SYSTEMS AND METHODS FOR REDUCING DRAG AND/OR VORTEX

INDUCED VIBRATION

Field of the Invention

The present invention relates to systems and methods for reducing drag and/or vortex-induced vibration ("VIV") with the use of a fairing. 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 and the length of the structure.

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.

Drilling in ever deeper water depths requires longer riser pipe strings which, because of their increased length and subsequent greater surface area, may be subject to greater drag forces which must be resisted by more tension. This is believed to occur as the resistance to lateral forces due to the bending stresses in the riser decreases as the depth of the body of water increases.

Accordingly, the adverse effects of drag forces against a riser or other structure caused by strong and shifting currents in these deeper waters increase

and set up stresses in the structure which can lead to severe fatigue and/or failure of the structure if left unchecked.

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 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 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.

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 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.

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.

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.

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; low drag fairings; high stability fairings; fairings which delay the separation of the boundary layer, which cause decreased drag, and/or decreased

VIV; fairings suitable for use at a variety of fluid flow velocities; and/or fairings that have a low drag and 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 Invention

One aspect of the invention provides a system comprising a structure; a long fairing comprising a chord to thickness ratio of at least about 1.7; and a short fairing comprising a chord to thickness ratio less than about 1.7.

Another aspect of the invention provides a method of suppressing vortex induced vibration and/or drag of a structure subject to a current, the method comprising installing a long fairing comprising a chord to thickness ratio of at least about 1.7 about the structure; and installing a short fairing comprising a chord to thickness ratio less than about 1.7 about the structure.

Advantages of the invention may include one or more of the following: improved VIV reduction; improved drag reduction; improved fairing stability; delaying the separation of the boundary layer over the fairing body; lower cost fairings; and/or lighter weight fairings.

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

Figure 1 shows a short fairing.

Figure 2 shows a long fairing.

Figure 3 shows a plurality of long and short fairings installed about a structure. Figure 4 shows a plurality of groups of long and short fairings installed about a structure.

Figure 5 shows a plurality of groups of long and short fairings and sleeves installed about a structure

Figure 6 shows displacement test results in the cross-flow and in-line directions for a structure without long and short fairings.

Figure 7 shows displacement test results in the cross-flow and in-line directions for a structure with long fairings installed about the structure.

Figure 8 shows displacement test results in the cross-flow and in-line directions for a structure with long and short fairings installed about the structure.

Figure 9 shows displacement test results in the cross-flow and in-line directions for a structure with short fairings installed about the structure. Figure 10 shows drag test results for a structure with long and short fairings installed about the structure.

Figure 1 1 shows drag test results for a structure with short fairings installed about the structure. Detailed Description of the Invention Referring now to Figure 1 , there is illustrated prior art short fairing 104 installed about structure 102 such as a pipe. Structure 102 may be subjected to an external flowing fluid environment where structure 102 is subject to vortex induced vibration (VIV). Short fairing 104 has chord 106 and thickness 108. The chord may be measured from the front to the tail and defines a major axis. The thickness may be measured from one side of the fairing to the other. Chord to thickness ratio of short fairing 104 may be less than about 1.5, or less than about 1.25. Short fairing 104 may be used to suppress VIV. While short fairing 104 is effective at reducing VIV, 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. Long fairing 204 has chord 206 and thickness 208. Chord to thickness ratio of long fairing 204 may be greater than about 1.7, greater than about 1.8, or greater than about 2.0. Compared to short fairing 104, long fairing 204 may have reduced drag when subjected to a flowing fluid environment. Long fairing 204 may experience lateral displacement 210 and/or torsional displacement

212. 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.

In one embodiment, there is disclosed a system for reducing drag and/or vortex induced vibration of a structure. In some embodiments, the system for reducing drag and/or vortex induced vibration includes a plurality of long and short fairings positioned along a structure. In some embodiments, the plurality of long and short fairings may be arranged in groups along the structure. Representatively, alternating groups of long fairings and groups of short fairings

may be positioned along the length of the structure. As will be shown in the experimental results set forth below, the combination of long and short fairings positioned along the structure as disclosed herein effectively reduces drag forces below that of short fairings while reducing VIV better than long fairings. It is further shown that these advantages may be achieved in the absence of any rotation dampening or reducing mechanisms that may be built into or attached to the fairings. In still further embodiments, one or more sleeves, in addition to long and/or short fairing groups may be positioned along the structure to further reduce VIV and drag forces. Referring now to Figure 3, structure 302 is shown with a plurality of fairings

