4.18 Determine the pressure at the bottom of the drill collars if the frictional loss in the drilistring is 900 psi, the flow rate is 350 gal/min, the mud density is 10-lbm/gal, and the well! depth is 8,000 ft. The in- ternal diameter of the drill collars is 2.75 in., and the pressure developed by the pump is 2,600 psig. Answer: 5,860 psig.