The string shot size shall be determined by the Logging Engineer so that it will be large enough to back-off the joint to be released without splitting the pipe. If the pipe does not back-off after the first shot, the charge can be increased on subsequent runs.

The physical size of the primer cord bundle shall be confirmed small enough to pass through the minimum pipe ID.

Weight, Torque and Turns on the String

Ideally the neutral point of the string weight distribution should be at the back-off point. The determination of this condition requires careful calculation and the Drilling Supervisor shall double check the results.

Working Torque Down the Hole in Straight Holes

• all non-essential personnel shall leave the drill floor
• apply right-hand first and work down prior to applying left-hand torque and work down
• in order to prevent the left-hand torque remaining in the upper hole section resulting in a back-off higher than planned, it is necessary to work the torque down the hole to the location where the back-off is required (this is especially critical in deviated or crooked hole)
• in order to work the left-hand torque down the hole:
• set the string to the determined weight for the back-off
• mark the pipe and use this mark for all subsequent readings, as wall friction can interfere with an accurate weight indicator reading
• apply half of the required left-hand torque and lock the top drive
• pick the string up off the slips and work the pipe up and down a few times being careful not to go below the weight mark as the pipe could break at a random point
• if the Drilling Supervisor determines that the remaining left-hand torque can now be applied to the string, it shall be applied

or...

• if the Drilling Supervisor determines that the string cannot take the remaining required left-hand torque, half of the value shall be applied in the above manner until the required amount of left-hand torque has been applied to the string.

Completing the Back-off

After the string shot has been detonated, a check for back-off should be made by picking up the string. The connection will usually (but not always) spin free completely unscrewing the pin and box.

However, in some cases the pipe may have only partially backed-off. To complete the back-off the following procedure must be followed:

Note :   Before applying any left-hand torque, the string shall be torqued to the right first and the ammeter reading shall be noted. When left-hand torque is applied later, the ammeter reading should not exceed the reading for right-hand torque.

1. APPLY approximately half the amount of left-hand torque originally used to back-off the string.

While the torque is being applied, the pipe should back-off, resulting in a corresponding loss of torque load.

2. If the pipe does not come free, RELEASE the torque in a controlled manner and NOTE any loss of torque in the process.

3.

If ...torque was lost during the application of 1... Then REPEAT the process until back-off is completed.

If ...torque was not lost during the application of 1...Then ...the weight used was probably incorrect. This can be corrected by applying half the torque originally calculated.

4. If the pipe is still not free APPLY more left-hand torque but DO NOT exceed the initial applied RH torque.

5. After accomplishing the completed back-off POOH.