Note: Descriptions are shown in the official language in which they were submitted.
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PIPE GRIPPING SYSTEM AND METHOD
This application is related to U.S. Patent Application No. 08/670,639
entitled Pipe Gripping Assembly and Method invented by Carlos A. Torres,
issued as U.S. Patent No. 5,732,909 on March 31, 1998.
Backgiround of the invention
The present invention relates generally to a system and method for
handling pipe that is to be run into or pulled from a well. More specifically,
the
present invention relates to a system and method for gripping such pipe
without
damaging the pipe's outer surfaces while simultaneously providing a secondary
gripping mechanism that automatically actuates to grip the pipe if the pipe
slips
through the primary gripping mechanism.
Brief Description of the Prior Art
Slip assemblies are customarily employed to temporarily grip and hold
pipe as it is being run into or pulled from a well. In a conventional slip
assembly,
tapered slips, which are carried in a tapered slip bowl, are "set" into
gripping
engagement with the pipe extending through the center of the bowl by moving
the
slips into contact with the pipe and then slightly lowering the pipe to allow
the
slips to support the pipe weight. The surface friction between the slips and
the
pipe causes the slips to move with the pipe, which pushes the tapered slips
axially downwardly into the tapered slip bowl. This relative movement between
the tapered slips and the tapered bowl forces the slips radially toward each
other
to grip the pipe extending through the center of the assembly. As the weight
of
the string increases, the downward force on the slips increases, which, in
turn,
acts through the engaged tapered surfaces to increase the radial pipe gripping
force exerted by the slips. The slips are released by first lifting the string
to
relieve the weight on the slips and then retracting the slips out of
engagement
with the pipe.
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The slips are typically equipped with replaceable, steel slip-dies that
contact and grip the pipe. Conventional steel dies are typically equipped with
radially projecting teeth that are designed to penetrate the outer pipe
surface to
increase the gripping force ~of the slips. The usual slip setting procedure
can
produce die-tooth cuts in the pipe surfaces that decrease the thickness and
structural strength of the pipe, provide a corrosion attack point, and
otherwise
detrimentally affect the pipe.
Efforts at reducing the scarring caused by die teeth include the use of slip
dies with very small teeth or specialty configured teeth or, in some cases,
with
no teeth at all. While the prior art designs produce reduced pipe damage, as
compared with conventional steel toothed-dies, a primary problem with these
designs is that the slips can sometimes fail to grip the pipe securely and
thus
permit the string to slide through the slip assembly. The problem is most
likely
to occur as the string weight increases or when the slip teeth become clogged
with debris or when the string or slips are contaminated with oil or other
slippery
substances.
if the pipe string slides through the dies, in many cases, the downward
slide is stopped suddenly when a pipe coupling at the end of a pipe joint
engages the slip assembly. Such slippage is objectionable in that it allows
the
string to be mispositioned, and also damages the pipe surface as the pipe
slides
through the slips. Moreover, if the impact of the coupling striking the slip
assembly is strong enough, the pipe may be knocked free of the coupling
allowing the string to fall into the well. .
One prior art design, described in U.S. patent no. 3,579,753, describes
a smooth die system that employs a special die carriage design to increase the
radial die forces acting on the pipe. The patented system requires a
relatively
complicated slip carrier design that can be expensive to produce and maintain.
No provision is made in the patented system for preventing pipe slippage if
the
smooth die slips should malfunction.
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Other prior art devices for holding pipe without damaging the pipe surface
have generally included complex mechanisms that are expensive to build and
maintain. These prior art devices also lack an effective backup holding
mechanism to prevent pipe movement if the primary holding device fails.
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Summarv of the Invention
Smooth, toothless slip-dies are used in a conventional tapered-bowl, slip
anchoring device as a primary assembly for gripping the pipe. A secondary
conventional anchoring device, with standard toothed dies, is used as a backup
assembly to automatically grip the pipe if the pipe slips through the primary
assembly.
