U.S. patent application number 10/820889 was filed with the patent office on 2004-12-09 for production riser with pre-formed curves for accommodating vessel motion.
This patent application is currently assigned to Deepwater Technologies, Inc.. Invention is credited to Horton, Edward E. III.
Application Number | 20040244985 10/820889 |
Document ID | / |
Family ID | 46301162 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040244985 |
Kind Code |
A1 |
Horton, Edward E. III |
December 9, 2004 |
Production riser with pre-formed curves for accommodating vessel
motion
Abstract
A system for establishing fluid communication between a floating
body and a wellhead on the seafloor includes a rigid,
self-tensioning riser for fluidly connecting the floating body and
the wellhead. The riser defines a major axis extending from the
floating body to the wellhead. The riser is formed with a series of
pre-formed curves that absorb and release energy in response to the
heave and surge of the floating-body by flexing in a direction
essentially parallel to the major axis. The series of curves may be
in a single plane (e.g., sinusoidal), or in multiple planes (e.g.,
helical).
Inventors: |
Horton, Edward E. III;
(Houston, TX) |
Correspondence
Address: |
KLEIN, O'NEILL & SINGH
2 PARK PLAZA
SUITE 510
IRVINE
CA
92614
US
|
Assignee: |
Deepwater Technologies,
Inc.
Houston
TX
|
Family ID: |
46301162 |
Appl. No.: |
10/820889 |
Filed: |
April 8, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10820889 |
Apr 8, 2004 |
|
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|
10213963 |
Aug 7, 2002 |
|
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Current U.S.
Class: |
166/367 ;
166/359 |
Current CPC
Class: |
E21B 17/015 20130101;
E21B 17/017 20130101 |
Class at
Publication: |
166/367 ;
166/359 |
International
Class: |
E21B 017/01 |
Claims
I claim:
1. A system for establishing a fluid communication between a
floating body and a wellhead, the system comprising: a rigid,
self-tensioning riser pipe providing fluid communication between
the floating body and the wellhead, the riser pipe defining a major
axis extending from the floating body to the wellhead; and a series
of pre-formed curves formed in the riser pipe so as to absorb and
release energy in response to the heave and surge of the floating
body by flexing in a direction essentially parallel to the major
axis.
2. The system of claim 1, wherein the series of pre-formed curves
comprises a series of single-planar, pre-formed curves in the
riser.
3. The system of claim 2, wherein the series of single-planar
curves comprises arcs having a substantially constant radius of
curvature.
4. The system of claim 2, wherein the series of single-planar
curves comprises sinusoidal shaped curves.
5. The system of claim 1, wherein the series of pre-formed curves
comprise a series of multi-planar, pre-formed curves in the
riser.
6. The system of claim 5, wherein the series of multi-planar curves
comprises form a substantially helical curve.
7. The system of claim 6, wherein the helical curve has a curve
spacing and a curve diameter, wherein the curve spacing is at least
double the curve diameter.
8. The system of claim 6, wherein the riser has an axial length
between the seafloor and the floating body, and wherein the helical
curve has a curve spacing that increases along the axial length of
the riser with the distance above the seafloor.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Continuation-in-Part of co-pending
application Ser. No. 10/213,963; filed Aug. 7, 2002, the disclosure
of which is incorporated herein by reference.
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] This invention is generally related to risers that convey
fluid from producing wells on the seafloor to a floating structure
or vessel on the sea surface. This invention is also related to a
conduit that is fixed to the seafloor, which must accommodate the
motion of a floating body, structure, or vessel that is connected
to it.
[0004] In offshore drilling and production operations carried out
from a floating vessel, fluid is conveyed from wells on the
seafloor to the vessel stationed on the surface by a conduit often
referred to as a "riser." Various methods and mechanisms are used
to reduce stresses in risers that are affixed to the moving vessel
on the surface and the stationary wellhead at the seafloor. These
include using flexible hose for the riser in lieu of steel pipe,
supporting a steel riser with hydraulic or elastomeric tensioners
that accommodate the relative movement of the vessel, buoyancy cans
that support the pipe at the top and allow the vessel to move (as
shown in, for example, U.S. Pat. No. 4,702,321, incorporated herein
by reference) or some combination of these techniques. Another
method is by using a steel catenary riser (often referred to as a
"SCR"), which comprises an extension of the steel riser pipe a
sufficient horizontal distance from the vessel such that the pipe
forms a rather deep catenary curve. Depending on a number of
factors, the SCR can be designed to accommodate some vessel
motion.
