Research and Application of Low-Pressure High-Passability Tri-Axial Magnetic Flux Leakage Internal Detection Technology

Abstract

Aiming at the practical problems of subsea long-distance pipelines, such as generally low operating pressure, widespread complex working conditions including local diameter reduction, oval deformation, and small-radius bends, conventional magnetic flux leakage (MFL) internal detectors cannot pass through some pipelines due to insufficient pressure. This paper conducts a research on the tri-axial MFL internal inspection technology with low pressure and high passing ability.

Firstly, the working conditions of subsea pipelines and the applicable conditions of traditional MFL internal detectors are analyzed. Based on the mechanism of magnetic flux leakage detection, key technical approaches including low-friction MFL structure and low-friction large-deformation dynamic cups are proposed. Secondly, the finite element method is used to compare and analyze steel-brush-type and yoke-type MFL structures. Combined with the B-H curve of steel and the requirements of MFL sensitivity, the dual-section yoke-type low-friction magnetic circuit is optimized from the balance among magnetization depth, peak leakage magnetic field, and friction resistance, and the structural design of the MFL unit system is completed. Based on contact mechanics theory, the contact stress and contact length of typical cup structures (conical, straight-plate, circular, and dished cups) under different interference fits are simulated. A cup optimization criterion indexed by "unit circumferential driving force vs. average contact stress per unit length" is proposed, and a multi-diameter dynamic cup combination is developed to meet the passing requirements of over 30% diameter reduction and 1.5D bends. The two major technical difficulties of stable operation at low pressure and high passing ability are solved. The structural reliability and passing performance of the internal detector are verified through 323 mm test pipeline and 12-inch loop tests. A low-pressure tri-axial MFL inspection of approximately 2.1 km pipeline is successfully conducted in the sea trial of a 12-inch water injection subsea pipeline in an offshore oilfield. The test and sea trial results show that the developed 12-inch tri-axial MFL internal detector with low pressure and high passing ability operates stably under low pressure and small pressure fluctuation conditions, and its passing capacity reaches the level of geometric detectors. It can effectively expand the applicable scope of MFL internal inspection for subsea pipelines and provide reliable technical support for the integrity management of offshore oil and gas long-distance pipelines.