Introduction

C303 bar-wrapped steel cylinder pipe (BWP) remains one of the least understood assets in many transmission systems. Installed decades ago, these pipelines were designed for durability and high operating pressures. Many are still in service today — often without clear visibility into their structural condition.

Unlike prestressed concrete cylinder pipe (PCCP), where wire distress is typically the primary failure indicator, C303 bar-wrapped pipe relies heavily on the structural integrity of its steel cylinder. When corrosion reduces wall thickness in that cylinder, pressure capacity declines — sometimes without visible warning signs.

Remote Field Testing (RFT) is one of the few in-line inspection technologies capable of measuring remaining wall thickness in metallic components within pipelines that are lined with non-magnetic materials such as cement mortar. For utilities managing aging bar-wrapped steel cylinder pipe, it provides measurable condition data instead of age-based assumptions.

This guide explains remote field testing in practical terms: how it works, what it detects, when to use it, and where its limitations lie.

What Is Remote Field Testing (RFT)?

Remote Field Testing (also referred to as Remote Field Eddy Current, or RFEC) is an electromagnetic inspection method used to evaluate metallic pipe walls from inside the pipeline.

At a simplified level, the process works like this:

• A low-frequency electromagnetic signal is generated by the inspection tool.
• The signal passes outward through the pipe wall.
• It travels along the exterior surface.
• It re-enters the wall downstream.
• Sensors measure how the signal has changed.

Those signal changes are directly influenced by wall thickness.

If the pipe wall is uniform and intact, the returning signal behaves predictably. If corrosion has reduced metal thickness, internally or externally, the signal’s strength and phase shift in measurable ways.

Through calibration and analysis, those variations are converted into:

• Quantified wall thickness measurements
• Corrosion mapping along the pipeline
• Location-specific anomaly identification

This behavior is consistent with established eddy current testing principles as outlined by the American Society for Nondestructive Testing (ASNT). Unlike CCTV or visual inspection tools, RFT evaluates structural integrity rather than surface appearance.

Why Bar-Wrapped Pipe Requires a Different Inspection Focus

Inspection strategy should always match failure mode.

In PCCP systems, the thin steel cylinder is supported by high-tension prestressing wires. Broken wire detection is therefore the dominant concern and typically the leading indicator for pipe segment failure (although still not 100% correlation, so pipe segment issues can be missed with broken wire only inspection technologies).

Bar-wrapped pipe is structurally different:

• Thicker steel cylinder
• Reinforcing bars rather than prestressing wire
• Greater structural reliance on cylinder wall thickness
• Ability for corrosion to impact steel cylinder without having any broken bars

For these assets, the steel cylinder is not just a liner; it is a primary load-bearing component.

That means corrosion-driven wall loss is often the leading structural risk.

Remote field testing (RFT) is particularly well-suited to detecting this condition, which is why it is frequently selected for bar-wrapped pipeline inspection programs.

What RFT Detects in Bar-Wrapped Pipeline Inspection

For C303 bar-wrapped steel cylinder pipe, RFT is primarily used to evaluate the condition of the steel cylinder itself.

It is effective at identifying:

• General wall thinning caused by long-term corrosion
• Localized pitting that may create stress concentrations
• Internal corrosion influenced by water chemistry
• External corrosion related to soil conditions or coating degradation
• Areas of accelerated section loss
• Detect number of broken bars if there are any

The key advantage is that RFT provides quantitative data — not just anomaly flags.

Instead of knowing that “something is present,” utilities receive measurable wall thickness information that can be incorporated into engineering analysis, safety factor calculations, and remaining useful life modeling.

In practice, the value of RFT lies not only in detection but in interpretation. Signal data must be evaluated in the context of pipe geometry, reinforcement configuration, and operating pressure to determine structural significance.

When Should Utilities Consider RFT?

RFT is most appropriate when structural integrity depends on remaining metal thickness in a pressurized metallic pipeline.

Utilities often consider remote field testing when:

• Transmission mains are 30–50+ years old
• Historical inspection data is limited or outdated
• Replacement costs require defensible justification
• External corrosion risk is uncertain
• Leadership or regulators require measurable integrity documentation

It is also particularly valuable when inspection must occur with minimal service disruption. Many RFT tools can be deployed through planned access points, reducing the need for extended outages.

For asset owners moving toward condition-based asset management, RFT provides a way to replace assumptions with data.

Understanding the Limitations of RFT

No inspection technology is universal, and informed decision-making requires an understanding of its limitations.

