The Magnetic Flux Leakage (MFL) Technique is the most commonly used technique to inspect large diameter transmission pipelines. A typical MFL inspection system uses permanent magnets to apply an axially oriented magnetic field to the ferromagnetic pipe material. The magnetic field is perturbed by a metal-loss region (usually caused by corrosion) to produce flux leakage outside the pipe, which can be measured by field sensors.
The magnetization system in an MFL inspection system should ideally produce a magnetic field that is strong enough to cause a measurable amount of magnetic flux to leak from the pipe material at metal-loss regions, uniform from inside to the outside surface of the wall thickness so that the measured signal is more linearly related to metal-loss depth, and consistent in magnitude along the length of a pipe so that flux leakage measurements can be compared at different locations during an inspection run.
In general, the field strength most strongly affects detection of metal loss defects while characterization of defect geometry requires a field that is strong, uniform, and consistent.
Improvements in the downhole hardware also provide more flexible and efficient data acquisition, reducing operating time while improving data accuracy and operational safety. In conventional magnetic flux leakage (MFL) tools, the flux leakage sensors are coils; in the “high-resolution” tool, the coil is replaced by multiple “Hall Effect” sensors.
The HR Vertilog service uses MFL measurements to identify and quantify internal and external corrosion defects. The overlapping arrays of flux-leakage sensors and discriminator sensors offer full circumferential inspection of the tubing or casing string. This process differentiates between metal-loss (corrosion) and metal-gain (hardware) Features, and distinguishes between general corrosion and isolated pitting. Paper represents technology overview and field cases history