INDUCED VIBRATION

Field of the Invention The present invention relates to systems and methods for reducing drag and/or vortex-induced vibration ("VIV"). Description of the Related Art

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.

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")-

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.

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.

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") mainly in a direction 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 the resonant frequency 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. 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.

Many types of devices have been developed to reduce vibrations 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. 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. Elongated structures in wind in the atmosphere 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. 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 thickness 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.

U.S. Patent Number 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. Patent Number 6,223,672 is herein incorporated by reference in its entirety.

U.S. Patent Number 4,398,487 discloses a fairing for elongated elements for reducing current-induced stresses on the elongated element. The fairing is made as a stream-lined shaped body that has a nose portion in which the elongated element is accommodated and a tail portion. The body has a bearing connected to it to provide bearing engagement with the elongated element. A biasing device interconnected with the bearing accommodates variations in the outer surface of the elongated element to maintain the fairing's longitudinal axis substantially parallel to the longitudinal axis of the elongated element as the fairing rotates around the elongated element. The fairing is particularly adapted for mounting on a marine drilling riser having flotation modules. U.S. Patent Number 4,398,487 is herein incorporated by reference in its entirety.

PCT Publication Number WO2009023624, published on February 19, 2009, and having attorney docket number TH 3245, discloses a system for reducing drag and/or vortex induced vibration of a structure, the system comprising a multiple sided device comprising from 4 to 6 sides. PCT Publication Number WO2009023624 is herein incorporated by reference in its entirety.

Co-pending PCT Publication Number PCT/WO2009/046166, filed on October 2, 2008, and having attorney docket number TH 3350, discloses a system for reducing drag and/or vortex induced vibration of a structure, the system comprising a multiple finned device comprising from 4 to 6 fins substantially aligned along a longitudinal axis of the structure. Co-pending PCT Publication

Number PCT/WO2009/046166 is herein incorporated by reference in its entirety.

Referring now to Figure 1 , there is illustrated prior art short fairing 104 installed about structure 102. Structure 102 may be subjected to a flowing fluid environment, where short fairing 104 may be used to suppress vortex induced vibration (VIV). Short fairing 104 has chord 106 and thickness 108. Chord to thickness ratio of short fairing 104 may be less than about 1.5, or less than about 1.25. While short fairing 104 is effective at reducing vortex induced vibration, short fairing 104 may be subject to drag forces 1 10 in a flowing fluid environment.

Referring now to Figure 2, prior art long fairing 204 is illustrated installed about structure 102. Structure 102 may be in a flowing fluid environment where structure 102 is subject to vortex induced vibration. Compared to short fairing 104, long fairing 204 may have reduced drag when subjected to a flowing fluid environment. Long fairing 204 has chord 206 and thickness 208. Chord to thickness ratio of long fairing 204 may be greater than about 1.7, or greater than about 1.8, greater than about 2.0, or greater than about 2.25. Although long fairing 204 may have lower drag than short fairing 104, long fairing 204 may be subject to flutter, galloping, and/or a plunge-torsional instability. Long fairing 204 may experience lateral displacement 210 and/or torsional displacement 212.

There are needs in the art for one or more of the following: apparatus and methods for reducing VIV on structures in flowing fluid environments, which do not suffer from certain disadvantages of the prior art apparatus and methods; improved VIV suppression devices; high stability devices; devices which delay the separation of the boundary layer, devices which provide decreased VIV and/or devices which provide reduced drag; devices suitable for use at a variety of fluid flow velocities; and/or devices that have a high stability. 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 Description

One aspect of the invention provides a system for reducing drag and/or vortex induced vibration of a structure, the system comprising a multiple finned device comprising from at least one long fin having a first average height and at least one short fin having a second average height, wherein the height is a radial distance extending outwardly from the structure, and wherein the first average height is at least 10% larger than the second average height.

Another aspect of the invention provides a system for reducing drag and/or vortex induced vibration of a structure, the system comprising a multiple sided device comprising from 3 to 10 sides; and a fin extending outwardly from the device. Brief Description of the Drawings

Figure 1 shows a prior art short fairing.

Figure 2 shows a prior art long fairing.

Figure 3 shows an embodiment of a three-sided VIV suppression device also having a fin.

Figure 4 shows an embodiment of a four-sided VIV suppression device also having a fin.

Figure 5 shows an embodiment of a six-sided VIV suppression device also having a fin. Figure 6 shows an embodiment of a three-finned VIV suppression device, in which one fin is substantially longer than all others.

Figure 7 shows an embodiment of a four-finned VIV suppression device, in which one fin is substantially longer than all others.

Figure 8 shows an embodiment of a six-finned VIV suppression device, in which one fin is substantially longer than all others.

Figure 9 shows an embodiment of a two-finned VIV suppression device, in which one fin is substantially longer than another.

Figure 10 shows an embodiment of a one-finned VIV suppression device.

Figure 1 1 shows an embodiment of a plurality of VIV suppression devices installed along a length of a structure. Detailed Description of the Drawings

Figure 3:

Figure 3 is a top cross-sectional view of three-sided device 304 having fin 316, according to one embodiment. The device is installed about structure 302. Structure 302 may be in a flowing fluid environment with flow 310.

Representatively, structure 302 may include a riser, marine riser, water intake riser, tendon, pipeline, umbilical, or other elongated structure exposed to a current, for example a generally tubular structure. The flow may subject the structure to the possibility of vortex-induced vibration (VIV). Device 304 may be used to help suppress the VIV of the structure, such as, for example, by providing streamlining and/or frequency de-correlation.

Structure 302 has a major or elongated axis, which in the illustration is into the page. The structure also has a diameter or other cross-sectional dimension transverse to the major axis. The cross section of the structure need not be circular, but may instead be oval, rectangular, or otherwise shaped.

Device 304, according to this embodiment, has three sides 318. In various embodiments, all of the sides may have the same length, two of the sides may have the same length, or each side may have a different length.

In one or more embodiments, brace members 320 may be connected to the sides to brace, support, or reinforce the sides. In the illustration, six brace members are shown, although fewer (e.g., three) or more brace members may alternatively be used. In one aspect, there may be twice as many brace members as sides, or the same number of brace members and sides.

