Innovation

 InterSub’s Acoustic Holography Non Destructive Testing System

InterSub’s 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 Technology Conference.

 InterSub’s Acoustic Holography equipment.

 The two-dimensional 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.

Diver operation of InterSub’s 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.  

 Diagram of typical nodes found in offshore steel jackets.

 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.