Drilling jars usage, calculations, specs etc.

1. DRILLING JAR OPERATIONS
Any jarring operation should start in the direction opposite to that got the drill string stuck.
However, there is an exception if the string is found stuck while picking from bottom (i.e. bit @
bottom), in which case the jarring up should be carried out to free the string.
Information Required for Making Jarring Calculations
• Normal pick-up weight
• Normal set down weight
• Free rotating weight
• BHA weight below jar
• Jar internal friction
• Pump open force
• Maximum jar load (maximum detent load)
• Maximum torsional strength, ft-lbs
Jarring Calculations
TO COCK JARS FROM THE FULL OPEN POSITION- Set weight indicators to:
• Normal set down weight
• (-) BHA weight below jar
• (-) Jar Friction
• (-) Pump Open Force (to be considered only if pumps are running)
TO COCK JARS FROM THE FULL CLOSED POSITION- Set weight indicators to:
• Normal pick-up weight
• (-) BHA weight below jar
• (+) Jar Friction
• (-) Pump Open Force (to be considered only if pumps are running)
TO JAR UP - Set weight indicators to:
• Normal pick-up
• (-) BHA weight below jar
• (-) Pump Open Force (to be considered only if pumps are running)
• (+) Desired up blow
TO JAR DOWN - Set weight indicators to:
• Normal set down weight
• (-) BHA weight below jar
• (-) Pump Open Force (to be considered only if pumps are running)
• (-) Desired down blow
Jar Calculation Example
In a well the d/string got stuck while making connection at 12500 ft in a vertical well. Normal
pick-up weight prior to sticking was 320 klbs & normal set down weight was 300 klbs. The free

2. rotating weight was 310 klbs. A hydraulic drilling jar is placed in the BHA, the details about
BHA weight, pump open force, jar friction etc. is as below:
BHA weight below jar – 80 klbs, Jar internal friction – 10 klbs, Pump open force – 20 klbs
Calculate the following:
Weight indicator reading to cock the jar (assume jar is full open & pumps are running)
Normal set down weight – BHA weight below jar – Jar internal friction – Pump open force
= 300 – 80 – 10 – 20 = 190 klbs
Weight indicator reading to cock the jar (assume jar is full open & pumps not running)
Normal set down weight – BHA weight below jar – Jar internal friction – Pump open force
= 300 – 80 – 10 = 210 klbs
Weight indicator reading for down jarring of 40 klbs (assume pumps are running)
Normal set down wt – BHA wt below jar – Pump open force – Desired down jar load
= 300 – 80 – 20 – 40 = 160 klbs
Weight indicator reading for down jarring of 40 klbs (assume pumps not running)
Normal set down wt – BHA wt below jar – Pump open force – Desired down jar load
= 300 – 80 – 40 = 180 klbs
Weight indicator reading to re-cock the jar (assume pumps not running)
Normal pick-up wt – BHA wt below jar + Jar internal friction - Pump open force
= 320 – 80 + 10 = 250 klbs
Weight indicator reading for up jarring of 70k (assume pumps are running)
Normal pick-up wt – BHA wt below jar - Pump open force + Desired up jar load
= 320 – 80 – 20 + 70 = 290 klbs
Note:
 ‘Pump Open Force’ is not to be considered in above calculations if pumps are not
running.
 While jarring down do not apply torque more than 50% of jar torsional strength. After each
jarring down with torque, release the torque before picking string for cocking the jar.
 Do not apply torque while jarring up.
 Jar manufacture’s usually recommend to begin with moderate jar load & increase the jar
load gradually. Therefore, for sticking mechanism that is time dependent e.g. differentially
stuck, start jarring with 50% of maximum jar load and may increase rapidly to maximum
permissible jar load say within ½ an hour.
However, whileapproaching to maximum jar load do consider margin of over pull while
jarring up for given drill string and weight of drill string above jar while jarring down.

