Production casing tieback connector assembly

A connector assembly for connecting a production casing (38) to a subsea wellhead (10) from a tensioned leg platform. A funnel (46) external to the wellhead, guides and aligns the conductor during installation and places a limit on the angular deflection of the conductor relative to the wellhead. The conductor includes at its lower end a tieback joint (44) which is pulled down, aligned, and sealed against the casing hanger (24) with a floating bushing (64) which also preloads the seal (66). The flexibility of the tieback joint (44) in relation to the angular deflection permitted is such that the surfaces of the seal (66) will not move; and preferably the preload on the seal is not removed.

BACKGROUND OF THE INVENTION 
The invention relates to oil production from the seafloor to a tensioned 
leg platform and in particular to a connector assembly for tying back the 
production casing from the seafloor to the TLP wellhead. 
Offshore oil wells may be drilled from a floating platform and therafter 
produced to a later-installed tensioned leg platform. Such a procedure 
requires the running of casing strings from the platform deck to the 
seafloor wellhead. Tubing is then run, surface production trees installed, 
and the well is produced in a conventional manner. 
When running a conductor and attaching it to the wellhead, axial alignment 
between the wellhead and the conductor is a problem. A solution for this 
is described in co-pending application Ser. No. 120,200, filed Feb. 11, 
1980, now Patent No. 4,343,495. A downwardly-extending funnel with two 
bearing surfaces surrounds the wellhead and achieves the angular 
alignment. Final alignment is achieved when the connector is brought into 
abutting relationship with the upper edge of the wellhead where the seal 
is effected. For production to be to a fixed platform, there would be no 
additional motion once the connection has been made. A tensioned leg 
platform, however, introduces movement of the upper end of the riser. 
Accordingly, a large diameter conductor experiences excessive strains and 
stresses for a given deflection. A smaller diameter production riser is 
preferable since this results in lower stresses for a given offset 
movement. 
Even with the smaller conductor, however, there are repeated and varying 
bending moments placed on the wellhead connection during normal operation. 
This will cause unloading, or even movement, of the seal and a loss of 
pressure integrity thereof. Accordingly, an apparatus is to be preferred 
which will avoid failure of the seal caused by the deflection of the 
production riser. 
SUMMARY OF THE INVENTION 
A connector assembly for tying back a subsea wellhead to a tensioned leg 
platform includes a casing hanger which is supported and sealed within the 
wellhead. The connector itself which is secured to the production casing 
includes both a tieback joint and a guiding means for limiting deflection. 
The tieback joint is a vertically elongated hollow cylinder which is 
rigidly connected at its upper end to the guiding means. The lower portion 
of this cylinder fits within the wellhead and has a sealing surface 
whereby a seal may be employed to effect a seal together with a sealing 
surface on the casing hanger. The guiding means includes a 
downwardly-extending funnel which surrounds the wellhead and interacts 
with the outer surface of the wellhead at an upper and lower elevation so 
as to limit the angular deflection of the tieback joint with respect to 
the wellhead. Means are also included for axially preloading the seal. 
The angular deflection permitted by the guiding means and the flexibility 
of the tieback joint is such that the bending moment transmitted on 
movement of the production riser does not cause movement of the sealing 
surfaces or preferably even loss of preloading on the seal. 
The joint is secured and the seal axially preloaded by the use of internal 
threads on the casing hanger below the sealing surface and a torque sleeve 
with external threads engaging these internal threads. The torque sleeve 
has a downwardly-facing shoulder which engages an upwardly-facing shoulder 
on the tieback joint so that the joint may be torqued down to effect the 
required preload. An abutment surface at the top of the casing hanger and 
at the bottom of the tieback joint limits the movement to compress the 
seal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with standard practice in drilling subsea wells, a wellhead 
10 is rigidly secured to an outer 50 cm casing 12 and through it secured 
to the seabed. Within the 50 cm casing there is supported a 34 cm casing 
14 by casing hanger 16. Seal 18 energized by packoff nut 20, effects the 
seal between the 34 cm casing string and the wellhead. 