RFT is designed specifically for lined metallic pipelines or concrete pressure pipes with metallic components (BWP/RCCP/PCCP). It does not apply to non-metallic materials such as HDPE or PVC.

Signal interpretation also requires experience. Reinforcement geometry, appurtenances, joints, and wall configuration all influence electromagnetic behavior. Accurate analysis depends on technical expertise in both inspection technology and pipeline engineering.

Finally, successful deployment requires planning. Access, insertion points, system conditions, and safety protocols must all be considered before inspection begins.

What RFT Data Looks Like

Electromagnetic signal variation forms the basis of RFT interpretation. Below is an example of RFT data collected during the inspection of a 36” Bar-Wrapped Pipe

Each horizontal green line in the above image represents data from a single detector on the inspection tool. Detectors are spaced evenly on the tool around the circumference of the pipe, with data from each detector displayed in the “unrolled” image above. 

In general, upward peaks in the data correlate to a reduction in metallic wall thickness, representing corrosion to the steel cylinder or broken bar wraps. Downward peaks in the data represent increased metallic wall thickness, typically found at joint connections or other pipeline features. The RFT tool is highly sensitive to even minor changes in the metallic components of a pipe. In the above image, the spiral weld of the pipelines steel cylinder can be seen as a series of small, repeating signal changes, running on an angle across the image. 

Analysts utilize specialized software to carefully measure all identified signal changes, and apply pipeline-specific calibrations to quantify the amount of wall loss or number of broken bars present at all identified locations. 

In the above example, three (3) corrosion indications were identified on the steel cylinder, two (2) of which presented in the data as potential through-holes (0% Remaining Wall). Additionally, approximately six (6) broken bar wraps were identified in the data overlapping with the potential through-holes. 

Upon completion of analysis, PICA provides detailed results to the asset owner, including:

• Location-referenced wall thickness measurements and broken bar indications
• Corrosion severity mapping
• Correlation to provided pipeline records (as-builts, GIS, etc.)
• Engineering-ready reporting

These results can be used to conduct verification work or targeted pipeline repairs. The pipe segment shown in the data above was excavated and removed from the pipeline, with the reported cylinder corrosion and broken bar wraps correlating directly with the RFT results. Photos of the excavated pipe segment with mortar coating removed are provided below.

When properly executed, RFT pipeline inspection provides a continuous structural profile along the length of the asset — allowing utilities to prioritize targeted repairs instead of defaulting to full replacement.

From Unknown Risk to Measured Condition

One of the greatest challenges facing utility leaders is capital justification. Infrastructure replacement is expensive and increasingly scrutinized by boards, regulators, and the public. Age alone is no longer a sufficient rationale.

Remote field testing transforms uncertainty into measurable condition data. Instead of asking whether a pipeline might be deteriorating, asset owners can evaluate how much wall remains, where corrosion is concentrated, and which sections warrant intervention.

For bar-wrapped steel cylinder pipe, where cylinder integrity directly affects pressure capacity, this clarity is essential.

Organizations specializing in advanced electromagnetic inspection of pressurized metallic pipelines, including bar-wrapped transmission mains, use RFT to support defensible integrity decisions grounded in data rather than assumptions.

Frequently Asked Questions

What does remote field testing measure?
It measures variations in electromagnetic signal behavior that correlate to changes in wall thickness, stresses, dents, broken wires, bars, or other anomalies in metallic pipelines.

Is RFT suitable for bar-wrapped pipe?
Yes. It is particularly effective for evaluating steel cylinder wall thickness in bar-wrapped steel cylinder pipe along with broken bar detection.

Does the pipeline need to be shut down?
Deployment depends on system conditions, but many RFT inspections can be performed with minimal service interruption.

How accurate is RFT?
When properly calibrated and interpreted, RFT provides repeatable, quantitative wall thickness data suitable for engineering analysis.

How is RFT different from RFEC?
They are often used interchangeably. RFEC (Remote Field Eddy Current) is the technical term; RFT (Remote Field Testing) is the common shorthand.

Speak with a Pipeline Integrity Specialist

If your utility manages bar-wrapped steel cylinder pipe and lacks quantified wall thickness data, the true risk profile of those assets may be unclear.

PICA supports utilities with advanced electromagnetic inspection of pressurized metallic pipelines, helping translate RFT data into actionable integrity insights.

To discuss your pipeline type, system constraints, and inspection objectives, contact the PICA team.