The Hidden Challenges of Low-Pressure Pipelines — and How to Manage Them
At first glance, low-pressure pipelines might seem easier to manage than their high-pressure counterparts. With operating stresses at only a fraction of the hoop stress associated with the pipe’s rated MAOP, and with a perception of lower consequence, they can appear to be less risky. In practice, however, these systems often prove to be some of the most difficult and costly to manage from an integrity standpoint.
Low-pressure pipelines bring a unique set of challenges that complicate inspection, assessment, and repair. But with the right strategies, operators can mitigate these risks and keep their assets safe and reliable.
1. ILI Behavior in Low-Pressure Systems
Corrosion assessment in low-pressure lines often produces counterintuitive results. Using models like RSTRENG, deeper corrosion defects can sometimes still “pass” because the operating hoop stress is relatively low. On the other hand, shallower but broad volumetric features may fail, generating calls that don’t always align with intuition.
Inspection technology adds another layer of difficulty. Magnetic Flux Leakage (MFL) tools in particular face challenges in low-pressure systems:
Metal loss clusters with complex morphology are difficult for MFL to size consistently, often producing variability in reported results — with some anomalies severely under-called and others over-called.
Ovality can cause sensors to lift off or misalign, either because debris collects in distorted areas or because the sensors cannot adequately reach and track the wall.
Higher volumetric coverage complicates anomaly characterization, further increasing sizing uncertainty.
The result is not necessarily “more calls,” but rather a different pattern of calls — with digs driven by variability and uncertainty rather than clear, consistent severity trends.
How to manage it:
Validate ILI results through targeted digs and develop a tool-specific error profile.
Use advanced assessment methods (e.g., API 579, probabilistic models) for clusters and complex features.
Where possible, select inspection technologies better suited to the asset, such as high-resolution MFL or ultrasonic tools.
2. Reduced Resistance to Buckling
Internal pressure does more than move product; it also stiffens the pipe wall, providing resistance to outside forces. In low-pressure systems, this stiffening effect is absent, leaving the pipe mechanically weaker.
A useful analogy is a balloon: inflated, it holds shape and resists bending; deflated, it buckles and kinks with ease. Low-pressure pipelines behave the same way. Without the support of internal pressure, they are more vulnerable to:
Soil movement or frost heave, which can lead to ovality or buckles.
External equipment loads, which can dent or deform the pipe.
Geometric distortion, which further undermines ILI accuracy.
How to manage it:
Maintain higher operating pressures where feasible to add stiffness.
Monitor ground conditions with strain gauges, settlement markers, or LiDAR in high-risk areas.
Stabilize or re-bed soil in locations prone to external loading or movement.
3. Slack Line Dynamics in Mountain Passes
One of the most overlooked risks in low-pressure liquid pipelines is the potential for slack line flow, especially in mountain passes where elevation changes are steep. When pressure drops below the vapor pressure of the product, sections of the pipeline transition from liquid to vapor.
In these slack regions, a pig may accelerate rapidly through the vapor pocket without the hydraulic support it was designed for. Upon re-entry into liquid, the tool can impact violently, creating multiple risks:
Tool damage – ILI pigs are delicate instruments, and sudden re-entry forces can cause substantial harm. At high velocities, sensor arrays themselves can be destroyed.
System impacts – The hydraulic shock can create pressure spikes, which in some cases may result in a temporary exceedance of MOP.
Inspection complexity – More resilient sensor materials or reinforced tool housings may be required, adding complexity and cost.
Data quality issues – High velocities can compromise the tool’s IMU performance, making it difficult to reliably capture curvature, bending strain, or movement data. Unfortunately, these sensitive mountain pass locations are often the exact areas where high-quality IMU data is most critical.
How to manage it:
Use hydraulic modeling to predict slack line conditions and plan runs during operating states that minimize vapor formation.
Adjust flow rates and batching strategies to better regulate pig velocity in elevation changes.
Partner with ILI vendors to design or select tools with reinforced sensors and materials for higher resilience.
In critical areas, consider a special mission: direct assessment, tethered crawler, or robotic inspection. These approaches may require substantial shutdowns or bypass, but they can provide reliable data when conventional ILI cannot.
4. Access and Logistics in Remote Terrain
Low-pressure pipelines frequently traverse mountain passes and remote regions, making access to dig sites a major challenge. Mobilizing equipment may require helicopters, seasonal access, or long mobilization windows.
What looks like a “routine” excavation on paper can turn into a major logistical event, with planning and cost far exceeding the technical scope of the repair. When combined with slack line risks in these same areas, mountain passes represent some of the toughest integrity challenges an operator can face.
How to manage it:
Pre-stage equipment in areas where access windows are short or seasonal.
Use remote monitoring technologies (strain sensors, corrosion probes, aerial inspection) to reduce unnecessary digs.
Where excavation is impractical, consider alternatives such as hydrotests or direct assessment methods to validate asset integrity.
Closing Thought
Low-pressure pipelines are not “low risk.” Instead, they combine a set of challenges that often make them more complex than high-pressure systems:
Counterintuitive corrosion assessment outcomes
Uncertainty in ILI performance due to complex morphology and ovality
Greater susceptibility to buckling
Slack line risks in mountain passes, including IMU data reliability issues
Logistical burdens of remote access
But these challenges can be managed. By validating ILI performance, applying advanced assessment techniques, modeling hydraulic behavior, reinforcing tool design, and planning for difficult access, operators can build integrity programs that anticipate the quirks of low-pressure pipelines instead of being surprised by them.
Low pressure doesn’t simplify pipeline integrity. In many ways, it makes it harder — but with the right strategies, it is manageable.
The information provided in this article is for general educational and informational purposes only. It reflects professional experience and industry practices but should not be interpreted as engineering advice, regulatory guidance, or a substitute for operator-specific assessments. Pipeline conditions vary, and integrity management decisions must be based on applicable codes, standards, regulations, and qualified engineering judgment specific to each system. Neither the author nor Gustafson Integrity Services LLC assumes any liability for actions taken based on the content of this article.
