Page 330 - Drilling Technology in Nontechnical Language

Chapter 13 – DRILLING PROBLEMS AND SOLUTIONS 321

and activities to remove the fish are termed fi shing. There are four main

causes for fish in the hole:
1. If there is a failure somewhere in the pipe in the hole that causes a
break, the lower part of the string will drop into the hole.
2. If stuck pipe cannot be freed, it has to be cut or
unscrewed downhole.
3. Something falls into the well.
4. Sometimes wells are worked over; they may require that the
existing completion is replaced with another completion. Often the
completion tubing has to be cut downhole and recovered in pieces.
There are five classes of tool that can be used to remove a fish or

junk from the hole. The choice of which one to use will depend on the
circumstances. The five classes are each briefly described.

Outside catch tools

If the junk has a round profile at the top, the preferred tool will normally

be an outside catch tool. These tools are pushed over the top of the fish.
When the fishing tool is moved upwards, it grips the fish and allows force

to be applied.
There are two types of outside catch tool that are commonly available:
the overshot and the die collar.
An overshot is very simple, strong, and versatile. It consists of a steel

barrel inside of which is a wedge-shaped profile machined on the inside

diameter. Fitting into this profile is a spiral element, called a grapple. The

grapple has a wedge-shaped outer profile that fits the ID of the outer barrel,

as can be seen in figure 13–6. On the grapple ID are hardened steel teeth
that grip the fish. As pull is applied to the tool, the wedge shapes cause the

grapple to close, and thus the more tension that is applied to the tool, the
tighter the grapple grips the fish.

Of all the fishing tools available, the overshot is overwhelmingly

preferred to catch anything tubular shaped, due to the following:

▫ It can exert a tremendous pull on the fish. A jar can be placed
above the overshot so that hammer blows can be applied to the
fish through the overshot.

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