Abstract:
A mechanism to be applied to an exterior surface of a cylindrical structure for reduction of the effect of Vortex Induce Vibration (VIV) in the cylindrical structure when immersed in flowing fluid. The mechanism is provided with a generally cylindrical column having a central axis, an interior surface corresponding in size and shape to the exterior surface of the cylindrical structure to which the mechanism is to be applied and an outer surface defining a wall thickness. A reduced wall thickness is formed into the outer surface in a pattern to produce a discontinuity that interrupts the lengthwise coherence of vortex shedding of moving fluid from the outer surface when the cylindrical column is attached to the exterior of the cylindrical structure in the flowing fluid. The effect of VIV on the cylindrical structure is effectively reduced.  
     A submergible cylindrical assembly for positioning in a flowing body of water and having enhanced resistance to vortex induced vibration is disclosed. The cylindrical assembly comprises a cylinder having an axis, an outer surface and a wall thickness. The cylinder has a pattern cut into the outer surface thereof that selectively reduces the wall thickness of the cylinder such that the formation of vortices is reduced, thereby reducing or eliminating the lift force on the cylinder and reducing or eliminating the vortex induced vibration that may weaken or damage the cylinder.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates in general to the field of solid and hollow cylinders, such as risers, hoses, pilings and pipes immersed in a fluid subject to relative motion between the cylinder and the fluid. In particular, the invention relates to a device and mechanism and method for reducing vortex-induced vibration caused by relative movement of water past a cylinder and also to a cylindrical assembly incorporating the inventive mechanism.  
         BACKGROUND OF THE INVENTION  
         [0002]    Without limiting the scope of the invention, its background will be described primarily with reference to offshore risers used in sub-sea production wells as an example. Submerged cylindrically-shaped objects, such as risers, spars, or other elongated cylindrical structures used for under-sea oil or gas production, pumping, or loading are often exposed to relative movement of a body of fluid, particularly moving sea currents. Such elongated cylindrical structures are common in offshore petroleum exploration, production and transportation. Sometimes such elongated cylindrical structures extend from the surface to hundreds of meters below the surface, as in the case of spar platforms for production. Sometimes the cylindrical structures extend from the seabed thousands of meters upward toward the surface and into sea currents, as in offshore production risers, loading and unloading risers or hybrid risers for petrochemical production or transport. Cylindrical riser structures may support on their exterior or encase one or more pipelines or risers extending from the seabed to a drilling or production platform, to a ship or to another offshore structure or vehicle. Such risers or cylindrical riser support structures are continuously exposed to ocean currents that produce vortexes or vortices that tend to travel downstream with the current as the water moves around and past the risers. These vortices produce oscillating “lift” forces on the cylindrical structure as a result of vortex shedding and the spanwise, or lengthwise, coherence of the vortex shedding can produce substantial cumulative lift force on the elongated cylindrical structure. The effect is particularly adverse in the case of a cylindrical riser support column extending several hundreds of meters in the path of the current.  
           [0003]    The lift forces due to vortex shedding act generally normal to the axis of the cylindrical structure and flow direction. As a vortex is produced and then separated in a “sheet” from the cylindrical surface along the length or span of the cylinder exposed to the current, the lift force can be significant and destructive. The vortices are swirling currents that repeatedly shed from the cylinder, sometimes called “Von Karman Vortex Sheets” and produce vortex-induced vibration. The vibratory movement or vortex-induced vibration (VIV) Von Karman Vortex caused by the repeated sheet separation from the cylinder is sometimes called “Aeolian Vibration.” This vortex-induced vibration creates cyclic stresses on the cylindrical structure that may be too small to cause immediate fracture, but upon constant repetition may weaken or damage the riser through material fatigue or stress-induced fracture. In certain relatively common current situations, a resonant vibration can be created, causing repetitive forces in phase with the vibratory motion that can overstress the cylindrical structure to potentially catastrophic failure.  
           [0004]    In the past, fins protruding from the peripheral surface of the cylinders exposed to the current or other fluid movement, as in production riser situations, were used to reduce the adverse effect of such vortex formation and vortex sheet shedding. For example, helically-arranged vortex-shedding ribs, or strakes, have been designed to be installed on submerged risers exposed to ocean currents. In one prior device, such strakes are to be incorporated as components of a flexible wrap or panel to be disposed about and secured to the submerged riser. Typically the strakes are to be clamped to the riser prior to its being submerged. Such strakes could be formed by pairs of clamping flanges mounted along the adjacent edges of elongated parallelogram-shaped wrap segments. The wrap segments could be positioned side-by-side, twisting around the outer surface of the riser, and then bolted to engage at the clamp flanges, forming a helical strake extending in a spiral around and along the length of the cylindrical structure that will be exposed to moving current.  
