Page 329 - Drilling Technology in Nontechnical Language

320 Drilling Technology in Nontechnical Language Second Edition

The energy that is used in creating this strain is stored in the pipe and is
called the strain energy.
A catapult works by using rubber strips to store strain energy. A stone
is placed in the catapult pouch, pulled back (to store energy), and let go.
The stored energy pulls the pouch and stone, accelerating it forwards.
The initial acceleration is fast (because the pull on the pouch is greatest),
and the acceleration reduces as the stone moves forward. When the
rubber becomes slack, the stone stops accelerating and has reached its
maximum speed.
A drillstring and jar is like a downhole catapult. The jar is a tubular
tool that is placed high up in the bottomhole assembly. It can open and
close just like a bicycle pump. However, there is a mechanism inside the

jar that stops the tool from opening until some condition is satisfied. The
condition might be that a certain amount of pull must be applied to the jar,
or that a period of time has to elapse after placing the jar in tension. Once

the condition is satisfied, the jar opens a certain amount, typically around
2 ft, and will not open further.
One or two drill collars are placed above the jar. The rest of the BHA
stays below the jar. If the pipe gets stuck (below the jar), strain energy is
applied to the drillstring by pulling upwards on it. The jar suddenly opens,
which allows the strain energy stored in the drillstring to accelerate the
heavy drill collars above the jar upwards. When the jar is completely open,
the weight of the drill collars moving quickly upwards exerts a hammer
blow on the jar and everything below it. In the catapult analogy, the drill
collars above the jar are equivalent to the stone that gets catapulted.
A jar is almost always run as part of a drilling bottomhole assembly.
Drilling jars are very robust so as to withstand many hours of rotating,
vibration, high internal pressure, and changes in pressure and temperature.
Fishing assemblies also incorporate jars.

Fishing

Sometimes, something gets into the hole that needs to be recovered.
For instance, a spanner might drop down the rotary table or part of the
drillstring might break. These items prevent normal operations from
continuing. The item that must be removed from the well is called a fi sh,

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