Pipeline in-line inspection in brief

Pipeline operators use a wide variety of methods to evaluate, inspect, and monitor the hundreds of thousands of miles of transmission pipelines now in operation worldwide. One of the most effective is to use in-line inspection tools that travel through the pipe.

Magnetic flux leakage (MFL) is the oldest and most commonly used in-line inspection method for finding metal-loss regions in gas-transmission pipelines. MFL can reliably detect metal loss due to corrosion and, sometimes, gouging.

Accurately determining the location of a defect is needed for field assessments and repairs. Identifying pipeline features such as girth welds, wall thickness changes, valves and off-takes can help in the location of defects and verifying the accuracy of as-built and maintenance documentation. Typically, requirements on location accuracy depend on the difficulty with which excavations are made and the ease with which marker systems can be placed during an inspection.

MFL pigs record flux leakage at specified intervals in both the axial and circumferential directions in the pipe. The data interval in the circumferential direction is defined by the number of sensors. A high-resolution 24-inch pig will typically tool will have between 150 and 300 sensors, thus the circumferential data interval be between 0.25 and 0.5 inches.

The axial data recording interval is defined by the data recording system, and is usually between 0.1 and 0.2 inches (2.5–5.0 mm). Over a billion flux leakage measurements are required for a 100 mile pipe inspection using a pig with 200 sensors and a data recording interval of 0.1 inches.

The flux leakage data record or "log" must be examined to detect the presence of possible defects. After a possible defect has been found, the log must be further analyzed to characterize the geometry. The detection and sizing process is usually performed manually.

Many display methods have been developed to aid log analysts in the process. Detection starts with visualization of the flux leakage data the over a large area. Once defects are identified additional data display methods are used including strip chart recording and computer generated displays in pseudo color and three dimensions either with wire frames or in color.