           [0005]    In another design, one or more ribs or strakes could be attached vertically or diagonally on a flat, rectangular panel of flexible wrapping material. The wrapping material would be dimensioned to encircle, by itself, an elongated segment of a single riser, piling, pipe or other cylindrical object. Clamping flanges were to be mounted along opposed vertical edges of the rectangular panel. The clamping flanges were to be brought together and clamped, thereby stretching the panel to wrap securely around and frictionally embrace the outer surface of the riser. A plurality of such wrapped panels with ribs or strakes were to be clamped in deployed positions, along the length of the cylindrical structure such that the strakes were aligned at either end of adjacent panels in a helical configuration encircling the wrapped riser structure.  
           [0006]    It is difficult to transport, handle and install a cylindrical riser support structure having protruding strakes. Further, it has been found that installation underwater at the riser site is extremely difficult and usually impractical. It has been found that fabrication of a cylindrical riser structure with a protruding strake of a prior design is costly. Additionally, it has been found that the protruding strake on a cylindrical riser support structure increases the viscous drag of the water against the riser assembly, thereby risking greater stress and requiring increased size and strength for the riser support design.  
           [0007]    In certain riser installations, a polymeric coating and, in particular, a polymeric foam layer is applied to the exterior surface of the risers and the riser support cylinder to provide protection from the undersea environment and advantageously to provide buoyancy to the assembly. The riser itself may be composed of a metal or a composite material. The riser support structure is normally a metal support cylinder with the metal or composite cylindrical riser pipe lines and polymeric foam coating material attached to the surface of the metal cylinder to facilitate maintaining the riser and support structure in an upright position by reducing the combined mass density (i.e., by adding buoyancy). It has been found that securing strakes, of any prior known design, to the exterior of a layer of polymeric foam is difficult. For example, clamping of strakes to the polymeric surface often fails due to insufficient compression strength of the foam. Particularly, in the case of a polymeric foam coating or bundle on the riser or riser support cylinder, clamping tension may not be sufficient to maintain the strakes in a secure position. Excessive clamping tension can significantly reduce the buoyancy by crushing the foam layer.  
           [0008]    A need has therefore arisen for a device, mechanism and method to reduce, resist or suppress vortex induced vibration (VIV), or the effect of VIV on submergible cylinders such as risers and riser support columns, without requiring the attachment of a protruding strake. A need has also arisen for a submergible riser assembly with a VIV reduction mechanism attached that is easy to transport, easy to handle and easy to install and that is not costly to fabricate. In addition, a need has arisen for such a VIV reduction mechanism for fluid immerse cylindrical structures and assemblies, including submergible riser assemblies that does not significantly increase the viscous drag of moving fluid or moving water against the immersed cylinder or submerged riser assembly.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention disclosed herein comprises a device, mechanism and method for use in a generally cylindrical assembly that is resistant to vortex-induced vibration when immersed in a moving fluid. The generally cylindrical assembly of the present invention, and particularly in the case of a cylindrical riser assembly, is easy to transport, handle and install and is not costly to fabricate. In addition, a feature of one embodiment of a cylindrical assembly according to certain inventive aspects of the present invention is that the cylindrical assembly is submergible in a body of water and resists or reduces vortex-induced vibration (VIV) and does not significantly increase the viscous drag of the fluid or water moving past the cylindrical assembly.  
           [0010]    The vortex induced vibration (VIV) reduction mechanism of the present invention and the submergible cylindrical assembly of the present invention having such VIV reduction mechanism combined therewith effectively reduce the adverse effect of vortex-induced vibration when positioned in a flowing body of fluid such as water. The VIV reduction mechanism comprises a generally cylindrical column having a central axis, an outer surface, a wall thickness and a length. A pattern is cut or formed into the outer surface of the generally cylindrical column to selectively decrease the distance of the outer surface from the central axis. The pattern may be formed with a plurality of columnar sections each having a notch cut into the outer surface. A plurality of columnar sections are placed in series or stacked along the length of the cylindrical column. The notch of each columnar section is positioned in a selected circumferential angular relationship with the notch of each other columnar section and extends partially along the length of the column, thereby selectively reducing the thickness of the wall and producing a discontinuity in the outer cylindrical surface at selected positions. The angular position of each notch or of each reduced thickness portion of a wall around the circumference of the generally cylindrical column sections is differently selected along the length of the column. The selected angular positions provide a pattern of discontinuities on the generally cylindrical outer surface of the column. It will be understood that for a solid cylinder the wall thickness is nominally equal to the nominal radius. For a riser support column comprising a hollow cylinder encased in a polymeric or foam material, the wall thickness is less than the nominal radius. Selectively decreasing the distance from the axis to the surface might also be considered the same as reducing the wall thickness at selected locations or in a desired pattern. The reduced radius or reduced wall thickness preferably provides a sharp discontinuity in the surface.  
