Note: Descriptions are shown in the official language in which they were submitted.
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PIPE STRING TIE-BACK CONNECTOR
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Field of_the Invention
This invention relates to pipe connectors, and
more particularly to tie-back connectors for joining a
riser pipe string to a ~asing hanger or other element in a
subsea wellhead.
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The product of oil and gas from offshore wells is
an established major endeavor of the petroleum industry,
and requires techniques and apparatus for connecting
strings of pipe to subsea wellheads to provide conduits
between the wellheads and the frilling and/or production
platforms at the water surface. Whereas divers can be
used to make up these connections at relatively shallow
depths, their employment is very costly and involves
undesirable elements of risk, thereby encouraging the
development of remotely operable connector systems that do
not require diver assist. The search for and production
of oil in deep water, and especially at depths beyond
practical diver operations, has increased the need for
well equipment that can be installed and operated entirely
by remote control from a surface facility, and the prior
ar~ reflects considerable development in that area.
One of the prior types of pipe connectors for
this p~rpose comprises a union nut s~yle o~ threaded
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components that requires rotation of a relatively large
ring or sleeve element to make up the connection. Not
only is it difficult to properly align the riser with the
wellhead so that cross-threading will not occur, it also
is txoublesome to xotate the ring or sleeve without the
aid of special equlpment and skilled personnel. Another
problem with union nut connectors is that their single
shoulder is highly loaded when the connection is
completed, and this stress results in an undesirably
short fatigue life.
Another type of known riser connector employs a
turnbuckle-style assembly with right and left hand
threads between a rotatable sleeve and the two pipe
elements that are to be connected. Although the
principle of this connector type is sound, in practice it
requires undesirably high torque in order to produce the
pre-load required for proper functioning.
In a third category of riser connectors the
entire riser must be rotated in order to make up the
connection at the wellhead. Not only is it difficult to
handle these very heavy, and often quite lengthy and
complex, strings of pipe, their rotation can result in
fatally galling the metal-to-metal seal that must be
employed at the wellhead, thereby requiring disconnection
and removal of the riser, replacement of the seal, and
~ another attempt to establish a fluid-tight joint.
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~ Summary of the ~ ve_tlon
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Accorcling to an aspect of the invention, a pipe
s-tring tie-bac]c connector ls provided for lnterconnecting
a riser pipe and subsea wellhead withol1t rotation of the
riser pipe. The connector comprises a housing connected
to the riser pipe. A landing body has means for seating
on a shoulder in a wellhead element. Axially reciprocal
sleeve means in-terconnects the housing and the landing
body and seats a locking split ring means for preventing
axial movement of the landing body.
The connector is characterized by the sleeve
means having differential threads on axial opposite ends
engaging complementary threads on the housing and the
landing body~ Means secures the landing body to the
wellhead element for preventing rotational movement o~
the landing body with respect to the wellhead element.
Means is provided for rotating the sleeve means relative
to the housing and the landing body for locking the
conn~ctor to the wellhead by expanding the split ring
means into a groove in the wellhead element and for
establishing a metal-to-metal seal between a lower
portion of the connector and another shoulder in the
wellhead element.
According to another aspect of the invention, a
method is provided for connecting the riser pipe to the
subsea wellhead withQut rotation of the riser pipe by
this apparatus. ~he method comprises attaching the pipe
connector assembly to the riser pipe; inserting the lower
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portion of the connector assembly into the wellhead and
seating the landing body on the first shoulder; rotating
the sleeve means and pulling the housing and landing body
towards each other and expanding the split ring and
locking the assembly to the wellhead, and continuing
rotating the sleeve means and establ:ishing the
metal--to-metal seal between the hous:ing and the second
shoulder of the wellhead.
In its preferred form, the tie-back connector
is associated with a guide assembly that is attached to
the riser to seat against the wellhead and serve as a
centrali~er for the connector housing. Trash seals
between the guide assembly and the wellhead protect the
tie-back connector and other components of the
installation from corrosion, a valuable additional
feature should it become necessary to disconnect and
remove the riser from the wellhead, for example if
reinstallation of a subsea blowout preventer is required.
Brief Desciption of the Drawings
Figure 1 is a side elevation, partially in
section, of a subsea wellhead installation and a riser
releasably attached thereto by means of a tieback
3~ connector according to the present invention.
Figure 2 is a fragmentary view, in side
elevation and on an enlarged scale, of the Figure 1
installation, showing in bet-ter detail the tie-back
connector components.
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The Preferred E~bodi~ent
Attention is directed first to Figure 1 which
illustrates a riser tie-back connector assembly 8
interconnecting a subsea wellhead assembly 10 and a riser
12. The wellhead assembly comprises a te~plate 14, a
guide assembly 16, guide posts 18 secured to the template
14 and anchoring guide cables 20 that extend to the
surface drilling platform (not shown), a conductor housing
22 mounted on top of a conductor pipe 24, a wellhead 26
within and supported on the conductor housing 22, a first
o.r outer casing hanger 28 supported in the wellhead 26
near the lower end thereof, and a second or inner casing
hanger 30 also supported in the wellhead 26.
