WO2006127718A1

WO2006127718A1 - Apparatus with strake elements and methods for installing strake elements

Info

Publication number: WO2006127718A1
Application number: PCT/US2006/019936
Authority: WO
Inventors: Donald Wayne Allen; Stephen Paul Armstrong; Dean Leroy Henning; Kenneth John Loch; David Wayne Mcmillan; Damon Michael Mcmillan
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2005-05-24
Filing date: 2006-05-23
Publication date: 2006-11-30
Also published as: US20060280559A1; BRPI0610116A2; GB2440083A; GB2440083B; NO20076620L; GB0721750D0

Abstract

There is disclosed an apparatus comprising a plurality of strake elements (206a), each strake element adapted to cover an arc-angle about a circumference of a structural element (204); and a mechanism adapted to attach the strake elements (206a) to each other to cover at least a portion of the circumference of the structural element (204).

Description

APPARATUS WITH STRAKE ELEMENTS AND METHODS FORINSTALLING

STRAKE ELEMENTS

Field of Invention: There is disclosed an apparatus with strake elements and methods for installing strake elements. Background:

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.

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 elements 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 disclosed invention provides an apparatus comprising a plurality of strake elements, each strake element adapted to cover an arc-angle about a circumference of a structural element; and a mechanism adapted to attach the strake elements to each other to cover at least a portion of the circumference of the structural element.

Another aspect of the disclosed invention provides a method of installing strake elements comprising providing a plurality of strake elements, each comprising an arc angle from 30° to 180°; and connecting each strake element to at least one other strake element about a circumference of a structural element. Improvements and advantages of the invention include one or more of the following: an improved strake element manufacturing system and method, an improved strake element installing system and method, a more efficient strake element installing system and method, and/or an improved system and method for installing strake elements about existing structural elements. Brief Description of the Drawings:

Figure 1 illustrates an offshore system.

Figure 2a is a cross-sectional view of a tubular with strake elements.

Figure 2b is a side view of a tubular with strake elements.

Figure 2c is an end view of a tubular with strake elements. Figure 3 a is a side view of a tubular with strake elements.

Figure 3b is an end view of a tubular with strake elements.

Figure 4a is an end view of a tubular with strake elements.

Figure 4b is a side view of a tubular with strake elements.

Figure 4c is a side view of a tubular with strake elements. Figure 5a is an end view of a tubular with strake elements.

Figure 5b is a side view of a tubular with strake elements.

Figure 5c is an end view of a tubular with strake elements.

Figure 5d is an end view of a tubular with strake elements.

Figure 6a is an end view of a strake element. Figure 6b is an end view of a tubular with strake elements.

Figure 6c is a side view of a tubular with strake elements. Figure 7a is an end view of a shell to form strake elements.

Figure 7b is a side view of a shell to form strake elements.

Figure 7c is a side view of a shell to form strake elements.

Figure 7d is an end view of a shell being used to form strake elements. Figure 7e is an end view of a shell being used to form strake elements.

Figure 8a is an end view of a die to form strake elements.

Figure 8b is an end view of a die to form strake elements.

Figure 8c is a side view of an extruder to form strake elements. Detailed Description of the Invention In one embodiment there is disclosed an apparatus comprising a plurality of strake elements, each strake element adapted to cover an arc-angle about a circumference of a structural element; and a mechanism adapted to attach the strake elements to each other to cover at least a portion of the circumference of the structural element. In some embodiments, the structural element comprises a structure selected from the group consisting of a tubular, a pipe, a rod, a buoyancy device, a riser, and a mooring line. In some embodiments, the mechanism for attaching comprises a plurality of bolts and nuts. In some embodiments, the mechanism for attaching the strake elements comprises a plurality of heat welds. In some embodiments, the apparatus also includes 4 helical strake elements, each strake element comprising an arc-angle from 70° to 120°, for example from 80° to 100°. In some embodiments, each strake element overlaps the next strake element, to form a plurality of strake shapes about the circumference of the cylindrical element, the strake shapes selected from the group consisting of triangles, rectangles, and trapezoids. In some embodiments, the cylindrical element comprises an outside diameter from 5 to 60 cm and/or wherein the strake elements have a height from 5% to 50% of an outside diameter of the structural element and/or wherein the strake elements are helical. In some embodiments, the apparatus also includes the structural element about which the plurality of strake elements has been attached. In some embodiments, the apparatus also includes from 2 to 8 strake elements. In some embodiments, the apparatus also includes a pitch between each strake element from 0.1 to 10 meters.