           [0011]    Preferably, the discontinuities will be selectively and appropriately positioned in a pattern, desirably a helical pattern, along the length of the column so that the VIV effect of vortex sheet separation from the cylindrical column is reduced. Forming or approximating a helical shaped discontinuity along the length of the cylindrical structure exposed to moving current facilitates reduction of VIV, or at least reduces its negative effects in the cylindrical structure. The discontinuity acts to shed the vortex at different times at different segments along the length of the cylinder. The various vortex-created lift forces are out of phase from each other and thus are out of phase with the oscillation that the forces would otherwise cause in the cylindrical structure at any given time. The “out of phase” forces tend to cancel each other out. Thus, the vibratory effect of vortex-induced lift forces on the cylinder are reduced.  
           [0012]    The abrupt reduction in thickness or the formation of a sharp discontinuity in the outer surface is generally accomplished using variously shaped notches or grooves. Preferably, notches or grooves having sharp corners have been found to be useful, such as a right angle triangular-shaped notch, an equilateral triangular-shaped notch, a rectangular-shaped notch, or other angular polygon. The notches or reduced thickness areas causing discontinuities in the outer surface of the cylindrical structure are either formed in a substantially continuous helical pattern or formed with segments of notches that are rotated to different angular positions at regular intervals along the length of the cylindrical structure. By forming relatively short segments of longitudinal notches and sequentially rotating each notch consistent angular amounts (between 10° and 90°) at regular intervals of length (between about 0.1 and 10 times the diameter), a long helical shape is approximated by the plurality of rotated notches or grooves. A series of partially rotated column sections, each column section having vertical or slightly angled notches or grooves may be provided along the length of the cylindrical column structure. By rotating the column sections at the time they are affixed to the support cylinder, a helically shaped groove is approximated by vertically elongated notches. A better approximation of a helical groove may be formed by a series of columnar sections having angled notch segments aligned end to end by rotating the columnar sections.  
           [0013]    A generally cylindrical column structure to which the present inventive VIV reduction mechanism is to be applied according to the disclosure herein, might typically be a support structure for drilling risers or production risers. It will be understood that this is by way of example only of the cylindrical structure to which the VIV reduction device and mechanism is applied. The resulting inventive VIV reduced cylindrical assembly may also be used for other cylindrical structures; i.e., it may be a drilling riser, a production riser, a hybrid riser and/or any number of other elongated cylindrical structures that may be subjected to the adverse effects of VIV. The cylindrical structure may comprise a solid metal outer surface or may comprise a composite material on which a VIV reduction mechanism is secured or formed. The VIV reduction mechanism may be notches or grooves formed in the solid surface. Preferably notches or grooves in helical pattern may be formed into a composite polymeric material or a polymer foam material secured, attached or formed onto the surface of a generally cylindrical support structure such as a riser support cylinder or cut or molded into the surface of a generally cylindrically shaped polymer foam material attached on the outer surface of any immersed cylindrical structure. The VIV reduction mechanism may also be formed in a composite structure with notches, grooves or other discontinuity formed into the outer surface or into the wall thickness, as described herein.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The foregoing objects, advantages, and features, as well as other objects and advantages, will become apparent with reference to the description, claims and drawings below, in which like numerals represent like elements and in which:  
         [0015]    [0015]FIG. 1 is a schematic perspective, partially cutaway view, depicting various undersea uses of cylindrical columns in moving fluid (i.e., vertical cylindrical columns in horizontal water currents).  
         [0016]    [0016]FIG. 2 is a schematic depiction of a cylindrical riser bundle support assembly provided with an upper buoyancy can, and a cylindrical support structure for the bundle of tubular risers with the cylindrical support structure having applicants&#39; VIV suppression invention applied to the cylindrical exterior surface;  
         [0017]    [0017]FIG. 3 is a schematic depiction of a hybrid riser assembly having a cylindrical riser support structure with a portion thereof having, for additional buoyancy, substantially cylindrical foam to which applicants&#39; inventive VIV suppression device has been applied;  
         [0018]    [0018]FIG. 4 is a schematic depiction of a representative segment of the upper enhanced buoyancy portion of the substantially cylindrical riser structure of FIG. 3 in which a plurality of risers are held together supported by a central support cylinder in segmented foam quadrants clamped in a substantially cylindrical shape and having segments of applicants&#39; VIV reduction devices applied and clamped to the exterior of the enhanced buoyancy foam riser bundle;  
         [0019]    [0019]FIG. 5 is a schematic cross-sectional depiction of one embodiment of applicants&#39; inventive VIV reduction device and mechanism in which four sections of the VIV suppression device are depicted for clamping around a riser, two of which in each cylindrical segment have a notch or sharp discontinuity formed therein with each notch at concentric opposed locations, the junctions at each end each section being concentric with the other ends and of the same width so that clamping engagement results in a smooth transition between one half and the other;  
         [0020]    [0020]FIG. 6 shows an embodiment of the VIV suppression device in which four discontinuities or four notches or four “step notches” are formed in four quadrants of the VIV columnar segments;  
         [0021]    [0021]FIG. 7 shows an embodiment similar to FIG. 6, except that each VIV reduction columnar segment is divided into two substantially identical pieces. The cut can be anywhere in the segment;  
         [0022]    [0022]FIG. 8 shows another embodiment similar to FIG. 7, except that each VIV reduction columnar segment is divided into four identical pieces which lock each other together. This embodiment will allow the load on the notches to be better distributed along the entire length of the segment.  
