U.S. patent application number 13/145349 was filed with the patent office on 2012-01-12 for vortex-induced vibration (viv) suppression of riser arrays.
Invention is credited to Donald Wayne Allen, Michalakis Efthymiou, Dean Leroy Henning, Guido Leon Kuiper, Li Lee.
Application Number | 20120006053 13/145349 |
Document ID | / |
Family ID | 42356364 |
Filed Date | 2012-01-12 |
United States Patent
Application |
20120006053 |
Kind Code |
A1 |
Allen; Donald Wayne ; et
al. |
January 12, 2012 |
VORTEX-INDUCED VIBRATION (VIV) SUPPRESSION OF RISER ARRAYS
Abstract
A system comprising an array of structures in a flowing fluid
environment, the array comprising at least 3 structures; and vortex
induced vibration suppression devices on at least 2 of the
structures.
Inventors: |
Allen; Donald Wayne;
(Richmond, TX) ; Efthymiou; Michalakis; (GD
Rijswijk, NL) ; Henning; Dean Leroy; (Needville,
TX) ; Kuiper; Guido Leon; (AV Delft, NL) ;
Lee; Li; (Houston, TX) |
Family ID: |
42356364 |
Appl. No.: |
13/145349 |
Filed: |
December 17, 2009 |
PCT Filed: |
December 17, 2009 |
PCT NO: |
PCT/US2009/068513 |
371 Date: |
September 26, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61146526 |
Jan 22, 2009 |
|
|
|
Current U.S.
Class: |
62/611 ;
405/211 |
Current CPC
Class: |
B63B 2021/504 20130101;
F25J 1/0022 20130101; B63B 13/00 20130101; F15D 1/10 20130101; F25J
1/0278 20130101; B63B 21/502 20130101; B63J 2/12 20130101; F25J
1/0297 20130101 |
Class at
Publication: |
62/611 ;
405/211 |
International
Class: |
F25J 1/00 20060101
F25J001/00; E02D 5/60 20060101 E02D005/60 |
Claims
1. A system comprising: an array of structures in a flowing fluid
environment, the array comprising at least 3 structures; and vortex
induced vibration suppression devices on at least 2 of the
structures, wherein at least one of the structures comprise no
vortex induced vibration suppression devices.
2. The system of claim 1, wherein the array of structures are
within a body of water.
3. The system of claim 2, wherein the structures comprise first
ends that are connected to a floating vessel, and second ends that
project generally downward into the water.
4. The system of claim 1, wherein the vortex induced vibration
suppression devices are installed on from 20% to 80% of the
structures.
5. The system of claim 1, wherein the vortex induced vibration
suppression devices are installed on from 30% to 60% of the
structures.
6. (canceled)
7. The system of claim 1, wherein the array comprises at least one
internal structure and a plurality of external structures that form
a periphery about the internal structures, wherein the vortex
induced vibration suppression devices are installed on from 40% to
65% of the external structures.
8. The system of claim 1, wherein the flowing fluid environment
comprises a predominant current direction, and wherein a structure
in the array that first encounters the predominant current
comprises at least one vortex induced vibration suppression
device.
9. The system of claim 1, wherein the vortex induced vibration
suppression devices are selected from strakes and fairings.
10. The system of claim 1, wherein the vortex induced vibration
suppression devices comprise at least two different types of
devices.
11. The system of claim 1, wherein a first structure of the array
of structures has a diameter at least 20% larger than a second
structure of the array of structures.
12. The system of claim 1, wherein the flowing fluid environment
comprises a predominant current direction, and wherein a structure
in the array that first encounters the predominant current
comprises a diameter at least 15% smaller than another structure in
the array.
13. The system of claim 1, wherein the array comprises at least 6
structures.
14. The system of claim 1, wherein the structures comprise a
tubular, each tubular comprising an opening therethrough for
transportation of a fluid.
15. A method of suppressing the vortex induced vibration of an
array of structures comprising: installing vortex induced vibration
suppression devices on from 10% to 90% of the structures.
16. The method of claim 15, further comprising connecting a
plurality of the structures to each other.
17. The method of claim 15, further comprising modifying a diameter
of at least one of the structures, so that a first structure of the
array of structures has a diameter at least 30% larger than a
second structure of the array of structures.
18. A system comprising: an array of structures in a flowing fluid
environment, the array comprising at least 3 structures, wherein
the structures are coupled to each other at a plurality of
locations along a length of the structures; and vortex induced
vibration suppression devices on at least 2 of the structures, and
wherein at least one of the structures comprise no vortex induced
vibration suppression devices.
19. The system of claim 18, wherein the vortex induced vibration
suppression devices are installed on from 20% to 90% of the
structures.
20. The system of claim 18, wherein the vortex induced vibration
suppression devices are installed on from 40% to 70% of the
structures.
21. The system of claim 1, wherein the at least 3 structures are
risers.
22. The system of claim 21, wherein the risers are arranged
parallel to each other, and wherein vertically spaced spacers are
provided along the risers to hold the risers in position relative
to each other.
23. A method of suppressing the vortex induced vibration of an
array of structures comprising: installing vortex induced vibration
suppression devices on at least 2 but not all of the
structures.
24. A process for liquefying natural gas, comprising: providing a
system comprising a floating vessel on a body of water, and an
array of structures, the array comprising at least 3 structures,
wherein the structures comprise first ends that are connected to
the floating vessel and second ends that project generally downward
into the water, and vortex induced vibration suppression devices on
at least 2 of the structures, wherein at least one of the
structures comprise no vortex induced vibration suppression
devices; using the at least 3 structures as water intake risers for
bringing water from the body of water to an FLNG plant on the
floating vessel; cooling and liquefying natural gas with the FLNG
plant, wherein the water is input to heat exchangers of the FLNG
plant in order to help liquefy the natural gas.
25. A process for gasifying liquefied natural gas, comprising:
providing a system comprising a floating vessel on a body of water,
and an array of structures, the array comprising at least 3
structures, wherein the structures comprise first ends that are
connected to the floating vessel and second ends that project
generally downward into the water, and vortex induced vibration
suppression devices on at least 2 of the structures, wherein at
least one of the structures comprises no vortex induced vibration
suppression devices; using the at least 3 structures as water
intake risers for bringing water from the body of water to an FLNG
plant on the floating vessel; using the water in the FLNG plant to
heat and gasify LNG.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to systems and methods for
reducing drag and/or vortex-induced vibration ("VIV") of a
plurality of structures.
BACKGROUND INFORMATION
[0002] Whenever a bluff body, such as a cylinder, experiences a
current in a flowing fluid environment, it is possible for the body
to experience vortex-induced vibration (VIV). These vibrations may
be caused by oscillating dynamic forces on the surface, which can
cause substantial vibrations of the structure, especially if the
forcing frequency is at or near a structural natural frequency.
[0003] Floating vessels may be used to liquify and gasify natural
gas. Sea water may be used to cool or heat the natural gas. It may
be desired to separate the water inlet from the water outlet due to
the temperature differences. A plurality of risers may be used to
collect or deposit water at a depth from the floating vessel. These
risers may be exposed to VIV.
