Fluid swivel structure

A fluid swivel is described, of the type which includes a ring-shaped outer structure (12, FIG. 2 ) that rotates about a ring-shaped inner structure (14), and which form an annular chamber (18) between them and a pair of gap passages (24, 26) between them which are sealed by pressure seals (30, 32). The outer structure includes a body (70) forming part of the annular chamber and a seal ring (72) mounted on an end of the body and abutting one side of the pressure seal. The seal ring is axially shiftable on the end of the body, so that when the body expands radially as a result of high pressure fluid being introduced into the annular chamber, the seal ring does not have to expand a similar amount and therefore does not have to cause a large increase in the width of the extrusion gap (62) at the downstream end of the pressure seal. A centering mechanism (110) urges the seal ring to remain substantially centered on the body, and can be formed by a ring of compressible material lying between shoulders (120, 122, FIG. 3 ) respectively on the body and on the seal ring, or can be formed by leaf spring portions (154, FIG. 5 ) of one of the parts. Registers (180, 182) on the seal ring and on the body can engage each other to assure concentricity when the body has expanded a predetermined amount under a predetermined high pressure of at least 50 atmospheres in the annular chamber.

BACKGROUND OF THE INVENTION 
Fluid swivels are commonly used in offshore installations to transfer gas 
and oil between a fixed underwater pipeline and a tanker that may drift 
around the installation. A typical fluid swivel includes ring-shaped inner 
and outer walls, or structures, forming an annular chamber between them, 
and forming a pair of gap passages extending from opposite sides of the 
chamber to the environment. A pressure seal is placed along each gap 
passage. A common type of pressure seal is a radial seal with opposite 
sides that press against radially spaced surfaces as one structure (e.g. 
the outer structure) slowly rotates about the other one. Although face 
seals are sometimes used which seal against axially-spaced surfaces, 
radial seals are often preferred because they commonly result in fewer 
separate parts that are easier to machine, and because fluid swivels with 
radial seals are generally easier to design. 
The radial pressure seal has an upstream side exposed to the same pressure 
as that which exists in the annular chamber, and has a downstream side 
which is generally at ambient pressure (one atmosphere). Where fluid in 
the annular chamber is at moderate to high pressure, there is a tendency 
for the pressure seal to extrude into the portion of the gap passage lying 
immediately downstream of the pressure seal, which can be referred to as 
the "extrusion gap". To avoid extrusion, the extrusion gap is made as 
narrow as possible. 
When high pressure fluid lies in the annular chamber formed between the 
inner and outer structures, the outer structure tends to slightly increase 
in diameter, while the inner structure tends to slightly decrease in 
diameter. Thus, the high pressure fluid tends to separate the inner and 
outer structures, which increases the thickness of the gap passage, 
including the extrusion gap. Any increase in thickness of the extrusion 
gap can result in a significant decrease in the life of the pressure seal. 
It may be noted that it is possible to orient the extrusion gap so it 
extends radially, but this can complicate construction of the fluid 
swivel. A fluid swivel that used radial seals and which was designed to 
hold high pressure fluid (at at least 50 atmospheres), which minimized 
changes in the width of the extrusion gap which lies immediately 
downstream of a pressure seal, would be of considerable value. 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of the present invention, a fluid swivel 
is provided, of the type that includes a radial seal along each gap 
passage, which minimizes increase in the thickness of the extrusion gap 
portion of the gap passage when a high pressure is applied to the fluid 
swivel. The fluid swivel includes inner and outer structures rotatable 
with respect to each other about an axis, and forming an annular chamber 
and gap passages extending from opposite sides of the chamber to the 
environment. One of the structures such as the outer one includes a body 
forming the outer portion of the annular chamber and a separate seal ring 
which seals against one side. of the pressure seal. The seal ring can 
shift position radially relative to the body, so as the body expands under 
the force of high pressure fluid, the seal ring does not have to similarly 
expand. A rubber sheet between the body and seal ring enables such 
shifting without substantial friction. By avoiding large expansion in 
diameter of the outer seal ring, the fluid swivel avoids a large increase 
in the thickness of the extrusion gap. 
