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Correcting IMU Data for Pipeline Movement Analysis

Managing the threat of pipeline movement, whether it be from geohazards or weather event, becomes an issue of data integration and alignment. A vital data element that can help pipeline operators understand whether a pipeline has moved is gathered via an inertial measuring unit (IMU), also sometimes referred to as Inertial Mapping Unit 

An IMU is often deployed or “run” with another technology during an in-line inspection (ILI), an inspection carried out by a device known as an ILI tool or pig that travels inside the pipe taking measurements. The IMU contains gyroscopes to measure and record angular rate (degrees/sec) and accelerometers to measure linear acceleration (m/sec2), typically taking multiple samples per second. This allows the system to measure the precise path the ILI tool has taken during its traverse of the pipeline. To this end, the IMU data can be used to provide an accurate 3D measurement of the pipeline centerline position. 

The Process 

To provide pipeline operators with accurate location information, IMU data is integrated with control point data provided from the pipeline operator, so that every anomaly and feature provided in the ILI report has an accurate GPS (X,Y, Z) coordinate assigned to it. Essentially if the GPS coordinate of the launcher (start of the ILI) and the GPS coordinate of the receiver (end point of the ILI) are known, the data from the IMU can be utilized to interpolate accurate location information for every point in between.  

Control points are made of “hard” or primary control points, considered permanent points that don’t move from ILI to ILI such as block valves and casings. Additionally, “soft” or secondary control points are provided to supplement the hard points and improve the accuracy of the interpolation e.g. above ground markers or AGMs. The ILI service provider uses this control information to tie or coordinate the ILI data with the positional data provided from the pipeline operator to create a centerline. A centerline is the imaginary axis which runs longitudinally along the pipe through the midpoint of its diameter.  

Theoretically, ILI centerlines can be compared from one inspection to the next to determine if or where the pipeline has moved. Understanding where or if the pipeline has moved can indicate an area of integrity concern and need for mitigation. Recent failures due to landslides show how critical it is to better understand even the slightest pipeline slippage or probability of pipeline movement. Unfortunately, problems arise when trying to align multiple ILI-provided centerlines that cannot be attributed to the pipeline physically moving, as described below.  

Problem 1: GIS vs ILI centerline misalignment  

After an IMU is run inside the pipeline, the ILI service provider provides a series of GPS coordinates which then make up the ILI-provided centerline. Meanwhile, the pipeline operator maintains a Geographic Information System (GIS) which provides spatial information for assets on the pipeline, typically in a version of the Pipeline Open Data Standard (PODS) database and visualization provided by an Environmental Systems Research Institute, Inc. (ESRI) platform.  Centerline data from GIS is often based on a Line, Route and Series model and is managed across the enterprise.   

When comparing the GIS centerline and ILI centerline and accounting for tool drift, both centerlines should be in good agreement. However when conducting this comparison for a pipeline operator, significant discrepancies were found between the two data sets that couldn’t be explained by tool drift. What is tool drift?  

Tool drift 

As the ILI moves past the control point the IMU unit experiences drift that can vary by ILI service provider. This drift ranges from 0.03 degrees per hour (top-end units) to 1.0 degrees per hour (lower quality units). As the tool travels between each control point, the drift continues. At the next control point downstream, the ILI service provider resets the calculated position to that of the known control point supplied by the pipeline operator. The ILI service provider then post-processes the data in an upstream direction to a mid-point in an effort to improve the positional accuracy.  This process continues for the balance of the ILI run.  The variables which affect drift include:   

  • Distance between control points: increase in time of travel increases drift factor.  
  • Speed of the tool: as speed decreases, the time of travel increases, which may increase the drift factor. Fluctuations in tool speed cause fluctuations in the drift factor. 
  • IMU specification: a higher drift rate increases the margin of positional error. 


Figure 1: Tool drift experienced by the IMU between Control Points.

The further the tool is from a control point, the more inaccurate the centerline may be. Unfortunately, it was found that the ILI service provider was using the first control point i.e. launcher block valve but wasn’t making use of the control points thereafter. Therefore, as the tool gets further from the first control point, the ILI centerline drifts increasingly further from the GIS centerline.

