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1. A free-standing riser system connecting a subsea hydrocarbon fluid source to a surface structure, the system comprising: a concentric free-standing riser comprising inner and outer risers defining an annulus there between, a lower end of the riser fluidly coupled to the subsea source through a lower riser assembly (LRA) and one or more subsea flexible conduits, and an upper end of the riser coupled to a subsea buoyancy assembly and fluidly coupled to the surface structure through an upper riser assembly (URA) and one or more upper flexible conduits;wh...
1. A free-standing riser system connecting a subsea hydrocarbon fluid source to a surface structure, the system comprising: a concentric free-standing riser comprising inner and outer risers defining an annulus there between, a lower end of the riser fluidly coupled to the subsea source through a lower riser assembly (LRA) and one or more subsea flexible conduits, and an upper end of the riser coupled to a subsea buoyancy assembly and fluidly coupled to the surface structure through an upper riser assembly (URA) and one or more upper flexible conduits;wherein the LRA comprises a first generally cylindrical member having a longitudinal bore, a lower end, an upper end, and an external generally cylindrical surface, wherein the first generally cylindrical member comprises a plurality of intake ports extending from the external surface to the bore and configured to flow hydrocarbons from the hydrocarbon fluid source and inflow of a functional fluid, wherein at least one of the intake ports is fluidly connected to an LRA production wing valve assembly, wherein the upper end of the first generally cylindrical member comprises a profile fluidly coupled to the free-standing riser, wherein the lower end of the first generally cylindrical member comprises a connector coupled to a seabed mooring;wherein the URA comprises a second generally cylindrical member having a longitudinal bore, a lower end, an upper end, and an external generally cylindrical surface, wherein the second generally cylindrical member comprises a plurality of outtake ports extending from the bore to the external surface and configured to flow hydrocarbons from the riser, and at least one port configured to flow a functional fluid into the annulus, wherein at least one of the outtake ports is fluidly connected to a URA production wing valve assembly to fluidly couple the second generally cylindrical member with the upper flexible conduit, wherein the upper end of the generally cylindrical second member comprises a connector coupled to the subsea buoyancy assembly, and wherein the lower end of the second generally cylindrical member comprises a profile fluidly coupled to the free-standing riser. 2. The free-standing riser system according to claim 1, wherein the LRA generally cylindrical member comprises a subsea wellhead housing having a lower end and an upper end, the lower end fluidly connected to a transition joint, the transition joint capped with a first pad eye end forging serving as an anchor point for the free-standing riser, the transition joint comprising said one or more intake ports, at least one of the intake ports fluidly connected to the LRA production wing valve assembly and to an internal tieback connector, the internal tieback connector fluidly connected to the inner riser, the upper end of the subsea wellhead housing fluidly connected to an LRA external tieback connector fluidly connecting the subsea wellhead housing to a riser stress joint, the riser stress joint in turn fluidly connected to the outer riser. 3. The free-standing riser system according to claim 1, wherein the URA comprises a drilling spool adapter fluidly connected at a first end to the free-standing riser and a second end fluidly connected to a tubing head comprising one or more outtake ports, the tubing head connected at a lower end to a casing head, and the casing head connected to a shackle flange adapter capped on its top with a second pad eye end forging serving as an attachment point of the free-standing riser to the buoyancy assembly, the URA production wing valve assembly fluidly connected to one of the outtake ports and to the collection vessel through one of the upper flexible conduits. 4. The free-standing riser system according to claim 1, wherein the subsea source is a failed subsea blowout preventer. 5. The free-standing riser system according to claim 1, wherein the LRA further comprises a hub assembly fluidly connecting the LRA production wing valve assembly with one of the subsea flexible conduits. 6. The free-standing riser system according to claim 2, the transition joint further comprising one or more hot stab ports for ROV intervention and/or maintenance. 7. The free-standing riser system according to claim 2, the transition joint further comprising one or more ports allowing pressure and/or temperature monitoring. 