Page 366 - Pipelines and Risers

Pipeline Jnspection, Maintenance and Repair 337

The principle of eddy current is based on the phenomenon that an alternating current in a
transmitter coil induces alternating currents or eddy currents in any nearby conductor through
inductive electromagnetic coupling. The eddy currents in the conductor will in turn induce
currents in other nearby conductors, establishing an indirect electromagnetic coupling from
the transmitter coil via the first conductor to the second conductor. Hence, a receiver coil can
be indirectly coupled to a transmitter coil via the pipe wall. By designing the receiver coil in a
figure eight shape, the direct electromagnetic coupling between transmitter and receiver coil is
canceled out and the receiver coil is only responsive to the indirect electromagnetic coupling
via the pipe wall. The phase and amplitude of receiver coil signal are highly sensitive to the
distance between the coils and the pipe wall. By a proper selection of frequency and phase of
the eddy currents, the signals have been made insensitive to pipe wall material properties.

Capabilities and Limitations
The sensor geometry has been optimized so that internal pitting and general corrosion with a
length exceeding 10 mm and a depth exceeding 1 mm should be detected and sized with an
accuracy of +/- 1 mm up to a maximum depth of 8 mm. Furthermore, the technique can
accurately measure ID reductions such as dents and ovalities

The HFEC technique can only measure internal defects, no measurement is obtained from
external defects. The measurement is insensitive to the pipeline product and to debris.

Applicability
HFEC pigs can be applied under the following conditions :-

Diameter range from 6-inch to 12-inch;
Velocities up to 5 ds;
All sorts of products;
When only internal corrosion is of concern.

Remote Field Eddy Current
The Remote Field Eddy Current (RFEC) dates back to the 1950’s (well bore inspection) but
use of the technique for pipeline inspection has not passed the experimental stage.

The RFEC technique utilizes a relatively large solenoidal exciter coil, internal to and coaxial
with the pipe, which is energized with a low frequency alternating current to generate eddy
currents in the pipe wall. At two to three pipe diameters distance (remote field) one or more
receivers are located detecting those eddy currents which have penetrated the pipe wall twice
(outward at exciter, inward at receiver). Both amplitude of the received signal and phase lag
between remote field and exciter field provides information on wall loss and changes in
material properties (electrical conductivity and magnetic permeability). Because of the double
wall transit the RFEC technique has equal sensitivity to internal and external wall loss.