U.S. patent application number 10/131658 was filed with the patent office on 2003-02-27 for composite tether and methods for manufacturing, transporting, and installing same.
This patent application is currently assigned to Conoco Inc.. Invention is credited to Hanna, Shaddy Y., Salama, Mamdouh M..
Application Number | 20030037529 10/131658 |
Document ID | / |
Family ID | 23101835 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030037529 |
Kind Code |
A1 |
Hanna, Shaddy Y. ; et
al. |
February 27, 2003 |
Composite tether and methods for manufacturing, transporting, and
installing same
Abstract
The present invention includes a nontwisted composite tether
comprising one or more composite rods encased in a jacket and a
method for manufacturing same. A portion of the rods may be bundled
into one or more strands, provided however that the rods comprising
the strands are not twisted into twisted strands in the assembled
nontwisted tether. Such untwisted strands, if any, additionally are
not twisted relative to each other. Temporary and/or permanent
buoyancy may be to the tether. The present invention includes
methods for preparing, transporting, and installing a composite
tether on a floating platform. The tether, preferably assembled at
a waterfront, is launched into the water and towed to an offshore
installation site, where the tether is upended and connected via a
bottom end connector on the tether to an anchor foundation in the
seabed and connected a top end connector on the tether to the
floating platform.
Inventors: |
Hanna, Shaddy Y.; (Houston,
TX) ; Salama, Mamdouh M.; (Ponca City, OK) |
Correspondence
Address: |
DAVID W. WESTPHAL
CONOCO INC.
P.O. BOX 4783
HOUSTON
TX
77210-4783
US
|
Assignee: |
Conoco Inc.
Houston
TX
|
Family ID: |
23101835 |
Appl. No.: |
10/131658 |
Filed: |
April 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60287191 |
Apr 27, 2001 |
|
|
|
Current U.S.
Class: |
57/7 ;
57/241 |
Current CPC
Class: |
Y10T 428/2933 20150115;
Y10T 428/2938 20150115; Y10T 428/2922 20150115; D07B 1/167
20130101; D07B 2201/2049 20130101; D07B 5/002 20130101; Y10T
428/2924 20150115; D07B 1/162 20130101; D07B 2801/10 20130101; D07B
2801/24 20130101; B63B 21/50 20130101; D07B 2201/1092 20130101;
D07B 2205/3007 20130101; D07B 2201/2049 20130101; D07B 2205/3007
20130101; Y10T 428/2936 20150115 |
Class at
Publication: |
57/7 ;
57/241 |
International
Class: |
D02G 003/36 |
Claims
1. A nontwisted composite tether.
2. The nontwisted composite tether of claim 1 further comprising
one or more composite rods encased in a jacket.
3. The nontwisted composite tether of claim 2 wherein at least one
composite rod is spoolable.
4. The nontwisted composite tether of claim 2 wherein at least one
composite rod is nonspoolable.
5. The nontwisted composite tether of claim 3 wherein at least one
composite rod is nonspoolable.
6. The nontwisted composite tether of claim 2 wherein a portion of
the composite rods is made from medium or high modulus carbon
fibers.
7. The nontwisted composite tether of claim 6 wherein the rods made
from medium modulus carbon fibers are circular and have a diameter
greater than about 5 mm.
8. The nontwisted composite tether of claim 6 wherein the rods made
from medium modulus carbon fibers are circular and have a diameter
of about 9 to about 25 mm.
9. The nontwisted composite tether of claim 6 wherein the rods made
from medium modulus carbon fibers are circular and have a diameter
of about 12 mm.
10. The nontwisted composite tether of claim 6 wherein the rods
made from high modulus carbon fibers are circular and have a
diameter of less than about 10 mm.
11. The nontwisted composite tether of claim 6 wherein the rods
made from high modulus carbon fibers are circular and have a
diameter of about 3 to about 9 mm.
12. The nontwisted composite tether of claim 6 wherein the rods
made from high modulus carbon fibers are circular and have a
diameter of about 5 mm.
13. The nontwisted composite tether of claim 2 wherein the total
number of rods is based on the required stiffness of the
tether.
14. The nontwisted composite tether of claim 2 wherein the total
number of rods is about 20 to about 1000.
15. The nontwisted composite tether of claim 2 wherein the total
number of rods is about 30 to 200.
16. The nontwisted composite tether of claim 2 wherein the total
number of rods is less than about 30.
17. The nontwisted composite tether of claim 2 wherein the total
number of rods is less than about 10.
18. The nontwisted composite tether of claim 2 wherein the tether
comprises a single rod.
19. The nontwisted composite tether of claim 2 wherein each rod has
a cross-section that is configured to fit closely with adjacent
rods.
20. The nontwisted composite tether of claim 2 wherein the
cross-section of at least one rod, the tether, or both is
round.
21. The nontwisted composite tether of claim 2 wherein the
cross-section of at least one rod, the tether, or both is
rectangular.
22. The nontwisted composite tether of claim 2 wherein the
cross-section of at least one rod, the tether, or both is
square.
23. The nontwisted composite tether of claim 2 wherein the
cross-section of at least one rod, the tether, or both is
hexaganol.
24. The nontwisted composite tether of claim 2 wherein the
cross-section of at least one rod, the tether, or both is
octagonal.
25. The nontwisted composite tether of claim 2 wherein the
cross-section of at least one rod, the tether, or both is
irregular.
26. The nontwisted composite tether of claim 2 wherein at least one
rod is hollow.
27. The nontwisted composite tether of claim 2 wherein a portion of
the rods is bundled into one or more strands.
28. The nontwisted composite tether of claim 27 wherein the rods
comprising the strands are not twisted into twisted strands.
29. The nontwisted composite tether of claim 27 wherein the strands
comprising the tether are not twisted relative to each other.
30. The nontwisted composite tether of claim 28 wherein the strands
comprising the tether are not twisted relative to each other.
31. The nontwisted composite tether of claim 1 wherein the tether
is neutrally or positively buoyant.
32. The nontwisted composite tether of claim 2 further comprising
buoyant material.
33. The nontwisted composite tether of claim 32 wherein a portion
the buoyant material is temporarily attached to the tether.
34. The nontwisted composite tether of claim 32 wherein a portion
the buoyant material is permanently attached to the tether.
35. The nontwisted composite tether of claim 34 wherein buoyant
material is encased within the jacket.