8. The free-standing riser system according to claim 1, further comprising an annulus vent sub coupled to the free-standing riser below the URA, wherein the annulus vent sub includes an annulus valve in fluid communication with the annulus, wherein the annulus valve is configured to be transitioned by a remote operated vehicle (ROV) between an open position allowing access to the annulus through the annulus vent sub and a closed position preventing access to the annulus through the annulus vent sub. 9. The free-standing riser system according to claim 1, wherein at least some portions of the inner and outer risers comprise sections of pipe joined by threaded joints. 10. The free-standing riser system according to claim 1, wherein the URA production wing valve assembly comprises at least one emergency shutdown valve (ESD) selected from the group consisting of one hydraulically-operated ESD, one electrically-operated ESD, and one hydraulically-operated ESD and one electrically-operated ESD where both ESDs are controlled using an umbilical connected to a collection vessel at the surface. 11. The free-standing riser system according to claim 1, wherein the URA production wing valve assembly comprises first and second flow control valves for controlling flow in the inner riser and in the annulus. 12. The free-standing riser system according to claim 1, wherein the inner and outer risers are constructed using high strength steel tubulars using threaded coupled connectors. 13. The free-standing riser system according to claim 1, wherein the lower subsea flexible conduits each comprise a lazy wave flexible jumper with distributed buoyancy modules connected from the base of the FSR to a subsea manifold on the seafloor, the manifold fluidly connected to the subsea source or sources. 14. The free-standing riser system according to claim 2, wherein the internal tieback connector comprises a nose seal which seals into a subsea wellhead profile of the subsea wellhead, the internal tieback connector also latching with dogs both to the subsea wellhead housing and to the riser stress joint in order to create a preloaded structural connection between the subsea wellhead housing and the internal and external tieback connectors, and optionally including an external connector latch which latches the internal tie-back connector to the subsea wellhead housing, the nose seal providing pressure integrity between an internal flow path in the inner riser and the annulus between the inner and outer risers. 15. The free-standing riser system according to claim 1, further comprising a suction pile foundation in the seabed, the suction pile foundation comprising a plunger and a chain tether connecting the plunger to the LRA. 16. The free-standing riser system according to claim 1, further comprising external wet insulation on the outer riser for flow assurance. 17. The free-standing riser system according to claim 1, further comprising a flow assurance fluid in the annulus between the inner and outer riser, the flow assurance fluid selected from the group consisting of nitrogen or other gas phase, heated seawater or other water, or organic chemicals such as methanol, and the like. 18. The free-standing riser system according to claim 1, further comprising external wet insulation on the outer riser and a flow assurance fluid in the annulus between the inner and outer riser for flow assurance. 19. The free-standing riser system according to claim 3, further comprising an inner riser adjustable hanger fluidly connecting the inner riser to the upper riser assembly. 20. The free-standing riser system according to claim 1, wherein the buoyancy assembly comprises one or more air cans. 21. The free-standing riser system according to claim 20, wherein the one or more air cans comprises a non-integral air can system comprising a primary and one or more auxiliary air cans to provide failed chamber redundancy. 22. The free-standing riser system according to claim 10, wherein the URA production wing valve assembly comprises both hydraulic and ROV-operated emergency shutdown valves. 23. The free-standing riser system according to claim 1, wherein the URA production wing valve assembly comprises one or more ROV hot-stab ports allowing a flow assurance fluid to flow into both the inner riser and the annulus, the flow assurance fluid selected from the group consisting of nitrogen or other gas phase, heated seawater or other water, or organic chemicals such as methanol, and the like. 24. The free-standing riser system according to claim 1, wherein the one or more upper flexible conduits comprises one or more flexible surface jumpers comprising a quick disconnect coupling allowing it to be disconnected from the floating production and storage vessel in an either an emergency or planned event (i.e. drive/drift off or hurricane evacuation). 