WO2009070483A1

WO2009070483A1 - Strake systems and methods

Info

Publication number: WO2009070483A1
Application number: PCT/US2008/084143
Authority: WO
Inventors: Donald Wayne Allen; David Wayne Mcmillan; Christopher Steven West
Original assignee: Shell Oil Company; Shell Internationale Research Maatschappij B.V.
Priority date: 2007-11-29
Filing date: 2008-11-20
Publication date: 2009-06-04
Also published as: BRPI0820366A2; MX2010005573A; GB201007942D0; GB2467676A; NO20100949L

Abstract

A strake system, comprising a flexible member (306) helically wrapped about an elongated structure (302); and a plurality of strake members (308) connected to the flexible member (306).

Description

STRAKE SYSTEMS AND METHODS

Field of the Invention

There is disclosed a strake system and methods for using and installing a strake system.

Background of the Invention

Production of oil and gas from offshore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on marine elements. Such marine elements may be employed in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; other mooring elements for deepwater platforms; and, the hull structure for spar type structures. These currents may cause vortexes to shed from the sides of the marine elements, inducing vibrations that can lead to the failure of the marine elements or their supports.

Deepwater production risers, drilling risers, platform export risers, import risers bringing in production from satellite wells, tendons for tension leg platforms, and other conduits for produced fluids and deepwater mooring elements formed from tubular goods may be typical of applications that may have vibration problems. Subsea pipelines traversing valleys on the ocean floor for extended, unsupported lengths and spar hulls moored at the end of long tethers and/or mooring lines provide additional examples.

When these types of structures, such as a cylinder, experience a current in a flowing fluid environment, it is possible for the structure to experience vortex- induced vibrations (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. The vibrations may be larger in the transverse (to flow) direction; however, in-line vibrations can also cause stresses which may be sometimes larger than those in the transverse direction.

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. Risers are discussed in this patent document as a non-exclusive example of an aquatic structure subject to VIV. A riser system may be used for establishing fluid communication between the surface and the bottom of a water body. The principal purpose of the riser is to provide a fluid flow path between a drilling vessel and a well bore and to guide a drill string to the well bore.

A typical riser system normally consists of one or more fluid-conducting conduits which extend from the surface to a structure (e.g., wellhead) on the bottom of a water body. For example, in the drilling of a submerged well, a drilling riser usually consists of a main conduit through which the drill string is lowered and through which the drilling mud is circulated from the lower end of the drill string back to the surface. In addition to the main conduit, it is conventional to provide auxiliary conduits, e.g., choke and kill lines, etc., which extend parallel to and may be carried by the main conduit.

The magnitude of the stresses on the riser pipe, tendons or spars is generally a function of and increases with the velocity of the water current passing these structures and the length of the structure.

There are generally two kinds of current-induced stresses in flowing fluid environments. The first kind of stress is 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 is 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 vibrations of the structure. For instance, a riser pipe that is vibrating due to vortex shedding will 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.

Some devices used to reduce vibrations caused by vortex shedding from sub-sea structures 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 strake elements, shrouds, fairings and substantially cylindrical sleeves. Currently available strake elements and fairings cover an entire circumference of a cylindrical element or may be clamshell shaped to be installed about the circumference.

Some VIV and drag reduction devices can be installed on risers and similar structures before those structures may be deployed underwater. Alternatively, VIV and drag reduction devices can be installed on structures after those structures may be deployed underwater. Elongated structures in wind in the atmosphere can also encounter VIV and drag, comparable to that encountered in aquatic environments. Likewise, elongated structures with excessive VIV and drag forces that extend far above the ground can be difficult, expensive and dangerous to install VIV and/or drag reduction devices. U.S. Patent Number 6,561 ,734 discloses a partial helical strake system and method for suppressing vortex-induced-vibration of a substantially cylindrical marine element, the strake system having a base connected to the cylindrical marine element and an array of helical strake elements projecting from the base for about half or less of the circumference of the cylindrical marine element. U.S. Patent Number 6,561 ,734 is herein incorporated by reference in its entirety.

U.S. Patent Number 4,722,367 discloses a pipeline vortex spoiler system comprising elongated foamed plastic strakes which are formed in sections approximately fifteen to twenty feet in length and have a somewhat inverted T- shaped cross sectional configuration. The strakes are disposed on the exterior surface of a cylindrical pipeline or the like to form a spiral or helical path and are secured to the pipeline section by flexible cylindrical bands or straps. The strake sections are adaptable to use on pipeline sections and other cylindrical bodies having a wide range of diameters, and are easily stored, shipped and attached to pipelines and similar bluff body structures in the field. U.S. Patent Number 4,722,367 is herein incorporated by reference in its entirety.