A 25 cm casing string 22 is supported from an upper casing hanger 24. This 
25 cm casing hanger rests on surface 26 of the 34 cm casing hanger. The 34 
cm casing hanger in turn is supported and located on shoulder 28 of the 
wellhead. Since this shoulder is machined into the wellhead, the precise 
location of each of the casing hangers is known, subjected to 
manufacturing tolerances of the components. 
An additional seal 30 energized by packoff nut 32 effects the seal between 
the 25 cm casing hanger 24 and the wellhead 10. The packoff energizes this 
seal by relative rotation of screw threads 34 external of the casing 
hanger so that the packoff nut 32 exerts a downward force on this seal. 
Lockdown ring 36 prevents any possibility of upward movement of the casing 
hanger. 
Accordingly, the inner 25 cm casing string 22 is sealed to the wellhead, 
with the problem remaining of connecting this casing string to a tensioned 
leg platform located at the surface of the water throughout a limited 
range of locations. Because of the range of platform locations and the 
concomitant movement of the riser, the major and central portion of the 
casing string conductor 38 is of 25 cm outside diameter. This use of the 
restricted diameter reduces the stress for a given movement of the 
platform and also reduces the bending moments imposed at the platform and 
on the wellhead. The lower portion 40 of the conductor 38 is tapered to a 
larger outside diameter to stiffen the portion of the conductor 
immediately above the wellhead. A flanged connection is provided to 
establish a strong and rigid connection between the conductor and the 
wellhead connector 42. 
The connector 42 includes a tieback joint 44 rigidly attached to a 
downwardly-extending funnel 46. The wellhead 10 has an upper bearing 
surface 48 with the funnel having an upper bearing surface 50 of a 
diameter only slightly larger than that of the wellhead bearing surface. 
The wellhead also has a lower bearing surface 52 with a corresponding 
lower bearing surface 54 on the funnel. The inner surface 56 of the funnel 
between the two bearing surfaces is of larger inside diameter with a 
sloped surface 58 located immediately below the upper bearing surface. 
The funnel may also include downwardly-extending funnel means (not shown) 
at the lower end to aid in the initial guidance. 
As the conductor with the connector is lowered toward the wellhead, the 
lower bearing surface 54 fits over the wellhead despite some angular 
misalignment. As it slides downwardly over the wellhead, the sloped 
surface 58 forces the conductor into alignment, until finally alignment is 
achieved with the interaction between bearing surfaces 48 and 50 at the 
top and bearing surfaces 52 and 54 at the bottom. 
More importantly, funnel 46 is a guiding means which operates to limit the 
angular deflection to itself and the rigidly-attached tieback joint 44 
with respect to the wellhead. As the moving tensioned leg platform causes 
the upper end of the conductor 38 to move horizontally, the tendency for 
horizontal movement and bending occurs at the wellhead. This is initially 
limited by the interaction between the funnel surface 50 and the wellhead 
surface 48. As a limit on further bending around this contact point, the 
lower funnel surface 54 abuts the lower wellhead surface 52, thereby 
limiting rotation of the wellhead connector 42. Further resistance of the 
bending moment depends on the strength of the wellhead 10. It can be seen 
that the limitation on angular deflection through the interaction of the 
funnel with the wellhead is a function of the clearances left between the 
discussed abutting surfaces. The clearance between the upper surfaces 48 
and 50 is the most critial. Only enough clearance should be left between 
the inside diameter of the funnel and the outside diameter of the wellhead 
at this upper location to assure the funnel fitting over the wellhead. 
Eight-tenths of a millimeter on a diameter is reasonable. The diametral 
clearance between the lower surfaces 52 and 54 is also important but not 
as critical as that above. A similar diametral clearance, however, of 
about 0.8 millimeters is also considered satisfactory. 
The tieback joint 44 is a vertically-elongated, hollow cylinder rigidly 
connecting at its upper end to the funnel or guiding means 46. The outside 
diameter of the lower portion of this joint is less than the ID of the 
wellhead so that it fits therewithin with a sealing surface 60 near its 
lower end. An abutting surface 62 is in contact with an abutting surface 
63 at the upper edge of the hanger 24 when the tieback joint 44 is 
completely tightened down. 
