Development of ultrasonic tomographic instrumentation system monitoring flaws on pipeline

Abstract

Online inspection and in situ monitoring for natural gas transmission and distribution pipeline is significant to prevent any fatal incident and to maintain pipe integrity. Most inspection technique conducted in industry is for the localized flaws detection. It is important to have an inspection system that is capable of producing an overall image of pipe profile (circumferentially) which can show the actual location of the flaws. Therefore, an ultrasonic tomographic instrumentation system was designed and fabricated for this study to monitor the existence of flaws circumferentially on pipeline. It consists of an ultrasonic sensing system, a data acquisition system and an image reconstruction system (software development). There were twenty-eight ultrasonic sensors arranged on a sensing ring that surrounded a carbon steel pipe NPS 8 (219 mm external diameter). The sensors were contactless to the pipe at a distance of 50 mm. Several testings were conducted to identify the ultrasonic beam pattern, the uniformity of ultrasonic beam intensity and the image grid covered by each sensor. A calibration test on distance measurement was conducted based on 40 mm to 60 mm range of distance, and the result showed ± 0.3 mm maximum error with the average accuracy of 99.82%. Three experiments on flaws detection around the pipe were carried out, which were the external flaws, internal flaws and a combination of external and internal flaws. The depth of flaws was ranging from 0.4 mm to 3.3 mm. From the output data, a tomogram image of the circumference pipe profile with flaws existence was reconstructed using linear back projection algorithm and direct method. The computer programming on image reconstruction was performed using MATLAB software. In brief, the developed ultrasonic tomography system was capable in detecting the flaws on pipeline, and visualizing the actual location of the flaws.