There is a need in the art for an improved apparatus and method for suppressing vibration. There is another need in the art for apparatus and methods for suppressing vibration which do not suffer from the disadvantages of the prior art.

There is another need in the art of apparatus for and new and improved methods of manufacturing and installing strake systems for suppressing vibration in a flowing fluid environment. These and other needs of the present disclosure 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 discloses a strake system, comprising a flexible member helically wrapped about an elongated structure; and a plurality of strake members connected to the flexible member.

Another aspect of the disclosed invention provides a method of suppressing vortex induced vibration of an elongated structure, comprising in any order: helically wrapping at least one flexible member about the structure; and attaching a plurality of strake members to the flexible member.

Improvements and advantages of the invention include one or more of the following: an improved strake system and method, an improved strake installing system and method, a more efficient strake installing system and method, and/or an improved system and method for installing strake systems about existing structural elements.

Brief Description of the Figures

Figure 1 illustrates an offshore system. Figure 2 illustrates a side view of a strake system.

Figure 3 illustrates a cross-sectional view of a strake system. Figure 4 illustrates a cross-sectional view of a strake system. Figure 5 illustrates a side view of an end termination of a strake system. Figure 6 illustrates a side view of a strake system and bands.

Figure 7 illustrates a cross-sectional view of a strake system.

Figure 8 illustrates a cross-sectional view of a strake system.

Figure 9 illustrates a cross-sectional view of a strake system.

Detailed Description

Figure 1 :

Referring now to Figure 1 there is illustrated an embodiment of an offshore system with which the strake system described herein may be used. Offshore system 100 includes floating platform 1 10 with facilities 105 on top. Platform 1 10 is floating in a body of water having water surface 1 15 and bottom of the body of water 135. Buoyancy device 120 keeps platform 1 10 from sinking. Riser 125 connects platform 1 10 with well 140. Mooring lines 130 anchor platform 1 10 to the bottom of the body of water 135. Vortex induced vibration (VIV) may cause vibration of a structural element, such as one or more of buoyancy device 120, riser 125, and/or mooring lines 130. In some embodiments of the invention, one or more strake elements and/or fairings may be applied to one or more structural elements of offshore system 100. Suitable structural elements that may benefit from such strake elements and/or fairings that may include, for example, tubulars, pipes, rods, buoyancy device 120, riser 125, and/or mooring lines 130. VIV may also cause vibration of other subsea structural elements to which the strake system described herein may be applied.

Figure 2:

Figure 2 illustrates a side view of strake system 200 on a structural element. In some embodiments, structural element 202 is a fluid-conducting conduit such as a tubular (e.g., cylindrical conduit). Representatively, structural element 202 may be an oil flowline, a pipeline, a drilling riser, a production riser, a steel tubular, import and export risers, subsea pipelines, 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, hull structures for tension leg platforms and for spar type structures, and/or column structures for tension leg platforms and for spar type structures.

Structural element 202 may extend from a surface of a body of water to a structure (e.g. a well) on the bottom of the water body. Representatively, in some embodiments, structural element 202 may be lowered onto the bottom of the water body having a depth of at least about 1000 meters, at least about 2000 meters, at least about 3000 meters or at least about 4000 meters. In some embodiments, the water body has a depth up to about 10,000 meters. Alternatively, structural element 202 may extend above a water body surface. Structural element 202 may be exposed to current and/or air flow patterns which may cause structural element 202 to experience VIV.

Structural element 202 may have an outer diameter D 212 of from about 0.1 to about 5 meters (m), and a length of from about 0.1 to about 200 kilometers (km). In some embodiments, structural element 202 may have a length to diameter ratio of from about 100 to about 100,000. In some embodiments, structural element 202 may be composed of from about 50 to about 30,000 tubular sections, each with a diameter of from about 10 centimeters (cm) to about 60 cm and a length from about 5 m to about 50 m, and a wall thickness from about 0.5 cm to about 5 cm. In some embodiments, structural element 202 may have a constant or variable outer diameter D 212. For example, structural element 202 may include tapered joints or other joints that decrease or increase an outer diameter of structural element 202 at the joint. In still further embodiments, structural element 202 may be a pipe having outer coverings such as an insulating layer, buoyancy, or other cover material which may expand and/or contract in response to environmental conditions.

