Page 379 - Pipeline Pigging Technology

Pipeline Pigging Technology

that the front cups will cross the feature 1.89m before the sonars detect it, and
the rear cups will be deflected 0.42m before the feature. By using the gyro and
diameter detail display, and selecting a zoom window at least 2m before the
dent, the gyro movement will verify that there is a true feature.
To check the size of the feature, the pig movement display is used. From
geometrical considerations, the size of the dent will be three times the total
movement of the front cup. This movement will be, for the NFS 30 pig, 1.89m
before the callipers measure the dent. The total movement can be calculated
by taking the square root of the sum of the squares of horizontal and vertical
movement. Dents that are short in the long-track direction (less than 0.15m
in length) will show movement as the dent falls between the pig cups. As the
rear cups pass over the dent, the pig will move in the opposite direction to
its initial deflection. The size of the reverse move is somewhat smaller than
the first movement.
The round or slice displays are useful in visualizing the calculation that is
used in determining the effective dent height. Pigco adapted the techniques
used to measure dents in the field to the measurements from the Geopig
calliper sonar. The technique used in the field was to measure the minimum
diameter with a pipeline calliper at the deepest part of the dent. The ovality
was measured by taking the calliper reading at right angles to the minimum
diameter and deducting the nominal diameter. The effective dent height was
then determined by taking the minimum diameter from the nominal diam-
eter, less half the ovality.
This calculation is done automatically in the Geodent program, and shows
on the left box of the display as Ovality, 1 through 5. The values shown as
Ovality are the effective dent height for the five largest dent readings within
the zoom box. The calculation is as follows:

maximum deviation inward from the nominal pipe radius and the
sensor number are determined for any particular scan line;
the deviation at 180° to the maximum deviation or, in the case of the
NFS 30 tool with 72 sonars, at the sensor number plus 35, is added;
the deviations at 90° and 270° or sensor number plus 17 and 53 are
averaged and subtracted;
the resulting effective dent height (called ovality in the display) is
shown and plotted.

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