In this paper we describe the development of a pipeline inspection gig (PIG) using an ultrasonic bubbler technique for monitoring the wall thickness, and hence corrosion, of a buried pipeline carrying the coolant water from the Indian nuclear reactor CIRUS. The main coolant loop of the reactor is made of carbon steel (ASTM Grade-A-53, Grade A) material with diameters of piping sections ranging from 12.5 mm (0.5") to 500 mm (20"). Seventy percent of the total length of this piping is 500 mm (20") in diameter and is embedded about 4m (156") below the ground.

The situation presents two interesting problems right at the outset. The first is that there is no easy access to the pipeline, in fact it is from the inside of a pit, and the second that the monitoring has to be carried out from the inside of the pipe, which is the more corroded surface. The first of these was overcome by developing a mobile vehicle to carry the ultrasonic probes so that it could be taken through a small pit which provides the access to the pipeline while the second was overcome by developing an ultrasonic water-bubbler technique. The transporter is a four wheeled vehicle driven by pneumatic motors. It houses the probe positioning head, which can be attached or retracted from the pipe surface. Sensors are mounted to know the position and orientation of the probe in the pipeline. The ultrasonic bubbler technique had to be used as the inside surface of the pipe has undergone nodular corrosion. This, besides impeding, would not allow the transducer to be moved smoothly and hence an attach-retract kind of operation was essential. In order to facilitate this, a probe holder was designed which would allow water to be bubbled for coupling. A unique method was also adopted to keep the water column near the probe's surface steady by using a membrane transparent to ultrasonic waves. Reference standards have been made from a section of the pipe to be inspected and the ultrasonic technique was capable of defecting a wall thickness variation of 10%. A number of trials have been successfully taken on a representative section of the pipeline. In this paper we present the results of these trials, the problems encountered and the modifications envisioned. Future development involves improving the mobile vehicle to negotiate steps encountered at flanges joining sections of the pipe and turn at bends.