In one embodiment there is disclosed a method of installing strake elements comprising providing a plurality of strake elements, each comprising an arc angle from 30° to 180°; and connecting each strake element to at least one other strake element about a circumference of a structural element. In some embodiments, at least one of the strake elements has been produced by extruding the element through a die. In some embodiments, at least one of the strake elements has been produced by forming the element in a mold. In some embodiments, at least one of the strake elements has been produced by placing a moldable sheet on a strake shape, forming the moldable sheet about the desired strake shape, and removing the moldable sheet from the shape. In some embodiments, the method also includes applying suction through the strake shape, to force the moldable sheet about the desired shape. In some embodiments, connecting each strake element to at least one other strake element comprises bolting the elements to each other. In some embodiments, connecting each strake element to at least one other strake element comprises welding the elements to each other. In some embodiments, connecting each strake element to at least one other strake element comprises overlapping adjacent elements to form a desired strake shape. In some embodiments, the structural element comprises a structure selected from the group consisting of a tubular, a pipe, a rod, a buoyancy device, a riser, and a mooring line.

Referring first to Figure 1, there is illustrated offshore system 100, with which the invention may be used. System 100 includes floating platform, 110 with facilities 105 on top. Platform is floating in a body of water having water surface 115 and bottom of the body of water 135. Buoyancy device 120 keeps platform 110 from sinking. Riser 125 connects platform 110 with well 140. Mooring lines 130 anchor platform 110 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 of more of buoyancy device 120, riser 125, and/or mooring lines 130. Suitable structural elements may include tubulars, pipes, rods, buoyancy device 120, riser 125, and/or mooring lines 130. Vortex induced vibration (VIV) may also cause vibration of other subsea structural elements to which the invention may be applied.

Referring now to Figures 2a-2c, in some embodiments of the invention, structural element 204 is illustrated. Structural element 204 encloses passage 202. Strake elements 206a, 206b, 206c, and 206d may be mounted about the circumference of structural element 204. Strake elements 206a-206d serve to inhibit vibration when structural element 204 is in a flowing fluid stream.

Structural element 204 has outside diameter D 218. Strake elements 206a-206d have a height H 220. Adjacent strake elements may be spaced apart by an arc-angle 222. In some embodiments of the invention, outside diameter D 218 may be from about 2 to 60 cm. In some embodiments of the invention, height H 220 may be from about 5% to about 50% of outside diameter D 218. hi some embodiments of the invention, height H 220 may be from about 1 to about 15 cm. In some embodiments of the invention, arc-angle 222 maybe from about 30 to about 180 degrees. In some embodiments of the invention, arc-angle L 222 may be from about 60 to about 90 degrees.

Referring now to Figures 3a-3b, in some embodiments of the invention, two helical- shape strake elements 306a and 306b may be installed about structural element 304. Structural element 304 encloses fluid passage 302. Strake elements 306a and 306b extend outside the circumference of structural element 304. Strake elements 306a and 306b have rectangular strake shapes that may be mounted at an angle relative to a longitudinal axis 305 of structural element 304. An end view of structural element 304 and strake elements 306 may be seen in Figure 3b showing rectangular strake shapes.

Structural element 304 has outside diameter D 328. Strake elements 306a and 306b have height H 330. Adjacent strake elements 306a-306d may be spaced apart by pitch L 332, and there may be two strake starts: strake elements 306a and 306b.

In some embodiments of the invention, outside diameter D 328 may be from about 2 to about 100 cm. In some embodiments of the invention, height H 330 may be from about 5% to about 50% of outside diameter D 328. In some embodiments of the invention, height H 330 may be from about 1 to about 20 cm. In some embodiments of the invention, pitch L 332 may be from about ID to about 10D. In some embodiments of the invention, pitch L 332 may be from about 10 to about 500 cm.

