Operating Manual

205
Crawler- and control technologies for on-
and offshore application are almost the
same. The choice of film can be different.
For onshore application traditional film is
used, in general type D4-film or equivalent
with a common developing process.
For lay barges, requiring the shortest pos-
sible cycle time, often high speed film
(RCF- rapid cycle film, see chapter 8) in
combination with special developer and
fixer is applied.
Instead of using standard film lengths it is
much more efficient to use a long strip of
film to be wrapped around the pipe as
illustrated in figure 14-18, e.g Agfa
Rollpac, with a length covering the cir-
cumference including a small overlap.
Strip film with and without lead screens exists in different widths. They are packed in boxes
with lengths up to 100 metres.
For large projects pre-cut films at specified lengths with sealed ends exist.
For onshore application of X-ray crawlers the required radiation safety is easily achieved by
distance and all other related safety measures and warning signs.
On a lay barge, crowded with people, this is not as easy. Here as a first measure radiation
reduction is largely achieved by a lead tunnel as shown in figure 15-18.
Moreover high speed film requiring a lower dose than traditional film, thus requiring a
lower source activity or lower mA-value, or shorter exposure times, contribute to reduce
radiation.
204
Positioning devices using different physical methods are used to stop the crawler, and thus
the source of radiation, at the correct position (at the weld plane). These positioning
devices have to transmit or receive a position signal through the (often thick) pipe wall.
Despite its radiation and related risks, low activity radioactive sources are often used
either in or outside the pipe in combination with collimated detectors to achieve the
required positioning accuracy.
Most attractive, for obvious reasons, are positioning devices not using radioactive isotopes.
With these control/positioning devices also the commands to the crawler are given for
“exposure”, “forward” to the next weld or “reverse”. Prior to arrival of the crawler at
the next weld the positioning device is accurately positioned on the pipe near this
next weld to stop the crawler. To increase its stop accuracy, at first the crawler is slowed
down, at reduced speed the crawler is stopped in order to achieve the highest possible posi-
tion accuracy of the radiation source.
Control
electronics
Battery
pack
Isotope
container
Fig. 13-18 Small diameter gamma-ray crawler
Fig. 14-18. Preparation of strip film on girth weld
Fig.15-18. Lead radiation protection tunnel
Lead
slab
Lead
shield
Girth weld
location
Pipe
Radiation
warning
device