25. The free-standing riser system according to claim 1, wherein the outer riser comprises one or more clamps for immobilizing the upper flexible conduits adjacent the outer riser. 26. The free-standing riser system according to claim 1, wherein the system comprises two or more concentric risers positioned laterally apart in the sea, each separately attached to its own respective ship-based floating production and storage facility, and to the same or different subsea source or sources. 27. The free-standing riser system according to claim 1, wherein the system comprises a hydrate inhibition system fluidly connected to the subsea source. 28. The free-standing riser system according to claim 1, wherein the LRA generally cylindrical member comprises a forged high-strength steel member fluidly connected to a production riser pup joint via a lower cross-over joint and threaded connector, and a pad eye flange allowing connection of the forged high-strength steel member to a pile assembly on the seabed. 29. The free-standing riser system according to claim 28, the LRA further comprising dual clamp supports for supporting respective dual subsea connectors, the connector fluidly connecting to two production wing valves assemblies, each fluidly connected to the forged high-strength steel member through respective block elbows. 30. The free-standing riser system according to claim 29, wherein each production wing valve assembly includes an ROV-operable valve. 31. The free-standing riser system according to claim 30, the LRA further comprising an additional assembly or sub fluidly connecting to the forged high-strength steel member through a third block elbow, the additional assembly or sub providing a fluid connection to a source of a functional fluid, such as a flow assurance fluid or other fluid. 32. The free-standing riser system according to claim 31, the LRA further comprising a hot stab assembly for injection of a functional fluid, the hot stab assembly allowing for a smaller flow rate of functional fluid than is possible through the additional assembly or sub. 33. The free-standing riser system according to claim 32, the forged high-strength steel member further comprising an internal surface, at least a portion of which is threaded, to threadedly engage mating threads of a tieback ring, the tieback ring including at least one set of internal threads which mate with a set of threads on the inner riser, and further including a seal element comprised of Inconel or other corrosion-resistant metal. 34. The free-standing riser system according to claim 33, the LRA further comprising an annulus vent sub for functional fluid injection (or circulation out) comprising dual inline ROV-operable valves, the annulus vent sub including a bore providing access to the annulus between the inner riser and the forged high-strength steel member and the lower cross-over joint. 35. The free-standing riser system according to claim 8, the URA further comprising components allowing circulation of a functional fluid, such as heated water, through the annulus between the URA and the annulus valve on the annulus vent sub. 36. The free-standing riser system according to claim 35, the URA comprises an offtake spool fluidly connected to a hanger spool, the hanger spool in turn fluidly connectable to a tapered stress joint of the free-standing riser. 37. The free-standing riser system according to claim 36, the URA further comprising a shackle and chain tether allowing the URA to be mechanically connected to a near-surface buoyancy device. 38. The free-standing riser system according to claim 37, the URA further comprising a first block elbow includes an inner bore which intersects with and is substantially perpendicular to a bore in the offtake spool, a second block elbow having an inner bore which is also substantially perpendicular to the offtake spool bore but which does not intersect the offtake spool bore, and a gooseneck conduit fluidly connected to the first block elbow providing a flow path for hydrocarbons in combination with first block elbow bore. 39. The free-standing riser system according to claim 38, the URA further comprising first and second emergency shutdown valves in the gooseneck conduit, the gooseneck conduit fluidly connected to a subsea connector in turn fluidly connected to an upper subsea flexible conduit of the one or more flexible subsea conduits. 40. The free-standing riser system according to claim 38, the URA further comprising a bleed valve in the gooseneck conduit allowing shutting in the URA, bleeding off contents of the gooseneck conduit, and retrieving the upper subsea flexible. 41. The free-standing riser system according to claim 35, wherein the components allowing circulation of a functional fluid through the annulus comprises a subsea connector, a conduit and one or more valves in the conduit, the conduit fluidly connected to the hanger spool. 