Knives for punching a number of apertures, through which oil may gain admittance to the inside of the casing, are so made that they cut a rectangular hole of the desired size. Those for ripping the pipe or a coupling have a cutting edge similar to the rounded blade of an ordinary knife (I) Fig. 202) so that when the knife has once made an incision it continues to rip the pipe as long as forced down by the weight of the tubing, or the jarring of the tools.
It is not uncommon for casing to part of its own accord at some point in the hole. This may result from the great strain of the weight of a long string, from the pull applied when trying to loosen a frozen string, or because of defective threads. Pipe rarely parts at the middle of a joint, the threaded portion directly where it enters the coupling appearing to be the most liable to break. Some styles of elevators, particularly when they have become worn, tend to pinch the casing directly below the coupling and weaken the bond between the pipe and the coupling at the thread. Such an injury to the pipe may not be noticeable at the time it is inserted and the weakened joint may be several hundred feet from the surface before an especially great strain is placed on the casing, causing it to part at this point.
The remedy in these cases is to withdraw the upper portion of the string and place on the bottom of it a steel die-nipple (Fig. 203) by means of which a thread may be cut on the top of the lost portion. The threaded parts of a die-nipple are usually 5 or 6 in. in length, with a slight taper and are grooved or fluted transversely to the direction of the thread in order to permit the steel cuttings to escape.
When the break occurs at the lower end of a coupling, all that is necessary is to run in the die-nipple and turn the casing until a sufficient thread has been cut on the outside of the lost pipe to insure a bond with the threads of the die-nipple. If the break is at the top of a coupling, leaving it in the hole, it may be that the outside threaded end of the die-nipple can be screwed into it; but unless the coupling is unusually long, enough threads cannot be cut to secure a tight hold and it is a more common practice to cut the pipe with a casing-cutter a short distance below the coupling and bring the loose piece holding the coupling out with the cutter when it is withdrawn. This leaves a cleaned end of the pipe ex
posed, over which the inside threaded end of the die-nipple may be screwed.
Some operators prefer to use, instead of a die-nipple, a casing bowl. The bowl, especially when equipped with two sets of slips (Fig. 173), supplies a much stronger hold on the lost pipe, and effects a saving in time, since the pipe need not be withdrawn for the removal of the bowl unless it is the string that is to exclude water from the oil-sand. When a die-nipple has been used to join the two ends, it is safer to pull the pipe and remove the nipple • and defective joint.
Another accident to which casing is subject is that of collapsing, either because of the pressure exerted against it by the column of water on the outside when it has been bailed dry, or through a rock or boulder falling in and grinding against the side. In the latter case, as the well is deepened and the pipe lowered the boulder becomes wedged between the wall of the hole and the pipe, directly below a coupling, forcing a portion of the 'pipeinward so that the tools or bailer are prevented from passing through at this point. Under ordinary circumstances the pipe may be pulled from the well and the damaged joint removed from the string. But when the string of casing has been landed and cannot be withdrawn, or the depression is only a slight one, a swage (Fig. 204) is run in on the drilling-tools and worked tip and down until it has forced back the pipe to its original position. Water-courses are provided by fluted channels diagonally along the side.
Another form of swage contains a hole bored diagonally from the bottom to a point on the side near the pin. Such a tool is necessary when the drilling-tools have become imprisoned by a collapse in the pipe that has occurred while the tools were in the hole. If it is deemed inadvisable to cut the drilling-line above the • weak place in the pipe, a new line is strung and the swage and a second string of tools are lowered in the hole, the swage passing down around the first line by sliding it through the opening. in this way the lost line does not interfere with the action of the swage. A third form of swage contains a series of rollers at the circle of its widest diameter, for rendering the swaging action more effective.