Page 366 - Pipeline Pigging Technology

High-accuracy calliper surveys

better than 100m. There are basically two ways to determine the curvature of
a pipeline by a SIMU: via cross-track acceleration (centrifugal force), or via
cross-track angular velocity.
Low-accuracy SIMUs are not accurate enough to use their acceleration to
determine the curvature. However, the acceleration is necessary to orientate
the curve with respect to the vertical (see Knickmeyer etal, 1988). Power
requirement was an important consideration due to the duration of pipeline
runs of a week or more.

Weld detection

Circumferential weld-detection sensors are mounted in one of the pig
rubbers, and sense the change in material at the weld. The output of each of
the three or four sensors is an electrical pulse. At any time, one of the sensors
may pick up the long seam weld or other changes in the metal, but only at the
girth weld will they all fire simultaneously. The resulting girth-weld indication
is used to correlate the pig data to as-built plans. The welds also build a log of
pipe joints for future comparisons. In epoch-to-epoch measurements, the
historical information on weld separation provides an indication of the axial
forces acting on the pipeline.

Odometers

Velocity information computed from odometer wheels bounds the errors
which occur in the time integration of the inertial data. At the same time, these
sensors provide a system chainage for the pig through its travel down the
pipeline. The hinged wheels maintain contact with the pipe wall by spring
tension; the pivot allows the wheels an additional degree of freedom to
maintain a tangential orientation when the pig is negotiating bends.

Data processing

Calliper processing

The sonar ring measures distances from the pig carrier to the pipe wall,
thus capturing a cross-section of the pipe. These ranges are processed using
adjustment techniques to compute the centre of the pipe with respect to the
pig, and the pig-to-pipe attitude, using circular and ellipsoidal models.

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