General Description
The Accelerator Jar was developed to run in conjunction with any hydraulic rotary jar. The function of the Accelerator Jar is to supply extra acceleration to the upper portion of the hydraulic jar and drill collars during the jarring (free) stroke of the hydraulic jar.
When pull is applied to the fishing string, fluid, or gas in the Accelerator is compressed storing high-intensity energy. At the free stroke of the jar, this energy is released, giving up this stored energy to accelerate the upward travel of the hydraulic jars and drill collars, resulting in a high-impact blow, delivered directly to the fish where it is most effective.
Use
The Accelerator Jar is, in a sense, a spring that should be run with the proper tension. Effectiveness of the jarring action of a hydraulic jar is proportional to the upward acceleration produced at the drill collars and lower end of the fishing string during the free stroke of the jar. This acceleration is produced, in turn, by the rebound of the running string having been stretched and suddenly released. It is clear, then, that any condition that diminishes this available stretch will diminish the effectiveness of the jarring action. Such conditions exist in very deep or very crooked holes where much of the stretch is given up to drag in the hole or “doglegs.” Also, at very shallow depths, very little stretch is available in the running string due to the inherent lack of stretchable length.
The great value of the Accelerator Jar or Booster Jar is that it stores energy at the point of greatest effectiveness.
An important secondary benefit is that after jarring, the Accelerator cushions the downward rebound of the fishing string. This reduced the shock to the string, tools, and rig, minimizing shock damage.
Construction
The tool utilizes the Mandrel Piston assembly within the fluid chamber, but has no tripping mechanism and no free-stroke. The Fluid chamber is filled with gas or hydraulic fluid under low precompression. During the pull stroke, this fluid is highly compressed, absorbing and storing the energy that is suddenly released by the tripping of the jar below it.
The Seals have Teflon anti-extrusion rings to assure long life even in hot hole conditions. Accelerator Jars and Booster Jars are available in all drill pipe sizes from 1 13/16” through 6 5/8” A.P.I. regular.
Operation
They are run above the Hydraulic Jars and the amount of weight run between them and the Hydraulic Jars determine the tension. When you pick up on the work string to begin jarring, the additional pull will completely open the Accelerator Jar (putting it in full tension). When the Hydraulic jars move upward to hit, the tension on the Accelerator Jars will increase the speed that the mass between the jars travel; therefore, it will increase the impact of the Hydraulic Jars.
There is a chart available to figure the weight that should be run between the jars. If you don’t have a chart, a good rule to follow is one drill collar per O.D. inch of the jars, as long as the O.D. of the jars and drill collars are close. If they are not, then you will have to make allowances for them.
Accelerator Jars are very helpful in deep crooked holes where drag is a problem. They will allow the lower part of the string to move much faster.
They are good to run when jarring shallow, where you have no stretch in the drill pipe. Regardless of where you run them, they reduce the shock on surface equipment.