Page 364 - Pipelines and Risers

Pipeline Inspection, Maintenance and Repair 335

Ultrasonics
Principle
Ultrasonic pigs utilize ultrasonic transducers that have a stand-off distance to the pipe wall. A
fluid coupling is required between the transducer and pipe wall. The transducers emit sound
pulses which are reflected at both the inner and outer surface of the pipe wall. The time
elapsed detection of these two echoes gives a direct measure of the remaining wall thickness
of the pipe. The time elapsed between pulse emittance and the first echo is used to determine
the stand-off distance. Any increase in stand-off distance in combination with a decrease in
wall thickness indicates internal metal loss. A decrease in wall thickness, while the stand-off
distance keeps constant, indicates external metal loss, laminations or inclusions. The outer
wall echo cannot be distinguished from the inner wall echo for too thin (remaining) wall
thickness.

Sensors
Ultrasonic pigs utilize piezoelectric ultrasonic transducers that emit 5 MIIZ sound pulses. Thej
transducers are placed in a stand-off distance to the pipe wall. Normally the transducer and
stand-off are chosen such that the ultrasonic beam at the pipe wall has a spread of below 10
mm. The circumferential sensor spacing of the state-of-the-art ultrasonic pigs is a little under
10 mm. Consequently the smallest detectable pits have a diameter of about 10 mm. A number
of measurements, about 4 or 5, must be made in the axial direction for a pit to be recognized.
The sampling frequency depends on the firing frequency of the ultrasonic transducers and the
speed of the pig. Under optimal circumstances, the axial sampling distance is about 3 mm.

For accurate metal loss monitoring in heavy wall pipelines the ultrasonic technique is better
suited than the Mm. technique. In gas or multiphase lines this can be achieved by running the
ultrasonic tool in a batch of liquid such as glycol. In view of the maximum allowable speed of
an ultrasonic tool the velocity excursions of the gas driven pig-slug train needs to be properly
controlled. The dynamics of a pig-slug train in a gas pipeline has been extensively studied to
determine the optimum parameter settings in order to avoid the pig-slug train from stopping
during the survey and subsequently shooting off at high velocities. The maximum allowable
speed of the ultrasonic tool is determined by the firing frequency of the ultrasonic sensors and
was in the past limited to about 1 ds. However, due to the improved electronics the firing
frequency has been increased which now allows a maximum velocity of around 3 ds.

Data Analysis
Interpretation of ultrasonic signals is more straight forward than MFL signals. The stand-off
and wall thickness signals give a direct mapping of the pipe wall, showing all corrosion
defects. A rough surface and internal debris may lead to loss of signal and can be recognized
as such. In addition laminations, inclusions, girth welds, valves and tees can be easily
recognized. Nowadays defect detection and sizing is fully automated however, the data is still
often checked manually.

Capabilities and Limitations
Ultrasonic pigs have the advantage that they provide a better quantification of the defect sizes
than MFL pigs. Detection of defects starts at lengths of 10 mm. The probability of detection