The second casing hanger 30 is locked into the
wellhead 26 by a lock-down assembly 34, which assembly 34
also is utilized to transfer riser loads through the
hanger 30 to the wellhead 26, and packoff assemblies 36
38 establish a fluid-tight seal between the wellhead 26
and the first casing hanger 28, and betwePn the wellhead
26 and the second casing hanger 30, respectively all in a
conventional manner.
The riser tie-back connector assembly 8, which
will be described in more detail with reference to Fig. 2,
includes an annular housing 40 that is attached at its
upper end by bolts 42 to the lower end of the riser 12.
In the illustrated embodiment, the housing 4V is
surrounded by a guide assembly 44, and the lower portion
40a of the housing 40 functions as an element of a
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lock-down assembly 46 for the tie-back connector, whereby
the housing 40, the guide assembly 44, and the lock-down
asse~bly 46 together constitute the tie-back connector
assembly 8. Mounted on this connector assembly 8 is a
guide frame 48 to which are fixed a plurality of guide
sleeves 50 (only two shown) for guiding ~he assembly 8 on
its descent from the surface platfor~ to the wellhead
assembly 10.
With reference now to Figure 2, the tie-back
csnnector lock-down asse~bly 46 comprises the lower
portion 40a of the housing 40, an annular landing body 52,
a rotatable sleeve 54 and a lock-down ring 56. The upper
end portion of the sleeve 54 is connected to the housing
40 by threads 58, and the landing body 52 is connected by
threads 60 to the lower portion of the sleeve 54. The
threads 58, 60 are so related that they constitute a
differential thread system, for example by differing in
their pitch, whereby rotation of the sleeve 54 with
respect to the housing 40 and landing body 52 in one
direction tends to pull the housing and landing body
towards each other, and rotation in the opposite direction
tends to push the housing and landing body apart. The
landing body 52 includes an annular shoulder 52a that
cooperates with a co~ple~entary shouldex 30a on the second
or inner casing hanger 30, and the landing body is keyed
to the hanger 30 at 62 to prevent rotation of the landing
body with respect to the hanger.
The lock-down ring 56, which has a single axial
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split and i6 of resilient construction, is carried on the
sleeve 54 surrounding its reduced outside diameter area
54a. The upper end of the area 54a is formed by an
annular frusto-conical shoulder 54b which functions to cam
the ring 56 out~ardly from a retracted condition in groove 54a (nGt
shown) into an expanded condition (Fig. 2~ as the sleeve
54 is threaded dowllwardly with respect to the landing body
52.
The lower end of the housing 40 is tapered to
cooperate with an annular shoulder 30a on the inside
surface of the hanger 30 to establish a metal-to-metal
seal at 66 between the housing and hanger. Annular
resilient seals 68 just above the metal-to-metal seal 66
provide a fluid-tight barrier between the housing 40 and
hanger 30 prior to establishing the metal-to-metal seal
66. The sleeve 54 similarly carries annulax resilient
seals 70 at its lower end to effect sealing engage~ent
with the hanger 30, and an annular resilient seal 72 at
the upper end of the sleeve 54 provides the requisite
trash seal between the sleeve and the housing 40O
O~eration
With respect to the structures illustrated in the
drawings, after the casing hanger 30 has been locked down
by means of the l~ck-down assembly 34 and the packoff
asse~bly 38, the tie-back connector assembly 8 and the
riser 12 to which it is attached are lowered to the subsea
wellhead assemb~y 10. The guide asse~bly 44 contacts and
seats on the wellhead 26, thus serving as a centralizer
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for the housing 40. A downward force i8 then applied to
the housing 40 through the riser 12, pushing the housing
and the tie-back connector lock-down assembly 46 further
into the wellhead 26 until the landing body shoulder 52a
lands and seats on the casing hanger shoulcler 30b (Fig. 2).
A torquing tool (not shown) is then lowered
through the riser 12 by means of a drill pipe string (not
shown). When the tool arrives at the grooves 80 (Fig, 2)
in the lock-down assembly sleeve 54, elements on the tool
expand into these grooves and releasably lock the tool to
the sleeve. The drill string is then rotated, thereby
rotating the running tool and the sleeve 54. As the
sleeve 54 rotates it move-~ downwardly within the landing
body 52, thereby expanding the lock-down ring 56 into the
mating grooves in the casing hanger 30 and securing the
body 52 against upward movement. As rotation of the
sleeve 54 is continued the differential threads 58, 60
cause the connector housing 40 to move downward until its
lower end comes to rest on the hanger shoulder 30a,
thereby establishing a metal-to-metal seal at 66 between
the sleeve and the hanger. Accordin~ly, at this final
position three seals exist between the connector assembly
and the hanger 30, i.e~ the two resilient seals 68 and the
metal-to-metal seal 66, thereby assuring the maintenance
of pressure integrity between the riser annulus and the
exterior of the wellhead installationO
Although the best mode contemplated for carrying
out the preserlt invention has been herein shown and
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described, it will be apparent that modif ication and
variation may be ~ade without departing f rom Yhat is
regarded to be the subject matter of the inventionO
