Back Off Procedure
Drill pipe or drill collars can be unscrewed downhole by exploding a charge inside a selected tool joint connection, close to the stuck point.
Requisites for a successful back-off are the following:
- There must be sufficient minimum inside diameter.
- The charge must be accurately placed across the connection.
- There must be sufficient string shot strength (Refer to table 16.a).
- Neutral or slightly positive tension is applied at the back-off point.
- Sufficient left hand torque must be applied at the back off point.
As a general rule, the first attempt to back-off should be made at the first connection above Free Point. If there is a failure, the second attempt should be performed on the first stand above the Free Point. Subsequent attempts should be made moving upward one stand at a time.
To ensure a safe operation, the Company Drilling and Completion Supervisor shall carry out the following checks:
- Ensure that tong and slips dies are clean, sharp and the proper size for the string above the rotary table.
- The tongs, snub and jerk lines are in working condition.
- The slip handles are tied together with strong line, to prevent the slips being kicked out of the table when the pipe break out.
- The elevators are latched around the pipe and loose under a tool joint with the hook locked when torque is being applied to the string.
- Ensure that no resilient torque remains in the string when it is picked up out of the slips, unless the pipe is properly held with a back-up tong.
- A wireline cutter must be on the rig floor during the entire operation.
A detailed standard back-off procedure cannot be used as there are too many variables. The following is a typical generic procedure:
- Keep non-essential personnel off the rig floor.
- Install one or two Kelly Cocks on the pipe above rotary the table.
- If it is required due to hole conditions, install a stuffing box and pump in sub.
- Tighten up all string connections applying right hand torque (max. 80% of nominal value). The torque should be worked down the string. This procedure should be repeated 4 to 5 times especially in crooked or deviated holes.
- Install the back-off tool in the string and run in the hole to around 150-300ft (50-100m) below the rotary table.
- Pick up the string to have a hook load equal to the weight, in air, of the pipe above the selected back-off point, plus 10 to 15%.
- Apply left hand torque in a series of steps. Work the pipe at each step to transfer the torque downhole.
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