Strake system 200 may include strakes 204a, 204b and 204c positioned around structural element 202 to reduce VIV. Although three strakes are illustrated in Figure 2, it is contemplated that any number of strakes sufficient for reducing VIV may be used. For example, in some embodiments, there may be about one to ten strake starts about a circumference of structural element 202. In other embodiments, there may be about two to six strake starts about a circumference of structural element 202. In some embodiments, the length of strakes 204a, 204b and 204c may vary depending upon the dimensions of the underlying element 202 they are to be used with. For example, the length of strakes 204a, 204b and 204c used on a structural element having a 5 m outer diameter may be greater than the length of strakes used on a structural element having a 1 m outer diameter. In some embodiments, for a structural element having a 13 m length and 5 m diameter with strakes extending over the entire length of the structural element, each strake 204a, 204b and 204c may have a length from about 0.5 to about 5 meters, for example from about 1 to 3 meters. In some embodiments, strakes 204a, 204b and 204c may be made of a composite structure that can accommodate changes in the diameter of underlying structural element 202. In one embodiment, the composite structure may include inner member 206 and outer member 208. Inner member 206 may be made of a flexible material suitable for wrapping around structural element 202 and providing reinforcement to strakes 204a, 204b and 204c. Representatively, inner member 206 may be a flexible elongated member such as a polymer rope (e.g., a polyester rope), stainless steel cable (e.g., a braided cable) or other similarly flexible rope or cable. In one embodiment, a diameter of inner member 206 is 0.25 to 5 cm, for example from about 1 to 3 cm. In other embodiments, inner member 206 may have any diameter suitable for connection to outer member 208 and wrapping around structural element 202. In some embodiments, outer member 208 may be secured to inner member 206 to provide strake height. In some embodiments, outer member 208 may have a lumen therethrough which accommodates a diameter of inner member 206. Outer member 208 may be made of any material suitable for reducing VIV when strakes 204a, 204b and 204c are applied to structural element 202. Representatively, outer member 208 may be made of a synthetic rubber, high density polyethylene (HDPE), other related plastic material or a composite material such as nylon reinforced rubber hose or a metal or metal alloy. Outer member 208 may be extruded or molded from the desired material.

Outer member 208 may have clamps, rings, hooks, or other attachment devices on the inside and/or outside to connect to inner member 206.

In some embodiments, outer member 208 may be made up of a plurality of segments which are connected along a length of inner member 206 to form strakes 204a, 204b and 204c. A thickness of outer member 208 may be greater than a tolerance on its diameter to avoid adjacent segments from sliding against each other. In other words, the diameter of a lumen of outer member 208 may be equal to or slightly less than a diameter of inner member 206 so that once the strake is formed, outer member 208 cannot slide on inner member 206. Gaps 216 may be provided between ends of segments of outer member 208 to maintain flexibility of strakes 204a, 204b and 204c. The length of a segment, L, and the size of gaps 216 between each segment of strakes 204a, 204b and 204c may vary depending upon the flexibility of the strake system desired. For example, the longer the outer members 208 are, the larger gaps 216 may be to allow inner member 206 to wrap around structural element 202. Smaller gaps 216 may be used with shorter outer members 208. Alternatively, instead of modifying the size of gaps 216 to accommodate longer outer members 208, outer members 208 may be made of a flexible material which allows them to curve around the outer surface of structural element 202. In some embodiments, the size of gaps 216 may vary depending upon components of the underlying structural element 202. For example, larger gaps may be used around connectors, anodes, etc. It is contemplated that the number of segments per strake 204a, 204b and 204c may vary depending upon the dimensions of the segments, the gap sizes and structural element 202. A representative gap for a strake on a 13 m structural element having a consistent 5 m outside diameter is from about 0.01 to 2 meters, for example from about 0.1 to 1 meters, or 0.2 to 0.5 meters.

In some embodiments, each segment of outer member 208 may have a height H 210 of approximately 25% of an outer diameter D 212 of the underlying structural element 202. In other embodiments, height H 210 may be from about 5% to about 50% of outside diameter D 212. In some embodiments, height H 210 may be from about 1 cm to about 15 cm.

Strakes 204a, 204b and 204c may extend longitudinally and helically about structural element 202 as illustrated in Figure 2. Adjacent strakes, for example strakes 204a and 204b, may be spaced apart by a pitch P 214. In some embodiments, pitch P 214 may be from about 1 diameter (D) to about 10 D. In other embodiments, pitch P214 may be from about 10 cm to about 500 cm.