3.  While carrying out prolonged jarring operations, do allow the jar to cool down, say for 0.25
hr after every 1 hr of jarring.
 Never exceed the max jar load (detent load) recommended by the jar manufacturer, as
doing so may lead to collapse of inner mandrel or rupture of seals or damage to detent
housing.
 During jarring operations keep the rig floor area clear of personnel & carry out
intermittent DROPS inspection.
Jar Internal Friction
For all sizes of Hydra jar the jar internal friction may be considered 10000 to 15000 lbs.
Pump Open Force
The pump open force comes into effect if pumps are running while operating jars. The pump open
force is an upward force that can be calculated for different sizes of Hydra jars from below chart.
Pl note that the differential pressure value shown in the chart (x-axis) is the drill bit pressure losses.

4. Maximum Detent Load
The maximum detent load is the maximum up/down jar load for a given size of jar and can be
obtained from below table for various sizes of Hydra jars. In other words this is the maximum pull
or slack-off at the jar before the jar fires up or down respectively. For instance, from below table,
the 6-1/4” Hydra jar has a maximum detent load of 150,000 lbs and which should not be exceeded
while deciding jar load (up or down) while jarring with this jar. However, after the jar has fired the
d/string can be pulled up further within the 90% of the limits of jar tensile load (i.e. 730,234 lbs
from table below) or maximum permissible over-pull for the drill pipe in use, whichever is less.
Maximum Torsional Strength
It is not recommended to torque the drill string during up jarring operations. However, while
jarring down, maximum torque equivalent to 50% of torsional strength of the jar may be applied
(pl see chart above for torsional strength of Hydra jars). After firing the jar down and before picking
up the string to cock the jar, the applied torque should be released to avoid any damage to the jar.
Possible Reasons for Malfunctioning of Drilling Jars

5.  Incorrect weight applied to the jar.
 Pump open force exceeds the compression force at jar.
 Stuck above the jar.
 Jar not cocked.
 Drag too high to allow sufficient force to be applied to the jar while jarring up.
 Well path is such that the compression cannot be applied to jar while jarring down.
 Not waiting long enough for the jar to fire.
 Hole is packed-off and the trapped pressure is not bled off.
Stuck Pipe Scenario & Recommended Jarring Procedure
 Differential Sticking
 Reduce pump rate (to reduce overbalance due to ECD) or stop the pump (may pump
intermittently at reduced rate).
 Apply torque equivalent to maximum of 50% of jar’s torsional strength and jar
down beginning with 50% of maximum jar load and thereafter increase the jar load
rapidly to maximum permissible within ½ an hour.
 Mechanical Sticking (borehole geometry)
 If stuck while moving up, apply right hand torque (maximum of 50% jar torsional
strength) and jar down beginning with 50% of maximum jar load and gradually
increase jar load to maximum permissible (stop or reduce circulation while jarring
down).
 If stuck while moving down, jar up beginning with 50% of maximum jar load and
gradually increase jar load to maximum permissible (do not apply torque while
jarring up). Normal circulation may be continued while jarring up, however may
reduce pump rate while cocking the jar, if required.
 Mechanical Sticking (pack-off)
 If circulation is not possible, leave 400-500 psi pressure under the pack off, pick up
string to free rotating weight and torque the string (maximum of 90% of d/string
make up torque). Continue torqueing the string while watching the pressure bleed
off & returns on shaker. Increase the pump rate if returns are seen on shaker.
 If circulation could not be established as above within first one hour, work on the
string between normal pick-up & set-down weights while torqueing the string and
maintaining 400-500 psi pressure under the pack-off. If pressure bleed-off does not
happen or returns are not seen, increase the pressure under pack off to 1500 psi and
continue working on string for another one hour.
 If situation does not improve or partial circulation is established but the string is
still stuck, start jarring in the direction opposite to that got the string stuck up as
explained above for mechanical sticking (borehole geometry).