What Is an Unpiggable Pipeline — and How Do You Inspect It?
More than half of the world's transmission pipelines cannot be inspected using conventional inline tools. The industry calls them unpiggable — and they represent one of the most significant blind spots in pipeline integrity management today.
For operators responsible for aging infrastructure carrying natural gas, crude oil, hydrogen, or industrial chemicals, an unpiggable pipeline is not just a pipeline inspection challenge. It is an unquantified risk.
This article explains what makes a pipeline unpiggable, why conventional methods fall short, and how above-ground inspection with CMI technology is changing the way operators assess these critical assets.
What Makes a Pipeline "Unpiggable"?
An unpiggable pipeline is a pipeline that cannot accommodate conventional inline inspection (ILI) tools — commonly known as smart pigs. These tools travel through the pipeline interior to measure wall thickness and detect defects. But they require specific physical conditions that many pipelines simply do not meet.
A pipeline is typically classified as unpiggable when it has:
- Tight bends (mitre bends, short-radius elbows) that block tool passage
- Diameter changes or reductions along the pipeline route
- Unbarred tees where tools can become stuck or misdirected
- No pig launcher or receiver stations — and no practical way to install them
- Continuous operation requirements that prevent the shutdowns needed for tool insertion
- Complex routing through urban areas, industrial facilities, or river crossings
Industry estimates suggest that over 50% of transmission pipelines worldwide fall into one or more of these categories. In gas distribution networks, the figure is even higher.
Why Unpiggable Pipelines Are a Growing Problem
The pipeline inspection challenge posed by unpiggable assets is not new — but it is becoming more urgent. Much of the world's pipeline infrastructure was built in the 1950s through 1980s and is now approaching or exceeding its original design life. Coating systems degrade. Cathodic protection conditions change. External corrosion develops at locations that have never been inspected.
Without reliable inspection data, operators cannot determine remaining wall thickness, cannot classify whether corrosion is active or stable, and cannot prioritise intervention. The result is either excessive conservatism — replacing or repairing pipelines that may still be fit for service — or undetected degradation that leads to failure.
Regulatory frameworks such as ASME B31.8S, CSA Z662, and European integrity management standards increasingly require operators to demonstrate that all pipelines, including those that cannot be pigged, are assessed using validated methods. Simply accepting "unpiggable" as a reason for no inspection is no longer viable.
Traditional Inspection Methods and Their Limitations
Inline Inspection (ILI) — The Pigging Approach
ILI tools deliver detailed wall thickness data and are the standard for piggable pipelines. However, they require physical access to the pipeline interior, launcher/receiver stations, and typically a shutdown or flow interruption. For unpiggable pipelines, ILI is simply not an option without costly retrofitting — which often exceeds the value of the inspection itself.
Hydrostatic Testing
Pressure testing can verify a pipeline's structural integrity at a specific point in time, but it provides no information about defect location, type, or progression. It requires a full shutdown, is water-intensive, and does not detect early-stage corrosion. It is a pass/fail test, not an inspection.
Direct Assessment (ECDA)
External Corrosion Direct Assessment methods such as DCVG and CIPS identify coating defects from above ground. However, as the DVGW NEMEK study demonstrated, these methods cannot determine whether corrosion is actually occurring at a detected defect. CIPS achieves only a 64.9% true positive rate for cathodic protection verification, and Intensive Measurement produces a 68.4% false negative rate — meaning up to 95% of excavations triggered by these methods may be unnecessary.
How CMI Technology Inspects Unpiggable Pipelines from Above Ground
EMPIT's patented Current Magnetometry Inspection (CMI) technology was developed specifically to solve the unpiggable pipeline problem. Unlike ILI, CMI does not require access to the pipeline interior. Unlike DCVG or CIPS, it does not stop at coating defects — it determines the corrosion state at each defect.
How It Works
CMI applies a multi-frequency AC current (2 Hz – 2 kHz) to the pipeline via existing cathodic protection test posts. Up to 70 tri-axial magnetometers, mounted on EMPIT's carrier systems, measure the resulting electromagnetic field from the surface — capturing 20,000 data points per second. Through patented spectrum analysis, the system maps the pipeline's condition without any excavation or service interruption.