1n a preferred embodiment, a relative soft, aluminum alloy is employed for
the slip dies so that the closing forces of the slip assembly cause the dies
to
conform to the outer surface of the harder pipe to thereby enhance the
gripping
force of the primary slip assembly. When the assembly is used with fiberglass
pipe and other very soft pipe, die materials harder than the pipe material may
advantageously be used.
In operation, the slips of the primary assembly are placed against the pipe
string and the string weight is transferred to the aluminum slips to set the
slip
assembly. Once the primary assembly has been set, the slips of the secondary
assembly may be closed to allow the steel toothed-dies to contact the pipe.
When the primary assembly is operating properly, the pipe string will be
stationary with the entire string weight supported by the primary assembly.
While the string is stationary, the steel dies of the backup assembly are in
contact with the pipe but are not set and therefore exert virtually no radial
force
on the pipe. If the string should slip through the closed primary dies, the
downward motion of the string will pull the steel dies of the secondary
assembly
down into the slip bowl, which will force the dies to move radially into firm
gripping engagement with the pipe to thereby set the secondary assembly to
prevent any continued downward string movement.
Under normal operating conditions, the pipe will be firmly held by the
primary slip assembly. On rare occasions, however, the primary assembly may
allow the pipe to move after the assembly has been set. On these rare
occasions, actuation of the secondary or backup slips to stop the string
slippage
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may produce external marking on the pipe. This damage can, however, be
repaired or, if necessary, the damaged joint may be extracted from the string
and
replaced with a new joint.
One of the advantages of the design of the present invention is that the
secondary system and the primary system share the string weight when pipe
slippage occurs. As a result, the amount of penetration produced by the teeth
of the secondary system is substantially less than is normally produced where
the full string weight is acting on the toothed-slips. Moreover, because the
secondary slips are engaged and operate immediately at the first onset of
string
slippage, the pipe does not have an opportunity to increase its falling speed.
As
a result, the impact of the secondary slips in the tapered bowl is held to a
minimum, which further reduces the likelihood of damage to the pipe.
The slippage of the pipe through the primary slip assembly may result, for
example, from the presence of oil, or grease, or other debris located between
the
primary slips and the pipe. Once the cause of the slippage is corrected, the
system may be reinitiated to continue running the pipe.
From the foregoing it will be understood that an important object of the
present invention is to provide a pipe gripping system that does not damage
the
external surface of the pipe.
A related object of the invention is to provide a system in which
conventional slip assemblies, using non-conventional, smooth-surface dies, are
used with conventional slip assemblies using conventional, toothed dies. The
two assemblies are employed together to reliably grip and hold fragile well
pipe
without, in most cases, harmfully damaging the pipe and without the risk of
dropping the pipe into the well.
An important object of the present invention, when running metal pipe, is
to employ a smooth, toothless slip-die of relatively soft material so that, as
the
weight of a pipe string increases, the die will increasely conform to surface
irregularities in the pipe and increase the gripping force between the slip
die and
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the pipe.
Another object of the present invention is to provide a fail-safe backup that
ensures the pipe string will not be dropped into the well if the primary pipe
gripping mechanism should fail. It is thus an object of the invention that the
backup pipe-gripping assembly function without damage to the pipe during
normal operation and only be actuated in the event of pipe slippage through
the
primary gripping system.
A general object of the present invention is to provide primary and
secondary pipe-holding mechanisms wherein the secondary mechanism is
automatically actuated to hold the pipe when the pipe slips through the
primary
mechanism.
In accordance with one aspect of the present invention there is provided a
slip assembly for gripping and holding pipe comprising: a tapered slip bowl
having a central axis; a tapered metal slip positioned in said bowl whereby
tapered surfaces between said slip and said bowl cause said slip to move
radially
relative to said central bowl axis as said slip moves axially relative to said
bowl; a
pipe contact portion of said slip comprising a smooth pipe contact surface
adapted to engage the outer cylindrical surface of a pipe extending axially
through said slip bowl, said pipe contact portion being constructed from a
material softer than the material of said pipe whereby said pipe contact
portion is
extruded into the surface irregularities of said pipe as said slip is moved
radially
into said pipe; and said slip being selectively moveable between a closed set
position holding said pipe and preventing axial pipe movement and an open
unset position releasing said pipe to permit axial pipe movement.
