Inserted-joint casing (Fig. 60) is sometimes placed in a hole where a small reduction of bore is desired rather than the greater strength of coupled pipe. It is made by swelling out one end of the joint and cutting this with an inside thread so that it screws over the outside thread end. The threads are usually 1172 to the inch.
As with riveted pipe, a steel shoe is placed on the lower end of the first joint in a string of casing (Fig. 61), and having an outside diameter slightly greater than that of the couplings so that the beveled cutting-edge insures a path large enough for the passage of the pipe and couplings (Fig. 62). The Baker shoe (Fig. 63) is made with a number of open spaces in the cutting end, and is a material improvement where conditions are such that the pipe is to be worked down through hard ground. When strings of casing are to be inserted in holes already drilled by the rotary method, a type of shoe having a saw-toothed end is frequently used. Any slight projections from the side of the hole encountered while low ering it are cut away by turning the pipe and milling off the irregu larities with the shoe.
All casing is presumably tested at the mill before shipping and is supposed to stand the internal test-pressure marked on the pipe. It is rarely, however, that pressure from the inside is at all im portant in well drilling operations, although the external or col lapsing pressure is often of vital importance. The most severe strain of this nature comes, after the water has been excluded by cementing or otherwise, when the well is hailed dry on the inside for the purpose of learning whether or not the attempt to shut off the superficial water was successful. The collapsing pressure ex erted against the pipe at this time is represented by the difference between the heights at which the fluids stand on the outside and the inside.
The following table* has been computed, from data determined by a great number of artificial tests on the collapsing pressure of casing, for the purpose of supplying an approximate idea as to the limit of depths to which casing may safely be carried under a factor of safety of 2, which while small yet seems to be warranted by the results of actual experience in the fields.