304a - 304e mounted about the structure. In this embodiment, long fairings 304a, 304c, and 304e, are alternated with short fairings 304b and 304d. Short fairings 304b and 304d may be a lower cost to long fairings 304a, 304c, and 304e, and/or may act to reduce correlation of vortices between adjacent long fairings. The fairings may have a chord and a thickness as defined in U.S. Patent Number 6,223,672. As previously discussed, the chord may be measured from the front to the tail and defines a major axis, and thickness may be measured from one side to the other. An example of a chord to thickness ratio of a long fairing is a ratio greater than 1.7. An example of a chord to thickness ratio of a short fairing is a ratio less than 1.7. In some embodiments, the fairing may have a cross-sectional shape selected from a teardrop, an airfoil, an ellipse, an oval, and/or a streamlined shape.

In some embodiments, connectors (not shown) may be provided between adjacent fairings or placed between every few fairings. In some embodiments, connectors may be springs, bungee cords, rubber straps, ropes, rods, cables, or combinations of two or more of the above.

In some embodiments, collars may be provided between adjacent fairings or placed between every few fairings. In some embodiments, fairings 304a-304e may be installed before structure (e.g., pipe) is installed, for example in a subsea environment. In some embodiments, fairings 304a-304e may be installed as a retrofit installation to structure 302 which has already been installed, for example in a subsea environment.

Referring to Figure 4, structure 402 is shown with a plurality of fairings 404 and 406 mounted about the structure. In this embodiment, short fairings 404 and long fairings 406 are arranged in groups 404a, 404b, 404c, 404d and 406a, 406b, 406c about structure 402. Groups of long fairings 406a, 406b and 406c are placed between groups of short fairings 404a, 404b, 404c and 404d in an alternating manner. Each of groups 404a, 404b and 404c include five short fairings 404 and group 404d includes two short fairings 404. Each of groups 406a, 406b and 406c include five long fairings 406.

Groups 404a, 406a and 404b are positioned above center line 408 of structure 402 and groups 406b, 404c, 406c and 404d are positioned below center line 408 of structure 402. Although Figure 4 illustrates 15 fairings positioned above center line 408 and 17 fairings positioned below center line 408, it is contemplated that any number of fairings capable of fitting along the length of structure 402 may be positioned above and below center line 408. Representatively, in some embodiments the number of fairings above and below center line 408 may be equal. It is further recognized that in embodiments where structure 402 is longer than 40 feet, more than thirty two fairings may fit along its length thereby allowing for the use of a larger number of fairings. Similarly, where structure 402 is less than 40 feet long, fewer than thirty two fairings may fit along the length of structure 402 and therefore fewer than thirty two fairings may be used. It is further noted that in addition to the length of structure 402, the number of fairings used may vary depending upon the height of the fairing or components positioned between the fairings (e.g. collars).

Although six groups having five fairings and one group having two fairings are illustrated in Figure 4, it is contemplated that more or less than seven fairing groups may be positioned along the structure and the number of fairings in the groups may vary. Representatively, in one embodiment, two groups having eight long fairings in each group and two groups having eight short fairings in each group may be positioned about structure 402 in an alternating manner. In other embodiments, one group having sixteen long fairings and one group having sixteen short fairings may be positioned about structure 402. In some embodiments, two groups having eight, ten, twelve or fourteen long fairings in each group may be positioned about structure 402 on opposite sides of one group

having sixteen, twelve, eight or four short fairings, respectively, such that a total of thirty two fairings are positioned about structure 402. Although the fairing groups tested herein include either long or short fairings having the same chord to thickness ratios, it is further contemplated that each group may include both long and short fairings, long fairings having the same or different chord to thickness ratios and short fairings having the same or different chord to thickness ratios.

In some embodiments, the long fairings in each group may have a chord to thickness ratio of about 2.24 and the short fairings in each group may have a chord to thickness ratio of about 1.36. In some embodiments, the chord to thickness ratio for the long fairings is greater than 1.7 and the chord to thickness ratio for the short fairings is less than 1.7. In some embodiments, the chord to thickness ratio for the short fairings may be at least about 1.10. In some embodiments, the chord to thickness ratio for the short fairings may be at least about 1.25. In some embodiments, the chord to thickness ratio for the short fairings may be at least about 1.5. In some embodiments, the chord to thickness ratio for the long fairings may be at least about 1.75. In some embodiments, the chord to thickness ratio for the long fairings may be greater than 2. In some embodiments, the chord to thickness ratio for the long fairings may be greater than 2.25. In some embodiments, the chord to thickness ratio for the long fairings may be up to about 10.0. In some embodiments, the chord to thickness ratio for the long fairings may be up to about 5.0. In some embodiments, the chord to thickness ratio for the long fairings may be up to about 4. In some embodiments, the chord to thickness ratio for the long fairings may be up to about 3. In some embodiments, the chord to thickness ratio for the long fairings may be up to about 2.75. In some embodiments, the chord to thickness ratio for the long fairings may be up to about 2.0.