[0005] The above methods all have disadvantages and limitations.
For example, flexible hose is costly, cannot withstand external
compressive loads without internal stiffening, and requires
bend-restrictor devices at the terminations. The SCR are much less
costly and have a long record of reliability; however, their
shortcoming lies in motion compensation. The tensioners and
buoyancy cans are expensive, and they both require a flexible hose
(referred to as a "jumper line") to accommodate the relative motion
between the top of the riser, which sometimes includes a "Christmas
tree," and a flow manifold (fixed to the vessel).
[0006] One proposed system that attempts to address the
aforementioned problems is disclosed in U.S. Pat. No.
5,553,976--Korsgaard. This reference discloses a riser formed into
a helical or sinusoidal configuration for decoupling axial stresses
in the riser resulting from internal fluid pressure and/or external
tension forces. The riser in this system, however, requires a
plurality of elastic tensioning members that extend along the
longitudinal direction of the riser, and that are secured to the
riser at spaced intervals. The need for such tensioning members
increases the cost of manufacturing and installing such
systems.
[0007] There is therefore a need for a relatively low-cost, simple
riser that compensates for the motion of a floating vessel, and
that does so without the need for separate tensioning members
attached to the riser.
SUMMARY OF THE INVENTION
[0008] The above issues are addressed by the present invention that
employs a self-tensioning curved riser in a system for establishing
fluid communication between a floating body and a wellhead on the
seafloor. The system comprises fluid-conducting means for fluidly
connecting the floating body and the wellhead, wherein the
fluid-conducting means defines a major axis essentially extending
from about the floating-body to about the wellhead. The system
further comprises a means for absorbing and releasing energy in
response to the heave and surge of the floating body that flexes in
a direction essentially parallel to the major axis. In the
preferred embodiments of the invention, the fluid-conducting means
comprises a self-tensioning steel riser, and the means for storing
and releasing energy comprises a series of pre-formed curves in the
steel riser. In some embodiments, the series of pre-formed curves
comprises a series of single-planar, preformed curves in the riser.
These single-planar curves comprise arcs having a substantially
constant radius of curvature in one embodiment and sinusoidal
curves in another embodiment. In some other embodiments, the series
of pre-formed curves comprises helical pre-formed curves.
[0009] In accordance with specific embodiments of the invention, an
apparatus for establishing a fluid connection between a floating
body and a wellhead fixed to the seafloor comprises a riser pipe
and a series of pre-formed curves in the riser pipe that defines a
substantially linear axis between the floating body and the
wellhead. In some embodiments the series of pre-formed curves
comprises multi-planar curves, which in one embodiment comprises
helical shaped curves. In some other embodiments, the series of
pre-formed curves comprises single-planar curves, which in one
embodiment comprises sinusoidal curves. The riser pipe is made of
steel in some embodiments, but in some embodiments, the pre-formed
curves may be of a different material than the remainder of the
riser pipe.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a side view of an exemplary embodiment of riser
with a series of pre-formed curves.
[0011] FIG. 2 is a side view of an exemplary embodiment with a
pre-formed helical curve.
[0012] FIG. 3 is a side view of an exemplary embodiment with a
pre-formed curve in a single plane.
DETAILED DESCRIPTION OF THE INVENTION
[0013] According to one exemplary embodiment of the invention, seen
in FIG. 1, a fluid conducting conduit between a floating structure
or vessel ("floating body") 16 and a wellhead 12 on the sea floor
13 is provided by a riser pipe 14, at least a substantial portion
of which is formed with a series of pre-formed curves 10. These
curves 10 accommodate the stress generated by the motion of the
floating body 16. The pre-formed curves 10 flex in response to the
motion of the floating body 16, so that the forces generated by the
motion of the floating body 16 are not transmitted to the wellhead
12. By using these pre-formed curves 10, a single, steel riser pipe
14 becomes feasible to connect the floating-body with the wellhead
12, without the need for a using catenary curve. The riser pipe 14
is a rigid conduit, preferably made of a suitable steel alloy or
some other similar alloy, as would be well-known to those skilled
in the pertinent arts.