36. The nontwisted composite tether of claim 34 wherein buoyant
material is encased in a second jacket.
37. The nontwisted composite tether of claim 35 wherein buoyant
material is encased in a second jacket.
38. The nontwisted composite tether of claim 1 further comprising
at least two connectable segments.
39. The nontwisted composite tether of claim 1 configured for
anchoring a TLP.
40. The nontwisted composite tether of claim 39 further comprising
top and bottom connectors connected to each end of the tether.
41. The nontwisted composite tether of claim 40 wherein at least
one end connector further comprises a resin filled cone.
42. A method for manufacturing a nontwisted composite tether
comprising: a) supplying one or more composite rods; b) arranging
the rods axially; and c) encasing the rods within a jacket such
that the resulting tether is nontwisted.
43. The method of claim 42 wherein a portion of the rods are
supplied on one or more spools.
44. The method of claim 42 wherein a portion of the rods are
supplied directly from a pultrusion device without being
spooled.
45. The method of claim 42 further comprising bundling a portion of
the rods into one or more strands prior to encasing.
46. The method of claim 45 wherein the rods comprising the strands
are not twisted relative to each other.
47. The method of claim 45 wherein the rods comprising the strands
are temporarily twisted into twisted strands and supplied on one or
more spools.
48. The method of claim 46 further comprising allowing the twisted
strands to untwist prior to encasing.
49. The method of claim 45 wherein the strands comprising the
tether are not twisted relative to each other.
50. The nontwisted composite tether of claim 46 wherein the strands
comprising the tether are not twisted relative to each other.
51. The method of claim 1 further comprising adding buoyant
material to the tether such that the tether is neutrally or
positively buoyant
52. The method of claim 51 wherein buoyant material is added inside
the jacket.
53. The method of claim 52 wherein buoyant material is added
outside the jacket.
54. The method of claim 53 further comprising adding a second
jacket over the buoyant material.
55. The method of claim 42 further comprising attaching top and
bottom connectors to each end of the tether.
56. The method of claim 42 further comprising segmenting the tether
into at least two connectable segments.
57. A nontwisted composite tether produced by the method of claim
42.
58. A method for installing a composite tether on a floating
platform, comprising: a) launching the composite tether; b) towing
the composite tether to an offshore installation site; c) upending
the composite tether and connecting a bottom end connector on the
tether to an anchor foundation in the seabed; and d) connecting a
top end connector on the tether to the floating platform.
59. The method of claim 58 further comprising increasing the draft
of the floating platform prior to connecting the top end connector
thereto and subsequently decreasing the draft after the top end
connector is connected thereto such that the composite tether is
placed under tension by the buoyancy of the floating platform.
60. The method of claim 58 wherein the floating platform is a
tension leg platform (TLP).
61. The method of claim 58 further comprising manufacturing the
composite tether at a waterfront.
62. The method of claim 61 wherein the composite tether is
manufactured in accordance with claim 42.
63. The method of claim 61 wherein the composite tether is not
spooled after manufacture.
64. The method of claim 63 further comprising looping the composite
tether on a substantially horizontal surface for storage prior to
launch.
65. The method of claim 58 wherein the composite tether is towed on
the surface.
66. The method of claim 58 wherein the composite tether is towed
below the surface.
67. The method of claim 58 wherein one or both end connectors are
added to the tether prior to launching.
68. The method of claim 58 wherein one or both end connectors are
added after launching.
69. The method of claim 68 wherein one or both end connectors are
added after towing.
70. The method of claim 58 wherein the tether comprises at least
two segments that are assembled prior to installation on the
floating platform.
71. The method of claim 70 wherein the segments are towed to the
installation site prior to assembly.
72. The method of claim 58 further comprising adding buoyancy to
the tether as needed for towing, post-installation service, or
both.
73. A method for transporting a composite tether over a body of
water, comprising: launching the tether into the body of water and
towing the tether to an offshore installation site.
74. The method of claim 73 wherein the tether is towed on or below
the surface of the water.
75. The method of claim 74 wherein the composite tether is
nontwisted.
76. A method for preparing a composite tether for transportation,
comprising: adding buoyancy to the composite tether and launching
the buoyant composite tether into a body of water.
77. The method of claim 76 wherein the buoyancy is added during
manufacture of the tether, after manufacture of the tether, or
both.
78. The method of claim 77 further comprising anchoring the buoyant
tether in the body of water for storage.
79. The method of claim 78 further comprising unanchoring the
buoyant tether and connecting the buoyant tether to a tow
vessel.
80. The method of claim 79 wherein the composite tether is
nontwisted.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Patent Application No. 60/287,191,
filed Apr. 27, 2001, which is hereby incorporated herein by
reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
[0003] Not applicable.
FIELD OF THE INVENTION
[0004] The present invention is a novel composite tether for use in
supporting or anchoring a structure such as a floating platform or
vessel, and in particular for use in anchoring a tension leg
platform (TLP) to the ocean floor in deepwater and methods for
manufacturing, transporting, and installing the tether. The novel
composite tether is nontwisted and comprises a plurality of large
diameter rods pultruded from a carbon fiber and polymer matrix
composite.
BACKGROUND OF THE INVENTION
[0005] Composite tethers (also referred to as cables, tendons,
support lines, mooring lines and the like) are useful for securing
floating structures such as TLPs in deepwater. Particularly in
depths over about 4000 feet, composite tethers offer significant
economic and technical advantages and reliability over steel
tethers. Composites such as carbon fibers embedded in a polymer
matrix material are lightweight and have high specific strength and
stiffness and excellent corrosion and fatigue resistance, which
make them attractive for water depth sensitive components such as
tethers and risers or umbilicals, which transport hydrocarbons from
a wellhead on the ocean floor. Furthermore, composites are easily
outfitted with instrumentation such as fiber optics integrated into
the composite for load and integrity monitoring.
