General Description and Construction
The Outside Backoff Tool is a drill collar ten to twelve feet long, with a slot cut in the side of it that tapers to the outside at the bottom. It also has a blade at the top just above and inline with the slot. This is to hold the tool away from the casing or hole on the side where the slot is. The tool should be counter bored on the pin end and two holes, approximately ½ inch drilled from the side, into the counter bore so pressure will not become trapped below it.
Use
The Outside Backoff Tool is used when a fish is plugged and can be run with a screw in sub, a box tap, a taper tap, or an overshot, or any other means of catching a fish. It is usually run just above the fishing tool, usually a screw in sub, overshot, or a spear.
Operation
Run it in the hole and engage the fish, then run a string shot through the work string and out the slot, down beside the fish, to make a backoff. Always be sure there is enough room between the fishing tool you use to engage the fish and the casing or hole that the string shot can pass. Example: You cannot run a 5 ¾” overshot inside 7” casing and pass the overshot with a string shot. Always be sure that any tools you run above the tool has a large enough I.D. to accommodate the string shot too.