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Inline Inspection (ILI) has long been a cornerstone of pipeline integrity management, yet its effectiveness depends heavily on maintaining a stable and predictable tool velocity. In today’s operating environment—where pipeline throughput is economically critical—operators are increasingly reluctant to reduce flow rates simply to accommodate inspection tools. This tension is especially pronounced in gas gathering systems, where low pressures, high flow rates, and variable operating conditions create significant challenges for traditional ILI technologies.
This article summarizes the development, deployment, and performance of a modern speed control ILI system in two low‑pressure gas gathering segments, demonstrating how tailored configurations and operational planning can overcome long‑standing inspection challenges.
The Operator’s Gathering System: A Complex Inspection Environment
The case study centers on a 65‑mile, 24‑inch gas gathering loop installed in 2016. The system collects natural gas from regional wells and delivers it to fourteen compression stations before sending it onward to nine processing plants. The loop is divided into multiple piggable segments, each with unique flow characteristics.
Two segments were selected for inspection:
Both segments also experience extremely low differential pressures—typically only 10–15 psig between launcher and receiver—making tool propulsion and velocity control especially difficult.
Why Low‑Pressure Gas Gathering Lines Are Hard to Inspect
ILI in gas gathering systems is notoriously challenging. Several factors contribute to this:
1. High Velocity Fluctuations
Low-pressure gas behaves unpredictably, causing rapid changes in tool speed. These fluctuations can distort magnetic fields in MFL tools, leading to data gaps. The figure below shows an example of an ILI tool velocity chart for a 280 psig gathering line inspection under more ideal operating conditions. There are several instances where the tool speed significantly exceeds the ideal range of 6.5 mph, with a ~+/- 5 mph range surrounding the target velocity, creating data gaps.
2. Risk of Tool Damage
High-speed surges can cause the tool to impact tight fittings or the receiver barrel, posing potential equipment risks.
3. Low Differential Pressure
With only 10–15 psig of differential pressure, launching and propelling an ILI tool becomes difficult. Tools may stall or surge unpredictably.
4. Limited Operational Flexibility
Increasing pressure or reducing flow—common strategies for stabilizing tool velocity—were not viable. Doing so would have required shutting in wells, disrupting production, and increasing costs.
5. Alternative Methods Were Not Feasible
Hydrostatic testing, robotic crawlers, tethered tools, and liquid batching were all evaluated and rejected due to cost, complexity, or incompatibility with the system’s geometry and flow conditions.
Given these constraints, the operator sought a solution that could maintain throughput while still delivering high-quality integrity data.
Speed Control Technology: How It Works
Speed control systems regulate ILI tool velocity by adjusting a bypass valve that allows gas to flow through the tool. The simplified block diagram outlines the primary function of a speed control system.
The logic module compares real-time tool speed to the target velocity. If the tool is too slow, the valve closes to increase speed; if too fast, the valve opens to allow more gas to bypass and reduce tool speed.
Key Factors Affecting Performance
There are several key factors impacting bypass performance, including:
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Line pressure and gas density
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Tool drag, influenced by debris and line cleanliness
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Pipeline geometry
A major challenge is predicting Δp (pressure drop across the tool), which can vary by up to 35% between pipelines. This variability makes it difficult to estimate bypass capability without real-world testing.
The plot below shows the estimated gas bypass curves for a large diameter MFL-based ILI tool as a function of line pressure. The red dashed line is the target velocity for the ILI, which the speed control will try to maintain. The volume of gas able to be bypassed decreases with increasing line pressure, as indicated in the plot. Furthermore, the plot includes a lower and upper gas velocity limit, the difference between these lines is to account for the potential Δ𝑝 variance between different pipelines.
Transmission Pipeline Benchmark: High Pressure, Predictable Dynamics
Before tackling the gathering system, the speed control system had been proven in high-pressure transmission pipelines. An example of the typical transmission pipeline operating conditions is shown in the table.
