Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
  • B63B21/50—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers
  • B63B21/502—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers by means of tension legs
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B13/00—Conduits for emptying or ballasting; Self-bailing equipment; Scuppers
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B17/00—Drilling rods or pipes; Flexible drill strings; Kellies; Drill collars; Sucker rods; Cables; Casings; Tubings
  • E21B17/01—Risers
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
  • F15D—FLUID DYNAMICS, i.e. METHODS OR MEANS FOR INFLUENCING THE FLOW OF GASES OR LIQUIDS
  • F15D1/00—Influencing flow of fluids
  • F15D1/10—Influencing flow of fluids around bodies of solid material
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
  • F25J1/0022—Hydrocarbons, e.g. natural gas
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
  • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
  • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
  • F25J1/0275—Construction and layout of liquefaction equipments, e.g. valves, machines adapted for special use of the liquefaction unit, e.g. portable or transportable devices
  • F25J1/0277—Offshore use, e.g. during shipping
  • F25J1/0278—Unit being stationary, e.g. on floating barge or fixed platform
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
  • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
  • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
  • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
  • F25J1/0296—Removal of the heat of compression, e.g. within an inter- or afterstage-cooler against an ambient heat sink
  • F25J1/0297—Removal of the heat of compression, e.g. within an inter- or afterstage-cooler against an ambient heat sink using an externally chilled fluid, e.g. chilled water
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
  • B63B21/50—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers
  • B63B21/502—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers by means of tension legs
  • B63B2021/504—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers by means of tension legs comprising suppressors for vortex induced vibrations
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63J—AUXILIARIES ON VESSELS
  • B63J2/00—Arrangements of ventilation, heating, cooling, or air-conditioning
  • B63J2/12—Heating; Cooling

Definitions

• VIV VORTEX-INDUCED VIBRATION
• the present invention relates to systems and methods for reducing drag and/or vortex-induced vibration ("VIV") of a plurality of structures.
• VIV vortex-induced vibration
• Floating vessels may be used to liquify and gasify natural gas.
• Sea water may be used to cool or heat the natural gas. It may be desired to separate the water inlet from the water outlet due to the temperature differences.
• a plurality of risers may be used to collect or deposit water at a depth from the floating vessel. These risers may be exposed to VIV.
• Drilling for and/or producing hydrocarbons or the like from subterranean deposits which exist under a body of water exposes underwater drilling and production equipment to water currents and the possibility of VIV.
• Equipment exposed to VIV includes structures ranging from the smaller tubes of a riser system, anchoring tendons, or lateral pipelines to the larger underwater cylinders of the hull of a mini spar or spar floating production system (hereinafter "spar").
• the magnitude of the stresses on the riser pipe, tendons or spars may be generally a function of and increases with the velocity of the water current passing these structures and the length of the structure.
• the second type of stress may be caused by drag forces, which push the structure in the direction of the current due to the structure's resistance to fluid flow.
• the drag forces may be amplified by vortex-induced vibration of the structure. For instance, a riser pipe that is vibrating due to vortex shedding will generally disrupt the flow of water around it more than a stationary riser. This may result in more energy transfer from the current to the riser, and hence more drag.
• Devices used to reduce vibrations caused by vortex shedding from sub-sea structures may operate by modifying the boundary layer of the flow around the structure to prevent the correlation of vortex shedding along the length of the structure.
• Examples of such devices include sleeve-like devices such as helical strakes, shrouds, fairings and substantially cylindrical sleeves.
• VIV and/or drag reduction devices Elongated structures in wind or other flowing fluids can also encounter VIV and/or drag, comparable to that encountered in aquatic environments. Likewise, elongated structures with excessive VIV and/or drag forces that extend far above the ground can be difficult, expensive and dangerous to reach by human workers to install VIV and/or drag reduction devices.
• Fairings may be used to suppress VIV and reduce drag acting on a structure in a flowing fluid environment. Fairings may be defined by a chord to length ratio, where longer fairings have a higher ratio than shorter fairings. Long fairings are more effective than short fairings at resisting drag, but may be subject to instabilities. Short fairings are less subject to instabilities, but may have higher drag in a flowing fluid environment.
• U.S. Patent Number 6,223,672 discloses an ultrashort fairing for suppressing vortex-induced vibration in substantially cylindrical marine elements.
• the ultrashort falling has a leading edge substantially defined by the circular profile of the marine element for a distance following at least about 270 degrees thereabout and a pair of shaped sides departing from the circular profile of the marine riser and converging at a trailing edge.
