Downhole power system

Abstract

The present invention relates to a downhole power system ( 1 ) comprising a tubing string ( 2 ) comprising a pressurised fluid ( 3 ), a turbine ( 4 ) for converting energy from the pressurised fluid into rotation of a shaft ( 5 ), a tool ( 6, 6 a ) powered by the turbine, wherein a constant flow downhole assembly ( 7 ) is arranged between the tubing string and the turbine for providing a substantially constant flow of the pressurised fluid to the turbine, wherein the constant flow downhole assembly has an assembly axis ( 9 ) and comprises a body ( 15 ) comprising a main bore ( 16 ) having a bore inlet ( 17 ) and at least one bore outlet ( 18 ), a hollow piston ( 19 ) having a piston inlet ( 32 ) and a piston outlet ( 33 ), said hollow piston being arranged in the main bore, and a spring ( 34 ) arranged in the main bore, said spring being compressed upon movement of the piston in a first direction ( 35 ), and wherein the bore inlet is in fluid communication with the piston inlet and the piston inlet is smaller than the bore inlet so as to force the piston to move in the first direction. The present invention also relates to a constant flow downhole assembly and to a method for proving a substantially constant flow of fluid into a turbine of the downhole power system.

1 . A downhole power system comprising:
 a tubing string comprising a pressurised fluid,   a turbine for converting energy from the pressurised fluid into rotation of a shaft,   a tool powered by the turbine,   wherein a constant flow downhole assembly is arranged between the tubing string and the turbine for providing a substantially constant flow of the pressurised fluid to the turbine,   wherein the constant flow downhole assembly has an assembly axis and comprises:   a body comprising a main bore having a bore inlet and at least one bore outlet,   a hollow piston having a piston inlet and a piston outlet, said hollow piston being arranged in the main bore, and   a spring arranged in the main bore, said spring being compressed upon movement of the piston in a first direction, and   wherein the bore inlet is in fluid communication with the piston inlet and the piston inlet is smaller than the bore inlet so as to force the piston to move in the first direction.   
     
     
         2 . A downhole power system according to  claim 1 , further comprising a generator arranged between the turbine and the tool, the generator comprising a rotor and a stator and the rotor being connected with the shaft. 
     
     
         3 . A downhole power system according to  claim 1 , wherein the piston inlet is smaller than the bore inlet so as to force the piston to move in the first direction. 
     
     
         4 . A downhole power system according to  claim 1 , wherein the piston has a piston wall adapted to partly cover the bore outlet when the piston moves in the first direction for reducing the flow of fluid into the bore outlet. 
     
     
         5 . A downhole power system according to  claim 1 , wherein the piston outlet is elongated and extends along the assembly axis. 
     
     
         6 . A downhole power system according to  claim 1 , wherein the main bore comprises first openings arranged upstream of the piston to allow pressurised fluid to flow in a bypass channel out through second openings and into apertures arranged in the piston wall. 
     
     
         7 . A downhole power system according to  claim 6 , wherein changeable flow restrictors are arranged in the first openings. 
     
     
         8 . A downhole power system according to  claim 1 , wherein the flow restrictors are nozzles. 
     
     
         9 . A downhole power system according to  claim 1 , wherein the tool is a driving unit, such as a downhole tractor. 
     
     
         10 . A downhole power system according to  claim 9 , wherein the driving unit comprises projectable arms having wheels. 
     
     
         11 . A downhole power system according to  claim 1 , wherein the system comprises two driving units, an electrically driven driving unit and a fluid-driven driving unit. 
     
     
         12 . A downhole power system according to  claim 1 , wherein the tubing string is a coiled tubing. 
     
     
         13 . A constant flow downhole assembly for controlling a substantially constant flow rate in the downhole power system according to  claim 1 , said constant flow downhole assembly having an assembly axis and comprising:
 a body comprising a main bore having a bore inlet and at least one bore outlet,   a hollow piston having a piston inlet and a piston outlet, and being arranged in the main bore, and   a spring arranged in the main bore, said spring being compressed upon movement of the piston in a first direction.   
     
     
         14 . A constant flow downhole assembly according to  claim 13 , wherein the piston inlet is smaller than the bore inlet so as to force the piston to move in the first direction. 
     
     
         15 . A method for providing a substantially constant flow of fluid into a turbine of the downhole power system according to  claim 1 , comprising the steps of:
 pressurising a fluid in a tubing string,   entering the pressurised fluid into the main bore of the constant flow downhole assembly,   letting the pressurised fluid flow past the hollow piston and into the turbine until the fluid flow exceeds a predetermined level,   moving a piston by the pressurised fluid pressing on a top of the piston, and   reducing a flow area of the bore outlet by the piston moving in a first direction until the fluid flow is substantially equal to the predetermined level.