How Do Magnetic Flux Leakage (MFL) Tools Determine if a Pipe Wall Defect is Internal or External?

In the first article of a multi-part series titled “Ultra-High Resolution MFL Sensor Technology: What to Look for in an Ultra-High Resolution Sensor,” Onstream's Dr. Stephen Westwood explores methods used to determine whether pipeline defects are internal or external, and how to identify if IDOD discrimination technology is being used by your inline inspection (ILI) vendor’s Magnetic Flux Leakage (MFL) tools.

Determining if a defect is Internal or External to the pipe wall is critical in determining the correct response, the active corrosion mechanism and potential inhibition program. From the vendor’s side, knowing whether the feature is internal or external is an important input into any sizing algorithm.

In the industry, there are three main methods used to determine whether defects are internal or external. This article outlines the three methods, and how to identify if your vendors have IDOD discrimination technology on their Magnetic Flux Leakage (MFL) tool.

Method 1: Analyst Inference (Tools without IDOD Sensors)

Some tools on the market do not have IDOD discrimination sensors. For tools without IDOD discrimination, the analyst is left to decide based on circumstantial evidence whether the corrosion is on the inside or the outside of the pipe. Typically, this is done by considering the product in the pipeline and the clock position of the corrosion. This method of determining whether corrosion is internal or external can be reliable when there is a singular, known corrosion mechanism. The follies in this method include:

1. Some pipelines have unknown corrosion histories.
2. Some pipelines have more than one corrosion mechanism, e.g. external coating failure causing an external defect and internal pitting in the same area.
3. There is little tolerance for internal corrosion and the presence of such can drive expensive mitigation actions.
4. Some pipelines have unusual corrosion patterns e.g. top of line corrosion, six o’clock external pitting (see Figure 1).

Many of Onstream’s competitors—in particular, newer startups without the technology and depth of experience that Onstream brings—frequently rely on the inference method to determine IDOD.

Method 2: Eddy Current Coil

This method relies on eddy current coil that is excited with a high frequency signal. This signal is only responsive to features on the internal surface of the pipeline as it decays exponentially due to the conductivity and permeability of the pipe steel. These eddy current sensors are typically within the main magnetic field of the MFL tool and are usually combined with main magnetic field sensors.

One consequence of this method is that the sensor housing that travels along the pipewall is made of a non-conductive material (typically ceramic or plastic). This is the method that Onstream uses, and the ceramic windows can be identified in Figure 2.

Figure 3 below shows the radial responses from a test spool for internal and external features, and how the IDOD sensor can be used to identify internal versus external features.

Method 3: Secondary Magnetic Sensing Ring

This method relies on a small permanent magnet and sensor being mounted in a sensor housing outside of the main magnetic field, typically as a secondary ring of sensors on the tool. ?A typical arrangement on a larger diameter MFL tool is shown below. The magnetic field from the smaller magnet does not saturate the pipewall and only responds to internal defects.

Conclusion

MFL Tool IDOD determination is an often overlooked but critical component of a successful integrity program. Less sophisticated MFL tools may rely on analyst inference alone. High resolution and ultra-high resolution MFL tool providers use one of the two physical sensor methods. Determination of internal vs external defects is a requirement based on industry standards such as API 1163.