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Feeler Snake Pig: A Simple Way to Detect and Size Internal Corrosion

Submarine pipelines inspection traditionally employs the same technologies used for onshore pipelines. One of such technologies is the instrumented pig, which usually runs under the same parameters and procedures as used for onshore inspections. However, it’s very common to find submarine pipelines with special characteristics, like 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 limits significantly the use of magnetic instrumented pigs. An alternative currently available to inspect these pipelines is the use of ultrasonic pigs. Nevertheless, this method has it’s 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 carries multi-phase f uids. Focusing in this context, a new method was developed to detect and size up loss of wall thickness associated to internal corrosion. A special pig was designed to bear large variation in diameter, have no practical limit of thickness to be inspected, and be able to navigate through curves and geometric accessories with small bend radius. The pig was called Feeler Pig, as it consists of several feeler-type sensors that measure internal corrosion. The system was tested in field and had its performance compared to a standard ultrasonic instrumented pig. Excellent defect correlation was observed between the Ultrasonic and Feeler Pig data, both in geometry and deepness of internal corrosion. With this high confidence, other prototypes of Feeler Pigs were developed. A new design, named Feeler Snake Pig, was implemented, mounting the feeler-type sensors over a flexible support yielding a tool with ultra high tolerance to geometric restrictions. The excellent results of the prototype and its robustness against line geometric restrictions immediately open a wide range of opportunities for the Feeler Snake Pig technology in field applications.

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