MFL inspections are typically used to detect, locate, and characterize metal-loss anomalies in natural gas-transmission pipelines. There are other many types of pipeline anomalies, and not all of these anomalies can be detected or characterized by MFL. This link gives examples and more details on the types of imperfections and defects that are commonly found in gas-transmission pipelines.

Pipeline operators typically use MFL to find metal loss due to corrosion or gouging. Corrosion is wastage or thinning of the pipe wall due to a chemical or electrochemical attack. Common forms of corrosion include external, internal, pitting (internal or external), selective seam corrosion, and stress corrosion cracking. Corrosion is found when an MFL tool measures a local change in the magnetic field.

Gouging is the mechanical removal of metal from a local area on the surface of a pipe. The remaining metal in a gouged area is often work hardened, which can change its magnetic properties. Also, a gouged area may contain shallow cracks, which further reduce the pressure-carrying capability of the pipe. Like corrosion, gouging is found when an MFL tool measures a local change in the magnetic field.

Metallurgical Anomalies

MFL can also detect some metallurgical anomalies, which are generally considered innocuous by pipeline operators. Here, metallurgical anomalies include hard spots, laminations, slivers, scabs, inclusions, and various other imperfections and defects.

Hard spots are local regions that have a considerably higher hardness than the bulk hardness of a pipe. Hard spots result from uneven cooling or quenching during the manufacture of the steel plate or skelp; cracks, which can lead to failure, can form in hard spots when atomic hydrogen is present.

Laminations are internal metal separations that are generally parallel to the surface of a pipe. Slivers and scabs are pieces of metal that have been rolled into the surface; scabs are usually due to casting defects while slivers are introduced during rolling. Inclusions are foreign or nonmetallic particles that are trapped during steel solidification.

MFL can find these anomalies when they change the magnetic properties of the pipe. Hard spots, scabs, and slivers significantly change the magnetic properties of a pipe, but inclusions and laminations generally do not. So, hard spots, scabs, and slivers are easier to find than inclusions and laminations. There is an advantage to an inspection system that is not capable of finding certain classes of imperfections. Because MFL systems are not very sensitive to inclusions and laminations (which are generally considered imperfections rather than defects), the number of indications that must be analyzed is less for MFL tools than for other inspection systems. The fact that MFL focuses on metal-loss defects makes it a preferred technology for many inspection applications.

Cracks

MFL tools occasionally detect cracks, even though they are not designed for this purpose. Crack-like imperfections or defects can be found only under limited conditions. Cracks are not common in pipelines, but when they occur, they are due to fatigue, stress corrosion, and weld defects. Selective weld corrosion is often grouped with cracks because it leads to a wedge-shaped groove that behaves somewhat like a crack. Cracks can sometimes be found when they are oriented in a direction that significantly affects the magnetic field around the defect.

Dents and Buckles

MFL tools can sometimes detect dents and buckles, although again, they are not designed for this purpose. Dents are depressions in the pipe surface, and buckles are a partial collapse of the pipe due to excessive bending or compression. MFL tools detect dents and buckles when the sensors lose contact with the wall. Detection of dents and buckles is not covered in detail in this report.