In some embodiments, the fairings may have a fixed chord to thickness ratio. In other embodiments, the fairings may have a modifiable chord to thickness ratio such that the fairing may be moveable from a short fairing configuration to a long fairing configuration as disclosed in co-pending PCT Patent Application No. PCT/US2007/084920, assigned Attorney Docket No. TH31 12, and herein incorporated by reference in its entirety.

Although, as will be shown in Example 1 below, a combination of long and short fairings mounted about a structure without any sort of rotation dampening mechanism coupled to the fairings, effectively reduces VIV and drag, in some embodiments, the fairing may include a nose or tail section including one or more dampening mechanisms to dampen the rotation of the fairing about the structure as disclosed in co-pending PCT Patent Application no. PCT/US2007/084918, assigned Attorney Docket No. TH3190, and incorporated herein by reference in its entirety. Representative dampening mechanisms may include, but are not limited to, perforations in a tail section of the fairing, a mass in a nose section of the fairing, a buoyancy module in the tail section of the fairing, perforations and balls and/or rods in the tail section of the fairing, a liquid container in the tail section of the fairing, a liquid container in the nose section of the fairing, friction pads between the fairing and the structure, and pins attached to the fairing within tracks moveably connected to the structure stabilizer fins and/or drag plates. In some embodiments, the fairing comprises a teardrop shape.

Referring to Figure 5, a structure is shown with a plurality of fairings and sleeves mounted about the structure. Similar to Figure 4, fairing groups 504a, 504b, 506a and 506b are mounted about structure 502. Groups 504a and 504b include short fairings 504 and groups 506a and 506b include long fairings 506. Fairings 504 and 506 may be substantially similar to fairings 404 and 406 described in reference to Figure 4. Although each of groups 504a, 504b, 506a and 506b are shown having five fairings, it is contemplated that any number of fairings of varying chord to thickness ratios may be included in each group.

Sleeves 508a and 508b are further mounted to structure 502 to help reduce drag and VIV. Sleeves 508a and 508b may be smooth sleeves as described in U.S. Patent No. 7,017,666 incorporated herein by reference in its entirety. In some embodiments, sleeves 508a and/or 508b may be made of gel-coated fiberglass, copper (when marine growth inhibition is required), carbon fiber, rubber or any sufficiently smooth thermoplastic, metal alloy or other material. In still further embodiments, a smooth sleeve surface may be obtained by a surface finish on an outside of structure 502 or maintained by a ablative paint or other coating applied to the surface of structure 502. Sleeves 508a and 508b may have any

dimension suitable for mounting sleeves 508a and 508b to structure 502 in combination with fairings 504 and 506.

It is recognized that the drag coefficient for flow past a cylinder sharply decreases as the Reynolds number is increased beyond about 200,000 (called the "critical" Reynolds number range) and then slowly recovers (called the "supercritical" Reynolds number range). Surface roughness can affect the Reynolds number at which this "dip" occurs and can add to the drag coefficient. A very smooth cylinder, however, does not experience VIV in this Reynolds number range, and furthermore the cylinder experiences very low drag. In addition, an "Ultra-smooth" sleeve can be effective in Reynolds number ranges from about 200,000 to over 1 ,500,000, perhaps more. In fact, benefits begin to be seen in the VIV and drag at a Reynolds number of about 100,000. This relationship of VIV and drag as a function of the level of surface roughness is quantifiable in a dimensionless roughness parameter, K/D, where: K is the roughness density and is defined as the average peak to trough distance of the surface roughness (e.g., as measured using confocal scanning with an electron microscope); and D is the effective outside diameter of the cylinder element, including any sleeve or coating. Substantial reduction in VIV can be observed where K/D is less than about 1.0x10 ^"4 and is most pronounced at about 1.0x10 ^"5 or less for fairly uniform roughness densities. Similar results may be achieved where the roughness density decreases, even though the overall K/D ratio may increase.

