Development of an Ultrasonic Inspection Pig for In-Service Wall Thickness Measurements of Gas Pipelines
Authors: Haugen Roald , Dalberg P. , Hlbgmoen K. 
Published: October 2008

I. ABSTRACT
In 1979, Elf Aquitaine Norge A/S and Det norske Veritas started a project for development of an ultrasonic inspection pig for offshore gas pipelines.
The project aimed at developing ultrasonic methods for wall thickness measurements suitable for a pig type inspection unit. The methods should be able to detect external as well as internal corrosion attacks, and give information for verification of pipeline integrity and analysis of possible corrosion attacks. The methods should further be capable of measuring during operational pressure in the pipeline, and not interfere with the normal gas production.
The project consists of three main phases. In the first phase the measurement principles were developed. Based on these prinicples, the pigging system and data handling system were outlined.
The second phase aimed at solving the technical problems associated with the operation of the measurement system in a pipeline. An on-stream pig was designed and built for evaluation of the system in an operational pipeline. The pig's capabilities and the accuracies of the measurements were tested in a 40 bar test section with artifical defects.
In the third project phase the complete data handling and presentation system will be built and the on-stream pig will be developed into a pig unit for complete coverage of the pipeline.
At the present stage, the phases one and two have been completed, and the presentation will describe the developed measurement system as well as test results and experiences.

I. INTRODUCTION
The present generation of offshore riser and pipeline inspection pigs are based on magnetic flux leakage, mechanical gouging or photographic techniques. Although suitable for detection of internal corrosion these techniques have limited abilities to measure the extent of the corrosion. For external corrosion, only the magnetic flux leakage technique will give some information, but the ability to measure the extent or corrosion is limited. Realizing the ultrasonic measurement technique's ability to measure pipe wall thickness and thus detect corrosion on either side of the pipeline, Det norske Veritas started to work on various pipeline and riser inspection concepts in 1975. As the ultrasonic immersion technique is well suited for automatic measurements, tools based on this technique were developed with an internal riser inspection tool named VIRIT as the first attempt. This tool used standard ultrasonic immersion sensors mounted on a hydraulic crawler unit for internal operation in oil or water filled risers. This concept proved to be impractical in use as it required production stop and depressurization of the pipeline, and therefore never came in offshore use.
A further development of the immersion technique followed with the externally operated CORRO-SCAN equipment. This equipment also uses conventional ultrasonic sensors, and has been successfully used for several years. This technique has, however, also operational limitations, as it requires the use of divers and removal of pipeline coatings. In order to develop more efficient inspection equipment, a pig type concept has been worked out parallel to the above mentioned equipments.