Page 365 - Pipeline Pigging Technology
P. 365

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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