In some embodiments, inner member 206 may be used to hold strakes 204a, 204b and 204c in place along structural element 202. For example, strakes 204a, 204b and 204c may be helically positioned along structural element 202 and ends of inner member 206 may be attached to tensioning devices (see Figure 5). The tensioning devices pull the ends of inner member 206 in opposite directions thereby tightening inner member 206 and in turn outer member 208 around structural element 202. Using inner member 206 to tighten strakes 204a, 204b and 204c about structural element 202 is particularly useful where diameter D 212, and in turn a circumference, of structural element 202 is variable. For example, structural element 202 may include tapered joints. Since inner member 206 of strakes 204a, 204b and 204c is made of a flexible rope or cable wrapped around structural element 202, each loop around structural element 202 may conform to the circumference of the portion of structural element 202 it is adjacent to. In some embodiments, structural element 202 may include insulation which cause changes to its circumference during installation or tolerances during manufacturing. Since strakes 204a, 204b and 204c can be adjusted along with changes in the surface of structural element 202 by increasing or decreasing the tension of inner member 206, strakes 204a, 204b and 204c remain in place along the underlying structural element 202.

The ability to modify the tension of inner member 206 along structural element 202 in this manner may eliminate the need for additional components typically used to secure strakes to an underlying member thereby decreasing manufacturing costs and installation time. Typically, strakes are attached to an underlying shell which is then secured around an underlying structural element. The shell covers an entire circumference of the structural element and requires several bands to hold the shell to the structural element. The materials and time it takes to fabricate and assemble additional strake components such as the shell and bands results in increased manufacturing and installation costs. Strakes 204a, 204b and 204c do not require a shell and require fewer bands thereby reducing manufacturing and installation costs.

Figure 3:

Figure 3 illustrates a cross-sectional view of a strake system. The strake system may include three strakes 304a, 304b and 304c spaced around an outer surface of structural element 302. Each of strakes 304a, 304b and 304c has a flexible inner member 306 and an outer member 308. In some embodiments, the height of outer member 308 may be from about 5% to about 50% of the outer diameter of structural element 302 and preferably about 25% of the outer diameter of structural element 302. In some embodiments, the height of outer member 308 may be from about 5 cm to about 20 cm. A diameter of inner member 306 may be about three centimeters or less. Outer member 308 of strakes 304a, 304b and 304c is held against an outer surface of structural element 302 by inner members 306 as shown.

In some embodiments, outer member 308 of strakes 304a, 304b and 304c may be cylindrical and have a round cross-section. Inner member 306 of strakes 304a, 304b and 304c may further have a round cross-section. In other embodiments, outer member 308 may have an elliptical, oval or substantially polygonal cross-section, for example a circle, square, pentagon, hexagon, octagon or trapezoidal shape. In some embodiments, inner member 306 may have an elliptical, oval or substantially polygonal cross-section, for example a circle, square, pentagon, hexagon, octagon or trapezoidal shape.

In some embodiments, strakes 304a, 304b and 304c may be attached to structural element 302 by stringing outer member 308 along inner member 306. Inner member 306 having outer member 308 attached thereto may then be helically wrapped around structural element 302 according to a desired pitch (see Figure 2 and pitch P) or until a tension in inner member 306 suitable for holding outer member 308 in position along structural element 302 is achieved. Ends of inner member 306 may then be secured to structural element 302 as will be described in more detail in reference to Figure 5.

Figure 4:

Representatively, Figure 4 illustrates a cross-sectional view of a strake system having strakes of different dimensions than that of Figure 3. The strake system may include three strakes 404a, 404b and 404c spaced around an outer diameter of structural element 402. Each of strakes 404a, 404b and 404c has a flexible inner member 406 and an outer member 408. Inner member 406 may be substantially the same as that described in reference to Figure 2 and Figure 3. Outer member 408 may include base portion 408a and fin portion 408b extending from base portion 408a. Base portion 408a may be dimensioned to provide a stable base for positioning fin portion 408b along structural element 402. Representatively, a foot print of base portion 408a may be greater than that of fin portion 408b to stabilize fin portion 408b along structural element 402. In some embodiments, base portion 408a may have a rectangular shape. In some embodiments, base portion 408a may have a bottom surface complimentary to a dimension of the outer surface of structural element 402. Representatively, when structural element 402 has a cylindrical shape, the bottom of base portion 408a may be curved. It is further contemplated that in some embodiments, base portion 408a is made of a flexible material (e.g., a polymer) such that base portion 408a may have a modifiable shape which conforms to an outer dimension of structural element 402.