[0006] Conventional composite TLP tethers comprise top and bottom
end connectors for connection to the TLP and a foundation on the
ocean floor, respectively, and a tether body having a plurality of
parallel twisted strands. The twisted strands herein referred to
are formed from a twisted assemblage of small, parallel rods having
a diameter of about 3-6 mm, and typically comprise in the range of
about 50 to 200 rods per strand, wherein the assemblage of rods is
subjected to a helical twist, typically about 2 to 3.degree. on the
outer rods. The plurality of parallel twisted strands, wherein each
strand is typically about 50 to 75 mm in diameter, are also twisted
slightly to achieve a helix in the conventional tether, also
referred to herein as a twisted tether. The size of the
conventional tether is determined by the number of twisted strands,
which is dictated by the strength and axial stiffness requirements
for a given tether service (e.g., size of the TLP, water depth,
ocean currents, storm history, etc.). The number of twisted strands
per conventional tether is typically from about 8 to 30 twisted
strands per assembled conventional tether. Conventional tethers are
twisted as described previously so that they may be wound upon
tether spools, typically having a diameter of greater than about
4.0 meters, and preferably from about 4 to 8 meters. In order for
the conventional composite tethers to be spoolable, small diameter
rods having a diameter of no greater than about 6 mm are required,
otherwise the size of the required spool becomes impractical, as
described below. The spooled tethers are transported upon reel
ships or barges for installation and anchoring of the TLP to the
ocean floor.
[0007] The manufacturing process of a conventional, spoolable
composite tether includes the following steps: fabrication of small
diameter composite rods, assembly of the rods into twisted strands,
assembly of the twisted tether from multiple twisted strands
(including addition of filler and profile material as needed), and
termination of the twisted strands in top and bottom end connectors
of the tether. The manufacturing of conventional, spoolable
composite tethers is described in the following conference paper,
which is incorporated by reference herein in its entirety:
Composite Carbon Fiber Tether for Deepwater TLP Applications,
presented at the Deep Offshore Technology Conference held in
Stavanger, Norway on Oct. 19-21, 1999.
[0008] Composite materials for rod manufacture consist of small
diameter fibers (from about 6 to about 10 microns) of high strength
and modulus, preferably carbon fibers, embedded in a polymer matrix
material, e.g., resins or glues. Commonly known thermoset or
thermoplastic polymeric matrices may be used. Preferred matrix
materials include vinylesters and epoxies. The resin materials have
bonded interfaces which capture the desirable characteristics of
both the carbon fibers and the matrix. The carbon fibers carry the
main load in the composite material while the matrix maintains the
fibers in the preferred orientation. The matrix also acts to
transfer load into the carbon fibers and protects the fibers from
the surrounding environment. Carbon fibers incorporated in the
matrix may be spun in long continuous lengths; however, short (from
about 25 to about 100 mm) discontinuous fibers may also be
used.
[0009] Composite rods are typically manufactured by pultruding the
composite material comprising the carbon fibers and the polymer
matrix material. Pultrusion is the pulling of the resin wetted
fibers through a die rather than pushing it through the die as in
extrusion processes used for metal manufacturing. The die size and
shape control the final size and shape of the pultruded composite
product. There are several commercial pultruders such as Glasforms,
Inc., DFI Pultruded Composites Inc., Exel Oyj, Strongwell Corp.,
Spencer Composites Corp., and others that are capable of producing
the composite rods. Rods used in conventional spoolable tethers are
typically round in cross-section. The composite rods produced
typically have a weight which is approximately 1/6 that of required
for an equivalent steel rod. As discussed previously, rods for use
in conventional composite tethers typically are from about 3 to
about 6 mm in diameter and are often wound onto rod spools, for
example a 1.8 or 2.2 m diameter rod spool, for transportation to a
strand and/or tether manufacturing facility.
[0010] In general, it is desirable to increase the stiffness of
rods used in a tether, and the stiffness of a rod may be calculated
according to the following equation: 1 E A = 4 2 L ( Vertical Mass
+ Added Mass ) n T 2
[0011] where E=axial stiffness of a rod (Pa); A=cross sectional
area of 1 tether (m.sup.2); L=water depth (m); n=number of tethers;
T=heave natural periods (s), typically from about 5 to about 5.5
seconds; vertical mass=mass of the platform (kg); and added
mass=mass of the water that moves when the platform moves (kg).
Typically, a stiffer rod cannot be bent as much as a less stiff
rod. Given that the rods typically must be wound onto a rod spool
for transportation, the bending stiffness of the rod is
proportional to the diameter of the rod (d) raised to the fourth
power (i.e., d.sup.4). Thus, it is necessary to use a small
diameter composite rod (i.e., from about 3 to about 6 mm) in order
for the resulting rod spool diameter to be a practical size for
handling and transport and the force necessary to spool the rod and
maintain it on the spool be practical. More specifically, in sizing
the rod spool, the strain in the spooled rod is equal to the
diameter of the composite rod divided by the diameter of the rod
spool. In a properly sized spool, the rod strain is less than 50%
of the ultimate strain to failure of the rod. Thus, if the
composite rod has 1% strain to failure, then the diameter of the
rod spool then needs to be larger than 200 times the diameter of
the rod to be able to spool the rod onto the rod spool without
damaging the rod. If the composite rod has 1/2% strain to failure,
then the diameter of the rod spool has to be larger than 400 times
the diameter of the rod. The diameter of a spool refers to the hub
or core (i.e., drum) of the spool. In sum, where the rod itself
must be spooled (or a strand or tether incorporating the rod must
be spooled, as discussed below), the diameter and/or the stiffness
of the rod must be engineered accordingly.
[0012] In a conventional, spoolable composite tether, the rods are
assembled into bundles to form twisted strands. The twisted strands
can be manufactured using typical wire rope stranding methods.
Specifically, the rods are uncoiled from the rod spools and pulled
through a guide plate for bundling. When the required number of
rods per strand are laid out, the guide plates are rotated to
impart a slight helical twist, typically 2 to 3.degree. on the
outer rods. Twisting the strand provides sufficient coherence to
the strand for handling, coiling and transportation without
significantly affecting the axial strength and stiffness. The rods
in the twisted strands are fixed into a position by wrapping with
tape or other securing device, cut to length and spooled onto
strand spools for use in the assembly of the tether body.
Generally, twisted strand spools include 1.8 or 2.2 m diameter
spools such as those used for rod spools.