Thus, it is contemplated that the incorporation of sleeves 508a and 508b between short fairings 504 and long fairings 506 coupled to structure 502 will provide further VIV and drag reduction advantages. Although two sleeves 508a and 508b are shown in Figure 5, it is recognized that any number of sleeves may be used. It is further recognized that sleeves 508a and 508b may be positioned between any combination of long fairings and short fairings or fairing groups. Representatively, an alternating arrangement of long fairings and sleeves or short fairings and sleeves may be positioned along a structure. Sleeves 508a and 508b may be pre-installed, they can be installed on structure 502 during its installation (e.g. while running a drilling riser); or they can be installed after structure 502 has already been installed (a post-installation). Sleeves 508a and 508b can be clam-shelled around structure 502 using hinges

and alternative latching mechanisms such as snaps, bolts, or other fasteners. Alternatively, sleeves 508a and 508b can be made with a continuous circumference and slid over structure 502.

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 comprises water, and may comprise seawater or fresh water, or may comprise a mixture of fresh water and seawater.

In some embodiments, fairings of the invention 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 subsea systems, such as for example, spar platforms, floating production systems, floating production storage and offloading, and subsea systems.

In some embodiments, fairings may be attached to marine structures such as subsea 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; and the hull and/or column structure for tension leg platforms (TLPs) and for spar type structures. In some embodiments, fairing may be attached to spars, risers, tethers, and/or mooring lines.

In some embodiments, the fairing may be mounted upon a structure for underwater deployment, the fairing comprising a fairing body which, when viewed along its length, may be substantially wedge-shaped or tear-drop shaped, having a relatively broad front tapering to a relatively narrow trailing edge, and optionally at least two collars which may be both secured to the fairing body and may be separated from each other along the length of the fairing body, the collars being positioned and aligned to receive the structure, thereby to pivotally mount the fairing body upon the structure such that it may be able to rotate about the axis of

the structure and so align itself with a water current, the fairing body defining, when viewed along the length of the fairing, a teardrop shape. The collar may be shaped to form a respective bearing ring for receiving the structure. Each bearing ring may have a substantially circular interior surface. A bearing surface of the collar, which faces toward the structure and upon which the collar rides, may comprise low friction material. The bearing surface may be self lubricating. The collar may comprise a plastics material with an admixture of a friction reducing agent.

In some embodiments, the fairing may be seen to be generally wedge shaped. Its front, lying adjacent the structure, may have a lateral dimension similar to that of the structure. Moving toward its rear the fairing tapers to a narrow trailing edge. This tapered shape may be defined by convergent walls, which meet at the trailing edge. The front of the fairing may be shaped to conform to the adjacent surface of the structure, being part cylindrical and convex. The fairing may form a streamlined teardrop shape. In a manner which will be familiar to the skilled person, this shape tends to maintain laminar flow and serves both to reduce drag and/or to prevent or reduce VIV.

In some embodiments, the fairing may be formed as a hollow plastic molding whose interior communicates with the exterior to permit equalization of pressure. In some embodiments, the fairing may be formed by a single plastic molding, such as by rotational molding, so that it may be hollow. The fairing 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 fairing to equalize internal and external pressures. The fairing could also be formed as a solid polyurethane molding. In some embodiments, the principal material used in constructing the fairing may be fiberglass. Other known materials may also be used which have suitable weight, strength and corrosion-resistant characteristics. In some embodiments, the fairings may be constructed from any metallic or non-metallic, low corrosive material such as an 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 fairings 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 fairing can be formed of vinyl ester resin with 7-10% elongation or polyurethane. The use of such materials eliminates the possibility of corrosion, which can cause the fairing shell to seize up around the elongated structure it surrounds.

Collars may be provided to connect the fairing to the structure and/or to provide spacing between adjacent fairings along the structure. Collars may be formed by a single plastics molding, 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 fairing 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 fairing 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 fairing may be mounted to the structure itself. That is, the fairing may be mounted directly upon the structure (or on a cylindrical protective sheath conventionally provided around the structure). A number of such fairings may be placed adjacent one another in a string along the structure. To prevent the fairings 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 fairings. 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 fairing 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 fairings 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 fairing that defines an opening to receive the structure. A low friction material may be provided on the portion of the fairing that surrounds a structure, for example strips of molybdenum 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 fairing or group of fairings. Buoyancy cans may also be installed above and/or below each fairing or group of fairings.

