Inspectioneering Journal

A History of In-Line Inspection Tools

By Matt Ellinger, Senior Engineer at DNV GL. This article appears in the March/April 2017 issue of Inspectioneering Journal.


The practice of inspecting pipelines by means of a “smart” inspection device (also known as a smart pig) began in the 1960’s. The first commercial smart in-line inspection (ILI) tool was built in 1964 and was equipped with magnetic flux leakage (MFL) inspection technology. During the 1970’s, the first “high-resolution” pigs were developed. These tools were capable of detecting pipeline defects more accurately and reliably. During the 1980’s, more MFL ILI vendors came to the market and new inspection technologies emerged in the form of ultrasonic wall thickness measurement (UTWM) ILI tools. During the 1990’s, ultrasonic crack detection (UTCD) and circumferential MFL tools capable of detecting narrow axially oriented defects (including open cracks) were developed. During the 2000’s, tool technologies were improving in ways that made them capable of accurately detecting and sizing a more diverse range of pipeline anomalies.

Today, The Code of Federal Regulations Title 49 Parts 192 (Transportation of Natural and Other Gas by Pipeline) and 195 (Transportation of Hazardous Liquids by Pipeline) prescribe three permissible methods by which a pipeline shall be assessed.

  • One is Direct Assessment, in which locations for examination for a given threat (internal corrosion, external corrosion, or stress corrosion cracking) are selected based on a screening process. The existence of the threat at other locations within the pipeline segment can be inferred based on the results of the direct assessments.
  • Another method is Hydrostatic Pressure Testing (hydrotest). During a hydrotest, the pipeline is filled with a medium (typically water) and the pressure is held at a predefined level (e.g., 125% of the maximum operating pressure) for a prescribed period of time (e.g., 8 hours). The hydrotest is designed to cause near critical defects to fail (i.e., leak), thus proving that the defects that remain in the pipeline (those that did not fail) are not critical and safe to operate at the maximum operating pressure (MOP).
  • The third method is In-Line Inspection. ILI is the only inspection technology that provides detailed information about defects within the pipeline segment that are sub-critical (i.e., not an imminent threat to the integrity of the pipeline). They also allow for the pipeline to be inspected without interrupting operation of the pipeline.

The purpose of this article is to describe the various ILI technologies that are currently available to the market. The pros, cons, and applicability of each type of tool will be discussed in greater detail. The intent is not to judge one ILI technology against another, rather it is to provide pipeline operators, and pipeline enthusiasts in general, with a better understanding of the capabilities of today’s fleet of ILI tools.

Available In-Line Inspection Tool Technologies

Today, there are a plethora of ILI tools and vendors available to the market. Different ILI vendors may focus their efforts on a specific ILI technology. Others may possess a fleet of ILI tools that cover a broad array of technologies. It is important to know the basics about the different ILI technologies so that the goals of the inspection can be met. For example, it is prudent to understand which ILI technologies are capable of accurately detecting and characterizing specific pipeline threats/indications/anomalies. In this manner, the pipeline operator can be assured that they are utilizing the most applicable ILI technology for their specific pipeline. The following paragraphs identify some of today’s ILI technologies, along with a brief description of the applicability of the technology type.

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Comments and Discussion

Posted by Shamik Chowdhury on August 10, 2017
Excellent. Really helpful Log in or register to read the rest of this comment.

Posted by Gerald Gerald on July 19, 2019
Really educative and enlightening. Thank you Log in or register to read the rest of this comment.

Posted by Mohamed Elganzory on July 27, 2019
Very good Log in or register to read the rest of this comment.

Posted by Leert Lane on February 15, 2020
Excellent Log in or register to read the rest of this comment.

Posted by Shaj M on October 17, 2020
Very informative article. Log in or register to read the rest of this comment.

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