A centering mechanism couples the body to the seal ring, and urges the seal 
ring radially to tend to keep it substantially centered on the body as the 
body expands in diameter but the seal ring does not expand as much. One 
centering mechanism comprises a compressible ring with radially inner and 
outer surfaces respectively abutting shoulders on the body and on the seal 
ring. Another centering mechanism includes a leaf spring extending from 
one of the parts such as the seal ring to the other part such as the body, 
to bias them towards concentricity while allowing the body to expand more 
than the seal ring. 
The body and seal ring can include registers that engage each other to 
precisely center the seal ring on the body, only after the body has 
expanded by a predetermined amount, which it attains only when the 
pressure in the annular chamber reaches a predetermined high pressure 
which is at least 50 atmospheres. 
The novel features of the invention are set forth with particularity in the 
appended claims. The invention will be best understood from the following 
description when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a fluid swivel 10 which includes an outer wall or 
structure 12 that can rotate relative to an inner wall or structure 14 
about a vertical axis 16. The structures form a fluid-carrying annular 
chamber 18 between them. Fluid can enter the chamber through a relatively 
stationary inlet pipe 20 and can exit from the chamber through an outlet 
pipe 22 that can rotate about the axis 16. The fluid swivel is constructed 
to enable the passage of high pressure fluid (e.g. at least about 50 
atmospheres) from a source 23 such as an undersea hydrocarbon well. The 
fluid swivel forms two gaps or gap passages, including an upper gap 
passage 24 and a lower one 26. As also shown in FIG. 2, the fluid swivel 
includes a pair of primary seals 30, 32 that each lies along one of the 
gap passages, to avoid the leakage of high pressure fluid from the chamber 
18 and along a gap passage into the environment. A pair of secondary seals 
34, 36 back up the primary seals. Bearings 40, 42 rotatably support the 
outer structure on the inner one. 
When fluid under high pressure, of at least about 50 atmospheres (735 psi) 
is admitted into the annular chamber 18, the high fluid pressure urges the 
outer structure 12 to expand and the inner structure 14 to contract. If 
each structure 12, 14 had all portions thereof rigidly fixed together, 
then there could be a substantial increase in the thickness of each gap 
passage such as 24 lying between the inner and outer structures. This 
could shorten the lifetime of the pressure seals such as 30. 
As shown in FIG. 3, the pressure seal 30 has radially-spaced (with respect 
to axis 16) outer and inner sides 50, 52 which bear respectively against 
surfaces of the outer and inner structure 12, 14. The pressure seal 30 
include a seal part 54 of relatively soft material such as moderate 
hardness rubber, and a backup ring 56 of harder material such as hard 
rubber. The pressure of fluid against the upstream end 60 of the seal 
causes the seal part 54 to tightly expand against opposite sides of the 
gap passage 24. The pressure on the pressure seal also presses the backup 
ring 56 upwardly, and tends to cause one side of the backup ring to 
extrude into a gap passage portion 62 at the downstream side of the 
pressure seal. The gap passage portion 62 is sometimes herein referred to 
as the "extrusion gap" because the pressure seal tends to extrude into 
this gap. The fluid swivel is constructed so the thickness T of the 
axially-extending extrusion gap 62 is as small as possible. 
As mentioned above, when high pressure fluid is applied to the annular 
chamber 18 (FIG. 2) the outer structure 12 tends to expand while the inner 
structure 14 tends to contract, all of which tends to increase the 
thickness of the gap passage 24. As also mentioned above, it is important 
to maintain the extrusion gap 62 of the gap passage as small as possible, 
to avoid extrusion of the pressure seal 30. Expansion of the extrusion gap 
62 can be minimized by constructing each structure 12, 14 of the fluid 
swivel, and especially the outer structure 12, of a plurality of separate 
parts. The outer structure 12 includes a body 70 which forms part of the 
annular chamber 18. The outer structure also includes a separate 
seal-holding ring 72 which forms one side 74 (FIG. 3) of the gap passage 
portion against which the pressure seal 30 seals, and which also forms one 
side 76 of the extrusion gap 62 into which the backup ring 56 of the 
pressure seal 30 is in danger of extruding. The seal ring 72 is mounted on 
an end 80 of the body 70 only to prevent the seal ring 72 from moving 
axially (parallel to the axis 16) relative to the body 70. 
