Pump Operating Cost 



Little data are published relating pump-operating cost to horsepower output. 

 In field tests, however, useful data have been compiled. For a pump in the 250- 

 horsepower class, suitable for conventional drilling to 5000 feet, the daily main- 

 tenance cost for expendable parts was 21 dollars. A 400-horsepower pump used 

 for jet-bit drilling to approximately the same depth operates for $30-per-day, 

 representing an increase of $9-per-day for the jet-bit operation. Major overhaul 

 costs are in addition to these daily maintenance costs ; and since no exact figures 

 are available, they must be estimated. It is reasonable to use a major overhaul 

 cost of $5-per-operating-day for the 250-horsepower pump, whereas $8-per-day 

 is estimated for the 400-horsepower pump. These costs are in the same ratio as 

 the horsepower of the pumps, and they show an additional $3-per-day expense 

 when jet bits are used. The sum of the expendable parts cost and the major over- 

 haul cost indicates additional costs of $12 for jet-bit drilling. 



Prime-Mover Operating Cost 



The larger pumps required for jet-bit operation necessitate additional horse- 

 power in the prime mover. For this example, it is assumed that power is supplied 

 by a single diesel engine or group of diesel engines compounded. The additional 

 operating costs resulting from the additional horsepower will be due to increased 

 diesel-fuel and lubricating-oil consumption and to increased overhaul costs. 



The present-day cost of diesel fuel is approximately one cent per brake horse- 

 power hour. Normally, the pump will be in operation 18 hours in each 24-hour 

 day, and the fuel cost would thus be $45-per-day for the 250-horsepower prime 

 mover, and $72-per-day for the 400-horsepower engine, an additional $27-per-day 

 cost for jet-bit operation. Additional lubricating-oil cost is estimated at $3-per-day 

 and major overhaul cost will approximate an additional $3-per-day, making a 

 total operating cost disadvantage of $33 for the jet bits. 



Miscellaneous Costs 



As a result of higher pumping pressure for jet bits, higher operating costs 

 will be realized for the mud lines and valves, rotary hose, and swivel. In the 

 absence of precise figures, $5-per-day additional cost is estimated. 



Depreciation 



The foregoing discussion has been concerned only with direct operating 

 costs. In addition, there must be higher depreciation rates charged against a rig 

 operating jet bits due to the increased capital investment in a larger pump and 

 prime mover. This capital investment can be determined by using cost figures 

 of $65 per installed horsepower for the pump, and $50 per installed horsepower 



661 



