Drilling Jars
About this chapter
Until recently, the Earthquaker Drilling Jar and the Shock Guard were the only two DD
tools which were provided directly by Anadrill for the drilling of directional wells.
The Earthquaker is still the most reliable and effective mechanical drilling jar on the
market. Clients have confidence in the Earthquaker because of its track record.
This chapter is designed to explain the theory and operation of the EQ Jars. Their
position in the BHA and the constraints thereon are covered here and in Chapter 11.
It is recommended that, in addition to this DD training manual, the Anadrill DD carry the
EQ Jar manual with him on every job. The manual has additional information (e.g.
rig-floor tripping load adjustment, EQ Jar specifications and fishing diagrams) which are
beyond the scope of this book.
Objectives of this Chapter
On completing this chapter the directional driller should be able to do the following
exercises:
1. Describe the objective of jarring.
2. Describe the different types of jars available.
3. Describe how the tripping mechanism of the EQ Jar operates.
4. List the advantages and disadvantages of the EQ Jar vis-a-vis hydraulic jars.
5. Describe the major constraints on EQ Jar position in the BHA.
6. Show what is meant by Pump Extension Force.
7. Calculate the weight indicator reading when the driller jars DOWN. Assume the
pumps are on and there is wall drag.
8. Show how the driller cocks the EQ Jars
· after jarring UP
· after jarring DOWN
9. Calculate the maximum allowable UP setting for the EQ Jar.
6.1 Using Drilling Jars
6.1.1 Drilling Jars
Drilling jars are run as part of most directional BHAs. On vertical wells, drilling jars
may or may not be run, depending on the client. Drilling jars are designed to deliver an
impact in either an upward or a downward direction. Some jars work in one direction
only.
Inside, a jar is basically a sliding mandrel that allows a brief and sudden axial
acceleration of the drillstring above the jar. Travel of this mandrel (the hammer) is
limited by a stop (anvil) on the outer sleeve (Figure 6-1).
6.1.1.1 Jarring Objective
To transfer the potential energy stored in the stretched drill pipe to kinetic energy in the
BHA above the jars. At the end of the jar stroke, a stress wave is sent to the stuck pipe.
The magnitude of the stress wave is related to the velocity of the accelerated BHA. The
duration of the wave is related to the length of the BHA.
Kinetic Energy =
1
2
MV2
M = Mass (weight) of BHA above the jar
V = Velocity (speed) at which the mass is moving when the jar fires (hammer strikes
anvil) in feet/second.
There are three types of drilling jars:
· Mechanical
· Hydraulic
· Hydromechanical.
6.1.1.2 Mechanical Jars
Mechanical jars operate using a series of springs, lock and release mechanisms.
Hydraulic jars operate using the controlled passage of hydraulic fluid. Hydromechanical
jars are a hybrid of both designs, usually hydraulic up and mechanical down.
A mechanical jar trips up at a preselected tensile force, and trips down at a preselected
compressional force. The jar trips only at the set threshold. This is normally beyond the
forces reached while drilling. The position of the mechanical jar while drilling is either
cocked (neutral) or extended. Drilling is never conducted with the jars tripped down as
unnecessary jarring might damage the bit and BHA components.
The release mechanism of a mechanical jar is set either downhole or at surface,
depending on jar design. There are two main designs. One uses the principle of the
torsion spring. These mechanical jars are delivered to the rig with specific up and down
tripping load settings. Their release force can be varied downhole by 10 - 15% by
applying torque to the drillstring. Left- hand torque decreases spring tension; right-hand
torque increases it. The Dailey L.I. Jar uses this design. Another design uses an
expanding sleeve with slots, lugs and ancillary springs. The overpull necessary to trip the
jar can be reduced downhole by increasing mud flow rate. The Anadrill Earthquaker
(EQ) Drilling Jar uses this latter design. It will be covered in some detail later in this
chapter.
6.1.1.3 Hydraulic Jars
A hydraulic jar consists of two reservoirs of hydraulic fluid separated by a valve. When
tension or compression is applied to the tool in the cocked position, fluid from one
chamber is compressed and passes through the valve at high flow resistance into the
second chamber. This allows the tool to extend or contract. The distance traveled is
called the metered stroke. When the stroke reaches a certain point, the compressed fluid
is allowed to suddenly bypass the valve. The valve trips as the fluid rushes into the
second chamber, instantly equalizing pressures between the two chambers. The greater
the force on the jar, the greater the compression of the fluid and the sooner and more
forceful the release. This is the principle of the Anadrill Hydraquaker Drilling Jar
(Figure 6-2).
Hydraulic jars do not trip at a preselected threshold. When, and how forcefully, the jar
trips is determined by the magnitude of the applied tension or compression. To trip up,
the force of the blow is proportional to the overpull. The greater the overpull on the
drillstring, the sooner the jars trip and the harder the blow. Thus, the hydraulic jar has the
advantage of having a continuously-variable jarring force, within its design limits.
Another advantage of hydraulic jars is that, for sizes 6 1/2" OD, they have a larger ID
than comparable mechanical jars.
Once a hydraulic jar is cocked, it will fire if given enough time to complete the metered
stroke. This gives hydraulic jars an advantage in high-angle and horizontal wells. Here,
excess drag may prevent the driller from applying sufficient tension or compression to
trip a mechanical jar. A cocked hydraulic jar will eventually fire, even with minimal
tension or compression. However, this can be a disadvantage also, as accidental jarring
(particularly in vertical wells) with the pipe in slips is dangerous and could lead to a
fishing job.