• the ultrashort fairing has dimensions of thickness and chord length such that the chord to thickness ratio is between about 1.20 and 1.10.
• U.S. Patent Number 6,223,672 is herein incorporated by reference in its entirety.
• U.S. Patent Number 3,978,804 discloses a structure floating on a body of water, and particularly a structure for drilling or producing wells from below the water. Buoyant members support at least a part of the structure above the surface of the water. The structure is connected to anchors in the floor of the body of water by a series of parallel leg members. Each leg member is composed of a plurality of elongated members, such as large diameter pipe usually called risers. These risers are parallel. Vertically spaced spacers are provided along the risers of each leg to (1 ) maintain the risers a fixed distance apart and (2) change the natural or resonant frequency of the individual riser pipes to be greater than the flutter frequency caused by the motion of the water past the risers.
• U.S. Patent Number 3,978,804 is herein incorporated by reference in its entirety.
• U.S. Patent Number 6,089,022 discloses a system and a method for regasifing LNG aboard a carrier vessel before the re-vaporized natural gas is transferred to shore.
• the pressure of the LNG is boosted substantially while the LNG is in its liquid phase and before it is flowed through a vaporizer(s) which, in turn, is positioned aboard the vessel.
• Seawater taken from the body of water surrounding said vessel is flowed through the vaporizer to heat and vaporize the LNG back into natural gas before the natural gas is off-loaded to onshore facilities.
• U.S. Patent Number 6,089,022 is herein incorporated by reference in its entirety.
• Patent Number 6,832,875 discloses a floating plant for liquefying natural gas having a barge provided with a liquefaction plant, member for receiving natural gas and with member for storing and discharging liquefied natural gas.
• the liquefaction plant involves a heat exchange in which heat is removed when liquefying natural gas is transferred to water.
• the barge is further provided with a receptacle; an open-ended water intake conduit having an inlet; a connecting conduit extending from the outlet of the water intake conduit to the receptacle; a pump for transporting water from the receptacle to the heat exchanger and a water discharge system for discharging water removed from the heat exchanger.
• the connecting conduit has the shape of an inverted "U" of which the top is located above the receptacle.
• U.S. Patent Number 6,832,875 is herein incorporated by reference in its entirety.
• apparatus and methods for reducing VIV and/or drag on structures in flowing fluid environments which do not suffer from certain disadvantages of the prior art apparatus and methods; apparatus and methods for reducing VIV and/or drag on multiple structures in flowing fluid environments; apparatus and methods for reducing VIV and/or drag on a riser array or bundle.
• One aspect of the invention provides a system comprising an array of structures in a flowing fluid environment, the array comprising at least 3 structures; and vortex induced vibration suppression devices on at least 2 of the structures.
• Another aspect of the invention provides a method of suppressing the vortex induced vibration of an array of structures comprising installing vortex induced vibration suppression devices on from 10% to 90% of the structures.
• Advantages of the invention may include one or more of the following: improved VIV reduction of a plurality of structures; improved drag reduction of a plurality of structures; lower cost VIV reduction; and/or VIV reduction of a plurality of structures with fewer VIV suppression devices.
• Figure 1 illustrates an example of a marine system in which embodiments may be implemented.
• Figure 2A is a cross-sectional top view illustrating one or more representative strakes installed along a length of tubular structure as VIV suppression device(s).
• Figure 2B is a cross-sectional top view illustrating a representative fairing installed along a length of tubular structure as a VIV suppression device.
• Figures 3A-3H illustrate several different examplary approaches or configurations for coupling VIV suppression devices with only a subset of tubular structures, according to various embodiments.
• Figure 4 illustrates an examplary approach or configuration of a plurality of tubular structures in which at least two, in this case at least three, of the tubular structures have different outer diameters, according to one or more embodiments.
• Figure 5 illustrates an example approach or configuration that is similar to that of Figure 4 except that, in addition to the different outer diameters, a subset of the tubular structures also have VIV suppression devices coupled therewith, according to one or more embodiments.
• Figure 6A illustrates an example of a marine system including a Floating Liquified Natural Gas (FLNG) plant, in which embodiments may be implemented.
• Figure 6B shows an example approach or configuration for a Floating Liquified Natural Gas (FLNG) plant in which nine tubular structures are arranged in a three-by- three rectangular array, according to one particular embodiment.
• FLNG Floating Liquified Natural Gas
• the marine system includes surface structure 102 near a water surface 104, for example a surface of the ocean.
