Blowout preventer with reduced fluid volume

1. A system for operating a blowout preventer (BOP), the system comprising:

a front piston positioned at least partially in a front chamber, wherein the front chamber comprises:

a front volume on a front side of the front piston; and

a back volume on a back side of the front piston;

a back piston connected to the front piston, wherein the back piston is positioned at least partially in a back chamber, wherein the back chamber comprises:

a front volume on a front side of the back piston; and

a back volume on a back side of the back piston;

a first valve configured to permit fluid flow into the front chamber during a free closing stroke of the BOP; and

a second valve configured to permit fluid flow between the front and back volumes of the back chamber during the free closing stroke.

2. The system of claim 1 , further comprising a third valve configured to cause the second valve to permit fluid flow between the front and back volumes of the back chamber during the free closing stroke in response to a pressure in the back volumes of the first and second chambers being less than a predetermined threshold.

3. The system of claim 1 , wherein a pressure differential is exerted on the front piston but not the back piston during the free closing stroke.

4. The system of claim 1 , wherein the second valve is configured to prevent fluid flow between the front and back volumes of the back chamber during a shearing stroke of the BOP.

5. The system of claim 4 , wherein the front piston is moving toward a tubular member to be sheared during the free closing stroke and the shearing stroke.

6. The system of claim 4 , further comprising a third valve configured to cause the second valve to prevent fluid flow between the front and back volumes of the back chamber during the shearing stroke in response to a pressure in the back volumes of the first and second chambers being greater than a predetermined threshold.

7. The system of claim 1 , wherein the second valve is configured to permit fluid flow between the front and back volumes of the back chamber during an opening stroke of the BOP.

8. The system of claim 7 , wherein the front piston is moving toward a tubular member to be sheared during the free closing stroke, and wherein the front piston is moving away from the tubular member during the opening stroke.

9. The system of claim 7 , further comprising a third valve configured to cause the second valve to permit fluid flow between the front and back volumes of the back chamber during the opening stroke in response to a pressure in the back volumes of the first and second chambers being less than a predetermined threshold.

10. The system of claim 7 , wherein a pressure differential is exerted on the front piston but not the back piston during the opening stroke.

11. A system for operating a blowout preventer (BOP), the system comprising:

a front piston positioned at least partially in a front chamber, wherein the front chamber comprises:

a front volume on a front side of the front piston; and

a back volume on a back side of the front piston;

a back piston connected to the front piston, wherein the back piston is positioned at least partially in a back chamber, wherein the back chamber comprises:

a front volume on a front side of the back piston; and

a back volume on a back side of the back piston;

a ram connected to the front piston;

a first valve configured to:

permit fluid flow from a tank to the back volume of the front chamber to push the front piston toward a closing position during a free closing stroke of the BOP;

permit fluid flow from the tank to the back volumes of the front and back chambers to push the front and back pistons toward the closing positions during a shearing stroke of the BOP, which causes the ram to shear a tubular member; and

permit fluid flow from the tank to the front volume of the front chamber to push the front piston toward an open position during an opening stroke of the BOP;

a second valve configured to:

permit fluid flow between the front and back volumes of the back chamber during the free closing stroke;

prevent fluid flow between the front and back volumes of the back chamber during the shearing stroke; and

permit fluid flow between the front and back volumes of the back chamber during the opening stroke; and

a third valve configured to:

cause the second valve to permit fluid flow between the front and back volumes of the back chamber during the free closing stroke in response to a pressure in the back volumes of the first and second chambers being less than a predetermined threshold;

cause the second valve to prevent fluid flow between the front and back volumes of the back chamber during the shearing stroke in response to a pressure in the back volumes of the first and second chambers being greater than the predetermined threshold; and

cause the second valve to permit fluid flow between the front and back volumes of the back chamber during the opening stroke in response to the pressure in the back volumes of the first and second chambers being less than the predetermined threshold.

12. The system of claim 11 , wherein the fluid in the front volume of the front chamber is transferred through the first valve into the tank during the free closing stroke, and wherein the fluid in the front volume of the back chamber is transferred through the second valve into the back volume of the back chamber during the free closing stroke.

13. The system of claim 11 , wherein the fluid in the front volume of the front chamber is transferred through the first valve into the tank during the shearing stroke, and wherein the fluid in the front volume of the back chamber is transferred through the second valve into the tank during the shearing stroke.

14. The system of claim 11 , wherein the fluid in the back volume of the front chamber is transferred through the first valve into the tank during the opening stroke, and wherein the fluid in the back volume of the back chamber is transferred through the second valve into the front volume of the back chamber during the opening stroke.

15. The system of claim 11 , wherein a pressure differential is exerted on the front piston and the back piston during the shearing stroke, and wherein a pressure differential is exerted on the front piston but not the back piston during the free closing stroke and the opening stroke.

16. A method for operating a blowout preventer (BOP), the method comprising:

performing a free closing stroke with front and back pistons, wherein the front piston is positioned at least partially within a front chamber, wherein the back piston is positioned at least partially within a back chamber, and wherein performing the free closing stroke comprises:

pumping fluid through a first valve and into a back volume of the front chamber to push the front piston toward a closing position; and

actuating a second valve to permit fluid flow between front and back volumes of the back chamber, wherein the second valve is actuated by a third valve in response to a pressure in the back volumes of the front and back chambers being less than a predetermined threshold.

17. The method of claim 16 , wherein a pressure differential is exerted on the front piston but not the back piston during the free closing stroke.

18. The method of claim 16 , further comprising performing a shearing stroke with front and back pistons, wherein performing the shearing stroke comprises:

pumping fluid through the first valve and into the back volumes of the front and back chambers to push the front and back pistons farther toward the closing position; and

actuating the second valve to prevent fluid flow between front and back volumes of the back chamber, wherein the second valve is actuated by the third valve in response to the pressure in the back volumes of the front and back chambers being greater than the threshold.

19. The method of claim 16 , further comprising performing an opening stroke with front and back pistons, wherein performing the opening stroke comprises:

pumping fluid through the first valve and into a front volume of the front chamber to push the front piston toward an opening position; and

actuating the second valve to permit fluid flow between front and back volumes of the back chamber, wherein the second valve is actuated by the third valve in response to the pressure in the back volumes of the front and back chambers being less than the threshold.

20. The method of claim 19 , wherein a pressure differential is exerted on the front piston but not the back piston during the opening stroke.