Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
517
BACKC,ROUND OF THE INVENTION
Field of the Invention
This invention relates to a structure floating on a body of
water. More particularly, the invention relates to a floating structure
from which drilling wells and/or production of oil and gas or like oper-
ations, or both, are carried out. In its more specific aspects, the
invention concerns a floating structure having buoyancy means to float
the structure and in which the structure is anchored by a plurality of
essentially parallel and vertical conduits commonly called "risers."
More specifically, the invention concerns such a structure in which con-
centric casing strings, within riser pipes, form an important part of
the anchoring system.
Setting
In recent years, it has become desirable to use a floating
vessel from which to drill wells in marine locations. Many of these
structures have been maintained on station by conventional spread caten-
ary mooring lines, or by propulsion thruster units. One system of
floating vessel receiving attention for drilling or production of wells
in water is the Vertically Moored Platform, such as described in U.S.
Patent 3,648,638, issued March 14, 1972, entitled "Vertically Moored
Platform," Kenneth A. Blenkarn, inventor. A key feature of Vertically
Moored Platforms is that the floating platform is connected to anchor
means in the ocean floor only by elongated, parallel members which are
preferred to be large diameter conduits, commonly called "riser pipes."
Thcse elongated members or riser pipes are held in tension by excess
buoyancy of the platform.
Prior Art
This invention is an improvement over the anchoring system
described in U.S. Patent 3,648,638, supra. This patent is considered
the closest prior art and, as stated above, our present invention is an
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improvement thereon O~her paten~s dealing with Vertically Moored Plat-
forms include U.S. 3,559,410; U.S. 3,559,411; U.S. 3,572,272; U.S.
3,9~6,021; U.S. 3,978,~04; U.S. 3,983,828; U.S. 3,993,273; U.S.
4,062,313; and U.S. 3,154,039. There are prior patents and art which
teach to have concen~ric strings of casing extending from an underwater
well to a platform above the water. In this latter regard, attention is
directed to U.S. Patent 3,971,576. U.S. Patent 3,705,623 shows concen-
tric pipes 33 and 17 connected to a buoyancy member 19; however, those
concentric pipes form no part of the anchoring system. None of these
patents or art to our knowledge teach to anchor a Vertically Moored
Platform by means of concentric tensioned casing strings within an outer
tensioned riser pipe. No prior art is known to do this.
BRIEF DESCRIPTION OF THE INVENTION
This invention concerns an anchoring system and method of con-
necting a vessel floating on a body of water to a subsea well having a
first string of casing set and secured in a hole in the bottom of said
body of water, and a second string of casing supported in the first
string and extending deeper than said first string of casing and secured
in said hole. A first riser conduit (commonly called a "riser pipe") is
connected at its lower end to said first string of casing in a sealing
relationship so that the first riser conduit and the first string of
casing form a fluid-tight conduit. The upper end of said first riser
conduit is supported from the vessel to apply a tension thereto. The
lower end of a second riser conduit or riser casing is connected to the
second string of casing in a sealing relationship so that said second
string of casing and the second riser conduit form a second fluid-tight
conduit. The upper end of the second riser conduit is supported from an
upper portion of the first riser conduit such that a tension is applied
to the second riser conduit when tension is applied to the first riser
conduit.
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The upper and lower ends of the f:irst riser conduit (or riser
pipe) are provided with terminators which are really stiffened sections
of the riser pipe to distribute curvature over a length or a portion of
the length of the riser pipe. The second or inner riser conduits are
provided with centralizers within the outer or first riser conduit ter-
minators. The upper and lower ends of the inner casing strings need no
terminators.
Various objects and a better understanding of the invention
can be had from the following description taken in conjunction with the
drawings.
DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a view of a Vertically Moored Platform.
FIGURE 2 illustrates, partly in cutaway view, one riser pipe
means of one leg of the Vertically Moored Platform of FIGURE 1.
FIGURE 3 illustrates an enlarged cross-sectional view of the
means of FIGURE 2 of connecting the top ends of the inner casing strings
to the riser pipe.
FIGURE 4 shows one means of connecting the riser pipe to the
string of casing anchored in the wellbore.
DETAILED DESCRIPTION
Reference is first made to FIGURE 1 which shows a side view of
a Vertically Moored Platform. Shown therein is a platform 10 supported
on a body of water 12 having a bottom 14. The structure 10 generally
includes a float means 16 which supports a working deck 18 above the
surface 20 of the body of water 12. It is to be noted that a Vertically
Moored Platform is described in detail in prior U.S. Patent 3,648,638,
supra. Float means 16 is, for example, composed of four bottle-shaped
buoyant legs 22. Each leg 22 is anchored by a plurality of riser pipes
24 which are provided with spacers 26. Riser pipes 24 connect to cas-
ings 28 which are cemented in holes in the bottom of the body of water.
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A template 30 is shown on the bottom 14 through which the wells for cas-
ings 28 were guided. Riser pipes 24 normally are made of hi8h quality
steel and typically are 20 inches in diameter. The riser pipes 24 sre
parallel and are held in tension by the vertical force exerted on the
buoyant structure. The typical length of these riser pipes 24 may be
from 500 feet up to several thousand feet from the base of the leg mem-
ber 22 of the Vertically Moored Platform to the sea floor 14.
