Definition - What does Cushion sub mean?
A cushion sub is a drill rig accessory designed to reduce vibration and increase a drill's lifespan. The cushion sub mates the threaded connections on a drill pipe without heavily loading the flanks of the threads. The cushion sub also absorbs the shock transmitted by the drill bit and reduces the impact on rotary drive bearings.
Cushion subs, sometimes called "float subs" or "floating cushion subs," are used to increase tooling life in boring and drilling, minimize rig downtime and reduce maintenance.
Trenchlesspedia explains Cushion sub
A cushion sub has a sliding spindle that allows the threads of the drill string parts to float together or float apart while rotating - much like the components of a car's automatic transmission - without allowing any changes in the alignment between the drill string of the rotary drive. This means you can add pipe to the drill string without fear of damaging the gearbox in the drilling rig's rotating top drive.
Cushion subs mean you don't have to start and stop the top drive every time you want to add another pipe to the drill string, maximizing the rig's efficiency and minimizing the opportunities for lost time through accidents.
As connectors between the rotating top drive and the drill string, cushion subs perform three functions.
- Cushion subs allow drillers to make up a drill string, adding or breaking out pipe without damaging threaded connections.
- Cushion subs decrease the load on the threads throughout the drill string.
- Cushion subs act as shock absorbers for the drill string. Two urethane cushions, an upper and a lower, reduce vibrations within the string that may result from damaged drill pipe, minor variations in string alignment a "wobble" in the rig. This means a lower impact - and longer service life - for the top drive bearings and the rig's gear box.