In one or more embodiments, device 304 may include a hinge 322 and latch 324 or other mechanism to allow the device to be opened and closed about structure 302. A portion between the hinge and the latch may swing open and closed about the hinge and the latch may secure the closed position. For example, the device may be opened, positioned around structure 302, and then closed around structure.

In one or more embodiments, device 304 may be able to rotate about structure 302. For example, device 304 may weathervane in flow 310. In one or more embodiments, device 304 may include a sleeve, jacket, hollow cylinder, other tubular structure, or other hollow structure 326 through which structure 302 is inserted. By way of example, a collar (not shown) mounted above and/or below device 304 may be used to secure the device at a fixed location along the length of structure 302, and provide a bearing surface, or other rotation mechanism or coupling, to allow device 304 to rotate relative to structure 302. Device 304 also has single fin 316. The single fin extends radially outward from device 304. In the illustrated embodiment, the fin is located proximate the middle of one of the three sides, proximate a brace member. In one aspect, the fin may be an extension of the brace member. Alternatively, the fin may be located at other positions, such as, for example, at an apex or corner of the device. Certain locations for the blade may offer better performance than others, but any location that encourages the device to rotate may provide advantages. A combination of different locations for different devices may also optionally be included to provide further de-correlation of frequencies. Some may provide more streamlining and some may provide more disruption.

The fin has a radial height. The radial height is the length that the fin extends radially outwardly from device 304. Typically, the radial height (h) of the fin may range from about 10 percent to about 400 percent of the diameter of structure 302. Often, the radial height of the fin may range from about 20 percent to about 300 percent of the diameter of the structure.

Device 304 may have a length along the length or major axis of structure 302, which in the illustration is into the page. Fin 316 may likewise have a length, not necessarily the same length, along the length or major axis of the structure. The length of the fin may be substantially aligned with the length or major axis of the structure. In one or more embodiments, the device and fin may each have a length ranging from about 0.5 to about 5 times a diameter of the structure, or from about 0.5 to about 3 times the diameter, or in some cases from about 0.5 to about 2 times the diameter of the structure. One potential advantage of fin 316 is that the fin may provide surface area subject to flow 310 that may help to encourage device 304 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 304. This may help to allow device to maintain a predetermined effective alignment or orientation relative to flow 310, which may help to maintain suppression of VIV.

In the illustration, a single fin has been shown and described. Alternatively, two or more fins, potentially of shorter radial length, may optionally be used. In one aspect, the two or more fins in combination may help to allow device 304 to weathervane or rotate or may provide a predetermined alignment or orientation to device 304 when in flow 310. For example, two fins may be provided at two corners of the device with the other corner not having a fin.

In some embodiments, one or more of the sides of device 304 (or the other devices in this disclosure) may be perforated to allow for reduced drag, increased heat transfer, and other benefits. The perforation may be from about 5% to about 50% of the surface area of the device 304, for example from about 10% to about 25%.

A three-sided device has been shown. Devices with other numbers of sides are also suitable. Figures 4-5 show four-sided and six-sided devices, respectively. These devices have certain similarities to the three-sided device just described. To avoid obscuring the description, the discussion of these devices will tend to focus primarily on different and/or additional features or aspects of these devices. Unless specified otherwise, or unless understood not to be the case, other features or aspects of these devices may be the same as, or analogous to, the corresponding features or aspects of the three-sided device. Furthermore, five- sided (e.g., pentagonal), more than six-sided, and other shaped devices may also optionally be used.

Figure 4:

Figure 4 is a top cross-sectional view of four-sided device 404 having fin 416, according to one embodiment. The device is installed about structure 402.

Four-sided device 404 and tubular structure 402 have certain similarities to three-sided device 304 and tubular structure 302 of Figure 3. To avoid obscuring the description, the discussion will tend to focus primarily on different and/or additional features or aspects of four-sided device 404 and tubular structure 402.

Unless specified otherwise, or unless understood not to be the case, other features or aspects of four-sided device 404 and tubular structure 402 may be analogous to, or the same as, corresponding features of three-sided device 304 or tubular structure 302.

Structure 402 may be in a flowing fluid environment with flow 410. Device 404 may be used to help suppress VIV of the structure due to the flow.

Device 404, according to this embodiment, has four sides 418. In various embodiments, all of the sides may have the same length, a subset of the sides may have the same length, or each side may have a different length. The shape of the device may be that of a square, rectangle, parallelogram, trapezoid, isosceles trapezoid, a trapezium, for example.

In one or more embodiments, brace members 420 may be connected to the sides to brace, support, or reinforce the sides. In the illustration, eight brace members are shown, although fewer (e.g., four) or more brace members may alternatively be used.

In one or more embodiments, device 404 may include a hinge 422 and latch 424, or other mechanism to allow the device to be opened and closed about structure 402.

In one or more embodiments, device 404 may be able to rotate about structure 402. In one or more embodiments, device 404 may include a hollow structure 426 through which structure 402 is inserted. Device 404 also has single fin 416. The single fin extends radially outward from the device. In the illustrated embodiment, the fin is located proximate the middle of one of the sides, proximate one of the brace members. In one aspect, the fin may be an extension of the brace member. Alternatively, the fin may be located at other positions, such as, for example, at one of the corners of the device. As yet another option, two or more fins may be included to encourage device 404 to weathervane or achieve a predetermined alignment or orientation when in flow 410. For example, two fins may be provided at two adjacent corners of the device, or at one corner and one side forming that corner. Certain locations for the fin may offer better performance than others, but any location that encourages the device to rotate may provide advantages. A combination of different locations for different devices may also optionally be included to provide further de-correlation of frequencies. Some may provide more streamlining and some may provide more disruption.

Typically, the radial height of the fin or fins may range from about 10 percent to about 400 percent of the diameter of structure 402. Often, the radial height of the fin may range from about 20 percent to about 300 percent of the diameter of the structure.

Device 404 and fin 416 may have a length along the length or major axis of structure 402, which in the illustration is into the page. The length of the fin may be substantially aligned with the length or major axis of the structure. In one or more embodiments, the device and fin may each have a length ranging from about 0.5 to about 5 times, or from about 0.5 to about 3 times, or from about 0.5 to about 2 times the diameter of structure 402.

One potential advantage of fin 416 is that the fin may provide surface area subject to flow 410 that may help to encourage device 404 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 404. This may help to allow device to maintain a predetermined effective alignment or orientation relative to the flow, which may help to maintain suppression of VIV.

