Note: Descriptions are shown in the official language in which they were submitted.
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ROD LOCK FOR RAM BLOWOUT PREVENTERS
BACKGROUND OF INVENTION
Field of the Invention
[0001] The invention relates generally to locking mechanisms for a blowout
preventer
as deployed in the petroleum exploration and recovery industry. More
particularly,
the invention relates to a roller lock mechanism to prevent the undesirable
reversal of
a thrust rod for a ram-type blowout preventer.
Background Art
[0002] Wellbores are drilled deep into the earth's crust to recover oil and
gas deposits
trapped in the formations below. Typically, these wellbores are drilled by an
apparatus that rotates a drill bit at the end of a long collection, or string,
of threaded
pipes known as a drillstring. Because of the energy and friction involved in
such an
operation, a drilling fluid, commonly referred to as drilling mud, is used to
lubricate
and cool the drill bit as it cuts the rock formations below. Furthermore, the
drilling
mud is capable of performing the secondary function of removing the drill
cuttings
from the bottom of the wellbore to the surface. Typically, drilling mud is
delivered to
the drill bit under high pressures through a central bore of the drillstring.
From there,
nozzles on the drill bit direct the pressurized mud to the cutters on the
drill bit where
the pressurized mud cleans and cools the bit. As the fluid is delivered
downhole
through the central bore of the drillstring, the fluid returns to the surface
in the
annulus formed between the outside of the drillstring components and the cut
wellbore. Therefore, a hydrostatic column of drilling mud typically extends
from the
surface to the bottom of the hole being cut.
[0003] As wellbores are drilled several thousand feet below the surface, the
hydrostatic column of drilling mud serves to help prevent blowout of the
wellbore as
well. Often, hydrocarbons and other fluids trapped in subterranean formations
exist
under significant pressures. Absent any flow control schemes, fluids from such
ruptured formations can blow out of the wellbore like a geyser and spew
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hydrocarbons and other undesirable fluids into the atmosphere. Several
thousand feet
of hydraulic head from the drilling mud column helps to prevent the wellbore
from
blowing out under normal conditions. However, under certain unforeseen
circumstances, the drill bit will encounter pockets of pressurized formations
and will
cause the wellbore to "kick" or experience a rapid increase in pressure.
Because
formation kicks are unpredictable and would otherwise result in disaster, flow
control
devices known as blowout preventers ("BOP's"), are mandatory on most drilling
rigs
in use today.
[0004] Blowout preventers are devices configured to seal the annular space
that
surrounds the drillstring. One of the most common types of blowout preventer
is
known as a ram-type blowout preventer. A ram-type blowout preventer includes a
large housing mounted atop the wellhead that includes a large passageway
through
which the drillstring (and any components connected thereto) is able to pass.
The
housing also includes two or more rams located in a plane substantially normal
to the
axis of the drillstring and wellhead that are configured to move between
retracted and
extended positions. The ends of the rams are configured so that when extended,
they
provide a complete annular hydraulic seal around the drillstring disposed
therethrough.
[0005] Referring initially to Figure 1, a typical ram-type BOP 100 is shown
schematically. Ram-type BOP 100 shown includes a main housing 102, two or more
piston ram housings 104, and top 106 and bottom 108 bolting flanges for
connection
to other wellhead components (not shown). Ram-type blowout preventer 100 is
preferably constructed such that a generally cylindrical through bore 110
allows
oilfield tubulars 112 to unobstructedly pass through along a drillstring axis
114.
Oilfield tubulars 112 are typically components of a drillstring and may
include, but
are not limited to, drill pipe, drill collars, measurement tools, coiled
tubing, or
wirelines. Under normal conditions, through bore 110 is open and not
obstructed
such that fluids pass through an annulus 116 formed between the outer profile
of
tubulars 112 and the inner profile of through bore 110. When the wellbore
below is to
be shut off such that fluids below BOP 100 can no longer communicate with the
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wellbore above BOP 100, ram assemblies 118 are activated to provide a 360
seal of
annulus 116 between bore 110 and tubulars 112.
