STRAKE SYSTEMS AND METHODS

Field of Invention:

The present disclosure relates to strake systems and methods. Background:

Structural elements can be installed at sea from a floating vessel using a J-lay configuration where the structural element is held vertically on the vessel and dropped vertically into the water and then when it reaches the bottom of the body of water, it lays horizontal, or alternatively structural elements can be installed in a S-lay configuration where the structural element is held horizontally on the vessel, drops to vertical through the body of water, and then rests on the bottom of the body of water in a horizontal configuration. Other configurations for installing a structural element from a vessel in a body of water are also known.

Referring now to Figure 1, system 100 for installing structural element 114 on bottom 116 of body of water 112 is illustrated. System 100 includes vessel 110 with tensioner 120 and stinger 118. Tensioner 120 holds structural element 114 in a horizontal configuration as it enters water, and then structural element 114 rolls down stinger 118, then drops to a vertical configuration, and then back to a horizontal configuration as it lays on bottom 116. Tensioner 120 and vessel 110 have a sufficient capacity to support structural element 114 as it is being installed.

Currents in body of water 112 may cause vortexes to shed from the sides of structural element 114. When these types of structural elements, such as a cylinder, experience a current in a flowing fluid environment, it is possible for the structural element 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 structural element, especially if the forcing frequency is at or near a structural natural frequency.

The magnitude of the stresses on a structural element is generally a function of and increases with the velocity of the water current passing these structural elements and the length of the structural element.

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 structural element ("vortex-induced vibrations") in a direction perpendicular to

the direction of the current. When fluid flows past the structural element, vortices may be alternately shed from each side of the structural element. This produces a fluctuating force on the structural element transverse to the current. If the frequency of this harmonic load is near the resonant frequency of the structural element, large vibrations transverse to the current can occur. These vibrations can, depending on the stiffness and the strength of the structural element 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 structural elements 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 structural element in the direction of the current due to the structural element's resistance to fluid flow. The drag forces may be amplified by vortex induced vibrations of the structural element. 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 structural elements operate by modifying the boundary layer of the flow around the structural element to prevent the correlation of vortex shedding along the length of the structural element. Examples of such devices include sleeve-like devices such as helical strake elements, shrouds, fairings and substantially cylindrical sleeves. Some VIV and drag reduction devices can be installed on risers and similar structural elements before those structural elements may be deployed underwater. Alternatively, VIV and drag reduction devices can be installed on structural elements after those structural elements have been deployed underwater.

When installing a structural element in an S-lay configuration, the structural element may travel over a stinger and encounter one or more rollers on the stinger. A pre- installed strake may be damaged if it passes over the stinger. One alternative is to install the strakes on the structural element after it passes over the rollers and the stinger. Another alternative is to protect the strakes as they are passed over the rollers and the stinger.

U.S. Patent Number 6,896,447 discloses a vortex induced vibration suppressor and method. The apparatus includes a body that is a flexible member of a polymeric (e.g., polyurethane) construction. A plurality of helical vanes on the body extend longitudinally along and helically about the body. Each vane has one or more openings extending transversely there through. A longitudinal slot enables the body to be spread apart for

placing the body upon a riser, pipe or pipeline. Tensile members that encircle the body and pass through the vane openings enable the body to be secured to the pipe, pipeline or riser. U.S. Patent Number 6,896,447 is herein incorporated by reference in its entirety.

Copending U.S. Patent Application having serial number 11/468,690 and Attorney Docket number TH2926 was filed on August 30, 2006, and discloses a system comprising a structural element, at least one helical strake about the structural element, and at least one ramp to provide a transition from the structural element to the helical strake. Copending U.S. Patent Application having serial number 11/468,690 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 for new and improved methods of installing strake elements for suppressing vibration in a flowing fluid environment. There is another need in the art for new and improved methods of installing strake elements for suppressing vibration in a flowing fluid environment on a structural element before the structural element is installed over a ramp or roller. There is another need in the art for new and improved methods of installing strake elements for suppressing vibration in a flowing fluid environment on a structural element before the structural element is installed in the flowing fluid environment which does not require intervention or adjustment of the strake elements once the structural element is in the 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 provides a system comprising a structural element; at least one strake holder connected to the structural element; and at least one flexible helical strake connected to the at least one strake holder.

Another aspect of the invention provides a method of installing a structural element in a body of water comprising attaching at least one flexible helical strake about the structural element; and moving the structural element and the flexible strake over a roller, so that the flexible strake is temporarily deformed when the flexible strake interfaces with the roller.