In some embodiments of the invention, the number of strake starts may be from about 2 to about 10, for example from about 4 to about 6.

Referring now to Figures 4a-4b, in some embodiments of the invention, a 4-start strake system may be installed about structural element 404. Structural element 404 encloses passage 402. Strake elements 406a, 406b, 406c, and 406d may be spaced about the circumference of structural element 404. In some embodiments of the invention, strake elements 406a-406d may be rigidly connected to first support 410 at one end, and second support 412 at the other end, as shown in Figure 4b. Strake elements 406a-406d may be helical strake elements mounted about structural element 404. Supports 410 and 412 may be locked relative to each other, for example by locking mechanism 414, as shown in Figure 4b. Structural element 404 has outside diameter D 458. Strake elements 406a-406d have height H 460. Strake elements 406a-406d may be spaced apart by pitch L 462.

In some embodiments of the invention, outside diameter D 458 may be from about 3 to about 50 cm. In some embodiments of the invention, height H 460 may be from about 5% to about 50% of outside diameter D 458. In some embodiments of the invention, height H 460 may be from about 2 to about 10 cm. In some embodiments of the invention, pitch L 462 may be from about ID to about 10D. In some embodiments of the invention, pitch L 462 may be from about 10 to about 100 cm.

Strake elements 406a-406d have a rectangular strake shape forming an angle relative to axis 450.

In some embodiments of the invention, referring to Figure 4c, strake elements 406a- 406d may be mounted about structural element 404. Bands 422, 424, 426, 428, and 430 may be placed about the circumference of structural element 404, to lock strake elements 406a- 406d in place. Strake elements 406a-406d form an angle with axis 450 of structural element 404.

In some embodiments of the invention, there may be about 2 to about 10 helical strake starts about a circumference of structural element 404. In some embodiments of the invention, there may be about 3 to about 6 helical strake starts about a circumference of structural element 404. rn some embodiments of the invention, there may be about 4 helical strake starts about a circumference of structural element 404.

Referring now to Figures 5a- 5b, in some embodiments of the invention, strake element system 500 is illustrated. System 500 includes structural element 504 enclosing passage 502. Strake elements 506a, 506b, 506c, and 506d may be placed about structural element 504, and connected to each other. In some embodiments of the invention, elements 506a-506d may be formed into the desired shapes, and then placed about structural element 504, for example by bending a sheet, molding the elements, vacuum forming the elements, and/or extruding the elements. Each of strake elements 506a, 506b, 506c, and 506d covers an arc-angle about a circumference of structural element 504 of about 90°.

In some embodiments, there may be provided about two strake elements each covering an arc-angle about a circumference of structural element 504 of about 180°. In some embodiments, there may be provided about three strake elements each covering an arc-angle about a circumference of structural element 504 of about 120°. In some embodiments, there may be provided about five strake elements each covering an arc-angle about a circumference of structural element 504 of about 72°. In some embodiments, there may be provided about six strake elements each covering an arc-angle about a circumference of structural element 504 of about 60°. In some embodiments, there may be provided from about two to about 12 strake elements each covering an arc-angle about a circumference of structural element 504 from about 30° to about 180°.

Structural element 504 has outside diameter D 558. Strake elements 506a-506d have height 560. Adjacent strake elements 506a-506d may be spaced apart by pitch L 562. In some embodiments of the invention, outside diameter D 558 may be from about 2.5 to 50 cm. In some embodiments of the invention, height H 560 may be from about 5% to about 50% of outside diameter D 558. In some embodiments of the invention, height H 560 may be from about 1.25 to 15 cm. In some embodiments of the invention, pitch L 562 may be from about ID to about 10D. In some embodiments of the invention, pitch L 562 may be from about 15 cm to 60 cm.

Referring now to Figure 5 c, in some embodiments of the invention, a method of connecting elements 506a-506d together is illustrated. Bolt 510a and nut 510b maybe used to connect element 506a and element 506b at the strake portion of those elements. Bolt 512a and nut 512b may be used to connect elements 506b and 506c. Bolt 514a and nut 514b maybe used to connect elements 506c and element 506d. Bolt 516a and nut 516b may be used to connect element 506d and element 506a.