The methods and systems of the invention may further comprise modifying the buoyancy of the fairing. This may be carried out by attaching a weight or a buoyancy module to the fairing. In some embodiments, the fairing may include filler material that may be either neutrally or partially buoyant. The tail portion of each fairing may be partially filled with a known syntactic foam material for making the fairing 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 fairing to discourage marine growth at the fairing - structure interface so that the fairing remains free to weathervane to orient most effectively with the current, for example a copper bar. In some embodiments, the fairings may be made of copper, or be made of copper and one or more other materials.

The height of the fairing can vary considerably depending upon the specific application, the materials of construction, and the method employed to install the fairing. In extended marine structures, numerous fairings may be placed along the length of the marine structure, for example covering from about 15% or 25%, to about 50%, or 75%, or 100% of the length of the marine structure with the fairings.

In some embodiments, fairings 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 multiple fairings around the structure. Alternatively, fairings 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, fairings may be attached to the connected sections before they are lowered into the ocean.

The fairings may comprise one or more members. Examples of two- membered fairings suitable herein include a clam-shell type structure wherein the fairing comprises two members that may be hinged to one another to form a hinged edge and two unhinged edges, as well as a fairing comprising 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 fairing. Clam-shell fairings may be positioned onto the marine structure by opening the clam shell structure, placing the structure around the structure, and closing the clam-shell structure around the circumference of the structure. The step of securing the fairing into position around the structure may comprise connecting the two members to one another. For example, the fairing 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 fairings may have a locking mechanism to secure the fairing about the structure, such as male-female connectors, rivets, screws, adhesives, welds, and/or connectors.

In some embodiments, fairings may be configured as tail fairings, for example as described and illustrated in co-pending U.S. application 10/839,781 , which was published as U.S. Patent Application Publication 2006/0021560, and is herein incorporated by reference in its entirety. In some embodiments, fairings may include one or more wake splitter plates. In some embodiments, fairings may include one or more stabilizer fins.

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 of the invention may further comprise positioning a second fairing, or a plurality of fairings around the circumference of a structure.

In the multi-fairing embodiments, the fairings may be adjacent one another on the structure, or stacked on the structure. The fairings may comprise end flanges,

rings or strips to allow the fairings to easily stack onto one another, or collars or clamps may be provided in between fairings or groups of fairings. In addition, the fairings 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 fairings of a stack of fairings may be connected to one another, or attached separately.

While the fairings 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.

The following example describes results for displacement and drag response testing performed on seven different fairing configurations.

Example 1 : Test Setup

Long and short fairings in various configurations were attached to a pipe as previously discussed. The long and short fairings tested had solid tails without any sort of rotation dampening mechanism built into or attached to the fairings to dampen rotation of the fairings about the structure. The fairings were subjected to an increasing flow speed of from about 1 to 7.5 feet per second in a current tank.

The tank included two honeycomb sections (straighteners) to minimize turbulence and fluid rotational effects, and a shear screen which was used to produce sheared velocity profiles when desired. A steel caisson was located in the test section to allow for test cylinders as long as about 18 m (59 feet). The excitation region of the test section was about 3.66 m (12 feet) deep by 1.07 m (3.5 feet) wide and was produced by a fixed steel insert with baffles that change the cross-sectional dimensions of the flow from about 2.13 m (7 feet) deep by 1.83 m (6 feet) wide to about 3.66 m (12 feet) deep by 1.07 m (3.5 feet) wide, and then back to 2.13 m (7 feet) deep by 1.83 m (6 feet) wide beyond the test section.

The structure used for testing was an Acrylonitrile Butadiene Styrene (ABS) pipe with an outside diameter of about 63.5 mm (2.5 inches), an inside diameter of 2 mm (0.08 inches), and a length of 3.71 m (12.2 feet). The structure was terminated at lower and upper ends by universal joints. The universal joints were ball joints designed such that they allow motion in all directions except torsional motion. A biaxial accelerometer was mounted inside the pipe at the center of the

structures tested. A bending load cell was mounted beneath the lower ball joint to monitor loads in the in-line and cross-flow directions. Similarly, a bending load cell was mounted above the upper ball joint. A tension load cell was placed on top of a spring that was linked at the top of the bending load cell. A rod was connected at the top of the tension load cell. Tension was applied on the structure by tightening a nut connected to the rod against a top frame. A tension of 222 N was applied at the top for all test cases. A current having an increasing flow speed from about 1 to 7.5 feet per second was generated in the tank. The center-to-center distance between the ball joints was slightly larger than the length of the pipe and the top end (ball joint) was about a foot above the still waterline.

