J

Jacket

The enormous steel pipe construction at the bottom of the sea that is used as foundation for the modules of the platform.

Jack up rig

A combination of a drilling rig and a floating barge with long supporting legs that can be raised or lowered to the seabed. 
 

Jar

A mechanical device, which is used to push an element down hole and typically used when this element is stuck. Two types of jars exist: mechanical jars and hydraulic jars, but they operate similarly. 
 

Jet bit

A drill bit with jets that are shaped in a way that fluid speed is increased when drilling mud is injected. This causes a flow effect, which in return increases the drilling speed.

Jet cutter

A fishing tool that uses shaped chargers to cut casings, tubing or drill pipes that have become stuck down hole.

Jet perforation

See perforating gun
 

Joint

A section of drill, casing or other pipes. Usually, joint come in lengths of 23-32 ft (7- 10 meters). When more sections are screwed together they are called a stand. 
 

Junked

Equipment lost down hole that is not worth fishing. Also a well that has been sealed and abandoned.