drill string. This estimate is probably low in wells less than 8000 feet deep, but 

 may be quite high for wells more than 10,000 feet deep. A number of experi- 

 ments indicate that under present rotary methods only 10 to 25 horsepower is 

 actually utilized in drilling deep wells. The remaining power is consumed in 

 transmission. 



In turbine drilling, rotating power is generated just above the bit, with 

 the result that the bit is rotated with greater efficiency. Since it is not necessary 

 for the drill string to rotate, fatigue failure of the pipe is greatly reduced, and 

 wear is almost eliminated. The loss that occurs in transmitting the hydraulic 

 power to the turbine is caused by drill-pipe pressure drop, which can be ade- 

 quately controlled through selection of the proper size of drill pipe. It can be 

 easily recognized that turbine drilling permits more power to be applied to the 

 bit with greater efficiency. 



For example, a turbine and hydraulic system may have an efficiency of 

 60 percent, and this system is powered by a mud pump having an efficiency of 

 70 percent. If 300 horsepower is applied to the mud pump, the power available 

 at the bit is 126 horsepower in turbine drilling. Since approximately only 10 

 percent of the power reaches the bit in a conventional rotary system, 300 horse- 

 power applied by the rotary table will provide only 30 horsepower to the bit. 

 If the bit can use the power efficiently, the turbodrill would penetrate the forma- 

 tion at least 4.2 times faster than the conventional rotary. Power output applied 

 to the bit by the turbodrill is reported to be 3 to 5 times the power applied by 

 rotary. 



In drilling with the turbodrill, the Russians usually drill a larger-diameter 

 hole (12i4"±) than is customary with rotary (8^4"±) in the U. S. Thus the 

 power per square inch of hole is not a direct ratio of the power applied to the 

 bit, and a comparison of probable penetration rates based on power available 

 at the bit may be misleading. For example, using rotary at a 10,000-foot depth 

 with 4i/9-inch drill pipe, and a 7i/2-inch bit turning at 125 revolutions per 

 minute, the Russians reported that the power transmitted to the bit was 40 

 horsepower or about 1.1 horsepower per square inch of hole. A turbodrill, using 

 a 6%-inch drill pipe, a 12^/2-inch bit and a pump capacity of 800 gallons per 

 minute, transmitted to the bit 200 horsepower or about 1.7 horsepower per 

 square inch of hole bottom. One report comparing the drilling rates of turbo- 

 drill and rotary showed an average increase of 11/2 to 2 times the penetration 

 capacity with turbine drilling under similar conditions. 



TURBODRILL The design of the turbodrill is relatively 



CONSTRUCTION simple. Only five basic components make up 



the unit (fig. 31-2) : (1) the body or housing, 

 (2) the thrust bearing, (3) the turbine, (4) the lower bearing, and (5) the bit. 



668 



