Remote field phenomenon was first discovered experimentally by W.R. MacLean in 1951. MacLean’s device consisted of axially oriented exciter and detector coils that are spaced apart by several pipe diameters inside a metallic pipe. A basic RFT setup including exciter and detector coils and a pipe or tube is shown in Figure 1.
Electromagnetic fields generated by the exciter coil propagate both inside and outside the pipe. The field inside the pipe propagates axially and its signal strength attenuates dramatically and becomes negligible after two to three pipe diameters from the exciter coil. This field is called the direct field and is in the direct field zone.
On the other hand, the field exiting the pipe continues to spread along the pipe axis and experiences much less attenuation compared to the direct field inside the pipe. At two to three pipe diameters away from the exciter coil, the outside field diffuses back to the inside of the pipe. This field component is called the remote field.
The transition zone is a buffer zone where the direct field zone ends and the remote field zone begins. Relative ID and OD magnetic field strength in corresponding zones are illustrated in Figure 1. Although the three zones are shown only on the right side of the exciter coil, the field is symmetric and the same zones exist on the left side of the exciter coil (not shown in the graph).
Figure 1. A basic RFT setup and plotting of ID and OD wall magnitude in three different zones: direct transition and remote field zones. 1D, 2D and 3D stands for one, two and three pipe diameter distance from the exciter coil.
The RFT technique can detect both ID and OD wall loss with equal sensitivity. RFT‑based inspection tools are sensitive to both local and general wall losses. Local wall loss includes localised defects, such as pitting defects, dents and cracks; whereas, general wall loss refers to wall loss spanning across a relatively large area both axially and circumferentially, such as erosion.
Multi‑sensor arrays are commonly used in RFT tools for pipeline inspection. Use of sensor arrays enhances tool sensitivity to local wall loss, especially localised pitting defects. Each sensor in the array can detect general wall loss at a particular circumferential position along the pipe axis. It is possible to generate a C‑Scan type of pipe condition colour map with sensor arrays in RFT tools, as shown in Figure 2.
Figure 2. Colour map display of a pipe joint inspected by a RFT tool.
Non‑wall loss indications, such as global stress/strain experienced by a pipe due to ground movement, uneven backfill, bridging and subsidence during pipeline construction can also be detected by the RFT technique. Stress/strain indications in a pipeline are not detected by other NDE techniques, such as UT or MFL.
Existence of global stress/strain does not affect the RFT technique’s capability to detect other defects in the same area. Local stress from a dent due to momentary impact from a heavy object (e.g. back‑hoe) or from a rock pushing against the pipe surface shows up in the RFT signal as magnetic permeability variations. If stress/strain and wall loss co‑exist in a pipeline, RFT can still distinguish them due to their differences in RFT signal phase angle.
Valves, girth welds, spiral welds, flanges and pipe tees are all clearly detected and identified by RFT technique with characteristic signals. RFT technique can detect metallic objects near a pipeline as long as the objects are in the propagation path of the electromagnetic field from the exciter coil to the sensors.
Some of the detected metallic objects in the past include a hammer left behind during pipeline construction, parallel (touching) or crossing pipelines, and casings under roads or railway tracks. Pipe bends and pipes on the slope of a hill can be detected as well if the inertial measurement unit (IMU) is incorporated in the RFT tool.
Field deployment and operation
A typical RFT tool consists of an exciter coil, detector arrays and supporting modules, such as electronics and centralisers. Exciter and detector coils and electronics are manufactured in sealed, pressure proof modules. A typical RFT tool is shown in Figure 3. The tool is flexible in order to negotiate elbows and fittings.
Figure 3. A photo of a SeeSnake™ RFT tool.
A tow pig and trailing stabilisation modules are usually attached to the tool to maintain constant travel speed and stability inside a pipe when pressurised water or other liquids push it during inspection runs. RFT tools can run in free‑swimming mode or can be towed using a tow line of steel or fibre rope. Either onboard distance encoders or an external encoder can be used for precise ocation of defects and other features.
The RFT tools can travel up to 10 m/min inside the pipeline, depending on pipe size, wall thickness and material properties. Onboard data storage and battery capacity allow the tools to run continuously for several days. RFT data is gathered and stored onboard while the tool is travelling inside the pipe. The onboard data is downloaded onto a computer for offline data analysis and reporting.
RFT tools can be launched through a standard, elongated pig launcher or an adaptor attached to a riser, and can be retrieved through a standard, elongated receiver or adaptor attached to a pipeline riser. Excavation sites can also be provided for tool launch and retrieval if standard pig launch and receive barrels are not available.
Pipe condition assessment
After the data has been collected, data analysis is performed offsite and a report is generated. Like other NDE techniques, defects are identified in terms of their location and severity. The RFT technique has a reported accuracy of ±15% for local wall loss and ±5% for general wall loss.
RFT tools have been used to inspect sour gas pipelines that have been internally lined by HDPE to prevent internal pipe corrosion. The linings have a typical thickness of about ¾ in. These pipes may be insulated and/or coated with ‘rock‑jacket’ on the OD surfaces.
Other NDE techniques, such as UT or MFL, do not work well with these types of pipe due to their requirement of sensor proximity to pipe ID surface. The RFT technique is most suitable for assessing the conditions of such pipes because RFT can tolerate large tool liftoff, and thick HDPE has only a minor effect on RFT tool operation and sensitivity.
A sample plot of pipe joint average remaining wall versus distance from reference for a 6 in. HDPE sour gas pipeline is shown in Figure 4. Remaining wall thickness of these pipe joints falls within manufacturing tolerance of ±10% of nominal wall thickness. Some localised wall loss indications were found on the same pipeline.
Figure 4. A plot of pipe joint average wall thickness versus pipeline distance for a 6 in. HDPE pipe.
Three local wall loss defects ranging from 25% to 40% deep were found near a girth weld. The HDPE lining thickness is 0.62 in. in this sour gas pipeline. A rock under a yellow jacketed gas pipe pushed against the pipe at bottom. The yellow jacket was damaged and the pipe was dented. The pipe started corroding over time. The corrosion resulted in 95% wall loss within the dented area at the time of the RFT inspection.
RFT signal of the defect and site photos of the pipe, rock, yellow jacket and corroded dent are shown in Figure 5. Similarly, RFT tools have been used for oil and gas pipeline inspection. So far, RFT inspections for oil pipelines have been primarily carried out on small to medium diameter pipes (less than 20 in.). Internal pitting defects (under‑deposit pitting) are commonly found in oil pipelines.
Figure 5. Corrosion within a dent caused by a rock pushing against a yellow jacketed pipe at the 6 o’clock position.
Multi‑sensor array RFT tools are designed specifically for detecting pitting defects. Figure 6 shows several pitting defects detected by a RFT tool in an 8 in. carbon steel oil pipeline. These pits come in different depth and volume wall losses. Some of them were detected by more than one sensor.
Figure 6. Pitting defects detected by a RFT tool in an 8 in. carbon steel pipe with a wall thickness of 0.156 in.
Summary
RFT‑based inspection tools are being used more frequently to inspect pipelines in oil and gas industries. In certain applications, such as HPDE lined sour gas pipes, RFT ILI tools are ideal for pipe condition assessment.
Free‑swimming RFT tools can inspect dozens of kilometers of pipes in one single run, while tethered RFT tools can inspect pipe lengths allowed by the tow line length (usually < 4 km). So far, RFT inspections have been performed mostly for pipes under 20 inches for Oil & Gas applications; however, RFT tools have been custom‑designed for larger pipe sizes and lengths in other industries like water and wastewater and now have RFT applications up to 84 inches in diameter.