Application of Acoustical Holography to the Inspection of Offshore Platforms

Abstract

ABSTRACT This paper describes a diver operated underwater inspection of internal defects in offshore platform weldments. The two-dimensional acoustic array is electronically programmed with digital techniques to simulate focused and non-focused source-receiver scanning. The electronic simulated reference beam is programmable using erasable proms. The imaging device consists of a diver hand held gun containing the acoustic array, miniature television camera and the L.E.D. display array. The gun is connected via the diver pack and cable to the control unit, digital memory display and data recording units located in the submersible. The television camera provides an optical view of the external weld surface, identification marks, etc., which is integrated with the acoustical image on a standard television monitor. The acoustical array provides complete real-time inspection by electronically scanning and constructing multiple focused holograms through the entire weld volume. The defect images are presented in side, plan and pseudo three-dimensional views with the options of rotation, tilt and zoom magnification. The system has two permanent recording techniques: the focused holographic defect images and optical views are stored on videotape and the basic r-f data on digital tape. This paper presents the results obtained during the laboratory trials, thus illustrating the unique capabilities of the system in flaw detection, location and sizing. This system is now undergoing tests in the, North Sea by International Submarine Services. INTRODUCTION The present paper describes a new equipment developed for inspection of underwater weldments, namely those critical welds connecting the .structural elements of offshore platforms. At the time when the task was initiated, the only techniques available for weld inspection were magnetic particle and the visual. Both inspection methods depend on the internal cracks voids, etc. protruding to the surface for examination. From our previous experience, after such defects have been discovered, the next most important parameter is knowledge of the defect internal geometry which is not feasible with the other techniques. On the other hand, cracks or flaws strictly located inside the weld or defects coming out on the hidden surface of the bracings escape completely to such inspection techniques. So, there was a real need for an instrument, operational underwater and able to detect flaws and cracks in the whole volume of the weldings. The major difficulty to overcome was the complex geometry of the weldments to inspect. They are joining cylindrical bracings, whose thicknesses and diameters can vary within wide limits. Moreover, on a given node of an offshore platform, the relative orientations of the bracings can vary and the shape of welds is undefined. These remarks concurred to eliminate radiographic techniques, only able to operate in very simple configurations and when both sides of the weld are exposed, and to choose a system based on acoustics. Another consequence of the adaptability required to deal with all the geometries likely to be met, was to reject automated systems, and to devote an important role to human intervention.