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Pipe cutoff methods
Tubing cutoff is important during any action that requires severing the tubing. The most common pipe cutoff methods involve either explosive or chemical cutters.
Tubing cutoff is important during any action that requires severing the tubing, including the following operations:
- Salvage operations
- Fishing operations
- Certain production operations
The most common pipe cutoff methods involve either explosive or chemical cutters. Explosive cutters use the same explosive technology used in perforating charges. Instead of a cylindrical cone, however, the explosive and the liner are arranged in a wedge so that the explosive front of the device will push out on all sides and sever the pipe. Although the technique is effective in most cases, the external part of the pipe is left with a flare that is often difficult to wash over during pipe recovery operations. Newer explosive cutters have largely reduced this flare to an acceptable level. Fig. 1 shows a flare produced after an explosive cutoff.
Chemical cutting has become one of the most common pipe cutoff methods, especially for tubing. The cutting fluid reacts extremely quickly and generates intense heat. It is sprayed through a nozzle assembly at the walls of the tubing all around the cutoff tool. As the fluid contacts the steel wall, a vigorous reaction occurs and the pipe is separated smoothly without leaving an external flare. Fig. 2 shows an example of a chemical cut pipe.
Chemical cutters can produce very smooth cuts but are very dependent on both orientation and even coverage or contact between the cutting chemical and the steel pipe. The following conditions can either slow the chemical cut on that side of the pipe or defeat it entirely so that pulling operations are needed to finally separate the pipe:
- Heavy walled pipe
- Higher alloy
- Increased depth
- Imperfections in the pipe
- Plastic liner
- Incorrect gun sizing
Fig. 3 shows an example of a partial chemical cut.
Radial explosive cutters
Radial explosive cutters, either continuous or segmented cutters, produce a pressure wave that is oriented outward and usually produce a flare in the steel at the cut point. Fig. 4 shows an example of a cut produced by a Thermite cutter. The severity of this flare can provide problems in recovering the pipe or in washing over the stuck section. A mill is often run to dress off the upward-looking connection before running the wash pipe.
Mechanical cutters based on mill design have been used successfully on both jointed and coiled tubing applications to sever pipe. These cutters are considerably slower than the chemical or explosive cutters but can be run on conventional equipment. The mechanical cutters are best used on softer, lower alloy pipes with a thinner wall. High alloy pipes and very thick pipes are more difficult to cut with a mechanical cutter.
Abrasive cutters have been reintroduced recently to the market and have the potential to rapidly sever almost any type of pipe at any depth. These cutters use a particulate such as the following:
- Glass beads
- Calcium carbonate
The particulate is pumped through a rotating nozzle, and the abrasion erodes the steel. Cuts through even heavy-walled drillpipe are possible if the cutter can be kept in the same place during the entire cutting operation. Cuts at surface with abrasive cutters are very fast; however, the cutting process is slowed because of backpressure when the cutters are applied downhole. Nonetheless, these cutters are beginning to see extensive use as pipe cutoff tools.
The cutting system necessary for a particular application depends on the well depth, temperature, and size of the tubing and alloy grade and weight of the tubing. However, the most important factor is any restriction above the cut point and the ability to pull tension on the pipe. Requirements for cutting tubing include knowledge of the specific design of the well and any restrictions above the point to be cut. Once the cut point is selected, the cutting method should be studied carefully to determine if a clean cut can be made that will requiring a minimum of overpull to separate the uncut sections of the pipe. Additional considerations include the conveyance system and the manner of depth control that will place the cutter at the correct position.
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