Lap-weld casing Fig. 30, p. 82) is made by welding the beveled edges of the steel or iron sheet which overlap. This is the type of casing most generally used.
Butt-weld tubes (see Fig. 30, p. 82) are made by welding the two sides of the sheet by direct contact. One-inch, two-inch and three-inch sizes are the general sizes for butt-weld pipe.
Seamless casing is made without any seams at all by pulling the metal through dies. Seamless casing is used principally for drill pipe with the rotary system.
The strength of welded pipe depends, of course, upon clean welding. The weakest points will be in the seams. If the weld has not been clean the pipe is weak. Seamless pipe does away that danger, of course.
Couplings.—Casing is threaded at both ends. Collars are used to couple the joints (see Fig. 30). The casing and the col lars have beveled threadg, which gives the greatest resistance to stripping.
Casing Shoe.—It was found that the bottom of the casing was liable to be bent or crushed when passing through hard beds, and also that where the casing was driven, the bottom would collapse. To overcome this difficulty, the casing shoe was invented. This is nothing more than an extra heavy and long steel collar, which is screwed on the bottom joint of a string of casing. It is beveled on the lower end. (See Fig. 30, p. 82.) With rotary pipe the shoe sometimes has a notched edge, with cutting teeth, so that, if desired, it may have a cutting action when the pipe is rotated.
Handling of Casing.—Skillful manipulation of a heavy string of casing requires careful attention.
A string of 10 in. casing 3000 ft. in length weighs 73 tons. The weight of such a heavy string sometimes causes the derrick to collapse. It requires at least nine to eleven casing lines to handle such a weight readily. The force required to move such a weight applied to the cable is roughly 7 to 8 tons.
The casing is handled in units or joints about 20 to 21 ft. in length. This casing is put in the hole a joint at a time.
Procedure.—A joint /Of casing is pulled into the derrick by a rope sling attached to the end of the big hook on which the elevators are also hung. The elevators are then set on the joint and it is lowered into the cellar.
The first joint of casing is held in a spider by four slips. The spider rests on a pair of sills (see Fig. 33, page 89) which are laid on the floor of the cellar. As each new joint is added the slips are pulled free and the upper joints are lowered into the hole until the top joint of casing is flush with the derrick floor. The slips are then put in again and the casing lowered on them.
One man of the crew stands on the roof of the derrick or on a cross board and balances each new joint of casing so that it will not bind while being coupled together.
This process is repeated until the complete string of casing is in the hole. Then other operations are resumed.
Care must be exercised that casing of proper diameter is used. In one case in Wyoming casing was used when 6%-in, was needed. The next string ordered was 5%-in., which would not pass through a 6%-in. casing as the outer diameter of the collars is 63-in. Such mistakes are expensive.
Driving Casing.—Sometimes it becomes necessary to drive casing. When that is done a pair of drive clamps is fastened to the auger item. These clamps (see Fig, 32a) are grooved pieces of iron bolted together around the stem. The tools are dropped upon the drive head (see Fig. 32b) which fits over the top collar of the casing, and protects the threads. There is a grooved slot in the drive head into which the collar of the casing fits. The driving is done by means of the jerk line as in the , "spudding" operation on page 50 and shown in Fig. 10, page 52.
Amount of Casing.—Some idea of the amount of casing used in , deep wells may be obtained from the following tables.
Pulling Excess Casing.—After an oil well is finished it is found in many instances that some of the casing in the hole can be withdrawn to advantage. For instance, if there are 12-in. 10-in., 8%-in. and 6%-in. strings and the 10-in. merely prevents caving of the formations during drilling operations, it is advisable to pull the 10-in. string and in some cases pull the leaving the 8%-in. to shut off water, and the 6%-in. casing as the oil-string.