• the surface structure may include a ship, a barge, a vessel, an FPSO (floating production storage and offloading), a TLP (tension leg platform), a spar, an offshore rig, an offshore platform, a floating plant, a floating liquefied natural gas plant, or other floating or surface structures as are known in the art.
• a plurality of tubular structures 106 are coupled with the surface structure.
• the tubular structures may be used in conjunction with providing cold water at depth to cool natural gas in a floating liquefied natural gas plant serving as the surface structure.
• the tubular structures may be connected to a marine riser tensioner, a swivel joint, a ball joint, or the like.
• a tubular structure has a circular or oval cross-section.
• a cross-section of a tubular structure need not be circular or oval, but can include other shapes such as, but not limited to, rectangular.
• tubular structures 106A, 106B are visible. More tubular structures may optionally be included, such as, for example, at least three, at least four, at least six, at least nine, or more.
• suitable tubular structures include, but are not limited to, cables, umbilicals, risers, marine risers, riser pipes, marine pipes, pipes, tubes, or the like, or combinations thereof.
• the structures may extend all the way to a seafloor 108, or only part way to the seafloor. In some cases, mud, crude, water, and/or other fluids or electricity or electrical signals may be conveyed through the structures.
• the tubular structures are physically connected together, or held in a position relative to one another, with one or more interconnected guide sleeves or other spacers 110A, 110B.
• the spacers connect or hold in position the tubular structures as an array, bundle, grouping, other ordered arrangement, or other joined plurality.
• the spacers may include a metal, plastic, or otherwise sufficiently strong material in a disc, plate, rectangle, interconnected polygonal bars, wheel and spoke shape, or other shape.
• the spacers may have holes or other openings therein. Each of the holes or openings may accommodate and have inserted therein one of the tubular structures.
• the spacers may help to keep the tubulars relatively close together, but separated so that they do not significantly strike into one another or otherwise damage one another.
• tubular structures may serve as a structural support for the spacers.
• a tubular structure serving as a structural support for a spacer may be connected (directly or indirectly) to the spacer.
• tubular structures in an array or grouping such tubular structures need not be connected to a spacer and in the case of a tubular structure having, for example, a circular or oval shape, may instead have an outer (outside) diameter (including or not including a VIV suppression device) less than a diameter of an opening in the spacer.
• the outer (outside) diameter of the tubular structure may be similar to a diameter of an opening so that the tubular structure or a VIV suppression device on a tubular structure and the spacer may be in contact (e.g., a force fit).
• VIV vortex- induced vibration
• VIV suppression devices may be used to help suppress the VIV.
• VIV suppression devices or structures suitable for implementing embodiments include, but are not limited to, strakes, fairings, Henning devices, shrouds, wake splitters, and other types of VIV suppression devices or structures.
• Patent Application Number 10/784,536 having attorney docket number TH1853.04
• U.S. Patent Application Number 10/848,547 having attorney docket number TH2463
• the VIV suppression devices may be installed on the tubular member (e.g. buoyancy material and riser) before or after the tubular member is placed in a body of water.
• the tubular member e.g. buoyancy material and riser
• the VIV suppression devices may have a clamshell configuration, and may be hinged with a closing mechanism opposite the hinge, for example a mechanism that can be operated with an ROV.
• VIV suppression devices may be provided with copper plates on their ends to allow them to weathervane with adjacent VIV suppression devices or collars. VIV suppression devices may be partially manufactured from copper.
• Figures 2A-2B are provided with copper plates on their ends to allow them to weathervane with adjacent VIV suppression devices or collars. VIV suppression devices may be partially manufactured from copper.
• FIGS. 2A-2B show two common types of VIV suppression devices or structures. Each of these devices or structures is suitable for implementing one or more embodiments.
• Figure 2A is a cross-sectional top view illustrating one or more representative strakes 220 installed along a length of tubular structure 206 as VIV suppression device(s).
• the strake(s) may be helical strakes, which are helically wrapped or coiled around the tubular structure and may be described as connected thereto.
• Figure 2B is a cross-sectional top view illustrating a representative fairing 222 installed along a length of tubular structure 206 as a VIV suppression device and may be described as connected thereto.
• the fairing has nose 224 and tail 226. The fairing may swivel around the tubular structure based on the ocean current.
• the leftmost tubular structure 106A has one or more VIV suppression devices or structures 114A, 114B connected thereto therewith.
• Conventional collars may be used to keep the VIV suppression devices from moving along the length of the tubular structures.
• the rightmost tubular structure 106B does not have any VIV suppression devices or structures coupled therewith.