In some embodiments of the invention, referring to Figure 5d, a method of connecting elements 506a-506d together is illustrated. Heat weld 520 is used to secure a portion of element 506a and 506b at the strake portion where the two elements meet. Heat weld 522 has been used to connect element 506b and 506c. Heat weld 524 has been used to connect element 506c and element 506d. Heat weld 526 has been used to connect element 506d and 506a.

In some embodiments of the invention, other methods of connecting elements 506a- 506d may be used, for example, rivets, male and female connections such as a pin and a groove, an end of one element fitting into a slot on an adjacent element, or other mechanical connections as are known in the art.

Referring now to Figures 6a-6c, in some embodiments of the invention, strake element system 600 is illustrated. System 600 includes structural element 604 enclosing passage 602. Individual strake elements 606 may be inter-connected, as shown in Figure 6b, so that element 606a overlaps element 606b, which overlaps element 606c, which overlaps element 606d, which overlaps element 606a. In some embodiments of the invention, elements 606a-606d may be identical. In some embodiments, elements 606a-606d overlap to form a trapezoidal strake shape.

Structural element 604 has outside diameter D 658. Strake elements 606a-606d have height H 660. Adjacent strake elements 606a-606d may be spaced apart by pitch L 662.

In some embodiments of the invention, outside diameter D 658 may be from about 3 to about 50 cm. In some embodiments of the invention, height H 660 may be from about 5% to about 50% of outside diameter D 658. In some embodiments of the invention, height H 660 may be from about 5 to 15 cm. In some embodiments of the invention, pitch L 662 may be from about ID to about 10D. In some embodiments of the invention, pitch L 662 may be from about 10 cm to 50 cm.

In some embodiments of the invention, four elements 606a-606d each encompassing a 90° arc-angle may be used about the circumference of structural element 604. In some embodiment, three elements, each encompassing a 120° arc-angle about circumference of structural element 604 may be used. In some embodiments of the invention, about 2, 5, 6, or more elements may be used to form strake elements about structural element 604.

In some embodiments of the invention, elements 606 when interconnected form a helical structure about structural element 604, as shown in Figure 6c.

In some embodiments of the invention, elements 606 form a strake shape having an angle with axis 650. Referring now to Figures 7a-7e, in some embodiments of the invention, a method and apparatus for forming strake elements will be illustrated. Strake element manufacturing system 700 includes shell 702 enclosing chamber 704 with holes 706 defined within shell 702 into chamber 704. Shell 702 defines strake shape 708. In some embodiments of the invention, as illustrated in Figure 7b, shell 702 may be used to manufacture a strake element with a long pitch and/or a large number of starts. In some embodiments of the invention, referring to Figure 7c, shell 702 may be used to manufacture a strake element with a short pitch and/or a small number of starts.

In operation, referring to Figures 7d-7e, moldable sheet 720 is placed above shell 702. Sheet 720 is heated or otherwise processed so that it is moldable, and then placed on top of shell 702. Suction is applied to chamber 704, which applies suction to holes 706, to force sheet 720 about shell 702 to take on desired strake shape 708. After sheet has taken on the desired shape, it may be cooled or otherwise processed and then removed from shell 702, and another sheet placed over shell 702. In some embodiments of the invention, sheet 720 may be made of a polymer, such as a thermoplastic polymer or a thermosetting polymer, for example polypropylene, polyethylene, other polyolefins, or co-polymers of olefins.

In some embodiments of the invention, the strake elements may be made in sections of varied length, based on oven capabilities at the molding facility. The strake elements may be manufactured in halves, or could be manufactured in thirds or more, for example quadrants. The molding process described is not limited to a specific number of starts.

In some embodiments of the invention, the manufacturing method uses state-of-the-art vacuum forming techniques. A pattern shell 702 may be built which represents the inside surface of the helical strake piece desired. The pattern shell 702 can be constructed from a wide array of materials.

