This article describes force multiplying fishing tools: Jars, Accelerators, Bumper Subs.
Drilling/ Fishing Jars
If a fishing specialist contractor is used, the contractor should be consulted on a case by case basis for the optimum selection and placement of jars.
Accelerators
A gas-charged (N2) accelerator or “intensifier” may be run to greatly increase the effectiveness of the jarring action. They are positioned above the jar separated by one joint of drill collars. Accelerators are also available in double-acting mode, to match the jar action. Typical accelerator strokes are 6in - 15in depending on the tool used.
Without an accelerator, the drillstring absorbs some of the impact force of the jar, reducing its effectiveness. The string is also subject to undesirable jarring shock loads. Both effects are increased when the hole is deviated, and pipe to wall friction exists.
Where an accelerator is used, less drill collars may be run without reducing the jarring impact force, which is beneficial in minimising wear on tools and running strings. Also, jarring can be conducted at shallower depths where less string stretch and overpull is available, preventing the jar from achieving a full blow.
When the jar trips, the accelerator imparts its stored energy to the drill collars and jar mandrel in the form of an accelerated force causing the jar to strike a high impact blow. The accelerator shall be matched to correspond to the stroking action of the jar below it. When the two have matched strokes, they are optimally set up to achieve the maximum jarring strokes per minute. Use of an accelerator, by maximising the impact of the jar and reducing the shock load effect on the string can enhance the jarring impact force by a factor of 1.5 - 2 in optimal conditions.
Bumper Subs
Bumper subs provide a means of delivering upward or downward blows.
This tool is primarily used to assist the release of an overshot or spear from a fish. Its secondary purpose is to bump down on the fish, since the majority of fishing jars only act in an upward direction.
The bumper sub shall be installed immediately above the fishing tool or safety joint.
The bumper sub is able to bump downwards as well as transmit torque that is required to break fishing tool engagement and release it from the fish.
To Bump Down in the Hole
1. PICK UP the string enough to open the tool completely and TAKE a strain or stretch in the string.
Note: This shall be the length of the stroke plus the permissible stretch in the fishing string.
2. DROP the string to within 6 in of the closed position of the tool and stop the string abruptly (maximum strength may not be required).
Note: If sufficient stretch has been taken in the string, the lower end of the fishing string shall bump downward, closing the bumper sub and due to the elasticity of the string, deliver a downward blow to the tool below the sub.
To Release Fish by Applying Torque
1. PICK UP the string enough to open the bumper sub to half of stroke. DO NOT take stretch. ROTATE string to the right to release fishing tool. If no release, MAINTAIN torque and PROCEED to Step 2.
2. DROP the string sharply this full length without braking. This causes the bumper sub to close quickly and the lower end of the mandrel body strikes the shoulder on the tool lower end of the mandrel, transmitting a single solid, downward blow to the tool below.
3. Cautionary Comment: Downward blows could cause an overshot to ‘bump out’ with the potential to get stuck on the fish. Always maintain right hand torque while ‘bumping down’ so grapple can move to release position.
Note: During fish operations always consult fishing specialist when available.