Screwed pipe is manufactured by rolling the ingots of metal into slabs and rolling the slabs again into plates of the proper length, thickness and width according to the size of pipe desired. The plates, known as `skelp,' are then bent to circular form and welded.
In the latter stage, two different processes are followed by which are made either the lap-weld or.the butt-weld pipe. The butt-weld is made by placing the two edges together as shown in Fig. 58; in the lap weld, before the skelp is bent the edges are scarfed so that when they are overlapped a much larger welding surface is obtained than with the butt-weld and a stronger bond insured at the weld. For this reason little butt-weld pipe is used for casing, although all ordinary low-pressure line-pipe for surface lines is made by this process.
Each size of an accepted standard weight, and when stronger and thicker pipe of this size is made for heavier duty, the additional metal is placed on the inside, reducing the actual inside diameter but retaining the same-outside measurements. Thus the so-called 6%-in. casings weighing 20, 24, 26 and 28 lb. per ft. all have the same outside diameter of 6.625 in., but iinernal diameters of 6.049, 5.921, 5.855 and 5.791 respectively. Permissible variations are 5% above and below the rated dimensions. The casing comes from the mills in random lengths ranging around 20 ft., and one make may also be obtained in lengths of 35 and 40 feet. These long joints are thought to be an advantage in reducing the friction of cavings against the collars, but the inconvenience in handling them has rather retarded their adoption.
Since it is desired, when more thari one string of casing is neces sary to finish a well, to reduce the bore of the hole as little as pos sible, a sequence of sizes is used so that one string will barely pass inside the next larger without unnecessary friction. The usual practice with both riveted pipe and screw casing is to use sizes that result in al loss of approximately 2 in. with each succeeding string. Wells using the larger sizes of riveted pipe may contain strings of 24, 22, 20 in., etc., and those with screw casing may have 10, 8%, in., etc. In many cases a combination of the two may be em
ployed so that a casing record shows 18 and 16-in. stovepipe, with 10 and 8%-in. screw casing; or screw casing; 13-in. stovepipe, and 10 and 8J4-in. screw-pipe, all depending on the drill ing conditions and personal preferences of the operators.
An idea of the range of sizes and weights of screw casing made may be obtained from the following table showing those manufac tured by one firm.* Several different kinds of screw casing are made for well work and the various forms differ somewhat in the sizes of collars, num ber of threads to the inch, etc. While the threads on ordinary line pipe in the sizes over 2Y2 in. nearly always number eight to the inch, this number has been found to take too much stock from the pipe at the threads to sustain the enormous weights of long strings of heavy casing, and 9, 10, and 14 threads have all been tried. The 11% and 14 thread cuts have been found to be so small that they permit the pipe to pull apart quite easily and present practice seems to have dropped back to the 10 thread for the greater portion of casing now made.
As a rule, the collar thread does not start at the end of the col lar, but begins from the end of a recess cut so that when the pipe has been screwed together the end of the collar fits snugly over the pipe and increases the rigidity of the completed string. The length of thread is usually from 3 to 33z in., with sufficient taper to insure a tight bond with the collar. The space inside the collar between the two ends is customarily from to in. after the joints have been screwed together. Pipe that is to be subjected to exception ally heavy driving is made so that the ends of the joints meet, and is known as 'drive pipe' (Fig. 59). Usually these threads have no taper and are cut coarser than the 10 thread of ordinary casing since butting the ends relieves the couplings of much of the strain. Drive pipe has small value for use where the ground caves into the hole to any extent, as after it has been driven severely it becomes weakened at the threads and pulls apart readily when a strong pull is applied.