• either all or only a subset of the plurality of risers or other tubular structures may have VIV suppression devices connected thereto.
• one or more other tubular structures of the plurality may not have VIV suppression devices connected thereto.
• VIV suppression devices Omitting the VIV suppression devices from some of the tubular structures (so that only a subset of the tubular structures have the VIV suppression devices) may offer certain potential advantages. For one thing, providing the VIV suppression devices on all of the tubular structures tends to increase the overall equipment cost. For another thing, it tends to be more difficult, time consuming, and/or more expensive to install tubular structures with VIV suppression devices as compared to tubular structures without VIV suppression devices. The VIV suppression devices may tend to make the tubular structures more bulky, difficult to maneuver, difficult to align, and/or difficult to couple with the spacers. It likewise tends to be more difficult, time consuming, and/or more expensive to retrieve tubular structures with VIV suppression devices, such as, for example, for cleaning, inspection, and/or repair.
• tubular structures typically from about 20 percent to about 80 percent may have the VIV suppression devices coupled with them. Often, from about 30 percent to about 70 percent of the tubular structures may have the VIV suppression devices coupled with them. In some cases, from about 40 percent to 60 percent of the tubular structures may have the VIV suppression devices coupled with them.
• the coverage density (the length of the structure covered with VIV suppression devices compared to the total length) may be less than 1.
• the coverage density may also be expressed as a percentage of the tubular structure length, and may be less than 100 percent. Typically, the coverage density may range from about 50 percent to about 100 percent, for example from about 60 to about 90%.
• the number or percentage of tubular structures having VIV suppression devices may be decreased by increasing the coverage density of a selection of the tubular structures. In converse, if desired, the coverage density may be decreased by increasing the number or percentage of tubular structures having VIV suppression devices.
• Figures 3A-3H illustrate several different example approaches or configurations of VIV suppression devices connected with a subset of tubular structures of an array, bundle, group, or other plurality of tubular structures, according to various embodiments. These figures represent cross-sectional views taken along section line 3/4/5 of Figure 1 through spacer 110A and the plurality of tubular structures 106. The spacers couple the tubular structures together, or hold the tubular structures in position relative to one another, as arrays, bundles, groups, other ordered arrangements, or other coupled pluralities.
• FIG. 3A-3F illustrate approaches or configurations for nine tubular structures arranged in a three-by-three rectangular array, in this particular case a substantially square array.
• Figures 3A illustrates a first configuration in which only tubular structures at all four corner positions of the three-by-three rectangular array have VIV suppression devices connected thereto, according to one embodiment.
• the three-by-three rectangular array has tubular structures at four corner positions 106A, 106C, 106G, and 1061, respectively. These corner positions are referred to herein as upper left corner 106A, upper right corner 106C, lower left corner 106G, and lower right corner 1061.
• the array also has tubular structures at four central side positions 106B, 106D, 106F, and 106H, respectively. These central side positions are referred to herein as upper side 106B, lower side 106H, right side 106F, and left side 106D, respectively.
• the corner positions and the side positions in combination form a periphery of the array.
• the three-by-three rectangular array also has one tubular structure at a center position 106E.
• the four tubular structures at the four corner positions are the only tubular structures that have VIV suppression devices connected thereto. These four tubular structures help to suppress VIV for the entire array.
• this arrangement is robust and does not show very much sensitivity to the angle of an oncoming ocean current.
• the vortex shedding frequency for the tubular structures with the VIV suppression devices are often lower than that of the tubular structure without the VIV suppression devices.
• the VIV suppression devices may tend to help "detune” or reduce the excitation frequency of the VIV of the tubular structures relative to bare tubular structures. This may increase the 'frequency dissociation' of the array. These tubular structures with different frequencies will be less likely to couple their vibrations. As a result, vibration of the array may be reduced.
• the tubular structures that have the VIV suppression devices are substantially interspersed, interleaved, or otherwise staggered with other tubular structures that do not have VIV suppression devices coupled with them.
• On the periphery of the array only every other tubular structure has a VIV suppression device connected thereto. In such a staggered arrangement, adjacent tubular structures tend to have different excitation frequencies. As discussed, with such frequency dissociation the vibrations on these tubular structures are less likely to couple and the overall vibration of the array may be reduced.
• four out of nine or about 44 percent of the tubular structures include VIV suppression devices connected thereto.
• four out of eight or a higher percentage of 50 percent of the tubular structures have VIV suppression devices connected thereto.
