In standard engineering practice a factor of safety of 5 to 1 is used to obtain the working load of a wire rope, but in drilling service the tensile strength of a line means little, for every drilling line is almost certain to be subjected at more or less frequent intervals to a load closely approximating its ultimate strength ; and since the elastic limit of steel is about 60% of its total strength the application of loads beyond this critical point, even though infrequent and of short duration, will tend to change the character of the steel and shorten its life, which would otherwise be determined by the normal condi tions of abrasion, etc.
No set rule obtains for deciding the proper size of line for any particular well or drilling conditions and operators follow individ ual tastes as to the one best suited to their needs. For fairly light work the 34 in. and in. are in common use. Deeper drilling and heavier tools require a 1-in, line, and recently considerable atten tion has been given to a study of the economic advantage of using extremely heavy tools and a line, under drilling conditions of such a nature that the time-factor and saving in labor-cost war rant the added expense of these heavier materials. Neither is it possible to state, except within very broad limits, the amount of drilling that may be expected of a line. Under favorable condi tions a light line may serve for the drilling of several 1000-ft. holes, while a heavier line in ground that is more severe on it may be come worn out in a few hundred feet of drilling. Fishing for lost tools and jarring on casing with a spear are especially trying, and a line deteriorates rapidly in such work.
Lines are shipped from the mills on heavy reels and when re ceived at the well are prepared for unwinding by placing a pipe through a centre opening in the reel and blocking up the end of this pipe so that the reel may turn on it. One end of the line is pulled up over its pulley in the crown block, then down and fas tened to the bull-wheel shaft and the line wound on the shaft by engine power. A space about 30 in. long at the centre of the shaft, with a frame built up at each end, is used to spool that part of the line in immediate use, the remainder being carried at one end of the shaft, with left-lay lines preferably at the end opposite the brake-band.
The practice of uncoiling a line from the shipping reel by plac ing the latter on its side and driving a stake in the ground to hold it in place while being turned places an undue strain on the line by reason of the tendency to kink, and should not be permitted.
Particular care should be taken when handling lines to prevent kinks by using as large snatch blocks as possible. Frequently lines are moved from one rig to another, not by coiling on reels and hauling them, but by pulling one end of the line to the new rig and coiling it directly from one shaft to the other. Unless pains are taken to prevent it the line may not kink but will `dog-leg,' that is, suffer a small sharp bend. In such a case the line at this point never becomes absolutely straight ; and it soon weakens from wearing on the side of the casing or hole and must be cut and spliced. The splice usually employed with drilling lines is that known as the 'blind' splice, in which the strands of each end of the line are opened for about 15 ft., the hemp core extracted and the strands woven together again, with one of the strands taking the place of the core.
In some fields a unique combination of wire and manila lines has been found very successful for drilling. It is known as the `cracker' line and consists of about 100 ft. of manila rope spliced on the end of a wire line nearest the drilling tools. In this way the benefit of the spring and stretch in the manila rope is obtained without the expense of running a line composed wholly of such rope, with the further advantage that it may be used in a 'wet' hole.
Casing-lines in almost all cases are standard hoisting ropes of cast steel wire, composed of 6 strands of 19 wires, right lay, with a hemp centre.
All the above sizes find use in different districts and it is probable that the factor of safety of 5 to 1 is rarely exceeded. The N and 1-in. sizes of this type are also used as hoisting ropes at rotary wells. After they have become worn so that they are unsafe for pulling casing they are used for tubing lines, for handling tubing and sucker-rods in producing wells.
Sand lines are identical with the standard coarse laid, transmis sion, or haulage rope. Like casing lines they are of cast steel wire, right lay, but differ from them in being composed of 6 strands of 7 wires each. They differ in construction because they are not sub jected to short bends, but do meet considerable abrasion while trav eling in and out of the hole, and the smaller number of coarser wires gives a longer life to the line and a lower first cost.