In accordance with another aspect of the present invention there is
provided a pipe gripping and holding system comprising: a primary pipe
gripping
mechanism, when set, grips and holds an axially extending pipe; and a
secondary pipe gripping mechanism, operable when said primary pipe gripping
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mechanism is set for automatically gripping said pipe when said pipe moves
axially through said primary gripping mechanism.
In accordance with yet another aspect of the present invention there is
provided a first pipe gripping apparatus for gripping and holding an axially
extending pipe comprising: a primary tapered slip bowl adapted to encircle
said
pipe, a primary pipe gripping apparatus having first tapered slip elements
carried
in said primary bowl and adapted to be forced radially inwardly as said first
slip
elements are moved axially downwardly relative to said primary bowl, and a
malleable pipe contact material, softer than the material of said pipe,
carried by
said first slip elements for engaging and plastically deforming against the
surface
of said pipe as said first slip elements are moved in a radial direction
toward said
pipe.
In accordance with still yet another aspect of the present invention there is
provided a pipe gripping apparatus adapted to encircle and grip an axially
extending pipe, comprising: a tapered slip bowl having a central axis and
adapted to encircle and align axially with said pipe; tapered slip elements
carried
in said slip bowl and adapted to move radially relative to said central bowl
axis
into engagement with said pipe as said slip elements move axially relative to
said
bowl; and pipe contact material, softer than the material of said pipe,
carried by
said slip elements for contacting said pipe and plastically deforming into the
surface irregularities of said pipe as said slip elements are forced radially
inwardly by axial movement of said slip elements relative to said bowl.
In accordance with still yet another aspect of the present invention there is
provided a pipe gripping and holding system comprising: first and second pipe
gripping mechanisms for gripping and holding a pipe; smooth pipe contact
elements in said first pipe gripping mechanism operable to be set for engaging
and holding said pipe to prevent axial pipe movement through said first pipe
gripping mechanism; and non-smooth pipe contact elements in said second pipe
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gripping mechanism automatically operable by pipe movement occurring after
said smooth contact elements are set to grip and hold said pipe.
In accordance with still yet another aspect of the present invention there is
provided a method of gripping and holding pipe comprising the steps of:
setting
smooth pipe contact elements against a pipe; resting the weight of said pipe
on
said smooth pipe contact elements; and placing non-smooth pipe contact
elements against said pipe whereby said non-smooth pipe contact elements are
set to grip and hold said pipe by pipe movement occurring after said smooth
pipe
contact elements are set.
These and other objects of this invention will be understood from the
following description taken with reference to the attached drawings.
Brief Description of the Drawings
Fig. 1 is a vertical elevation, partially in section, illustrating the pipe
gripping system of the present invention;
Fig. 2 is a partial, horizontal cross sectional view taken along the line 2-2
of Fig. 1, illustrating the primary gripping system of the present invention
in set
position; and
Fig. 3 is a vertical elevation illustrating details in the construction of a
smooth-face die insert used in the primary gripping system of the present
invention.
Description of the Preferred Embodiments
The pipe gripping assembly of the present invention is illustrated generally
at 10 in Fig. 1. The assembly 10 includes a moveable slip assembly 11 and a
stationary slip assembly 12 that operate to selectively grip, hold, release
and
raise or lower a pipe string 13.
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The movable slip assembly 11 is attached to conventional elevator links
14, which, in turn, are connection to a conventional block (not illustrated)
that
moves up and down in the derrick (not illustrated).
The stationary slip assembly 12 is employed with a work structure 15 that
is positioned about a central floor opening 16 formed in a floor 17 of a
conventional drilling or workover rig. The stationary assembly 12 includes a
primary slip assembly 18 supported on an upper work structure floor 19 and a
secondary slip assembly 20 positioned on the rig floor 17. The assembly 20 is
aligned below the assembly 18 such that both assemblies are positioned to grip
the pipe 13 extending through the central floor opening 16.
