Snubbing
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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 Xmas tree from the wellhead.
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[edit] Rigup
A snubbing rigup is a very tall structure. It consists of a hydraulically powered snubbing unit, which provides the force on a pipe, above 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 powers the pipe into an 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.
Primary pressure control during snubbing operations is provided by a self-energising stripper and through an assembly of stripping rams. The stripping rams consists of two sets of rams, one on top of the other. There are opened one at a time to allow the collar of the connections between joints of pipe to pass through the rams. Below the stripping rams are the safety rams, which may consist of a shear/blind blowout preventer and an annular BOP. The tertiary barrier, a shear/seal BOP, sits below this directly above the Xmas tree.
[edit] Stripping the pipe
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 and 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.
This process is repeated as successive collars are lowered into the well. When pulling out of hole, this procedure is reveresed.
[edit] Heavy-pipe and light-pipe
Because snubbing is normally done under pressure, initially, the weight of pipe in wellbore is less than the force due to the wellbore pressure. This is light-pipe. This means that downward force is required on the pipe to force it in against resistance. Once a sufficient amount of pipe has been run in hole, the weight becomes sufficient to overpower the wellbore pressure and the pipe naturally wants to fall in the hole. This is heavy-pipe. At this point, the snubbing mechanism is changed over to the one, which provided upward force to hold the pipe and lower it controllably in the well.
When pulling out of hole, upward force is initially used to lift the pipe until the equilibrium point, when the downward force is used to prevent wellbore pressure from blowing the light-pipe out of hole.
[edit] Risks
The more complex method of pressure control, as compared to coiled tubing and wireline, naturally invites more opportunity for things to go wrong. One such peril was seen in June 2007 on the Shearwater platform. Snubbing was being used to clean out large pebble, which had entered the well through a collapsed liner. While pulling out of hole, one stripping ram was not opened sufficiently and a collar on the pipe string caught on the ram. The excessive force applied to the pipe caused it to break apart, dropping the string below the failure into the well. In the time it took to prepare to fish out the pipe, the pebbles in the process of being circulated out, settled on the pipe, preventing successful fishing.