• Figures 3B illustrates a second configuration in which only a tubular structure at a center position and tubular structures at all four corner positions of the array have VIV suppression devices connected thereto, according to one embodiment.
• This configuration is similar to the configuration of Figure 3A except that the tubular structure at the center position also has VIV suppression devices connected thereto.
• tubular structures with VIV suppression devices are substantially staggered with tubular structures without VIV suppression devices.
• tubular structures without VIV suppression devices are substantially staggered with tubular structures without VIV suppression devices.
• VIV suppression devices are only every other tubular structure. Such staggering may tend to increase the amount of frequency dissociation, which may also help to reduce damage due to VIV.
• Figures 3C illustrates a third configuration in which only tubular structures at four central side positions of the array have VIV suppression devices connected thereto, according to one example embodiment.
• This configuration is substantially opposite to the configuration of Figure 3A in that tubular structures at the sides positions instead of at the corner positions have VIV suppression devices.
• tubular structures with VIV suppression devices are substantially staggered with tubular structures without VIV suppression devices.
• FIG. 3D illustrates a fourth configuration in which only tubular structures at three corner positions of a front row of the array that would first experience a predominant ocean current have VIV suppression devices connected thereto, according to one example embodiment. This configuration is similar to the configuration of Figure 3A except that the tubular structure at the lower left corner position does not have VIV suppression devices.
• a 'predominant' ocean current is the average or most common ocean current including its average or most common direction.
• Five tubular structures on the top and right sides of the array constitute a front row. This front row first experiences the predominant ocean current.
• three out of nine, or about 33 percent of the tubular structures include VIV suppression devices coupled therewith.
• three out of eight, or a higher percentage of about 37 percent of the tubular structures include VIV suppression devices.
• On the front row three out of five, or an even higher percentage of about 60 percent of the tubular structures, have VIV suppression devices.
• the array is aligned so that the tubular structure at the upper right corner position first experiences the predominant ocean current before all other tubular structures.
• This tubular structure would tend to experience the ocean current at its highest velocity and would tend to have a relatively large amount of VIV.
• this tubular structure has one or more VIV suppression devices.
• a wake refers to a region of separated flow, in some cases turbulent, downstream of a solid body caused by flow of the fluid around the body. Average fluid velocity tends to be lower in a wake. As a result, these downstream tubular structures tend to experience lesser velocity ocean currents and tend to have less VIV. In addition, tubular structures in the wake of other tubular structures, and experiencing a lower velocity current, tend to have a lower vortex shedding frequencies and/or excitation frequencies. This adds frequency dissociation to the array, which helps to reduce vibrations.
• the tubular structure at the lower left corner position does not have one or more VIV suppression devices.
• This tubular structure is downstream from several upstream tubular structures and should tend to experience the ocean current at a relatively reduced velocity. This makes it a relatively good candidate to omit VIV suppression device(s).
• a tubular structure that would last experience a predominant ocean current, after all other tubular structures of the array may not have one or more VIV suppression device(s).
• Figures 3E illustrates a fifth configuration in which four tubular structures at positions on a front row of the array that would first experience a predominant ocean current have VIV suppression devices connected thereto, according to one example embodiment.
• arrows are used to indicate a predominant ocean current.
• the array is aligned so that the tubular structure at the upper right corner position first experiences the predominant ocean current. This alignment places more of the tubular structures in the wakes of upstream tubular structures.
• Five tubular structures on the top and right sides of the array constitute a front row that first experiences the predominant ocean current. In this embodiment, all four of the tubular structures with VIV suppression devices are on the front row.
• Figures 3F illustrates a fifth configuration in which only one tubular structure on a front row of the array and three tubular structures at positions downstream from the front row have VIV suppression devices connected thereto, according to one example embodiment. This design relies more upon frequency dissociation than upon strong suppression on the front row.
• Five tubular structures on the top and right sides of the array constitute a front row that first experiences the predominant ocean current. In this case, only one tubular structure with VIV suppression devices is on the front row.
• the array is aligned so a tubular structure at the upper right corner position, which first experiences the predominant ocean current, has VIV suppression devices connected thereto. This alignment also places more of the tubular structures in the wakes of upstream tubular structures. Notice that the lower left tubular structure, which would last experience the predominant ocean current after all other tubular structures of the array, does not have a VIV suppression device(s).
• Figure 3G illustrates another example configuration for twelve tubular structures arranged in a four-by-three rectangular array of twelve tubular structures, according to one example embodiment.