In accordance with the present invention, expansion of the extrusion gap 62 
is minimized by allowing the seal ring 72 to shift radially (so one side 
or all of the seal ring moves towards or away from the axis 16) with 
respect to the body 70. As a result of this construction, when high 
pressure fluid enters the annular chamber 18, causing the body 70 to 
expand radially outwardly, the seal ring 72 does not have to similarly 
expand. Instead, all portions of the seal ring slide radially inwardly 
relative to the end of the body 70, to avoid radially outward expansion of 
the seal ring 72. This enables the side 76 (FIG. 3) of the extrusion gap 
to remain close to the other side 78 of the extrusion gap, to minimize 
increase in the thickness T of the extrusion gap. Thus, even when the body 
70 expands in diameter, the seal ring 72 does not have to undergo a 
corresponding expansion in diameter, and the extrusion gap 62 can retain a 
small thickness T. 
The forces tending to expand the body 70 (FIG. 2) equal the pressure of the 
fluid 82 in the chamber 18, times the height H of the body 70, times the 
average circumferential length (the length 2.pi.R along 360.degree. of the 
annular chamber) of the inner side of the body. In one fluid swivel that 
applicant has designed, where the radius R of the annular chamber was 
about two feet (about 0.6 meter) and the height H of the body was also 
about two feet, a 50 atmospheres pressure of fluid in the chamber 18 
resulted in a radially outer force on the body 70 of about 18,000 pounds. 
Where the body 70 is constructed with walls of only moderate thickness to 
minimize weight, the expansion of the body can be substantial, especially 
at higher pressures which may exceed 5,000 psi (340 atmospheres). In one 
example, the pressure seal 30 has a radial thickness A of one inch (2.5 
cm) and the extrusion gap has a thickness T of 0.010 inch (0.25 mm) when 
the pressure of fluid (air) in the chamber is at one atmosphere. At a high 
pressure such as 50 atmospheres or more, the body 70 whose inner diameter 
is about four feet, may expand in radius by 0.010 inch or more. This could 
cause the thickness T of the extrusion gap 62 to double in thickness, and 
thereby greatly decrease the lifetime of use of the pressure seal. By 
avoiding expansion of the seal ring 72 in the same amount as the body 70, 
applicant minimizes increase in the thickness of the extrusion gap. 
Each of the bearings 40, 42 has one side connected to an outer bearing ring 
90, 92 and an opposite side connected to an inner bearing ring 94, 96. The 
particular bearings shown comprise rollers (which may be tapered) that 
roll against surfaces on the opposed bearing rings. The bearings such as 
40 are constructed to keep all parts of the outer structure 12 
substantially concentric with the inner structure 14, to avoid rubbing of 
steel surfaces anywhere else along the gap passage 24, except at the seals 
30-36. Any hard rubbing of steel surfaces can cause high friction which 
resists turning of the outer structure about the inner one. A group of 
outer bolts 100 uniformly spaced about the axis 16, serve as a connecting 
device that connects the bearing ring 90 to the seal ring 72 and to the 
body 70. The bolts 100 are constructed to keep the bearing ring 90 and 
seal ring 72 precisely concentric, to avoid rubbing of closely spaced 
surfaces at the extrusion gap 62. However, the thickness of the gap 
passage portion 102 at the body 70 is much larger, than that of the 
extrusion gap 62 and the outer bolts 100 enable some sideward shifting of 
the body 70 with respect to the seal ring 72. 
