Holography Non Destructive Testing System
InterSubís Acoustic Holography system enabled a lockout diver to inspect
internal defects in offshore platform welds. Of particular benefit in the nodal
area, where multiple tubular sections converged, it was demonstrated to wide
acclaim by Alain Stankoff of InterSub Development at the 1980 Houston Offshore
InterSubís Acoustic Holography equipment.
acoustic array was electronically programmed with digital techniques to simulate
focused and non-focused source-receiver scanning. The electronic simulated
reference beam was programmable using erasable proms. The imaging device, shown
top right in the above photograph, consisted of a diver hand held gun, which
contained the acoustic array, miniature television camera and L.E.D. display.
The gun was connected, via a diver pack
and cable, to the control unit, digital memory display and data recording units
located in the submersible. The television camera provided an optical view of
the external weld surface, identification marks, etc., which was then integrated
with the acoustical image on a standard television monitor. The acoustical array
provided complete real-time inspection by electronically scanning and
constructing multiple focused holograms through the entire weld volume. The
defect images were then presented in side, plan and pseudo three-dimensional
views with the options of rotation, tilt and zoom magnification.
operation of InterSubís hand-held Acoustic Holography Equipment.
The system had two
permanent recording techniques: the focused holographic defect images and
optical views were stored on videotape and the basic r-f data on digital tape.
At the time the development task was initiated, the only techniques available
for weld inspection were magnetic particle - MPI, and visual. Both inspection
methods depended on the internal cracks voids, etc. protruding to the surface
for examination. From operational experience, InterSub realised that once
defects had been discovered, the next most important parameter was knowledge of
the defect internal geometry which is not feasible with MPI and visual
techniques. On the other hand, cracks or flaws located inside the weld - or
defects coming out on the hidden surface of the bracings - completely escaped
identification by established techniques. Therefore there was a real need for an
underwater instrument, capable of detecting flaws and cracks in the whole volume
of welds. The major difficulty to overcome was the complex geometry of the welds
that had to be inspected. The
convergence of tubular steel sections, where thicknesses and diameter can vary
widely, coupled with the orientation of the bracings themselves, resulted in
complex inspection sites, where accuracy of inspection and location was
paramount. Such requirements served
to eliminate radiographic techniques that were only able to operate in very
simple configurations and when both sides of the weld were exposed, in favour of
a system based on acoustics.
Optical view of the
external weld surface, identification marks, etc, integrated with the acoustical
image on a standard television monitor.