• tubular structures having VIV suppression devices As a result, in relatively larger arrays it is generally possible to use comparatively smaller numbers or percentages of tubular structures having VIV suppression devices than with small to moderate numbers of tubulars. For arrays with more than 12 risers even 20 percent or 25 percent of the tubular structures may have VIV suppression devices depending upon the particular implementation.
• Non-rectangular arrays are also suitable.
• Figures 3H illustrates yet another example configuration for nine tubular structures arranged in a concentric array, according to one embodiment.
• the concentric array is circular.
• the array may be elliptical, oval, star shaped, triangular, etc.
• Arrows are used to indicate a predominant ocean current. The array is aligned so that a tubular structure that has VIV suppression devices coupled thereto first experiences the predominant ocean current.
• Tubular structures with VIV suppression devices are substantially staggered with tubular structures that lack VIV suppression devices. As before, on a periphery, only every other tubular structure has VIV suppression devices.
• the VIV suppression devices may optionally be conventional, and may be constructed of any suitable material conventionally used for VIV suppression devices.
• protective structures such as, for example, covers, caps, bumpers, or the like, may optionally be included on the VIV suppression devices to help prevent mechanical damage, if bumping or contact with a VIV suppression device were to occur.
• the protective structures may comprise pliable, elastic, or soft materials, such as, for example, rubber, plastic, foam, or the like.
• the ends of the sections of the VIV suppression devices may optionally be tapered to a smaller outside diameter than the outside diameter of a remainder of a section of a suppression device, which may facilitate installation and/or insertion through spacers.
• a tubular structure having a VIV suppression device installed thereon is inserted through an opening in a spacer, if an end of the suppression device that is to be initially inserted through the opening is tapered to a smaller outside diameter, it tends to be easier to align the suppression device/tubular structure with the opening and insert it into the opening.
• VIV suppression devices rather than using a single type and/or size of VIV suppression device, multiple, different types and/or sizes of VIV suppression devices may optionally be used, although this is not required.
• some of the subset of risers that have VIV suppression devices may have a first type of VIV suppression device (e.g., strakes), and others of the subset may have a second, different type of VIV suppression device (e.g., fairings).
• One strategy for using different types of VIV suppression devices might be to change the excitation frequencies of the risers and/or increase the frequency dissociation of the array.
• the different types may optionally be staggered relative to one another to provide additional frequency dissociation.
• tubular structures having a plurality of different outer diameters Another way of reducing vibrations is by using tubular structures having a plurality of different outer diameters. Accordingly, other embodiments pertain to a plurality of risers or other tubular structures, in which at least two of the risers or other tubular structures have different outer diameters. In one or more embodiments, at least three of the tubular structures may have different outer diameters.
• the diameter of a tubular structure affects its vortex shedding frequency and its VIV resonant frequency.
• tubular structures that have relatively larger hydrodynamic diameters will tend to have lower vortex shedding frequencies and lower excitation frequencies compared to tubular structures that have relatively smaller hydrodynamic diameters.
• including tubular structures with different outer diameters in a group, array, bundle, or other coupled plurality may help to "detune" the vibratory frequency of the group, array, bundle, or other plurality by increasing the level of frequency dissociation of the array.
• an upstream tubular structure typically has a higher shedding frequency than a downstream tubular structure that is in its wake. As a result, greater frequency dissociation is generally possible when a relatively larger diameter tubular structure is downstream of a relatively smaller diameter tubular structure.
• a tubular structure having a relatively larger diameter may be in a wake of, or downstream from, a structure having a relatively smaller diameter.
• an average diameter of a plurality of upstream tubular structures may be less than the average diameter of a plurality of downstream tubular structures in their wake.
• the largest diameters may range from 5 percent to 200 percent larger than the smallest diameters (expressed as a percentage of the smallest diameters).
• the largest diameters may range from 10 percent to 150 percent larger than the smallest diameters. In cases, the largest diameters may range from 25 percent to 100 percent larger than the smallest diameters.
• the scope is not limited to any known difference between the diameters.
• a sheath or other coating may be included on the outside of a tubular structure in order to increase the outside diameter of the tubular structure.
• the term coating is not limited to a paint-like application process or the like but more broadly encompasses a material coupled with the outside of the tubular structure.
• coatings having a plurality of different thicknesses may be included on the outside of different tubular structures in order to increase the outside diameters of the tubular structures, and to provide a plurality of different outside diameters.
• the coatings may potentially serve a purpose other than to increase the diameter.
• the coatings may include thermal insulation to thermally insulate a fluid within the tubular structures.
• the coatings may include a buoyancy material.