In order to prevent considerable eccentricity of the body 70 with respect 
to the bearing ring 90 and the seal ring 72 (which remains precisely 
concentric with the bearing ring 90) applicant provides a centering 
mechanism 110 which urges the seal ring 72 radially (urges locations on 
the seal ring towards or away from the axis 16) to keep the seal ring 72 
substantially centered on the body 70, as the body expands and contracts. 
The centering mechanism shown in FIG. 3 appears to be in the form of a 
block 112 as seen in a sectional view, and is preferably in the form of a 
ring having a largely rectangular cross section (although the ring can 
have gaps). The block form or ring 112 is resiliently compressible in a 
radial direction. If only light forces were encountered, the block ring 
112 could be formed of rubber, but because of the large forces 
encountered, applicant constructs the block ring 112 of a harder material 
which is less stiff than steel, such as aluminum, which has a modulus of 
elasticity of 10 million psi, as compared to the modulus of elasticity of 
steel which is about 30 million psi. As shown in FIG. 4, the elasticity of 
the ring 112 in the radial direction can be increased by forming numerous 
holes 114 which reduce the effective cross sectional area to be 
compressed. Referring to FIG. 3, the ring 112 lies between shoulders 120, 
122 respectively on the seal ring 72 and on the body 70. As the body 70 
expands in radius, its shoulder 122 moves from the initial position 122A 
to the position 122, thereby causing radial compression of the ring 112. 
Due to the resilience of the ring material, it encourages the seal ring 
and body to remain substantially concentric even as the shoulder 122 
approaches the shoulder 120. 
The outer structure 12 shown in FIG. 2 is substantially symmetrical about a 
horizontal plane 130, and includes a lower seal ring 132 which is 
substantially rigidly fixed to the lower bearing ring 92, and which is 
coupled by a centering mechanism 134 to the lower end of the body 70. The 
inner structure 14 is constructed somewhat similarly to that of the outer 
structure, in that it includes a body 140 upper and lower seal rub rings 
142, 144 each coupled to an end of the body by a centering mechanism 146, 
148. Each seal rub ring 142, 144 is substantially rigidly connected to a 
corresponding bearing ring 94, 96. It is noted that the outer seal ring 72 
may be referred to as a seal-holding ring because it has grooves which 
hold the pressure seals 30, 34. the inner seal ring 142 may be referred to 
as a rub seal ring or rub ring, because it merely rubs against the 
pressure seals 30, 34 as they turn with the outer seal ring 72. 
Each seal ring 72, 142 may be referred to as a "seal abutting ring" to 
indicate that it abuts one side of a pressure seal such as 30. 
There is some tendency for the seal ring such as 72 to expand when high 
pressure is applied to the annular chamber 18. This is because the high 
pressure is applied to the seal ring along most of the height B of the 
pressure seal. However, most of the height of the seal ring 72 is not 
exposed to the high pressure, so the expansion of the seal ring 72 is much 
less than that of the outer body 70. The parts of the inner structure 14 
contract less than the amount by which corresponding parts of the outer 
structure 12 expand. 
FIG. 5 illustrates part of another fluid swivel 150, where the centering 
mechanism 152 includes a leaf spring 154 formed as part of the seal ring 
156 and extending around the outside of the body 158. The leaf spring 154 
can extend continuously around the body 158, so the leaf spring tends to 
not expand even as the seal ring 156 expands slightly. The leaf spring 
will resist expansion even if there are gaps in it, though not as 
strongly. The leaf spring 154 has a lower or outer end 160 which bears 
against a surface 162 on the body 158. As the body 158 expands radially 
with respect to the axis of rotation 164, the surface 162 deflects the 
outer end 160 of the leaf spring 154 radially outwardly to bend the 
spring. The leaf spring 154 and body surface 162 each preferably extend 
360.degree. around the axis 164. If the body 158 should tend to shift so 
it is eccentric with respect to the axis 164, then one side of the body 
would tend to deflect one side of the leaf spring 154 excessively, and 
that side of the leaf spring would greatly resist such deflection and tend 
to center the body 154 on the seal ring 156. 