In the test set up for structures having long and short fairings connected to the structure, the fairings were free-to-rotate and positioned along an entire length of the structure. Collars having a height of about 0.75 inches were placed between the fairings. The blockage ratio (ratio of cylinder diameter to width of the flow section) was about 0.06 (=0.0635/1.07). The aspect ratio (UD) was about 58 (=3.71/0.0635).

Data Acquisition and Test Parameters Columbia Model HEVP biaxial accelerometers were used to collect acceleration response data. During the testing, two accelerometers were used and were positioned at either the center of the pipe or the center and quarter point of the pipe. The accelerometers were calibrated using a shaker assembly prior to the tests and found to be accurate when compared to a simultaneous LVDT measurement (mounted on top of the calibrator). As mentioned above, two bending load cells were mounted to measure the in-line and cross-flow forces at both ends, and a tension load cell was used to monitor the top tension. A total of seven channels of data were acquired. The data was sampled at a rate of 64 hertz (Hz), and each duration was 32 seconds. Low-pass frequency filters were used with a setting of 53 Hz. A commercially available data acquisition system was used to collect these data.

The fundamental natural frequency of the bare pipe (i.e., pipe without fairings) in its lateral bending motion was 1.72 Hz. The flow speeds of the fresh

water were chosen such that the nominal vortex-shedding frequency ranged over the fundamental natural frequency of the pipe's lateral bending motion in water for the bare pipe (up to a reduced velocity of 15). In the suppression tests, the flow speeds went higher (to a reduced velocity of 25 and greater) to check the stability performance. Note that with the addition of long and short fairings along the pipe, the natural frequency was reduced to 1.25 Hz. Reynolds numbers varied from approximately 20,000 to 1 12,000.

The test results for seven different fairing system configurations are set forth below. The following results show that the combination of long and short fairings positioned along a structure, without rotational dampening mechanisms built into or attached to the fairings, effectively reduces VIV and drag.

Test 1:

In test 1 , thirty two aluminum fairings were positioned about a pipe having a 2.5 inch outer diameter (OD) and a length of about 12.2 feet. Fifteen of the fairings were long fairings having a chord to thickness ratio of 2.24. Seventeen of the fairings were short fairings having a chord to thickness ratio of 1.36. Each of the fairings had a height of approximately 3 inches. The fairings were arranged in alternating groups along the pipe in the following order from the bottom to the top of the pipe: 2 short, 5 long, 5 short, 5 long, 5 short, 5 long, 5 short. The fairings were positioned such that the first four groups (17 fairings) were below the pipe center and the remaining three groups (15 fairings) were above the pipe center. Collars were positioned between the fairings.

Test 2:

In test 2, thirty two aluminum fairings were positioned about a pipe having a 2.5 inch OD and a length of about 12.2 feet. Sixteen of the fairings were long fairings having a chord to thickness ratio of 2.24. Sixteen of the fairings were short fairings having a chord to thickness ratio of 1.36. Each of the fairings had a height of approximately 3 inches. The fairings were arranged in alternating groups along the pipe in the following order from the bottom to the top of the pipe: 8 short, 8 long, 8 short, 8 long. The fairings were positioned such that the first two groups

(16 fairings) were below the pipe center and the remaining two groups (16 fairings) were above the pipe center. Collars were positioned between the fairings.

Test 3: In test 3, thirty two aluminum fairings were positioned about a pipe having a

2.5 inch OD and a length of about 12.2 feet. Sixteen of the fairings were long fairings having a chord to thickness ratio of 2.24. Sixteen of the fairings were short fairings having a chord to thickness ratio of 1.36. Each of the fairings had a height of approximately 3 inches. The fairings were arranged in alternating groups along the pipe in the following order from the bottom to top of the pipe: 16 short, 16 long. The fairings were positioned such that the first group (16 fairings) was below the pipe center and the remaining group (16 fairings) was above the pipe center. Collars were positioned between the fairings.

Test 4:

In test 4, thirty two aluminum fairings were positioned about a pipe having a 2.5 inch OD and a length of about 12.2 feet. Sixteen of the fairings were long fairings having a chord to thickness ratio of 2.24. Sixteen of the fairings were short fairings having a chord to thickness ratio of 1.36. Each of the fairings had a height of approximately 3 inches. The fairings were arranged in alternating groups along the pipe in the following order from the bottom to the top of the pipe: 8 long, 16 short, 8 long. The fairings were positioned such that eight of the long fairings and eight of the short fairings were below the pipe center and the remaining fairings (8 short and 8 long) were above the pipe center. Collars were positioned between the fairings.