• Figure 4 illustrates an example approach or configuration of a plurality of tubular structures in which at least two, in this case at least three, of the tubular structures have different outer diameters, according to one or more embodiments.
• this figure is a cross-sectional view taken along section line 3/4/5 of Figure 1 through spacer 110A and the plurality of tubular structures 106.
• tubular structures are arranged in a three-by- three rectangular array.
• a tubular structure at a center position 106E and four tubular structures at four corner positions 106A, 106C, 106G, and 1061, all have a first outer diameter.
• a tubular structure 106D at a left center side position and a tubular structure 106H at a lower center side position both have a third outer diameter.
• the first outer diameter may be less than the second outer diameter
• the second outer diameter may be less than the third outer diameter.
• one or more of the tubular structures may have yet another fourth outer diameter different than the other three diameters.
• FIG. 5 illustrates an example approach or configuration that is similar to that of Figure 4 except that, in addition to the different outer diameters, a subset of the tubular structures also have VIV suppression devices connected thereto, according to one or more embodiments.
• the hatched circles are tubular structures with VIV suppression devices.
• the un-hatched circles represent tubular structures without VIV suppression devices.
• the hatched circles are tubular structures without VIV suppression devices.
• the un-hatched circles represent tubular structures with VIV suppression devices.
• Both the different outer diameters and the VIV suppression devices may contribute to reducing vibrations.
• the greater the variation in the outer diameters the lesser the number of VIV suppression devices that would be needed to sufficiently reduce vibrations a particular implementation (including potentially none).
• devices or structures may be part of or connected with an array or other associated or connected plurality of tubular structures. This may help to increase the overall suppression of the system. Examples of such devices include helical axial fins, axial non-helical fins, and circumferential fins.
• the fins or other devices or structures may be flexible or include a flexible material to further provide dampening.
• a coating that attracts marine growth may be added to the outside of the tubular structure to enhance dampening.
• a method of assembly may include initially installing a structural support tubular structure having one or more spacers connected therewith.
• a center tubular structure e.g., structure 106E
• a center tubular structure may be initially connected to one or more spacers along a length of the structural support (e.g., a length of multiple sections of the structural support).
• one or more of other tubular structures may be separately threaded, inserted, or otherwise introduced through openings in the spacers.
• the tubular structures may either have VIV suppression devices connected thereto before being introduced into the openings or the VIV suppression devices may be connected afterwards.
• tubular structures in the final assembly, some but not all, or a subset, of the tubular structures have VIV suppression devices connected thereto. In one or more embodiments, in the final assembly, a number of the tubular structures, in some cases at least three of the tubular structures, have different outer diameters. Other embodiments pertain to methods of suppressing vibrations in the connected or associated tubular structures. In one or more embodiments, vibrations are suppressed with VIV suppression devices connected with only a subset of the tubular structures of a coupled or associated array or grouping. In one or more embodiments, tubular structures with a number of outer diameters (e.g. at least three different outer diameters) may be vibrated at a plurality of different frequencies.
• Figures 6A & 6B
• FIG. 6A illustrates an example of a marine system 600 in which embodiments may be implemented.
• the marine system includes a Floating Liquefied Natural Gas (FLNG) plant 602 on/in a surface of the ocean 104.
• FLNG Floating Liquefied Natural Gas
• the FLNG plant is one particular example of a surface structure.
• the FLNG plant may cool and liquefy natural gas, or alternatively heat and gasify LNG.
• One or more tubular structures of an array or grouping connected with FLNG plant 602 may be used to bring water from the ocean to the plant.
• riser arrays may be used as drilling riser arrays, production riser arrays, TLP tendons, etc.
• Marine system 600 in this embodiment, includes a number of tubular structures or risers 606 (e.g., nine tubular structures).
• the risers each have first ends and second ends.
• the first ends are connected to with the FLNG plant.
• the second ends project generally downward into the ocean but not necessarily to the seafloor.
• the second ends may have depths of around 130 to 170 meters, although this is not required.
• tubular structures 606 may deflect from vertical by around 40 degrees or so (not shown).
• tubular structures 606 may be connected with the FLNG plant through a swivel joint, a ball joint, a riser hanger, or other pivotable or hingeable coupling.
• Tubular structures 606 are physically associated or connected together with a plurality of guide sleeves or spacers 610A, 610B, 610C.
• the spacers may have openings through which respective ones of the tubular structures are disposed. In one embodiment, enough spacers may be provided to keep the tubular structures from striking into one another.
• tubular structures 606 may serve as water intake risers.
