Page 608 - Instrumentation Reference Book 3E

590 Non-destructive testing
equipment. The microphone picks up the voice flood- or flashlights. The photography is normally
within the helmet. Further clarity is lost by the on 35 or 70mm format using color film with
effect of air pressure altering the characteristic stereoscopic recording where later analysis and
resonance of the voice, effectively increasing the templating will assist repair techniques.
frequency band so that, even with frequency shift- Camera types are normally waterproofed ver-
ers, intelligible information is lost. Hence any sions of land-based equipment. When specifically
communication with the diver is monosyllabic designed for underwater use, the infrequent load-
and repetitive. ing of film and power packs is desirable with
For initial surveys and in areas of high ambient built-in flash and single-hand operation or
danger the mounting of sensor arrangements on remote firing from a submersible on preset or
remote-controlled vehicles (RCV) is on the automatic control.
increase. CCTV provides immediate and recordable
Additional requirements for robust techniques data to the surface. It is of comparatively poor
and equipments are imposed by 24-hour operation resolution and, in black and white, lacks the extra
and continual changes in shift personnel. Equip- picture contrast given by color still photography.
ment tends to become common property and not
operated by one individual who can undertake 24.6.4 Magnetic particle inspection (MPI)
maintenance and calibration.
Surface preparation prior to examination is MPI is the most widely accepted technique for
undertaken to remove debris, marine growth, cor- underwater non-destructive testing. As a robust
rosion products, and, where required, the existing system with wide operator latitude and immedi-
surface protection. The preparation is commonly ate confirmation of a successful application by
15cm on either side of a weld and down to bare both diver and surface, the technique is well sui-
metal. This may be carried out by hand scraping, ted to cope in the hostile environment.
water jetting, pneumatic/hydraulic needle descaler Where large equipment cannot gain access,
guns, or hydraulic wire brushing. Individual com- magnetization is by permanent magnets with a
pany requirements vary in their estimation of the standard pull-off strength, although use will be
effects of surface-cleaning methods which may limited away from flat material, and repeated
peen over surface-breaking cracks. application at 6-inch intervals is required.
When working near to the air-sea boundary, the
magnetization is derived from flexible coils driven
24.6.2 Visual examination
from surface transformers and the leakage flux
The initial and most important examination is from cracks disclosed by fluorescent ink supplied
visual in that it provides an overall and general from the surface. A.c. is used to facilitate surface
appreciation of the condition of the structure, the crack detection. The flexible cables carrying the
accumulation of marine growth and scouring. magnetization current are wrapped around a mem-
Whilst only the most obvious cracks will be ber or laid in a parallel conductor arrangement
detected, areas requiring supplementary non- along the weld. At lower levels the primary energy
destructive testing will be highlighted. The exam- is taken to a subsea transformer to minimize power
ination can be assisted by closed-circuit television losses. The transformer houses an ink reservoir
(CCTV) and close-up photography. CCTV nor- which dilutes the concentrate 1O:l with seawater.
mally uses low-light level silicon diode cameras, Ink illumination is from hand-held ultraviolet
capable of focusing from 4 inches to infinity. lamps which also support the ink dispenser (Figure
In order to assist visual examination, tem- 24.38). At depth the low ambient light suits fluor-
plates, mimics, and pit gauges are used to relay escent inspection whilst in shallow conditions
information on the size of defects to the surface. inspection during the night is preferred. Photo-
Where extensive damage has occurred, a template graphic recording of indications is normal along
of the area may be constructed above water for with written notes and any CCTV recording avail-
evaluation by the inspection engineer who can able.
then design a repair technique.
24.6.5 Ultrasonics
24.6.3 Photography
Ultrasonic non-destructive testing is mainly con-
Light is attenuated by scatter and selective absorp- cerned with simple thickness checking to detect
tion in seawater and the debris held in suspension erosion and corrosion. Purpose-built probes
so as to shift the color balance of white light detect the back-wall ultrasonic echo and will be
towards green. Correction filters are not normally hand-held with rechargeable power supplies pro-
used as they increase the attenuation. Correct bal- viding full portability. The units may be self-acti-
ance and illumination levels are achieved with vating, switching on only when held on the test