[0004] Drilling for and/or producing hydrocarbons or the like from
subterranean deposits which exist under a body of water exposes
underwater drilling and production equipment to water currents and
the possibility of VIV. Equipment exposed to VIV includes
structures ranging from the smaller tubes of a riser system,
anchoring tendons, or lateral pipelines to the larger underwater
cylinders of the hull of a mini spar or spar floating production
system (hereinafter "spar").
[0005] The magnitude of the stresses on the riser pipe, tendons or
spars may be generally a function of and increases with the
velocity of the water current passing these structures and the
length of the structure.
[0006] It is noted that even moderate velocity currents in flowing
fluid environments acting on linear structures can cause stresses.
Such moderate or higher currents may be readily encountered when
drilling for offshore oil and gas at greater depths in the ocean or
in an ocean inlet or near a river mouth.
[0007] There are generally two kinds of current-induced stresses in
flowing fluid environments. The first kind of stress may be caused
by vortex-induced alternating forces that vibrate the structure
("vortex-induced vibrations") in a direction mainly perpendicular
to the direction of the current. When fluid flows past the
structure, vortices may be alternately shed from each side of the
structure. This produces a fluctuating force on the structure
transverse to the current. If the frequency of this harmonic load
is near one of the natural frequencies of the structure, large
vibrations transverse to the current can occur. These vibrations
can, depending on the stiffness and the strength of the structure
and any welds, lead to unacceptably short fatigue lives. In fact,
stresses caused by high current conditions in marine environments
have been known to cause structures such as risers to break apart
and fall to the ocean floor.
[0008] The second type of stress may be caused by drag forces,
which push the structure in the direction of the current due to the
structure's resistance to fluid flow. The drag forces may be
amplified by vortex-induced vibration of the structure. For
instance, a riser pipe that is vibrating due to vortex shedding
will generally disrupt the flow of water around it more than a
stationary riser. This may result in more energy transfer from the
current to the riser, and hence more drag.
[0009] Many types of devices have been developed to reduce
vibrations and/or drag of sub sea structures. Some of these devices
used to reduce vibrations caused by vortex shedding from sub sea
structures operate by stabilization of the wake. These methods
include use of streamlined fairings, wake splitters and flags.
[0010] Devices used to reduce vibrations caused by vortex shedding
from sub-sea structures may operate by modifying the boundary layer
of the flow around the structure to prevent the correlation of
vortex shedding along the length of the structure. Examples of such
devices include sleeve-like devices such as helical strakes,
shrouds, fairings and substantially cylindrical sleeves.
[0011] Elongated structures in wind or other flowing fluids can
also encounter VIV and/or drag, comparable to that encountered in
aquatic environments. Likewise, elongated structures with excessive
VIV and/or drag forces that extend far above the ground can be
difficult, expensive and dangerous to reach by human workers to
install VIV and/or drag reduction devices.
[0012] Fairings may be used to suppress VIV and reduce drag acting
on a structure in a flowing fluid environment. Fairings may be
defined by a chord to length ratio, where longer fairings have a
higher ratio than shorter fairings. Long fairings are more
effective than short fairings at resisting drag, but may be subject
to instabilities. Short fairings are less subject to instabilities,
but may have higher drag in a flowing fluid environment.
[0013] U.S. Pat. No. 6,223,672 discloses an ultrashort fairing for
suppressing vortex-induced vibration in substantially cylindrical
marine elements. The ultrashort falling has a leading edge
substantially defined by the circular profile of the marine element
for a distance following at least about 270 degrees thereabout and
a pair of shaped sides departing from the circular profile of the
marine riser and converging at a trailing edge. The ultrashort
fairing has dimensions of thickness and chord length such that the
chord to thickness ratio is between about 1.20 and 1.10. U.S. Pat.
No. 6,223,672 is herein incorporated by reference in its
entirety.
[0014] U.S. Pat. No. 3,978,804 discloses a structure floating on a
body of water, and particularly a structure for drilling or
producing wells from below the water. Buoyant members support at
least a part of the structure above the surface of the water. The
structure is connected to anchors in the floor of the body of water
by a series of parallel leg members. Each leg member is composed of
a plurality of elongated members, such as large diameter pipe
usually called risers. These risers are parallel. Vertically spaced
spacers are provided along the risers of each leg to (1) maintain
the risers a fixed distance apart and (2) change the natural or
resonant frequency of the individual riser pipes to be greater than
the flutter frequency caused by the motion of the water past the
risers. U.S. Pat. No. 3,978,804 is herein incorporated by reference
in its entirety.
[0015] U.S. Pat. No. 6,089,022 discloses a system and a method for
regasifing LNG aboard a carrier vessel before the re-vaporized
natural gas is transferred to shore. The pressure of the LNG is
boosted substantially while the LNG is in its liquid phase and
before it is flowed through a vaporizer(s) which, in turn, is
positioned aboard the vessel. Seawater taken from the body of water
surrounding said vessel is flowed through the vaporizer to heat and
vaporize the LNG back into natural gas before the natural gas is
off-loaded to onshore facilities. U.S. Pat. No. 6,089,022 is herein
incorporated by reference in its entirety. U.S. Pat. No. 6,832,875
discloses a floating plant for liquefying natural gas having a
barge provided with a liquefaction plant, member for receiving
natural gas and with member for storing and discharging liquefied
natural gas. The liquefaction plant involves a heat exchange in
which heat is removed when liquefying natural gas is transferred to
water. The barge is further provided with a receptacle; an
open-ended water intake conduit having an inlet; a connecting
conduit extending from the outlet of the water intake conduit to
the receptacle; a pump for transporting water from the receptacle
to the heat exchanger and a water discharge system for discharging
water removed from the heat exchanger. The connecting conduit has
the shape of an inverted "U" of which the top is located above the
receptacle. U.S. Pat. No. 6,832,875 is herein incorporated by
reference in its entirety.
[0016] There are needs in the art for one or more of the following:
apparatus and methods for reducing VIV and/or drag on structures in
flowing fluid environments, which do not suffer from certain
disadvantages of the prior art apparatus and methods; apparatus and
methods for reducing VIV and/or drag on multiple structures in
flowing fluid environments; apparatus and methods for reducing VIV
and/or drag on a riser array or bundle.
[0017] These and other needs in the art will become apparent to
those of skill in the art upon review of this specification,
including its drawings and claims.
SUMMARY OF THE INVENTION
[0018] One aspect of the invention provides a system comprising an
array of structures in a flowing fluid environment, the array
comprising at least 3 structures; and vortex induced vibration
suppression devices on at least 2 of the structures.
[0019] Another aspect of the invention provides a method of
suppressing the vortex induced vibration of an array of structures
comprising installing vortex induced vibration suppression devices
on from 10% to 90% of the structures.
[0020] Advantages of the invention may include one or more of the
following: improved VIV reduction of a plurality of structures;
improved drag reduction of a plurality of structures; lower cost
VIV reduction; and/or VIV reduction of a plurality of structures
with fewer VIV suppression devices.
[0021] These and other aspects of the invention will become
apparent to those of skill in the art upon review of this
specification, including its drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention may best be understood by referring to the
following description and accompanying drawings that are used to
illustrate embodiments of the invention. In the drawings:
[0023] FIG. 1 illustrates an example of a marine system in which
embodiments may be implemented.
