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Title: The feeler-snake pig: a simple way to detect and size internal corrosion
Category: Technical papers from the Journal of Pipeline Engineering
Downloadable: Yes 
Project No.:
Research Agency:
Catalog No.: 2117s
Date of Publication: December, 2007
Price: $25.00 US
Authors: Claudio Camerini, Jean Pierre von der Weid, Miguel Freitas, and Thiago Salcedo
Abstract: SUBMARINE PIPELINE inspection traditionally employs the same technologies used for onshore pipelines, one of which is the instrumented pig, which usually runs under the same parameters and procedures as those used for onshore inspections. However, it is very common to find submarine pipelines with special characteristics, such as varying diameters and small-radius bends, that may prevent the use of conventional instrumented pigs. Other equally-relevant factors that make the inspection difficult are the increased pipeline wall thickness, which significantly limits the use of magnetic instrumented pigs. An alternative currently available for inspecting these pipelines is the use of ultrasonic pigs. Nevertheless, this method has its own limitations, such as the need of a homogeneous liquid, with good acoustic properties, to serve as couplant. On crude oil pipelines this couplant is not always available, as these lines may carry multi-phase fluids.

Focusing on this issue, a new method has been developed to detect and size the loss of wall thickness associated with internal corrosion. A special pig has been designed that can negotiate large variations in diameter, with no practical limit on the wall thickness to be inspected, and which can navigate through bends with small radiuses. The pig has been called a ‘feeler’ pig, as it consists of several feeler-type sensors that measure internal corrosion. The system has been tested in the field and its performance compared to that of a standard ultrasonic instrumented pig. Excellent defect correlation was observed between the ultrasonic and feeler pig data, both in geometry and depth of internal corrosion. With this high confidence, other prototypes of the feeler pig have been developed. A new design, named the ‘feeler-snake’ pig, was implemented, in which the feeler sensors are mounted over a flexible support, yielding a tool with high tolerance to geometric restrictions. The excellent results from the prototype, and its robustness against in-line geometric restrictions, open a wide range of opportunities for the feeler-snake pig technology in field applications.

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