[0014] By accommodating the motion of the floating body 16, the
preformed curves 10 also eliminate the need for a flexible section
of pipe to connect the riser pipe 14 and the floating-body 16.
Using a metal (particularly steel) riser pipe 14 further eliminates
the need for the external stiffening associated with using a
flexible pipe section because the steel can withstand the external
compressive loads exerted by the environment. In one embodiment,
the pre-formed curves 10 are fashioned from a different material
than the remainder of the riser pipe 14.
[0015] By absorbing the forces exerted by the floating body 16
without using a catenary curve, the pre-formed curves 10 also
eliminate the need for additional buoyancy devices. As a result, in
one embodiment, the riser pipe 14 connecting the wellhead 12 to the
floating body 16 is only suspended from the floating body 16. In
one embodiment, the suspension from the floating body 16 supports
the entire weight of the riser pipe 14, while in another
embodiment, part of the weight of the riser pipe 14 is supported by
the riser pipe 14 itself.
[0016] As shown in FIG. 1, while mooring lines 15 may be provided
to attach the floating body 16 to the sea floor, the riser pipe 14
is self-tensioning and thus needs no external anchoring or
tensioning means attached to it. This self-tensioning, or
pre-tensioning, is accomplished by first lowering the riser pipe 14
to the seafloor 13 and anchoring it at the wellhead 12. This is
typically accomplished by means such as derrick or any conventional
equivalent apparatus (not shown) on the floating body 16. Tension
is then applied to the upper end of the riser pipe 14 by means of
the derrick (or equivalent apparatus), and upper end of the riser
pipe 14, in its pre-tensioned state, is secured to the appropriate
structure on the floating body by conventional means, as is
well-known in the art. Thus, the riser pipe 14, when installed, is
in a self-tensioned or pre-tensioned state, and needs no external
tensioning means.
[0017] In some embodiments, the pre-formed curves 10 do not affect
the overall orientation or direction of the riser pipe 14.
Therefore, in one embodiment, the floating body 16 from which the
riser pipe 14 is suspended is positioned directly above the
wellhead 12. The riser pipe 14 thereby defines an axis 21
essentially from about the floating body 16 to about the wellhead
12. For the riser pipe 14 to accommodate the motion of the floating
body 16, the pre-formed curves 10 flex in a direction essentially
parallel to the axis 21 defined by the riser pipe 14. In another
embodiment, positioning the floating body 16 closer to the wellhead
12 simplifies the installation and design of the sub-sea systems,
in part by enabling a vertical connection between the riser pipe 14
and the wellhead 12. Tools pass more easily through a vertical
wellhead 12 connection than through a horizontal connection.
[0018] In one embodiment, the portion of the riser pipe 14 in which
the series of pre-formed curves 10 is formed is at or near the
bottom end of the riser pipe 14, and is thus connected between the
wellhead 12 and the remainder of the steel riser pipe 14. In other
embodiments, the series of pre-formed curves 10 extends along
substantially the entire length of the riser pipe 14, from the
wellhead 12 to the floating body 16. In still other embodiments,
segments of relatively straight riser pipe 14 are on either end of
the portion having the series of pre-formed curves 10.
[0019] In the example shown in FIG. 1, the riser pipe 14 connects
with a floating body 16 (in this example, a SPAR-type
semi-submersible), and has a series of pre-formed curves 10 at its
lower end, near the juncture with the wellhead 12. Other types of
floating body 16 that can be used with the invention include
floating production storage and offloading (FPSO) systems,
semi-submersible platforms, tension leg platforms, and others known
to those of ordinary skill in the art. The connection between the
wellhead 12 and the floating body 16 provided by the
self-tensioning curved riser allows fluid communication
therebetween. In some examples, this connection also allows tools
to be passed from one section to another, and in one specific
embodiment, the riser pipe 14 is raised using some lifting means
(not shown) located on the floating body 16, stretching the series
of pre-formed curves 10 and allowing tools to pass more easily
through the series of preformed curves 10.
