Snubbing

See snubber for a device used to suppress ("snub") voltage transients in electrical systems

Snubbing is a type of heavy well intervention performed on oil and gas wells. It involves running the BHA on a pipe string using a hydraulic workover rig. Unlike wireline or coiled tubing, the pipe is not spooled off a drum but made up and broken up while running in and pulling out, much like conventional drill pipe. Due to the large rigup, it is only used for the most demanding of operations when lighter intervention techniques do not offer the strength and durability. Unlike conventional drilling and completions operations, snubbing can be performed with the well still under pressure (not killed). When done so, it is called hydraulic workover. It can also be performed without having to remove the Christmas tree from the wellhead.

A snubbing rigup is a very tall structure. It consists of a hydraulically powered snubbing unit, which provides the force on a pipe, above a string of multi-layered pressure control components.

At the top of the snubbing unit is the basket, which serves as the control post for the rigup. Below the basket are the hydraulic jacks, which power the pipe into and out of the hole. It consists of two mechanisms for applying force to the pipe in either direction. Each mechanism consists of travelling and stationary slips. The travelling slips are used to move the pipe, while the stationary slips are used to hold the pipe while the travelling slips are repositioned between strokes.

Unlike coiled tubing or wireline, where the wire or tubing is always the same diameter allowing for a single unmoving primary barrier (stuffing box or stripper), snubbing uses a pipe, which will have an enlarged collar at the connection between the joints. Therefore, the pressure control system must be able to accommodate this variable diameter. The stripping rams accomplish this. The first stage of lowering a collar through the stripping system is to close the lower rams so as to seal off the mechanism above from wellbore pressure. The space between the rams can then be bled off allowing the upper rams to be opened. The collar can then pass through the opened upper rams. Once the collar is in between the rams, the upper rams are closed and pressure is equalised either side of the lower rams. The lower rams are then safely opened and the collar is lowered through the rams.

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