Solution: Compare ILI centerline with hard points

Check that the ILI service provider is making use of all of the hard points by comparing the alignment of GIS vs ILI data and ensuring the ILI centerline is going through every hard point. In Figure 2 below, the GIS-provided centerline (yellow line) and ILI centerline (blue dots) show good alignment. The first control point, the launcher valve, Valve 1, is identified with a red dot. In Figure 3, the GIS centerline and ILI centerline do not align. The ILI data does not go through the control point, Valve 2, which is ~900 feet downstream from Valve 1. Instead, the ILI centerline is approximately 6 feet to the east of the GIS centerline. Therefore, the solution is to have the ILI service provider re-calculate the centerline utilizing the provided hard points.


Figure 2: GIS vs ILI at Valve 1


Figure 3: GIS vs ILI at Valve 2

This is a straightforward and relatively easy solution to execute. Determining whether the ILI service provider used the secondary control points i.e. above ground markers or understanding the accuracy of the AGM location information is a more complex problem to solve. However, to discern areas of pipeline movement, it may be necessary.

Problem 2: ILI vs ILI centerline misalignment

Not only was it found that ILI centerlines weren’t matching to GIS centerlines, but ILI centerlines weren’t aligning from one inspection to the next i.e. the centerline provided from the 2017 inspection was not aligning with the centerline provided with the 2024 inspection. If the hard points don’t change between the two runs, what would cause such misalignment? Enter secondary control points. . .

If you’ve ever had the (distinct and honorary) privilege of “running pigs,” no explanation may be necessary for why two ILI centerlines may be misaligned. However, for everyone else, let me explain. AGMs are temporary control points that supplement hard points. They’re often required to lend more accuracy to the calculated centerline, depending on the distance between hard points. Typically, a surveyor stakes these secondary control points meaning they identify the pipeline centerline and then put a (typically non-permanent) marker or wooden stake in the ground to mark the location of this temporary control point. They then survey the exact location of the marker to be provided to the ILI service provider. During the IMU/ILI run, a special marker or AGM box is placed at the staked location above-ground, that detects when the pig passes in the pipeline below ground.


Figure 4: 4 ILI centerlines with a 30 meter offset

As you can imagine, many issues arise with these secondary control points i.e. stakes being lost or destroyed by landowners, non-high resolution GPS survey (depending on the technology used – this can be an issue with hard points as well), missed AGMs because the pig is traveling faster than expected and a box couldn’t be placed before the pig passed, etc. AGM boxes can provide their own GPS coordinates (if it seems the box wasn’t placed where it should have), but the accuracy can vary based on location, weather, and type of box.

Solution: Revise Secondary Control Points i.e. AGMs

The primary cause of these ILI runs not aligning run-to-run was due to discrepancies in the secondary control points being used. The example in Figure 5 illustrates how the location of AGM 4 for one ILI centerline is in a significantly different location than AGM 4 in a different ILI centerline. Therefore, using hard point and girth weld alignment, the AGM locations can be revised to ensure the secondary control points match run-to-run.


Figure 5: Revising AGM locations.

  • Identify the master IMU tally or log
  • Match girth welds between master log and historic log.
  • Match historic AGMs to AGMs in master log
  • CIM generates revised GPS coordinates for each AGM that is in a different location and creates a new set of control points.
  • Have ILI service provider revise IMU results with the new control points.


Correcting AGMs ensure that the ILI centerline is created using the same control points thereby allowing for proper and accurate ILI alignment, as can be seen in the following figures.


Figure 6: ILI centerline before revised secondary control points applied.


Figure 7: ILI centerline after revised secondary control points applied.


Once the secondary control points are corrected, ILI pipeline centerlines can be more accurately compared, allowing for easier pipeline movement identification. As mentioned in a recent blog, spatial data is the basis of a pipeline movement and geohazard management program. Combine these centerline comparisons with alignment of aerial photography, soil data, seismicity information, etc. and you have yourself a comprehensive pipeline movement / geohazard and weather pipeline integrity program.

Comments or questions? We’d love to hear from you!