[0024] FIG. 2A is a cross-sectional top view illustrating one or
more representative strakes installed along a length of tubular
structure as VIV suppression device(s).
[0025] FIG. 2B is a cross-sectional top view illustrating a
representative fairing installed along a length of tubular
structure as a VIV suppression device.
[0026] FIGS. 3A-3H illustrate several different exemplary
approaches or configurations for coupling VIV suppression devices
with only a subset of tubular structures, according to various
embodiments.
[0027] FIG. 4 illustrates an exemplary approach or configuration of
a plurality of tubular structures in which at least two, in this
case at least three, of the tubular structures have different outer
diameters, according to one or more embodiments.
[0028] FIG. 5 illustrates an example approach or configuration that
is similar to that of FIG. 4 except that, in addition to the
different outer diameters, a subset of the tubular structures also
have VIV suppression devices coupled therewith, according to one or
more embodiments.
[0029] FIG. 6A illustrates an example of a marine system including
a Floating Liquified Natural Gas (FLNG) plant, in which embodiments
may be implemented.
[0030] FIG. 6B shows an example approach or configuration for a
Floating Liquified Natural Gas (FLNG) plant in which nine tubular
structures are arranged in a three-by-three rectangular array,
according to one particular embodiment.
DETAILED DESCRIPTION OF THE DRAWINGS
[0031] In the following description, numerous specific details are
set forth. However, it is understood that embodiments may be
practiced without these specific details. In other instances,
well-known structures and techniques have not been shown in detail
in order not to obscure the understanding of this description.
[0032] FIG. 1:
[0033] FIG. 1 illustrates an example of a marine system 100 in
which embodiments may be implemented.
[0034] The marine system includes surface structure 102 near a
water surface 104, for example a surface of the ocean. By way of
example, the surface structure may include a ship, a barge, a
vessel, an FPSO (floating production storage and offloading), a TLP
(tension leg platform), a spar, an offshore rig, an offshore
platform, a floating plant, a floating liquefied natural gas plant,
or other floating or surface structures as are known in the
art.
[0035] A plurality of tubular structures 106 are coupled with the
surface structure. In one particular aspect, the tubular structures
may be used in conjunction with providing cold water at depth to
cool natural gas in a floating liquefied natural gas plant serving
as the surface structure. By way of example, the tubular structures
may be connected to a marine riser tensioner, a swivel joint, a
ball joint, or the like. In one embodiment, a tubular structure has
a circular or oval cross-section. In another embodiment, a
cross-section of a tubular structure need not be circular or oval,
but can include other shapes such as, but not limited to,
rectangular.
[0036] In the illustration of FIG. 1, two tubular structures 106A,
106B are visible. More tubular structures may optionally be
included, such as, for example, at least three, at least four, at
least six, at least nine, or more. Examples of suitable tubular
structures include, but are not limited to, cables, umbilicals,
risers, marine risers, riser pipes, marine pipes, pipes, tubes, or
the like, or combinations thereof. The structures may extend all
the way to a seafloor 108, or only part way to the seafloor. In
some cases, mud, crude, water, and/or other fluids or electricity
or electrical signals may be conveyed through the structures.
[0037] The tubular structures are physically connected together, or
held in a position relative to one another, with one or more
interconnected guide sleeves or other spacers 110A, 110B. The
spacers connect or hold in position the tubular structures as an
array, bundle, grouping, other ordered arrangement, or other joined
plurality. By way of example, the spacers may include a metal,
plastic, or otherwise sufficiently strong material in a disc,
plate, rectangle, interconnected polygonal bars, wheel and spoke
shape, or other shape. The spacers may have holes or other openings
therein. Each of the holes or openings may accommodate and have
inserted therein one of the tubular structures. The spacers may
help to keep the tubulars relatively close together, but separated
so that they do not significantly strike into one another or
otherwise damage one another. One or more of the tubular structures
may serve as a structural support for the spacers. A tubular
structure serving as a structural support for a spacer may be
connected (directly or indirectly) to the spacer. For other tubular
structures in an array or grouping, such tubular structures need
not be connected to a spacer and in the case of a tubular structure
having, for example, a circular or oval shape, may instead have an
outer (outside) diameter (including or not including a VIV
suppression device) less than a diameter of an opening in the
spacer. Alternatively, the outer (outside) diameter of the tubular
structure (including or not including a VIV suppression device) may
be similar to a diameter of an opening so that the tubular
structure or a VIV suppression device on a tubular structure and
the spacer may be in contact (e.g., a force fit).
[0038] It is not uncommon that the tubular structures will be
disposed in water having current 112. Current 112 may tend to cause
hydrodynamic drag and/or vortex-induced vibration (VIV) of the
tubular structures. Further, in an array of tubular structures
coupled or positioned together with a spacer (e.g., spacer 110A),
VIV directly induced by current 112 on one tubular structure of the
array may be imparted to other tubular structures of the array.
Such VIV is generally undesirable, and if not suppressed, may
result in damage, fatigue, or even premature failure of the tubular
structures. Accordingly, it is generally desirable to reduce the
VIV of the tubular structures.
[0039] In some embodiments, VIV suppression devices may be used to
help suppress the VIV. Examples of VIV suppression devices or
structures suitable for implementing embodiments include, but are
not limited to, strakes, fairings, Henning devices, shrouds, wake
splitters, and other types of VIV suppression devices or
structures.
[0040] Suitable VIV suppression devices are disclosed in U.S.
patent application Ser. No. 10/839,781, having attorney docket
number TH1433; U.S. patent application Ser. No. 11/400,365, having
attorney docket number TH0541; U.S. patent application Ser. No.
11/419,964, having attorney docket number TH2508; U.S. patent
application Ser. No. 11/420,838, having attorney docket number
TH2876; U.S. Patent Application No. 60/781,846 having attorney
docket number TH2969; U.S. Patent Application No. 60/805,136,
having attorney docket number TH1500; U.S. Patent Application No.
60/866,968, having attorney docket number TH3112; U.S. Patent
Application No. 60/866,972, having attorney docket number TH3190;
U.S. Pat. No. 5,410,979; U.S. Pat. No. 5,410,979; U.S. Pat. No.
5,421,413; U.S. Pat. No. 6,179,524; U.S. Pat. No. 6,223,672; U.S.
Pat. No. 6,561,734; U.S. Pat. No. 6,565,287; U.S. Pat. No.
6,571,878; U.S. Pat. No. 6,685,394; U.S. Pat. No. 6,702,026; U.S.
Pat. No. 7,017,666; and U.S. Pat. No. 7,070,361, which are herein
incorporated by reference in their entirety.
[0041] Suitable methods for installing VIV suppression devices are
disclosed in U.S. patent application Ser. No. 10/784,536, having
attorney docket number TH1853.04; U.S. patent application Ser. No.
10/848,547, having attorney docket number TH2463; U.S. patent
application Ser. No. 11/596,437, having attorney docket number
TH2900; U.S. patent application Ser. No. 11/468,690, having
attorney docket number TH2926; U.S. patent application Ser. No.