[0006] Each ram assembly 118 includes a sealing rain 120 having a leading edge
122
connected to a hydraulic piston 124 through a thrust rod 126. Leading edge 122
is
preferably contoured such that it corresponds with an outer profile of
oilfield tubular
112 so that a tight seal can be formed therebetween. Connected to sealing rams
120
through thrust rods 126, hydraulic pistons are activated by an external
hydraulic
source (not shown) to engage leading edges 122 against tubular 112 and seal
off
annulus 116. While it may be typical for there to be two ram assemblies 118,
each
with a corresponding semi-circular profile for leading edges 122, it should be
understood that a ram-type BOP 100 may employ three or more ram assemblies 118
with corresponding circular portions for leading edges 122 to shut off and
seal
annulus 116.
[0007] To seal off annulus 116, pressurized fluids are applied to hydraulic
port 128 in
communication with a reservoir 130 of ram housing 104. Increases in pressure
in
reservoir 130 urge back face 132 of piston 124, causing piston 124, rod, 126,
and ram
120 to be thrust toward tubular 112. The higher the pressure in communication
with
reservoir 130, the higher the loads transmitted through rod 126 to ram 120. To
retract
rams 120 out of annulus 116, pressure to port 128 is reduced and piston 124 is
able to
retract into reservoir 130. Retraction of pistons 124 may be assisted through
the use
of retraction springs (not shown), hydraulic retraction, or through any other
means
known in the art. Particularly, a retraction port 129 can be used to provide
hydraulic
access to a retraction reservoir 131, such that an increase in hydraulic
pressure
displaces piston 124 to retract ram 120.
[0008] With rams 120 extended and leading edges 122 engaging oilfield tubular
112,
a strong hydraulic seal prevents fluid from escaping the wellbore through
annulus
116. As long as hydraulic pressure is maintained in reservoir 130, rams 120
will
continue to seal annulus 116. However, there are circumstances where it is
desired to
maintain the annular seal regardless of the operational abilities of ram-type
BOP 100.
For example, it is desirable to maintain seal integrity in the event of a
power failure at
the rigsite. Furthermore, if wellbore is to be shut-in for an extended period
of time,
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maintaining hydraulic pressure over that time period is not always reliable or
prudent.
Therefore, systems and mechanisms to "lock" hydraulic rams 120 in place once
activated are highly desirable in the oilfield.
[0009] One former method to lock hydraulic rams 120 in place involves the
tightening of mechanical screws to lock pistons 124 in place once displaced in
reservoir 130. Such screws were tightened either manually or through power
devices
and would effectively lock rams 120, thrust rods 126, and pistons 124 in
place.
However, accessibility concerns make such a solution less than optimal.
Particularly,
in deep-sea installations, these locking screws must be activated by remotely
operated
vehicles or through electrical actuators. As such, their reliability is
suspect in depths
of several hundred feet or more. Furthermore, on land-based rigs, the BOP 100
is
typically located beneath the rig floor. As such, engaging and disengaging the
locking screws takes considerable time, time that is not always available in
the event
of an emergency.
[0010] Additional solutions to lock hydraulic rams are available to lock
thrust rods
126 in place. Formerly, ratchet profiles (e.g. U.S. Patent No. 3,941,141 to
Robert,
hereby incorporated by reference herein) have been used upon the outer
profiles of
thrust rods 126 in conjunction with matching locking members to retain thrust
rods
126 in place. Furthermore, various internal threaded mechanisms (e.g. U.S.
Patent
Nos. 4,052,995 to Ellison and 4,076,208 to Olson, both hereby incorporated by
reference herein) have been employed to secure thrust rods 126 in place using
whereby thrust rods 126 are threaded and corresponding jam-nut devices lock
thrust
rods 126 in place. Finally, various wedging solutions (e.g. U.S. Patent Nos.
