Page 356 - Pipeline Pigging Technology

Corroston surveys with the 'UltraScan' pig

Flg.3. Ultrasonic module with a 40-in sensor carrier.

Fig.l shows the basic principle: the ultrasonic sensor, which is perpen-
dicular to the wall of the pipe, emits a series of short ultrasonic pulses. These
pulses are reflected by both the internal and external surfaces of the pipe. The
distance of the sensor from the wall, stand-off, A, and the wall thickness, D,
can be determined by the time interval between the transducer exit pulse, the
wall penetration echo, and the rear wall echo. The diagram shows the test
readings of a sensor which has run across the two indicated defects. The line
representing wall thickness clearly shows both defects; the remaining wall
thickness can be read directly off the diagram. It is however, not possible to
differentiate between internal and external corrosion merely on the basis of
the wall-thickness data; for this reason, the distance between the sensor and
wall (stand-off), A, is also indicated. The stand-off value does not change when
the defect is on the exterior; when the defect is internal, it will be shown as
a mirror image on the stand-off trace. Consequently, it is possible to differen-
tiate between an internal and an external defect by combining the wall
thickness and stand-off information. This differentiation is very important to
the pipeline operator, since corrosion prevention measures are quite differ-
ent for the two types of defects.

337