Top Sub

I. Functions and Roles of Top Sub

The top sub is mainly used to connect the drilling jar and the drill string, playing a role in transmitting drilling pressure, torque and impact force. During the drilling process, when a stuck pipe accident occurs, the shock force generated by the drilling jar is transmitted upward to the drill string through the top sub, helping to eliminate the stuck pipe fault.

II. Structural Features of Top Sub

The top sub is usually cylindrical in shape, with either an internal or external thread at one end, which is used to connect with the drill string or other components of the drilling jar. For instance, in some mechanical drilling jars, an top sub is set at the top of the outer cylinder, and an upper stop block is set at the bottom of the top sub. The drilling jar is sleeved inside the upper stop block and is in contact with the top sub. The drilling jar can move up and down within the outer cylinder and complete the impact action by colliding with the top sub.

III. Material Requirements of Top Sub

As the top sub needs to withstand considerable pressure, tensile force and impact force during the drilling process, it is usually made of high-strength alloy steel, such as 42CrMo, to ensure it has sufficient strength, toughness and wear resistance. Meanwhile, to prevent corrosion, the surface of the top sub usually undergoes special anti-corrosion treatment, such as hard chromium plating, etc.

IV. Coordination with Other Components of Top Sub

The top sub is used in conjunction with other components of the drilling jar, such as the cover and bearings, to jointly form the working system of the drilling jar. For instance, in a snap-removing device for a drilling jar, the top sub includes shaft sections of different radial dimensions. The cover is fitted onto the first shaft section and is press-fitted and sealed with it. Bearing one is fitted onto the first shaft section and is axially pressed against the cover. Bearing two is fitted onto the second shaft section and is axially pressed against bearing one. Through the coordinated action of each component, the snap-removing shock force is effectively transmitted.