11/612,203, having attorney docket number TH2875; U.S. Patent
Application No. 60/806,882, having attorney docket number TH2879;
U.S. Patent Application No. 60/826,553, having attorney docket
number TH2842; U.S. Pat. No. 6,695,539; U.S. Pat. No. 6,928,709;
and U.S. Pat. No. 6,994,492; which are herein incorporated by
reference in their entirety.
[0042] The VIV suppression devices may be installed on the tubular
member (e.g. buoyancy material and riser) before or after the
tubular member is placed in a body of water.
[0043] The VIV suppression devices may have a clamshell
configuration, and may be hinged with a closing mechanism opposite
the hinge, for example a mechanism that can be operated with an
ROV.
[0044] VIV suppression devices may be provided with copper plates
on their ends to allow them to weathervane with adjacent VIV
suppression devices or collars. VIV suppression devices may be
partially manufactured from copper.
[0045] FIGS. 2A-2B:
[0046] FIGS. 2A-2B show two common types of VIV suppression devices
or structures. Each of these devices or structures is suitable for
implementing one or more embodiments.
[0047] FIG. 2A is a cross-sectional top view illustrating one or
more representative strakes 220 installed along a length of tubular
structure 206 as VIV suppression device(s). The strake(s) may be
helical strakes, which are helically wrapped or coiled around the
tubular structure and may be described as connected thereto.
[0048] FIG. 2B is a cross-sectional top view illustrating a
representative fairing 222 installed along a length of tubular
structure 206 as a VIV suppression device and may be described as
connected thereto. The fairing has nose 224 and tail 226. The
fairing may swivel around the tubular structure based on the ocean
current.
[0049] Referring again to FIG. 1, the leftmost tubular structure
106A has one or more VIV suppression devices or structures 114A,
114B connected thereto therewith. Conventional collars (not shown)
may be used to keep the VIV suppression devices from moving along
the length of the tubular structures. The rightmost tubular
structure 106B does not have any VIV suppression devices or
structures coupled therewith.
[0050] In accordance with some embodiments, either all or only a
subset of the plurality of risers or other tubular structures may
have VIV suppression devices connected thereto. In these later
embodiments, one or more other tubular structures of the plurality
may not have VIV suppression devices connected thereto.
[0051] Omitting the VIV suppression devices from some of the
tubular structures (so that only a subset of the tubular structures
have the VIV suppression devices) may offer certain potential
advantages. For one thing, providing the VIV suppression devices on
all of the tubular structures tends to increase the overall
equipment cost. For another thing, it tends to be more difficult,
time consuming, and/or more expensive to install tubular structures
with VIV suppression devices as compared to tubular structures
without VIV suppression devices. The VIV suppression devices may
tend to make the tubular structures more bulky, difficult to
maneuver, difficult to align, and/or difficult to couple with the
spacers. It likewise tends to be more difficult, time consuming,
and/or more expensive to retrieve tubular structures with VIV
suppression devices, such as, for example, for cleaning,
inspection, and/or repair.
[0052] Typically from about 20 percent to about 80 percent of the
tubular structures may have the VIV suppression devices coupled
with them. Often, from about 30 percent to about 70 percent of the
tubular structures may have the VIV suppression devices coupled
with them. In some cases, from about 40 percent to 60 percent of
the tubular structures may have the VIV suppression devices coupled
with them.
[0053] It is not required that a tubular structure have VIV
suppression devices along its entire length. In other words, the
coverage density (the length of the structure covered with VIV
suppression devices compared to the total length) may be less than
1. The coverage density may also be expressed as a percentage of
the tubular structure length, and may be less than 100 percent.
Typically, the coverage density may range from about 50 percent to
about 100 percent, for example from about 60 to about 90%. The
number or percentage of tubular structures having VIV suppression
devices may be decreased by increasing the coverage density of a
selection of the tubular structures. In converse, if desired, the
coverage density may be decreased by increasing the number or
percentage of tubular structures having VIV suppression
devices.
[0054] FIGS. 3A-3H:
[0055] FIGS. 3A-3H illustrate several different example approaches
or configurations of VIV suppression devices connected with a
subset of tubular structures of an array, bundle, group, or other
plurality of tubular structures, according to various embodiments.
These figures represent cross-sectional views taken along section
line 3/4/5 of FIG. 1 through spacer 110A and the plurality of
tubular structures 106. The spacers couple the tubular structures
together, or hold the tubular structures in position relative to
one another, as arrays, bundles, groups, other ordered
arrangements, or other coupled pluralities.
[0056] In these illustrations, circles indicate tubular structures.
It is to be appreciated that a tubular structure need not occupy an
entire cross-sectional area of an opening in spacer 110A. Hatched
circles indicate tubular structures that have VIV suppression
devices coupled therewith. Un-hatched circles indicate tubular
structures without VIV suppression devices. While FIGS. 3A-3H show
different examples of VIV suppression devices connected with a
subset of tubular structures of an array (e.g., less than all
tubular structures), in another embodiment, VIV suppression devices
may be connected to each of the tubular structures in the array.
Any of the aforementioned VIV suppression devices are suitable.
[0057] FIGS. 3A-3F illustrate approaches or configurations for nine
tubular structures arranged in a three-by-three rectangular array,
in this particular case a substantially square array.
[0058] FIGS. 3A illustrates a first configuration in which only
tubular structures at all four corner positions of the
three-by-three rectangular array have VIV suppression devices
connected thereto, according to one embodiment.
[0059] The three-by-three rectangular array has tubular structures
at four corner positions 106A, 106C, 106G, and 106I, respectively.
These corner positions are referred to herein as upper left corner
106A, upper right corner 106C, lower left corner 106G, and lower
right corner 106I. The array also has tubular structures at four
central side positions 106B, 106D, 106F, and 106H, respectively.
These central side positions are referred to herein as upper side
106B, lower side 106H, right side 106F, and left side 106D,
respectively. The corner positions and the side positions in
combination form a periphery of the array. The three-by-three
rectangular array also has one tubular structure at a center
position 106E.
[0060] The four tubular structures at the four corner positions are
the only tubular structures that have VIV suppression devices
connected thereto. These four tubular structures help to suppress
VIV for the entire array. Advantageously, this arrangement is
robust and does not show very much sensitivity to the angle of an
oncoming ocean current.
[0061] In addition to suppressing VIV, the vortex shedding
frequency for the tubular structures with the VIV suppression
devices are often lower than that of the tubular structure without
the VIV suppression devices. In other words, the VIV suppression
devices may tend to help "detune" or reduce the excitation
frequency of the VIV of the tubular structures relative to bare
tubular structures. This may increase the `frequency dissociation`
of the array. These tubular structures with different frequencies
will be less likely to couple their vibrations. As a result,
vibration of the array may be reduced.
[0062] The tubular structures that have the VIV suppression devices
are substantially interspersed, interleaved, or otherwise staggered
with other tubular structures that do not have VIV suppression
devices coupled with them. On the periphery of the array only every
other tubular structure has a VIV suppression device connected
thereto. In such a staggered arrangement, adjacent tubular
structures tend to have different excitation frequencies. As
discussed, with such frequency dissociation the vibrations on these
tubular structures are less likely to couple and the overall
vibration of the array may be reduced.
