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Figure 5 – Discontinuities section position. Detected by conventional UT, at joint 00 of one FPSO turret.

3.1 ACFM

The use of ACFM (Alternating Current Field Measurement) is very conventional in oil production activities to detect fatigue crack at tubular welded joints (maritime production structures). The application of ACFM came as an alternative to underwater MPI that has a lot disadvantage in that case. Initially PETROBRAS tried to use Eddy Current as an alternative NDT method but ACFM advantages surpass both methods (MPI and EC) in use in underwater inspection. Nowadays several international papers present the use of ACFM industrial above water inspections.

The ACFM technology was developed in the 1980's from the successful ACPD contacting technique to provide a system for crack detection and sizing without the need for any electrical contact.

The crack sizing capability has resulted from the use of a uniform input field which allowed theoretical studies at the NDE Centre at University College London to predict crack depth from knowledge of the surrounding a.c. electromagnetic fields.

The technique was initially developed to allow crack sizing underwater where the ACPD technique was hindered by the need for good electrical contact. However, the other advantages arising from non-contact and a uniform input current (ease of scanning, little adverse effect from material property changes or probe lift-off) meant that the technique was quickly applied to topside inspections as well, particularly on painted or coated welded structures.

An ACFM sensor probe is placed on the surface to be inspected and an alternating current is induced into the surface. When no defects are present the alternating current produces a uniform magnetic field above the surface. Any defect present will perturb the current forcing it to flow around and underneath the defect; this causes the magnetic field to become non-uniform and sensors in the ACFM probe measure these field variations.

Two components of this magnetic field are measured (figure 6) - one provides information about the depth or aspect ratio of the defect(s), the other provides information on the positions of the ends of each defect. The two signals are used together to confirm the presence of a defect and, together with a sizing algorithm, measure its length and depth.

The 'Y' component, By, is parallel to the input cur-rent, the 'X' component, Bx, is perpendicular to the current and parallel to the metal surface, and the 'Z' component, Bz, is perpendicular to the metal surface. For deployment on fatigue cracked weld toes for

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