Figure 5:

Figure 5 is a top cross-sectional view of six-sided device 504 having fin 516, according to one embodiment. The device is installed about structure 502.

Structure 502 may be in a flowing fluid environment with flow 510. Device 504 may be used to help suppress VIV of the structure due to the flow.

Device 504, according to this embodiment, has six sides 518. In various embodiments, all of the sides may have the same length, a subset of the sides may have the same length, or each side may have a different length. As shown, in one embodiment, the shape of the device may be that of a hexagon or generally that of a hexagon.

In one or more embodiments, brace members 520 may be connected to the sides to brace, support, or reinforce the sides. In the illustration, six brace members are shown, although fewer or more (e.g., twelve) brace members may alternatively be used.

In one or more embodiments, device 504 may include a hinge 522 and latch 524, or other mechanism to allow the device to be opened and closed about structure 502. In one or more embodiments, device 504 may be able to rotate about structure 502. In one or more embodiments, device 504 may include a hollow structure 526, such as a tube, through which structure 502 may be inserted.

Device 504 also has a single fin 516. The single fin extends radially outward from the device. In the illustrated embodiment, the fin is located proximate one of the corners, proximate one of the brace members. In one aspect, the fin may be an extension of the brace member. Alternatively, the fin may be located at other positions, such as, for example, along one of the sides. As yet another option, two or more fins may be included to encourage the device to weathervane or achieve a predetermined alignment or orientation when in flow 510. For example, two fins may be provided at two adjacent corners of the device, or one may be provided at one corner and one may be provided along a side forming that corner. Certain locations for the blade may offer better performance than others, but any location that encourages the device to rotate may provide advantages. A combination of different locations for different devices may also optionally be included to provide further de-correlation of frequencies. Some may provide more streamlining and some may provide more disruption.

Typically, the radial height of the fin or fins may range from about 10 percent to about 400 percent of the diameter of structure 502. Often, the radial height of the fin may range from about 20 percent to about 300 percent of the diameter of the structure.

Device 504 and fin 516 may have a length along the length or major axis of structure 502, which in the illustration is into the page. The length of the fin may be substantially aligned with the length or major axis of the structure. In one or more embodiments, the device and fin may each have a length ranging from about 0.5 to about 5 times, or from about 0.5 to about 3 times, or from about 0.5 to about 2 times the diameter of structure 502.

One potential advantage of fin 516 is that the fin may provide surface area subject to flow 510 that may help to encourage device 504 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 504. This may help to allow device to maintain a predetermined effective alignment or orientation relative to flow 510, which may help to maintain suppression of VIV.

Figure 6:

Figure 6 is a top cross-sectional view of three-finned device 604, according to one embodiment. The three-finned device is installed about structure 602.

Structure 602 may be in a flowing fluid environment with flow 610. Representatively, structure 602 may include a riser, marine riser, water intake riser, tendon, pipeline, umbilical, or other elongated generally tubular structure.

The flow may subject the structure to the possibility of vortex-induced vibration

(VIV). Device 604 may be used to suppress the VIV of structure 602.

Structure 602 has a major or elongated axis, which in the illustration is into the page. The structure also has a diameter or other cross-sectional dimension transverse to the major axis. The cross section of the structure need not be circular, but may instead be oval, square, etc.

Device 604, according to this embodiment, includes three fins 630A 630B and 630C. The fins extend radially outward from structure 602. In this embodiment, the angular distance between adjacent fins may range from about 40 degrees to 160 degrees and the angular distance need not be the same between each fin. Device 604 shows three fins each separated by an angular distance of approximately 120 degrees. In this embodiment, each of fin 630B and fin 630C (the shorter fins) are disposed 60 degrees on either side of device 604 and fin 630A is disposed 180 degrees from a point on the device representative of current flow 610. In other embodiments, an angular distance between fin 630B and fin 630C is less than 120 degrees. Representative examples include an angular distance of 90 degrees, 70 degrees and 50 degrees (e.g., 45, 35 and 25 from a point representative of current flow, respectively). These examples translate to angular distance of each of fin 630B and fin 630C to fin 630A of 135 degrees, 145 degrees and 155 degrees, respectively.

In one or more embodiments, device 604 may include a hinge 622 and latch 624 or other mechanism to allow the device to be opened and closed about structure 602. A portion of the device between the hinge and the latch may swing open and closed about the hinge, and the latch may secure the closed position. For example, the device may be opened, positioned around structure 602, and then closed around structure. In one or more embodiments, device 604 may be able to rotate about structure 602. For example, device 604 may weathervane in flow 610. In one or more embodiments, device 604 may include a sleeve, jacket, hollow cylinder, other tubular structure, or other hollow structure 626 through which structure 602 is inserted. By way of example, a collar (not shown) mounted above and/or below device 604 may be used to secure the device at a fixed location along the length of structure 602, and provide a bearing surface, or other rotation mechanism or coupling, to allow device 604 to rotate relative to structure 602.

Each of fins 630A-C has a radial height. The radial height is the length that a fin extends radially outwardly from hollow structure 626. The radial heights of the fins may vary from one implementation to another, depending upon such factors as the amount of vibration expected, the number of such devices included along the length of the structure, the length of the structure, etc.

Typically, each of the fins radial height may range from about 5 percent to about 500 percent of the diameter of the structure. Often, each of the fins radial height may range from about 10 percent to about 400 percent of the diameter of the structure.

In one or more embodiments, the radial height of one of the fins may be substantially longer than the radial height of the other fins. For example, in the illustration, the radial height of a longest first fin 630A is substantially longer than the radial height of shorter second fin 630B and the radial height of shorter third fin

630C.

In one or more embodiments, depending upon the lengths of the shorter fins, the radial height of longest first fin 630A may be from 20 percent to 600 percent longer than the radial height of shorter second fin 630B and third fin 630C. In one or more embodiments, the radial height of longest first fin 630A may be from 30 percent to 400 percent longer than the radial height of shorter second fin 630B and third fin 630C. In various embodiments, the longest fin may be at least 25 percent, at least 100 percent, at least 250 percent, or at least 500 percent longer than the next longest fin.

Second fin 630B and third fin 630C may have the same radial height, or each of these fins may have a different radial height. Often, the radial height of each of the second and third fins may range from about 5 percent to about 50 percent of the diameter of structure 602, often from about 10 percent to about 25 percent of the diameter.