Fin portion 408b may be of any shape and size to provide a strake height sufficient to reduce VIV when strakes 404a, 404b and 404c are attached to structural member 402. In some embodiments, the height of fin portion 408b, alone or in combination with base portion 408a, may be from about 5% to about 40% of the outer diameter of structural element 402, for example from about 10% to 30% of the outer diameter of structural element 402. In some embodiments, the height of fin portion 408b may be from about 5 cm to about 25 cm.

Outer member 408 of strakes 404a, 404b and 404c is held against an outer surface of structural element 402 by inner members 406 as shown. In some embodiments, channel 412 may be formed within outer member 408 for receiving inner member 406. In this aspect, channel 412 may have any dimension suitable for accommodating a diameter of inner member 406. An opening 410 to channel 412 may be formed along a joint region between base portion 408a and fin portion 408b. Base portion 408a may be positioned along an outer surface of structural element 402. Strakes 404a, 404b and 404c may be assembled by inserting inner member 406 through opening 410 and into channel 412. In some embodiments, strakes 404a, 404b and 404c may be attached to structural element 402 by loosely fitting inner member 406 around structural element 402. Outer member 408 may then be snapped onto inner member 406 by positioning base portion 408a along an outer surface of structural element 402. Base portion 408a may then be slid between inner member 406 and structural element 402 until inner member 406 slides through opening 410 and into channel 412. Since inner member 406 now wraps around structural element 402 and base portion 408a, inner member 406 is tightened to secure strakes 404a, 404b and 404c to structural element 402. It is further contemplated that in some embodiments, outer member 408 and inner member 406 are assembled prior to wrapping strakes 404a, 404b and 404c around structural element 402. Alternatively, outer member 408 may not include opening 410 to channel 412. Outer member 408 may be connected to inner member 406 by inserting an end of inner member 408 into channel 412 and maneuvering inner member 406 through channel 412.

Figure 5:

Figure 5 illustrates a side view of an end termination of a strake. In some embodiments a three-start strake system including strakes 504a, 504b and 504c may be installed about structural element 502. Strakes 504a, 504b and 504c may be helical strakes mounted about structural element 502. Strakes 504a, 504b and 504c may be spaced about the circumference of structural element 502 as previously described. Strakes 504a, 504b and 504c may be composed of segmented outer members 508 positioned along inner member 506. In some embodiments, strakes 504a, 504b and 504c may be connected to first support 510 at one end and second support 512 at the other end as shown in Figure 5. In some embodiments, first support 510 and/or second support 512 may be a collar. Supports 510 and 512 may be locked relative to each other, for example by locking mechanism 514 as shown in Figure 5.

Tension devices 516 and 518 may be attached to supports 510 and 512, respectively. Each end of inner members 506 of strakes 504a, 504b and 504c may be attached to a tension device. Although two tension devices 516 and 518 are illustrated in Figure 5 it is contemplated that in some embodiments, the number of tension devices used may correspond to the number of inner member ends in the strake system such that each end is attached to a separate tension device. In other embodiments, one end of each inner member may be attached to a support and an opposite end may be attached to a tension device such that the number of tension devices is less than the number of inner member ends in the strake system.

In Figure 5, ends of inner members 506 of strakes 504a and 504c are shown attached to tension devices 516 and 518, respectively. In some embodiments, tension devices 516 and 518 may be bolts rotatably connected to supports 510 and 512. Supports 510 and 512 may have openings therethrough such that ends of inner members 506 may be inserted through one side of supports 510 and 512 to an opposite side attached to tension devices 516 and 518. The ends may be attached to tension devices 516 and 518 by a knot, clamp or any other similarly suitable securing technique. Rotating tension devices 516 and 518 in one direction causes inner members 506 to wind around tension devices 516 and 518 thereby tightening inner members 506 about structural element 502. Rotating tension devices 516 and 518 in an opposite direction causes inner members 506 to unwind thereby loosening inner members 506 about structural element 502.

In some embodiments, a spring mechanism may also be used to maintain a desired tension of inner members 506. The spring mechanism may be attached to the tension device and/or the inner member. Representatively, in some embodiments the spring mechanism is a compression spring positioned around a portion of the tension device. Representatively, in embodiments where the tension device is a bolt, the spring is positioned around the bolt body, between the bolt head and the support. In some embodiments, the spring mechanism may be an extension spring. In this embodiment, one end of the extension spring may be attached to an end of the inner member and an opposite end of the spring may be attached to the tension device (e.g. a bolt head).

