General Description
There are several manufacturers of Outside External Cutters, but there are only two basic types. The Mechanical Cutter and the Pump Pressure Operated Cutter.
Some Mechanical External Cutters are automatic spring-fed cutters; others are positive feed. The spring-fed feature prevents excessive strain being applied to the knives, which could cause the knives to burn or break before the cut is made.
The Pressure Operated External Cutter is a hydraulic actuated mechanical tubing drill pipe cutter. The knives are activated by pump pressure.
Use
Outside External Cutters are used in conjunction with washover pipe, to go over and cut pipe from the outside.
Construction
The Mechanical Outside Cutter has many parts. The outer body is usually made up of three parts or more; the top sub, middle body, and guide. Inside the cutter, there are the flipper dogs or spring dogs or slips, bearings, bearing rings, pusher sleeve, tension (or feed spring), knife wedge block, knives and knife pins. These are just some parts you will hear in relation to the outside cutter. It is very important that you learn the names of each part, the location of each, and it’s function in relation to the other parts. It is very important to understand the mechanics of each tool, as well as how to operate it, because the cutter often must be redressed in the field.
The Pressure Operated External Cutter is fairly simple and is used basically to cut slimhole or flush pipe. It is composed of a top sub, body, guide, a set of knives, and segmented piston assembly. A pair of shear pins holds the piston in running-in position until they are sheared by pump pressure, to begin a cut.
The Piston Assembly is composed of a feed ring and a set of interlocking, conical piston segments. The segments are held in contracted position by a tough rubber ring stretched around the upper ends of the segments in a suitable groove. Each piston segment has a fluid passage that is engineered to simultaneously pass fluid for flushing and cooling, while restricting the fluid flow sufficient to have ample pump down force to feed the knives to effect the cut.
Operation
We will discuss the Mechanical Cutter first. You should never run a cutter in the hole without breaking it down far enough to be sure each part is in the right place and working properly. When you get over the fish, run the cutter slowly until you reach the desired depth you plan to cut. Begin rotating and rotate several minutes to let the pipe relax. You can circulate while doing this. When you think the pipe is relaxed, kick rotary out, and pick up the cutter slowly until you pull under a collar or tool joint. This is where the skill of the man will be tested. A good rule of thumb when using a cutter with spring dogs or slip dogs is to pick the weight of the fish you expect to cut plus one thousand pounds. For example, if you were cutting 500 feet of pipe that weighs 6.7 lbs per foot, you would have 3,350 lbs (500 x 6.7), plus 1,000 lbs. You would pick up 4,350 lbs. If you have a pump that is pulsating badly, then do not use the pump, for this would increase or decrease the weight at the cutter; therefore, moving the knives up and down the fish, causing them to dig into the fish.
Forty to sixty RPM of rotary is a good cutting speed, but do not keep trying to adjust the rotary. You should be able to determine how to set the rotary while you are rotating prior to picking up under the collar or tool joint. Some rotaries have a Jack Rabbit start. If you have one like this, then load the rotary before beginning to rotate. You can do this son some rigs by killing one engine. Some may have to let the remaining engine dread the one you killed. Another solution may be to put a pump on the mud guns. However, some you may just have to find your own way. This is the work of a good fishing tool hand. If the cutter you are using is dressed properly and working properly, and you have thought everything out to your advantage, then you should cut any nominal size and weight pipe in less than five minutes. When you use the method above with the weight of the fish you are cutting plus 1,000 lbs, you should notice on the weight indicator when the cut is made, for you will loose the 1,000 lbs.
When you run a cutter with flipper dogs, you are using very weak cutters, and one that should be handled with great care. Pull under the tool joint, and then rotate very carefully. You may have to load the draw works or kill one or two engines to be able to pick up and rotate like you want. If everything works well after about five or ten minutes of rotating, pick up on the pipe about one inch while still rotating. Do this every five or ten minutes until you have raised the pipe four or five inches. If you don’t gain weight, you should have a cut made. If you have succeeded in making the cut, pick on up and come out of the hole. Always caution the rig crew about jerking the pipe off the slips or setting down real hard on the slips. When pulling a cut out of the hole, it should be handled very smoothly.
Some Mechanical Cutters are designed to be released in the event that you miss a cut. Others you tear up the slips or dogs to pull them out of the hole. I suggest you learn which one releases and how to release them as soon as possible.
The Pressure Operated Outside Cutter is run in the hole to the desired depth, rotation is then established, giving the pipe time to relax, and the jars time to open. Check for back torque so you will know how much it takes to turn the cutter. When all this has been achieved, adjust the rotary RPM to approximately sixty RPM. Put pump on and level the pressure out as soon as possible. Any fluctuation in pressure will cause the cutter to move up and down the fish and highly increase the chances of breaking the knives. Any pressure you level out at, between 800 and 1,200 lbs. is satisfactory to operate the cutter. I will repeat, Get the pressure needed as quickly as possible and do not try to adjust if afterwards. You should be able to cut the pipe in approximately 15 minutes. After 15 minutes of cutting, pick up one inch while rotating with the pump on. Rotate 5 minutes and pick up again the same way. After picking up four or five inches in this manner, and not having gained any weight, then a cut should be made. Pull out of the hole very carefully. The knives are all you have carrying the fish. Never circulate with a pressure-operated cutter before reaching the depth where you intend to make a cut. Never try to pick a cutter up over a collar or tool joint unless it is one designed to do so.
Some thought should be given to the job beyond just making a cut. If the tubing is plugged and you cut below the plug, do you have enough mud weight to hold the well? Sometimes the tubing will be plugged with sand or Barite. Caution should be taken to prevent a flash fire if it kicks after reaching the surface. Plans should be made ahead of time to be sure everything will go smoothly. When retrieving the fish from the washpipe, do you have a bowl and slips? Do you have a stripping plate? Do you have a clamp to secure the fish? Do you have a collar or tool joint on top to accommodate the elevators? Or do you have to make other plans? All these should be thought out in advance for two good reasons. First, to save time, and secondly, whether you have or have not thought these things out in advance will be a direct reflection on your ability to supervise a job.
In closing, I would like to mention again, don’t ever take it for granted that someone has dressed the cutter properly. Always check it for yourself first before ever using it.