Device 604 may have a length along the length or major axis of structure 602, which in the illustration is into the page. Each of the fins may likewise have a length along the length or major axis of the structure. The length of the fins may be substantially aligned with the length or major axis of the structure. In one or more embodiments, each of the fins may have a length from about 0.5 to about 5 times a diameter of structure 602, or from about 0.5 to about 3 times the diameter, or in some cases from about 0.5 to about 2 times the diameter.

One potential advantage of including at least one fin 630A that is substantially larger than other fins 630B-C is that the long fin may provide extra surface area subject to flow 610 that may help to encourage device 604 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 604. This may help to allow device to maintain a predetermined effective alignment or orientation relative to the flow, which may help to maintain suppression of VIV.

In the illustration, a single fin is longer than the other two. Alternatively, two fins may be longer than a third fin, as long as the combination of the two longer fins allows the device to weathervane with the flow and/or achieve a predetermined orientation in the flow. By way of example, two longer fins may be included at two corners, two longer fins may be included close to each other on two adjacent sides, etc. Any configuration that encourages the device to weathervane in the flow and/or a predetermined alignment or orientation in the flow relative to if all fins had the same length may potentially be used.

A three-finned device has been shown. Devices with other numbers of fins are also suitable. Figures 7-10 show four-finned, six-finned, two-finned, and one- finned devices, respectively. These devices have certain similarities to the three- finned device just described. To avoid obscuring the description, the discussion of these devices will tend to focus primarily on different and/or additional features or aspects of these devices. Unless specified otherwise, or unless understood not to be the case, other features or aspects of these devices may be the same as, or analogous to, the corresponding features or aspects of the three-sided device. Furthermore, five-finned devices are also suitable.

Figure 7:

Figure 7 is a top cross-sectional view of four-finned device 704, according to one embodiment. The four-finned device is installed about structure 702. Structure 702 may be in a flowing fluid environment with flow 710. Device 704 may be used to suppress VIV of structure 702 due to flow.

Device 704, according to this embodiment, includes four fins 730A, 730B, 730C and 730D. The fins extend radially outward from structure 702. In this embodiment, the angle formed between adjacent fins may range from about 40 degrees to 130 degrees, and the angular distance need not be the same between each fin. Figure 7 shows four fins each separated by an angular distance of 90 degrees.

In one or more embodiments, device 704 may include a hinge 722 and latch 724 or other mechanism to allow the device to be opened and closed about structure 702.

In one or more embodiments, device 704 may be able to rotate about structure 702. In one or more embodiments, device 704 may include a hollow structure 726 through which structure 702 is inserted. Each of fins 730A-D has a radial height. Typically, each of the fins radial height may range from about 5 percent to about 500 percent of the diameter of the structure. Often, each of the fins radial height may range from about 10 percent to about 400 percent of the diameter of the structure.

In one or more embodiments, the radial height of one of the fins may be substantially longer than the radial height of the other fins. For example, in the illustration, the radial height of a longest first fin 730A is substantially longer than the radial height of shorter fins 730B-D.

In one or more embodiments, depending upon the lengths of shorter fins 730B-D, the radial height of longest fin 730A may be from 25 percent to 600 percent longer than the radial height of shorter fins 730B-D. In one or more embodiments, the radial height of longest fin 730A may be from 30 percent to 400 percent longer than the radial height of shorter fins. In various embodiments, the longest fin may be at least 10 percent, at least 20 percent, at least 50 percent, or at least 75 percent longer than the next longest fin. The other shorter fins may have the same radial height, some may have the same radial height, or each of these fins may have a different radial height. Often, the radial height of each of the shorter fins may range from about 5 percent to about 40 percent of the diameter of structure 702, often from about 10 percent to about 30 percent of the diameter. One potential advantage of including at least one fin 730A that is substantially larger than other fins 730B-D is that the long fin may provide extra surface area subject to flow 710 that may help to encourage device 704 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 704. This may help to allow device to maintain a predetermined effective alignment or orientation relative to the flow, which may help to maintain suppression of VIV.

In the illustration, a single fin is longer than the other fins. Alternatively, two fins may be longer than the other fins, as long as the combination of the two longer fins encourages the device to weathervane with the flow and/or achieve a predetermined orientation in the flow. By way of example, two longer fins may be included at two adjacent corners, two longer fins may be included close to each other on two adjacent sides, two longer fins may be included on opposite corners or sides, etc. Any configuration that encourages the device to weathervane in the flow and/or a predetermined alignment or orientation in the flow relative to if all fins had the same length may potentially be used.

Figure 8: Figure 8 is a top cross-sectional view of six-finned device 804, according to one embodiment. The six-finned device is installed about structure 802.

Structure 802 may be in a flowing fluid environment with flow 810. Device

804 may be used to suppress VIV of structure 802 due to flow.

Device 804, according to this embodiment, includes six fins 830A, 830B, 830C, 830D, 830E and 830F. The fins extend radially outward from structure 802.

In this embodiment, the angular distance between adjacent fins may range from about 45 degrees to 75 degrees, and the angular distance need not be the same between each fin. Figure 8 shows six fins each separated by an angular distance of about 60 degrees. In one or more embodiments, device 804 may include a hinge 822 and latch

824 or other mechanism to allow the device to be opened and closed about structure 802.

In one or more embodiments, device 804 may be able to rotate about structure 802. In one or more embodiments, device 804 may include a hollow structure 826 through which structure 802 is inserted.

Each of fins 830A-F has a radial height. Typically, each of the fins radial height may range from about 5 percent to about 500 percent of the diameter of the structure. Often, each of the fins radial height may range from about 10 percent to about 400 percent of the diameter of the structure. In one or more embodiments, the radial height of one of the fins may be substantially longer than the radial height of the other fins. For example, in the illustration, the radial height of a longest first fin 830A is substantially longer than the radial height of shorter fins 830B-F. In one or more embodiments, depending upon the lengths of the shorter fins, the radial height of longest fin 830A may be from 20 percent to 600 percent longer than the radial height of the shorter fins. In one or more embodiments, the radial height of longest fin 830A may be from 30 percent to 400 percent longer than the radial height of shorter fins 830B-F. In various embodiments, the longest fin may be at least 10 percent, at least 15 percent, at least 35 percent, or at least 75 percent longer than the next longest fin.

