Abstract:
A casing seal for use with a source of high pressure fluids with apparatus for preventing shredding of the seal with variations in fluid pressure. A T-shaped annular metal ring provides a multiplication of fluid pressure so pressure exerted on an elastomeric sealing ring is greater than the fluid pressure. This multiplication prevents gases from entering the elastomer and prevents shredding of the elastomer when pressure is released. The multiplication of pressure also forces the elastomer firmly against a casing and a casing head to provide a fluid-tight seal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention pertains to apparatus for sealing leaks between a casing and a casing head and more particularly, to a seal which intensifies the pressure inside the casing seal to create improved sealing action. 
     Crude oil and natural gas wells drilled offshore often have their wellheads positioned on the ocean floor. The wellhead is normally installed by mounting it on the upper end of a string of casing which is lowered into the bore hole and cemented in place. The wellhead is generally in the shape of a cylinder having a bore which is adapted to sealably support an inner concentric string of casings and casing hangers that are affixed to the casing string prior to lowering the string and seating the hanger within the wellhead. The casing hangers are adapted to be received and supported within the wellhead. Each casing hanger is provided with a packoff element around its outer periphery to provide a pressure-tight seal between the outside of the casing and the inside of the wellhead. 
     In many present day well completion systems annular packoff seal elements are positioned between the wellhead and each casing to provide a pressure-tight seal therebetween. It is important that gas, water and other fluids which may be forced into some of the outer casings be prevented from moving into the inner casing and that fluids from the inner casing be prevented from moving into the outer casings. Each of the packoff seals must be designed to withstand the maximum oil pressure since pressure abnormalities are controlled by closing a blowout preventer situated above the wellhead, and this will cause high pressure to be exerted across the various seals. 
     Some of the prior art seals include an elastomeric material which is compressed by a high pressure exerted against the elastomeric material. This high pressure forces the elastomeric material against the casing and against the wellhead to provide a seal between the casing and the wellhead. This pressure can cause gases to be forced into the elastomeric material. Any subsequent release of the pressure allows the gases to rapidly move out of the material which causes shredding and destruction of the material and causes the seal to fail. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a casing seal which can be used with high pressure fluids without shredding the elastomeric portion of the seal when the high pressures are released. The casing seal includes a first annular metal ring having a generally T-shaped cross section with a stem portion of the T in a generally vertical direction and a cross bar portion of the T in a generally horizontal direction. The first metal ring is adapted for mounting between an annular well casing and a casing head with the stem portion of the T generally parallel to a vertically extending casing. First and second annular elastomeric rings are mounted adjacent to the stem portion of the T with the first elastomeric ring between the casing and the stem portion of the T, and with the second elastomeric ring between the casing head and the stem portion of the T. A second metal ring is mounted between the casing and the stem portion of the first metal ring below the first elastomeric ring. A third metal ring is mounted between the casing head and the stem portion of the first metal ring below the second elastomeric ring. An annular support is positioned beneath the second and third metal rings to support the casing seal when a downward pressure on the cross bar of the first metal ring compresses the first elastomeric ring between the first metal ring and the second metal ring to expand the first elastomeric ring against the casing, and the downward pressure also compresses the second elastomeric ring between the first metal ring and a third metal ring to expand the second elastomeric ring against the casing head. The cross bar prevents gases from moving downward into the elastomeric material and prevents shredding of the material when pressure is suddenly reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary diagrammatic central section of a well casing and casing head in which the casing seal of the present invention can be used. 
     FIG. 2 is an enlargement of a portion of FIG. 1 with the casing removed. 
     FIG. 3 is an enlarged fragmentary perspective view of the casing seal of the present invention. 
     FIG. 4 is a cross section of the casing seal of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, FIG. 1 discloses a cross sectional view of a portion of an annular casing head 10 having an upper portion 11 and a lower portion 12. Portions 11 and 12 are interconnected by a plurality of bolts 16 (only one being shown) and a plurality of nuts 17. Casing head 10 has an axially extending bore 18 having an inwardly projecting flange 22 in upper portion 11. An annular drill bit guide ring 23 having an inwardly projecting flange 24 is mounted inside bore 18 below flange 22. An annular well casing 28 is mounted inside guide ring 23 with an upper end 29 adjacent to flange 24 of guide ring 23. A hold-down nut 30 is threaded inside bore 18 at a lower end of upper portion 11 of casing head 10 to support a casing seal 34 of the present invention. Seal 34 is mounted above nut 30 in a space 35 between an outer wall 36 of casing 28 and an inner wall 37 of casing head 10. A ring gasket 41 mounted in a pair of grooves 42, 43 provides sealing between upper and lower portions 11, 12 of casing head 10. A test port 47 connected between the lower end of casing seal 34 and the outer portion of casing head 10 can be used to check for leakage from bore 18. A test port bleeder plug 48 threaded into port 47 provides a seal when leakage is not being checked. 