The primary slip assembly 18, illustrated in its open condition, includes a
tapered slip bowl 21 and multiple tapered slip elements 22, 23, 24 and 25.
When the assembly is in its closed condition, the tapered slip elements are
forced to move radially as they move axially within the bowl 21 to provide an
increasingly greater radial pipe gripping force with increasing pipe string
weight.
As thus far described, the assembly 18 is of conventional design and
operation.
Each of the slip elements 22, 23, 24 and 25 carries, respectively, a slip
die 26, 27, 28, and 29 of the present invention which is devoid of die teeth
or
other significant surface irregularity in the pipe contact area. As best
illustrated
in Fig. 2, the four dies 26-29 cooperate to substantially completely encircle
the
pipe to maximize the circumferential surface contact area between the pipe and
the dies. When used for running chrome alloy and other relatively soft, metal
pipe, the dies 26-29 are preferably constructed of a material that is
malleable
and can be deformed into the surface irregularities of the pipe as the radial
gripping forces are increased to improve the grip of the smooth surface dies
with
the pipe. When used for running non-metal pipe, however, the die may
preferably be constructed of a material that is harder than the pipe material.
In
any event, the material of construction of,the die may be selected as a
function
of the pipe material and the pipe string weight to optimize system
performance.
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Thus, the die material for fiberglass pipe may also preferably be an aluminum
alloy even though the aluminum alloy is harder than the pipe material.
Fig. 3 illustrates details in a smooth or toothless die 26 which may be
employed in a conventional sip a~sembly as described herein. The die 26
includes a curved pipe contact surface 26a that has a curvature matching the
outer surface of the pipe 13. in a preferred embodiment, the die 26 has a
circumferential development of approximately 90° so that four such dies
provide
almost 360° coverage of the pipe circumference. Maximum contact surface
bcttNeen the slip die 26 and pipe is desired to obtained optimum gripping
force.
Except for the material of construction and the absence of teeth, the die 26
is
similar to a conventional replaceable die employed in conventional slip
assemblies.
1n one embodiment of the system and method of the invention, dies
constructed of 6061-T6 bare aluminum in a "full circle" pattern having a
tensile
strength of 45,000 psi and a yield strength of 40,000 psi were used in a
manually
operated Cavens Model "C" spider for the primary slip assembly 18. The pipe
being run into the well was a 13% chrome alloy.
The secondary slip assembly 20, which is conventional in all respects,
includes conventional steel dies 30, 31,32, and 33 with die teeth formed on
their
pipe contact surfaces. As will be hereinafter more fully described, the
secondary
slip assembly 20 is designed to automatically grip the pipe 13 in the event
the
pipe slips through the primary slip assembly 18. In one embodiment of the
system used to run a 13% chrome pipe string, the assembly 20 was a Cavens
Model "C" spider with conventional steel, full- circle, slip inserts.
The moveable slip assembly 11, which is illustrated schematically in its
closed condition, includes a primary slip assembly 34 and a secondary slip
assembly 35. The primary assembly 34 includes smooth surface slips 36 and
the lower assembly 35 includes toothed slips 37. In operation and basic
construction, the assembly 11 is similar to the assembly 12. The dies in slips
36
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of the primary moveable slip assembly 34 are smooth, tooth-free elements
similar to the dies 26-29 of the stationan~ assembly 18. The dies in slips 37
are
conventional toothed dies similar to the dies used in the stationary slip
assembly
20. The assembly 11 may be constructed of stacked assemblies such as
illustrated for the assembly 12 or may be constructed of a single structure
having
two separate bowl sections as schematically illustrated in Fig. 1.
Although not specifically illustrated herein, it will be understood that the
slips of the assemblies 18, 20, 34, and 35 may be manually operated between
open and closed positions or may be operated between such positions with the
use of hydraulic or air control systems. The construction and operation of
such
operating methods and controls are well known in the art.