Advantages of the invention include one or more of the following: improved apparatuses and methods for suppressing vibration;

improved systems and methods of installing strake elements for suppressing vibration in a flowing fluid environment; improved systems and methods of installing strake elements for suppressing vibration in a flowing fluid environment on a structural element before the structural element is installed over a ramp or roller; and improved systems and methods of installing strake elements for suppressing vibration in a flowing fluid environment on a structural element before the structural element is installed in the flowing fluid environment which does not require intervention or adjustment of the strake elements once the structural element is in the flowing fluid environment.

Brief Description of the Drawings

Figure 1 illustrates a system for installing a structural element in a body of water in an S-lay configuration.

Figure 2 illustrates a system for installing a structural element in a body of water in an S-lay configuration.

Figures 3a and 3b illustrate a structural element with strakes.

Figures 4a-4c illustrate a structural element with strakes traveling over a stinger.

Figures 5a-5b illustrates a structural element with strakes Detailed Description of the Invention In one embodiment, there is disclosed a system comprising a structural element; at least one strake holder connected to the structural element; and at least one flexible helical strake connected to the at least one strake holder. In some embodiments, the structural element is selected from the group consisting of a shell, a collar, 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 column structures for tension leg platforms and for spar type structures. In some embodiments, the structural element comprises a plurality of sections welded or threaded to each other. In some embodiments, the at least one flexible helical strake comprises a flexible material having a Young's Modulus E from 0.01 to 0.5 GPa. In some embodiments, the at least one flexible helical strake comprises at least three flexible helical strakes. In some embodiments, the at least one strake holder comprises a high strength

material having a Young's Modulus E from 0.5 to 500 GPa. In some embodiments, the at least one flexible helical strake comprises a rubber strake having a t-shaped cross section, at least a portion of the strake extending out of the strake holder. In some embodiments, a first end of the at least one strake holder is attached to a first collar, and a second end of the at least one strake holder is attached to a second collar, the first collar and the second collar attached about the structural element.

In one embodiment, there is disclosed a method of installing a structural element in a body of water comprising attaching at least one flexible helical strake about the structural element; and moving the structural element and the flexible strake over a roller, so that the flexible strake is temporarily deformed when the flexible strake interfaces with the roller. In some embodiments, the structural element is selected from the group consisting of a shell, a collar, 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 column structures for tension leg platforms and for spar type structures. In some embodiments, the structural element comprises a plurality of sections welded to each other. In some embodiments, the structural element comprises a plurality of sections threaded to each other. In some embodiments, attaching at least one helical strake about the structural element comprises attaching at least three helical strakes about the structural element. In some embodiments, attaching at least one helical strake about the structural element comprises attaching a plurality of flexible sheets to each other. In some embodiments, attaching at least one helical strake about the structural element comprises attaching a plurality of flexible sheets to each other with an adhesive. In some embodiments, the first roller is azimuthally spaced apart from the second roller by 90 to 150 degrees measured as an arc angle of the structural element.

In one embodiment, there is disclosed a system comprising a structural element; at least one strake sheet connected to at least one other strake sheet about the structural element to form at least two strakes. In some embodiments, the at least one strake sheet comprises a flexible material having a Young's Modulus E from 0.00001 to 0.5 GPa. In some embodiments, the at least one strake sheet comprises a rubber strake sheet. In some embodiments, the system also includes at least three strakes.

Referring now to Figure 2, in one embodiment of the invention, system 200 is illustrated. System 200 includes vessel 210 in body of water 212, installing structural element 204 in body of water 212 and resting a portion of structural element 204 on bottom 216. Vessel 210 may include tensioner 220 to keep tension on structural element 204 so that it does not sink in water 212. Flexible strakes 206 are attached about structural element 204 to dampen any vortex induced vibration of structural element 204.