4,305,565
to Abbe and 4,969,390 to Williams, both hereby incorporated by reference
herein)
have been proposed to lock thrust rods 126 in place. While promising, each of
these
solutions are considered by many to be less than optimal in that they exhibit
a slight
amount of slip or "play" in the reverse direction known as backlash before
they
engage and lock the thrust rods 126 in place. In the event of a high pressure
"kick" to
a wellbore, even infinitesimal displacements in rams 120 connected to thrust
rods 126
can result in a catastrophic release of wellbore fluids.
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[0011] Therefore, there is a long-felt need in the industry for an apparatus
to quickly,
positively, and solidly lock ram-type blowout preventers in an engaged
position with
minimal operator interaction and with minimal backlash of the rams before
locking
occurs.
SUMMARY OF INVENTION
[0012] In one embodiment, a blowout preventer includes a housing configured to
be
positioned above a wellhead and to surround a drillstring, a plurality of rams
positioned perpendicular to an axis of the drillstring, and a roller lock
positioned
about a thrust rod of each ram. The rams may be configured to engage the
drillstring
and hydraulically isolate an annulus between the wellbore and the drillstring
from
components located above the housing when the rams are in a closed position.
The
roller locks may be configured to maintain the rams in the closed position.
[0013] In one aspect, the present invention related to a locking apparatus to
be used
with a blowout preventer. In one embodiment, the locking apparatus includes a
plurality of spherical locking elements to engage and restrict movement of
thrust rods
connected to rams of the blowout preventer. The locking apparatus may also
include
a plurality of receptacles for the spherical locking elements, wherein each
receptacle
includes an inclined surface configured to thrust the spherical locking
elements into
compressive contact with the thrust rods when the operating rams are urged
open.
Furthermore, the locking apparatus may also include a release cage to retract
and
retain the spherical locking elements into the receptacles and to direct them
out of
compressive contact with the thrust rods when the operating rams are to be
opened.
[0014] In one aspect, the present invention relates to a method to lock rams
of a
blowout preventer. The method may include positioning spherical locking
elements
inside receptacles located adjacent to thrust rods of the rams, wherein the
receptacles
include inclined surfaces configured to engage the spherical locking elements
into the
thrust rods when the rams are urged open. The method may further include
locking
the thrust rods with compressive engagement of the spherical locking elements.
The
method may further include retracting the spherical locking elements into
their
respective receptacles with a release cage to unlock the rams.
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[0015] Other aspects and advantages of the invention will be apparent from the
following description and the appended claims.
BRIEF DESCRIPTION OF DR.AWINGS
[0016] Figure 1 is a schematic section-view drawing of a ram-type blowout
preventer.
[0017] Figure 2 is a schematic drawing of a thrust rod retainer in accordance
with an
embodiment of the present invention.
[0018] Figure 3 is a schematic drawing of a thrust rod retainer in accordance
with an
embodiment of the present invention.
DETAILED DESCRIPTION
[0019] Referring now to Figure 2, a schematic representation of a thrust rod
lock 250
in accordance with an embodiment of the present invention is shown. In this
embodiment, thrust rod lock 250 is preferably located within a piston ram
housing
(e.g. 104 of Figure 1) in a fixed position surrounding a thrust rod 226.
Thrust rod lock
250 is desirably configured to allow displacement of thrust rod 226 in a
locking
direction 252, while disallowing displacement of thrust rod 226 in an
unlocking
direction 254. Thrust rod lock 250 includes a main body 256, into which one or
more
roller receptacles 258 are formed. Roller receptacle 258 is shown including a
deep
section 260 that extends to a shallow section 262 through an inclined surface
264.
[0020] As such, each roller receptacle 258 is configured to retain and deploy
a roller
266 within and from receptacle 258 when locking of thrust rod 226 is
selectively
desired. Particularly, when roller 266 is located within deep section 260,
substantially
no contact occurs between roller and thrust rod 226, but as roller 266 travels
down
inclined surface 264, thrust rod 226 is increasingly locked into a bind by
roller 266
and surface 264. Furthermore, once so locked, further increases in load upon
thrust
rod 226 in unlocking direction 254 tend to cause roller 266 in contact with
inclined
surface 264 to compress against thrust rod 226 even tighter, resulting in an
even
stronger locked position. Displacements of thrust rod 226 in locking direction
252
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tend to roll roller 266 up inclined surface 264 toward deep section 260 of
main body
256, such that'rod 226 is free to move in locking direction 252.
