A tubular jar is a downhole jarring tool incorporated into the drill string as a section of standard tubular that delivers high-impact mechanical blows (jarring loads) to free stuck pipe by converting the elastic energy stored in the stretched or compressed drill string into a rapid impact force that exceeds the sticking force holding the pipe in place; tubular jars (also called string jars or drill string jars) are distinguished from downhole jars positioned in the bottom hole assembly (BHA) by their position in the drill string above the BHA, their larger outer diameter (designed to match the drill pipe or drill collar outer diameter with a reduced-profile connection), and their ability to apply jarring loads over the full length of the pipe string between the jar and the stuck point, using the elastic stretch or compression of that pipe string as the energy storage mechanism; tubular jars are activated by applying overpull (upward force exceeding the drill string weight) to store energy in the stretched pipe above the stuck point, then suddenly releasing the collet or clutch mechanism inside the jar that allows the mandrel to travel rapidly upward and strike the upper impact shoulder, delivering an upward impact to the stuck portion of the string; modern tubular jars include both mechanical jars (that trigger at a fixed load based on a spring or collet mechanism) and hydraulic jars (that use a metered fluid bypass to control the time delay between applying the trigger load and the jar firing, allowing the operator to build maximum elastic energy in the string before the impact is delivered).
Key Takeaways
The energy delivered by a jar blow depends on the length of string between the jar and the stuck point (the "stretch string"), the axial stiffness of that string, and the overpull applied at surface: the elastic energy stored in the stretch string is E = F^2 * L / (2 * A * E_steel), where F is the overpull force, L is the length of the stretch string, A is the cross-sectional area of the pipe, and E_steel is Young's modulus of steel (30 x 10^6 psi); increasing the overpull force increases the stored energy quadratically (doubling the overpull quadruples the stored energy), while increasing the stretch string length increases the energy linearly; the jar impact force delivered to the stuck point is approximately F_impact = F_overpull + F_string_weight, because when the jar fires, the stored elastic energy is converted to kinetic energy of the moving string mass above the stuck point, and the kinetic energy is dissipated over the distance of the jar travel (typically 12-24 inches) producing an impact force significantly higher than the overpull alone; the peak impact force can be 5-10 times the static overpull for a well-optimized jar configuration, making jarring effective even when the stuck load requires forces that would exceed the drill string yield strength if applied statically.
Hydraulic jars provide a significant operational advantage over mechanical jars by allowing the operator to control the timing of the jar blow through the hydraulic time delay mechanism: the hydraulic jar contains a metered orifice that restricts the flow of hydraulic fluid from one chamber to another as the mandrel moves in response to the applied overpull load, introducing a time delay (typically 2-30 seconds) between reaching the trigger load and the jar firing; this time delay allows the operator to apply the maximum permissible overpull load and hold it for the full duration of the delay, ensuring that the string is at maximum stretch (maximum stored energy) at the moment the jar fires; mechanical jars fire immediately when the trigger load is reached, which may be before the string has reached maximum stretch if the load was applied rapidly, resulting in a less energetic blow; the hydraulic time delay also prevents premature firing due to pipe weight fluctuations or rig heave (on floating drilling vessels), which can momentarily apply the trigger load for shorter periods than the mechanical jar's firing threshold; the trade-off is that hydraulic jars require the drilling fluid to be clean enough to flow through the metered orifice without plugging (abrasive particles or barite settling can block the orifice and disable the jar), and the time delay changes with temperature and fluid viscosity (the delay is shorter at high temperature when the hydraulic fluid is less viscous), requiring the operator to understand the jar's performance envelope under actual wellbore conditions.
Jar placement in the drill string is a critical design decision that balances the need for maximum energy delivery to the stuck point against the mechanical limitations of the drill string and the desire to keep the BHA configuration simple: the jar should be placed as close to the expected stuck point as possible to maximize the length of stretch string above the jar (more stored energy) and to minimize the length of pipe between the jar and the stuck point (which would absorb impact energy through its own elastic deformation); however, placing the jar too deep in the string (close to the bit) means that the jar itself may be in the stuck zone and unable to fire, which is the fundamental limitation of BHA jars that are positioned near the bit; tubular jars placed higher in the drill string (in the heavyweight drill pipe or drill pipe string above the BHA) are less likely to be in the stuck zone (which is usually the BHA or lower drill collars) and can fire freely, but they are farther from the stuck point and deliver less impact to the stuck section because the intermediate pipe between the jar and the stuck point absorbs some of the impact energy; typical industry practice places the jar 1,000-2,000 feet above the anticipated stuck point, with accelerator tools (tandem accelerators or bumper subs) added between the jar and the drill collars to increase the jar travel and the impact energy delivered.
