In the turbine body are 80 to 100 turbine stages, each composed of a fixed 

 stator mounted in the main body. Each stator has blades to direct the fluid 

 toward similar curved blades of the rotors on the rotating shaft. The fluid is 

 forced through the turbine blading and imparts reaction torques on the blade of 

 each stator and rotor. The fluid also lubricates and cools the bearings. The 

 cumulative effect of the torque reaction created by all the stator-rotor stages 

 in the turbine drives the bit. 



The pressure drop of the liquid as it passes through the turbine generates 

 two useful forces: the driving torque and the axial thrust toward the bit. 

 Because all stator-rotor stages in the turbine are identical, the pressure drop 

 across each stage is always the same. For instance, a turbine with a pressure 

 drop of 6 pounds per square inch per stage would have a total pressure drop 

 of 600 pounds per square inch in a 100-stage unit. 



Unlike previous experimental models, the modern turbodrill has sufficient 

 power and torque to drive the bit. Power of the turbodrill now ranges up to 

 300 horsepower; bit speeds are from 500 to 900 revolutions per minute with 

 bit loads of 20 to 35 tons. The running torques which are on the order of 2000 to 

 3000 pound feet, are sufficient to rotate the bit under all normal drilling condi- 

 tions. 



TURBODRILL The operation of the turbodrill differs from 



OPERATION the turbine in a hydroelectric plant in one 



way: the turbodrill operates at a constant 

 flow and variable speed, whereas the turbines in hydroelectric plants work at 

 variable flow and constant speed. 



Usually the turbodrill requires no more pressure for its mud circulation 

 than can be supplied by a rotary type rig equipped for jet drilling. Since the 

 fluid pressure supplies the power in turbodrilling, all details of the mud lines 

 must be studied with extreme care. 



Where the formation permits, the specific gravity of circulating fluid used 

 to operate the turbine should be nearly that of water in order to enable the 

 pumps to supply high pressures and flows and to reduce both turbine and pump 

 maintenance. Although turbines have been used successfully with mud having 

 a density of 1.6 to 1.8, fluid with a density and viscosity nearly that of water — 

 helps reduce bearing friction — often as much as 50 percent — and thereby re- 

 leases considerable additional turbine power to the bit. 



A method of obtaining even greater torques than are possible with one 

 turbodrill is to use several turbodrills coupled together. They then cumulate 

 torque and increase the axial thrust due to high pressure drop. Where a single 

 10-inch turbine gives an 18-ton axial thrust, the double turbine gives up to 25 

 tons for an equal flow of fluid. 



670 