The other shorter fins may have the same radial height, some may have the same radial height, or each of these fins may have a different radial height. Often, the radial height of each of the shorter fins may range from about 5 percent to about 75 percent of the diameter of structure 802, often from about 10 percent to about 60 percent of the diameter.

One potential advantage of including at least one fin 830A that is substantially larger than other fins 830B-F is that the long fin may provide extra surface area subject to flow 810 that may help to encourage device 804 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 804. This may help to allow device to maintain a predetermined effective alignment or orientation relative to the flow, which may help to maintain suppression of VIV. In the illustration, a single fin is longer than the other fins. Alternatively, two or three fins may be longer than the other fins, as long as the combination of the longer fins encourages the device to weathervane with the flow and/or achieve a predetermined orientation in the flow more than if the fins all had equal length. By way of example, two longer fins may be included at two adjacent corners, two longer fins may be included close to each other on two adjacent sides, two longer fins may be included on opposite corners or sides, etc. Any configuration that encourages the device to weathervane in the flow and/or a predetermined alignment or orientation in the flow relative to if all fins had the same length may potentially be used.

Figure 9:

Figure 9 is a top cross-sectional view of two-finned device 904, according to one embodiment. The two-finned device is installed about structure 902. Structure 902 may be in a flowing fluid environment with flow 910. Device 904 may be used to suppress VIV of structure 902 due to flow.

Device 904, according to this embodiment, includes two fins 930A and 930B. The fins extend radially outward from structure 902. In this embodiment, the angle formed between adjacent fins may range from about 150 degrees to 210 degrees, or as shown may be about 180 degrees.

In one or more embodiments, device 904 may include a hinge 922 and latch 924 or other mechanism to allow the device to be opened and closed about structure 902. In one or more embodiments, device 904 may be able to rotate about structure 902. In one or more embodiments, device 904 may include a hollow structure 926 through which structure 902 is inserted.

Each of fins 930A and 930B has a radial height. Typically, each of the fins radial height may range from about 5 percent to about 500 percent of the diameter of the structure. Often, each of the fins radial height may range from about 10 percent to about 400 percent of the diameter of the structure.

In one or more embodiments, the radial height of one of the fins may be substantially longer than the radial height of the other fins. For example, longer fin 930A may have a substantially longer radial height than shorter fin 930B. Often, the radial height of shorter fin 930B may range from about 5 percent to about 60 percent of the diameter of structure 902, often from about 10 percent to about 45 percent of the diameter.

In one or more embodiments, depending upon the radial height of the shorter fin, the radial height of the longer fin 930A may be from 20 percent to 600 percent longer than the radial height of the shorter fin 930B. In one or more embodiments, the radial height of the longer fin may be from 30 percent to 400 percent longer than the radial height of the shorter fin. In various embodiments, the longer fin may be at least 5 percent, at least 15 percent, at least 25 percent, or at least 50 percent longer than the shorter fin. One potential advantage of including longer fin 930A that is substantially longer than shorter fin 930B is that the longer fin may provide extra surface area subject to flow 910 that may help to encourage device 904 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 904. This may help to allow device to maintain a predetermined effective alignment or orientation relative to the flow, which may help to maintain suppression of VIV.

Figure 10:

Figure 10 is a top cross-sectional view of one-finned device 1004, according to one embodiment. The one-finned device is installed about structure 1002.

Structure 1002 may be in a flowing fluid environment with flow 1010. Device 1004 may be used to suppress VIV of structure 1002 due to flow. Device 1004, according to this embodiment, includes one fin 1030. The fin extends radially outward from structure 1002.

In one or more embodiments, device 1004 may include a hinge 1022 and latch 1024 or other mechanism to allow the device to be opened and closed about structure 1002. In one or more embodiments, device 1004 may be able to rotate about structure 1002. In one or more embodiments, device 1004 may include a hollow structure 1026 through which structure 1002 is inserted.

Fin 1030 has a radial height. Typically, the radial height may range from about 25 percent to about 200 percent of the diameter of the structure. Often, each of the fins radial height may range from about 50 percent to about 100 percent of the diameter of the structure.

One potential advantage of the fin is that the fin may provide extra surface area subject to flow 1010 that may help to encourage device 1004 to continue to weathervane or rotate even after marine growth, fouling, or debris has accumulated on or around device 1004. This may help to allow device to maintain a predetermined effective alignment or orientation relative to the flow, which may help to maintain suppression of VIV.

Regarding the fins shown and described for Figures 3-10, in one or more embodiments, the fins may be substantially straight along the length of the structure. In other words, the fins may not be helically wound about structure, or at least not to the same extent that a helical strake would be. For example, the fins may rotate or twist less than about 15 degrees, less than about 10 degrees, or less than about 5 degrees along a one-diameter length of the structure. The fins may either be substantially flat, or they may have a slight convex or concave curvature.

The devices of Figures 3-10 may be molded, welded, bent, cast, glued, assembled, fastened, or otherwise formed with manufacturing techniques as are known in the art. Representatively, these devices may be made of plastic, metal, or a combination thereof. Examples of suitable plastics include, but are not limited to, polyalkenes (e.g., polyethylene, polypropylene, etc.), polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS), fiberglass, other fiber reinforced plastics, other plastics used in similar marine environments, and the like, and combinations thereof, to name just a few examples. Examples of suitable metals include, but are not limited to, steel, stainless steel, copper, aluminum, coated metals, other metals used in similar marine environments, and the like, and combinations thereof.

Figure 1 1 : Referring now to Figure 1 1 , structure 1 102 is illustrated with a plurality of

VIV suppression devices 1 104A-D, of the types disclosed elsewhere herein, installed about structure 1 102 in order to suppress vortex induced vibration of the structure, when the structure is subjected to fluid flow 1 110. In some embodiments, collars (not shown) may be provided between adjacent devices or placed between every few devices.

In some embodiments, devices 1 104A-D may be installed before structure is installed, for example in a sub-sea environment. In some embodiments, devices 1 104A-D may be installed as a retrofit installation to structure 1 102 which has already been installed, for example in a sub-sea environment. Device 1 104A has height 1 130A and distance 1 132A between adjacent devices 1 104A and 1 104B. Portion of structure 1 102 covered with devices 1 104A- D has height 1 134. Device 1 104B has height 1 130B, device 1 104C has height 1 13OC, and device 1 104D has height 1 13OD.