During most of the useful lifetime of the fluid swivel, when the outer 
structure 170 might turn with respect to the inner structure 172, the 
fluid swivel is carrying fluid under high pressure (at least about 50 
atmospheres). It is during such time when it is most desirable that the 
seal ring 156 (which is always closely concentric with the bearing 174 and 
bearing ring 176) remain closely concentric with the body 158. This is 
especially so because the lower seal ring 176 is not directly mounted on 
any lower bearing, but depends on concentricity of the body 158 to 
maintain it concentric with the lower seal ring 178 of the inner 
structure. To maintain concentricity when high fluid pressure is present, 
applicant provides registers 180, 182 respectively on the body 158 and on 
the seal ring 156 which engage each other only when a predetermined high 
fluid pressure lies in the annular chamber 184. Preferably, the registers 
180, 182 engage each other when nearly full pressure is applied. 
In one example, where the fluid swivel is designed to carry fluid from an 
underground well which is known to supply fluid at a pressure of about 200 
atmospheres (2940 psi), applicant forms the registers 180, 182 so they 
will tightly engage each other when a pressure of 193 atmospheres (2800 
psi) is reached. This assures that at design pressure of 200 atmospheres, 
the body 158, seal ring 156, and bearing ring 176 will be closely 
concentric. Actually, when fluid passes at a substantial flow rate, the 
pressure in the fluid swivel will be somewhat below design pressure. 
Applicant prefers to provide a pair of bushings 190, 192 formed of bearing 
material such as aluminum bronze, at the seal ring 156 and seal rub ring 
194, to assure concentricity of the rings 156, 194. Preferably, there is a 
small gap such as 0.010 inch between the bushings 190, 192 at zero 
pressure, to minimize friction, so the bushings serve only to prevent 
great eccentricity of the rings 156, 194 in a worse case condition. The 
lower seal ring 176 is maintained concentric with the body 158 by another 
set of registers 196, 198 which engage when the predetermined high 
pressure (e.g. 193 atmospheres) is reached. 
FIG. 6 shows how the thickness of the extrusion gap 200 can vary, between 
an especially small amount at C and an especially large amount at D if 
there is even moderate eccentricity of the two seal rings 156, 194. In the 
case of FIG. 6, the small amount of eccentricity E will have the 
undesirable effect of allowing the side of the pressure seal which is of 
the thickness D to extrude thereat. Thus, maintaining close concentricity 
can prevent a large extrusion gap anywhere around the 360.degree. extent 
of the pressure seal to minimize extrusion and assure a long lifetime of 
use of the pressure seal. 
FIG. 5 shows that the inner body 210 of the inner structure 172 is 
maintained substantially concentric with the rub ring 194 by another leaf 
spring 212 which has an end part 214 that lightly touches a surface on the 
rub ring 194. The leaf spring 212 has a top forming a flange 216 for 
mounting another fluid swivel on the fluid swivel 150. When the pressure 
of fluid in the annular chamber 184 increases, the inner body 210 moves 
inwardly (by about half as much as the outer body moves outwardly). 
However, the inner spring 214 tends to not move (because it is a 
preferably continuous ring) and resists inward motion of inner body 210. 
As a result, the inner spring 214 remains lightly touching the rub ring 
194, to keep it concentric with the inner body. Should the rub ring 194 
start to shift eccentrically, the inner leaf spring 214 would oppose such 
eccentric movement and urge the rub ring 194 to remain concentric with the 
inner body 210. 
The fluid swivel 150 is symmetric about an imaginary central horizontal 
plane 220, so the lower part of the fluid swivel also has leaf springs 
shown at 222 and 224. Concentricity of the inner structure 172 with the 
outer one is assured at maximum, or design pressure, by registers 226-229. 
The gap between pairs of registers such as 226 and 229, is designed to 
close at a pressure just below the design pressure. 
