A Study on Application-Oriented MFL Magnetizer Design for Safer Inspection of Coastal and Offshore Petroleum Facilities

Abstract Magnetic flux leakage (MFL) testing is a well-established method for detecting metal loss in ferromagnetic structures, including pipelines, storage tanks, and offshore facilities. The detection performance depends not only on the sensing probe but also on the coupled behavior of permanent magnets (PMs), magnetic bridge, air gap, and specimen properties, as well as the operating point on the material’s B–H curve. This study introduces a design framework that systematically links inspection requirements, such as material type, thickness, target defect, and lift-off, to the development of a compact, high-performance magnetizer. The framework involves: (i) selecting the specimen’s operating point via B–H data; (ii) establishing a magnetic-circuit load line that couples pole geometry, air gap, and specimen field to the PM demagnetization curve; (iii) optimizing PM dimensions via constrained optimization to minimize magnet mass while satisfying field and saturation constraints; and (iv) defining a bridge-thickness criterion to avoid premature saturation. Validation through finite element simulations and a bench test on steel plates with artificial defects demonstrates close agreement with theoretical predictions and confirms the dominant role of bridge thickness in determining signal contrast. The proposed workflow provides a reproducible path from inspection requirements to hardware realization, thereby enhancing MFL sensitivity for safer evaluation of coastal and offshore petroleum infrastructure.