• the water intake risers may take in cold water 640 at depth, and convey the cold water upward to the FLNG plant.
• the cold water may be input to heat exchangers of the FLNG plant in order to cool natural gas to help liquefy the natural gas.
• the heated ocean water from the outlet of the heat exchangers may be discharged back into the ocean at the surface, or alternatively conveyed back to depth with a different riser or set of risers.
• filters may optionally be coupled to each of the bottoms of tubular structures 606. The filters may help to prevent soil, marine life (e.g., seaweed, algae, fish, etc.), and the like, from entering the tubular structures.
• an array or grouping may include surplus water intake tubular structures (risers) so that the surplus water intake tubular structures may optionally be included to provide adequate water intake even after some of the filters clog.
• a tubular structure may be used until its filter clogs and then may be taken off line and a new tubular structure having a clean filter may be newly brought online.
• tubular structures with VIV suppression devices may not be used for water intake and may not have filters, but rather may be used primarily for VIV suppression, while bare tubular structures without VIV suppression devices may have filters and be used for water intake. These bare tubular structures tend to be easier to retrieve when their filters become clogged.
• Figure 6B shows an example approach or configuration for nine tubular structures (risers) arranged in a three-by-three rectangular array, according to one particular embodiment. This figure is a cross-sectional view taken along section line 6B of Figure 6A through the plurality of tubular structures 606.
• the array has eight tubular structures along the periphery and one riser at the center.
• the eight tubular structures along the periphery may serve as water intake risers to provide cold water to the FLNG plant.
• the tubular structure at the center may serve as a structural support structure (riser) for the spacers.
• the riser at the center may, or may not, convey water to the surface (i.e., may or may not serve as a water intake riser).
• the eight tubular structures along the periphery may have outer diameters of about 42 inches and wall thicknesses of about 1 inch, while the structural tubular structure at the center may have an outside diameter of about 24 inches and a thickness of about 0.75 inches.
• the eight tubular structures along the periphery may be equally spaced apart by a distance of about one outer diameter or about 42 inches.
• each of tubular structures may not be necessary to be in operation at any one time.
• one or more of the tubular structures may serve as a surplus water intake riser.
• Example 1 In this example approach or configuration only tubular structures at all four corner positions of the three-by-three rectangular array have VIV suppression devices connected therewith. Alternatively, other arrangements or configurations disclosed herein may optionally be used. Example 1
• Speed range pertains to the current flow rate.
• Max rms A/D refers to the maximum motion magnitude (root-mean-square value) in the speed range tested and measures vibration. The lower the max rms A/D, the lower the amount of vibration on the risers.
• the scope of the invention is not limited to achieving any known particular amount of VIV suppression or dampening.
• the amount of VIV suppression or dampening appropriate for a particular implementation may vary widely from one implementation to another. This may be due in part to variation in ocean current, tubular length, materials of construction, amount of overdesign, and the like.
• a system comprising an array of structures in a flowing fluid environment, the array comprising at least 3 structures; and vortex induced vibration suppression devices on at least 2 of the structures.
• the array of structures are within a body of water.
• at least one end of the structures are connected to a floating vessel.
• the vortex induced vibration suppression devices are installed on from 20% to 80% of the structures.
• the vortex induced vibration suppression devices are installed on from 30% to 60% of the structures.
• the structures are coupled to each other at a plurality of locations along a length of the structures.
• the array comprises at least one internal structure and a plurality of external structures that form a periphery about the internal structures, wherein the vortex induced vibration suppression devices are installed on from 40% to 65% of the external structures.
• the flowing fluid environment comprises a predominant current direction, and wherein a structure in the array that first encounters the predominant current comprises at least one vortex induced vibration suppression device.
• the system also includes the vortex induced vibration suppression devices are selected from strakes and fairings.
• the vortex induced vibration suppression devices comprise at least two different types of devices.
• the array comprises a first structure having a diameter at least 20% larger than a second structure.
• the flowing fluid environment comprises a predominant current direction, and wherein a structure in the array that first encounters the predominant current comprises a diameter at least 15% smaller than another structure in the array.
• the array comprises at least 6 structures.
• the structures comprise a tubular, each tubular comprising an opening therethrough for transportation of a fluid.
• a method of suppressing the vortex induced vibration of an array of structures comprising installing vortex induced vibration suppression devices on from 10% to 90% of the structures.
• the method also includes connecting a plurality of the structures to each other.
• the method also includes modifying a diameter of at least one of the structures, so that a first structure has a diameter at least 30% larger than a second structure.

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• 2009
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