[0063] In this configuration four out of nine or about 44 percent
of the tubular structures include VIV suppression devices connected
thereto. On the periphery, four out of eight or a higher percentage
of 50 percent of the tubular structures have VIV suppression
devices connected thereto.
[0064] FIGS. 3B illustrates a second configuration in which only a
tubular structure at a center position and tubular structures at
all four corner positions of the array have VIV suppression devices
connected thereto, according to one embodiment. This configuration
is similar to the configuration of FIG. 3A except that the tubular
structure at the center position also has VIV suppression devices
connected thereto.
[0065] As before, in this configuration, tubular structures with
VIV suppression devices are substantially staggered with tubular
structures without VIV suppression devices. As before, on a
periphery, only every other tubular structure has VIV suppression
devices. Such staggering may tend to increase the amount of
frequency dissociation, which may also help to reduce damage due to
VIV.
[0066] In this configuration five out of nine, or about 55 percent
of the tubular structures, have VIV suppression devices connected
thereto. On the periphery, four out of eight, or a higher
percentage of 50 percent of the tubular structures, have VIV
suppression devices connected thereto.
[0067] FIGS. 3C illustrates a third configuration in which only
tubular structures at four central side positions of the array have
VIV suppression devices connected thereto, according to one example
embodiment. This configuration is substantially opposite to the
configuration of FIG. 3A in that tubular structures at the sides
positions instead of at the corner positions have VIV suppression
devices.
[0068] As before, in this configuration, tubular structures with
VIV suppression devices are substantially staggered with tubular
structures without VIV suppression devices. As before, on a
periphery, only every other tubular structure has VIV suppression
devices. Such staggering may tend to increase the amount of
frequency dissociation, which may also help to reduce damage due to
VIV.
[0069] In this configuration four out of nine, or about 44 percent
of the tubular structures, have VIV suppression devices connected
thereto. On the periphery, four out of eight, or a higher
percentage of 50 percent of the tubular structures, have VIV
suppression devices connected thereto.
[0070] Sufficient suppression or dampening may be achieved with
even lower numbers or percentages of tubular structures having VIV
suppression devices when a predominant ocean, river, or other
flowing fluid current is known. In particular, in one or more
embodiments, the array may be aligned so that a higher percentage
of the tubular structures having the VIV suppression devices are on
a front row that faces the predominant ocean current.
[0071] FIGS. 3D illustrates a fourth configuration in which only
tubular structures at three corner positions of a front row of the
array that would first experience a predominant ocean current have
VIV suppression devices connected thereto, according to one example
embodiment. This configuration is similar to the configuration of
FIG. 3A except that the tubular structure at the lower left corner
position does not have VIV suppression devices.
[0072] Arrows are used to indicate a predominant ocean current. As
used herein, a `predominant` ocean current is the average or most
common ocean current including its average or most common
direction.
[0073] Five tubular structures on the top and right sides of the
array constitute a front row. This front row first experiences the
predominant ocean current.
[0074] In this configuration three out of nine, or about 33 percent
of the tubular structures, include VIV suppression devices coupled
therewith. On the periphery, three out of eight, or a higher
percentage of about 37 percent of the tubular structures, include
VIV suppression devices. On the front row, three out of five, or an
even higher percentage of about 60 percent of the tubular
structures, have VIV suppression devices.
[0075] Notice that a higher percentage of the front row tubular
structures have VIV suppression devices than the rest of the
non-front row tubular structures. It is these tubular structures
that would first experience the predominant ocean current, and that
would tend to experience the ocean current at its highest velocity.
These higher velocities would tend to make these tubular structures
have the most severe VIV. However, using the VIV suppression
devices on a higher percentage of these front row tubular
structures tends to suppress a large part of the VIV. Additionally,
staggering has been used along the front row. This helps to
increase the amount of frequency dissociation.
[0076] Moreover, the array is aligned so that the tubular structure
at the upper right corner position first experiences the
predominant ocean current before all other tubular structures. This
tubular structure would tend to experience the ocean current at its
highest velocity and would tend to have a relatively large amount
of VIV. However, advantageously, this tubular structure has one or
more VIV suppression devices.
[0077] Notice also that this alignment places more of the tubular
structures immediately downstream from or immediately in the wakes
of other upstream or front row tubular structures. A wake refers to
a region of separated flow, in some cases turbulent, downstream of
a solid body caused by flow of the fluid around the body. Average
fluid velocity tends to be lower in a wake. As a result, these
downstream tubular structures tend to experience lesser velocity
ocean currents and tend to have less VIV. In addition, tubular
structures in the wake of other tubular structures, and
experiencing a lower velocity current, tend to have a lower vortex
shedding frequencies and/or excitation frequencies. This adds
frequency dissociation to the array, which helps to reduce
vibrations.
[0078] As compared to FIG. 3A, the tubular structure at the lower
left corner position does not have one or more VIV suppression
devices. This tubular structure is downstream from several upstream
tubular structures and should tend to experience the ocean current
at a relatively reduced velocity. This makes it a relatively good
candidate to omit VIV suppression device(s). Accordingly, in one or
more embodiments, a tubular structure that would last experience a
predominant ocean current, after all other tubular structures of
the array, may not have one or more VIV suppression device(s).
[0079] Arrangements or configurations that have an even stronger
amount of suppression on the front row are contemplated. FIGS. 3E
illustrates a fifth configuration in which four tubular structures
at positions on a front row of the array that would first
experience a predominant ocean current have VIV suppression devices
connected thereto, according to one example embodiment.
[0080] As before, arrows are used to indicate a predominant ocean
current. The array is aligned so that the tubular structure at the
upper right corner position first experiences the predominant ocean
current. This alignment places more of the tubular structures in
the wakes of upstream tubular structures.
[0081] Five tubular structures on the top and right sides of the
array constitute a front row that first experiences the predominant
ocean current. In this embodiment, all four of the tubular
structures with VIV suppression devices are on the front row.
[0082] In this configuration four out of nine, or about 44 percent
of the tubular structures, have VIV suppression devices. On the
periphery, four out of eight, or a higher percentage of 50 percent
of the tubular structures, have VIV suppression devices. On the
front row, four out of five, or an even higher percentage of 80
percent of the tubular structures, have VIV suppression devices.
Accordingly, in this arrangement or configuration, an even higher
percentage of the front row tubular structures have VIV suppression
devices than the rest of the non-front row tubular structures.
[0083] FIGS. 3F illustrates a fifth configuration in which only one
tubular structure on a front row of the array and three tubular
structures at positions downstream from the front row have VIV
suppression devices connected thereto, according to one example
embodiment. This design relies more upon frequency dissociation
than upon strong suppression on the front row.
[0084] Arrows indicate a predominant ocean current. Five tubular
structures on the top and right sides of the array constitute a
front row that first experiences the predominant ocean current. In
this case, only one tubular structure with VIV suppression devices
is on the front row.
[0085] The array is aligned so a tubular structure at the upper
right corner position, which first experiences the predominant
ocean current, has VIV suppression devices connected thereto. This
alignment also places more of the tubular structures in the wakes
of upstream tubular structures. Notice that the lower left tubular
structure, which would last experience the predominant ocean
current after all other tubular structures of the array, does not
have a VIV suppression device(s).
