Pipelines live in the landscape – and the landscape is not always as benign or hospitable as operators would like it to be. Extreme weather events occur with increasing frequency, some areas are prone to earthquakes or volcanic activity, and rivers are constantly shifting.
Geohazards represent a major threat to pipelines because the resulting loading can be hard to identify, difficult to predict and extremely challenging to control.
Abnormal loadings can result in damage to – or even failure of – the pipeline from tensile fracture or buckling. In this article, experts Chris Holliday and Andy Young from the ROSEN Group, Carrie Murray from Stantec Consulting Ltd. and Terri Funk from Husky Midstream General Partnership explore how strain and pipeline movement assessments help to identify and mitigate threats posed by geohazards.
Following a loss of containment incident in July 2016 on a 16-inch diameter pipeline on the south slope of the North Saskatchewan River located in Saskatchewan, Canada, Husky completed an extensive study to understand and learn from the failure. The cause of the incident was ground movement resulting from a landslide complex on the slope. One aspect of this study was to conduct a structural analysis of the pipeline response to the loading imposed from the ground movement to minimize the potential of a similar occurrence happening in the future and determine the integrity of the pipeline at the time of the assessment.
Figure 1 – A pipeline exposed in a rotational landslide
Given the scale and complexity of the landslide, slope stabilization measures were impractical to implement, so repeat ILI using caliper and inertial measurement technology (IMU), in addition to a robust monitoring program, was implemented. Combined with documented risk thresholds that identified when to proactively shut in the pipeline, real-time monitoring of ground movements, pipe strain and precipitation levels provided a monitoring and early-warning system.
The study involved modelling of the pipeline history on the slope, including loads that had accumulated in the original pipeline sections based on historical in-line inspection (ILI) results and slope monitoring. The pipeline orientation was parallel with the ground movement in the landslide complex, so the development of axial strain in the pipeline was the dominant load component, which is particularly damaging in the compression zone and challenging to detect with IMU technology.
LINKING GROUND MOVEMENT TO PIPELINE STRAIN
A key element of geohazard management for pipelines is the development of the link between ground movement and the strain in the pipeline. This can be achieved by structural modelling, which enables the condition of the pipeline to be evaluated using results of surface and sub-surface measurements of ground movement data. In addition, the future movement that can be tolerated before the strain capacity limits are reached can be estimated from the modelling. This information provides a framework to manage the pipeline and protect it from damaging strains by scheduling intervention works such as stress relief or replacements.
The link between ground movement and the strain in the pipeline can be used with strain capacity limits to determine maximum allowable ground movement before interventions such as stress relief or replacements should be conducted. Due to the uncertain nature of future rates of movement, plots based on movement are preferred to plots based on time. In addition, this enables direct comparison with measurements of slope movement from slope inclinometer (SI) or monument surveys.
Slope movements have been recorded by on-site instrumentation since September 2016. Following the 2017 replacement (to repair the loss-of-containment site), the structural modelling predicted the continued development of high compressive strain at the location of the 2016 loss-of-containment event, i.e. the field overbend located near the base of the slope. As a result of this finding, the decision was taken in late 2018 to take the pipeline out of service rather than proceed with another proposed replacement project.
Findings from this loss of containment event highlight the need for operators to review the geometry of trenchless river crossings – or indeed trenched crossings – installed near the toe of valley slopes; ground movement can accumulate strain at the overbend, as it forms an anchor for the pipeline.
MONITORING GEOLOGICAL CONDITIONS
This incident also illustrates that current pipeline integrity risks are imposed by historical states of practice. Furthermore, it underscores the need to identify and routinely assess the geological conditions during geohazard assessments – both on the right of way (ROW) and at a regional scale.
Intense localized high-rainfall/fast-snowfall melt events are triggering mechanisms for re-activating landslides. An on-site weather station is recommended for high-risk geohazard sites to capture and monitor local rainfall events.
Despite ground movement resulting in a loss of containment in 2016 and subsequent monitoring indicating continued slope movements, only one open ground crack was observed.
Installation and monitoring of SI and regular surveys of near-surface survey monuments were most effective in monitoring ground movements when integrated with terrain ground traverses on and off ROW with regular LiDAR change detection analysis to observe landslide behavior and surface movements.
The selection and positioning of slope monitoring instrumentation should be based on a comprehensive analysis of the ILI, the pipeline structural analysis and the geohazard analysis.
This investigation demonstrates that the most robust evaluation of pipelines located in landslide hazards calls for a combination of investigative, monitoring and assessment methods.
A single ILI can identify areas of bending strain linked to geohazard events, and consecutive ILIs can quantify the pipeline movement between inspections. But ILI may not identify geohazard sites that have been dormant since the pipeline was constructed.
Geohazard analysis can be conducted to identify likely sites of ground movement but may not identify all sites of interest.
USE ALL INFORMATION AND MEANS AVAILABLE
However, by aligning and overlaying both the ILI and geohazard analysis (and routinely updating the analysis after each subsequent ILI), a more thorough understanding can be achieved. Therefore, one recommendation from this study is to use all information and means available to identify complex threats to pipeline integrity rather than rely on one hazard identification method alone.