Marine Maintenance Technology International - April 2016

NDT (^) ⠿
MARINE MAINTENANCE TECHNOLOGY INTERNATIONAL | APRIL 2016 ⠿ 77
has a high tolerance to variations in pass
speed and lifting from the surface, which
means fast and reliable data is achievable
consistently. The length and depth of
any crack defect found is measured and
recorded instantly. As ACFM detects metal
defects with an electromagnetic field, it can
be easily applied underwater with reliability
and is becoming the technique of choice for
underwater inspection work.
UWILDs are often carefully scrutinized
by supervising surveyors, as the underwater
inspector is the only one with direct
access to the structure being examined.
Traditionally this will be an inspection diver,
who needs to be certified by a regulatory
body, which can be expensive and increases
the risk factors involved. Safety concerns for
divers in the water are now being answered
by the use of ROVs and robotic crawlers.
Remote underwater inspection
In response to this and other environmental
factors, TSC’s engineering solutions team
developed the ACFM MagCrawler. This
unit is designed specifically to enable
ROV deployment of ACFM Array Probe
Technology underwater.
The robotic unit can inspect a wide
variety of subsea geometries including
circumferential, fillet and full penetration
welds in pipes and plates. Ideal for restricted
access areas, the unit is rated for depths
to 150m (493ft) and is a robust, magnetic-
tracked, remotely operated vehicle equipped
with cameras.
Traction is achieved by two rubber
caterpillar tracks that are easily
maneuverable on horizontal and vertical
planes, as well as inverted surfaces. Probes
are mounted onto the crawler with a metal
sprung arm for passive compliance with the
surface; a 360° rotational head allows full
weld coverage. An electronic rotary encoder
is also on board to determine the position
of the crawler and simultaneously collect
distance measurements while scanning.
Gulf of Mexico UWILD
Working with Meridian Ocean Services on
a recent UWILD campaign aboard a semi-
submersible drilling vessel operating in the
Gulf of Mexico, the engineering solutions
team from TSC used the MagCrawler to
complete the specified NDT inspection
requirements. Meridian’s ROV deployed the
MagCrawler to the UWILD inspection sites.
Directed by an ABS surveyor, the work
required inspection for surface cracking in
welds on the three port-side columns where
they join the pontoons 18m (59ft) below the
surface of the sea.
The data collected at all locations allowed
for accurate signal analysis by ACFM
operators. As function checks were carried
out before and after each daily inspection
shift, a comparison between signals
containing a defect and live inspection
signals could be detected.
All of the UWILD survey work was
conducted remotely without the need for
any diver deployment. \
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MAIN IMAGE: A TSC
field service manager
conducts last-minute
checks on the
MagCrawler unit
before deployment
LEFT: A function
check scan performed
before an inspection
contains three
defects (20 x 2mm) in
different orientations
RIGHT: Meridian’s ROV
deployed the TSC
MagCrawler to the
UWILD inspection site
LEFT: Comparative
data from the column
2 aft weld with no
defects detected.
This was consistent
throughout the
campaign
RIGHT: ACFM crack
inspection on the
leg-to-pontoon weld
using the TSC
MagCrawler and
array weld probe