Test 5:

In test 5, thirty two aluminum fairings were positioned about a pipe having a 2.5 inch OD and a length of about 12.2 feet. Twenty of the fairings were long fairings having a chord to thickness ratio of 2.24. Twelve of the fairings were short fairings having a chord to thickness ratio of 1.36. Each of the fairings had a height of approximately 3 inches. The fairings were arranged in alternating groups along the pipe in the following order from the bottom to top of the pipe: 10 long, 12 short,

10 long. The fairings were positioned such that ten of the long and six of the short fairings were below the pipe center and the remaining fairings (6 short and 10 long) were above the pipe center. Collars were positioned between the fairings.

Test 6:

In test 6, thirty two aluminum fairings were positioned about a pipe having a 2.5 inch OD and a length of about 12.2 feet. Twenty four of the fairings were long fairings having a chord to thickness ratio of 2.24. Eight of the fairings were short fairings having a chord to thickness ratio of 1.36. Each of the fairings had a height of approximately 3 inches. The fairings were arranged in alternating groups along the pipe in the following order from the bottom to top of the pipe: 12 long, 8 short, 12 long. The fairings were positioned such that twelve of the long and four of the short fairings were positioned below the pipe center and the remaining fairings (4 short and 12 long) were above the pipe center. Collars were positioned between the fairings.

Test 7:

In test 7, thirty two aluminum fairings were positioned about a pipe having a 2.5 inch OD and a length of about 12.2 feet. Twenty four of the fairings were long fairings having a chord to thickness ratio of 2.24. Four of the fairings were short fairings having a chord to thickness ratio of 1.36. Each of the fairings had a height of approximately 3 inches. The fairings were arranged in alternating groups along the pipe in the following order from the bottom to top of the pipe: 14 long, 4 short, 14 long. The fairings were positioned such that fourteen of the long and two of the short fairings were positioned below the pipe center and the remaining fairings (2 short and 14 long) were above the pipe center. Collars were positioned between the fairings.

All of the fairing systems examined in tests 1-7 above were found to be very stable. In particular, as can be seen from the test data found in Table 1 below, each of the configurations had extremely small displacement response (rms A/D) and drag coefficient (Cd) measurements. For example, the fairing configuration tested in test 1 (2 short, 5 long, 5 short, 5 long, 5 short, 5 long, 5 short) had a maximum displacement response of 0.09 and maximum drag coefficient of

approximately 0.6. This is in comparison to, for example, the cross-flow and in-line displacement response measurements for a pipe with long fairings alone which may reach more than 1.5 and about 0.25, respectively, as reduced velocity increases (see Figure 7) and drag coefficient for a pipe with short fairings alone which may reach approximately 1 (see Figure 1 1 ).

Figure 6 shows displacement test results in the cross-flow and in-line directions for a bare structure without long and short fairings. The structure tested was an ABS pipe as discussed in the above example. The displacement is expressed as a fraction of the pipe outside diameter and is indicated in the vertical axis. The horizontal axis is the reduced velocity. Reduced velocity refers to the flow speed divided by the product of the first still water natural frequency of structure and its outside diameter. As can be seen from Figure 6, in the reduced velocity zone tested, the bare structure vibrates violently. The second peak corresponds to the second modal frequency of the structure. The magnitude of the second mode was smaller than that of the first mode, but did not vanish.

Figure 7 shows displacement test results in the cross-flow and in-line directions for a structure with long fairings. The tested structure included thirty two long fairings having a chord to thickness ratio of 2.24 positioned about an ABS pipe as previously discussed. As can be seen from Figure 7, a very unstable response (commonly referred to as galloping) was seen when only long fairings were coupled to the structure. In particular, as the reduced velocity reached 20 or above, the motion began to build up quickly and increased with flow speed. It is

further noted that the cross-flow motion remained greater than that of the in-line direction.

Figure 8 shows displacement test results in the cross-flow and in-line directions for a structure with long and short fairings installed about the structure. The tested structure included short and long fairings as described in Test 1 above. As can be seen from Figure 8, when both long and short fairings are coupled to the structure, the responses for both in-line and cross-flow directions were extremely small and very stable. It is noted that there existed a slow upward trend for the responses as flow speed increased. This was likely due to the unstable nature of the long fairings. The responses, however, never went out of control as observed in Figure 7 when only long fairings were coupled to the pipe.

