drill bit the pressure gauge to monitor hot tap drill pre charge pressure the pressure pre charge control valve the hot tap vent valve and the union the hot tap drilling subassembly should be pressured up to the pre charge value that was specified by the engineer in charge of the plug and abandon operation the casing or pipe is drilled with u00bd to u00bc inch diameter bit when the bit breaks through the pipe wall the pre charge pressure will drop to a casing or pipe internal pressure the casing or pipe internal pressure is vented and the hot tap drilling sub assembly 10 is removed next the front donut 30 with the hot tap saddle 32 is removed the rear donut 38 with the hot tap rear support anvil 40 is removed the rail system which is used to support and position the large diameter drill 4 inch to 6 inch in diameter or hole saw is then installed on the outer casing 24 below the multiple string hot tap clamp assembly 10 if the multiple string hot tap clamp assembly 10 has to be moved or rotated to provide better support during hot tapping of an inner casing string 26 the rail system can also be used as a support while the clamp is repositioned next a probe is then run into hot tap hole and the location of the next inner casing string or production tubing 26 is determined if the inner casing or pipe 26 is not located on the center line the hot tap clamps 18 36 the hot tap clamps must be repositioned to insure.