[0020] Referring now to FIGS. 2 and 3, examples of pre-formed
curves 10 are shown. In FIG. 2, the pre-formed curves 10 are
three-dimensional curves forming an open coil, which advantageously
may be a helical curve. As shown in FIG. 2, the vertical distance
between equivalent points in the helical curve is called the curve
spacing 17, and the curve diameter 18 describes the diameter of the
cross-sectional area of the curve. In some embodiments, the curve
spacing 17 is at least double the curve diameter. In one
embodiment, the curve spacing 17 increases with the distance along
the axial length of the riser 14 above the seafloor.
[0021] The characteristics of one set of exemplary embodiments of a
riser pipe 14 with helical pre-formed curves 10 are shown in Table
1below.
1TABLE 1 Riser Pipe Embodiments With Helical Pre-Formed Curves No L
A TL OD Wt D/t RF RFr St Kr PS.sub.ksi PS.sub.psf P.sub.ksf *
10.sup.2 L.sub.30 ksi 1 30 3.4 240 6.625 0.4321 15.3 24.6 1.0 1.2
1.0 127.1 1.83E+07 183.0 1016.7 2 20 3.4 240 6.625 0.4321 15.3 19.5
0.8 1.0 0.8 85.1 I.23E+07 122.5 680.6 3 30 2 240 6.625 0.4321 15.3
84.6 3.4 4.2 3.4 294.4 4.24E+07 424.0 2355.6 4 30 5 240 6.625
0.4321 15.3 9.2 0.4 0.5 0.4 59.0 8.49E.div.06 84.9 471.7 5 40 3.4
240 6.625 0.4321 15.3 27.5 1.1 1.4 1.1 153.9 2.22E+07 221.6 1231.1
6 20 3.4 240 6.625 0.2161 30.7 10.8 0.4 0.5 0.4 88.1 1.27E+07 126.8
704.4 7 30 2 240 6.625 0.2161 30.7 46.9 1.9 2.3 1.9 304.9 4.39E+07
439.0 2438.9 8 30 3.4 240 6.625 0.2161 30.7 13.6 0.6 0.7 0.6 131.3
1.89E+07 189.0 1050.0 9 30 5 240 6.625 0.2161 30.5 5.1 0.2 0.3 0.2
61.0 8.79E+06 87.9 488.3 10 40 3.4 240 6.625 0.2161 30.7 15.3 0.6
0.8 0.6 159.0 2.29E+07 229.0 1272.2 11 30 3.4 240 8 0.5229 15.3
52.1 2.1 2.6 2.1 153.6 2.21E+07 221.3 1229.2 L represents the curve
spacing measured in feet. A represents the curve radius measured in
feet. TL represents the total length of the curve used for
simulation purposes measured in feet. OD represents the outer
diameter of the riser pipe measured in inches. Wt represents the
wall thickness of the riser pipe wall measured in inches. D/t
represents the ratio of the outer diameter to the thickness of the
riser pipe wall. RF represents the reaction force necessary to
displace the top of the planar sine wave riser 20 feet. RFr
represents a normalization of the reaction forces to a base case
scenario. K represents the stiffness of the riser model. Kr
represents a normalization of the stiffness to a base case
scenario. PS.sub.ksi represents the peak stress in kips per square
inch. PS.sub.psf represents the peak stress in pounds per square
foot. PS.sub.ksf*100 represents the peak stress in kips per square
foot multiplied by 100. L.sub.30 ksi represents the length of a
curved section with necessary to maintain a maximum stress of 30
ksi in the riser.
[0022] FIG. 3 shows an exemplary embodiment in which the series of
pre-formed curves 10 comprises curves in a single plane. In some
embodiments, these single-planar, pre-formed curves 10 are
sinusoidal; and, in other embodiments, the pre-formed curves 10
have semi-circular or other shapes. Combinations of such shapes of
varying complexity are included in still further example
embodiments. In one embodiment, the pre-formed curves 10 comprise
several connected segments of pipes. As shown in FIG. 3, the
vertical distance between equivalent points in the sinusoidal curve
is called the wavelength 19, and the amplitude 20 describes the
width of the curve.
[0023] The characteristics of one set of exemplary embodiments of a
sinusoidal riser pipe 14 are shown in Table 2 below.