In a typical pipe "running-in", operation in which pipe is being run into the
well, the stationary assembly 12 holds and supports the string 13 while the
movable assembly 11 is used to pick up and place a single joint of pipe {not
illustrated) at the top of the string 13. After the newly added joint is
screwed into
the top of the string, the slips 36 of the movable assembly 11 are set to grip
the
top of the new joint and the slips 37 are then closed. The block is raised
slightly
to raise the joint and attached string 13 to take the string weight off of the
stationary slip assembly 18. Once the string weight is removed, the slips 18
and
20 of the assembly 12 are opened and the movable slips 11 and gripped string
13 are lowered into the position illustrated in Fig. 1. The stationary
assembly ~12
is set by first setting the slips 18 and then resting the string weight on the
slips
18. The slips 20 are then closed. Because the weight of the string is being
supported by the primary slips assembly 18, there is no downward pipe force
acting on the slips of the secondary assembly 20 to cause the slip dies to
bite
into the pipe. After the two slip assemblies 18 and 20 are respectively set
and
closed, the movable slip assembly 11 may release the string 13 to pick up
another single joint and repeat the "running in" process. The described
process
is repeated until the entire string has been lowered into the well.
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Pulling or removing the pipe string from the well is a similar procedure, run
in reverse. Thus, the slips of the stationary assembly 12 are open as the
slips
36 of the moveable assembly 11 grip and move the string to pull one joint
above
the stationary assembly. The slips 18 and then 20 of the stationary assembly
12
5 are respectively set and closed, the entire string weight is rested on this
stationary assembly 12, and the slips 37 and then 36 of the movable assembly
35 are respectively opened and onset to release the pipe. The top joint is
unscrewed from the string and the movable assembly is lowered to grab the new
top of the string. The movable assembly grips the string 13 and lifts the
string
10 up slightly and the stationary assembly is opened once the string weight is
taken
by the movable slip assembly. The described procedure is repeated for each
joint until the entire pipe string is removed from the well.
In the described method of operating the slips of the stationary and
moveable assembly, it will be understood that the slips in either assembly may
be released from the pipe after the string weight has been taken by the other
assembly. The setting procedure uses the closing of the slips as well as the
application of string weight to produce the force required to grip and hold
the
string. Preferably, the toothed-die slips are set, or moved into position
between
the bowl and the pipe in preparation to being set, after the smooth die slips
have
firmly gripped and are independently holding the string stationary. The amount
of force exerted by the toothed die against the pipe when the conventions!
slips
are closed and set is sufficient to cause the toothed die to move downwardly
in
the event the pipe slips down but is not great enough to produce any
penetration
or other damaging marking on the pipe under normal situations where there is
no slippage of the pipe through the smooth dies.
From the foregoing, it will be appreciated that the pipe gripping system
and method of the present invention provides a safe and efficient procedure
for
running and pulling fragile pipe strings. Conventional slip and elevator
designs
may be employed in combination with unique, smooth-surface slip dies to grip
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and hold the pipe strings without damage to the pipe surface. In a preferred
embodiment, the smooth surtace dies are constructed of a relatively soft
material
as compared to the material of the pipe. The danger of string loss is
prevented
by employing conventional slip assemblies with toothed slip dies as secondary
gripping and holding assemblies that actuate only when slippage of pipe
through
the set primary slip assembly occurs.
As used herein, the terms smooth and non-smooth are intended to be
comparative terms that distinguish the primary pipe gripping elements from the
backup or secondary pipe gripping elements, It will be understood that the
smoothness of the pipe contact area is a matter of degree and that a pipe
contact surface with small irregularities is considered "smooth" when compared
with the pipe contact surface of conventional pipe dies. The comparative terms
used are employed to distinguish the pipe gripping elements as a function of
the
amount of damage or potential damage each may do to the pipe surface when
used as a gripping element. The less smooth the surface, the greater the
likelihood of damage. It will also be understood that, white the preferred
form of
the toothless dies of the present invention have been described as being
constructed of an aluminum alloy, other materials may also be advantageously
employed, even those which may not be malleable or softer than the pipe which
is being handled.
Accordingly, while a preferred embodiment of the system and method~of
the present invention has been described herein, it will be appreciated that
various modificatie~~s in the construction and operation of the described
system
and method may be made without departing from the spirit and scope of the
invention.
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