| 摘要 |
A measurement while drilling tool has upper and lower centralizers secured to an upper end. Each centralizer comprises three major components: a housing, a tapered nose, a shaft which is slides in a bore of the housing, and a set of fingers around the shaft. The nose of the lower centralizer is tapered at a greater angle than the nose of the upper centralizer. An upper end of each finger is pivotally attached to the shaft. The fingers slidingly engage the nose when they are not in a retracted position. The centralizers are suspended in a drill pipe so that the shafts are fully extended from their housings. With the shafts extended, the fingers retract. Prior to drilling, the tool is lowered until it lands in a sub which causes the lower shaft to slides downward into the housing. The lower fingers move outward to engage the drill pipe without wedging against it. The upper shaft then slides downward into its housing, causing its fingers to move to an extended position and frictionally wedge between the upper nose and the pipe. The tool may be retrieved by running the line back down to the tool to jar the upper fingers loose from the pipe. Once the upper fingers disengage, the upper shaft slides upward out of its housing to allow the lower shaft to move upward. The lower fingers then fall into the retracted position before the tool is lifted out of the pipe.
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