Bull-ropes have diameters of 2 to 23. in., and weigh 1%0 to 2% lb. per foot.
Steel-drilling cables are of the common, 7-ply type with 19 strands and a center. (See Fig. 28.) They have a left hand lay and will stand the following strains. See Table 6.
Sheaves.—The main or crown sheave (see Fig. 29, page 80) over which the drilling cable passes, the casing sheave, and the sand-line sheave over which the sand line passes differ to corre spond with the demands on them. The crown sheave is a heavy pulley of a diameter which permits rapid revolving with a large surface exposed to cooling and a minimum of flexing strains. A crown pulley with a diameter of 26 in. must stand a speed of over 1000 ft. per minute. This means for a 6.8 ft. circumference nearly 150 r.p.m.
The casing sheaves are of smaller diameter as they revolve at a slower rate of speed. The casing sheaves are seldom used for speeds over 50 r.p.m.
Sand-line sheaves must stand high speeds of 150 to 200 r.p.m. The weight on them is light, however, and the line used is in. in diameter, so that the sheave is of light construction.
The main problem with pulleys is the lubrication of the pulley grooves. The sheaves are set in hardwood grooves which are kept well greased with a good heavy axle grease. Where steel-crown blocks are used the metal seats are kept well greased. Many fires in derricks have started from ungreased pulleys on wooden-pulley grooves. It is one of the toolies' duties to see that the pulleys are greased.
Casing.—Casing is used primarily as a lining for producing oil and gas wells and as an auxiliary in drilling. Casing consists principally of heavy stvel tubes, though occasionally some iron tubes and in rare instances wooden tubes are employed. It must be clearly borne in mind that the amount of casing is governed by the number of caving shales and sands encountered, by the water sands, and also by accidents in drilling.
The exclusion of water from oil and gas sands and the "shutting off" of dry sands that might absorb the oil or gas are made possible by the use of casing, and also by the use of mud-fluid. The
casing acts as a lining for the hole and keeps back the cavings and the water, and also serves to keep the oil and gas within proper bounds.
Casing serves: 1. To exclude water, mud, or sand from the drill hole while drilling is progressing; 2. To exclude water or mud from oil and gas sands; 3. To separate the various oil and gas producing horizons from one another; 4. To confine oil or gas to its immediate producing horizon.
Casing to meet these requirements should be both gas- and water-tight. It should have sufficient resistance to external water pressure so that the casing will not collapse, and it must have sufficient tensile strength in the threads, so when suspended it will hold together.
Casing sizes must be selected to suit the varying diameters of holes.
The deeper the hole, the greater the water and the rock press ures encountered. Casing must then be correspondingly heavy, not only to withstand greater collapsing pressures, but also greater tensile strains, for as the casing is lowered and raised, greater strains are put upon it. In the past the use of light "hen skin" pipe has caused more lost holes than any other one cause.
Table 29 in the Appendix gives some idea as to the sizes, weights and strengths of casing used.
Casing is of four types.
1. Riveted.
2. Lap weld.
3. Butt weld.
4. Seamless.
Riveted or stove-pipe casing (see Fig. 30, p. 82) was formerly quite widely used for large-sized pipe at the top of the hole. The pipe came in flat sheets about 2 ft. long that were rolled to form cylinders which were then riveted at the well. Short sections were put in a hole until the desired length was secured, usually from 10 to 15 sections. Such pipe is used to hold back gravel and surface alluvium or to shut off surface water. The diameter of this riveted casing ranges from 12M to 20 in. Good practice now calls for real screw casing. Dimensions of 104, 18 and 20 in. are available.