In some embodiments, inner members 506 may be tensioned and/or un- tensioned by rotating support 510 relative to support 512, until a desired level of tension is achieved and then locking supports 510 and 512. In some embodiments, a biasing mechanism may be provided to keep supports 510 and 512 rotated relative to each other and keep a level of tension on inner members 506. In some embodiments, a suitable level of tension for inner members is from about 10 to 5000 newtons, for example 50 to 1000 newtons, or 100 to 500 newtons.

Figure 6:

Figure 6 illustrates a side view of a strake system and bands. In some embodiments, the strake system includes strakes 604a, 604b and 604c helically positioned about structural element 602. Strakes 604a, 604b and 604c include segmented outer member 608 and inner member 606. Bands 612, 614 and 616 may be placed about the circumference of structural element 602 to hold strakes 604a, 604b and 604c in place. In some embodiments, bands 612, 614 and 616 are positioned within gaps 626 between segments of outer member 608 to facilitate positioning of strakes 604a, 604b and 604c about structural element 602. In some embodiments, bands 612, 614 and 616 may be made of any suitable synthetic material. In other embodiments, bands 612, 614 and 616 may be made of a metal or metal alloy which is resistant to high temperatures and corrosion (e.g. Inconel®, a nickel-based superalloy). Although three bands 612, 614 and 616 are shown, it is contemplated that any number of bands may be used. In some embodiments, inner member 606 may be used instead of or in addition to bands to hold strakes 604a, 604b and 604c. In this aspect, the higher the tension of inner member 606, the fewer the number of bands required to hold strakes 604a, 604b and 604c in position.

Figure 7: Figure 7 illustrates a cross-sectional view of a strake system. The strake system as shown includes three strakes 704a, 704b and 704c spaced around an outer surface of structural element 702. In other embodiments, there may be from about 1 to about 10 strakes, for example from about 2 to about 6, or about 3 to about 4 strakes spaced around an outer surface of structural element 702. Each of strakes 704a, 704b and 704c has a flexible inner member 706 and an outer member 708. In some embodiments, the height of outer member 708 may be from about 5% to about 50% of the outer diameter of structural element 702 and preferably about 25% of the outer diameter of structural element 702. In some embodiments, the height of outer member 708 may be from about 5 cm to about 20 cm. A diameter of inner member 706 may be about three centimeters or less. Outer member 708 of strakes 704a, 704b and 704c is held against an outer surface of structural element 702 by inner members 706 as shown. In some embodiments, outer member 708 of strakes 704a, 704b and 704c may have a triangular cross section, or an A-shaped cross section as shown with a middle and/or a lower brace member. Inner member 706 of strakes 704a, 704b and 704c may have a round cross-section. In some embodiments, inner member 706 may have an elliptical, oval or substantially polygonal cross-section, for example a circle, square, pentagon, hexagon, octagon or trapezoidal shape.

In some embodiments, strakes 704a, 704b and 704c may be attached to structural element 702 by stringing outer member 708 along inner member 706. Inner member 706 having outer member 708 attached thereto may then be helically wrapped around structural element 702 according to a desired pitch or until a tension in inner member 706 suitable for holding outer member 708 in position along structural element 702 is achieved. Ends of inner member 706 may then be secured to structural element 702.

Figure 8: Figure 8 illustrates a cross-sectional view of a strake system. The strake system as shown includes three strakes 804a, 804b and 804c spaced around an outer surface of structural element 802. In other embodiments, there may be from about 1 to about 10 strakes, for example from about 2 to about 6, or about 3 to about 4 strakes spaced around an outer surface of structural element 802. Each of strakes 804a, 804b and 804c has a flexible inner member 806 and an outer member 808. In some embodiments, the height of outer member 808 may be from about 5% to about 50% of the outer diameter of structural element 802 and preferably about 25% of the outer diameter of structural element 802. In some embodiments, the height of outer member 808 may be from about 5 cm to about 20 cm. A diameter of inner member 806 may be about three centimeters or less. Outer member 808 of strakes 804a, 804b and 804c is held against an outer surface of structural element 802 by inner members 806 as shown. In some embodiments, outer member 808 of strakes 804a, 804b and 804c may have a trapezoidal cross section, or an upside-down A-shaped cross section as shown with a middle and/or a lower brace member. Inner member 806 of strakes 804a, 804b and 804c may have a round cross-section. In some embodiments, inner member 806 may have an elliptical, oval or substantially polygonal cross-section, for example a circle, square, pentagon, hexagon, octagon or trapezoidal shape.

