Pipeline pigging explained: cleaning pigs, smart pigs and in-line inspection

By About Pipelines editors · November 4, 2026

A pipeline cannot be opened up and looked at along its whole length, so the industry sends tools through the inside instead. These devices are called pigs, and running them is called pigging. Some pigs simply clean the line. Others, known as smart pigs or in-line inspection (ILI) tools, carry sensors that measure the pipe wall from the inside and record any damage.

This guide explains how pigging works, the main types of pigs, and what inspection tools can and cannot find. For a closer look at one operator's inspection program, see the article Inside the smart pig.

How a pig travels through a pipeline

A pig is built to fit the inside diameter of the pipe snugly, using cups, discs or a foam body. The basic sequence is the same for every type:

1. Launch. The pig is loaded into a launcher, a section of pipe that can be isolated, depressurized and opened. Once it is closed and pressurized again, valves direct flow behind the pig.
2. Travel. The product in the line (natural gas, crude oil or refined products) pushes the pig along. Its speed depends on the flow rate, and operators manage flow so that inspection tools move at a speed their sensors can work with.
3. Tracking. Crews follow the tool's progress with above-ground markers or transmitters, so they know where it is and can find it if it stops.
4. Receive. At the far end, a receiver traps the pig so it can be taken out. For inspection tools, the recorded data is downloaded and sent for analysis.

Because the pig rides with the product, the pipeline usually keeps operating during a run.

Utility pigs: cleaning and maintenance

Most pig runs are routine maintenance rather than inspection. Utility pigs are used to:

• Clean the line. Brushes and scrapers remove wax, sludge, scale and debris that reduce flow and can contribute to internal corrosion.
• Remove water and liquids. In gas lines, liquids that collect at low points can be swept out.
• Separate batches. Liquids pipelines often carry different products one after another; pigs can be used to keep batches apart.
• Prepare for inspection. A clean pipe gives an inspection tool better data, so cleaning runs normally come first.

A simple gauging pig, often fitted with a soft metal plate, can also be sent ahead of a smart pig to confirm the line has no restriction that would trap a more expensive tool.

Smart pigs: in-line inspection tools

In-line inspection tools are the main way operators find problems inside and outside a buried pipe before they become leaks. The U.S. Pipeline and Hazardous Materials Safety Administration describes them as tools that travel through the pipe and measure and record irregularities that may represent corrosion, cracks, laminations, dents, gouges or other defects. Different technologies look for different threats:

Operators often run more than one tool type, or a combined tool, because no single technology finds every kind of defect.

What happens with the data

A smart pig run produces a large data set. Specialists analyse it to identify and size each feature, then engineers assess whether the feature is acceptable or needs action. Where it does, the operator excavates the pipe at that spot, an integrity dig, to examine it directly and repair or replace it if needed. Comparing results from successive runs shows whether corrosion is growing and how fast. The process is described in the article What is pipeline integrity?

Regulation

In Canada, the Canada Energy Regulator's Onshore Pipeline Regulations require federally regulated companies to run integrity management programs, and CSA Z662 sets out how those programs work. In-line inspection is one of the main assessment methods those programs use, alongside hydrostatic pressure testing and direct assessment. In the United States, PHMSA rules require new and replaced gas transmission lines to be designed to accommodate in-line inspection tools in most cases, and integrity management rules require periodic assessment of lines in high-consequence areas.

Limits of pigging

Smart pigs are powerful, but they are not perfect. Each technology has detection thresholds, so very small features may be missed or mis-sized. Some lines cannot be pigged at all because of tight bends, diameter changes or fittings. And pigs inspect the pipe, not what is happening outside it, so operators combine ILI with right-of-way patrols, cathodic protection monitoring, leak detection systems and damage prevention programs. You can read more about those in How are leaks detected in an underground pipeline? and our guide to types of pipelines.

Sources

• U.S. Pipeline and Hazardous Materials Safety Administration, in-line inspection ("smart pig") fact sheet
• Canada Energy Regulator, Safety and environment
• CSA Group, CSA Z662, Oil and gas pipeline systems
• U.S. Code of Federal Regulations, 49 CFR 192.150, Passage of internal inspection devices