Figure 9 shows displacement test results in cross-flow and in-line directions for a structure with short fairings installed about the structure. The tested structure included thirty two short fairings having a chord to thickness ratio of 1.36 positioned about an ABS pipe as previously discussed. It can be seen from Figure 9 that at lower reduced velocities, the short fairings exhibited somewhat higher responses that what is seen when both short and long fairings are used as discussed in reference to Figure 8.

Figure 10 shows drag test results for a structure with long and short fairings installed about the structure. The tested structure included short and long fairings as described in Test 1 above. A very important parameter in evaluating the merit of suppression devices is their response to drag. In the case of offshore drilling, it is known that large drag will increase the deflection of the structure (e.g. riser, pipe, etc.). Large drag loads on the structure can make drilling impossible. Drag loads on an array of production structures could further adversely affect a floating production unit. As can be seen from Figure 10, excellent drag characteristics are achieved when both long and short fairings are coupled to the structure. In the reduced velocity range tested, the maximum drag coefficient is approximately 0.6. This is in comparison to helical strakes attached to a cylinder to suppress VIV which are known to have drag coefficient reaching 1.8 or more.

Figure 1 1 shows drag test results for a structure with short fairings installed about the structure. The tested structure included thirty two short fairings having a chord to thickness ratio of 1.36 positioned about an ABS pipe as previously

discussed. As can be seen from Figure 1 1 , the drag coefficient for the short fairings is higher than that of the long/short fairing configuration, reaching approximately 1.0. On average, the drag load on the long/short fairing configuration is about 65% of that of the short fairing configuration. Thus, as is shown by the above results, the drag load can be reduced by using both long and short fairings as described herein.

As can be seen from the foregoing test results, a bare cylinder may experience significant VIV if it is not protected. In addition, long fairings are not effective at reducing VIV, and furthermore, may make the riser responses unstable. The coupling of long and short fairings to a structure without any rotation dampening mechanism built into or attached to the fairings is shown by the above experimental results to be very efficient at reducing VIV. Structures having long and short fairings attached thereto exhibit very stable response behaviors. In addition, the disclosed structures are very effective at reducing drag loads. The drag coefficient may be reduced to one-third of that of helical strakes and better than that of short fairings. Further advantages with respect to VIV and drag may further be achieved by mounting combinations of long fairings, short fairings and sleeves about the structure as described herein.

Illustrative Embodiments:

In one embodiment, there is disclosed a system comprising a structure; a long fairing comprising a chord to thickness ratio of at least about 1.7; and a short fairing comprising a chord to thickness ratio less than about 1.7. In some embodiments, the long fairing comprises a chord to thickness ratio of at least about 1.8. In some embodiments, the long fairing comprises a chord to thickness ratio of at least about 2.0. In some embodiments, the long fairing comprises a chord to thickness ratio of at least about 2.5. In some embodiments, the short fairing comprises a chord to thickness ratio less than about 1.5. In some embodiments, the short fairing comprises a chord to thickness ratio less than about 1.25. In some embodiments, the system also includes a sleeve located between the long fairing and the short fairing. In some embodiments, the sleeve comprises a smoothness having a K/D value of less than about 1 X 10 ^"4 . In some embodiments, the sleeve comprises a smoothness having a K/D value of less than

about 1 X 10 ^"5 . In some embodiments, the system also includes a coupling mechanism connected to the long fairing and the short fairing. In some embodiments, the system also includes a rotation dampening mechanism connected to the long fairing. In some embodiments, the system also includes a group of at least 3 long fairings adjacent to a group of at least 3 short fairings. In some embodiments, the system also includes a group of at least 5 long fairings adjacent to a group of at least 5 short fairings. In some embodiments, the system also includes a plurality of groups of long fairings interspersed with a plurality of groups of short fairings. In one embodiment, there is disclosed a method of suppressing vortex induced vibration and/or drag of a structure subject to a current, the method comprising installing a long fairing comprising a chord to thickness ratio of at least about 1.7 about the structure; and installing a short fairing comprising a chord to thickness ratio less than about 1.7 about the structure. In some embodiments, the method also includes installing a sleeve about the structure. In some embodiments, the method also includes installing a plurality of sleeves about the structure. In some embodiments, the method also includes installing a plurality of long fairings about the structure. In some embodiments, the method also includes installing a plurality of short fairings about the structure. In some embodiments, the method also includes forming groups of at least 3 long fairings next to each other, and forming groups of at least 3 short fairings next to each other.

While the illustrative embodiments of the invention 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.