Devices 1 104A-D may cover from about 10 percent to about 100 percent of height 1 134, for example from about 10 percent to about 70 percent, or from about 15 percent to about 50 percent. Height 1 130A may be from about 0.5 times the diameter of structure 1 102 to about 10 times, for example from about 1 to about 6 times the diameter, or from about 2 to about 5 times the diameter.

Distance 1 132A may be from about 0 times the diameter of structure 1 102 to about 10 times, for example from about 1 to about 6 times the diameter.

Illustrative Embodiments:

In one embodiment, there is disclosed a system for reducing drag and/or vortex induced vibration of a structure, the system comprising a multiple finned device comprising from at least one long fin having a first average height and at least one short fin having a second average height, wherein the height is a radial distance extending outwardly from the structure, and wherein the first average height is at least 10% larger than the second average height. In some embodiments, the fins are substantially aligned along a longitudinal axis of the structure. In some embodiments, the first average height is from about 20% to about 100% larger than the second average height. In some embodiments, the device is installed about the structure. In some embodiments, the device comprises a length along a major axis of the structure from 0.5 to 10 times a diameter of the structure. In some embodiments, the device comprises 3 fins, comprising two short fins and one long fin. In some embodiments, the device comprises 4 fins, comprising three short fins and one long fin. In some embodiments, the device comprises an even number of fins. In some embodiments, the device comprises a first collar at a top of the device and a second collar at a bottom of the device, the fins connected to the first collar at the top of the device and the second collar at the bottom of the device. In some embodiments, the system also includes a plurality of multiple finned devices along a length of the structure. In some embodiments, the system also includes a collar connected to a first set of fins above the collar and a second set of fins below the collar. In some embodiments, a coverage density of the devices is from about 10% to about 80%.

In one embodiment, there is disclosed a method for modifying a structure subject to drag and/or vortex induced vibration, said method comprising positioning at least one multiple finned device around the structure, the multiple finned device comprising at least one long fin having a first average height and at least one short fin having a second average height, wherein the height is a radial distance extending outwardly from the structure, and wherein the first average height is at least 10% larger than the second average height. In some embodiments, the method also includes locking the device at a preferred angular orientation based on ambient expected currents acting on the structure.

In one embodiment, there is disclosed a system for reducing drag and/or vortex induced vibration of a structure, the system comprising a multiple sided device comprising from 3 to 10 sides; and a fin extending outwardly from the device. In some embodiments, the fin extends from a position located in a middle of one of the sides. In some embodiments, the fin extends from an intersection of two of the sides. In some embodiments, the device comprises a chord to thickness ratio of greater than 1.25. In some embodiments, the device is installed about the structure. In some embodiments, the fin comprises a height extending in a radial direction away from the structure, the height from about 0.5 to about 10 times a diameter of the structure. In some embodiments, the device comprises 4 sides. In some embodiments, the device comprises 3 sides. In some embodiments, the device comprises an even number of sides. In some embodiments, the system also includes the device comprises a square shape. In some embodiments, the system also includes a plurality of multiple sided devices along a length of the structure. In some embodiments, the system also includes at least 3 corners, each corner having a radius of curvature less than a radius of the structure.

In one embodiment, there is disclosed a system for reducing drag and/or vortex induced vibration of a structure. The system may include at least one of: (1 ) a multiple sided device having a fin; and (2) a finned device including from 1 to 6 fins with one of the fins substantially longer than all other fins. The device may be substantially aligned along a longitudinal axis of the structure. The device may be free to rotate about the structure. In some embodiments, the device is installed about the structure. In some embodiments, the device has a length along a major axis of the structure from 0.5 to 10 times a diameter of the structure. In some embodiments, the device includes a multiple finned device having from 3 to 6 fins. In some embodiments, the device includes a multiple sided device having from 3 to 6 sides. In some embodiments, the system includes a first collar at a top of the device and a second collar at a bottom of the device, the device connected to the first collar at the top of the device and the second collar at the bottom of the device. In some embodiments, the system also includes a plurality of other such devices along a length of the structure. In some embodiments, the system also includes a collar connected to a first device above the collar and a second device below the collar.

In one embodiment, there is disclosed a method for modifying a structure subject to drag and/or vortex induced vibration. The method may include positioning around the structure at least one of: (1 ) a multiple sided device having a fin; and (2) a finned device having from 1 to 6 fins with one of the fins substantially longer than all other fins. In one aspect, the device may include a multiple finned device having from 3 to 6 fins substantially aligned along a longitudinal axis of the structure. In one aspect, the device may include a multiple sided device having from 3 to 6 sides. In some embodiments, the positioning includes positioning at least two of such devices about the structure. In some embodiments, the method also includes positioning a collar, a buoyancy module, and/or a clamp around the structure.

In one embodiment, there is disclosed a system for reducing drag and/or vortex induced vibration of a structure. The system may include at least a first device and a second device. Each of the first and second devices, independently of the other, may be either: (1 ) a multiple sided device having a fin; or (2) a finned device having from 1 to 6 fins with one of the fins substantially longer than all other fins. The first and second devices may be spaced at a distance along a longitudinal axis of the structure. In some embodiments, a space between the first device and second devices is from about 0.25 to about 20 times a diameter of the structure. In some embodiments, the system also includes one or more collars located about the structure, above and/or below at least one of the first and second devices. In some embodiments, at least one of the first and second devices has a length measured along a longitudinal axis of the structure from 0.5 to 10 times a diameter of the structure. In some embodiments, at least one of the first and second devices is a multi-finned device that has from 3 to 6 fins. In some embodiments, at least one of the first and second devices is a multiple sided device that has from 3 to 6 sides. In some embodiments, at least one of the first and the second device has a collar connected to and holding the device about and adjacent to the structure. In some embodiments, the system also includes a plurality of other such devices spaced along a length of the structure. In some embodiments, a coverage density of the devices on the structure is from about 5 percent to about 90 percent. In some embodiments, a coverage density of the devices on the structure is from about 10 percent to about 70 percent. In some embodiments, a coverage density of the devices on the structure is from about 15 percent to about 50 percent. In one embodiment, there is disclosed a system for reducing drag and/or vortex induced vibration of a structure. The system may include at least one of: (1 ) a multiple sided device having a fin; and (2) a finned device having from 1 to 6 fins with one of the fins substantially longer than all other fins. The device may be substantially aligned along a longitudinal axis of the structure. In some embodiments, the device is installed about the structure. In some embodiments, the device has a height from 0.5 to 10 times a diameter of the structure. In some embodiments, the device includes a multiple finned device having from 3 to 6 fins. In some embodiments, the device includes a multiple sided device having from 3 to 6 sides. In some embodiments, the system also includes a first collar at a top of the device and a second collar at a bottom of the device, the device connected to the first collar at the top of the device and the second collar at the bottom of the device. In some embodiments, the system also includes a plurality of other such devices along a length of the structure. In some embodiments, the system also includes a collar connected to a first device above the collar and a second device below the collar.

