Constant motion swivel seal assembly

A constant motion swivel seal assembly having greatly improved useful life without leakage of fluid from a swivel joint. The seal assembly includes one or more annular sealing rings rotatably positioned between an outer joint member and an inner joint member. A wedge-shaped anti-extrusion ring mounted between the annular sealing ring and a gap between the inner and outer joint members prevents the sealing ring from extrusion into the gap. The anti-extrusion ring has a relatively large surface area in contact with the inner joint member and a relatively small surface area in contact with the outer joint member so the anti-extrusion ring remains in substantially the same position relative to the inner joint member. The relatively small area of contact between the outer joint member and the anti-extrusion ring allows the anti-extrusion ring to slide over the surface of the outer joint member as the outer joint member rotates relative to the inner joint member. Hoop stress also prevents fluid pressure from forcing the anti-extrusion ring radially outward against the outer joint member while fluid pressure axially presses the anti-extrusion ring tightly against an end portion of the inner joint member. A relatively small area of contact between the sealing ring and the inner member and a larger area of contact between the sealing ring and the outer member allows the sealing ring to slide over the inner member and rotate with the outer member.

BACKGROUND OF THE INVENTION 
This invention relates to pipe swivel joints, and more particularly to 
swivel seal assemblies especially adapted for use in constant motion 
service. 
A production of oil and gas from offshore wells is a common endeavor in the 
petroleum industry. A well or cluster of wells is drilled in the ocean 
floor and fluid from these wells transported by conduit to marine tankers 
which transport the fluid to shore facilities. A system of pipelines 
convey the fluid from the wells to a platform or floating buoy to which a 
marine tanker may be attached. The pipeline system includes one or more 
pipes or conduits extending generally horizontally across the ocean floor 
from the wells to a point below the floating buoy and a generally vertical 
pipe or hose extending from the buoy to the horizontal pipe. At a 
plurality of locations in the pipeline system connections are needed 
between the various pipes. A flexible hose or an articulated loading arm 
secured between the buoy and the marine tanker may include one or more 
connections. Some of these connections are used to permit one pipe or hose 
to rotate relative to an adjacent pipe or hose by the use of swivel 
joints. 
Some of the swivel joints include one joint portion which rotates relative 
to another joint portion on occasional basis and other swivel joints 
include a first joint portion which is in almost constant motion relative 
to another joint portion. One location where a constant motion swivel 
joint is often used is in the flexible hose extending between the tanker 
and the buoy anchored to the ocean floor. In prior art swivel joints the 
seals of the joints wear rapidly due to the constant motion so the joint 
quite often fails and the seals must be replaced. Some of the prior art 
seals include a thin annular anti-extrusion ring mounted between the main 
seal and a gap between an inner joint member and an outer joint member. 
These anti-extrusion rings usually fail after a few thousand oscillations 
of the joint because high pressure forces a portion of the anti-extrusion 
ring into the gap between the joint members causing the anti-extrusion 
rings to break. When the joint is located beneath the surface of the 
ocean, especially at a great depth, a replacement of these seals is 
usually difficult and very expensive. 
SUMMARY OF THE INVENTION 
The present invention comprises a constant motion swivel seal assembly for 
use with a pipe swivel joint. This invention overcomes some of the 
disadvantages of the prior art by providing a swivel seal assembly for use 
with a generally cylindrical outer member having an axially extending 
passage to receive a generally cylindrical inner member, a radial flange 
extending inward from the axial passage, a bore extending axially through 
the inner member and an annular seal assembly mounted between the radial 
flange and an end of the inner member. The seal assembly includes an 
annular sealing ring having a generally U-shaped cross-section and an 
annular anti-extrusion ring having a generally wedge-shaped cross-section, 
the anti-extrusion ring being mounted between the sealing ring and a 
junction between an outer wall of the inner member and the axial passage 
of the outer member. The thicker portion of the anti-extrusion ring is 
mounted adjacent a gap at the junction between the inner and outer 
members. The thick portion of the anti-extrusion ring prevents the ring 
from being forced into the gap at the juction of the joint members and 
greatly extends the life of the anti-extrusion ring. A relatively small 
surface of the sealing ring is against a smooth surface of the inner 
member and a relatively large surface of the sealing ring is against a 
surface of the outer member causing the sealing ring to stick with the 
outer member and to slide over the smooth surface of the inner member as 
the inner member rotates relative to the outer member. Conversely, a 
relatively large surface of the anit-extrusion ring is against a surface 
of the inner member so the anti-extrusion ring stays with the inner 
member. The sealing ring and the anti-extrusion ring are each made of a 
low friction material so the surface of the sealing ring can easily slide 
over the surface of the anti-extrusion ring.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A constant motion swivel joint 11 using a constant motion swivel joint 
assembly of the present invention is illustrated in detail in FIG. 2. The 
swivel joint 11 includes an inner or male member 12 for connection to a 
first pipe or hose (not shown) and an outer or female member 13 for 
connection to a second pipe or hose (not shown). An axial bore 17 extends 
through the length of the inner member 12 to provide a path for fluid to 
flow. 
The outer member 13 (FIG. 2) includes an enlarged upper portion 18 having 
an axially extending passage 19 to receive the inner member 12 in position 
to transfer fluid between bore 17 and an axially aligned bore 23 of the 
outer member 13. As indicated in FIG. 2 the members 12 and 13 are 
rotatably held in axial alignment by a plurality of ball bearings 24 in a 
plurality of annular bearing races 25, 26. A constant motion swivel 
assembly 30 (FIGS. 1, 2) is mounted between a lower end 12a of the inner 
member 12 and a lower end 19a of the passage 19. 
