In a Nutshell:
For the detection of geohazard-induced bending strain and pipeline movement, the inertial measurement unit (IMU) has proven itself an indispensable component of pipeline integrity. This is especially true for operators whose pipelines are located in areas that are susceptible to external loading due to landslides, earthquakes, washouts, seabed movement and other geohazards. ROSEN’s RoGeo PD (PipeDrift) service is a dedicated IMU platform that affords operators the ability to run a simple and efficient tool for addressing geohazard threats in cases where only bending-strain and pipeline-movement assessments are required. The simplicity of the platform allows operators to successfully mobilize and deploy the IMU with significantly less effort and time than what is required with combination in-line inspection (ILI) technologies. The robustness of the chassis provides the ability to inspect at substantially higher velocities when compared to combination tool counterparts, resulting in reduced impact on flow/operations. Ultimately, implementation provides a rapid and economical answer to the question, Has my pipeline moved?
Geohazard Evaluation Process
The relevant data collected by an inertial measurement unit consists of two components: a time record and inertial data. Inertial responses are collected by three accelerometers (linear) and three gyroscopes (rotational), which provide six degrees of freedom defined by a Cartesian coordinate reference system. By measuring inertia and time at high frequency and aligning the dataset to a master distance system, bending strains can be calculated and pipeline movement can be detected. The RoGeo PD service platform does not incorporate an odometer wheel; therefore, a native distance log does not exist within a raw dataset. To overcome the challenge posed by this, the IMU data is linearly aligned and geospatially tied to a master distance system belonging to a dataset with an integrated distance log. This process is similar to the typical alignment methods utilized when comparing features between two pipelines. The primary difference is that raw IMU signals are used to align the data.
To obtain an aligned PD and IMU dataset, the raw data is first processed without linear or geospatial correction. An integrity engineer aligns the data linearly and generates a series of offset points, which correlate it to a master distance log belonging to a baseline inspection. A data analyst then ties the dataset to the baseline inspection at a specified interval to ensure geospatial accuracy without muting of pipeline-movement signal responses. Bending strains are then calculated, and all final adjustments to the data are performed. Once these steps are complete, full bending strain and pipeline movement assessments are possible.
External influences such as extreme weather events, subsurface hydrology, settlement and construction activity can lead to the displacement of a pipeline from its originally constructed position and induce flexural strain into a pipe body. At elevated magnitudes, bending strains may lead to plastic deformation of a pipeline and compromise the integrity of longitudinal and girth welds. Calculating and understanding the bending strains present in a pipeline is paramount in determining the overall integrity of the system.
Utilizing a predefined methodology, a related set of assumptions and the data collected from an inertial measurement unit, the vertical and horizontal curvatures at an instantaneous point in the line can be calculated using Equations 1 and 2.
The maximum bending strain components can then be calculated by employing Equations 3 and 4.
Once bending strains have been calculated, it is possible to screen for anomalies (based on set criteria), identify strain interactions with pipe appurtenances and compare the calculated strains to those of additional datasets to determine the likelihood of pipeline movement (as shown in Figure 1).
Figure 1: Example bending strain comparison
Pipeline deviations can be quantified by comparing the trajectories of two pipelines across a specified distance interval. These trajectories are derived using the differential spatial information generated once the IMU data has been fully processed. To achieve a meaningful comparison, the PD and baseline trajectories are projected onto a common reference frame such that the start and end of a chosen segment in both datasets are coincident with the horizontal axis. This is achieved by calculating normalized coordinate values, generating a straight-line plot passing through the origin and the termination of the coordinate set, and calculating the orthogonal distance from the original coordinates to the straight line. Utilizing the resultant values, out-of-straightness (OOS) and elevation differences (∆E) may be calculated as
where ∆X, ∆Y and ∆Z are the components of distance from a given coordinate point to the line passing through the origin and termination of the coordinate set, and i ̂ and j ̂ are unit vectors corresponding to the straight line. By plotting the values calculated using Equations 8 and 9 against a specified master distance system, the trajectories of both datasets can be compared, and deviations can be characterized. This is illustrated in Figure 2 below. All deviations quantified within this paper were calculated with an accuracy of ±1.31 ft at 80% confidence.
Figure 2: Example pipeline movement area
IMU Data Quality Verification
During its early stages of development, the PipeDrift process was dynamic and continually evolving. Increases in efficiency through revisions in post-processing methods occurred throughout the first year of the technology’s advancement. However, the accuracy and reliability of the technology has been apparent from the beginning. Two RoGeo PD services, taking place in April and June 2021, serve as sufficient examples. Through quantitative and qualitative observation, it can be shown that the data produced by the PD technology platform is acceptable when compared to its historically reliable combination-tool counterparts. Figures 3 and 4 illustrate the accuracy of the technology by comparing the bending strains produced by both RoGeo PD and RoGeo XT services for an identified area of pipeline movement.
Figure 3: RoGeo PD – RoGeo XT bending strain comparison
Figure 4: RoGeo PD – RoGeo PD bending strain comparison
Technology Benefits & Limitations
Combination ILI technologies offer a host of advantages. These advantages make combination tools ideal for standard inspection intervals in areas where there is no known elevated threat due to geohazards and where multiple assessment types are not desired. However, in cases where an elevated threat exists and only an IMU-based assessment is needed, the RoGeo PD service offers the following benefits:
- Simple, robust design
- Rapid redeployment capability
- Reduced impact to product flow/operations
- Simpler set-up and deployment
- Higher frequency of geohazard screening
- Utilization of above-ground markers not required (baseline coordinates are used for geospatial referencing)
Ultimately, RoGeo PD can answer the question, Has my pipeline moved? with minimal effort and low impact on operations. To date, sustained average speeds of up to 8.7 m/s second have been achieved with no adverse influence on data quality or tool integrity. Due to the nature of the tool’s construction, a baseline inspection is required with a tool that generates a native distance system and girth weld log, and replacement areas must be identified prior to a RoGeo PD service.
Multiple 2022 RoGeo PD services are currently scheduled to support proactive geohazard monitoring programs as the service continuous to grow and improve. Signal-matching algorithms and machine learning are actively being implemented into the PipeDrift linear alignment process to reduce reporting times, and additional research is being performed with regard to the effects of physical parameters on IMU data capture. Additionally, improved reporting accuracies are currently being evaluated; they are listed in the table below.
Table 1: Draft Revised Pipeline Movement Accuracies
Now and into the future, implementation of the RoGeo PD services in the geohazard management programs of susceptible locations can offer valuable insights while significantly reducing cost.
Daniel Bahrenburg is a Senior Engineer with ROSEN Integrity Services, where he currently works on projects and service development related to bending strain, pipeline movement and geohazards management. He holds a bachelor’s degree in offshore and coastal systems engineering from Texas A&M University.