In some embodiments of the invention, for small quantity runs, the pattern shell 702 used for the vacuum forming can be constructed from a material such as fiberglass. A fiberglass mold is laid up over the pattern, with a parting flange, which allows the fiberglass mold to be separated from the pattern. The fiberglass mold may be then dressed and used to lay up fiberglass tooling shell 702 which will have appropriate holes 706 and vacuum access attached. The tooling shell 702 can then be used to form helical strake sections from any of a variety of thermoplastic compounds, for example polypropylene, polyethylene, or copolymers of olefins.

In some embodiments of the invention, for large production runs, the pattern can be used to make a sturdy tooling shell 702 from aluminum or other materials using sand casting or other state-of-the-art manufacturing techniques.

Referring now to Figures 8a-8c, in some embodiments of the invention, a method and apparatus for forming strake elements will be illustrated. Strake element manufacturing system 800 includes die 806a through which a strake element 814 may be extruded. One suitable strake shape 808a is shown in die 806a, which may be used to form helical strake elements 814, which may be connected to each other and installed about a structural element. Another suitable strake shape 808b is shown in die 806b, which may be used to form helical strake elements 814, which may be connected to each other and installed about a structural element. System 800 also includes extruder 810, for example a screw extruder or a piston which may be used to melt and pressurize a polymer or other meltable material such as metal, then force it through die 806. Hopper 812 may be used to store the extrudable material prior to being introduced into extruder 810.

In operation, a material may be introduced into hopper 812. Gravity and/or extruder 810 then pull material into extruder 810. Material may be melted and forced through die 806 to form desired strake shape 814. A cooling means, for example a cooling bath or a fan may be provided downstream of the die 806 to cool the strake 814.

In some embodiments of the invention, strake sections may be applied to one side of a riser in a manner that all of the strake sections may be mounted to the riser away from the rollers. After the riser, with the attached strake sections, passes through the rollers, the strake sections may be spaced equidistant around the riser with the correct spacing and pitch, for example manually or by an ROV.

In some embodiments of the invention, strake sections may be attached to the riser with an interference fit, which may eliminate the need to use collars. The flanges of the strake sections could be shaped in an "A configuration" rather than flat to allow some flange flexibility to provide an interference fit to the riser. The flanges would be fastened using snaps or flange wraps. In some embodiments of the invention, a hinged upper collar and a hinged lower collar may be mounted to cylindrical elements, such that individual strake sections would insert into the collars and lock on the riser without the use of bands or other fastening devices. Velcro may be used on the upper flanges of the strake section to temporarily hold the strake sections to the riser while the upper collar may be installed. The sequence of operations might be to install the lower collar first. Then install the strake sections about a cylindrical element. Last install the upper collar to lock the strake sections in place about the cylindrical element. The collars could be designed such that the lower portion of the collar would lock in the upper flanged strake sections and the upper portion of collar would provide the base to install the next section of flanged strake sections.

In some embodiments of the invention, there is disclosed a system of reducing vibration, including a structural element, a plurality of strake elements spaced about a circumference of the cylindrical element, wherein the cylindrical element comprises a longitudinal axis, and wherein the strake elements may be substantially aligned with the axis, and wherein the strake elements may be attached at a first end to a first support, and wherein the stake elements may be attached at a second end to a second support, wherein the first support and the second support may be adapted to be rotated relative to one another, so that the stake elements form a helix about the cylindrical element. In some embodiments of the invention, the number of stake elements may be about 2 to about 6 stake elements. In some embodiments of the invention, the number of stake elements may be about 4 stake elements. In some embodiments of the invention, the system also includes a locking device to rotationally lock the first support and the second support relative to one another. In some embodiments of the invention, the stakes have a height of about 5% to about 50% of an outside diameter of the cylindrical element. In some embodiments of the invention, the stake elements have a height of about 0.5 to about 15 cm. In some embodiments of the invention, the stake elements may be spaced apart by a length of about 15 to about 150 cm. In some embodiments of the invention, the stake elements may be spaced apart by length of about 1 to about 20 outside diameters of the cylindrical element.