[0013] The twisted strands are assembled to form a conventional,
spoolable composite tether [5] (i.e., a twisted tether) as shown in
FIG. 1. The conventional, spoolable tether [5] is made up of
multiple twisted strands [15], the twisted strands being further
twisted with each other to form the twisted tether [5]. It can be
seen that there are a large number of composite rods [10], which
are bundled together to form individual twisted strands [15]. In
this particular figure, there are fifteen twisted strands [15]
making up the twisted tether [5], and a typical conventional tether
may include from about 8 to about 30 twisted strands. The twisted
strands [15] are held in place by a profiled member [20], which
fills the voids between the twisted strands [15] and also provides
a means for imparting a helical twist to the plurality of twisted
strands [15] (thereby forming the twisted tether [5]). The profiled
member [20] is preferably made from a plastic such as
polyvinylchloride (PVC) or polypropylene and may be divided into
segments such as center profile [25], intermediate profile [30],
and outer profile [35]. The profiled member [20] may also contain
void spaces [40]. A filler material may be placed in the void space
between the twisted strands [15]. Preferred filler according to
this invention is foam, which is used to give the tether buoyancy
as described below.
[0014] The twisted strands [15] are free to move individually in
the length direction, allowing individual adjustment and hence a
better distribution of axial loads. The composite rods [10] and
twisted strands [15] are free to act or move independently in the
twisted tether [5]. In other words, there is relative axial
movement between adjacent composite rods [10] within a twisted
strand [15] and between adjacent composite twisted strands [15]
within a twisted tether [5]. Otherwise the entire diameter of the
conventional, spoolable tether [5] must be considered in
calculating the diameter of the tether spool, since strain relates
to the diameter of the body that is being spooled divided by the
diameter of the spool, as described previously. By puffing a twist
in the composite rods [10] (via twisted strands [15] and twisted
tether [5]) and keeping them separate and independent, the diameter
of the individual composite rods [10] can roughly be used to
calculate the diameter of the tether spool rather than the entire
diameter of the twisted tether [5]. Typically, however, the tether
spool is made somewhat larger to account for the friction between
adjacent composite rods [10] as the conventional tether is spooled
onto the tether spool.
[0015] As shown in FIG. 2, different size twisted strands [16] and
[17] may be used to better fit all the twisted strands [16] and
[17] inside an outer jacket or casing [45]. Preferably, the area
within the casing [45] is filled by twisted strands [16] and [17]
and empty void spaces are minimized. The twisted strands [16] and
[17] are typically at least 30% of the area of the conventional,
spoolable tether [5] and more typically 50% of the area of the
tether [5]. Typically, the profiled member [20] and any filler
material do not add any performance characteristics to the twisted
tether [5], and thus it is desirable to minimize such components as
much as possible to avoid unwanted additional weight and increased
size.
[0016] The assembly of the conventional, spoolable tether [5] is
performed using a conventional umbilical closing machine. Spools
containing the twisted strands [15] and the profiled members [20]
are lifted onto the closing machine. The twisted strands [15] and
the profiled members [20] are then pulled through closing plates.
During this process, the machine rotates to impart a helical twist
in the twisted strands [15] to form the twisted tether [5]. A yarn
or other securing device is then applied to hold the assembly
together prior to extrusion of the protective, outer jacket [45]
such as high density polyethylene (HDPE), nylon, or the like over
the twisted strands to hold the twisted strands in place and
protect the tether during handling. Conventional, spoolable tethers
may be manufactured as a single continuous body that is spooled
onto a spool. Alternatively, the tether body may be manufactured as
a plurality of body lengths or segments that are spooled onto a
spool. The segments are connected with connectors (e.g., couples or
collars) to create a continuous tether. Segmenting the tether is
helpful in accommodating production of rods, strands, and tethers,
in limiting spool size, and in readjusting tether length for
re-installations.
[0017] The final step of manufacturing a conventional, spoolable
composite tether is the termination process that includes
connecting the twisted tether to top and bottom end connectors.
Termination using resin-potted cones has been extensively used in
the wire rope industry. Resin terminations have been proven to be
successful for terminating composite twisted strands as well. The
twisted strands are fastened to a steel end connector using a
potted cone technique similar to that used for termination of steel
wires. The twisted strands are spread with a specific angle in the
steel cones, and the cone is then filled with epoxy resin. A vacuum
injection method is used in this process to avoid air gaps and to
ensure consistent molding. Use of a flexible cone and cylindrical
metal connector with spacers can minimize the effect of termination
bending and provide better rod distribution inside the end
conector. Alternatively, the twisted strands may be individually
terminated and then assembled into a tether. After termination, the
tether is spooled onto an appropriately sized conventional tether
spool having a drum diameter of from about 4 to 8 m and a width of
about 5 m, for transportation and installation offshore. An
appropriately sized tether spool should be selected based upon the
characteristics of the composite rods as described previously.
[0018] When conventional tethers are spooled, the inside rods
comprising the twisted strands (and the inside twisted strands
comprising the twisted tether) are spooled at a smaller diameter
than the outside rods comprising the twisted strands (and the
outside twisted strands comprising the twisted tether), thereby
affecting the positioning of the rods (and twisted strands) within
the conventional tether and the compression and strain forces
acting thereon. Twisting the individual strands (i.e., twisted
strands) and the conventional tether itself (i.e., twisted tether)
subject the rods comprising the twisted strands and the twisted
strands comprising the twisted tether to an effective "average
diameter," meaning that no individual rod or twisted strand is
always on the inside or outside of the spool. Thus, all of the rods
comprising the twisted strands and the twisted strands comprising
the twisted tether maintain relative position to one another and
experience about the same forces while on the spool.
[0019] A number of problems exist with conventional, spoolable
composite tethers. Attempts to maximize the tether stiffness are
limited by the requirement that the rod diameter and/or stiffness
be engineered such that the rods (as well as the resultant twisted
strands and twisted tether) may be spooled without damage to the
rods. Spoolable tethers incorporating a large number of rods are
more difficult to manufacture and handle, and result in larger
diameter tethers that are more susceptible to adverse affects from
wave action such as fatigue and possible failure over time. Rod
strands typically result in more undesirable void space within the
tether since the strands often cannot be tightly spaced, further
requiring more filler material and/or profiled members that add
undesirable weight and increase size. The required twist in the
twisted strands and in the twisted tether to facilitate spooling
also adds to the difficulty and cost of manufacture and reduces the
axial stiffness of the spoolable tether, thus requiring a larger
number of rods to compensate for the stiffness loss. Expensive reel
ships are required for transport and installation of spoolable
tethers on TLPs. The novel composite tether of the present
invention solves these various problems.
SUMMARY OF THE INVENTION
[0020] The present invention includes a nontwisted composite
tether, a method for manufacturing the nontwisted composite tether,
a method for installing a composite tether on a TLP, and methods
for transporting a composite tether and preparing for such.
