There are many examples of geotechnical hazards that may affect buried pipelines: settlement, erosion, landslides, and seismic events, just to name a few. Those geohazards frequently cause loss of support or significant external loads, bending the pipeline and moving it from its original position. This article explains how strain and pipeline movement assessments help identify and mitigate threats posed by geohazards.
IMPORTANCE OF MONITORING EXTERNAL LOADS
A case study illustrates how these assessments provided an operator with the information needed to manage a potentially serious loading problem. Considerable external bending loads and the resulting high strains can threaten the integrity of the pipeline, as it can crack or bend. Therefore, it is crucial for the safe operation of a pipeline to monitor external loads through strain and pipeline movement assessments. Bending strain assessments can identify areas where the pipeline is deformed beyond allowable code limits. Pipeline movement assessments can use geo-spatial data and repeated inertial measuring unit runs to identify areas of active bending movement, allowing a calculation of a rate of change in bending strain.
CAPTURING THE DATA
An inertial measurement unit (IMU) is used to capture the data for pipeline movement or bending strain assessments. This technology is often paired with caliper or magnetic flux leakage (MFL) technologies on combo in-line inspection (ILI) tools. Although IMU is most commonly utilized to locate the pipeline and provide GPS coordinates, a key advantage of this technology is its ability to quantify pipeline movement and bending strains.
Further, this method detects curvature in the pipeline, meaning the change in angle of the pipe centerline with distance. The curvature associated with formed bends can be easily identified by its characteristic profile.
It is assumed that any remaining curvature that is measured has been imposed on the pipeline. Most of this curvature will result from the construction of the pipeline, since it usually follows the contours of the ground. The other source of imposed curvature are external loading effects such as ground movement, which is therefore important for integrity management programs.
This method shows if active loading is taking place on a pipeline even before damage occurs and enables appropriate decision-making.
The three assessment levels applicable to bending strain data include:
Level 1: Locations of bending strain above a threshold level are reported and plotted.
Level 2: Assessment of strain acceptability and evaluation for active movement of source of strain.
Level 3: Detailed assessment of locations of active movement to develop a monitoring and mitigation plan.
With data from several tool runs, it is possible to perform an evaluation of pipeline movement within the Level 1 assessment. This offers more certainty in identifying active geohazards that cause pipeline displacement. Figure 1 shows a plan view of a pipeline inside a landslide. Clearly, the centerline of the pipe has shifted under the influence of the ground movement between the two inspections.
Figure 1 – Pipeline movement assessment using IMU inspection data
The Level 2 strain acceptability evaluation process can take into account the condition of the pipeline, including geometric features identified during a caliper inspection or the presence of metal loss in the bending strain area detected by the MFL tool. The degree of strain severity or type of strain source is associated with a prioritization process and a set of recommended actions adequate to the situation.
If active movement is detected, a Level 3 assessment is recommended.
This includes structural calculations of the effect of ground movement on the pipeline to allow a full assessment of the structural integrity of the asset and predictions of the expected future condition as movement continues. Integrating the analysis with IMU data of the pipeline centerline geometry and strain changes allows for a development of a robust representation of the actual mechanism at site. The result will provide a framework for decisions to protect the pipeline from further damaging stresses.
MAKING USE OF COLLECTED DATA
Although all operators request GPS coordinates for their tool runs, many do not take advantage of post-ILI bending strain calculations. However, once the IMU data is collected, it can be used to perform a bending strain assessment at any point.
In this case, the operator previously completed an inspection using a combo ILI tool. After experiencing a product release due to settlement-related issues, the available IMU data was used to perform a bending strain assessment. The presence of high bending strains was detected in the area where the failure subsequently occurred and at another location.
Based on the results of the bending strain analysis, the operator was able to take decisive measures to prevent the possibility of another failure.
The area of high strain was excavated and the pipeline section was cut out and replaced. As part of the pipeline rehabilitation project, ROSEN experts collected in-situ strain measurements to compare with the reported bending strains. This collected data was utilized to validate the accuracy of the data and the bending strain assessment process. The in-situ strain was measured using strain gages placed on the pipe close to the locations where the pipeline was cut.
ROSEN provided assistance in data acquisition before and during the excavation phase. Prior to the main excavation for the pipeline cut-out, our experts evaluated the site to identify suitable locations for the monitoring exercise, including an evaluation of bending strain data and site conditions. Installing strain gages requires small excavations to expose the pipe surface. However, this proved to be challenging since the area of bending strain was located below a business driveway crossing a swamp area.
The location may partially explain the loading on the pipeline, as swampland is made up of soils with low strength. These can shift under surface loads such as the construction of new roadways or earthworks.
SELECTING THE LOCATION
It is recommended to excavate as little as possible before strain gages are placed to avoid relieving any in-place strain while at the same time taking into account factors such as equipment access and personnel safety when selecting the location for gage installation.
Our experts provided on-site support throughout the instrumentation and data collection exercise, including the installation of the strain gages.
This included correlating the profile of measured bending strains with the in-field pipeline. The location of the final cut was selected based on the distribution of bending strains along the pipeline. The pipe experienced significant movement after the cut, confirming the presence of external loads.
Figure 2 – Installation of strain gages on pipeline surface at two locations
ROSEN processed the collected strain gage data to provide comparisons with the bending strain data that had been previously recorded during the IMU inspection (results see Figure 3). The bending strain calculations indicated a total bending strain of approximately 1,000 microstrain (0.1%) was present in the pipeline two years before the field works. Further, the data gathered from the strain gages attached to the pipeline indicated a total strain of 800 microstrain (0.08%) was present prior to the cut-out.
The difference between the measured strains is attributed to some stress relief occurring during excavation prior to the placement of the gages. Nevertheless, the measured strains are within the reported accuracy of 0.02% for the bending strain calculations, confirming the excellent performance of the IMU tool. This data justified the decisions made on the basis of the IMU data and provided confidence in the use of this data.
Figure 3 – Location of strain gages and pipe cut plotted on the IMU bending strain profile
This article underlines that the assessment of bending strains utilizing IMU inspection tools provides an effective method to identify, evaluate and monitor the presence of geohazards that can affect pipelines. Detecting the early development of active movement will reduce the likelihood of pipeline failure and provides more options to mitigate the loading threat.