(eBook version)
Need: A system
for mechanized ultrasonic inspection of girth welds during
pipeline construction, the RTD Rotoscan, has already been
available for twenty years. Its development started back
in 1977. First commercial use started around 1981. 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. Although the
early version of the Rotoscan system, as already stated,
was already commercially in use and improvements were considered,
it was thought that a thorough and structured evaluation
of the potentials of mechanized UT systems for pipeline
girth welds was highly desirable. A major obstacle in the
introduction of this kind of systems was found to be the
fact that in industry only a limited generic insight was
present on the performance of mechanized UT systems in terms
of Probability Of Detection (POD) and sizing capabilities
on one hand, and the likelihood of ?false calls?, causing
unnecessary and costly repairs on the other hand. It had
been experienced that, rather than generic acceptance, potential
customers therefore wish to evaluate the system for their
own specific applications and on their own representative
test welds for the purpose of system qualification.
Benefit: The
work performed in this project can be regarded as a continuation
of an earlier PRCI project: ?Evaluation of Ultrasonic Inspection
Techniques for the Root Region of Girth Welds? (project
PR-220-9123), which was completed by the end of 1995. In
that project, the performance of modern systems for mechanized
ultrasonic inspection of girth welds in pipelines was evaluated
with the emphasis on the weld root, which can be considered
historically as the most difficult part of the weld when
it comes to conventional ultrasonic inspection. In the current
project, this was extended to the entire weld. Already in
report PR-220-9123, it was stated that a structured evaluation
would also help to arrive at a generic approval of mechanized
UT systems for use in pipeline industry, to reduce or replace
job-specific qualification tests. This point is still valid.
The results of the first project, and hopefully also of
this one, will be an important basis for international efforts
currently being undertaken to develop such approvals, as
well as to arrive at adapted acceptance criteria for weld
defects. The current project was, similarly to the previous
one, built around a number and a variety of representative
test welds, using different welding techniques. These were
inspected with Rotoscan, also using improvements (such as
simultaneous TOFD), partly initiated on the basis of the
work done in the previous project, and with radiography.
They were then destructively tested for correlation with
the NDE results.
Result: The results
of this project confirm that mechanized ultrasonic inspection
can be considered as a mature alternative for radiography.
In terms of inspection quality, mechanized ultrasonic inspection
can even be superior over radiography, if applied with optimum
inspection parameters. Moreover, ultrasonic methods can,
contrary to radiography, be applied with Engineering Critical
Assessment (ECA) acceptance criteria.