[0021] The nontwisted composite tether comprises one or more
composite rods encased in a jacket. A portion of the rods may be
bundled into one or more strands, provided however that the rods
comprising the strands are not twisted into twisted strands in the
assembled nontwisted tether. Such strands within the nontwisted
tether, if any, are untwisted, and such untwisted strands
additionally are not twisted relative to each other. In an
embodiment, rods for use in nontwisted tethers comprise medium
modulus carbon fibers (from about 32 to about 35 msi) and have a
circular cross section with a diameter of greater than about 5 mm,
preferably about 9 to about 25 mm, and more preferably about 12 mm.
In another embodiment, rods for use in nontwisted tethers comprise
high modulus carbon fibers (from about 55 to about 80 msi) and have
a circular cross section with a diameter of less than about 10 mm,
preferably about 3 to about 9 mm, and more preferably about 5 mm.
The nontwisted tethers typically comprise from about 20 to about
1000 total rods, preferably from about 30 to about 200 total rods,
and more preferably from about 30 to 80 total rods. Additional
embodiments include nontwisted tethers wherein the total number of
rods is less than about 30; wherein the total number of rods is
less than about 10; and wherein the tether comprises a single rod.
The nontwisted tether may further comprise buoyant material added
temporarily or permanently inside and/or outside the jacket to
increase the buoyancy of the nontwisted tether (preferably such
that the nontwisted tether is neutral or positively buoyant). The
nontwisted tethers may further comprise end connectors for
connecting to the TLP and an anchoring foundation on the ocean
floor, and the nontwisted tethers may be sized to a predetermined
length and segmented into connectable segments for further ease of
handling and transport.
[0022] The method for manufacturing the nontwisted composite tether
comprises supplying one or more composite rods, arranging the rods
axially, and encasing the rods within a jacket such that the
resulting tether is nontwisted. The rods may be supplied on a spool
or pultruded directly at a manufacturing site, preferably located
at a waterfront. The rods may be supplied as temporarily twisted
strands on spools, provided the strands are allowed to untwist
prior to final assembly into the nontwisted tether. Buoyant
material may be added temporarily or permanently inside and/or
outside the jacket to increase the buoyancy of the nontwisted
tether (preferably such that the nontwisted tether is neutral or
positively buoyant). End connectors for connecting to a TLP and an
anchoring foundation on the ocean floor may be added, and the
nontwisted tethers may be sized to a predetermined length and
segmented into connectable segments for further ease of handling
and transport.
[0023] The method for installing a composite tether on a floating
platform comprises launching the composite tether, towing the
composite tether to an offshore installation site, upending the
composite tether and connecting a bottom end connector on the
tether to an anchor foundation in the seabed, and connecting the
top end connector on the tether to the floating platform. In an
embodiment, the installation process further comprises increasing
the draft of the floating platform prior to connecting the top end
connector thereto and subsequently decreasing the draft after the
top end connector is connected thereto such that the composite
tether is placed under tension by the buoyancy of the floating
platform. Preferably, the floating platform is a tension leg
platform (TLP). The tether may be towed at the surface or below the
surface to the installation site and may be anchored offshore for
storage before or after towing.
[0024] The method for transporting a composite tether over a body
of water comprises launching the tether into the body of water and
towing the tether to an offshore installation site. The tether may
be towed on or below the surface of the water, and preferably is a
nontwisted tether.
[0025] The method for preparing a composite tether for
transportation comprises adding buoyant material to the composite
tether and launching the buoyant composite tether into a body of
water. The buoyant material may be added during manufacture of the
tether, after manufacture, or both. The tether, preferably a
nontwisted tether, may be anchored offshore for storage before or
after towing.
DESCRIPTION OF DRAWINGS
[0026] FIG. 1 is a cross-sectional view of a conventional,
spoolable composite tether having same sized strands of rods.
[0027] FIG. 2 is a cross-sectional view of a conventional,
spoolable composite tether having differing sized strands of
rods.
[0028] FIGS. 3A-G are cross-sectional views of nontwisted tethers
according to the present invention.
[0029] FIGS. 4A-D depict the manufacture of a nontwisted tether
according to the present invention.
[0030] FIGS. 5 and 6 are cross-sectional views of tether
terminations.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The novel composite tethers according to the present
invention are nontwisted as compared to conventional, twisted
composite tethers. The nontwisted composite tether comprises one or
more composite rods encased in a jacket, and typically comprises a
plurality of composite rods encased within the jacket. Within the
nontwisted tether, the rods are arranged in parallel, axial
alignment and are not subjected to twist either individually or
relative to one another. More specifically, the rods may be placed
in bundles or strands within the nontwisted tether, but the bundles
or strands are not subjected to helical twisting to form twisted
strands. Furthermore, bundles or strands within the nontwisted
tether are not subjected to twist relative to one another (i.e.,
are not twisted into a twisted tether, as described previously).
Typically, the nontwisted composite tethers of the present
invention, upon assembly, are not capable of being spooled onto
spools as described previously for conventional tethers.
[0032] Rods for use in nontwisted tethers may be made of the same
or similar materials (e.g., carbon fibers in a polymer matrix) and
by the same or similar methods (e.g., pultrusion) as rods for use
in conventional, spoolable tethers, as described previously. Rods
for use in nontwisted tethers preferably (but not necessarily) have
a relatively larger diameter as compared to the 3 to 6 mm spoolable
rods described previously in a conventional, spoolable tether. The
cross-sectional area of individual rods for use in nontwisted
tethers is preferably greater than about 28 mm.sup.2. Rods for use
in nontwisted tethers typically (but not necessarily) are not
capable of being spooled onto spools as described previously for
spoolable rods used in conventional, spoolable tethers. Provided
that the rods are spoolable, the rods may be spooled onto rod
spools for transportation to the manufacturing site of the
composite nontwisted tether. If the rods are not spoolable (due to
size, stiffness, cross-sectional shape, composite composition, or
combinations thereof), then the larger rods are preferably
manufactured at the nontwisted tether manufacturing site, which
ideally is located near a waterfront as discussed below. A portion
of the rods may be bundled into one or more strands, provided
however that the rods comprising the strands are not twisted into
twisted strands in the assembled nontwisted tether. In sum, strands
within the nontwisted tether, if any, are untwisted, and such
untwisted strands additionally are not twisted relative to each
other.