In some embodiments, strakes 804a, 804b and 804c may be attached to structural element 802 by stringing outer member 808 along inner member 806. Inner member 806 having outer member 808 attached thereto may then be helically wrapped around structural element 802 according to a desired pitch or until a tension in inner member 806 suitable for holding outer member 808 in position along structural element 802 is achieved. Ends of inner member 806 may then be secured to structural element 802.

Figure 9:

Figure 9 illustrates a cross-sectional view of a strake system. The strake system as shown includes three strakes 904a, 904b and 904c spaced around an outer surface of structural element 902. In other embodiments, there may be from about 1 to about 10 strakes, for example from about 2 to about 6, or about 3 to about 4 strakes spaced around an outer surface of structural element 902.

Each of strakes 904a, 904b and 904c has a flexible inner member 906 and an outer member 908. In some embodiments, the height of outer member 908 may be from about 5% to about 50% of the outer diameter of structural element 902 and preferably about 25% of the outer diameter of structural element 902. In some embodiments, the height of outer member 908 may be from about 5 cm to about 20 cm. A diameter of inner member 906 may be about three centimeters or less. Outer member 908 of strakes 904a, 904b and 904c is held against an outer surface of structural element 902 by inner members 906 as shown. In some embodiments, outer member 908 of strakes 904a, 904b and 904c may have a triangular cross section with a tail section, or an upside-down Y- shaped cross section as shown (with an optional middle brace member, not shown). Inner member 906 of strakes 904a, 904b and 904c may have a round cross-section. In some embodiments, inner member 906 may have an elliptical, oval or substantially polygonal cross-section, for example a circle, square, pentagon, hexagon, octagon or trapezoidal shape.

In some embodiments, strakes 904a, 904b and 904c may be attached to structural element 902 by stringing outer member 908 along inner member 906. Inner member 906 having outer member 908 attached thereto may then be helically wrapped around structural element 902 according to a desired pitch or until a tension in inner member 906 suitable for holding outer member 908 in position along structural element 902 is achieved. Ends of inner member 906 may then be secured to structural element 902.

Illustrative Embodiments:

In one embodiment, there is disclosed a strake system, comprising a flexible member helically wrapped about an elongated structure; and a plurality of strake members connected to the flexible member. In some embodiments, the system also includes a plurality of flexible members helically wrapped about the structure. In some embodiments, the system also includes a plurality of strake members connected to each of the plurality of flexible members. In some embodiments, the system also includes a gap along the flexible member between adjacent strake members. In some embodiments, at least one of the strake members comprises a lumen therethrough, the flexible member within the lumen. In some embodiments, at least one of the strake members comprises a connector comprising at least one of a loop, a hook, and a bracket, wherein the connector is attached to the flexible member. In some embodiments, at least one of the strake members comprises a channel open to an outside of the strake member, the flexible member within the channel. In some embodiments, the structure comprises a length to diameter ratio of at least 100. In some embodiments, the flexible member comprises at least one of a rope, a cable, and a chain. In some embodiments, at least one of the strake members comprises a pipe section, the flexible member located in an interior opening of the pipe. In some embodiments, the system also includes at least one tensioning device, the device connected to the flexible member in order to set a desired level of tension of the flexible member. In some embodiments, the system also includes a biasing mechanism, the biasing mechanism connected to the at least one tensioning device, the biasing mechanism adapted to maintain a desired level of tension of the flexible member. In some embodiments, the system also includes a first support and a second support, the first support connected to the flexible member at a first end, and the second support connected to the flexible member at a second end. In some embodiments, the first support and second support can be rotated relative to one another in order to selectively increase or decrease a level of tension of the flexible member. In some embodiments, the system also includes a biasing mechanism connected to at least one of the first support and the second support, the biasing mechanism adapted to maintain a desired rotation level of the first support and the second support, in order to maintain a desired level of tension in the flexible member. In some embodiments, the system also includes a plurality of straps about at least one of the flexible member and the strake members, the straps adapted to secure the flexible member and the strake members to the structure. In some embodiments, the structure is located in a body of water subject to currents.