Referring now to Figures 3a-3b, in some embodiments of the invention, structural element 304 is illustrated. Structural element 304 encloses passage 302. Strake holders 306a, 306b, and 306c may be mounted about the circumference of structural element 304. Flexible strake elements 308a, 308b, and 308c are inserted into strake holders 306a-306c, respectively. Strake holders 306a, 306b, and 306c provide a high strength structure attached to structural element 304 to retain flexible strake elements 308a-308c. Flexible strake elements 308a-308c serve to inhibit vortex induced vibration when structural element 304 is in a flowing fluid stream. In some embodiments, flexible strake elements 308a, 308b, and 308c fit fully inside the strake holders 306a, 306b, and 306c. In some embodiments, flexible strake elements 308a, 308b, and 308c are of the same cross section as the inside of the strake holders 306a, 306b, and 306c. In some embodiments, flexible strake elements 308a, 308b, and 308c extend out of the strake holders 306a, 306b, and 306c as shown in Figure 3a. In some embodiments, flexible strake elements 308a, 308b, and 308c may alternately fit inside the strake holders 306a, 306b, and 306c when compressed, for example by a roller, and extend out of the strake holders 306a, 306b, and 306c as shown in Figure 3a when not compressed. In some embodiments, flexible strake elements 308a, 308b, and 308c may comprise an elastic material that can be compressed into strake holders 306a, 306b, and 306c, and then regain shape outside of strake holders 306a, 306b, and 306c when not under compression.

In some embodiments, strake holders 306a, 306b, and 306c may be cut along their longitudinal axis to reduce their effective stiffness or to allow the flexible strake elements 308a, 308b, and 308c to extend outside of the strake holders. These cuts may vary in length and density along the strake holder.

Structural element 304 has outside diameter D 328. Strake elements 308a-308c have height H 330. Adjacent strake elements may be spaced apart by a pitch L 332. In some embodiments of the invention, outside diameter D 328 may be from about 2 to 60

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 15 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, strake elements 308a-308c may comprise a flexible material, for example rubber, poly butadiene, or polyurethane. In some embodiments, strake elements 308a-308c may have a Young's Modulus E from about 0.01 to about 0.5 giga- pascals (GPa), for example from about 0.1 to about 0.4 giga-pascals (GPa), or for example from about 0.001 to about 0.05 giga-pascals (GPa).

In some embodiments, strake holders 306a-306c may comprise a high strength material, for example aluminum, steel, stainless steel, copper, nylon, polyethylene, polypropylene, a thermoset polymer, and polyvinyl chloride. In some embodiments, strake holders 306a-306c may have a Young's Modulus E from about 0.6 to about 400 giga- pascals (GPa), for example from about 0.75 to about 200 giga-pascals (GPa), or for example from about 1 to about 50 giga-pascals (GPa).

In some embodiments of the invention, there may be about 1 to about 10 helical strake starts about a circumference of structural element 304. In some embodiments of the invention, there may be about 2 to about 6 helical strake starts about a circumference of structural element 304. In some embodiments of the invention, there may be about 3 helical strake starts about a circumference of structural element 304.

In some embodiments of the invention, strakes 308a-308c may be made of a flexible material, such as a polymer, for example a thermoplastic polymer: polypropylene, polyethylene, polybutylene, other polyolefins, or co-polymers of olefins. In some embodiments of the invention, strakes 308a-308c may be made of a composite, such as fiberglass or carbon fiber composite. In some embodiments of the invention, strakes 308a- 308c may be made of a metal, such as steel or aluminum.

In some embodiments of the invention, strakes 308a- 308c may be attached to a support apparatus. The support apparatus and strakes 308a-308c may then be installed into strake holders 306a-306c about structural element 304.

Referring now to Figures 4a-4c, in some embodiments of the invention, stinger 418 and structural element 404 are illustrated. Stinger 418 includes roller 419a and roller 419b which are adapted to transport structural element 404. Structural element 404 is able to roll

down stinger 418 while resting on rollers 419a and 419b. In some embodiments of the invention, rollers 419a and 419b may be azimuthally spaced from about 90 to about 150 degrees apart, measured as an arc-angle of structural element 404.

Referring now to Figure 4b, structural element 404 is shown in cross-section moving along stinger 418. Structural element 404 encloses passage 402 and has attached to its exterior strake holders 406a, 406b, and 406c. Flexible strakes 408a, 408b, and 408c are attached to strake holders 406a, 406b, and 406c. Stinger has rollers 419a and 419b, which interface with an exterior of structural element 404 to support structural element 404 and allow structural element to roll along stinger 418. Flexible strake 408c is bent as it is interfacing with stinger 418. In some embodiments, flexible strakes 408a, 408b, and 408c are made of an elastic material that is able to deform when it encounters other structures and then return to its original shape.

Referring now to Figure 4c, in some embodiments of the invention, structural element 404 of Figure 4b has moved further along so that strake 408b is interfacing with roller 419b, and strake 408c is interfacing with roller 419a. Strake 408b has temporarily deformed and strake 408c has temporarily deformed, so that the strakes are not damaged when they encounter the rollers.

