More fatalities occur during free point indication/ backoff than during any other wire-line operation except perforating.
In Free Point Indication/ Backoff (FPI/ BO) operations, the travelling block is required for working the drill pipe and consequently the upper sheave has to be installed in the certified, permanent upper sheave support in the Drilling Mast. Precautions such as safety slings and shackle pin locking devices etc. shall be incorporated.
All non-essential personnel shall be kept clear of the rig floor at all times during this operation.
Free Point Indication
1. Never assume that the string is stuck at the bit. The approximate depth of the stuck point may be estimated from the stretch generated in the drill pipe by a given overpull, but this only speeds up the free point determination by giving an approximate target depth.
2. Check the tool operation (stretch and torque) as soon as the tool is in mud.
3. Descend to 300 m (1000 ft) above the estimated stuck point and make a check (stretch and torque). Never check with the tool straddling a drill collar. Keep a record of stretch and torque readings at each level.
4. If free, descend 300 m (1000 ft) at a time, checking for free point (stretch and torque) at each level.
5. Once an indication of stuck pipe is confirmed, pull up 150 m (500 ft) and check again. Continue to yo-yo until the lowest depth with pipe free in stretch and torque is determined.
6. Usually there is a very sharp change from free to stuck pipe and in this case the first free tool joint can be selected for a successful back-off.
7. Sometimes either stretch or torque freedom disappears over considerable depth and a decision must be made where to attempt a back-off. Back-off should not be attempted if 15% of the free pipe signal (stretch or torque) has been lost. There are two reasons for this:
a.) Either the neutral point will not be at the back-off point, or insufficient torque may be transmitted to unscrew.
b) The reason for gradual disappearance of the pipe signal is probably that the hole is filling up. In this case, in the time taken to pull the FPI tool out of the hole and replace it with a back-off charge the stuck point will almost certainly have moved higher.
In highly deviated holes, determination of stuck pipe is quite difficult. The sensor of the free point indicator is located between two centralisers having rough-coated arms which locate the top and bottom of the sensor. The stretch/ torque signal is generated by relative movement between the two centralisers. If the tool slips down the hole, or movement of the cable is transmitted to the upper centraliser, the instrument will give a false reading of free pipe. In highly deviated holes there is much greater likelihood of the cable moving with the pipe. This phenomenon can be recognised by an experienced engineer: it is therefore recommended that an expert in Free Point Indication/ Backoff (FPI/ BO) techniques should be called in for operations in highly deviated wells.