Method and apparatus for drilling and casing a large diameter borehole

Claim 1

In a method of impact drilling and casing a large diameter borehole in overburden comprising: subjecting an annular bit body secured to the lower end of a double walled drill pipe consisting of an inner and an outer pipe in axial alignment with an annular passage therebetween and a central passage in said inner pipe to a steady axially directed force while transmitting impact energy to said bit body by a series of blows at a rate in excess of 500 foot pounds per blow per inch of borehole diameter on an anvil cap mounted at the upper end of the drill pipe, said annular bit body having radial cutting teeth and an axial bore communicating with the central passage in the drill pipe, passages in said teeth communicating with the annular passage of the double walled drill pipe, and opening into said axial bore; passing fluid down the annular passage of the double walled drill pipe and the passages in thE teeth into said axial bore and then passing said fluid and overburden fragments upwardly through the axial bore of the bit body and central passage of the drill pipe; the improvement comprising: a. casing the borehole from the upper end of the borehole to a point immediately above the cutting teeth of said bit body with a casing having an outside diameter equal to the diameter of said borehole and sealing the upper end of the borehole between the casing and the double walled drill pipe, b. passing liquid at relatively high pressure downwardly between the casing and the double walled drill pipe, c. continuing the passage of liquid at relatively high pressure between the cutting teeth and into the axial bore of the bit body, d. continuing the passage of fluid at relatively low pressure down the annular passage of the double walled drill pipe, the passages in the teeth and into the axial bore adjacent the said teeth, e. passing the fluid and liquid upwardly through the axial bore and central passage of the drill pipe thereby assisting the removal of overburden fragments upwardly through said axial bore and central passage, and f. subsequently withdrawing the double walled drill pipe and bit body from the borehole.

Claim 6

An apparatus for impact drilling and casing a large diameter borehole in overburden comprising: a. a double walled drill pipe having an outer diameter smaller than the diameter of the resulting borehole and capable of sustaining repeated axial blows of impact energy in excess of 500 foot pounds per inch of borehole diameter, said drill pipe consisting of an outer pipe with an inner pipe coaxially fitted therein and an annular passage therebetween and a central passage through said inner pipe, b. an annular bit secured to the lower end of the drill pipe having radial cutting teeth and an axial bore communicating with the central passage in the drill pipe, fluid passages in said teeth in communication with the annular passage of the drill pipe and opening into said axial bore, said annular bit having a diameter at the cutting edge of said radial cutting teeth substantially equivalent to the diameter of said borehole; c. a casing having an outside diameter equal to the diameter of said borehole and coaxially mounted over the drill pipe and drill bit from at least the upper end of said borehole to a point immediately above the cutting teeth of the bit; d. an anvil cap assembly mounted at the upper end of the drill pipe and casing, said assembly including an anvil for receiving impact energy and transmitting said energy to the drill pipe drill bit and casing, means for introducing fluid down the annular passage of the drill pipe, means for discharging the effluent of the central passage of the drill pipe from the apparatus and means for introducing liquid at a pressure greater than the pressure of the fluid in the drill pipe down the space between the drill pipe and the casing; e. a fluid tight seal between the anvil cap assembly and casing and a second fluid tight seal between said assembly and drill pipe whereby fluid introduced down the annular passage of the drill pipe is discharged into the axial bore of the bit via fluid passages in said cutting teeth and liquid introduCed down the space between the drill pipe and casing is discharged between the drill bit and the casing in an area immediately adjacent the teeth; f. a fluid tight seal between the means for discharging the effluent of the said central passage and the inner pipe whereby fluid in said axial bore, liquid from the space between the drill pipe and casing and borehole fragments from said borehole may be passed upwardly through said axial bore and central passage and discharged as effluent from the apparatus.

Claim 7

An apparatus for impact drilling and casing a large diameter borehole in overburden comprising: a. a double walled drill pipe having an outer diameter smaller than the diameter of the resulting borehole and capable of sustaining repeated axial blows of impact energy in excess of 500 foot pounds per inch of borehole diameter, said drill pipe consisting of an outer pipe with an inner pipe coaxially fitted therein and an annular passage therebetween and a central passage through said inner pipe, b. an annular bit secured to the lower end of the drill pipe having radial cutting teeth and an axial bore communicating with the central passage in the drill pipe, fluid passages in said teeth in communication with the annular passage of the drill pipe and opening into said axial bore, said annular bit having a diameter at the cutting edge of said radial cutting teeth substantially equivalent to the diameter of said borehole; c. a casing having an outside diameter equal to the diameter of said borehole and coaxially mounted over the drill pipe and drill bit from at least the upper end of said borehole to a point immediately above the cutting teeth of the bit; d. an anvil cap assembly mounted at the upper end of the drill pipe and casing, said assembly including an anvil for receiving impact energy and transmitting said energy to the drill pipe drill bit and casing, means for introducing fluid down the annular passage of the drill pipe, means for discharging the effluent of the central passage of the drill pipe from the apparatus and means for introducing liquid at a pressure greater than the pressure of the fluid in the drill pipe down the space between the drill pipe and the casing; e. a fluid tight seal between the anvil cap assembly and casing and a second fluid tight seal between said assembly and drill pipe whereby fluid introduced down the annular passage of the drill pipe is discharged into the axial bore of the bit via fluid passages in said cutting teeth and liquid introduCed down the space between the drill pipe and casing is discharged between the drill bit and the casing in an area immediately adjacent the teeth; f. a fluid tight seal between the means for discharging the effluent of the said central passage and the inner pipe whereby fluid in said axial bore, liquid from the space between the drill pipe and casing and borehole fragments from said borehole may be passed upwardly through said axial bore and central passage and discharged as effluent from the apparatus.