Taper Tap

Key Takeaways

• Taper tap selection criteria require knowledge of the fish's internal diameter: the taper tap must be sized so that the maximum outer diameter of the tapered thread profile matches the fish's nominal ID (or slightly larger, to ensure the threads bite into the pipe wall rather than passing through without engaging); taper taps are manufactured in a range of OD sizes corresponding to common drill pipe, drill collar, and tubing internal diameters, and the appropriate tap size is selected from the fish's pipe weight and grade specifications (which determine the ID from the standard API or manufacturer dimensions); for non-standard or damaged fish, the fish ID may be measured by a gauging trip (running a precisely dimensioned gauge tool to measure the minimum bore restriction in the fish) before the taper tap is selected; the taper tap length must be sufficient to provide adequate thread engagement for load transfer (typically 150 to 300 mm of engaged thread length, providing the bearing area needed for the pulling or jarring load without shear failure of the engaged threads), but must not be so long that it extends past the end of the fish when fully engaged, which would waste thread engagement potential.
• Taper tap engagement and setting procedure involves running the taper tap to the top of the fish on the drill string, slowly rotating the drill string at low RPM (typically 5 to 20 RPM) while simultaneously applying downward weight (WOB of 2 to 5 thousand pounds) to advance the tap into the fish's bore; as the tapered thread profile cuts into the soft pipe steel interior, the resistance increases (visible on the surface rotary torque indicator) and the tap advances deeper into the fish; after the tap has advanced until the thread engagement length is sufficient (confirmed by the depth measurement from the taper tap tip to the fish top), rotation is stopped and the connection is tested by gradually applying upward pull to confirm the tap has gripped the fish securely; right-hand taps (the most common) are screwed in by clockwise rotation and back-unscrewed by counter-clockwise rotation; left-hand taps are occasionally used when right-hand rotation would tend to unscrew the fish's joints (for left-hand threaded drill pipe or in situations where the fish's box-pin thread orientation makes right-hand rotation counterproductive).
• Over-tapping (advancing the taper tap too deep into the fish) is a significant operational risk that can wedge the tap irreversibly in the fish bore, creating a combined fish (taper tap and original fish) that may be more difficult to retrieve than the original fish alone; over-tapping most commonly occurs when the tap advances without adequate torque resistance (indicating that the thread is not engaging the pipe wall as expected) and the operator continues rotating until the tap has advanced past the optimal engagement length; prevention requires monitoring the torque signature during tapping (a sudden torque increase indicates the tap has reached the fish OD and begun biting into the steel, confirming correct engagement), limiting the WOB to the minimum required for engagement, and stopping rotation immediately when the engagement length target is reached; if the taper tap becomes stuck in the fish, the only options are to attempt counter-rotation to back the tap out (which may fail if the thread is fully engaged), to disconnect the drill string above the taper tap (by backing out the safety joint or box-pin connection above the tap) and mill the tap-fish assembly in place, or to work the combined fish with a conventional overshot after adding a milling shoe to clean up the top of the assembly.
• Jar action during taper tap fishing provides the impact force needed to free stuck fish after the tap has established a mechanical connection: once the tap is set in the fish, the drill string above the tap is configured with a downhole jar (a mechanical or hydraulic jar that stores potential energy and releases it as a sharp impact when tripped) and intensifier (a tool that increases the jar impact force by adding the stored compression of the drill string above the jar to the jar impact); upward jarring pulls on the fish in the direction needed to free it from its stuck point (scale buildup, collapsed formation, stuck packer, or mechanical binding); downward jarring is used when the fish must be pushed down to unset a packer or release a stuck mechanical component before the pull-out direction is effective; the jar trip weight, cock weight, and strike intensity are calculated from the string weight, jar specification, and the estimated force required to free the fish (which depends on the sticking mechanism and is estimated from the overpull observed when the drill string was pulled before the fish parted).
• Taper tap alternatives and complementary fishing tools include the spear (a slip-type internal grapple that engages the fish ID with slip elements rather than threads, used when the taper tap engagement is insufficient due to a damaged or corroded fish bore), the die collar (an external tool with hardened threaded dies that engage the fish OD, used for non-hollow fish or when the clearance between the fish OD and the wellbore is sufficient), and the overshot (a bell-shaped external grapple with spring-loaded slip segments that engage the fish OD when the overshot is pushed over the fish top, used for the majority of fishing operations where the fish OD is accessible and the fish is not too close to the wellbore wall); the choice between taper tap (internal engagement) and overshot (external engagement) depends primarily on the clearance available outside the fish -- if the fish OD is within 10 to 15 mm of the wellbore ID or casing ID, an overshot cannot be run over the fish without jamming, requiring an internal engagement tool such as the taper tap; in complex fishing scenarios (heavily corroded fish, parted fish with no clean top for engagement, or fish in a deviated wellbore where the fish is lying against the low side), a specialized fishing engineer from a downhole fishing service company (SLB, Halliburton, Weatherford, National Oilwell Varco) is consulted to design the optimal fishing program.