What CMI Detects
- Corrosion defects with a resolution down to less than 1 mm²
- Coating holidays and coating condition assessment
- Pipeline alignment, depth of cover, and 3D positioning
- Bending strain and pipeline movement
- Girth welds, crossings, and encroachments
Active vs. Passive Corrosion Classification
This is CMI's defining capability. At every detected coating defect, CMI analyses the frequency-dependent spread resistance to classify the corrosion state:
- Active corrosion: The spread resistance remains constant across frequencies, indicating ohmic current flow through bare steel in direct soil contact. The corrosion cell is active. Excavation and repair are required.
- Passivated corrosion: The spread resistance increases with decreasing frequency, indicating a capacitive barrier — a protective calcareous layer has formed. The corrosion process has stopped. Monitoring is sufficient.
The DVGW NEMEK study validated this capability across 6,672 coating defects on operational pipelines of six European gas transmission operators, confirming a method reliability of 99.96%.
Inspection Method Comparison
| Method | Internal Access Required | Shutdown Needed | Detects Corrosion State | Works on Unpiggable Pipelines | Typical Coverage per Day |
|---|---|---|---|---|---|
| ILI (Smart Pig) | Yes | Yes | No (wall loss only) | No | Variable |
| Hydrostatic Test | Yes | Yes | No | Limited | N/A |
| DCVG / CIPS | No | No | No (coating defects only) | Yes | Limited |
| CMI (EMPIT) | No | No | Yes (active vs. passive) | Yes | Up to 10 km |
Where CMI Is Already Solving the Unpiggable Challenge
CMI-based above-ground inspection is deployed worldwide across a range of unpiggable pipeline scenarios:
- Gas transmission and distribution networks with multiple diameter reductions, bends, and branch connections
- Industrial facilities — refineries, chemical plants, LNG terminals, and airport fuel systems where pipeline routing prevents pig launching
- Urban and road-crossing pipelines where excavation is restricted and above-ground access is the only viable option
- Water and wastewater pipelines made of ductile iron or grey cast iron
- Hydrogen pipelines — an emerging application as operators convert existing infrastructure for hydrogen transport
EMPIT's inspection approach aligns with ISO 15589-1 and is ECDA-ready, making it suitable for integration into existing pipeline integrity management systems. To date, EMPIT has inspected pipelines in over 30 countries, processing more than 300 TB of inspection data from over 10,000 pipeline kilometres.
What Operators Should Know Before Choosing an Inspection Method
Not every unpiggable pipeline requires the same pipeline inspection approach. The right method depends on the pipeline's material, operating conditions, regulatory requirements, and the specific integrity questions the operator needs to answer.
Key considerations include:
- Do you need to know whether corrosion is active or stable? Only CMI provides this classification from above ground.
- Can the pipeline be shut down? If not, ILI and hydrostatic testing are excluded. CMI operates without any service interruption.
- What is the pipeline material? CMI works on all ferromagnetic materials — steel, ductile iron, and grey iron.
- What depth is the pipeline buried at? CMI covers standard depths up to 7 metres onshore and up to 50 metres offshore.
- Is the inspection ECDA-compatible? CMI data integrates directly into ECDA workflows per NACE SP0502 and ISO 15589-1.
For a detailed comparison, see also: Aboveground Pipeline Inspection for Unpiggable Pipelines.
Inspect Your Unpiggable Pipeline — Without Excavation or Shutdown
If your pipeline cannot be pigged, it does not mean it cannot be inspected. EMPIT's CMI technology provides the data operators need to make informed integrity decisions — without excavation, without shutdowns, and with the only above-ground method validated to classify active versus passive corrosion.
Whether you operate a gas transmission network, a water distribution system, or an industrial facility — contact our team to discuss how CMI can assess your unpiggable pipeline.
For answers to common questions about CMI and pipeline inspection, visit our FAQ.