A bushing or torque sleeve 64 provides a means for tightening down the 
tieback joint 44 and axially preloading the seal 66. This bushing has 
external threads 68 at its upper end which are engageable in threads 70 
located in the wellhead connector 42. These threads hold the bushing in a 
raised position while running the conductor downwardly from the platform. 
After running the conductor so that engagement between the funnel and the 
wellhead is effected, the bushing is rotated by means of a tool lowered 
down through the conductor and engaging in slots 72. The bushing 64 then 
drops into a floating position and upon further rotation the external 
threads 74 on the lower end of the bushing 64 engage internal threads 76 
on the casing hanger 24. Upon further rotation of the bushing, the 
downwardly-facing shoulder 78 on the bushing engages an upwardly-facing 
shoulder 80 on the tieback joint. This draws the connector down until the 
lower abutment surface 62 contacts the upper abutment surface 63 of the 
casing hanger. During this operation, the seal 66 is preloaded with the 
spacing of the components being such that proper preloading is 
accomplished with the given physical movement. The compression forces 
applied through torquing bushing 64 exceed those required to preload the 
seal for reasons discussed below. 
If it were not for the limitation on deflection established by the 
interaction between the guide funnel and the wellhead, any movement of the 
upper end of the conductor 38 would place the full bending movement on the 
joint at seal 66. This would cause relaxation of the prelead on the 
tension side of the joint and possibly even cause movement and separation 
of the seal. With some types of seal, loss of preload would produce a 
potential leakage condition; and rubbing of the seal surface would 
deteriorate the surface. 
With the funnel limiting the angular deflection permitted, a lesser bending 
moment is transmitted to the seal. The upper end of the tieback joint 
moves to the extent permitted and rotates to the limited angle from the 
centerline of the wellhead. This movement causes the bending moment to be 
transmitted back to the seal located which is a function of the stiffness 
of the tieback joint 44 in bending. The amount of bending moment 
transmitted is reduced where the length of the tieback joint is long, its 
diameter small, and its wall thickness small. Accordingly, the flexibility 
of the tieback joint should be such that with the given limited 
deflection, the bending moment transmitted to the seal location will not 
cause loss of preload on the seal. The axial load set in the joint when 
the bushing 64 is tightened down should be sufficient to supply the 
preload of the seal plus resisting of the bending moment due to this 
limited deflection as well as any vertical forces which may be placed on 
the production conductor be the tensioner from the platform. 
Where the seal does not lose its effectiveness by loss of preload, the 
relationship between the permitted deflection and the flexibility of the 
tieback joint should be such that there is no movement of the sealing 
surfaces relative to the seal. 
The actual elevation of the funnel with respect to the wellhead may vary 
slightly from predicted because of the accumulated tolerances between the 
lower casing hanger support 28 through the various components up to the 
funnel 48. Accordingly, the upper interacting surfaces 48 and 50 in 
particular and also the lower interacting surfaces 52 and 54 should be 
vertical. This provides the maintenance of the constant clearance despite 
some variations in the relative elevations of the two components. 
In order to maintain the close tolerance at the upper point of interaction 
on the wellhead while still maintaining ease in lowering the funnel over 
the wellhead, the outside diameter of the wellhead at the upper surface 48 
should be somewhat less than the nominal diameter of the wellhead and its 
central body 82. 
It can be seen that a seal and rigid structural joint has now been effected 
between the casing hanger and the conductor string. The preload on the 
joint is greater than that required for the seal, and the flexibility of 
the tieback joint 44 limits the amount of bending movement transmitted to 
the seal location for a fixed axial deflection at the upper end of the 
tieback joint. This angular deflection is in itself limited by the 
interaction between the wellhead and the guiding means or funnel 46. 
Accordingly, a sealed connection is achieved which permits the movement of 
the tensioned leg platform to occur without damage of the seal to the 
casing hanger.