         [0023]    [0023]FIG. 9 shows the arrangement of the segments and notches depicted in FIGS.  6 - 8  in the longitudinal direction. For clarity only one notch on each columnar segment is shown.  
         [0024]    FIGS.  10 - 13  show cross-sections of the segments of FIG. 9 taken along the lines  10 - 10 ,  11 - 11 ,  12 - 12 , and  13 - 13 , respectively.  
         [0025]    [0025]FIG. 14 shows another arrangement of the notches depicted in FIGS.  6 - 8  in the longitudinal direction. In this embodiment, successive notches form a spiral line. For clarity, only one notch on each columnar segment is shown.  
         [0026]    FIGS.  15 - 18  show cross-sections of the segments of FIG. 14 taken along the lines  15 - 15 ,  16 - 16 ,  17 - 17  and  18 - 18 , respectively.  
         [0027]    [0027]FIG. 19 shows another embodiment in which the outline of the columnar segment is not a circle, with the phantom line in the drawing showing a circle (that is not part of a structure) for comparison. At one side, the surface extends beyond the circular phantom line and at the other side it is inside the circular phantom line. The notch arrangement of successive segments in the longitudinal direction can be the same as depicted in FIGS. 9 and 14.  
         [0028]    [0028]FIG. 20 shows another embodiment similar to FIG. 19, except that the columnar VIV reduction segment is divided into two identical pieces. The notch arrangement of successive segments in the longitudinal direction can be the same as depicted in FIGS. 9 and 14.  
         [0029]    [0029]FIG. 21 shows another embodiment of a segment that has a notch of a different shape. The notch arrangement in the longitudinal direction can be the same as depicted in FIGS. 9 and 14.  
         [0030]    [0030]FIG. 22 is a side view of longitudinally arranged segments with triangular notches. The triangular notches cover entire cylindrical surface and in the longitudinal direction, the notches forming spiral (helical) lines.  
         [0031]    [0031]FIG. 23 is a cross-sectional view of one of the segments of FIG. 22, taken along the line  23 - 23 .  
         [0032]    [0032]FIGS. 24 and 25 show another embodiment where the cross section of the segment is an ellipse and the angular orientation of the long axis of each rotates, as shown in the cross-section in FIG. 25, to form a spiral (twisted) shape.  
         [0033]    [0033]FIGS. 26 and 27 show another embodiment where the cross section is a triangle with rounded corners. The angular orientation of each triangle rotates, as shown in the cross-section in FIG. 27, to form a spiral (twisted) shape.  
         [0034]    [0034]FIGS. 28 and 29 show another embodiment where the cross section is a square with rounded corners. The angular orientation of the square rotates, as shown in FIG. 29, to form a spiral (twisted) shape.  
         [0035]    [0035]FIGS. 30 and 31 show another embodiment where the cross section is an ellipse. The angular orientation of the long axis of the ellipse rotates, as shown in FIG. 31, to form a discontinuous stepped pattern.  
         [0036]    [0036]FIGS. 32 and 33 show another embodiment where the cross section is a triangle with rounded corners. The angular orientation of the triangle rotates, as shown in FIG. 33, to form a discontinuous stepped pattern.  
         [0037]    [0037]FIGS. 34 and 35 show another embodiment where the cross section is a square with rounded corners. The angular orientation of the square rotates, as shown in FIG. 35, to form a discontinuous stepped pattern.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applications for the inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.  
         [0039]    Referring to FIG. 1, which is a schematic depiction of floating production systems on the sea surface  10  and extending from the seabed  12  through a distance of ocean, including a portion  14  having sea currents and a portion  15  without significant sea currents. Examples of various ocean equipment to which the invention may be usefully applied are depicted, including a sea floor drilling rig  16 , a ship  18 , a columnar-supported drilling platform  20 , a floating production platform  22  and a spar platform  24 , as well as a collection vessel  26 . Risers  28  are shown extending from the seabed  12  to the collection ship  18  where hydrocarbons are pumped on board from the risers and transported to an appropriate port facility where similar risers may offload the petroleum products to a refinery. The drilling or production platform  20  is schematically depicted with a drill casing  30  extending to the floor surface and also support legs  32  on which the drilling or production platform is secured to the sea floor  12 .  