[0086] In this configuration four out of nine, or about 44 percent
of the tubular structures, have VIV suppression devices connected
thereto. On the periphery, three out of eight, or about 37 percent
of the tubular structures, have VIV suppression devices. On the
front row, one out of five, or 20 percent of the tubular
structures, have VIV suppression devices.
[0087] The three-by-three rectangular array of FIGS. 3A-3F is not
required. In alternate embodiments, the plurality of tubular
structures may have various other numbers of tubular structures
and/or various other shapes (e.g., circular, star, triangular,
etc.).
[0088] FIG. 3G illustrates another example configuration for twelve
tubular structures arranged in a four-by-three rectangular array of
twelve tubular structures, according to one example embodiment. In
this configuration, a mix of a sufficient amount of front row
suppression and a sufficient amount of frequency disassociation has
been utilized.
[0089] Arrows indicate a predominant ocean current. The array is
aligned so that a tubular structure at the upper right corner
position, which has VIV suppression devices coupled thereto, first
experiences the predominant ocean current. This alignment also
places more of the tubular structures in the wakes of upstream
tubular structures.
[0090] Moreover, in relatively larger arrays, there tends to be a
relatively larger amount of natural frequency dissociation due to
the interferences and wake effects for the larger numbers of
tubular structures. In addition, fewer VIV suppressed tubular
structures may effectively reduce vibrations. As a result, in
relatively larger arrays it is generally possible to use
comparatively smaller numbers or percentages of tubular structures
having VIV suppression devices than with small to moderate numbers
of tubulars. For arrays with more than 12 risers even 20 percent or
25 percent of the tubular structures may have VIV suppression
devices depending upon the particular implementation.
[0091] In this configuration six out of 12 or 50 percent of the
tubular structures have VIV suppression devices connected thereto.
On the periphery, four out of 10 or 40 percent of the tubular
structures have VIV suppression devices. On the front row, two out
of six or about 33 percent of the tubular structures have VIV
suppression devices.
[0092] Non-rectangular arrays are also suitable. FIGS. 3H
illustrates yet another example configuration for nine tubular
structures arranged in a concentric array, according to one
embodiment. In this case, the concentric array is circular.
Alternatively, the array may be elliptical, oval, star shaped,
triangular, etc.
[0093] Arrows are used to indicate a predominant ocean current. The
array is aligned so that a tubular structure that has VIV
suppression devices coupled thereto first experiences the
predominant ocean current.
[0094] Tubular structures with VIV suppression devices are
substantially staggered with tubular structures that lack VIV
suppression devices. As before, on a periphery, only every other
tubular structure has VIV suppression devices.
[0095] In this configuration four out of nine or about 44 percent
of the tubular structures have VIV suppression devices connected
thereto. On the periphery, four out of eight or 50 percent of the
tubular structures have VIV suppression devices. On the front row,
three out of five or 60 percent of the tubular structures have VIV
suppression devices.
[0096] In any of the configurations of FIGS. 3A-3H, the VIV
suppression devices may optionally be conventional, and may be
constructed of any suitable material conventionally used for VIV
suppression devices. If desired, in one or more embodiments,
protective structures, such as, for example, covers, caps, bumpers,
or the like, may optionally be included on the VIV suppression
devices to help prevent mechanical damage, if bumping or contact
with a VIV suppression device were to occur. The protective
structures may comprise pliable, elastic, or soft materials, such
as, for example, rubber, plastic, foam, or the like. In one aspect,
the ends of the sections of the VIV suppression devices may
optionally be tapered to a smaller outside diameter than the
outside diameter of a remainder of a section of a suppression
device, which may facilitate installation and/or insertion through
spacers. Representatively, if a tubular structure having a VIV
suppression device installed thereon is inserted through an opening
in a spacer, if an end of the suppression device that is to be
initially inserted through the opening is tapered to a smaller
outside diameter, it tends to be easier to align the suppression
device/tubular structure with the opening and insert it into the
opening.
[0097] In any of the configurations of FIGS. 3A-3H, in one or more
embodiments, rather than using a single type and/or size of VIV
suppression device, multiple, different types and/or sizes of VIV
suppression devices may optionally be used, although this is not
required. For example, some of the subset of risers that have VIV
suppression devices may have a first type of VIV suppression device
(e.g., strakes), and others of the subset may have a second,
different type of VIV suppression device (e.g., fairings). One
strategy for using different types of VIV suppression devices might
be to change the excitation frequencies of the risers and/or
increase the frequency dissociation of the array. The different
types may optionally be staggered relative to one another to
provide additional frequency dissociation.
[0098] Another way of reducing vibrations is by using tubular
structures having a plurality of different outer diameters.
Accordingly, other embodiments pertain to a plurality of risers or
other tubular structures, in which at least two of the risers or
other tubular structures have different outer diameters. In one or
more embodiments, at least three of the tubular structures may have
different outer diameters.
[0099] The diameter of a tubular structure affects its vortex
shedding frequency and its VIV resonant frequency. In particular,
tubular structures that have relatively larger hydrodynamic
diameters will tend to have lower vortex shedding frequencies and
lower excitation frequencies compared to tubular structures that
have relatively smaller hydrodynamic diameters. As a result,
including tubular structures with different outer diameters in a
group, array, bundle, or other coupled plurality, may help to
"detune" the vibratory frequency of the group, array, bundle, or
other plurality by increasing the level of frequency dissociation
of the array. Additionally, an upstream tubular structure typically
has a higher shedding frequency than a downstream tubular structure
that is in its wake. As a result, greater frequency dissociation is
generally possible when a relatively larger diameter tubular
structure is downstream of a relatively smaller diameter tubular
structure.
[0100] In one or more embodiments, for a predominant ocean current,
a tubular structure having a relatively larger diameter may be in a
wake of, or downstream from, a structure having a relatively
smaller diameter. In one or more embodiments, for a predominant
ocean current, an average diameter of a plurality of upstream
tubular structures may be less than the average diameter of a
plurality of downstream tubular structures in their wake.
[0101] Typically, the largest diameters may range from 5 percent to
200 percent larger than the smallest diameters (expressed as a
percentage of the smallest diameters). Often, the largest diameters
may range from 10 percent to 150 percent larger than the smallest
diameters. In cases, the largest diameters may range from 25
percent to 100 percent larger than the smallest diameters. However,
the scope is not limited to any known difference between the
diameters.
[0102] In one or more embodiments, a sheath or other coating may be
included on the outside of a tubular structure in order to increase
the outside diameter of the tubular structure. The term coating is
not limited to a paint-like application process or the like but
more broadly encompasses a material coupled with the outside of the
tubular structure. In one or more embodiments, coatings having a
plurality of different thicknesses may be included on the outside
of different tubular structures in order to increase the outside
diameters of the tubular structures, and to provide a plurality of
different outside diameters. The coatings may potentially serve a
purpose other than to increase the diameter. For example, the
coatings may include thermal insulation to thermally insulate a
fluid within the tubular structures. As another example, the
coatings may include a buoyancy material.