In some embodiments of the invention, there is disclosed a method of reducing vibration in a cylindrical element, including providing a structural element, placing a plurality of stake elements about a circumference of the cylindrical element, substantially in alignment with a longitudinal axis of the cylindrical element, attaching a first end of the strake elements to a first support, attaching a second end of the strake elements to a second support, and rotating the first support relative to the second support, to impart a helical twist to the strake elements. In some embodiments of the invention, the method also includes locking the first support relative to the second support. In some embodiments of the invention, the number of strake elements may be from about 2 to about 6. In some embodiments of the invention, the number of strake elements may be about 4.

In some embodiments of the invention, there is disclosed a system for reducing vibration, including a cylindrical element, a plurality of strake elements about a circumference of the cylindrical element, the strake elements substantially aligned with a longitudinal axis of the cylindrical element, and a device adapted to receive the strake elements and impart a desired pitch to the strake elements about the circumference of the cylindrical element, hi some embodiments of the invention, the system also includes a plurality of locking devices adapted to secure the strake elements at the desired pitch. In some embodiments of the invention, there is disclosed a method of minimizing vibration, including providing a cylindrical element, placing a plurality of strake elements substantially aligned with a longitudinal axis of a cylindrical element, the strake elements spaced about a circumference of the cylindrical element, sliding a device over the strake elements to impart a desired pitch to the strake elements, hi some embodiments of the invention, the method also includes locking the strake elements in place at the desired pitch. In some embodiments of the invention, the number of strake elements may be from about 2 to about 6, for example about 4. In some embodiments of the invention, the desired pitch comprises an angle of about 15 to about 75°. In some embodiments of the invention, the desired pitch comprises an angle of about 30 to about 60°. In some embodiments of the invention, there is disclosed an apparatus, including a structural element, a plurality of strake elements, each covering an arc segment about a circumference of the cylindrical element, and a mechanism for attaching the strake elements to each other to cover the entire outer circumference of the cylindrical element. In some embodiments of the invention, the mechanism for attaching comprises a plurality of bolts and nuts. In some embodiments of the invention, the mechanism for attaching the strake elements comprises a plurality of heat welds. In some embodiments of the invention, the apparatus includes 4 strake elements, each strake element comprising an arc angle of about 90°. In some embodiments of the invention, each strake element overlaps the next strake element, to form a plurality of strake shapes about the circumference of the cylindrical element. In some embodiments of the invention, there is disclosed a system for manufacturing strake elements, including a shell, the shell defining an interior chamber and an exterior surface, the exterior surface defining a desired strake shape, the exterior surface comprising a plurality of holes defined therethrough, wherein the system may be adapted to receive a moldable sheet which can be forced about the desired strake shape. In some embodiments of the invention, there is disclosed a method of manufacturing a strake element, including placing a moldable sheet on a strake shape, heating the moldable sheet to form about the desired strake shape, cooling the moldable sheet, and removing the moldable sheet from the shape. In some embodiments of the invention, the method also includes applying suction through the strake shape, to force the moldable sheet about the desired shape. In some embodiments of the invention, the strake shape defines a helix in a longitudinal direction.

In some embodiments of the invention, clamshell type strake elements maybe mounted around a pipe according to the method disclosed in United States Patent No. 6,695,539, which is herein incorporated by reference in its entirety. In some embodiments of the invention, strake elements may be installed about a pipe according to the method disclosed in United States Patent No. 6,561,734, which is herein incorporated by reference in its entirety.

In some embodiments of the invention, strake elements may be installed about a pipe according to the method disclosed in United States Patent Application Publication No. 2003/0213113, which is herein incorporated by reference in its entirety.

Ih some embodiments of the invention, the outside diameter of a pipe to which strake elements can be attached may be from about 10 to about 50 cm. In some embodiments of the invention, the height of strake elements may be from about 5% to about 50% of the pipe's outside diameter. In some embodiments of the invention, the height of strake elements may be from about 5 to about 20 cm. In some embodiments of the invention, arc-angle between adjacent strake elements maybe from about 30 to about 180 degrees. In some embodiments of the invention, arc-angle between adjacent strake elements may be from about 60 to about 90 degrees.

In some embodiments of the invention, the structural element maybe cylindrical, or have an elliptical, oval, or polygonal cross-section, for example a square, pentagon, hexagon, or octagon.