Repeated jarring with a hydraulic jar can lead to overheating of the hydraulic fluid. This
reduces its viscosity, shortening the metering time and tripping the jar before the desired
tension can be applied. Consequently, jarring force decreases over time. Although some
changes have occurred recently, an adequate design for hydraulic jars has yet to be
proven i.e. visco jets.
One major advantage of mechanical jars is that they will not fire until the threshold
setting is reached. They are often perceived as being more rugged and durable than
hydraulic jars.
6.1.1.4 How does the Earthquaker (EQ) Drilling Jar Work ?
The major external components of the EQ Jar are shown in Figure 6-3. Note that some
EQ Jars (older design) have a Jack Nut. The new design dispenses with the Jack Nut.
The function of the Jack Nut was to extend the EQ Jar prior to running in the hole to
drill. This is no longer deemed necessary (See below).
The EQ Jar has a relatively simple tripping mechanism that can be adjusted on surface, if
required. In the EQ Jar, the relationship between the Trip Mandrel, Trip Sleeve, Friction
Sleeve, Adjusting Sleeves and Spring Tubes (Figure 6-4) is what determines whether the
jar is cocked or tripped. The Trip Sleeve acts as a radial spring along its length. The
Spring Tubes are sets of three concentric tubular springs having a very high spring rate
(only 0.1" reduction in length for a compressive load of 100,000 lbs.).
Figure 6-4 is an expanded view of the EQ Jar tripping mechanism in four different
positions, one of which (position 2) is a "snapshot". In the cocked position, the Trip
Sleeve is in its normal state. It is closed around the Trip Mandrel, with the teeth and
grooves on the ID of the Trip Sleeve meshed with the teeth and grooves on the OD of the
Trip Mandrel. The teeth on the OD of the Trip Sleeve contact the teeth on the Friction
Sleeve crest-to-crest. The Friction Sleeve is held securely inside the Middle Housing and
acts as an integral part of the housing.
The housings are free to slide up and down a fraction of an inch over the Trip Mandrel
until the Spring Tubes contact the end of the Trip Sleeve. Adjusting Sleeves, threaded
into the housings at top and bottom, control the amount of free movement. Rotation of
the Adjusting Sleeves controls the amount of free movement and changes the point
where resistance is encountered. It is the Adjusting Sleeves which are rotated to a
number of different positions (corresponding to specific tripping loads) when the EQ Jar
is calibrated in the workshop. On the rig, if the "UP" or "DOWN" trip settings have to be
adjusted, the lower or upper adjusting sleeve, respectively, is rotated to the desired
position, corresponding to a specific tripping load on the EQ Jar calibration sheet (Refer
to EQ Jar Manual).
When contact is made between the Trip Sleeve and the Spring Tubes, additional pushing
or pulling of the EQ Jar causes compression to begin in either the Upper or Lower Spring
Tube, respectively. As tension on the tool increases, pressure grows between the Trip
Sleeve and Trip Mandrel teeth. The high spring rate of the Spring Tubes at the end of the
Trip Sleeve and the contact angle of the Trip Mandrel teeth on the Trip Sleeve teeth,
forces the Trip Sleeve to expand. This expansion is restricted by the Friction Sleeve teeth
which are positioned to confine the Trip Sleeve. The Spring Tubes compress until the
Housing and Friction Sleeve have moved enough that the Trip Sleeve is no longer
constrained by the Friction Sleeve teeth. This point is determined by the EQ Jar setting.
At this point, the Trip Sleeve instantaneously expands off the Trip Mandrel and engages
with the teeth and grooves on the ID of the Friction Sleeve. The Housings, now free to
move, accelerate up or down until the Hammer strikes the Anvil. This completes the
jarring cycle. The Trip sleeve expansion is maintained through the free stroke by
Expansion Pads at either end of the Trip Sleeve.
Recocking of the jar occurs as the Trip Sleeve, still engaged with the Friction Sleeve, is
repositioned over the Trip Mandrel. When the teeth of the Trip Sleeve realign with the
teeth on the OD of the Trip Mandrel, the Trip Sleeve returns to its normal position by
closing around the Trip Mandrel and disengaging form the Friction Sleeve. With the
tension no longer applied, the Spring Tubes extend to their full length, completing the
recocking. The cycle is now ready to repeat, in either direction, as often as possible.
To summarize, following are the three “major” positions of the EQ Jar (Figure 6-5):
1. Jar Tripped Down The mandrel has been released from the Trip Sleeve and
expanded into the Friction Sleeve in the “jarred down” position.
2. Jar Cocked The mandrel is locked in position inside the Trip Sleeve, with preset
loads restrining the Trip Sleeve crest-to-crest with the teeth of the Friction Sleeve.
The Spring Tubes are relaxed.
3. Jar Tripped Up The mandrel has been released from the Trip Sleeve and expanded
into the Friction Sleeve in the jarred up position. The jar is in tension for running in
the hole and drilling.
6.1.1.5 Features of the EQ Jar
1. With the EQ Jar in the drillstring, the driller can start jarring up or down
immediately if the pipe becomes stuck.
2. Both up and down tripping loads can be independently adjusted to a maximum of
180,000 lbs. (for some sizes) either in the workshop or at the rig-site (Refer to EQ
Jar manual). Torque has no effect on these preset hitting loads.
3. The EQ Jar spline system (for rotation) means that there is no torsional slack in the
mandrel. This is useful in DD work, especially when orienting.
4. The working parts are enclosed in oil to minimize wear or malfunction due to mud
solids etc.
5. A compensating piston minimizes pressure differentials on all seals. Preloaded
V-packing on moving seal areas eliminates both high and low pressure leaks.
6. The jar will trip at the same preset load, regardless of time in the hole or downhole
temperature.
7. Internal compensation allows for high extension force and compression placements.
It also provides a means for downhole load adjustment.