[0103] FIG. 4:
[0104] FIG. 4 illustrates an example approach or configuration of a
plurality of tubular structures in which at least two, in this case
at least three, of the tubular structures have different outer
diameters, according to one or more embodiments. As before, this
figure is a cross-sectional view taken along section line 3/4/5 of
FIG. 1 through spacer 110A and the plurality of tubular structures
106.
[0105] Similarly to FIGS. 3A-3F, nine tubular structures are
arranged in a three-by-three rectangular array. In this
configuration, a tubular structure at a center position 106E, and
four tubular structures at four corner positions 106A, 106C, 106G,
and 106I, all have a first outer diameter. A tubular structure 106B
at an upper center side position and a tubular structure 106F at a
right center side position both have a second outer diameter. A
tubular structure 106D at a left center side position and a tubular
structure 106H at a lower center side position both have a third
outer diameter. As shown, the first outer diameter may be less than
the second outer diameter, and the second outer diameter may be
less than the third outer diameter. If desired, one or more of the
tubular structures may have yet another fourth outer diameter
different than the other three diameters.
[0106] Arrows indicate a predominant ocean current. The larger
diameter tubular structure 106D at the left side position is
downstream from and in the wake of the smaller diameter tubular
structure 106B at the upper side position. Likewise, larger
diameter tubular structure 106H at the lower side position is
downstream from and in the wake of smaller diameter tubular
structure 106F at the right side position. An average diameter of
the upstream tubular structures (e.g., 106B, 106C, and 106F) is
less than the average diameter of the downstream tubular structures
in their wake (e.g., 106D, 106H, and 106G).
[0107] FIG. 5:
[0108] It is also possible to use tubular structures having
different diameters in combination with including VIV suppression
devices on a subset of the tubular structures. FIG. 5 illustrates
an example approach or configuration that is similar to that of
FIG. 4 except that, in addition to the different outer diameters, a
subset of the tubular structures also have VIV suppression devices
connected thereto, according to one or more embodiments.
[0109] In one embodiment, the hatched circles are tubular
structures with VIV suppression devices. The un-hatched circles
represent tubular structures without VIV suppression devices.
[0110] In another embodiment, the hatched circles are tubular
structures without VIV suppression devices. The un-hatched circles
represent tubular structures with VIV suppression devices.
[0111] Both the different outer diameters and the VIV suppression
devices may contribute to reducing vibrations. In general, the
greater the variation in the outer diameters, the lesser the number
of VIV suppression devices that would be needed to sufficiently
reduce vibrations a particular implementation (including
potentially none). Likewise, the greater the number of VIV
suppression devices, the lesser the variation in the outer
diameters needed to sufficiently reduce vibrations for a particular
implementation (including potentially no variation).
[0112] The scope is not limited to the particular configurations
shown in FIGS. 4 and 5. A wide variety of other arrangements or
configurations will be apparent to those skilled in the art and
having the benefit of the present disclosure.
[0113] As yet another approach for dealing with vibrations, it is
also contemplated that devices or structures, whose primary purpose
is to add damping, produce frequency disassociation, or cause a
different shedding frequency, as opposed to primarily VIV
suppression, may be part of or connected with an array or other
associated or connected plurality of tubular structures. This may
help to increase the overall suppression of the system. Examples of
such devices include helical axial fins, axial non-helical fins,
and circumferential fins. In one or more embodiments, the fins or
other devices or structures may be flexible or include a flexible
material to further provide dampening. In one or more embodiments,
a coating that attracts marine growth (instead of suppressing
marine growth) may be added to the outside of the tubular structure
to enhance dampening.
[0114] Other embodiments pertain to methods of assembly of the
associated or connected tubular structures. A method of assembly
may include initially installing a structural support tubular
structure having one or more spacers connected therewith. For
example, in a rectangular array such as illustrated in FIGS. 3A-3F,
a center tubular structure (e.g., structure 106E) may be initially
connected to one or more spacers along a length of the structural
support (e.g., a length of multiple sections of the structural
support). Then one or more of other tubular structures, may be
separately threaded, inserted, or otherwise introduced through
openings in the spacers. The tubular structures may either have VIV
suppression devices connected thereto before being introduced into
the openings or the VIV suppression devices may be connected
afterwards. In one or more embodiments, in the final assembly, some
but not all, or a subset, of the tubular structures have VIV
suppression devices connected thereto. In one or more embodiments,
in the final assembly, a number of the tubular structures, in some
cases at least three of the tubular structures, have different
outer diameters.
[0115] Other embodiments pertain to methods of suppressing
vibrations in the connected or associated tubular structures. In
one or more embodiments, vibrations are suppressed with VIV
suppression devices connected with only a subset of the tubular
structures of a coupled or associated array or grouping. In one or
more embodiments, tubular structures with a number of outer
diameters (e.g. at least three different outer diameters) may be
vibrated at a plurality of different frequencies.
[0116] FIGS. 6A & 6B:
[0117] FIG. 6A illustrates an example of a marine system 600 in
which embodiments may be implemented. The marine system includes a
Floating Liquefied Natural Gas (FLNG) plant 602 on/in a surface of
the ocean 104. The FLNG plant is one particular example of a
surface structure. The FLNG plant may cool and liquefy natural gas,
or alternatively heat and gasify LNG. One or more tubular
structures of an array or grouping connected with FLNG plant 602
may be used to bring water from the ocean to the plant.
Alternatively, riser arrays may be used as drilling riser arrays,
production riser arrays, TLP tendons, etc.
[0118] Marine system 600, in this embodiment, includes a number of
tubular structures or risers 606 (e.g., nine tubular structures).
The risers each have first ends and second ends. The first ends are
connected to with the FLNG plant. The second ends project generally
downward into the ocean but not necessarily to the seafloor. By way
of example, the second ends may have depths of around 130 to 170
meters, although this is not required. Due to the ocean current,
tubular structures 606 may deflect from vertical by around 40
degrees or so (not shown). To accommodate for such deflection,
tubular structures 606 may be connected with the FLNG plant through
a swivel joint, a ball joint, a riser hanger, or other pivotable or
hingeable coupling.
[0119] Tubular structures 606 are physically associated or
connected together with a plurality of guide sleeves or spacers
610A, 610B, 610C. The spacers may have openings through which
respective ones of the tubular structures are disposed. In one
embodiment, enough spacers may be provided to keep the tubular
structures from striking into one another.
[0120] In one embodiment, some or all of tubular structures 606 may
serve as water intake risers. The water intake risers may take in
cold water 640 at depth, and convey the cold water upward to the
FLNG plant. The cold water may be input to heat exchangers of the
FLNG plant in order to cool natural gas to help liquefy the natural
gas. The heated ocean water from the outlet of the heat exchangers
may be discharged back into the ocean at the surface, or
alternatively conveyed back to depth with a different riser or set
of risers.
