General Description
There are many manufacturers of Inside Cutters, but only two basic types. One is the Mechanical Cutter and the second one is the Pump Pressure or Pressure Operated Cutter.
Mechanical Cutters are simple, efficient tools for cutting tubing, casing or drill pipe. Manufactured to exacting standards, Mechanical Cutters permit the redressing of a basic assembly in order to cut alternate sizes of pipe.
The Pressure Operated cutter is a superior tool for cutting casing internally. Since no slips or wiper springs are required for engagement, the tool is simple to operate and easy to maintain. This tool is so effective that multiple cuts may be made in a single run, often in crooked holes where other tools would malfunction.
Use
The Mechanical Cutter works by setting slips in the casing or tubing. The Mechanical Cutter is run to the desired depth, and then begins rotation. As you rotate, slack off on the work string very slowly (within four or five inches the slips will set), and at the same time the knives will extend outward and contact the pipe you are cutting. Continue to rotate until pipe is cut, usually five to seven minutes. Some Mechanical Cutters are spring fed. This meaning the tension on the knives controlled by a spring and the weight you put on the slips has no effect on the knives. Other Mechanical cutters are positive cutters. This means that the weight you apply to the slips is also applied to the knives. If too much weight is applied to the knives it will break them or burn them causing it to miss a cut. To avoid this, you predetermine the weight you want to apply and run a set of bumper jars above the cutter with the drill pipe or drill collars between the jars and cutter in the amount of weight you want.
The Pressure Operated Inside Cutter works with a piston. When pressure is applied to the cutter, it moves the piston and in turn the piston extends the knives or blades outward to contact the pipe you are cutting.
Construction
The Mechanical Cutter consists of a wiper block (or drag spring) assembly, to accomplish setting in the pipe, slips and cone assembly to anchor the tool, a main spring to assist in maintaining uniform feed to the knives, wedge-like knife blocks to drive the knives downward and outward to engage the pipe, and hardened and ground knives for easy, efficient cutting.
An important feature of the Mechanical Cutter is the “Automatic Bottom”. This device permits the operator to set the cutter at any desired depth, to release the tool, and to reset at another depth, all without coming out of the hole.
The Pressure Operated Cutter consists essentially of a Body, Top Sub, Piston, Neutralizing Spring, and Piston Stop. The assembly for 9 5/8” casing and larger uses an Adapter Block to place the cutters in best-cut positions.
Operation
You run the Pressure Operated Cutter to the desired depth you want to cut, begin rotating when you have established the RPM you want, and then apply the pump until you get the pressure you want. Forty to sixty RPM is very suitable for both types of cutters.
When using a positive feed cutter, less than one thousand weight will be sufficient.
Another type of Pressure Operated Inside Cutter is the multi-string cutter. This cutter is designed to cut one or more strings of casing at the same time, and is used mostly for plug and abandon work.
The multi-string cutter operates on the same principle as the other Pressure Operated Cutters, except it has large blades instead of knives. You run it to the desired depth where you want to make a cut and establish rotation, start the pump, and you will know immediately when the blades are extended because of the chattering and torqueing. You will have to regulate the torque by increasing the pump pressure. If everything is working right, you should be able to cut each string in less than 30 minutes.
The multi-string cutters are equipped with various length blades. If you are cutting more than one string of casing, you will have to start with a short set of blades. When the first string is cut, pull out of the hole and change to a longer set. It is very easy to get back in the same cut with this cutter by going in the hole until you are near the cut, apply pressure to the cutter and slack off on the work string. When the new blades get to the cut, they will expand into it and you will not be able to slack off any deeper, adjust your weight, and begin rotating.
I would like to caution everyone about cutting casing at a shallow depth. Sometimes there is gas trapped between the different strings of casing when the cut is made. You have no way of controlling this gas. A blow out, as well as a fire, could occur. Several men have lost their lives this way. Before making a cut, be sure that the valves between the casings are open. I have seen them plugged with cement, and/or Barite, and they still hold pressure. So do not accept the fact that you are safe because the valve has been opened. Run a water hose in the valve to be sure that is open. As a rule, you will get a surge of fluid inside or outside the casing or pipe when the cut is made. So it is a good idea to have someone watch for this.
To retract a Mechanical Inside Cutter, you stop the rotary and pick up, it will automatically retract.
To retract a Pressure Operated Inside Cutter, you stop the pump and it retracts. It is possible that cuttings could be under the blades or knives and cause some trouble. This happens more often on the multi-string cutters. When it does, work the cutter up and down easily with just enough pump to circulate.