42. The free-standing riser system according to claim 1, the URA further comprising a production bore offtake spool fluidly and mechanically connected to a substantially vertical conduit and to a production tubing, the production tubing in turn fluidly connected to a bend restrictor through a subsea API flange, a high pressure subsea connector, another subsea API flange connection, and optionally a QDC subsea connector, the bend restrictor connected to the upper subsea flexible conduit which extends in a catenary loop to the collection surface vessel, and wherein the substantially vertical conduit fluidly connects in series to an adapter which in turn fluidly connects to a hanger spool an API flange, a casing head via another API flange, a stem joint welded to the casing head, and to the outer riser via a threaded connection into a stem joint, the offtake spool including a shackle flange allowing connection to the subsea buoyancy device. 43. The free-standing riser system according to claim 42, further comprising an ROV-operable ESD fluidly connected in a section of the conduit. 44. The free-standing riser system according to claim 43, further comprising a support bracket which supports production tubing an angle σ to the conduit, and also supports a bend shield which provides a mechanical barrier between the production tubing and the conduit, where the angle σ ranges from 0 to about 180 degrees. 45. The free-standing riser system according to claim 44, further comprising a connection to the hanger spool for connecting a gooseneck for delivery of heated water to the hanger spool from a surface vessel. 46. The free-standing riser system according to claim 45, wherein the gooseneck comprises, in order starting at the hanger spool, an API flange, a section of tubing, a high pressure subsea connector, a subsea API connector and API flange, and a bend restrictor. 47. The free-standing riser system according to claim 46, wherein the inner riser is positioned inside of the adapter, the hanger spool, and the casing head, creating the annulus between an inner surface of the hanger spool and the inner riser. 48. The free-standing riser system according to claim 47, comprising a pair of O-ring seals which seal the inner riser into the adapter, and one or more slips which wedge between an inner slanted surface of the hanger spool and the inner riser, firmly securing the inner riser in the hanger spool. 49. The free-standing riser system according to claim 1, the LRA further comprising a forged, high-strength steel intake spool fluidly connected a gooseneck assembly, the gooseneck assembly fluidly connected to the lower flexible conduit, the intake spool also comprising a connector allowing connection to a source of a functional fluid. 50. The free-standing riser system according to claim 49, wherein the gooseneck assembly comprises a subsea API flange connected in series to a tubing spool, a high-pressure subsea connector, another subsea API flange, and a bend restrictor. 51. The free-standing riser system according to claim 50, wherein the intake spool comprises an internal surface adapted to accept and fluidly connect with an internal tieback connector landed in the internal surface of intake spool, the intake spool further comprising a latching mechanism allowing the internal tieback connector to releasably connect to the intake spool, while an O-ring seal provides a fluid-tight seal between a bore of the internal tieback connector and the annulus. 52. The free-standing riser system according to claim 1 wherein the vessel comprises a dynamic positioning system. 53. A free-standing riser system connecting a subsea source to a surface structures, said system comprising: a concentric free-standing riser comprising an inner and outer riser defining an annulus there between;wherein a lower end of the free-standing riser is fluidly coupled to the subsea source through a lower riser assembly (LRA), one or more subsea flexible conduits, and one or more subsea manifolds;wherein an upper end of the free-standing riser is coupled to a buoyancy assembly and fluidly coupled to the surface structure through an upper riser assembly (URA) and one or more upper flexible conduits, and wherein the URA includes a first connection port in fluid communication with the inner riser and a second connection port in fluid communication with the annulus;an annulus vent sub coupled to the free-standing riser below the URA, wherein the annulus vent sub includes connection port in fluid communication with the annulus;wherein the connection port of the annulus vent sub and the second connection port of the URA are configured to circulate a fluid through the annulus between the URA and the LRA. 54. The free-standing riser system according to claim 53, wherein the LRA further comprises a subsea wellhead housing having a lower end and an upper end, the lower end fluidly connected to a transition joint, the transition joint capped with a first pad eye end forging serving as an anchor point for the free-standing riser, the transition joint comprising the first connection port on the LRA, the first connection port on the LRA fluidly connected to an LRA production wing valve assembly, the wing valve assembly fluidly connected to the subsea source through the one or more subsea flexible conduits, and the upper end of the subsea wellhead housing fluidly connected to an LRA external tieback connector fluidly connecting the subsea wellhead housing to a riser stress joint, the riser stress joint in turn fluidly connected to the annulus. 