What are Drilling Jars and How They Work?

What are Drilling Jars and How They Work?

We have talked about Mechanical Pipe Stuck in the Learn section. Jarring the string is one of the freeing actions to the mentioned problem. In this post, you can get brief answers to the questions below:

1. What are Drilling Jars?

2. How Drilling Jars work?

3. How to place Jars in the drill string?

Also, you will find information about the Neutral Zone in the drill string, and potential damage to the Drilling Jars if placed near this zone. So, let’s get started with the first question:

What are Drilling Jars?

Jars look like drill collars, but inside they are sliding mandrels that allow brief and sudden acceleration of the drillstring above the jar. The same effect is pulling a rubber band and letting it go: You build up energy, either by tension in the up-direction or gravity in the down-direction, to affect the hammering action.

Drilling jars serve a single purpose: to free stuck pipe. Jarring is the process of dynamically transferring strain energy stored in the drillstring to the jar – that concentrates kinetic energy at the point where the pipe is stuck. 

How Drilling Jars work?

What’s inside a jar is a mandrel that slides within a sleeve. The free end of the mandrel is shaped in the form of a hammer to provide a striking action against the face of the anvil.

Jars have a detent mechanism that restricts the movement of the mandrel briefly before releasing it. The time delay enables the drill pipe to store potential energy. The sudden release of the detent mechanism causes the mandrel to accelerate rapidly for a distance before it slams against the sleeve, releasing the stored energy and imparting an impact force at the stuck point.

The upward motion of the mandrel causes the hammer to slam into the anvil, producing an upward force on the drillstring. Downward mandrel motion produces the opposite effect.

How to place Jars in the drill string?

In this section we’ll see some basic, yet important considerations for placing the jars in the drill string:

Stay away from the neutral point
Consider stuck pipe mechanisms

No stabs above jars, no x-sectional change in direct connection with the jar
Avoid placing stabilizers or other BHA components above the tool that have a larger Outer Diameter (OD) than the jar

Neutral Point Explained

The neutral point is defined as a point along the drillstring where it is divided into two parts, an upper part, being suspended from the elevators and which is under tension as well as a lower part that generates the appropriate WOB and is under compression. The location of this neutral point depends on the weight-on-bit and the buoyancy factor of the drilling fluid. In practice, since the WOB fluctuates, the position of the neutral point changes. It is therefore quite common to refer to a “transition zone” as the section where axial stress changes from compression to tension.

So, why should we avoid placing the jar in the Neutral Point?

In Neutral Point (NP), or even close to it, lateral vibrations tend to be more severe. Placing the Drilling Jar near NP will result in the jar continuously cycling between compression and tension. This in turn accelerates fatigue damage, even might cause jar fire unexpectedly.

As a general rule 3 to 4 drill collars should be run above the jar. This will ensure that sufficient weight is available to cock the jar and allow sufficient down weight to be used for effective down-jarring.

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