The swivel assembly 30 includes an annular sealing ring 31 having a 
generally U-shaped cross-section with an internally mounted expander such 
as an O-ring 32 to bias a pair of fingers 33a, 33b axially against end 12a 
of member 12 and against end 19a of the passage 19 to provide a 
fluid-tight seal. A wedge-shaped anti-extrusion ring 37 is mounted between 
sealing ring 31 and a gap G between an outer wall 38 of the inner member 
12 and an inner wall 39 of outer member 13 with the thicker portion of 
ring 37 adjacent the gap G. Pressure from fluid in the bores 17, 23 
presses the anti-extrusion ring 37 tightly against the lower end 12a of 
inner member 12, while the hoop stress of anti-extrusion ring 37 prevents 
the fluid pressure from pressing the ring 37 as tightly against the inner 
wall 39 of outer member 13. Also, a relatively large surface of the 
anti-extrusion ring 37 is pressed against the surface at the end 12a of 
member 12 and a relatively small surface S of ring 37 (FIG. 2) is against 
the wall 39 of the outer member 13. As a result, anti-extrusion ring 37 
rotates along with inner member 12 and slides over the surface of inner 
wall 39 as inner member 12 rotates relative to outer member 13. The 
wedge-shape of anti-extrusion ring 37 and the corresponding mating shape 
of sealing ring 31 causes fluid pressure to press anti-extrusion ring 37 
upward and outward toward gap G to enhance the sealing action between ring 
37 and members 12 and 13. Sealing ring 31 may be made from a plastic 
material such as Teflon and anti-extrusion ring 37 may be made of metal or 
of a firm plastic material such as Ryton (polyphenylene sulfide). 
The sealing ring 31 has a relatively small surface against the surface at 
the lower end 12a of the inner member 12. This surface at end 12a can be 
polished to further reduce friction and the Teflon sealing ring 31 has a 
low coefficient of friction so it is able to slide over the adjacent 
surface of the member 12. The sealing ring 31 has a relatively large 
surface against the end 19a of passage 19 causing the sealing ring 31 to 
stick with the outer member 13 as outer member 13 rotates relative to 
inner member 12. A rougher surface at the end 19a of the passage 19 can be 
used to insure that sealing ring 31 does rotate with the outer member 13. 
The amount of friction between the Teflon sealing ring 31 and the Ryton 
anti-extrusion ring 37 is extremely low so ring 37 can rotate relative to 
ring 31. High pressures inside the axial bores 17, 23 would cause the 
Teflon material to flow into gap G without the Ryton or metal ring 37. 
Ring 37 could be formed from a metal impregnated with Teflon to provide 
both a firm material which would not flow into the gap G (the metal 
portion of the ring) and a low friction, from the Teflon portion of the 
ring. Any metal in ring 37 which contacts the surfaces of the members 12, 
13 would be worn away by motion of the members 12, 13 and the Teflon would 
remain as a low friction material in contact with the surface of members 
12, 13. 
A second constant motion swivel assembly 30a (FIG. 2) is mounted between a 
shoulder 43 on the outer portion of inner member 12 and a shoulder 44 in 
passage 19 of outer member 13. The pressure in gap G presses a sealing 
ring 31a of swivel assembly 30a against an anti-extrusion ring 37a to 
secure ring 37a adjacent a gap G1, between members 12 and 13. 
Constant motion swivel assembly 30 (FIG. 2) provides an excellent seal when 
installed as shown, but it does not seal well if the assembly should 
inadvertently be installed upside down with ring 37 adjacent end 19a of 
passage 19 and with ring 31 adjacent gap G. In this upside down position a 
portion of sealing ring 31 may be forced into gap G and this portion 
pulled away from the remainder of the ring when the inner member 12 
rotates relative to the outer member 13. A second embodiment of the 
present invention (FIGS. 3, 4) alleviates this problem when the swivel 
assembly is installed by inexperienced personnel. 
The embodiments of FIGS. 3 and 4 includes a swivel assembly 130 having an 
annular sealing ring 131 with a generally U-shaped cross-section with an 
O-ring 132 mounted inside ring 130 to bias a pair of fingers 133a, 133b 
axially against end 12a of member 12 and against end 19a of passage 19 to 
provide a fluid-tight seal. A pair of anti-extrusion rings 137a, 137b are 
mounted adjacent the sealing ring 131 with ring 137a mounted above ring 
131 and anti-extrusion ring 137b mounted below sealing ring 131. However, 
the entire assembly 130 can be turned over with anti-extrusion ring 137b 
on top of the sealing ring 131 and an anti-extrusion ring 137a below the 
sealing ring 131 and work just as well. The wedge-shaped anti-extrusion 
ring 137a has an upper surface 166 having a greater area than a side 
surface 167 thereby providing a greater amount of friction between surface 
166 and a lower end 12a of member 12 than is provided between surface 167 
and an inner wall 39 of outer member 13. This also aids in having ring 
137a rotate with inner member 12 and sliding along the iner wall 39 of 
outer member 13. 
Thus, the U-shaped sealing ring and the O-ring provide a fluid-tight seal 
between the inner and outer joint members, and the wedge-shaped 
anti-extrusion ring has a thick portion adjacent a gap between the inner 
and outer joint members to prevent the sealing ring from being pressed 
into the gap and damaged. 
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.