[0033] Rods for use in nontwisted tethers preferably (but not
necessarily) comprise medium or high modulus carbon fibers.
Preferred low cost, medium modulus (from about 32 msi to about 35
msi, and preferably about 33 msi) carbon fibers are
polyacrylonitrile (PAN) carbon fibers such as those available from
Grafil Inc., Toray Industries, Inc., Akzo Nobel, and ZOLTEK, among
others. Preferred low cost, high modulus (from about 55 msi to
about 80 msi, and preferably 70 msi) carbon fibers are those
available from Conoco Inc. and Mitsubishi Corp.
[0034] In an embodiment, rods for use in nontwisted tethers
comprise medium modulus carbon fibers and have a circular cross
section with a diameter of greater than about 5 mm, preferably
about 9 to about 25 mm, and more preferably about 12 mm. In another
embodiment, rods for use in nontwisted tethers comprise high
modulus carbon fibers and have a circular cross section with a
diameter of less than about 10 mm, preferably about 3 to about 9
mm, and more preferably about 5 mm.
[0035] Typically, nontwisted tethers of the present invention
comprise a total number of rods that is less than the total number
of rods in a conventional, spoolable tether, the total number of
rods being based upon the required stiffness of the tether, as
described previously. The nontwisted tethers typically comprise
from about 20 to about 1000 total rods, preferably from about 30 to
about 200 total rods, and more preferably from about 30 to 80 total
rods. Additional embodiments include nontwisted tethers wherein the
total number of rods is less than about 30; wherein the total
number of rods less than about 10; and wherein the tether comprises
a single rod.
[0036] The cross-section of the rods may have any suitable shape,
including irregular. Preferred rod cross-sectional shapes include
those which allows adjacent rods to closely fit against one another
such as round and polygonal (e.g., hexagonal, octagonal, square,
triangular, and rectangular), thereby minimizing the void space
between rods. Likewise, the nontwisted tether itself, as well as
any strands therein, may have a wide variety of cross-sectional
shapes in comparison to conventional, spoolable tethers that are
almost exclusively circular. Close fitting rods provide for a much
more compact tether which is less susceptible to wave action and
eliminates or minimizes filler material and profiled members,
thereby reducing the weight of the tether. The rods may be solid or
hollow, that is having at least one cross-section with a hole or
aperture in it. Hollow rods are preferably open at either end and
hollow across their entire length like a tube or pipe. To account
for water pressure, hollow rods preferably have hoop wind or the
bores thereof are filled with a support material such as foam.
[0037] Examples of cross-sections for nontwisted tethers of the
present invention are shown in FIGS. 3A-G, such examples being a
small sample of the many possible combinations and are not to be
construed as limiting the available combinations. Preferred
configurations are shown in FIGS. 3A and 3D. In an embodiment
employing high-modulus, discontinuous carbon fibers, a preferred
configuration is shown in FIG. 3B. Referring to FIG. 3A, a
nontwisted tether [125] comprises a plurality of solid hexagonal
rods [127] arranged in an abutting, close fitting relationship and
protected within a jacket [128]. A filler material [129] (and/or a
profiled member as described previously) fills the space between
outer surfaces of the rods [127] and the inner surface of the
jacket [128]. Referring to FIG. 3B, a nontwisted tether [130] has a
square cross section and comprises a plurality of stacked,
rectangular solid rods [132] protected within a jacket [134]. Given
the close fitting relationship between the rectangular rods [132]
and the jacket [134], no filler material or profiled member is
required in nontwisted tether [130]. Referring to FIG. 3C, a
nontwisted tether [135] comprises a plurality of solid circular
rods [142] that are not bundled into strands. Nontwisted tether
[135] is protected within a jacket [146], and a filler material
[144] (and/or a profiled member) fills the space between the outer
surface of the rods [142] and the inner surface of the jacket
[146]. Referring to FIG. 3D, a nontwisted tether [140] comprises a
plurality of solid circular rods [132] bundled into strands [134],
which are not twisted. The strands [134] are protected within a
jacket [136], and a filler material [138] (and/or a profiled
member) fills the space between the outer surface of the strands
[134] and the inner surface of the jacket [136]. Referring to FIG.
3E, a nontwisted tether [150] comprises a plurality of solid square
rods [152] arranged in an abutting, close fitting relationship
(shown in partial fill for clarity) and protected within a jacket
[153]. A filler material [154] (and/or a profiled member) fills the
space between outer surfaces of the rods [152] and the inner
surface of the jacket [153]. Referring to FIG. 3F, a nontwisted
tether [155] comprises a plurality of solid rectagular rods [156]
arranged in an abutting, close fitting relationship (shown in
partial fill for clarity) and protected within a jacket [157]. A
filler material [158] (and/or a profiled member) fills the space
between outer surfaces of the rods [156] and the inner surface of
the jacket [157]. Referring to FIG. 3G, a nontwisted tether [160]
comprises a plurality of small, solid circular rods [162] (shown in
partial fill for clarity) bundled into strands [164], which are not
twisted. The strands [164] surround a centrally positioned, large
irregularly shaped rod [165] having a bore [166], indicating that
the hexagonal rod is hollow. The nontwisted tether [160] further
comprises a plurality of medium circular rods [168] positioned
along the inner circumference of the tether. The nontwisted tether
[160 ] is protected by a jacket [167], and void space within the
tether is filled with a filler material [169] (and/or a profiled
member).
[0038] As described in detail below, nontwisted tethers of the
present invention, and in particular those designed and configured
for service in anchoring a TLP to the ocean floor, may further
comprise buoyant material added temporarily or permanently to
increase the buoyancy of the nontwisted tether (preferably such
that the nontwisted tether is neutral or positively buoyant). The
nontwisted tethers may further comprise end connectors for
connecting to the TLP and an anchoring foundation on the ocean
floor, and the tethers may be segmented into connectable segments
for further ease of handling and transport.
[0039] Referring to FIGS. 4A-D, in producing the nontwisted tether
[50] of the present invention, a plurality of rods [55] are
supplied, for example from rod spools [60]. Alternatively, the rods
[55], and in particular rods that are nonspoolable due to size,
stiffness, cross-sectional shape, composite composition, or
combinations thereof, can be produced on-site with protrusion
equipment (not shown) installed at the tether manufacturing site.
