Stripping well control is moving the pipe into or out of the well against well pressure when the force of that pressure is less than the weight of pipe being stripped. Remember additional influx and/or excessive pressures can occur if pressure is not monitored and corrected for the displacement of the pipe being stripped and gas expansion.
Exercise care when stripping. If necessary pipe weight (tripping in or out under pressure) is not maintained, pipe can be blown from well. Stripping complications can occur due to some preventers being wellbore pressure assisted to various degrees. Also, the wear factor on sealing elements may lead to element failure and pressure venting to rig floor.
If preventer develops a leak, this may lead to rapid failure of a sealing element and/or preventer and may jeopardize the operation. There is also the possibility of the wrong preventer being opened if speed exceeds caution. All stripping operations should be performed carefully, with all personnel briefed and familiar with their responsibilities.
Stripping policies and procedures vary. Depending on pressure, pipe, collars and tool joints may not strip down of their own weight, but require a pull-down (snub) force. String weight must be greater than computed force or pull down force (snubbing) will be required. The equation shows why it might be necessary to start pipe with a few stands of ram to ram stripping rather than with the annular preventer. When using ram to ram stripping, tool joint is never in the preventer so the termD is smaller.
Travelling blocks have been used to push pipe down. This is dangerous because pipe might slip back up and start to unload out of the hole. Be careful about the beginning of stripping operations. If pipe is not heavy enough to go into the hole against the well pressure, it needs to be kept under restraint at all times while stripping, until it is heavy enough to overcome upward forces.
Stripping well control process
When stripping in or out of the hole it is necessary to have a float or inside BOP in the string. Also, a safety valve should be on the open box as a joint or stand is pulled/lowered. Two safety valves may be used. One is on the string and another is either taken off the last joint pulled or made up on the next to be run. These valves must be in place in the event the float or inside BOP fails, so the string can be shut in. Safety valves should be left open so pipe will not pressurize without warning Stripping operations require excellent communications between the choke operator and driller. As tool joint nears the floor, driller must inform choke operator that he will be slowing and stopping pipe. Choke operator must dictate the overall rate of pipe movement, as it will be his responsibility to maintain pressures as close as possible according to calculations.
Some operators close off the accumulator bank and strip using accumulator pumps for pressure. This is a bad technique as pumps are used too erratically. A better procedure would be to close off one-half of the bank and keep it for reserve or to turn off either electric or air pumps and keep one type of pump for reserve.