         [0040]    The spar platform hull  24  is supported on a large cylindrical spar hull  40  having a heavy end  39  and an upwardly buoyant end  37  so that the platform  24  is floating in a desired position and may be anchored in position with mooring lines  41 . Top tension risers and steel catenary riser pipes  42  extend upward to the spar platform  24  and through or about the spar hull  40  to the production platform  24 . The collection vessel  26  is shown receiving hydrocarbon from a hydrocarbon collection system  44  for sub-sea wells on the seabed  12  and providing the produced hydrocarbons through upwardly extending risers  46  and also collecting hydrocarbons from the well  16  through elongated recovery pipes  48  that may extend flexibly along the seabed  12  and upward to collection vessel  26 .  
         [0041]    The foregoing floating production systems are depicted by way of background so that uses of the inventive VIV reduction mechanism according to various embodiments of the present invention may be more fully understood as to the wide ranging applications to riser cylinders drill casings, riser support columns, pipes, platform legs, cylindrical spars and other similar immersed cylindrical structures.  
         [0042]    With reference to FIG. 2, a production/transport vessel  50 , in this case a ship  50 , is shown in position for receiving hydrocarbons above a buoyancy canister  52  attached to a riser support cylinder  54  so that the riser support cylinder  54  may be held upright and having a connection in  53  held adjacent to the sea surface  10 . Depicted in FIG. 2 is one embodiment of VIV reduction mechanism  56  attached along a length  14  exposed to current  58  that is depicted as horizontal arrows  58 . In shallow waters, the current  58  may extend from the sea surface  10  to the seabed  12 , however, in deep waters as is often the case, the current  58  may extend a length  14  that may be several hundred to several thousand meters deep. In situations where the sea depth is thousands of meters, there will also be a length  15  of riser  54  that is not exposed to any significant current. In situations where no VIV reduction mechanism  56  is applied to the cylindrical riser support, the current  58  will form vortexes or a sheet of vortex material along substantially the entire length  14  exposed to the current  58 . With vortex reduction mechanisms  56  applied to riser support structure  54 , the vortices  60   a, b, c, d, e, f,  and  g  will each shed from the column surface at different times and/or different locations such that the lifting force at each longitudinal position along the riser support structures is out of phase with the oscillation of the entire riser  54  thereby canceling out the vibration. This effectively reduces the vibration.  
         [0043]    The vessel  50  is shown held in place with anchor cable  62  attached to sea anchors  64  so that the conduits  66  from the connection head  53  to the production vessel  50  are retained in a relatively stable position. The VIV reduction mechanism  56  applied along cylindrical riser  54  comprises a plurality of VIV reduction column segment  70 . These have been labeled starting at the topmost as VIV reduction column segment  70   a  with the next columnar segment  70   b,    70   c  and etc. Each columnar segment is rotated relative to the next such that a sharp notches, grooves or discontinuities  72   a, b, c, d, e,  etc. are provided in each columnar segment.  
         [0044]    Advantageously, the discontinuity areas are rotated angularly with each successive columnar segment to a different angular position relative to the adjacent columnar segments. Desirably, for example, segment  70   b  is rotated an angle of between about 10° and 90° relative to segment  70   a.  Also desirably segment  70   c  is also rotated to the same angular amount relative to  70   b  as  70   b  is rotated relative to  70   a.  Thus, a consistent rotational interval is provided along each VIV reduction column segment.  
         [0045]    As will be described more fully below, the column segments may have an axial length that is between about ½ times the diameter to about 10 times the diameter. In particular, it has been discovered that columnar segments having a length of approximately 1½ times the diameter each rotated about 30° relative to each other will advantageously break up the vortex sheet. Vortex shedding at one column will be out of phase with the next so that vortex induced lifting forces are out of phase and cancel each other. By rotating each columnar segment, a consistent rotational angle between about 10 and 90°, a helical design is approximated. Each VIV reduction columnar segment may comprise one or a plurality of longitudinal VIV reduction discontinuities. Generally speaking, the greater number of discontinuities per columnar segment, the longer the columnar segment may be and still have a desired VIV reduction effect. Various embodiments, constructions and manufacturing of VIV reduction columns will be discussed more fully below with reference to FIGS.  5 - 43 .  