The individual rods [55] are bundled together to form rod bundle
[67], which can be facilitated by passing them through a template
[65] having holes to guide each of the rods [55] together. In
contrast to manufacture of a conventional, spoolable tether, the
template [65] is not rotated to impart a helical twist in the
tether (i.e., a twisted tether). A cross-sectional view taken along
line A-A of the rod bundle [67] is indicated by reference numeral
[75]. Alternatively, the rods may be placed in untwisted strands as
described previously. Such strands may be assembled at a remote
location and transported to the manufacturing site on spools, in
which case the strands may be temporarily twisted into twisted
strand for spooling and transport, but allowed to untwist prior to
integration into the nontwisted tether. The individual rods [55]
may be laid upon soft supports or rollers [70] spaced to prevent
unacceptable deflections and abrasion in the rods. The rods [55]
may be added to the bundle [67] individually or simultaneously and
held in place using temporary means such as tape or yarn. A
protective jacket [80], for example polyethylene, nylon, or the
like, is extruded over the rod bundle [67] using a jacket machine
[85] to form the nontwisted tether [85]. The nontwisted tether [85]
is terminated by cutting the tether to length and adding end
connectors [90] and [95].
[0040] The termination used for the addition of end connectors [90]
and [95] on the nontwisted tether [85] is similar to that used for
a steel tether (e.g., a potted termination). Referring to FIG. 5, a
metal cone [120] receives the ends [122] of the composite rods
[124] and the cone is filled with a resin system, as for example,
an epoxy system. Alternatively as shown in FIG. 6, with larger
diameter composite rods (or strands of rods) used in the tether,
each of the individual rods (or strands of rods) [124] can be
terminated separately, which should produce a stronger connection.
A metal sleeve [123] is bonded to (with epoxy or other resin
system) and protrudes from the end of each individual larger
diameter composite rods (or strands of rods) [124] and the ends of
these metal sleeves [123] then are attached together such as by
putting the ends of the metal sleeves [123] into an end connector
[126].
[0041] The nontwisted tether [85] may be further segmented into two
or more connectable segments (not shown) for ease of handling, the
segments having connector means such that the segments are capable
of being reconnected prior to installation. Buoyant material may be
temporarily and/or permanently added to the nontwisted tether [85].
Permanently attached buoyant material [105], for example foam, can
be placed inside the jacket [80], for example over the bundle of
rods [67] and/or into the void space there between. Another method
for permanently adding buoyant material is to wrap a buoyant
material such as foam around a first jacket and then place a second
jacket over the foam. Permanent buoyant material is preferably
added during manufacture of the nontwisted tether. Any suitable
type of buoyant material may be used as known to those skilled in
the art, for example syntactic foam or foamed polypropylene.
[0042] Temporarily attached buoyant material [105] can be attached
to the outside of the nontwisted tether [85] and removed during or
after the tether installation. Additional external temporary
buoyancy modules (TBM) [107] may be required, for example after
securing a nontwisted TLP tether to the foundation and before
arrival of the TLP at the installation site. The nontwisted tether
may include a special collar (not shown) to support the TBM, or the
TBM can be supported against the top end connector as shown in FIG.
4D. TBM can be installed after upending the tether, or towable TBM
can be installed on the tether body before launching. For example,
towable metal or composite air cans can be pre-installed at the
fabrication site to eliminate the need for offshore crane to handle
and attach the TBM.
[0043] Preferably, nontwisted tethers according to the present
invention, and in particular TLP tethers, are manufactured at a
shoreline, waterfront, or waterside where they can be launched and
towed to an installation site offshore. The terms shoreline,
waterfront, and waterside are synonymous and mean in close
proximity to a continuous water passage from the manufacturing site
to the site where the tether is to be installed offshore, for
example a stretch of beachfront or a manufacturing facility located
at a dock, pier, or harbor. Preferably, the shoreline manufacturing
site will have relatively unrestricted direct access to open water
(in contrast to a route requiring turns or bends during navigation)
to minimize the imposition of high curvatures on the nontwisted
tethers during towing. The nontwisted tether may be laid out
parallel, perpendicular, or at an angle to the shoreline, and may
be gently looped back and forth (for example in a figure eight
pattern) along a substantially horizontal surface if necessary to
save space provided that the design limits of the nontwisted tether
are not exceeded by bending in the loops. The nontwisted tether may
be launched for example by using cranes where the nontwisted tether
is positioned parallel to the shoreline or by using a winch and
rollers where the nontwisted tether is positioned perpendicular or
at an angle to the shoreline. The nontwisted tether may be anchored
offshore for storage before or after towing. Tugs or towboats
suitable for towing the launched tether are more commonly available
and cheaper than the relatively rare, specialized reel ships that
are used in installing conventional spooled tethers.
[0044] Buoyancy may be added or removed from the tether as needed
for transportation and/or post-installation service. Typically, a
nontwisted tether may or may not require additional temporary
buoyancy for towing the tether to the offshore platform and/or for
installation thereon. Buoyancy during towing avoids sagging and
associated stresses on the tether and eases towing. The tether may
be on the surface or below the surface for towing, and the buoyancy
adjusted as required for the desired tow. Buoyancy during
installation avoids placing an excessive load on the tether.
Permanent buoyancy may be used during post-installation service to
minimize the tendon weight on the floating platform. Preferably,
the composite tethers are nearly neutral buoyant upon installation,
hence the displacement of the TLP is constant and need not be
increased to account for additional weight of a negatively buoyant
tether. Thus, a composite tether can be used in extremely large
depths without appreciably adding additional weight to the floating
platform.
[0045] Typically, the tether is outfitted for towing prior to being
placed in the water, for example with buoyancy elements to support
the top and bottom end connectors; temporary, removable buoyancy
elements spaced intermittently along the length of the tether, if
the tether is not neutrally-buoyant; temporary, removable
marker-type buoys, if the tether is neutrally buoyant; navigation
lights and radar reflectors; and towing bridles forward and aft.
The outfitted tethers may be stored on land and launched shortly
before commencement of tow, or alternatively may be launched and
moored in a sheltered location for storage before or after tow.
Typically, the tethers are towed one at a time to minimize risk of
loss, and three vessels are used to tow the tether: a lead tug, a
trail tug, and an escort vessel to reduce the risk of other
waterborne traffic hitting or riding over the tether while in tow.
