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Once this has Figure 8-4. Short-catch overshot been indicated by an increase in pump (Courtesy of National Oilwell). pressure, the pump should be stopped to avoid kicking the overshot off the fish. If jarring is to be done, it should be started with a light blow and gradually increased, as this tends to "set" the grapple on the fish. A hard impact may strip the grapple off the fish and cause the wickers to be dulled if not properly engaged. To release bowl-and-grapple overshots, the tapered surfaces of the bowl and grapple must first be freed from each other.

If the jarring method is used, extra pipe adds elasticity to the fish. BACKOFF Backing off is the procedure of applying left-hand torque to a pipe string while firing a shot of prima cord (Figure 7-1) explosive to unscrew the pipe at a selected threaded joint above the stuck point. The explosion produces a concussion that partially unscrews the threads. The backoff method of parting pipe is probably the most popular, particularly in drill pipe and drill collars, as this is the only method that leaves a threaded connection at the top of the remaining pipe.

Pressure in the casing/tubing annulus tends to straighten the tubing and put more weight on the packer. Casing pressure will also increase the pack-off force in the packing element of a partially packed-off set-down-type packer. 7 Parting the Pipe String After determining the stuck point in a pipe string, it is normal procedure to part the string so fishing tools such as a jarring string or washpipe string may be run. There are four acceptable methods of parting the pipe string. 9 9 Backoff Chemical cut 9 Jet cut 9 Mechanical cut The cutting method for each particular job should be carefully selected.