Detection Of External Pipe Defects With A Modified Borehole Televiewer

The conventional ultrasonic televiewer is capable of precise and detailed mapping of the topography of the borehole wall or of the inner surface of casing and production tubing. ARCO has recently designed, built and tested a new televiewer that is also capable of determining pipe wall thickness with the same detail. The major functional modifications in ARCO'S tool relative to the previous televiewer are(1) that the ultrasonic transducer is exposed directly to the borehole fluid without an intervening window; and (2) that the ultrasonic signal is digitized downhole for later transmission up the wireline. A micro-processor in the tool controls the measurement process and can perform significant portions of the signal processing downhole, Although the system was designed to do real-time calculations, the data are currently recorded at the surface and are subsequently processed to obtain thicknesses. A lateral resolution of 1/4" and a wall thickness accuracy of 0.01" are possible. The minimum resolvable wall thickness depends on the ultrasonic bandwidth and is about O. 1" for the transducers used to date. The prototype tool has been tested in pipe with well-characterized external defects, and some examples of test results are shown. The thickness data are displayed as pseudo-color images or as cross-sections or polar plots. The areal extent of defects can be mapped out with precision, but quantitative thickness data can be obtained only where the inner and outer ipe surfaces and the ultrasonic wavefronts are approximately parallel. Fortunately this normally includes wall thickness minima. Although the initial goal was to detect exterior corrosion on pipe, other applications such as cement bond logging are possible.