The "rat hole" on a drilling rig refers to a cylindrical hole in the rig floor where the kelly (a key component of the drill string) is stored when it is not in use. It is an essential part of the rig setup and has specific functions:

　　1. Location and Construction:

　　The rat hole is typically a steel casing installed vertically into the rig floor, extending a few feet below ground level.

　　2. Purpose:

　　- It provides a secure place to store the kelly, which is the long steel pipe that connects the rotary table to the drill string.

　　- It keeps the kelly out of the way during certain rig operations, such as adding or removing sections of the drill string.

　　3. Safety and Efficiency:

　　By storing the kelly securely in the rat hole, the rig crew minimizes clutter and reduces the risk of accidents. It also helps streamline operations, saving time when transitioning between tasks.

　　The rat hole is a standard feature on drilling rigs and plays a vital role in maintaining an organized and efficient workspace.