In this article, it describes type of mechanical cutters used for fishing operation. When running fishing tools to retrieve stuck or damaged downhole equipment, the pipe or casing often needs to be cut or parted. This allows the fishing tools to latch onto and retrieve the fish. If wireline tools are available, the pipe can be parted using wireline cutting methods to minimize rig time. However, when wireline is not practical, the pipe must be cut using mechanical cutters run on the end of a workstring.
Internal Mechanical Cutters
Mechanical Internal Cutter
One common method is an internal mechanical cutter. This tool is mounted on a mandrel with an automatic slips release mechanism that allows it to be set at the desired depth. Friction blocks or drag springs provide backup for the release.
The cutter works by slowly rotating right-hand and applying weight, which feeds out knives on tapered blocks to cut into the inside of the pipe. Springs in the feed mechanism absorb shocks to prevent the knives from gouging or breaking. A bumper jar is usually run above to control the cutting weight.
The knife tips are made of brass to prevent breaking when contacting the pipe wall. Internal cutters are available for most tubing and casing sizes.
Internal Hydraulic Cutters
An alternative is an internal hydraulic cutter designed for single strings. It uses hydraulically-activated knives for a smooth, efficient cut. An indicator signals when the cut is complete by a drop in pump pressure. Stabilizer slips anchor the tool before cutting.
Hydraulic Internal Cutter
The tool is run to depth, then rotation and circulation are initiated. Increasing torque indicates cutting has started. When complete, a control dog drops into a recess, reducing pump pressure to signal the cut is done. Straight pickup then retracts the slips and knives.
For multiple strings or open holes, a pressure-activated multiple string casing cutter can be used instead of a hydraulic cutter. Pump pressure forces the knives into the pipe to make the cut.
External Cutter (Washover Cutters)
When circumstances demand cutting from the outside, the washover outside or external cutter steps in as a versatile solution. Ideal for addressing scenarios where internal obstructions impede the use of wireline tools, this cutter operates on the bottom of a washover string, executing cuts from the exterior.
Washover outside cutter
Adaptability is the hallmark of the external cutter, with configurations tailored to catch various tool joints or couplings on the fish. From spring fingers to flipper dog cages, each design caters to specific requirements, ensuring a secure grip. Flush-joint pipe necessitates a hydraulically actuated catcher, with pump pressure activating the cutter’s knives.
With careful calibration of rotation and fluid flow, the cut is initiated, culminating in the retrieval of the fish and subsequent extraction from the washpipe at the surface. Vigilance is paramount throughout the process to prevent surges in pump pressure and ensure a smooth operation from start to finish.
References
The Guide to Oilwell Fishing Operations: Tools, Techniques, and Rules of Thumb (Gulf Drilling Guides) by Joe P. DeGeare, David Haughton, Mark McGurk
