A form of accident liable to occur when drilling with the rotary tools and which may develop into serious difficulties, is that which arises from dirt binding the drill stem, either through the unexpected heav ing of sand when a gas-stratum is encountered, or through the sides of the hole caving in. The simplest way out of trouble of this kind is to run an overshot on a string of pipe that is large enough to pass over the lost drill-pipe. The overshot is preceded by a rotary casing-shoe and the circulating fluid is pumped down inside' the larger string, removing the caved material as fast as it is loosened by slowly turning the shoe. In this way the caved ground is cleaned out and when the larger string is withdrawn the overshot pulls the drill-pipe with it. But in many such cases the small space betwee the lost pipe and the fishing-string, and be tween the latter and the side of the hole, hinders the free circulation of mud and not infrequently causes the fishing-string itself to become frozen, thus complicating matters still further.
For this reason it is generally considered preferable, although requiring more time, to recover the lost pipe in single joints, by un screwing them. The fishing string is left
hand-thread pipe and the tool run on the bottom of it is a wash-down spear (Fig. 216), with a circular slip or with two ordi nary bulldog tubing-slips. In addition to these, and the opening for the passage of the' circulating fluid, it is equipped with another slip, which is moved horizontally by a spring, the duty of which is to grip the inside of the lost pipe when the fishing-string is turned to the left. When the body of the spear has entered the top of the lost pipe, the is turned to the left until one or more joints of the lost pipe have been unscrewed. The fishing-string is then with drawn, pulling with it, by means of the vertical slips, the un screwed sections.
If the well is remote from where left-hand pipe may be ob tained, the ordinary pipe may be used by boring a hole through the coupling and pipe at each point where the two come together and inserting pins in these openings when the pipe is being run into the hole. The pins thus prevent the pipe from unscrewing when the left-hand turn is given it in unscrewing the lost pipe.