Operating Manual

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This positioning device is used to stop the crawler at the correct X-ray tube position (within
a few millimetres) with regard to the weld plane in order to make a true panoramic
exposure of the weld.
Usually the crawlers are self-contained and electrically driven. Most of them are powered by
heavy duty batteries or sometimes by a motor-generator for the larger diameters, see figure 12-18.
Such crawlers must be very reliable to limit cut-outs in case of brake down onshore or avoid
costly loss of time on lay barges. Such crawlers are equipped with X-ray tubes with true
panoramic (conical) beam) to create a full circumferential exposure in one shot.
Although the resulting image quality is less than with X-ray sometimes radioactive sources
are allowed. Figure 13-18 shows a battery powered gamma-ray crawler.
Usually crawlers are built according to a “fail safe” design to avoid spontaneous radiation,
not triggered by the operator.
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18.8 Radiography of welds in large diameter pipes
To create an on- or offshore pipeline individual pipes (length usually 12 m) or pipe sections
(double or multiple joints) are welded together by a circumferential weld, a so-called girth weld.
Onshore production rates can be far beyond 100 welds per day dependent on pipe
diameter and terrain conditions. On lay barges, used for production of offshore pipelines,
more than 300 welds per day (24 hours) are no exception.
According to applicable (mandatory) norms and specifications these welds have to be
inspected either with radiography (RT) or automated ultrasonic testing systems (AUT).
In the past the inspection was exclusively done by RT, in more recent years AUT
increasingly replaces RT. One attractive feature of AUT is that, contrary to traditional RT -
using film - the results are instantly available.
Nevertheless for many pipelines RT - using X-rays or sometimes gamma rays - is still in use
to check weld quality. To eliminate development time of the film several attempts in the past
to replace traditional RT by RTR (real time radiography) providing instant results have only
resulted in limited success, mainly due to lack of image quality.
In the meantime some of such systems have entered the market and are in use. Although no
EN standards exist (essential to conquer a market share) some other standards accept digi-
tal real time radiography providing one can prove that the required image quality can be
achieved, see chapter 16.
Attempts continue to develop better RTR systems than already exist.
Development of radiation sensitive sensor technology is still in full progress and has the
potential to ultimately meet the required image quality which at present only can be
achieved by using traditional film.
To cope with the high weld production rates, thus limited time available for inspection,
the RT-process has been fully optimised. On land, with sufficient exposure time available, it
is common practice to develop and judge the films only once or twice the same day.
Repair can be done afterwards.
For offshore work this process is impossible. Each weld has to be judged instantly and if
necessary be immediately repaired before a new one can be made, because a lay barge in
general only moves forward. For a full cycle, exposure and development of the film and its
interpretation, approximately five minutes are available in case of an S-lay situation. For the
more complex J-lay situation - applied in deep waters - in general more time is available, so
inspection time, although still at the critical path, is less critical.
To optimise the inspection process first of all X-ray crawlers with control units have been
developed in combination with very accurate positioning devices as illustrated in
figure 11-18.
Fig. 11 -18. Concept of girth weld inspection
Fig. 12-18. Large diameter X-ray crawler with motor generator.
Receiver
Positioning device
Transmitter
Batteries
Wrapped film
Source
Weld
Weld plane
Pipe wall
Crawler
Beam
boundaries
X-ray unit
Motor
generator