55. The free-standing riser system according to claim 53, wherein the URA further comprises: a drilling spool adapter fluidly connected at a first end to the concentric riser and a second end fluidly connected to a tubing head comprising the first connection port on the URA, the tubing head connected to a casing head, and the casing head connected to a shackle flange adapter capped on its top with a second pad eye end forging serving as an attachment point of the concentric riser to the buoyancy assembly, the URA further comprising one or more URA production wing valve assemblies, the URA wing valve assemblies fluidly connected to the collection vessel through one of the upper flexible conduits. 56. A free-standing riser system connecting one or more subsea sources to one or more surface structures, said system comprising: a free-standing riser comprising inner and outer risers defining an annulus there between, wherein a lower end of the riser is coupled to one of the subsea sources through a lower riser assembly (LRA) and one or more subsea flexible conduits, and wherein an upper end of the free-standing riser is coupled to a buoyancy assembly and to one or more of the surface structures through an upper riser assembly (URA) and one or more upper flexible conduits;an annulus vent sub disposed along the free-standing riser below the URA, wherein the annulus vent sub is configured to allow the annulus of the free-standing riser to be open to facilitate circulation of a flow assurance fluid through the annulus between the URA and the annulus vent sub; anda hydrate inhibition system fluidly connected to the one or more subsea sources. 57. The free-standing riser system of claim 56 wherein the free-standing riser is in a near-vertical position beneath their respective buoyancy assemblies. 58. The free-standing riser system of claim 56 wherein the annulus vent sub is coupled to the outer riser of the free-standing riser at a point axially between the LRA and the URA. 59. The free-standing riser system of claim 56 wherein the outer riser of the free-standing riser comprises plurality of annulus vent subs fluidly connected thereto in separate longitudinal locations axially spaced along the outer riser. 60. The free-standing riser system according to claim 58, wherein the annulus vent sub comprises one or more valves controllable by an ROV. 61. The free-standing riser system according to claim 58, further comprising external wet insulation on the outer riser. 62. The free-standing riser system of claim 57 wherein the hydrate inhibition system is based on a surface vessel, and the fluid connection comprises a plurality of umbilicals. 63. The free-standing riser system of claim 62 wherein one of the subsea sources is a malfunctioning subsea BOP, and one of the umbilicals is fluidly connected to locations on the subsea BOP selected from the group consisting of a kill line of the subsea BOP, a choke line of the subsea BOP, and both the kill and choke lines of the subsea BOP. 64. The free-standing riser system of claim 62 wherein one of the subsea sources is a malfunctioning subsea BOP, and one of the umbilicals is fluidly connected to a subsea BOP stack manifold. 65. The free-standing riser system of claim 62 wherein one of the umbilicals is fluidly connected to a subsea manifold. 66. The free-standing riser system of claim 56 wherein the hydrate inhibition system comprises: (a) a vessel;(b) one or more tanks secured to the vessel containing a liquid chemical suitable for inhibiting hydrate formation in subsea components;(c) one or more primary pumps fluidly connected to one or more of the tanks;(d) one or more booster pumps fluidly connected to one or more of the tanks and to one or more of the primary pumps; and(e) one or more umbilicals fluidly connected to the one or more primary pumps and to one or more subsea components. 67. The free-standing riser system of claim 66 wherein the primary pumps are diesel-driven, the booster pumps are air-driven, and comprising a subsea, ROV-controlled umbilical distribution box fluidly connecting the umbilicals to a subsea ROV-controlled hot stab patch panel, the patch panel in turn fluidly connected to one or more subsea sources. 