         [0046]    Turning now to FIG. 3, which is a configuration of hybrid riser, an additional application of the inventive VIV reduction mechanism may be more fully understood in connection with a support riser  76  having structural steel pipe inside the bundle, by which a plurality of riser pipes  68  may be supported vertically upward from the seabed  12  to a position close to sea surface  10 , for providing flexible riser  82  connection to floating platform  74 . In this embodiment, the VIV reduction mechanism  77  comprises of a plurality of VIV reduction columnar segments,  78   a, b, c,  and  d  etc., each having a VIV reduction notch  84   a, b, c,  and  d  etc. preferably a plurality of angled notches or discontinuities  84   a, b, c,  and  d  etc. The angle of the notch relative to the longitudinal axis of a columnar segment  78 , desirably provides a segment of a helical notch  84 . Adjacent VIV reduction columnar segments  78   a  and  b  are each simultaneously merged and are each rotated relative to each other at appropriate angular interval so that the notches  84   a  and  84   b  are lined end to end form a cylindrical notch comprised of a plurality of segments  84   b, c, d, e, f, g,  and etc. The number of columnar segments required to provide the VIV reduction system along the length of riser support  76  that is exposed to currents will depend upon the depth of the currents and the length of each columnar segment.  
         [0047]    In the embodiment shown in FIG. 3, additional buoyancy polymeric foam segments  80   a, b, c  and etc. are also provided secured to the cylindrical riser support structure  76  toward the top thereof where it may be tethered through cables  88  to a production platform  74  floating on the sea surface  10 . A connection head  90  is provided by which the risers  68  are in fluid communication with flexible risers  82  to provide hydrocarbons to the surface vessel.  
         [0048]    Referring now to FIG. 4, one embodiment of a riser support column with risers encased in a foam retaining material is schematically depicted with a partial perspective view of one portion of a riser support cylinder assembly having foam material in cylindrical quadrants encasing a plurality of risers and further providing additional buoyancy VIV reduction mechanisms clamped around the periphery of the cylindrical foam structure. Particularly, a metal cylinder  102  provides the main riser support and a plurality of petroleum recovery risers  104   a,    104   b,    104   c,    104   d  are provided along with control cables  106   a  and  106   b  as well as additional pressurizing pipes  108   a, b  and  108   c  and  d  as well as gas recovery pipes  110   a  and  110   b  ( 110   b  not shown in FIG. 4). The VIV columnar segments  70   a,    70   b,    70   c  and  70   d  are shown constructed of four VIV reduction column sections, the risers, conduits and control cables extending along the length of support cylinder  102  being are encased within four molded polymeric foam sections  120 ,  122 ,  124  and  126  making up each of the columnar segments  70   a,    70   b,    70   c  and  70   d.  Adjacent ones of sections  120 ,  122 ,  124  and  126 , need not be the same cross-sectional shape, although it is preferred that respectively opposing sections, i.e.,  120  and  126 , and  122  and  124 , be the same shape as their opposed section. These sections are respectively “split” at junctions  146  and  148  (not shown if FIG. 4, see FIG. 5) for petroleum recovery risers  104   a,    104   b,    104   c  and  104   d  and include half-circle cutouts for these risers. Sections  122  and  126  include outwardly open cut-outs for cables  106   a  and  106   b,  and sections  120  and  124  include inwardly open cut-outs for gas recovery lines  110   a  and  110   b.  The construction of these sections will be more fully understood with reference also to FIG. 5 which is a cross-sectional view of VIV reduction riser assembly according to FIG. 4 taken along section line  5 - 5 . two of which  128  and  130 .  
         [0049]    Each VIV reduction segment  70   a,    70   b,    70   c  and  70   d  has a discontinuity  132   a,    132   b,    132   c  and  132   d  in its outer surface, and a corresponding discontinuity  132   a′,    132   b′,    132   c′  and  132   d′  on the outer surface of its back side. As depicted in FIG. 4, each of these discontinuities comprises a substantially radially directed face  134  extending inward from the exterior surface  142 , a distance approximating between {fraction (1/10)}th and {fraction (3/10)}ths the diameter thereby decreasing the wall thickness of VIV reduction columnar half  130  as depicted at  136 . A substantially flat surface  140  is formed projecting substantially at right angles to face  134  thereby providing a right triangular notch  132 . Subsequent columnar segments  70   a,    70   b  and  70   c  also have a similar notches  132   a,    132   b  and  132   c,  respectively. In the embodiment depicted in FIGS. 4 and 5, two opposed ones of the four columnar segments also has a discontinuity or a notch  132  formed in its face. These sections are clamped using clamps  142  and  144  to securely hold the additional buoyancy foam, into which the VIV reduction mechanism has been formed, onto the exterior of the cylindrical riser assembly  80 . At junctions  146  and  148  (not shown in FIG. 4, see FIG. 5) between the sections, the wall thickness of the adjacent VIV reduction column sections is the same.  
