Page 75 - Reliability and Maintainability of In service Pipelines

64 Reliability and Maintainability of In-Service Pipelines

can be used to determine quantitative data and presence of defects in pipes. More
details of the magnetic method are explained in Section 2.3.4.
The ultrasonic method is able to obtain quantitative data relating to the depth
of defects with precision to the millimeter. This process is based on propagation
delay time of an acoustic wave emitted by ultrasonic transducers that travel in a
liquid medium and reflected by internal pipe walls to detect and evaluate corro-
sion within pipes. The types of defects that can be identified depends on the char-
acteristics of the proposed ultrasonic transducer that is used, the precision of time
measurement of propagation of acoustic wave, and synchronization of transdu-
cer’s trigger with measurement of the pig’s movement.
The pigs normally have a mechanical part which consists of a cylindrical cap-
sule supported by rubber discs. The capsule rotates freely around its longitudinal
axis to keep ultrasonic transducers directed to the lower half of the pipe. The
function of the rubber discs is to keep the capsule centered, block fluid, and pro-
pel the pig forward in the inspection process. An odometer wheel is also held out-
side the capsule to keep track of the total distance covered by the pig whilst
inside the pipe by generating pulses every 10 cm traveled. The pig uses a number
of ultrasonic transducers in a mechanical support in the capsule’s plug. The trans-
ducers’ function is to measure echo time propagation. Transducers are kept away
from internal pipeline wall by implementation of mechanical support and rubber
discs. A microcomputer is connected to the pig for the calibration process before
inspection begins. The beginning of inspection is detected by generation of pulses
from the odometer wheel. Once pulses are detected again the inspection will
resume. This process is designed to optimize energy consumption while the pig
travels extremely long distances of pipeline infrastructure in order to save battery
power for more important uses. The pig can acquire 250 measurements per sec-
ond per transducer, however, the electronics are developed to cope with a rate of
500 measurements per second per transducer. If there are 16 transducers traveling
with the pig, then 4000 measurements per second are able to be collected as data
acquisition. The echoes detected by the pig classify defects at certain depths and
the classification is based on the percentage of corrosion within internal walls.
While data is being collected by the pig, the software uses a data compression
technique to save storage space as space is limited on the disc. The basic principle
of this mechanism is that there is no purpose in storing data and information for
pipelines when no defects are present. Improved axial and lateral resolutions can
be achieved by increasing the frequency among transducers since it produces
shorter wavelengths.
The flow of product can continue whilst routine checks and cleans are occur-
ring, allowing for minimal disturbance to be caused. Fig. 2.7 depicts the intelli-
gent pigging device.