68. A method of installing a subsea marine free-standing riser-based system, the method comprising the steps of: (a) constructing one or more concentric free-standing riser systems, each concentric free-standing riser system comprising a concentric free-standing riser, a lower riser assembly (LRA) coupled to a lower end of the free-standing riser, and an upper riser assembly (URA) coupled to an upper end of the free-standing riser, wherein the inner and outer risers defining an annulus there between;(b) installing the concentric free-standing riser system at a subsea location;(c) connecting an upper flexible conduit to the URA;(d) installing a suction pile in the seabed and tensioning the free-standing riser system to the suction pile;(e) connecting a subsea flexible conduit to the LRA and to a subsea source using a subsea installation vessel;(f) injecting a flow assurance fluid into the annulus through a first port of the URA;(g) releasing the flow assurance fluid from the annulus through a second port coupled to the riser below the URA after (f); and(h) maintaining riser tension by connecting the URA to a near-surface subsea buoyancy assembly. 69. The method of claim 68 comprising clamping the upper flexible to a side of the concentric free-standing riser during or after step (c). 70. The method of claim 68 wherein step (b) is performed using a mobile offshore drilling unit (MODU). 71. The method of claim 70 wherein step (b) is performed using a vessel comprising a dynamic positioning system. 72. The method of claim 68 wherein riser tension is maintained using a non-integral aircan system chain tethered above the riser to the buoyancy assembly. 73. The method of claim 72 wherein the aircans provide at least 100 kips (445kilonewtons) effective tension at the base of the riser under all loading conditions, including failure of one or more aircan chambers. 74. The method of claim 68 comprising disconnecting the upper flexible conduit using a quick disconnect (QDC) coupling. 75. The method of claim 68 comprising attaching a disconnectable buoy to the upper flexible near the vessel. 76. The method of claim 68 comprising, in the event of an unplanned or planned disconnect, disconnecting the upper flexible conduit from the vessel in a controlled manner and lowering the conduit using a support vessel to hang the conduit along a side of the free-standing riser. 77. The method of claim 76 comprising clamping the conduit in place substantially adjacent the free-standing riser using an ROV. 78. The method of claim 68 wherein step (a) is performed using existing dry tree riser components and subsea wellhead inventory. 79. The method of claim 68 comprising fluidly attaching a hydrate inhibition system to the subsea marine system. 80. The method of claim 68 wherein step (a) comprises constructing the inner and outer risers using high strength steel tubulars using threaded coupled connectors. 81. A method of producing a fluid from a subsea source, the method comprising the steps of: (a) deploying a subsea marine system comprising at least one concentric free-standing riser including inner and outer risers defining an annulus there between, a lower riser assembly (LRA), and an upper riser assembly (URA);(b) fluidly coupling the free-standing riser to the subsea source and to a surface structure;(c) initiating flow of the fluid from the subsea source though the free-standing riser;(d) maintaining flow of the fluid though the free-standing riser; and(e) circulating a flow assurance fluid through the annulus between an inlet port proximal an upper end of the free-standing riser and an exit port positioned along the free-standing riser axially below the URA. 82. The method of claim 81 comprising shutting down flow of the subsea source by closing at least one emergency shutdown valve in the URA wing valve assembly. 83. The method of claim 81 wherein the URA comprises one or more production wing valve assemblies, the method comprising controlling flow in the inner riser and in the annulus using first and second flow control valves in the URA production wing valve assemblies. 84. The method of claim 81 wherein step (b) comprises fluidly connecting the free-standing riser to the subsea source and to a surface collection vessel using a subsea flexible conduit comprising a lazy wave flexible jumper with distributed buoyancy modules connected from the base of the free-standing riser to a subsea manifold on the seafloor, the manifold fluidly connected to the subsea source or sources. 85. The method of claim 81 comprising fluidly connecting the inner riser to the LRA employing an internal tieback connector. 86. The method of claim 81 comprising securing the free-standing riser using a suction pile foundation on the seafloor, the suction pile foundation comprising a plunger, and a chain tether connecting the plunger to the LRA. 87. The method of claim 81 comprising assuring flow of fluid through the riser using external wet insulation on at least a portion of the outer riser for flow assurance. 88. The method of claim 81 comprising assuring flow of fluid through the riser using external wet insulation on at least a portion of the outer riser and a flow assurance fluid in the annulus between the inner and outer riser for flow assurance. 