Attention is next directed to FIGURE 2 which illustrates an
improved anchoring connection means between the Vertically Moored Plat-
form and the sea floor. Shown thereon is leg 22 which is one of the
four float me0bers of the Vertically Moored Platform of FIGURE 1. For
simplicity and ease of understanding, we have shown only one riser pipe
means extending between the leg 22 and the sea bottom 14. A vertical
passage 32 extends through the lower part or enlarged portion of leg 22.
The upper end of riser pipe 24 is provided with an upper riser
terminator 34. As a word of explanation, it is known that if a tubular
member is held under tension and subject to bending, stresses concen-
trated in the ends. One way of meeting this problem is to make the end
section sufficiently strong to distribute the bending deformation which
may concentrate therein over a longer length. This is what is done here
and we call the strengthened portion "a terminator," in this case, "the
upper riser terminator 34." Thus, a terminator is a stiffened section
of riser pipe to distribute curvature over a selected portion of the
riser pipe.
Upper horizontal bearings 36 and lower horizontal bearings 38
are provided between upper terminator 34 and the wall of passage 32
through jacket 22. Above the horizontal bearing 36 is a vertical bear-
ing means 40. Details of this vertical bearing 40 are shown in Patent
3,976,021, Figures 17 and 18. It includes primarily a jack 42, bracket
44, engaging shoulders 46 of the upper end of the upper riser terminator
11~5517
34, and shims and bearings 48. The vertical force of the tension in
riser pipe 24 is transmitted through vertical bearing 40 to the Verti-
cally Moored Platform ~acket 22.
The lower end of riser pipe 24 is connected to a lower termi-
nator 50 which passes through a drive pipe 52 in template 30. A 20-inch
conductor casing 54 is hung from drive pipe 52 through mudline suspen-
sion 56, which in reality may bean upwardly facing shoulder 58 on drive
pipe 52, and a shoulder 60 having a downwardly facing shoulder attached
to the outer wall of 20-inch conductor casing 54. If the bottom 14 is
sufficiently soft, drive pipe 52 can be driven the required depth into
the bottom 14; otherwise, a hole can be drilled through the guide tube.
A hole can be drilled through drive pipe 52 and the 20-inch conductor
casing 54 set and cemented in place using conventional sea-drilling
equipment.
After the 20-inch casing has been cemented in place, a smaller
diameter hole to accommodate the next smaller size of casing can be
drilled in the bottom thereof. This may be a 13-5/8 inch casing, which
is illustrated as intermediate casing 62, which is supported by mudline
suspension 64, which is similar to mudline suspension 56. The second
or 13-5/8 inch intermediate casing 62 is then run and cemented in place.
Then, the 13-5/8 intermediate riser conduit 78 is run and connected to
casing 62. After this, an additional hole is drilled to accommodate the
next smaller size of casing, which may be 9-5/8. The innermost casing
66 is run and cemented in place and is suspended by mudline suspension
68. Any desired number of casing strings may be set in place in drilled
holes in a manner described above which is well known. The upper ends
of each of casings 54, 62, and 66 are provided with a locking means,
such as J-slots 70, 72, and 74.
The lower end of riser pipe 24 is connected to the upper end
of conductor casing 54 by a J-lug 76 which fits into the J-slot 70.
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Sealing means are also provided so that a fluid-tight conduit is formed
from the conductor casing 54 upwardly to the floating structure as
examplified by jacket 22. Latching means, not shown, between conductor
casing string 54 and drive pipe 52 can be installed to restrain
vertical movement between casing 54 and drive pipe 52. A similar
device can be installed for succeeding pairs of casing strings such
as casing 62 and 66.
Within riser pipe 24 are shown two concentric strings of
casing, and an intermediate riser conduit 78 and the innermost riser
conduit 80. Of course, any reasonable number of inner casing strings
can be used. The lowermost end of intermediate riser conduit 78 is
connected through J-slot 72 to the cement casing 62 in the borehole,
and, likewise, the lower end of innermost riser conduit 80 is connected
to the cemented innermost casing 66, which is shown as the smaller one
in the drawing. Thus, we have a casing 62 and intermediate riser con-
duit 78 forming a fluidtight conduit extending from the bottom of the
casing to the top of the intermediate riser conduit 78; likewise, a
smaller fluid-tight conduit is formed from the lower end of the inner-
most casing 66 through riser conduit 80 to the top of the platform. If
desired, intermediate riser conduit 78 can be run before the hole for
the inner casing 66 is drilled.
The connecting arrangement between the riser pipe and the
casing set in the wellbore is shown in FIGURE 4. Shown thereon also
is the J-slot 70 on the upper end of the enlarged end portion of
conduit casing 54 and a J-lug 76, which is on the lower end of riser
24. Seal means 82 are provided between the lower end of riser 24 and
the enlarged portion of the upper end of conduit casing 54. Connect-
ion 72 for intermediate riser conduit 78 and cemented casing 62 and
connection 74 for innermost riser conduit 80 and cemented casing 66
can be like that shown in FIG~RE 4.