2TABLE 2 Riser Pipe Embodiments With Sinusoidal Pre-Formed Curves
No L A TL OD Wm D/t RF RFr St Kr PS.sub.ksi PS.sub.psf
P.sub.ksi*102 L.sub.30 ksi 1 30 2.0 210 6.625 0.4321 15.3 425.4 2.8
21.3 2.8 815.5 1.17E+08 1174.3 5708 2 30 3.4 210 6.625 0.4321 15.3
150.4 1.0 7.5 1.0 461.9 6.65E+07 665.1 3233 3 20 3.4 220 6.625
0.4321 15.3 135.5 0.9 6.8 0.9 411.0 5.92E+07 591.8 3014 4 40 3.4
200 6.625 0.4321 15.3 85.8 0.6 4.3 0.6 508.8 7.33E.div.07 732.7
3392 5 30 3.4 420 6.625 0.4321 15.3 72.1 0.5 3.6 0.5 228.3 3.29E+07
328.7 3196 6 30 5.0 210 6.625 0.4321 15.3 63.2 0.4 3.2 0.4 288.7
4.16E+07 415.8 2021 7 30 2.0 210 6.625 0.2161 30.7 235.4 1.6 11.8
1.6 844.3 1.22E+08 1215.8 5910 8 30 3.4 220 6.625 0.2161 30.7 75.0
0.5 3.7 0.5 425.4 6.13E+07 612.5 3119 9 40 3.4 200 6.625 0.2161
30.7 85.8 0.6 4.3 0.6 508.8 7.33E+07 732.7 3392 10 20 3.4 220 6.625
0.2161 30.7 75.0 0.5 3.7 0.5 425.4 6.13E+07 612.5 3119 11 30 3.4
420 6.625 0.2161 30.7 39.9 0.3 2.0 0.3 236.3 3.40E+07 340.3 3308 12
30 5.0 210 6.625 0.2161 30.7 34.0 0.2 1.7 0.2 298.9 4.30E+07 430.4
2092 13 30 3.4 210 8 0.5229 15.3 318.8 2.1 15.9 2.1 559.0 8.05E+07
805.0 3913 L represents the wavelength measured in feet. A
represents the amplitude measured in feet. TL represents the total
length of curves used for simulation purposes measured in feet. OD
represents the outer diameter of the riser pipe measured in inches.
Wt represents the wall thickness of the riser pipe wall measured in
inches. D/t represents the ratio of the outer diameter to the
thickness of the riser pipe wall. RF represents the reaction force
necessary to displace the top of the planar sine wave riser 20
feet. RFr represents a normalization of the reaction forces to a
base case scenario. K represents the stiffness of the riser model.
Kr represents a normalization of the stiffness to a base case
scenario. PS.sub.ksi represents the peak stress in kips per square
inch. PS.sub.psf represents the peak stress in pounds per square
foot. PS.sub.ksi*100 represents the peak stress in kips per square
foot multiplied by 100. L.sub.30 ksi represents the length of a
curved section with necessary to maintain a maximum stress of 30
ksi in the riser.
[0024] One important benefit derived from including pre-formed
curves 10 is that they add an additional layer of safety for the
structural integrity of the whole riser pipe 14. If, for example,
the top end of the riser pipe 14 should move beyond its normal
operating design limits either horizontally or vertically, the
pre-formed curves 10, in various exemplary embodiments, flex,
without local buckling, and the riser 14 still maintains structural
integrity. This situation might occur if, for example, the floating
body 16 should lose buoyancy due to a damaged tank, if the moorings
were to come loose or some other mishap were to occur.
[0025] In addition to the characteristics of a riser pipe 14 with
pre-formed curves 10 shown in the tables above, a number of
additional design factors are considered to develop a site-specific
design. A non-exhaustive list of these additional factors
includes:
[0026] Water depth
[0027] Envelope of surface vessel motion
[0028] Physical properties of the riser
[0029] Ocean currents
[0030] Envelope of deflection curve of the riser to avoid
clashing
[0031] Method of installation and removal of riser
[0032] Limitation of curvature of riser to allow passage of
through-tubing tools (e.g. "pigs").
[0033] The specific embodiments described above and shown in the
drawings are given by way of example only. Other aspects and
examples of the invention will be understood to be within the
spirit of the present invention and with the scope of or equivalent
to that described by the claims.
* * * * *