Page 365 - Pipeline Pigging Technology
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Pipeline Pigging Technology
Two rings of sonar sensors in liquid, or ultrasonic sensors in gas, scan the
wall of the pipe and determine the pig-to-pipe translation and attitude. The
use of sonar or ultrasonic technology increases the reliability and accuracy
without the dependency on mechanical detectors and without contact on the
wall.
Configuration of the sensors
The sonar or ultrasonic devices that range to the pipe wall are designed to
stand-off 10 to 15cm (4 to 6in). Shorter distances cause an inability to read the
transit time with sufficient accuracy for precise distance measurements. If the
distances are much longer, it is difficult to obtain sufficient signal strength to
correctly pick the return time.
In the NFS 30-in tool, the sensors were spaced to cover 3cm (1,25in) of the
pipe wall circumference. At a run speed of Im/s (3.3fps) and a recorded
sample rate of 32Hz, the length of the sample window is 3cm (1.25in). The
footprint of a sonar on the wall has a diameter of about 1cm (0.4in).
Timing
To minimize interference from one sensor with the others and the effect
of reflecting signals, the sensors are pulsed opposite the last one plus one.
Originally, the Geopig sonar sensors were sampled at 32Hz using a
medium-strength power level. The return signal was recorded regardless of
the amplitude. In the early runs, single sensor spikes occurred on one or two
scan lines. These were most probably reflections off particles in the oil or
refraction off the side of a small dent.
On subsequent runs the sampling rate was increased to 64Hz. The first
pulse was low power; if the return signal amplitude was too low, a more
powerful second pulse was sent out. If the amplitude of the second return
signal was stronger, it was recorded as the pipe wall return.
This sequence of power pulsing significantly reduced the false returns on
the later runs.
Strapdown inertia! navigation system
The selection of a particular SIMU was based primarily on size, accuracy,
power requirements, and cost. The size requirement was dictated by the
smallest pipeline diameter and the ability to negotiate bends in the line. The
accuracy requirement was to provide radius of curvature measurement to
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