In some embodiments of the invention, suitable risers 125, buoyancy devices 120, and/or mooring lines 130 have an outside diameter of about 5 to 100 cm. In some embodiments of the invention, suitable risers 125, buoyancy devices 120, and/or mooring lines 130 have a outside diameter of about 10 to 50 cm. In some embodiments of the invention, suitable risers 125, buoyancy devices 120, and/or mooring lines 130 have a outside diameter ofabout 20 to 30 cm.

In some embodiments of the invention, suitable risers 125, buoyancy devices 120, and/or mooring lines 130 have a wall thickness of about 0.1 to 5 cm. In some embodiments of the invention, suitable risers 125, buoyancy devices 120, and/or mooring lines 130 have a wall thickness of about 0.2 to 3 cm. In some embodiments of the invention, suitable risers 125, buoyancy devices 120, and/or mooring lines 130 have has a wall thickness of about 0.5 to 2 cm.

In some embodiments of the invention, suitable risers 125, buoyancy devices 120, and/or mooring lines 130 may be made of a carbon steel pipe.

Those of skill in the art will appreciate that many modifications and variations are possible in terms of the disclosed embodiments, configurations, materials and methods without departing from their spirit and scope. Accordingly, the scope of the claims appended hereafter and their functional equivalents should not be limited by particular embodiments described and illustrated herein, as these are merely exemplary in nature.

Claims

C L A I M S

1. An apparatus, comprising: a plurality of strake elements, each strake element adapted to cover an arc-angle about a circumference of a structural element; and a mechanism adapted to attach the strake elements to each other to cover at least a portion of the circumference of the structural element.

2. The apparatus of claim 1, wherein the structural element comprises a structure selected from the group consisting of a tubular, a pipe, a rod, a buoyancy device, a riser, and a mooring line.

3. The apparatus of one or more of claims 1-2, wherein the mechanism for attaching comprises a plurality of bolts and nuts.

4. The apparatus of one or more of claims 1-3, wherein the mechanism for attaching the stake elements comprises a plurality of heat welds.

5. The apparatus of one or more of claims 1-4, comprising 4 helical stake elements, each stake element comprising an arc-angle from 70° to 120°, for example from 80° to 100°.

6. The apparatus of one or more of claims 1-5, wherein each strake element overlaps the next stake element, to form a plurality of stake shapes about the circumference of the cylindrical element, the stake shapes selected from the group consisting of triangles, rectangles, and trapezoids.

7. The apparatus of one or more of claims 1-6, wherein the cylindrical element comprises an outside diameter from 5 to 60 cm and/or wherein the stake elements have a height from 5% to 50% of an outside diameter of the structural element and/or wherein the stake elements are helical.

8. The apparatus of one or more of claims 1-7, further comprising the structural element about which the plurality of strake elements has been attached.

9. The apparatus of one or more of claims 1-8, comprising from 2 to 8 strake elements.

10. The apparatus of one or more of claims 1-9, comprising a pitch between each strake element from 0.1 to 10 meters.

11. A method of installing strake elements comprising: providing a plurality of strake elements, each comprising an arc angle from 30° to

180°; and connecting each strake element to at least one other strake element about a circumference of a structural element.

12. The method of claim 11, wherein at least one of the strake elements has been produced by extruding the element through a die.

13. The method of one or more of claims 11-12, wherein at least one of the strake elements has been produced by forming the element in a mold.

14. The method of claim 11, wherein at least one of the strake elements has been produced by placing a moldable sheet on a strake shape, forming the moldable sheet about the desired strake shape, and removing the moldable sheet from the shape.

15. The method of claim 14, further comprising applying suction through the strake shape, to force the moldable sheet about the desired shape.

16. The method of one or more of claims 11-15, wherein connecting each strake element to at least one other strake element comprises bolting the elements to each other.

17. The method of one or more of claims 11-16, wherein connecting each strake element to at least one other strake element comprises welding the elements to each other.

18. The method of one or more of claims 11-17, wherein connecting each strake element to at least one other strake element comprises overlapping adjacent elements to form a desired strake shape.

19. The method of one or more of claims 11-18, wherein the structural element comprises a structure selected from the group consisting of a tubular, a pipe, a rod, a buoyancy device, a riser, and a mooring line.