Lifting Blocks.--The casing lines run through a block which carries the traveling pulleys. The big hook hangs from this block, and upon this hook the bails of the casing elevators are hung. The block may have 3 or 4 pulleys depending upon the size.
Elevators.—Elevators are classed as: 1. Casing, 2. Tubing, 3. Rod.
The working principle is similar for all three types. The tubing and rod elevators are the lightest. A good type of elevator is the Fair. (See Fig. 13.) This elevator consists pri marily of two wings or jaws that are latched around the casing. The inside diameter of these jaws is just a little larger than the casing. Bails are attached to the sides of the jaws and these fit over the casing hook. The elevators when in use are latched around the casing and slide up the joint until they catch under the casing collar, which furnishes a lifting medium.
Several other types of elevators are used, namely the Scott, which has the bail or link locking over the nose of the elevator. (See Fig. 13b.) Another type of elevator is used with slips. This elevator has a single bail which carries a solid spider. This spider fits over the top of the casing, and slips are put in to lock the elevator. This is excellent for heavy pulls,
but has little value where it is desirable to handle casing rapidly.
The Fair elevator is the simplest and quickest to operate.
Underreamers.—Underreamers are used to enlarge a hole in special cases where the ordinary bit can no longer be used.
If for any reason the hole has been cased to a certain depth, and it is necessary to carry the casing deeper, a bit of diameter small enough to go inside the casing must be used. The hole is drilled as far as necessary with a smaller bit, and then the under reamer is used.
This underreamer, really an expan sion bit (see Fig. 14, page 57) has two lugs, or cutters, that can be con tracted to fit the casing. When the tool emerges from the bottom of the casing, the lugs, driven by a powerful spring, expand outward. Drilling with underreamers enlarges the diameter of the hole so the casing may be lowered. A 10-in. under reamer will cut a 13%-in. hole, allow ing 10-in. casing, and a casing-shoe, 12 in. over all, to pass freely down ward in the hole.
The underreamer lugs when pulled upward are contracted as they hit the inside of the casing-shoe and are then raised to the surface.