The Free Pipe Indicator (FPI) instrument is part of a system used for recovering free drill pipe/casing/tubing that is stuck in a borehole. The primary function of the instrument is to find the freepoint, i.e., the downhole location where the string is no longer stuck and therefore free to move.In accomplishing this, the instrument operates inside the string to detect pipe wall distortion as the string is torqued, stretched or compressed at the surface. An electronic cartridge in the instrument signals the presence and relative amount of distortion to a surface panel for observation and interpretation. A direct-reading digital counter is utilized as a surface indicator of the percent of free pipe.
After the freepoint has been found, the FPI system can be used to separate the free portion of the string from the stuck portion. One procedure utilizes the string shot backoff tool to detonate an explosive charge at the collar above the freepoint while uncoupling torque is applied to the pipe string at the surface. The explosion is intended to loosen the connection of the free and stuck portions of the string so they separate at the collar. If this procedure proves to be unsuccessful or impractical, jet or chemical cutters can be lowered into the pipe string to sever the pipe.