Page 484 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 18.1
18.1 Nondestructive testing definition and sensors 479
(a)
Demagnetized Y
(b)
Z
X
S
N
Magnetized
Lines of magnetic flux upon a (a) demagnetized surface as well as a (b) magnetized surface.
Bar magnet
NS
Z
X Y
FIGURE 18.2
Lines of magnetic flux upon magnetization.
metal. Once externally magnetized, the magnets then snap to the new magnetic flux and align themselves with the magnetic field (Figure 18.1).
Once the magnetic field is stimulated (by one of the three methods above), a magnetic ink (fer- romagnetic powder in liquid suspension—hence the term “magnetic particle”) is introduced onto the metal, and then the lines of magnetic flux are visually observed (Figure 18.2). Any anomaly within the magnetic field becomes readily obvious, thus revealing the metal defect.
18.1.2 Alternating current field measurement
In recent years, alternating current field measurement (ACFM) has become the subsea NDT “weapon of choice” for the magnetic flux leak detection inspection sensor. ACFM is an electro- magnetic technique whereby an alternating current is induced into the metal surface of a test sub- ject (e.g., the jacket leg of an oil and gas production platform), and then the magnetic field is mapped for uniformity (or lack thereof). Once the AC magnetic field is induced, the metal produces a uniform magnetic field above the surface. Any discontinuity in the metal’s surface will produce a perturbation in this field (Figure 18.3), forcing the field to flow around or under/over the fault.