Real-Time Radiographic Imaging for Submerged-Arc Welded Pipe

Abstract

According to American Petroleum Institute (API) Standards 5L and 5LX, the longitudinal weld in submerged-arc-welded (SA-welded) pipe may be nondestructively inspected by either radiological or ultrasonic methods. Each method has particular merits for detection of the weld imperfections normally encountered; the ultrasonic method has high sensitivity to planar imperfections, and the radiographic method has high sensitivity to volumetric imperfections. Although both methods have proven capable of meeting customer requirements, recent specifications for pipelines in the Arctic and offshore require automatic ultrasonic inspection. The trend toward ultrasonic inspection was anticipated; hence, there was sufficient lead time to study the effects that it would have on flaw discrimination and rejection rates and to develop an automatic system that could meet all current and anticipated requirements. This new inspection system combines ultrasonic and radiographic inspection units in a complementary manner, utilizing each to detect the type of weld imperfection to which each is most sensitive and without confirmation by the other. Thereby, the combined system will exceed the individual performance of either unit for the detection of weld imperfections. To satisfy the design objectives for this new inspection system, the radiological portion was to be capable of displaying a consistent 2 percent 2T sensitivity image, equivalent to radiographic specifications. Accordingly, a reasearch effort was initiated to develop an improved fluoroscopic system design. Analysis of the performance limitations for a wide variety of fluoroscopic system designs indicated that designs employing low-light-level television (TV) systems with direct X-ray to light conversion screens showed the best long-range potential for development. The concurrent development of scan-conversion equipment capable of storing and integrating TV images gave considerable impetus to this effort, and a new system design was proposed that would be capable of providing both improved real-time imaging and stored images and could meet the requirements of equivalent radiographic sensitivity. This new fluoroscopic system design was developed through several prototypes. The final version, available through Inspection Technology, Inc., Twinsburg, Ohio, is now installed in two pipe mills for the inspection of SA-welds. These systems are used in conjunction with automatic ultrasonic inspection systems concurrently developed by U.S. Steel. They provide acceptable 2 percent 2T sensitivity, with additional features of automatic inspection data display, image integration on command, 35-mm film image recording, and correlated display of ultrasonic alarms on the TV monitor. This paper describes the development of the fluoroscopic system, analyzes the critical system parameters, describes tests performed, and describes the results obtained.