At these operating parameters, the speed control tool was expected to run as a steady state system, with limited dynamic movement maintaining the target tool speed. In addition, given the maximum flow conditions, the speed control was anticipated not to reach full bypass capacity. The figure below shows the target velocity (red dashed), the tool run speed (blue solid), and the operation of the speed control valve (black dotted). Reviewing the tool speed, the velocity of the tool exceeded the target range, as indicated in the plot. The speed control valve opened, bypassing gas, until the tool velocity came back within the target tool velocity range. The overspeed and 100% opening of the speed control valve were due to a hard tool launch. Most notable from the plot is the limited movement of the speed control valve; the dynamic response of the valve, once the tool was in the target speed range, was very limited. This speed control valve's dynamic response is characteristic of the steady flows expected in gas transmission pipelines. Lastly, the speed control valve was open approximately ~50% throughout the inspection, indicating the speed control still had capacity to bypass more gas, aligning with the predictions outlined in the table.
This steady-state behavior is typical of transmission systems, where pressures and flows are more consistent. Below are images of the ILI tool post transmission pipeline inspection.
Applying Speed Control in Low‑Pressure Gathering Pipelines
The pipeline owner operates a super gathering system that requires routine ILI to manage the asset integrity. It was understood by all parties that while speed control tools have the capability to reduce tool speed under high flow conditions, there were upper limits to the flow. The Operator’s Integrity engineers, Operations and Control Center, along with the ILI Vendor, agreed upon a plan where the flow would be reduced for the duration of the ILI run to the least possible without shutting in any wells. Similarly, the average line pressure would also be increased to the maximum possible without shutting in any wells, along with trying to maintain a large enough differential to keep the tool moving without stopping.
With agreed-upon plans in place, the first ILI speed control tool run was attempted in Segment-1 in May 2025. The average operating pressure was maintained at 167 psig with a differential of 60 psig. The run was completed with the ILI tool travelling at an average speed of 5.8mph, which was below the tool’s maximum specification. The tool experienced some short areas of speed excursion totaling the equivalent of 3.86% of the total distance. Overall, the tool run was successful with 100% sensor coverage.
Encouraged by the success of the tool run in Segment-1, a run was conducted in Segment-2 in June 2025. The pipeline operating parameters for Segment-2 are included in the table below. The average operating pressure was maintained at 181 psig with a differential of 20 psig.
The graph below shows the tool velocity and speed control valve dynamics for this gathering line. The target tool velocity is indicated by the dashed red line, which was set to 6 mph. The speed control was able to maintain the tool velocity close to the target set point. There was an ~+/- 1 mph range surrounding the target velocity. The speed control valve was very active in maintaining this velocity, as indicated by the valve position (black dotted line) in Figure 9. The speed control valve dynamics were as expected, given the low line pressure and high gas flows. Contrasting the speed control valve dynamics in the transmission example, the valve was making 30-50% position changes to maintain velocity, as expected. The tool experienced minimal areas of speed excursion, totaling the equivalent of 0.72% of the total distance. This tool run was also deemed successful with 100% sensor coverage.
The line conditions were dry as expected based on the cleaning results. The tool came out very clean with no debris. The images below show the condition of the ILI tool pre and post-inspection. The dry conditions contributed to the speed control performance and improving bypass capability. Throughout the inspection, the speed control was never 100% open or limited in bypass capability, resulting in the ability to maintain the target speed and handle the significant gas dynamics from the low-pressure line.
Conclusion: A New Path Forward for Low‑Pressure ILI
The successful deployment of speed control in these gathering segments represents a meaningful advancement for operators facing similar challenges. Historically, low-pressure gas gathering lines have been difficult—and sometimes impossible—to inspect using conventional ILI tools. This often forced operators to choose between:
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Costly hydrostatic testing
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Production shutdowns
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Increased excavation and NDT
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Shortened re-inspection intervals
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Accepting reduced data quality
Speed control offers a practical alternative that avoids these trade-offs. The trials in Segment‑1 and Segment‑2 demonstrate that adaptive speed control systems can reliably manage tool velocity in low-pressure, high-flow gas gathering pipelines—environments once considered unsuitable for MFL ILI. The plots below show the tool speed comparison of an ILI in a 400 psi line without speed control, compared to Segment 2 at 180 psi with speed control. The technology maintained near-target speeds, minimized excursions, and delivered full sensor coverage without requiring throughput reductions or operational disruptions.
For operators managing aging infrastructure, this capability is transformative. It enables proactive integrity management, reduces operational risk, and supports safer, more efficient pipeline operations—all without sacrificing production.
Interested in adaptive speed control? Contact our Onstream team today: Contact - ONSTREAM | Pipeline Inspection Services