Overshot

Key Takeaways

• The grapple engagement mechanism of an overshot converts tensile load applied at the surface into a radial gripping force on the fish through a wedging or camming action: in a spiral grapple overshot (the most common design), the grapple is a hardened steel element with a helical or spiral outer surface that contacts the tapered inner bore of the overshot body when the grapple is engaged against the fish; as upward tension is applied at the surface, the grapple is pulled upward relative to the overshot body, the spiral outer surface wedges against the tapered bore, and the radial pressure of the grapple against the fish OD increases proportionally with the applied tension; the harder the pull, the tighter the grip — a positive feedback that makes the spiral grapple self-tightening and capable of holding loads up to the tensile rating of the overshot body or the fishing string, whichever is lower; the catch is released by setting down weight on the fish (reducing the upward tension to zero and allowing the grapple to relax) and rotating the drill string in the releasing direction (typically right-hand rotation for a left-hand release overshot) to back the grapple off the fish and free the tool; the releasing mechanism is critical for safe fishing because if the overshot cannot be released after catching a fish that proves impossible to recover, the overshot itself becomes part of the fish and the situation is worsened; releasing overshots (designed to always allow controlled release after catch) are preferred over non-releasing overshots for this reason.
• Overshot sizing and selection requires accurate knowledge of the fish OD before the tool is made up and run: the nominal fish OD determines the overshot size (the overshot must be large enough to pass over the fish OD and small enough that the grapple engages before the fish exits through the bottom of the overshot), and the condition of the fish OD (whether it is smooth, corrugated, swaged by fishing jars, damaged by impact, or coated with scale, sand, or cement) determines whether a spiral grapple, basket grapple, or extension mill guide is the appropriate design; for a fish with an unknown OD (such as a wireline tool that separated at an unknown depth and the fish end configuration is uncertain), the fishing engineer runs a dress mill on the first trip to clean up the fish top and measure the OD impression (an impression block of lead is run on the next trip and lowered to the fish, where the fish top impression is recorded in the lead, providing a physical measurement of the fish top OD and configuration); a fish with irregular top (bent, mushroomed by impact, or corroded) may require a mill guide shoe on the overshot to dress the top to cylindrical form before the grapple can engage; running the wrong overshot size (too large to grip or too small to pass over the fish) results in a failed catch and a wasted trip, emphasizing the importance of pre-job fish OD determination.
• The distinction between overshots and spears as fishing tools divides the recovering approach into external gripping (overshot, which goes outside the fish) and internal gripping (spear, which goes inside a tubular fish and grips on the inner diameter): overshots are used when the fish OD is accessible (when the fish is a drill collar, heavy drill pipe, or any tool string with accessible outer surface), while spears are used when the fish has a hollow bore that allows the spear to be inserted and expanded against the inner surface (used for drill pipe, tubing, and casing where the inner bore is accessible but the outer surface is inside a tighter casing string that prevents an overshot from passing over the fish); the choice between overshot and spear is governed by the geometry of the wellbore (the casing ID must be large enough to accommodate the overshot OD above the fish OD), the fish configuration (whether the fish top is accessible from above for overshot engagement or the fish bore is accessible for spear insertion), and the load capacity required (overshots typically have higher load capacity than spears for the same fish size because the wedging contact area on the outer surface is larger than the wedging contact area on the inner surface at the same OD); in some fishing scenarios, both tools are run in the same fishing string (an overshot on one assembly, a spear on another) as backup or in tandem to improve the probability of catching an irregular or collapsed fish on the same trip.
• Rotary overshots are used when the fish must be backed off from a stuck point rather than simply pulled free: the rotary overshot (also called a rotary catch-and-release overshot) is engaged on the fish OD and then the drill string is rotated to unscrew the lowermost free joint of the fish from the stuck assembly below, recovering the free portion while the stuck portion remains to be fished in a subsequent trip; the rotary overshot must transmit the torque required for backing off the fish joint (calculated from the make-up torque of the connection, reduced by the torque reduction from a string shot or chemical solvents applied to the stuck connection) through the grapple-fish contact interface, which requires that the grapple engagement be sufficiently secure to prevent slip under torque before the connection breaks out; rotary overshots with both catching and torque transmission capability are more complex and expensive than simple pulling overshots, but they enable a single-trip combination of catch, backoff, and recovery that would otherwise require separate trips with a free-point indicator, a string shot, and then an overshot on separate runs.
• Overshot fishing in open hole (below the casing shoe, in uncased formation) presents additional challenges compared to fishing in cased hole because the borehole wall is not smooth and the fish can be buried in cuttings, cave-ins, or formation debris that partially fill the hole above the fish and prevent the overshot from passing over the fish top: pre-conditioning the hole by circulating the overshot to the fish top (pumping through the overshot while slowly lowering it into the debris pile, allowing the returning fluid to clean the cuttings and debris from around the fish top) is the standard approach to removing the fill from above the fish before attempting engagement; the rate of circulation must be controlled to avoid excessive annular velocity that would erode the formation walls and cause additional caving that would refill the hole around the fish; jars (downhole impact tools that apply upward or downward jarring loads to help free a differentially stuck fish) are sometimes incorporated in the fishing string above the overshot to provide impact loading at the fish in addition to the steady upward pull, increasing the probability of freeing a differentially stuck fish that cannot be pulled free with steady tension alone.