Page 557 - Corrosion Engineering Principles and Practice

520 C h a p t e r 1 2 C o r r o s i o n a s a R i s k 521

Metal Loss (Corrosion) Tools
Metal loss tools are used to detect defects that have resulted in wall
thinning in a pipeline. They can discriminate to some extent between
manufacturing defects, corrosion defects and mechanical damage.
There are two main types of tools:
1. The most commonly used and established ILI tool is without
any doubt MFL (Fig. 12.19). Magnetic flux leakage tools use a
circumferential array of MFL detectors embodying strong per-
manent magnets to magnetize the ppe wall to near saturation
flux density. As the tool travels down the pipe it records mag-
netic flux leakages that are converted into information reveal-
ing anomalies i the pipe wall, such as corrosion pits (Fig. 12.20).
The leakage fluxes are detected by Hall probes or induction
coils that are part of the MFL ILI tool.
Two types of MFL tools are available, i.e. high resolution MFL
and standard resolution MFL. The main difference between
the two is in the number of sensors and the effective resolution.
Most MFL tools can determine the location and o’clock
position of the metal loss anomaly and detect if a corrosion
anomaly is nternal or external to the pipe wall. MFL tools also
provide data of each corrosion anomaly including its length
and maximum depth to determine the pipe remaining
srength. MFL tools are typically capable of detecting corrosion
depth greater than 20% of the pipe wall thickness. Axially-
oriented flaws such as SCC, selective seam corrosion and
axial gouges are difficult to detect with MFL tools.
2. Ultrasonic (UT) wall measurement tools use the ultrasound
echo time technique for measuring the remaining wall thick-
ness. The UT tool transmits an ultrasonic pulse into the pipe

Cups
Wheels (propulsion)
(stability)

Gas bypass valve
Magnetizer body (speed control)
(magnetic circuit)
FIGURE 12.19 Magnetic flux leakage ILI tool designed for high-speed gas
pipeline applications. (Courtesy of BJ Pipeline Inspection Services)