Towage speeds typically range between about 6 to about 8 knots,
depending in part upon towage distance and weather conditions.
[0046] Upon installation of adequate permanent and/or temporary
buoyancy and launch of the completed nontwisted tether as described
previously, the tether is towed by a tow vessel spread to the
offshore site where the floating platform (e.g., TLP) is to be
tethered to the ocean floor. Anchor foundations, for example a
concrete foundation or suction anchor, are preset in the seabed at
the installation site. Upon reaching the installation site, the
nontwisted tether is disconnected from the tow vehicles, upended,
and the bottom end connector connected to the preset anchor. More
specifically, a support vessel having a suitable crane, ROV spread,
and TBMs is stationed at the installation site. Upon arrival of the
tether towing vessels, the lead tug transfers forward, top end
portion of the tether to the crane on the support vessel. The trail
tug remains attached via an anchor winch wire to the aft, bottom
end portion of the tether. The buoyancy elements are removed from
the bottom end portion of the tether, which is subsequently
supported by the winch wire. To up-end the tether, the trail tug
plays out the winch wire to lower the bottom end portion of the
tether toward the ocean floor, and the top end portion of the
tether is held on place by crane. During up-ending, other removable
elements such as marker buoys and intermediate buoyancy elements
are removed, for example by pull lines, acoustically activated
release triggers, or automatically activated, depth-sensitive
release mechanisms. When the tether reaches a substantially
vertical position, the winch line is disconnected from the bottom
end portion of the tether via an ROV from the support vessel. A TBM
is attached to the top end portion of the tether, and the support
vessel maneuvers the bottom end portion of the tether over the
appropriate tether foundation receptacle located in the ocean
floor, as monitored by a ROV. The bottom end connector of the
tether is stabbed into the foundation and latched into position, at
which time the buoyancy of the TBM is adjusted via a blowdown hose
that displaces seawater in the TBM with air from a compressor on
the support vessel. Once the tugs have transferred the tether to
the support vessel, the towage vessel spread may return to base for
towing of the next tether. The tether up-ending operations continue
until all tethers are up-ended and freestanding. Typically, the
support vessel remains on site during the time between completion
of upending and final installation of the TLP to monitor the
tethers and adjust buoyancy of the TBM as needed.
[0047] Before connecting the top end connector of the tether to the
TLP, a constant tension winch is connected to tether and is
activated in combination with the addition of ballast to cause the
TLP to sink lower in the water (i.e., increase the draft of the
TLP). The top end connector of the tether is connected to the TLP,
and the draft is reduced through deballasting until the correct
draft and tension on the tethers are maintained. Typically, a
plurality of tethers are installed to hold the floating platform
securely in position. Installation of the composite tether via
towing and upending is similar to towing and upending steel
tethers, as discussed in the following articles each of which is
incorporated by reference herein in its entirety: Drilling and
Production Risers Can be Effectively Installed at a Much Lower Cost
Using the Piperlines Towing Techniques, presented at the Deep
Offshore Technology 12.sup.th International Conference held in New
Orleans, La. on Nov. 7-9, 2000; OTC 8100: The Heidrun
Field--Heidrun TLP Tether System, presented at the Offshore
Technology Conference held in Houston, Tex. on May 6-9, 1996 (p.
677-688); OTC 8101: The Heidrun Field--Marine Operations, presented
at the Offshore Technology Conference held in Houston, Tex. on May
6-9, 1996 (p. 689-717); OTC 6361: Materials, Welding, and
Fabrication for the Jolliet Project, presented at the Offshore
Technology Conference held in Houston, Tex. on May 7-10, 1990 (p.
159-166); and OTC 6362: Installation of the Jolliet Field TLWP,
presented at the Offshore Technology Conference held in Houston,
Tex. on May 7-10, 1990 (p. 167-180).
[0048] While is it preferred that the preparation, transportation,
and installation methods described herein be used to install
nontwisted tethers of the kind described herein, such methods may
also be used to install conventional composite tethers. For
example, the spool containing a conventional tether may be placed
near the waterside and the tether towed out therefrom and installed
as described previously.
EXAMPLE
[0049] The following example is a comparison of the dimensions of a
conventional, spoolable composite tether identified as round tether
A with two nontwisted tethers, each of which is produced in
accordance with this invention, identified as square tether NS-1
having a plurality of solid rectangular rods and round tether NS-2
having a plurality of solid circular rods.
[0050] Two important parameters for sizing a tether in response to
a given load and to provide the needed stiffness are the total
cross-sectional area of the composite rods in the tether and the
elastic modulus of the rods. In general, if the elastic modulus of
the composite rod is increased (thus increasing the stiffness of
the composite), the required cross-sectional area of the composite
that is carrying the load is reduced. The total cross-sectional
area of the rods that is carrying the load is equal to the
cross-sectional area of each rod times the number of rods. Stated
alternatively, the number of rods required can be determined by
dividing the total cross-sectional area of the rods required to
carry a given load and achieve a specific stiffness by the
cross-sectional area of each rod. From this relationship, it can be
seen that for a given total cross-sectional area, use of larger
rods, as is preferred according to the present invention, results
in a fewer number of rods that must be produced, handled, and
incorporated into the tether. The following table compares the
dimensions of the three tethers where the elastic modulus of the
composite and the cross-sectional area of the composite that is
carrying the load are held constant:
1 Tether A Tether NS-1 Tether NS-2 Rod Dimensions 6 mm 150 mm 12 mm
diameter wide and diameter 6 mm thick Number of Rods 781 24 195
Surface Area per Rod 28.3 900.0 113.0 (mm.sup.2) Total Surface Area
22102 21600 22035 (mm.sup.2) Tether Dimensions 270 150 mm 177 (mm)
diameter square diameter
[0051] As can be seen form the table, the nontwisted tethers NS-1
and NS-2 of the present invention have significantly fewer total
rods and are significantly smaller in overall size as compared to
the conventional, spoolable composite tether A.
[0052] While preferred embodiments of the invention have been shown
and described, modifications thereof can be made by one skilled in
the art without departing from the spirit and teachings of the
invention. The embodiments described herein are exemplary only, and
are not intended to be limiting. Many variations, combinations, and
modifications of the invention disclosed herein are possible and
are within the scope of the invention. Accordingly, the scope of
protection is not limited by the description set out above, but is
defined by the claims which follow, that scope including all
equivalents of the subject matter of the claims.
* * * * *