89. The method of claim 81 comprising assuring flow of fluid through the riser using external wet insulation on at least a portion of the outer riser, and injecting a second flow assurance fluid into the flow stream inside the inner riser. 90. The method of claim 81 comprising fluidly connecting the inner riser to the upper riser assembly employing an inner riser adjustable hanger. 91. The method of claim 81 comprising maintaining buoyancy of the riser using one or more buoyancy assemblies. 92. The method of claim 91 wherein the one or more buoyancy assemblies comprises a non-integral air can system comprising a primary and one or more auxiliary air cans to provide failed chamber redundancy. 93. The method of claim 82 wherein closing at least one emergency shutdown valve in the URA wing valve assembly comprises closing both an hydraulic and an electrically-operated emergency shutdown valve using an umbilical from the surface. 94. The method of claim 81 comprising wherein the URA production wing valve assembly comprises one or more ROV hot-stab ports allowing nitrogen or other flow assurance fluid injection into both the inner riser and the annulus. 95. The method of claim 81 comprising wherein the LRA production wing valve assembly comprises one or more ROV hot-stab ports allowing nitrogen or other flow assurance fluid injection into both the inner riser and the annulus. 96. The method of claim 81 comprising disconnecting one or more upper flexible conduits using a quick disconnect coupling allowing it to be disconnected from a floating production and storage vessel in an either an emergency or planned event (i.e. drive/drift off or hurricane evacuation). 97. The method of claim 81 comprising immobilizing the upper flexible conduits adjacent the outer riser using one or more clamps attached to the riser. 98. The method of claim 81 comprising positioning two or more concentric free-standing risers vertically and laterally apart in the sea, each separately attached to its own respective ship-based floating production and storage facility, and to the same or different subsea source or sources. 99. The method of claim 81 comprising fluidly connecting a hydrate inhibition system to the subsea source. 100. The method of claim 81 comprising dynamically positioning the surface collection vessel. 101. A method of inhibiting hydrate formation in a subsea free-standing riser-based system, the method comprising the steps of: (a) installing a concentric free-standing riser comprising inner and outer risers defining an annulus there between, a lower riser assembly (LRA), and an upper riser assembly (URA), wherein the LRA includes an annulus vent sub configured to provide access to the annulus at the LRA;(b) flowing a flow assurance fluid into the annulus at the URA and out of the annulus at the annulus vent sub; and(c) flowing a hydrate-inhibitor liquid chemical from a surface structure to one or more subsea components. 102. The method of claim 101 wherein the flow assurance fluid is a gas atmosphere consisting essentially of nitrogen. 103. The method of claim 101, further comprising installing a wet insulation on the free-standing riser, wherein the wet insulation comprises a syntactic material. 104. The method of claim 103 wherein the syntactic material comprises a plurality of layers of syntactic polypropylene. 105. The method of claim 101 wherein the hydrate-inhibitor liquid chemical is selected from the group consisting of alcohols and glycols. 106. A free-standing riser system connecting a subsea hydrocarbon source to a surface structure, said system comprising: a concentric free-standing riser comprising inner and outer risers defining an annulus there between, wherein a lower end of the riser is coupled to the subsea source through a lower riser assembly (LRA) and one or more subsea flexible conduits, and wherein an upper end of the riser is coupled to a buoyancy assembly and the surface structure through an upper riser assembly (URA) and one or more upper flexible conduits, wherein the riser is maintained in an erect substantially vertical position by tension applied by the buoyancy assembly;wherein the LRAincludes a first flow path configured to flow a hydrocarbon fluid from the subsea hydrocarbon source to the inner riser, and a second fluid path configured to flow a functional fluid into or out of the annulus, wherein the first fluid flow path is isolated from the second fluid flow path;wherein the URAincludes a third fluid flow path configured to flow the hydrocarbon fluid out from the inner riser to a surface vessel, and a fourth fluid flow path configured to flow the functional fluid into or out of the annulus, wherein the third fluid flow path is isolated from the fourth fluid flow path. 107. The system of claim 106 wherein the surface structure comprises a dynamic positioning system.
