Nested cyclinder compact blowout preventer

1. A blowout preventer which comprises:

an elongated BOP housing;

a BOP bore which extends axially through the BOP housing; and

a pair of ram assemblies which are mounted on diametrically opposite sides of the BOP housing, each ram assembly comprising:

a cylinder housing which comprises a recess that extends generally transversely relative to the BOP bore from a radially inner end of the cylinder housing to a transverse first wall located at a radially outer end of the cylinder housing;

a cylinder-ram which is slidably received in the recess and which comprises a cavity that extends from a radially outer end of the cylinder-ram to a transverse second wall located at a radially inner end of the cylinder-ram;

a back plate which is connected to or formed integrally with the outer end of the cylinder-ram, the cylinder-ram being sealed to the cylinder housing proximate the back plate;

a piston head which is positioned in the cavity between the second wall and the back plate and is sealed to the cylinder-ram, the piston head being connected to the cylinder housing;

a first piston chamber which is defined between the back plate and the first wall; and

a second piston chamber which is defined between the piston head and the second wall;

wherein application of hydraulic pressure to the first and second piston chambers will move the cylinder ram from an open position in which the cylinder-ram is retracted from the BOP bore to a closed position in which the second wall of the cylinder ram extends across the BOP bore to approximately the centerline of the BOP bore.

2. The BOP of claim 1 , wherein the second wall comprises a radially inwardly directed front face to which a sealing insert is connected.

3. The BOP of claim 1 , wherein the second wall comprises a radially inwardly directed front face to which a shearing insert is connected.

4. The BOP of claim 1 , wherein the second wall comprises a radially inwardly directed front face to which a pipe sealing insert is connected.

5. The BOP of claim 1 , wherein the second wall comprises a radially inwardly directed front face to which a combination of at least two of a sealing insert, a shearing insert and a pipe sealing insert are connected.

6. The BOP of claim 1 , wherein the piston head is connected to a support rod which extends through a hole in the back plate and is connected to the first wall.

7. The BOP of claim 1 , wherein the hydraulic pressure is conveyed from the first piston chamber to the second piston chamber.

8. The BOP of claim 7 , wherein the hydraulic pressure is conveyed through a number of fluid passages which extend through the cylinder-ram.

9. The BOP of claim 8 , wherein each fluid passage communicates with the second piston chamber via a corresponding port in the cylinder ram which is located adjacent the front wall.

10. The BOP of claim 8 , wherein each fluid passage communicates with the first piston chamber via a corresponding hole which extends through the back plate.

11. The BOP of claim 1 , wherein each ram assembly further comprises:

a third piston chamber which is defined between the piston head and the back plate;

wherein application of hydraulic pressure to the third piston chamber will move the cylinder ram from the closed position to the open position.

12. The BOP of claim 11 , wherein the hydraulic pressure is conveyed through a first fluid passage which extends through the piston head.

13. The BOP of claim 12 , wherein the piston head is connected to a corresponding support rod which extends through a hole in the back plate and is connected to the cylinder housing, and wherein the hydraulic pressure is conveyed through a second fluid passage which extends through the support rod and is connected to the first fluid passage.

14. The BOP of claims 1 , further comprising a second pair of ram assemblies which are mounted on diametrically opposite sides of the BOP housing.