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Attention is now directed to means for supporting the upper
end of the intermediate riser conduit 78 and innermost riser conduit
80 to the upper end of the riser pipe such that the inner riser
conduit 78 and 80 form a psrt of the anchoring system. This is shown
clearly in FI W RE 3. The upper end of riser pipe extension 24A is
provided with a flange 81. A casing hanger spool 84 is provided to
sit on top of flange 81. Means are provided to connect the casing
hanger spool 84 to the intermediate riser conduit 78. This includes
a slip means 86. Screw 88 is used to set a seal of the annulus bet-
ween casing 78 and casing hanger spool 84. Thus, the upper end of
intermediate riser conduit 78 is supported from riser extension 24A
through casing hanger spool 84. Casing hanger spool 84 has an upper
flange 92 which supports casing hanger spool 94;thus, innermost
casing string 80 is supported from riser extension 24A through
casing hanger spools 84 and 94. Bolts 100,102 and 104 with proper
machining and sealing are provided to assure fluid-tight annular
spaces 106 between riser extension 24A and riser conduit 78 and annul-
us 108 between the two inner riser conduits 78 and 80. Plugs 110 and
112 may be removed and pressure gauges installed to determine the
pressure in these annuli. Conventional valves and other equipment
may be placed on extension 114 in which to produce the well drilled
through these casings.
The preferred installation procedure is to first pre-tension
the riser 24 to a predetermined value with the jack 42 and then shim
it in place on bearing 48. The hole for the casing 62 is drilled. The
casing 62 is run and cemented in. The intermediate riser conduit 78 is
run and latched to casing 62 at the J-slot 72; then the intermediate
riser conduit 78 is tensioned with the draw work of the drilling rig to
a predetermined value which is a function of the riser 24 tension.
The locking means 86 is set, locking the upper end of intermediate riser
conduit 78 to casing hanger spool 84. Other inner strings are installed
in a similar manner.
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Within riser 24 and riser terminators 34 and 50, we have
provided centralizers 35 between the riser 24 and terminators 34
and 50 and the first or intermediate riser conduit 78 and central-
izers 37 between riser conduits 78 and 80. By thus doing so, we
control the frictional wear caused by the relative motion between
the two strings. Also, the casing string, being inside the riser,
does not require a terminator.
By the system that we have just described, a substantial
part of the mooring is by the inner casing strings 78 and 80. This
provides a much stronger anchoring means for a Eiven size of riser
pipe and will afford more protection in the event of any very severe
storm. The amount of mooring by the outer riser pipe 24 compared to
the inner casing riser conduits 78, 80, etc., is a function of the
cross-sectional area or, more accurately, a function of their respec-
tive axial flexibility. The part of the mooring carried by the riser
conduits may vary from as low as about 25% to about 70% of the total
mooring forces.
An example of where the casing risers carry 27% of the moor-
$ng force in calm water is:
Riser (24) 18-5/8" OD 0.625" W.T. 610 kips
Riser Conduit (78) 9-5/8" OD 0.352" W.T. 128 kips
Riser Conduit (80) 7" OD 0.272" W.T. 72 kips
Tubing Riser 2-7/8" OD 0.217" W.T. 32 kips
NOTE: A KIP ls 1000 pounds.
An example of where the riser conduits carry 45% of the moor-
ing force in calm water is:
Riser 18-5/8" OD 0.625" W.T. 455 kips
Riser Conduit 13-3/8" OD 0.380" W.T. 144 kips
Riser Conduit 9-5/8" OD 0.452" W.T. 126 kips
Riser Conduit 7" OD 0.453" W.T. 86 kips
Tubing 2-7/8" OD 0.276" W.T. 21 kips
ll;~SSi'~
~ n example of where the casiDg riscrs carry 60X of the mooring
force in ca~m water is:
Riser18-5/8" OD 0.625" W.T. b,60 lcipS
Riser Conduit 13-3/8"OD 0.380" W.T. 208 kips
Riser Condult 9-5l8"OD 0.972" W.T. 239 kips
Riser Conduit 7 OD 0.276" W.T. 194 lcips
Tuhillg ~iser 2x2-3l8"OD 0.190" W.T. 45 ~ips
An example o~ where the casing risers carry 67~ of the mooring
force in calm watcr is:
Riser18-5l8'' OD 0.625" W.T. 460 kips
Riser Conduit 13-3l8"OD 0.719" W.T. 383 kips
Riser Conduit 9-5l8''OD 0.545" W.T. 271 kips
Riser Condult 7" OD 0.54" W.T. 231 Icips
~ul)lng Riser 2x2-3/8"OD 0.218" W.T. 60 kips
These distributions are determined by the axial flexibility of
the riser strings and by the expected temperature and pressure effect.
They will change when the temperature and tbe pressure distribution
between each string vary. They will also change when the total mooring
force changes under the influence of the wind, the waves, and the cur-
rent.
While the above embodiments have been described in greattetail, it is possible to incorporate variations therein without de;art-
' ing from the spirit or scope of the invention.
mG:el (4)
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