In one embodiment, there is disclosed a method for modifying a structure subject to drag and/or vortex induced vibration. The method may include positioning at least one device around the structure. In some embodiments, the device may be one of: (1 ) a multiple sided device having a fin; and (2) a finned device having from 1 to 6 fins with one of the fins substantially longer than all other fins. The device and fins may be substantially aligned along a longitudinal axis of the structure. In some embodiments, the positioning includes positioning at least two of such devices about the structure. In some embodiments, the method also includes positioning a collar, a buoyancy module, and/or a clamp around the structure. In some embodiments, the device may include a multiple finned device having from 3 to 6 fins. In some embodiments, the device may include a multiple sided device having from 3 to 6 sides. In one embodiment, there is disclosed a system for reducing drag and/or vortex induced vibration of a structure. The system may include at least one of: (1 ) a multiple sided device having a fin; and (2) a finned device having from 1 to 6 fins with one of the fins substantially longer than all other fins. The device may be free to rotate about the structure. In some embodiments, the system also includes one or more collars located about the structure, above and/or below the device. In some embodiments, the device has a length from 0.5 to 10 times a diameter of the structure. In some embodiments, the device may be a multiple finned device having from 3 to 6 fins. In some embodiments, the device may be a multiple sided device having from 3 to 6 sides. In some embodiments, the system may include a collar connected to and holding the device about and adjacent to the structure. In some embodiments, the system also includes a plurality of such devices along a length of the structure.

The VIV systems and methods disclosed herein may be used in any flowing fluid environment in which the structural integrity of the system can be maintained. The term, "flowing-fluid" is defined here to include but not be limited to any fluid, gas, or any combination of fluids, gases, or mixture of one or more fluids with one or more gases, specific non-limiting examples of which include fresh water, salt water, air, liquid hydrocarbons, a solution, or any combination of one or more of the foregoing. The flowing-fluid may be "aquatic," meaning the flowing-fluid includes water, and may be seawater or fresh water, or may include a mixture of fresh water and seawater.

In some embodiments, devices may be used with most any type of offshore structure, for example, bottom supported and vertically moored structures, such as for example, fixed platforms, compliant towers, tension leg platforms, and mini- tension leg platforms, and also include floating production and sub-sea systems, such as for example, spar platforms, floating production systems, floating production storage and offloading, and sub-sea systems. The devices are also suitable for suppressing VIV of cooling water intake risers used to provide cooling water from depth to a floating liquefied natural gas plant.

In some embodiments, devices may be attached to marine structures such as sub-sea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; space- frame members for platforms; cables; umbilicals; mooring elements for deepwater platforms; the hull and/or column structure for tension leg platforms (TLPs) and for spar type structures; and water intake risers used to provide cooling water from depth to a floating liquefied natural gas plant. In some embodiments, device may be attached to spars, risers, tethers, and/or mooring lines.

In some embodiments, the devices may be formed as a hollow plastic moulding whose interior communicates with the exterior to permit equalization of pressure. In some embodiments, the device may be formed by a single plastic moulding, such as by rotational moulding, so that it may be hollow. The device may be manufactured of polythene, which may be advantageous due to its low specific gravity (similar to that of water), toughness and low cost. Openings may be provided to allow water to enter the device to equalize internal and external pressures. The device could also be formed as a solid polyurethane moulding. In some embodiments, the principal material used in constructing the device may be fiberglass. Other known materials may also be used which have suitable weight, strength and corrosion-resistant characteristics. In some embodiments, the device may be constructed from a metallic or non-metallic, low corrosive material such as a aluminum or multi-layer fiberglass mat, polyurethane, vinyl ester resin, high or low density polyurethane, PVC or other materials with substantially similar flexibility and durability properties. These materials provide the device with the strength to stay on the structure, but enough flex to allow it to be snapped in place during installation. The fiberglass may be 140-210 MPa tensile strength (for example determined with ISO 527-4) that may be formed as a bi-directional mat or the device can be formed of vinyl ester resin with 7-10 percent elongation or polyurethane. The use of such materials eliminates the possibility of corrosion, which can cause the device shell to seize up around the elongated structure it surrounds. Collars may be provided to connect the device to the structure and/or to provide spacing between adjacent devices along the structure, and/or as a mechanism to hold the devices in place. Collars may be formed by a single plastics moulding, such as nylon, or from a metal such as stainless steel, copper, or aluminum. In some embodiments, the internal face of the collar's bearing ring may serve as a rotary bearing allowing the device to rotate about the structure's longitudinal axis and so to weathervane to face a current. Only the collar may make contact with the structure, its portion interposed between the device and the structure serving to maintain clearance between these parts. This bearing surface may be (a) low friction and even "self lubricating" and/or (b) resistant to marine fouling. These properties can be promoted by incorporation of anti-fouling and/or friction reducing materials into the material of the collar. The material of the collar may contain a mixture of an anti-fouling composition which provides a controlled rate of release of copper ions, and/or also of silicon oil serving to reduce bearing friction.

In some embodiments, there may not be provided a collar, and the device may be mounted to the structure itself, or flanges can be used on the devices to keep them from sliding off each other, and then a single collar (or bend restrictor, connector, etc.) can be used to hold a long string of devices in place. That is, the device may be mounted directly upon the structure (or on a cylindrical protective sheath conventionally provided around the structure). A number of such devices may be placed adjacent one another in a string along the structure. To prevent the devices from moving along the length of the structure, clamps and/or collars may secured to the structure at intervals, for example between about every one to five devices. The clamps and/or collars may be of a type having a pair of half cylindrical clamp shells secured to the structure by a tension band passed around the shells.

