Risk-Based Inspection Technique - Offshore Structures: Design, Construction and Maintenance - page 616

total cost), it is often necessary to put a constraint on the reliability index to ful-
fill code requirements.
8.4.6 Flooded Member Inspection
The two principal means of flooded member detection (FMD) are ROV-
deployed gamma FMD and diver-deployed ultrasonic FMD.
Gamma FMD requires a gamma source and a detector to be fixed to either
end of a yoke that is attached to an ROV. The ROV pilot then positions the
yoke over the member and notes the reading count from the detector. This is
compared with an expected count that is calculated taking account of the
absorption characteristics of water between the yoke and the member, the mem-
ber steel thickness and the air within the member. If the member is flooded, the
count is lower than expected due to the absorption of the gamma radiation by
the water and the member is flagged as flooded.
Ultrasonic FMD employs an ultrasound probe that is fixed to the outside of
the member after suitable cleaning to provide good acoustic contact. ( Figure 8.7
provides a schematic.)
Water is a better transmitter of ultrasound than air is, and a flooded member
is detected by observing echoes that are received back at the probe and viewed
on a screen. For an air-filled member, the echo will be that of the back wall. For
a water-filled member, the ultrasound is transmitted through the water and a
second echo will be generated at the interface between the water and the
back face of the tube. This back-face echo is an indication that the member
is flooded. Figure 8.8 shows a typical UT trace for a flooded member.
Water-filled tubular member
Ulrasonic probe
Electrical
package
Umbilical cable
FIGURE 8.7 Typical arrangement of UT FMD equipment.