[0021] Furthermore, a retainer cage 268 of thrust rod lock 250 retains roller
266
within receptacle 258 and is configured to retrieve roller 266 into deep
section 260
when thrust rod 226 is to be released. Retainer cage 268 preferably includes a
slot
(not shown) adjacent to roller 266 to allow retainer cage 268 to displace
roller 266
without interfering with the roller's engagement with thrust rod 226. In the
case
where roller 266 is a spherical roller, slot can be a longitudinal slot or a
spherical
section to match the outer profile of roller 266. Alternatively, in the case
where roller
266 is cylindrical, the slot can be a transverse slot approximately the same
width as
roller 266. Regardless of configuration, when thrust rod 226 is to be
displaced in
unlocking direction 254, retainer cage 268 is displaced in direction 270 to
retrieve
roller 266 up inclined surface 264 and into deep section 260, away from the
engagement with outer surface of thrust rod 226. Retainer cage 268 may be
biased so
that roller 266 is biased in the direction opposite 270 and into thrust rod
226.
Alternatively, cage 268 may be unbiased allowing movement of thrust rod 226 to
be
the sole force in causing roller 266 to be engaged therewith.
[0022] It should be understood that any means to displace or bias retainer
cage known
in the art may be employed, including, but not limited to, hydraulic lines,
springs, and
tension cables. Particularly, retainer cage can be constructed to be displaced
when
hydraulic pressure is applied to a hydraulic actuator attached thereto.
Furthermore, if
a hydraulic device is employed to retract thrust rod 226 into unlocking
direction 254,
a control system (not shown) can be used to direct such hydraulic pressure to
either
the retainer cage actuator, the thrust rod retractor, or both. Alternatively,
a hydraulic
system to release thrust rod lock 250 can be distinct from a hydraulic system
to
displace thrust rod 226 in unlocking direction 254.
[0023] Furthermore, it should be understood that inclined surface 264 can be
any of
various types known in the art. Particularly, surface 264 can be a mere planar
surface
or can be profiled to fit the contours of spherical rollers 266. Furthermore,
while
rollers 266 are described generically, it should be understood that they can
be
constructed as spherical or cylindrical devices and can be constructed with
various
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hardness and friction values to facilitate contact and engagement between
thrust rod
226 and inclined surface 264.
[0024] Referring now to Figure 3, a schematic of a thrust rod lock 350 in
accordance
with an embodiment of the present invention is shown. Like thrust rod lock 250
of
Figure 2, thrust rod lock 350 of Figure 3 is preferably located within a
piston ram
housing 304 in a fixed position surrounding a thrust rod 326. While a single
thrust
rod lock 350 is shown, it should be understood that a plurality of rod locks
350 can
surround thrust rod 326 radially and axially.
[0025] Thrust rod lock 350 is desirably configured to allow displacement of a
thrust
rod 326 in a locking direction 352, while resisting displacement of thrust rod
326 in
an unlocking direction 354. Thrust rod lock 350 includes a main body 356, into
which two series of roller receptacles 358A, 358B are formed. Like receptacles
258
of Figure 2, inner roller receptacle 358A is profiled to urge a roller 366A
into
engagement with thrust rod 326 when thrust rod 326 is displaced in unlocking
direction 354. At the same time, outer roller receptacle 358B is profiled to
urge a
roller 366B into engagement with an outer wear plate 380 when main body 356 is
displaced in unlocking direction 354. Therefore, for each location about and
along
thrust rod 326, one roller 366A bites with thrust rod 326 and another roller
366B bites
with wear plate 380 to resist displacement of thrust rod 326 in unlocking
direction
354. As such, for each incremental displacement in unlocking direction 354,
thrust
rod 326 of Figure 3 will experience double the radial compression as would be
experienced by a thrust rod in a single roller configuration (e.g. Figure 2).