In one embodiment, there is disclosed a method of suppressing vortex induced vibration of an elongated structure, comprising in any order: helically wrapping at least one flexible member about the structure; and attaching a plurality of strake members to the flexible member. In some embodiments, the attaching comprises feeding the flexible member through a lumen in the strake members. In some embodiments, the attaching comprises feeding the flexible member into a channel in the strake members. In some embodiments, the strake members are attached to the flexible member before the flexible member is wrapped about the structure. In some embodiments, the flexible member is wrapped about the structure before the strake members are attached to the flexible member. In some embodiments, the method also includes tensioning the flexible member about the structure when installing. In some embodiments, the method also includes maintaining a desired level of tension on the flexible member about the structure. In some embodiments, the method also includes securing a plurality of straps about the structure and at least one of the flexible member and the strake members. 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 the 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.

Claims

C L A I M S

1. A strake system, comprising: a flexible member helically wrapped about an elongated structure; and a plurality of strake members connected to the flexible member.

2. The system of claim 1 , further comprising a plurality of flexible members helically wrapped about the structure.

3. The system of claim 2, further comprising a plurality of strake members connected to each of the plurality of flexible members.

4. The system of one or more of claims 1-3, further comprising a gap along the flexible member between adjacent strake members.

5. The system of one or more of claims 1-4, wherein at least one of the strake members comprises a lumen therethrough, the flexible member within the lumen.

6. The system of one or more of claims 1-5, wherein at least one of the strake members comprises a connector comprising at least one of a loop, a hook, and a bracket, wherein the connector is attached to the flexible member.

7. The system of one or more of claims 1-6, wherein at least one of the strake members comprises a channel open to an outside of the strake member, the flexible member within the channel.

8. The system of one or more of claims 1-7, wherein the structure comprises a length to diameter ratio of at least 100.

9. The system of one or more of claims 1-8, wherein the flexible member comprises at least one of a rope, a cable, and a chain.

10. The system of one or more of claims 1-9, wherein at least one of the strake members comprises a pipe section, the flexible member located in an interior opening of the pipe.

1 1. The system of one or more of claims 1 -10, further comprising at least one tensioning device, the device connected to the flexible member in order to set a desired level of tension of the flexible member.

12. The system of claim 1 1 , further comprising a biasing mechanism, the biasing mechanism connected to the at least one tensioning device, the biasing mechanism adapted to maintain a desired level of tension of the flexible member.

13. The system of one or more of claims 1-12, further comprising a first support and a second support, the first support connected to the flexible member at a first end, and the second support connected to the flexible member at a second end.

14. The system of claim 13, wherein the first support and second support can be rotated relative to one another in order to selectively increase or decrease a level of tension of the flexible member.

15. The system of claim 14, further comprising a biasing mechanism connected to at least one of the first support and the second support, the biasing mechanism adapted to maintain a desired rotation level of the first support and the second support, in order to maintain a desired level of tension in the flexible member.

16. The system of one or more of claims 1 -15, further comprising a plurality of straps about at least one of the flexible member and the strake members, the straps adapted to secure the flexible member and the strake members to the structure.

17. The system of one or more of claims 1-16, wherein the structure is located in a body of water subject to currents.

18. The system of one or more of claims 1 -17, wherein at least one of the flexible member and the strake members comprise a coating adapted to inhibit marine growth.

19. The system of one or more of claims 1-18, wherein the strake members comprise a first strake member having a first size, and a second strake member having a second size, the first size larger than the second size.

20. The system of one or more of claims 1-19, wherein the strake members comprise a first strake member having a first shape cross section, and a second strake member having a second shape cross section, the first shape cross section different than the second shape cross section.

21. A method of suppressing vortex induced vibration of an elongated structure, comprising in any order: helically wrapping at least one flexible member about the structure; and attaching a plurality of strake members to the flexible member.

22. The method of claim 21 , wherein the attaching comprises feeding the flexible member through a lumen in the strake members.

23. The method of claim 21 , wherein the attaching comprises feeding the flexible member into a channel in the strake members.

24. The method of one or more of claims 21 -23, wherein the strake members are attached to the flexible member before the flexible member is wrapped about the structure.

25. The method of one or more of claims 21 -23, wherein the flexible member is wrapped about the structure before the strake members are attached to the flexible member.

26. The method of one or more of claims 21 -25, further comprising tensioning the flexible member about the structure when installing.

27. The method of one or more of claims 21 -26, further comprising tensioning the flexible member about the structure after installing.

28. The method of one or more of claims 21 -27, further comprising maintaining a desired level of tension on the flexible member about the structure.

29. The method of one or more of claims 21 -28, further comprising securing a plurality of straps about the structure and at least one of the flexible member and the strake members.