         [0050]    Referring to FIG. 5 that is a cross-sectional view of the VIV reduction riser assembly of FIG. 4, it can be seen that the VIV reduction columns according to this embodiment have substantially concentric notches at opposite sections where the thickness of the wall is reduced an equivalent amount D on each side and the wall thickness progressively increases from that notch  132  toward the opposing section, where the diameter continues to increase until the second notch  132  on that opposing section is reached. Again, the discontinuity wall thickness is decreased the distance D and again the wall thickness progressively increases past the junction  148  until the subsequent notch  132  on the other side is reached. Similar structure is provided with respect to each of the VIV reduction columnar segments  70   a,    70   b,    70   c  and  70   d,  in which successive segments are mounted sequentially adjacent to each other except rotated a predetermined angular interval between zero and 90°. It has been found that rotation of approximately 30° provides good VIV reduction, thus discontinuity  132   b  is offset from the prior discontinuity  132  by an angle of approximately 30°. Subsequent columnar segment  70   c  is likewise formed with four sections. The foam segments of these successive of these columnar segments are molded such that each successive discontinuity  132  is rotated about 30°. with respect to the next. It has further been found that the length  144  of each columnar segment  170   a, b, c,  etc. may be desirably about 1.5 times the nominal diameter of the VIV reduction columnar segments.  
         [0051]    Turning now to FIG. 6, a cross-section another embodiment of the VIV suppression device surrounding a pipe  108 ′ is depicted having four discontinuities or “notches”  158 ,  159 ,  160  and  161  formed in four quadrants of the VIV columnar segment. The eccentric exterior shape retains or approximates a substantially cylindrical columnar shape. In this embodiment, the VIV suppression device may conveniently be molded onto the pipe, or slipped onto its end prior to installation of the pipe.  
         [0052]    [0052]FIG. 7 shows an embodiment similar to FIG. 6, except that each VIV reduction columnar segment is divided into two substantially identical pieces, to facilitate assembly. The cuts  163  and  164  can be anywhere in the segment.  
         [0053]    [0053]FIG. 8 shows another embodiment similar to FIG. 7, except that the discontinuities  158 ,  159 , 160  and  161  are, for example, at or near the junctions between each quadrant. In this embodiment, each VIV reduction columnar segment is divided into four identical pieces which lock each other together at zig-zag split lines  166 ,  167 ,  168 ,  169 . This embodiment permits the load on the notches to be better distributed along the entire length of the segment.  
         [0054]    [0054]FIG. 9 is a schematic depiction of a VIV reduction mechanism  180  formed of a plurality of VIV reduction columnar segments  181   a, b, c, d, e, f, g, h, i, j, k  and  l  stacked in an elongated column each having a longitudinal discontinuity  182  in the form of notches  182   a, b, c, d, e, f, g, h, i, j, k  and  l.  For clarity only one notch on each columnar segment is shown. Each columnar segment is rotated 30° degrees relative to each other. By sequentially rotating the columnar segments  181 , the notches  182  are arranged in a pattern that approximates a helical pattern. The rotation angle of 30° provides twelve columnar segments for one complete helical rotation of the vertical notch positions.  
         [0055]    [0055]FIGS. 10, 11,  12  and  13  are schematic cross-sectional views taken at section lines at  10 - 10 ,  11 - 11 ,  12 - 12  and  13 - 13 , respectively. Each cross-sectional depiction represents 90° rotation or each third one of the columnar sections each rotated 30°. In FIG. 10 an indication of a perspective view is depicted in phantom lines in combination with the solid line cross-sectional view to assist in visualization of the construction of the discontinuity or notch  182   a.  Although the embodiment depicted shows a cross-section of a substantially cylindrical column segment that is slightly eccentric rather than perfectly cylindrical, the construction may be understood in terms of a nominal diameter D represented by numeral  184 . Referring again to FIG. 9 the height of each column  185  is conveniently in a range of between one half times D to about five times D, to permit offsetting of the discontinuities by the desired rotation angle, however, the ratio is not critical to the invention. Longer columnar segments might be used, for example, where a plurality of notches  182  are formed in each columnar segment rather than the single notch as depicted in FIGS. 9 through 13. The notch or discontinuity has a substantially flat face  183  that provide a corner along the length of  185  of the column. The face has a depth B represented by numeral  187  into the eccentric surface of the cylindrical column  181  a. Depth B consist of a portion C represented by numeral  188  that accomplishes the eccentricity of the columnar segment and the remainder which corresponds to the reduction in the radius less than the nominal diameter D. The size of the notch depends upon the specific conditions of use. Of course, the rotation need not be 30 degrees, as any offset sufficient to create any pattern of notches effective to diminish VIV will suffice. Again with reference also to FIGS. 10, 11,  12  and  13  each of which depicts a cross-sectional view of the VIV reduction mechanism  190  at Section lines  10 - 10 ,  11 - 11 ,  12 - 12 , and  13 - 13 , respectively. In the embodiment depicted in FIGS. 19 through 13 as more specifically set forth with reference to FIGS. 10 and 11, the cylindrical columnar segments  192  have a diameter D represented by numeral  194 . The longitude and the length of each column is between one-half times D and five times D as represented by reference rule  195 . The discontinuity or notch  192  a has a flat face  193  that is radiantly aligned with the central axis of the VIV columnar segment  191   a  and has a flat surface  195  projecting at right angles from face  193 . This produces a sharp exterior corner at  198  that facilitate initiation of the shear shedding as discussed previously. The depth of the phase B represented by numeral  197  may be in the range of 0.1 to 0.3 times the diameter D. The face  195  has a width A represented by numeral  196  that may be in the range of 0.3 to 0.8 times the nominal diameter D.  