Jar accelerators (also called bumper subs or string accelerators) are companion tools placed immediately above the jar to increase the effective mass of string that participates in the jar blow and to provide additional travel for the jar mandrel: the accelerator contains a sliding mandrel that pre-loads against a mechanical stop when the overpull is applied, and releases simultaneously with the jar to add the kinetic energy of the accelerator mass to the jar blow; the combined energy of the jar and accelerator is higher than the jar alone, and the longer travel of the combined jar-accelerator pair (up to 48 inches combined) extends the time over which the impact force is applied, increasing the impulse (force times time) delivered to the stuck point; the impulse is the quantity that most directly determines whether the stuck pipe will break free, because the sticking force must be overcome over a finite displacement, not instantaneously; without an accelerator, the jar travel (12-24 inches) may not provide enough displacement for the stuck pipe to break free from its sticking mechanism (differential pressure sticking against the filter cake, for example, requires the pipe to move laterally away from the permeable formation face before the sticking force drops); with an accelerator providing additional travel, the pipe has a better chance of moving far enough in the firing direction to escape the stuck condition.
Jarring program design for a stuck pipe situation requires selecting the jarring direction (up, down, or alternating), the overpull or setdown weight, the number of blows at each load level, and the jar configuration (hydraulic versus mechanical, with or without accelerator): upward jarring (applying overpull to stretch the string and delivering an upward impact) is used for most stuck pipe situations including differential sticking (where the pipe is held against the permeable formation face by the pressure difference between the wellbore and the formation), and mechanical sticking due to keyseating, ledges, or debris packing around the drill collars; downward jarring (applying setdown weight to compress the string and delivering a downward impact) is used when the pipe is stuck in a manner that allows downward movement (junk in the hole below the BHA, tight spot that can be reamed through) and when upward jarring has failed to free the pipe after multiple attempts; alternating upward and downward jarring can be effective when the stuck mechanism is not well understood, as it attempts to break the sticking force in both directions and may dislodge packing material or break filter cake adhesion that is resistant to single-direction loading; the maximum safe overpull for jarring is limited by the tensile strength of the weakest connection in the stretch string above the jar (typically 80-90% of the connection's rated tensile yield strength), and exceeding this limit risks parting the drill string above the stuck point and compounding the fishing problem.
Fast Facts
Downhole jarring tools for freeing stuck drill strings have been used in rotary drilling since the early 20th century, with the first mechanical jars developed in the 1920s and 1930s in the US oilfields. Hydraulic jars were developed in the 1950s and 1960s and became widely adopted as drilling moved into deeper wells and more challenging formations where stuck pipe events were more frequent and more costly to resolve. The economics of stuck pipe in the modern drilling industry are substantial: the average stuck pipe incident costs tens of thousands to hundreds of thousands of dollars in rig time, fishing service costs, and well directional correction, and severe stuck pipe events that require sidetracking add millions of dollars to well cost. Proper jar selection, placement, and operation represents one of the highest return-on-investment design decisions in drill string engineering for challenging wells.
What Is a Tubular Jar?
A tubular jar is a specialized section of drill string that can fire a powerful mechanical blow to free stuck pipe. When the drill string gets stuck — held against the formation by differential pressure, jammed by a keyway, packed off by cuttings, or wedged against a ledge — the options are limited: apply force and hope the pipe breaks free, inject lubricants and chemicals to reduce the sticking force, or use a jar to deliver an impact that momentarily exceeds the stuck load. The tubular jar does the last of these by functioning as a mechanical snap. The driller applies overpull — pulling up on the drill string with more force than it weighs — which stretches the pipe above the stuck point like a spring. When the jar's internal mechanism triggers, the mandrel travels rapidly and strikes the impact shoulder, converting the stored elastic energy of the stretched string into a sharp blow that can dislodge stuck pipe that static pulling cannot move. The key is that the impact force is far greater than the overpull that generated it — the elastic energy stored in thousands of feet of stretched pipe all releases in the fraction of a second of the jar travel, generating an impulse that can break the stuck pipe free before the string returns to its un-stretched length.
Tubular jar is also called a string jar, drill string jar, or simply a jar in drilling parlance. Related terms include stuck pipe (a condition in which the drill string becomes immovable in the wellbore, caused by differential pressure sticking against a permeable formation, mechanical sticking due to keyseating or wellbore collapse, or packing off by drill cuttings, requiring jarring, chemical spotting, or fishing operations to resolve), differential sticking (the most common cause of stuck pipe in overbalanced drilling operations, where the differential pressure between the wellbore and a permeable formation holds the drill string against the filter cake with a force proportional to the contact area and the pressure differential, mitigated by reducing mud weight, spotting lubricant, and upward jarring), fishing (the downhole operation of retrieving stuck, lost, or dropped objects from the wellbore using specialized tools run on the end of a work string, required when jarring operations fail to free stuck pipe and the drill string must be cut and the remaining portion retrieved separately), overpull (the additional upward force applied to the drill string above the hook load needed to maintain the string in tension, used to stretch the string for jarring operations and to detect the onset of pipe sticking by monitoring whether the string requires abnormal force to move), and jar accelerator (a companion tool placed above the jar in the drill string that stores additional elastic energy and provides extended travel to increase the impact energy and impulse delivered by the jar blow to the stuck pipe).