[0026] Furthermore, inner and outer retainer cages 368A, 368B retain rollers
366A,
366B inside receptacles 358A, 358B and allow for rod lock 350 to be released
once
retraction of thrust rod 326 is desired. As with retainer cage 268 of Figure
2, retainer
cages 368A, 368B allow rollers 366A, 366B to contact thrust rod 326 and wear
place
380 through slots or other forms of apertures (not shown) therethrough. As
before,
such apertures in retainer cages 368A, 368B can take the form of longitudinal
slots or
spherical sections in the case where rollers 366A, 366B are spherical or can
be
transverse slots if rollers 366A, 366B are cylindrical. A pair of bias springs
382, 384
is shown working in conjunction with main body 356 and retainer cages 368A,
368B
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to thrust rollers 366A, 366B into locking engagement with thrust rod 326 and
wear
plate 380 by default. As inner retainer cage 368A is fixed relative to housing
304,
spring 382 urges main body in the direction of arrow 352 such that inclined
surface
364A urges roller 366A into contact with thrust rod 326. Likewise, spring 384
between main body 356 and outer retainer cage 368B urges cage 368B and roller
366B in the direction of arrow 352 such that inclined surface 364B urges
roller 366B
into contact with wear plate 380.
[0027] To release rollers 366A, 366B from their engagement with thrust rod 326
and
wear plate 380, an unlocking mechanism 386 is employed. Unlocking mechanism
386 can be constructed any number of ways, but is preferably configured to
retract
rollers 366A, 366B into their respective receptacles 358A, 358B so that thrust
rod 326
can be retracted in unlocking direction 354. Unlocking mechanism 386 of Figure
3 is
shown within a recess 388 of housing 304. A hydraulic seal 390 surrounding
mechanism 386 ensures that when hydraulic pressure is increased to a hydraulic
port
392, mechanism 386 is displaced in the direction of 354 such that a thrust
face 394
engages a corresponding load shoulder 396 of outer retainer cage 368B. When
pressure to port 392 is sufficiently elevated, springs 384 and 382 are
compressed such
that rollers 366A, 366B are retained within recesses 358A, 358B to enable
thrust rod
326 to be retracted. Once thrust rod 326 is retracted, pressure to port 392
can be
released so that thrust rod 326 can be quickly engaged and held in place
without any
secondary locking step necessary. In its ordinary, equilibrium state, rod lock
assembly 350 resists disengagement of thrust rod 326 without any supplemental
steps,
even in the event of total loss of hydraulic power.
[0028] Rod lock assemblies 250 and 350 of the present invention exhibit many
advantages over locking mechanisms currently available. In particular, rod
lock
assemblies 250, 350 are capable of securing thrust rods 226, 326 almost
instantaneously and with little or no backlash or slippage. Furthermore, rod
lock
assemblies 250, 350 are disclosed as "fail safe" devices, in that they lock by
default.
No affirmative steps are necessary to lock thrust rods 226, 326 in place once
they are
extended. Rod lock assemblies 250, 350 automatically engage and resist
disengagement of thrust rods 226, 326. As no external power source is
necessary to
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engage rod locks 250, 350, their effectiveness is not compromised by power
failures.
In contrast, hydraulic (or other) power is only necessary to disengage rod
locks 250,
350. Finally, as rod locks 250, 350 are configured to engage smooth outer
profiles of
thrust rods 226, 326, no obstructive features are necessary on thrust rods
226, 326.
Former solutions required special profiles that could obstruct thrust rod 226,
326
operation and engagement.
[0029] While the invention has been described with respect to a limited number
of
embodiments, those skilled in the art, having benefit of this disclosure, will
appreciate
that other embodiments can be devised which do not depart from the scope of
the
invention as disclosed herein. Accordingly, the scope of the invention should
be
limited only by the attached claims.