In some embodiments of the invention, strakes 408a-408c may be attached to a collar, pipe, shell, or other support apparatus. The support apparatus and strakes 408a- 408c may then be installed about structural element 404.

Referring now to Figures 5a and 5b, in some embodiments of the invention, structural element 504 is illustrated. Structural element 504 encloses passage 502. Flexible sheets 506a, 506b, and 506c may be mounted about the circumference of structural element 504. Connector 508a holds together a portion of flexible sheets 506a and 506b, to form flexible strake element 510a. Connector 508b holds together a portion of flexible sheets 506b and 506c, to form flexible strake element 510b. Connector 508c holds together a portion of flexible sheets 506c and 506a, to form flexible strake element 510c. Flexible strake elements 510a-510c serve to inhibit vortex induced vibration when structural element 504 is in a flowing fluid stream. Structural element 504 has an outside diameter D 528. Strake elements 510a-510c have height H 530. Adjacent strake elements may be spaced apart by a pitch L 532. In some embodiments of the invention, outside diameter D 528 may be from about 2 to 60 cm. In some embodiments of the invention, height H 530 may be from about 5% to about

50% of outside diameter D 528. In some embodiments of the invention, height H 530 may be from about 1 to about 15 cm. In some embodiments of the invention, pitch L 532 may be from about ID to about 10D. In some embodiments of the invention, pitch L 532 may be from about 10 to about 500 cm. In some embodiments of the invention, there may be about 1 to about 10 helical strake starts about a circumference of structural element 504. In some embodiments of the invention, there may be about 2 to about 6 helical strake starts about a circumference of structural element 504. In some embodiments of the invention, there may be about 3 helical strake starts about a circumference of structural element 504. In some embodiments of the invention, flexible sheets 506a, 506b, and 506c may be made of a flexible material, such as a polymer, for example a thermoplastic polymer: polypropylene, polyethylene, polybutylene, other polyolefins, or co-polymers of olefins. In some embodiments of the invention, connectors 508a - 508c may be an adhesive, a staple, a bolt and a nut, a rivet, a weld, or other suitable connection for holding together flexible sheets 506a, 506b, and 506c.

In some embodiments of the invention, flexible sheets 506a, 506b, and 506c may comprise a flexible material having a Young's Modulus E from about 0.01 to about 0.5 giga-pascals (GPa), for example from about 0.01 to about 0.3 giga-pascals (GPa), or for example from about 0.01 to about 0.1 giga-pascals (GPa). In one embodiment, reinforced rubber sheet having a thickness from about a 0.02 diameter to about a 0.10 diameter thickness (measured as outside diameter D 528) is provided. The reinforced rubber sheet is cut in long strips to form flexible sheets 506a, 506b, and 506c. These flexible sheets 506a, 506b, and 506c are formed into a U shaped cross sections. The U shaped sections are placed on structural element 504 with a pitch from about 4 to about 30 times outside diameter D 528. The height of strake elements 510a- 510c that are formed with the adjacent U shaped sections is from about 0.1 to about 0.3 times outside diameter D 528. The adjacent U shaped sections are coated with a vulcanizing adhesive. The structural element 504 with the strakes attached is placed in an oven and heated to obtain rubber cure. In some embodiments of the invention, clamshell type strake elements may be mounted around a structural element 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 structural element 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 structural element according to the method disclosed in United States Patent Application Publication No. 2003/0213113, which is herein incorporated by reference in its entirety.

In some embodiments of the invention, the outside diameter of a structural element 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 structural element'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, the structural element may be cylindrical, or have an elliptical, oval, or polygonal cross-section, for example a square, pentagon, hexagon, or octagon. In some embodiments, portions of structural element 204 may be lowered onto bottom 216 of water 212. In some embodiments, water 212 has 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, water 212 has a depth up to about 10,000 meters.

In some embodiments of the invention, structural element 204 may be a pipeline, a crude oil flowline, a mooring line, a riser, a tubular, or any other structural element installed in a body of water. In some embodiments, structural element 204 may have a diameter from about 0.1 to about 5 meters, and a length from about 1 to about 200 kilometers (km). In some embodiments, structural element 204 may have a length to diameter ratio from about 100 to about 100,000. In some embodiments, structural element 204 may be composed from about 50 to about 30,000 tubular sections, each with a diameter from about 10 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.

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.