In the 80’s, there was a need for a more adequate technique to estimate the penetration depth of fatigue cracks in underwater welded tubular intersections of offshore oil platforms. That’s when the Alternating Current Field Measurement (ACFM) non-destructive technique was invented.
What exactly is ACFM? It’s an electromagnetic technique that can detect surface-breaking cracks without removing the coating and can then predict the crack’s length and depth using an advanced mathematical model. More precisely, the material under test is submitted to an AC magnetic field, produced by the probe, which induces a uniform current to flow in the material surface. The magnetic flux over the material’s surface is measured by sensors in the probe nose. Two components of the magnetic flux density are measured: one is primarily related to the depth of a defect and the other is sensitive to the location of the ends of the defect and hence predicts the defect’s length.
Eddyfi Technologies, via it’s TSC product line, offer ACFM instruments and probes that can be deployed for topside inspections by inspection technicians or rope access specialists and subsea, deployed by divers or ROVs.
Due to its insensitivity to lift-off, the ACFM technique is particularly suited for subsea applications including node weld inspection, pipe welds and load bearing points. This inspection technique leads to more efficient and less costly inspections because cleaning marine growth is reduced and removing coating is not necessary.
For situations where placement accuracy is limited, multi-element array probe suits perfectly since they can inspect a wide area in one pass and can be coupled with remotely operated vehicles (ROV).
Recently, Eddyfi have developed an ACFM array probe, which can inspect butt welds under coatings up to 40 mm (1.5 in) thick and is able to detect defects 20 mm (0.8 in) long by 2mm (0.08 in) deep and greater.