe elbow. The Opt circuit demonstrated the best performance, is more compact and can be used for a range of inspection object shapes, including plates, pipes and elbows. The discrepancy between experimental and simulation results was below 2% for all tests. The sensitivity analysis of the novel devices was performed for defects of different depths, radii, locations and sensor lift-offs.