     Casing seal 34 (FIGS. 3, 4) includes a generally T-shaped annular metal ring 49 having a stem portion 53 mounted generally parallel to the vertical walls 36, 37 (FIGS. 1, 4) of casing 28 and casing head 10. A cross bar portion 54 of ring 49 having a pair of sloping surfaces 55, 56 on the underside of bar portion 54 extends at right angles to stem portion 53. A pair of elastomeric rings 60, 61 are mounted adjacent to stem portion 53 to provide a seal between casing 28 and ring 49 and between casing head 10 and ring 49. A pair of annular metal rings 62, 63 are mounted on either side of stem portion 53 and are supported by hold-down nut 30. 
     When fluids and/or gases under high pressure are present in bore 18 (FIGS. 1, 2) some of these fluids and gases move through a space 67 between guide ring 23 and wall 37 of casing head 10. These fluids and gases exert a downward pressure on a top portion 54a of cross bar 54 (FIGS. 2, 4) forcing cross bar 54 downward against elastomeric rings 60, 61. Since the area of upper surface 54a is greater than the area of sloping surfaces 55, 56, the pressure per square inch exerted on the elastomeric rings 60, 61 is greater than the downward pressure per square inch on top of cross bar 54. This difference in pressure could cause material in rings 60, 61 to be extruded into a space 68 between wall 36 and cross bar 54 and into a space 69 between wall 37 and cross bar 54. A pair of wedge-shaped non-extrusion rings 73, 74 move over sloping surfaces 55, 56 to press tightly against walls 36, 37 due to the upward pressure from elastomeric rings 60, 61. This prevents elastomeric material from rings 60, 61 from moving into spaces 68, 69. The greater pressure per square inch on the elastomeric rings 60, 61 cause the stress in the elastomer to be higher than the pressure applied to the upper end of seal 34 and prevents the diffusion of gas into the elastomer. This prevents shredding of the elastomer when pressure in bore 18 is suddenly reduced. 
     A plurality of wedge-shaped non-extrusion rings 78 - 81 (FIG. 4) prevent elastomeric material in rings 60, 61 from moving downward. A downward pressure from elastomeric ring 60 causes a lower surface 78a of non-extrusion ring 78 to slide over an upper surface 80a of non-extrusion ring 8 so ring 80 presses tightly against wall 36 and non-extrusion ring 78 passes tightly against stem 53 of ring 49. This prevents elastomeric material from being forced into a space 85 between ring 62 and wall 36 and into any space (not shown) which may develop between ring 62 and stem 53 of ring 49. Downward pressure from elastomeric ring 61 causes non-extrusion ring 79 to move tightly against stem 53 and causes non-extrusion ring 81 to move tightly against wall 37 of casing head 10. This prevents elastomeric material from being forced into a space 86 between ring 63 and wall 37 and into any space (not shown) which may develop between ring 63 and stem 53 of ring 49. 
     The downward pressure on cross bar 54 (FIGS. 1, 4) causes elastomeric ring 60 to be compressed between rings 78, 80 and cross bar 54 and ring 73. The compressed ring 60 presses against wall 36 of casing 28 and against stem 53 of ring 49 to provide a fluid tight seal. The downward pressure on cross bar 54 also causes elastomeric ring 61 to be compressed between rings 79, 81 and cross bar 54 and ring 74. Compressed ring 61 provides a fluid tight seal between wall 37 of casing head 10 and stem 53 of ring 49. 
     The present invention discloses a casing seal which can withstand high pressure and raising and lowering pressures inside the casing without damage to the seal. A multiplication of casing bore pressure prevents the diffusion of gases into an elastomeric portion of the seal and causes the elastomeric portion of the casing seal to provide a fluid tight seal. A plurality of wedge-shaped non-extrusion rings adjacent to the elastomeric portion of the seal prevents movement of the elastomer into spaces adjacent to the casing seal. 
     Although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modification and variation may be made without departing from what is regarded to be the subject matter of the invention.