Magnetic flux leakage (TFI or Transverse Field Inspection technology) is a magnetic method of nondestructive testing that is used to detect corrosion and pitting in steel structures, most commonly pipelines and storage tanks. The basic principle is that a powerful magnet is used to magnetize the steel. At areas where there is corrosion or missing metal, the magnetic field "leaks" from the steel. In an MFL (or Magnetic Flux Leakage) tool, a magnetic detector is placed between the poles of the magnet to detect the leakage field. Analysts interpret the chart recording of the leakage field to identify damaged areas and to estimate the depth of metal loss.
Introduction to pipeline examination
There are many methods of assessing the integrity of a pipeline. In-line-Inspection (ILI) tools are built to travel inside a pipeline and collect data as they go. The type of ILI we are interested in here, and the one that has been in use the longest for pipeline inspection, is the magnetic flux leakage inline inspection tool (MFL-ILI). MFL-ILIs detect and assess areas where the pipe wall may be damaged by corrosion. The more advanced versions are referred to as "high-resolution" because they have a large number of sensors. The high-resolution MFL-ILIs allow more reliable and accurate identification of anomalies in a pipeline, thus, minimizing the need for expensive verification excavations (i.e. digging up the pipe to verify what the problem is). Accurate assessment of pipeline anomalies can improve the decision making process within an Integrity Management Program and excavation programs can then focus on required repairs instead of calibration or exploratory digs. Utilizing the information from an MFL ILI inspection is not only cost effective but, as well, can also prove to be an extremely valuable building block of a Pipeline Integrity Management Program.
The reliable supply and transportation of product in a safe and cost-effective manner is a primary goal of most pipeline operating companies and managing the integrity of the pipeline is paramount in maintaining this objective. In-line-inspection programs are one of the most effective means of obtaining data that can be used as a fundamental base for an Integrity Management Program. There are many types of ILI tools that detect various pipeline defects, but high-resolution MFL tools are becoming more prevalent as its applications are surpassing those to which it was originally designed. Originally designed for detecting areas of metal loss, the modern High Resolution MFL tool is proving to be able to accurately assess the severity of corrosion features, define dents, wrinkles, buckles, and, in some cases, cracks. Having a device that can perform simultaneous tasks reliably is more efficient and ultimately provides cost saving benefits.
MFL pipeline inspection tools
Background and origin of the term "pig": In the field, a device that travels inside a pipeline to clean or inspect it is typically known as a pig. PIG is a bacronym for "Pipeline Inspection Gauge". The acronym PIG came later as the nickname for "pig" originated from cleaning pigs (first designed pigs) that actually sounded like squealing or screeching pigs when they passed through the lines scraping, scrubbing and "squeegeeing" the internal surface. The name serves as common industry jargon for all pigs, both intelligent tools and cleaning tools. Pigs, in order to fit inside the pipeline, are cylindrical and are necessarily short in order to be able to negotiate bends in the pipeline. Many other short, cylindrical objects, such as propane storage tanks, are also known as pigs and it is likely that the name came from the shape of the devices. In some countries a pig is known as a "Diablo", literally translated to mean "the Devil" relating to the shuddering sound the tool would make as it passed beneath people's feet. The pigs are built to match the diameter of a pipeline and use the very product being carried to end users to transport them. Pigs have been used in pipelines for many years and have many uses. Some separate one product from another, some clean and some inspect. An MFL tool is known as an "intelligent" or "smart" inspection pig because it contains electronics and collects data real-time while travelling through the pipeline. Sophisticated electronics on board allow this tool to accurately detect features as small as 1 mm by 1 mm, dimensions of the wall of a pipeline as well as depth or thickness of wall (helps indicate potential wall loss).