Page 331 - Drilling Technology in Nontechnical Language

322 Drilling Technology in Nontechnical Language Second Edition

▫ It can be unlatched down hole if the fish cannot move.

Latching and unlatching does not damage the top of the
fish significantly.

▫ As it is hollow down the middle, logging tools and explosives

can be run on wireline into the fish.

▫ If the fish is not plugged off, it is possible to circulate down
the overshot and through the fish—a major advantage.

Fig. 13–6. Cutaway of an overshot

A die collar is a tubular piece of steel (fig. 13–7). Inside it has a slightly

conical profile facing downwards. Threads are cut into this profile. To

latch onto the fish, the die collar is lowered over the fish and a little weight

applied. By rotating the drillstring to the right, these threads cut into the
top of the fish and grip it. However, once the die collar is on the fish, it

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