[0121] If desired, filters may optionally be coupled to each of the
bottoms of tubular structures 606. The filters may help to prevent
soil, marine life (e.g., seaweed, algae, fish, etc.), and the like,
from entering the tubular structures. Over time, the filters may
tend to become clogged. It tends to be relatively difficult to
clean the filters. For example, removing one or more tubular
structures 606 from an array so that the filters may be cleaned
tends to be costly, labor intensive, and/or time consuming. In one
or more embodiments, rather than removing the tubular structures
each time the filters become clogged, an array or grouping may
include surplus water intake tubular structures (risers) so that
the surplus water intake tubular structures may optionally be
included to provide adequate water intake even after some of the
filters clog. In one aspect, a tubular structure may be used until
its filter clogs and then may be taken off line and a new tubular
structure having a clean filter may be newly brought online. In
another aspect, tubular structures with VIV suppression devices may
not be used for water intake and may not have filters, but rather
may be used primarily for VIV suppression, while bare tubular
structures without VIV suppression devices may have filters and be
used for water intake. These bare tubular structures tend to be
easier to retrieve when their filters become clogged.
[0122] FIG. 6B shows an example approach or configuration for nine
tubular structures (risers) arranged in a three-by-three
rectangular array, according to one particular embodiment. This
figure is a cross-sectional view taken along section line 6B of
FIG. 6A through the plurality of tubular structures 606.
[0123] The array has eight tubular structures along the periphery
and one riser at the center. The eight tubular structures along the
periphery may serve as water intake risers to provide cold water to
the FLNG plant. The tubular structure at the center may serve as a
structural support structure (riser) for the spacers. The riser at
the center may, or may not, convey water to the surface (i.e., may
or may not serve as a water intake riser).
[0124] In one particular embodiment, the eight tubular structures
along the periphery may have outer diameters of about 42 inches and
wall thicknesses of about 1 inch, while the structural tubular
structure at the center may have an outside diameter of about 24
inches and a thickness of about 0.75 inches. The eight tubular
structures along the periphery may be equally spaced apart by a
distance of about one outer diameter or about 42 inches. To provide
sufficient cooling water to FLNG plant 602, in one embodiment, each
of tubular structures may not be necessary to be in operation at
any one time. Thus, one or more of the tubular structures may serve
as a surplus water intake riser.
[0125] In this example approach or configuration only tubular
structures at all four corner positions of the three-by-three
rectangular array have VIV suppression devices connected therewith.
Alternatively, other arrangements or configurations disclosed
herein may optionally be used.
Example 1
[0126] Tank tests have been performed on a three-by-three riser
array scaled down model with and without helical strakes. The
helical strakes, when included, were included only on four corner
risers of the array. This configuration is similar to that shown in
FIG. 3A. The tests and test results are summarizes in Table 1.
TABLE-US-00001 TABLE 1 Water Speed Range max rms Temp (F.) Test
Description (ft/sec) A/D 61 bare pipe bundle (9 pipes with 0.2-3.2
0.602 3 spacers 0-deg heading 78 strakes (0.2D height) on 4 corner
0.2-3.6 0.059 risers, 0-deg heading 79 strakes (0.2D height) on 4
corner 0.2-3.6 0.008 risers, 22.5 deg heading 82 strakes (0.2D
height) on 4 corner 0.2-3.6 0.008 risers, 45 deg heading
[0127] Different water temperatures were tested. Speed range
pertains to the current flow rate. Max rms A/D refers to the
maximum motion magnitude (root-mean-square value) in the speed
range tested and measures vibration. The lower the max rms A/D, the
lower the amount of vibration on the risers. These results indicate
that including helical strakes on only four corner risers in a
nine-riser array is sufficient to significantly reduce VIV.
[0128] The scope of the invention is not limited to achieving any
known particular amount of VIV suppression or dampening. The amount
of VIV suppression or dampening appropriate for a particular
implementation may vary widely from one implementation to another.
This may be due in part to variation in ocean current, tubular
length, materials of construction, amount of overdesign, and the
like.
[0129] In the description above, for the purposes of explanation,
numerous specific details have been set forth in order to provide a
thorough understanding of the embodiments. It will be apparent
however, to one skilled in the art, that one or more other
embodiments may be practiced without some of these specific
details. The particular embodiments described are not provided to
limit the invention but to illustrate it. The scope of the
invention is not to be determined by the specific examples provided
above but only by the claims below. In other instances, well-known
structures, devices, and operations have been shown in block
diagram form or without detail in order to avoid obscuring the
understanding of the description. Where considered appropriate,
reference numerals or terminal portions of reference numerals have
been repeated among the figures to indicate corresponding or
analogous elements, which may optionally have similar
characteristics.
Illustrative Embodiments
[0130] In one embodiment, there is disclosed a system comprising an
array of structures in a flowing fluid environment, the array
comprising at least 3 structures; and vortex induced vibration
suppression devices on at least 2 of the structures. In some
embodiments, the array of structures are within a body of water. In
some embodiments, at least one end of the structures are connected
to a floating vessel. In some embodiments, the vortex induced
vibration suppression devices are installed on from 20% to 80% of
the structures. In some embodiments, the vortex induced vibration
suppression devices are installed on from 30% to 60% of the
structures. In some embodiments, the structures are coupled to each
other at a plurality of locations along a length of the structures.
In some embodiments, the array comprises at least one internal
structure and a plurality of external structures that form a
periphery about the internal structures, wherein the vortex induced
vibration suppression devices are installed on from 40% to 65% of
the external structures. In some embodiments, the flowing fluid
environment comprises a predominant current direction, and wherein
a structure in the array that first encounters the predominant
current comprises at least one vortex induced vibration suppression
device. In some embodiments, the system also includes the vortex
induced vibration suppression devices are selected from strakes and
fairings. In some embodiments, the vortex induced vibration
suppression devices comprise at least two different types of
devices. In some embodiments, the array comprises a first structure
having a diameter at least 20% larger than a second structure. In
some embodiments, the flowing fluid environment comprises a
predominant current direction, and wherein a structure in the array
that first encounters the predominant current comprises a diameter
at least 15% smaller than another structure in the array. In some
embodiments, the array comprises at least 6 structures. In some
embodiments, the structures comprise a tubular, each tubular
comprising an opening therethrough for transportation of a
fluid.
[0131] In one embodiment, there is disclosed a method of
suppressing the vortex induced vibration of an array of structures
comprising installing vortex induced vibration suppression devices
on from 10% to 90% of the structures. In some embodiments, the
method also includes connecting a plurality of the structures to
each other. In some embodiments, the method also includes modifying
a diameter of at least one of the structures, so that a first
structure has a diameter at least 30% larger than a second
structure.
[0132] It should also be appreciated that reference throughout this
specification to "one embodiment", "an embodiment", or "one or more
embodiments", for example, means that a particular feature may be
included in the practice of the invention. Similarly, it should be
appreciated that in the description various features are sometimes
grouped together in a single embodiment, Figure, or description
thereof for the purpose of streamlining the disclosure and aiding
in the understanding of various inventive aspects. This method of
disclosure, however, is not to be interpreted as reflecting an
intention that the invention requires more features than are
expressly recited in each claim. Rather, as the following claims
reflect, inventive aspects may lie in less than all features of a
single disclosed embodiment. For example, unless specified or
claimed otherwise, the floating structure or floating liquefied gas
plant shown in a Figure is not intended to be a part of the
invention. Thus, the claims following the Detailed Description are
hereby expressly incorporated into this Detailed Description, with
each claim standing on its own as a separate embodiment of the
invention.
* * * * *