The bumper sub (Drilling Subs) is usually installed in the drill string just above the fishing tool (overshot) or safety joint. Its presence in the string enables the operator to release the fishing tool in the event that it is needed in the fishing operations. It will deliver the sharp downward blow that is required to release from the fish.
Applications In Fishing Operations
Uses for bumper subs include:
- Aiding in the release of grappling tools (overshot or spears)
- Jarring down on a fish to free it
- Providing a means to move the work string approximately 18″ if the string is attached to a stuck fish
- Helping to get over the top of a fish
- Applying a predetermined amount of weight to tools such as internal mechanical cutters
- Swaging out collapsed casing (Stuck in Collapsed Casing)
- Running above a marine swivel to compensate for rig movement on floating rigs.
- A bumper sub may also be used as a “feed off” tool.
Bumper Sub Types
There are two types of the subs:
- Mechanical (slack joint)
IF very high temperatures are expected, THEN the mechanical type should be used. High temperatures may destroy the seals in the lubricated bumper sub.
IF pump pressure greater than 500 psi is expected, THEN the lubricated type should be used because it is in a closed housing protected by seal rings. Because the lubricated type has more sealing elements, it is not as likely to wash out.
Bumper Sub Operation
- Close the jar mandrel and mark the drill pipe for reference.
- Pick up on the string until the mandrel is fully extended. Note: You will know that the mandrel is fully extended when a rapid weight gain is shown on the weight indicator.
- Take a stretch in the string to the length of the stroke plus the permissible stretch in the fishing string.
- Drop the string as fast as possible and stop abruptly a few inches above the referenced mark. The elasticity of the string, plus the law of inertia, will cause the bumper sub to move downward rapidly, delivering a sharp blow to the fish.
Operating The Bumper Sub As A “Feed Off” Tool
Install it in the cutting string (Check Free point Indicator tool – BackOff Operations), a number of joints above an internal mechanical cutter. Since it has a 20-inch stroke length, the cutting string is lowered only 10 inches after the cutter is set. Therefore, only the predetermined weight of the pipe between the cutter and the bumper sub is applied to the cutter.