In some embodiments, the device may be designed so that it can freely rotate about the structure in order to provide more efficient handling of the wave and current action and VIV bearing on the structure. The devices may not be connected, so they can rotate relative to each other. Bands of low-friction plastic rings, for example a molybdenum impregnated nylon, may be connected to the inside surface of the device that defines an opening to receive the structure. A low friction material may be provided on the portion of the device that surrounds a structure, for example strips of molydbodeum impregnated nylon, which may be lubricated by sea water.

In some embodiments, a first retaining ring, or thrust bearing surface, may be installed above and/or below each device or group of devices. Buoyancy cans may also be installed above and/or below each device or group of devices.

The methods and systems may further include modifying the buoyancy of the device. This may be carried out by attaching a weight or a buoyancy module to the device. In some embodiments, the device may include filler material that may be either neutrally or partially buoyant. The device may be partially filled with a known syntactic foam material for making the device partially buoyant in sea water. This foam material can be positively buoyant or neutrally buoyant for achieving the desired results.

In some embodiments, at least one copper element may be mounted at the structure and/or the device to discourage marine growth at the device- structure interface so that the device remains free to weathervane to orient most effectively with the current, for example a copper bar. In some embodiments, the devices may be made of copper, or be made of copper and one or more other materials.

The height and/or length of the device can vary considerably depending upon the specific application, the materials of construction, and the method employed to install the device. In extended marine structures, numerous devices may be placed along the length of the marine structure, for example covering from about 5 percent, 10 percent, 15 percent, or 25 percent, to about 50 percent, or 75 percent, or 100 percent of the length of the marine structure with the devices. In some embodiments, devices may be placed on a marine structure after it is in place, for example, suspended between a platform and the ocean floor, in which divers or submersible vehicles may be used to fasten the devices around the structure. Alternatively, devices may be fastened to the structure as lengths of the structure are assembled. This method of installation may be performed on a specially designed vessel, such as an S-Lay or J-Lay barge, that may have a declining ramp, positioned along a side of the vessel and descending below the ocean's surface, that may be equipped with rollers. As the lengths of the structure are fitted together, devices may be attached to the connected sections before they are lowered into the ocean.

The devices may have one or more members. Examples of two-membered devices suitable herein include a clam-shell type structure wherein the device has two members that may be hinged to one another to form a hinged edge and two unhinged edges, as well as a device having two members that may be connected to one another after being positioned around the circumference of the marine structure. Optionally, friction-reducing devices may be attached to the interior surface of the device. Clam-shell devices may be positioned onto the marine structure by opening the clam shell device, placing the device around the structure, and closing the clam-shell device around the circumference of the structure. The step of securing the device into position around the structure may include connecting the two members to one another. For example, the device may be secured around the structure by connecting the two unhinged edges of the clam shell structure to one another. Any connecting or fastening device known in the art may be used to connect the member to one another.

In some embodiments, clamshell type devices may have a locking mechanism to secure the device about the structure, such as male-female connectors, rivets, screws, adhesives, welds, and/or connectors.

Of course, it should be understood that the above attachment apparatus and methods are merely illustrative, and any other suitable attachment apparatus may be utilized.

The methods and systems may further include positioning a second device, or a plurality of devices around the circumference of a structure. In the multi- device embodiments, the devices may be adjacent one another on the structure, or stacked on the structure. The devices may have end flanges, rings or strips to allow the devices to easily stack onto one another, or collars or clamps may be provided in between devices or groups of devices. In addition, the devices may be added to the structure one at a time, or they may be stacked atop one another prior to being placed around/onto the structure. Further, the devices of a stack of devices may be connected to one another, or attached separately. While the devices have been described as being used in aquatic environments, they may also be used for VIV and/or drag reduction on elongated structures in atmospheric environments.

While the illustrative embodiments have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the invention, including all features which would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Example 1 : Three Finned Devices

Experiments were performed to determine the effectiveness of three finned devices at suppressing VIV. A 4.5 inch outside diameter pipe having a length of 12.2 feet was secured in a current tank and exposed to currents ranging from 2.5 to 7.5 feet per second of water flow. The water temperature in the tank ranged from about 59 to 69°F. Tests were conducted when the pipe was bare and when the device had various configurations of three finned VIV suppression devices attached to the pipe. The three finned devices had one fin that had a longer radial height than two other fins (e.g., at least more than twice as long). The two shorter fins both had radial heights of 0.67 inches. The radial height of the longer fin varied from one test to another. In each case, the devices each had a height along the pipe of one pipe outside diameter (i.e., 4.5 inches). Different numbers of devices and spacing between the devices were tested. "O' spacing refers to devices being placed one next to another along a length of the pipe. Other spacing configurations include devices separated by a distance equivalent to outside diameter of the pipe (1 D), a distance equivalent to three times a diameter of the pipe ('3O), and six times a diameter of the pipe ('6D). The results are provided in Table 1. Table 1.

a All fins have equal length b initially longest fins staggered 90° to flow c initially longest fins started downstream

Max RMS A/D measures the extent of motion of the pipe, it is the motion magnitude expressed as a fraction of pipe outside diameter. The Cd is the drag coefficient.

These results indicate that compared to a bare pipe, three finned devices having one fin longer than the others are able to reduce VIV of the pipe. Additionally, these results suggest that having one fin substantially longer than the others reduces VIV more than if all fins are of equal length. Further, devices where the shorter fins are separated less than an angular distance of 120 degrees (e.g., 50 degrees, 70 degrees or 90 degrees) offer reduced VIV and drag than shorter fins separated by an angular distance of 120 degrees.

Example 2: One Finned Devices

An experiment was performed to determine the effectiveness of one finned devices at suppressing VIV. The conditions were basically the same as in the experiments described above except that the currents ranged from 2.5 to 5.75 feet per second of water flow. The water temperature in the tank was about 56°F. The devices were round sleeves or tubular structures having each having a single fin. The devices had heights along the pipe of one pipe diameter (i.e., 4.5 inches). The results are provided in Table 2.

Table 2.

These results indicate that compared to a bare pipe, one finned devices are able to reduce VIV of the pipe.