Drilling Jars and hammers are the mechanical devices used in the well drilling, completions, well workover and intervention operations. This equipment is also know by the term “Hammer Drilling Jars”. This equipment is useful when any downhole tool or equipment assembly gets stuck inside the borehole and when a high impact hit is needed to un-jam the downhole tool assembly.

The principle of working of Drilling Jars and Hammers is very much similar to that of a carpenter using a hammer to put the nail inside the board. A hammer stores kinetic energy in it when it is brought down forcefully to hit the nail on the board. On the same principle, these hammer drilling jars also delivers a high impact load on the stuck downhole tool in the borehole. Hammer drilling jars are commonly classified into two categories namely:

• Drilling Jars or
• Fishing Jars

These can either operate hydraulically or mechanically and are designed in such a way that they can strike in upward direction or downward direction or in both directions simultaneously.

When a jarring up operation is carried, the driller pulls the drill string upward, but the bottom hole assembly does not move i.e. it is stuck. As drill string moves up, it keeps on stretching and storing potential energy. Drilling jars & hammers are inside the drill string which also keeps on stretching. When the jar reaches to its ultimate stretching point or fire point, this stored high potential energy gets converted into kinetic energy as it comes down with great velocity and slams into the steel shoulder of the stuck downhole tool assembly imparting a high impact load, thereby freeing the jammed tool.

Drilling jars and hammers are commonly used in fishing operations.