Pigging is an in-line inspection (ILI) technique in which devices referred to “pigs” are inserted into pipelines to perform cleaning and inspection activities. Initially, pigging was used purely as a way to clean pipelines.
For inspection, these are often referred to as “intelligent pigs” or “smart pigs.” What makes intelligent pigging different is that smart pigs are capable of performing advanced inspection activities as they travel along the pipe, in addition to just cleaning it. Smart pigs use nondestructive examination techniques such as ultrasonic testing and magnetic flux leakage testing to inspect for erosion, corrosion, metal loss, pitting, weld anomalies, and hydrogen induced cracking, among others. They are also able to gather data on the pipeline’s diameter, curvature, bends, and temperature.
Inspecting Pipelines Methods
Traditionally, for inline inspection, a sizing pig with one (or more) notched round metal plates (as gauges) are being used. The notches allow different parts of the plate to bend when a bore restriction is encountered.
More complex systems exist for inspecting various aspects of the pipeline. Intelligent pigs are used to inspect the pipeline with sensors and record the data for later analysis. These pigs use technologies such as Magnetic Flux Leakage (MFL), Transverse Flux Leakage (TFI) and ultrasonic to inspect the pipeline. Intelligent pigs may also use calipers to measure the inside geometry of the pipeline.
It is a tool to measure and identify the defections on the pipe (like corrosion, gouges and so on) through sensing a powerful axial magnetic field which is generated to saturate the pipe wall. If there is any defection (thickness reduction) inner or outer of the pipe, the amount of saturated magnetic field will be reduced and it will be measured by sensors mounted around the pig’s body and then recorded in the memories of electronic boards for later analysis.
To detect longitudinally oriented anomalies such as tunnel corrosion, longitudinal weld anomalies, it is utilized and induced a flux field in the circumferential direction. It also has a good performance in measuring some sort of crack-like anomalies. The usage of this tool is not used on its own and it usually pass through pipeline accompanying with MFL to achieve better accuracy in detecting anomalies in pipeline.
MFL+TFI inspection tools induce a magnetic field along the pipe by means of permanent magnets, sensors fitted to the tool covering the full circumference of the pipe measure and record leakage and disturbances of the magnetic flux induced.
The Caliper employs to inspect all geometry deviations on a pipeline. It is very important to locate such deviation because:
- Variation in pipeline circular shape has negative effect on passing the product and increase consumption of power require for product transportation.
- Caliper can be considered as a survey of pipeline mapping and it will be used for future inspection data synchronization and fitness-for-purpose assessments
The principle of ultrasound inspection is commonly known – using echo signals and registering reception time difference between them. In intelligent pigging application these high frequency radial signals are emitted by UT transducers perpendicularly to a pipe wall. These signals produce two major echoes – from inner surface and outer surface of a pipe wall. The anomalies are detected through the registered difference between time periods of receiving the echo signals from healthy pipe (calibration values) and pipe wall with metal loss or any in-homogeneity.
The data recorded is then interpreted with specialized analytical software to give the position and depth and sizes of pipe anomalies such as metal loss, dents, general and pitting corrosion, laminations, girth weld defects, mechanical damage, Odometer position and so on.