(e-book version)
Need: A system
for mechanized ultrasonic inspection of girth welds during
pipeline construction, the RTD Rotoscan, has already been
available for almost twenty years. In fact, its development
started back in the seventies, after initial trials going
as far back as the fifties. First commercial use started
in the beginning of the eighties. Today, RTD Rotoscans are
being used all over the world whereby, for a number of pipeline
projects, it replaces radiography as a sole girth weld inspection
technique. It has been known for many years, that the main
difficulty in mechanized ultrasonic inspection of pipeline
girth welds is in a reliable inspection of the root region.
On one hand, the possibility exists that serious planar
defects are over-looked (missed defects) because they are
interpreted as geometry signals, on the other hand geometrical
echoes (caused by e.g. hi-lo) can easily give rise to interpretation
as defect signals false calls). This difficulty is mainly
caused by the fact that defect signals and geometry echoes
are received with only very small differences in transit
distance, and thus are difficult to separate by means of
a gating system.
Benefit: This
project was initiated to evaluate the performance of the
existing system(s) / method(s) and the improvements contemplated.
It was decided to build the project around a number and
variety of representative test welds. These were inspected
with Rotoscan 1990, also using improvements and radiography,
and then destructively tested for correlation with the NDE
results. This document reports improvements to the RTD mechanized
ultrasonic inspection system for pipeline girth welds. Describes
how the addition to the ROTOSCAN 1990 system of ROTOMAP
AND ROTOTOFD have led to improvements in the accuracy of
the inspection of the girth welds in the root region.
Result: The work
done within this project shows, that by proper implementation
of improvements such as Rotomap and RotoTOFD (here still
used in early prototype versions) the initial drawbacks
of mechanized ultrasonic inspection in the root can be overcome.
Together with the technology and experience that form the
basis for the existing Rotoscan 1990, mechanized ultrasonic
inspection of pipeline girth welds can now be considered
as a mature technique, capable of replacing radiography
in many cases.