When the outer body 158 expands substantially but the seal ring 156 does 
not expand as much, then locations on the top of the body must shift 
radially outwardly with respect to adjacent locations on the bottom of the 
seal ring. Although friction could be reduced by assuring good surface 
finishes, such friction is unpredictable. Applicant assures that such 
shifting will occur by locating a thin elastomeric (rubber) layer or sheet 
186 between them. An elastomeric material has a modulus of elasticity on 
the order of 3,000 psi or less, and typically of about 500 psi. The 
opposite faces of the elastomeric sheet are preferably vulcanized to a 
pair of thin metal sheets to facilitate handling. The elastomeric sheet is 
compressed between the body and seal ring and deforms as the body expands 
more than said seal ring while sealing the space between them. The low 
resistance to shear of the elastomeric sheet 186, provides very low 
equivalent resistance to shifting. Its presence can sometime eliminate the 
need for the springs, as the rubber sheet 186 can apply sufficient force 
to maintain concentricity. In that case, zero pressure registers are 
desirable to assure concentricity at zero pressure. Similar other 
elastomeric sheets 187-189 are provided for the other rings 176, 178 and 
194. 
It is highly desirable that a simple way be available to confirm that the 
seal holding ring 156 and seal rub ring 194 remain closely concentric. 
This can be accomplished by providing at least three locations for 
measuring the size of the extrusion gap. Applicant provides three drilled 
inspection ports 195 which extend from the outside to the extrusion gap, 
through which a gap measurement instrument can be inserted, such as a 
depth micrometer. The ports 195 are uniformly spaced (by 120.degree. ) 
about the swivel axis 164, and lie at the same height (below the flange 
216). The inspection ports each extend to a location immediately 
downstream of a main pressure seal 30. The particular depth micrometer 240 
shown, has opposite surfaces 242, 244 which engage radially opposite 
surfaces of the gap passage to measure the radial gap between them. Three 
gap measurements at the three locations (when the swivel is substantially 
nonrotating but pressurized) enables a determination of degree of 
concentricity. It may be noted that U.S. Pat. No. 4,828,292 owned by the 
present assignee shows a single inspection passage, but a gauge therein 
measures only the short radially-extending extrusion gap of a face seal. 
Such short extrusion gap has axially-spaced surfaces, so the width of the 
gap between them does not indicate concentricity of swivel parts. 
FIG. 7 illustrates a modified form of the fluid swivel of FIGS. 1-3, with 
the body 70X and seal ring 72X similar to the body 70 and seal ring 72 of 
FIG. 2. However, the body 70X is formed with a body register 230 which can 
engage a seal ring register 232. Surfaces 234, 236 on the registers are 
initially spaced from each other, but engage one another to hold the body 
and seal ring closely concentric, when a pressure close to that of the 
design pressure is reached. Thus, for the example given above where the 
design pressure is 200 atmospheres, the surfaces 234, 236 engage each 
other when a pressure such as 193 atmospheres is reached. 
It should be noted that while terms such as "vertical", "horizontal", 
"upper", "lower", etc. are used herein to aid in the description of the 
invention, that the fluid swivel can be operated in other orientations 
with respect to gravity. 
Thus, the invention provides a fluid swivel of the type that has at least 
one radial pressure seal, which minimizes increase in the thickness of the 
extrusion gap lying on the downstream side of the seal. This is 
accomplished by forming one of the structures such as the outer structure, 
with a separate seal ring or seal abutting ring and separate body. The 
seal abutting ring, which engages one side of the pressure seal and forms 
one side of the extrusion gap that lies on the downstream side of the 
pressure seal, can shift radially with respect to the body. A rubber sheet 
between the seal ring and body can assure low friction shifting. A 
centering mechanism urges the seal ring to remain substantially concentric 
with the body even as the body expands in radius by more than the seal 
ring. Registers can be located on the inner and/or outer body and on the 
rub and/or seal ring, which engage each other to hold the ring concentric 
to the body, when a predetermined high pressure of at least about 50 
atmospheres lies in the annular chamber of the fluid swivel. A plurality 
of inspection ports extending to the gap can measure concentricity of the 
inner and outer seal rings. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art, and consequently, it is 
intended that the claims be interpreted to cover such modifications and 
equivalents.