         [0056]    [0056]FIG. 14 depicts a side view of sequentially arranged segments with notches formed at an angle into the outer surface of the VIV reduction device, so that when the segments are successively arranged, the notches form a substantially longitudinally continuous spiral notch. Each columnar segment rotate at 30° relative to the other as with 90 degrees of rotation. The arrangement of each third segment is depicted in cross-sections in FIGS. 15, 16,  17  and  18 .  
         [0057]    [0057]FIG. 19 shows another embodiment in which the outline of the columnar segment is not exactly a circle; i.e., it is somewhat spiral-shaped. The phantom line  199  in the drawing shows a circle but is not part of a structure. At one side of the surface extends beyond the circular phantom line and at the other side it is inside the circular phantom line. The notch sequential off-setting arrangement in the longitudinal direction can be the same as depicted in FIGS. 9 and 14; i.e., approximately 30 degrees..  
         [0058]    [0058]FIG. 20 shows another embodiment similar to FIG. 19, except that the columnar VIV reduction segment is divided into two identical pieces at cut lines  163 ′ and  164 ′. The notch arrangement in the longitudinal direction can be the same as depicted in FIGS. 9 and 14; i.e., approximately 30 degrees.  
         [0059]    [0059]FIG. 21 shows another embodiment that has a notch  158 ″ of a different shape; i.e., a square. The notch arrangement in the longitudinal direction can be the same as depicted in FIGS. 9 and 14. Although only one notch  158 ″ is depicted, four or any number could be used, as in FIGS. 9 and 14.  
         [0060]    [0060]FIG. 22 is another embodiment which has a cross section as shown in FIG. 23. The triangular notches  300  cover entire cylindrical surface and in the longitudinal direction, the notches form spiral (helical) lines.  
         [0061]    This embodiment uses a VIV reduction mechanism in which a plurality of V-type notches  300  are equilateral triangles are formed into the surface of the substantially cylindrical column. Again the star-shaped cross-section of FIG. 23 continuously spirals along the length of the column depicted in FIG. 23. This may be created by a long columnar section longer than the one-half to ten times the diameter columns that might be more appropriate with vertically aligned notches. However for ease of manufacture and for clamping onto cylindrical risers or cylindrical riser support structures or the like columnar sections might still be used and alignment will be easily accomplished because of the uniform star shape provided by the plurality of V-shaped notches.  
         [0062]    [0062]FIG. 24 and  25  show another embodiment where the cross section  250  is slightly twisted, an ellipse, successive segments being offset about 45 degrees so the long axis of the ellipse “spirals,” as shown in FIG. 25, to form a spiral (twisted) shape.  
         [0063]    [0063]FIGS. 26 and 27 show another embodiment where the cross section  255  is a slightly twisted triangle with rounded corners. Successive segments are offset about 45 degrees, the direction of the triangle, as shown in FIG. 27, to form a spiral (twisted) shape.  
         [0064]    [0064]FIGS. 28 and 29 show another embodiment where the cross section is a square with rounded corners. The angular orientation of the square rotates, as shown in FIG. 29, to form a spiral (twisted) shape.  
         [0065]    [0065]FIGS. 30 and 31 show another embodiment where the cross section is an ellipse. The angular orientation of the long axis of the ellipse rotates as shown in FIG. 31, to form a discontinuous stepped pattern.  
         [0066]    [0066]FIGS. 32 and 33 show another embodiment where the cross section is a triangle with rounded corners. The angular orientation of the triangle rotates, as shown in FIG. 33, to form a discontinuous stepped pattern.  
         [0067]    [0067]FIGS. 34 and 35 show another embodiment where the cross section is a square with rounded corners. The angular orientation of the square rotates, as shown in FIG. 35, to form a discontinuous stepped pattern.  
         [0068]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments. Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading the present disclosure, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventors are legally entitled.