Abstract: An underwater installation for use in offshore drilling of and production from, respectively, oil and/or gas wells at deep and moderate deep water comprises an intermediate station in the form of a buoyancy based hollow body (16) tension strut anchored at the seabed (10) and located at a depth substantially closer to the surface of the sea than the seabed (10). At least one hydrocarbon conveying pipeline extends between the buoyancy body (16) and seabed depth, for the transfer of hydrocarbons from the reservoir to said buoyancy body (16). At least one casing extension pipe string (18a-18h) constitutes the tension leg/strut anchoring the buoyancy body (16). The invention also relates to a method for the building of an underwater installation.

Abstract: A riser guide and support mechanism for a floating vessel, and particularly a spar type structure, where the buoyancy can stem extends nearly the entire length of the floating vessel. A riser centralizing element is provided on the riser near the lower end of the buoyancy guide stem. Riser bend limiting elements are positioned on the riser so as to extend above and below the riser centralizing element. Since the buoyancy cans and buoyancy can stem are not required to rotate relative to the stem guides on the floating vessel, the stem guides can be formed from pipe sections that provide a much larger bearing area than is customary.

Abstract: There are disclosed two embodiments of a connector for use in supporting a riser from cable which extend about sheaves intermediate the riser and a tensioning device located on a floating rig above a subsea wellhead to which the lower end of the riser is to be anchored. Each connector is of such construction that the cable may be advanced through an opening in a body of the connector in order that the wear point on the cable where it passes over the sheave may be changed from time to time without having to detach and reattached it from and to the riser.

Abstract: A pressurized slip joint for a marine intervention riser decouples the flowhead assembly in the moon pool of a vessel from the riser string, enabling safe changeover of equipment during workover operations. One part of the slip joint assembly is coupled to the flowhead assembly through a flexible joint assembly. A second part of the slip joint assembly supports the riser string and is coupled to the tensioning mechanism. The first part may be inserted into the second part and locked in place during workover operations except when equipment changeover is taking place. When changeover is being carried out, the first and second parts are unlocked, so that the flowhead assembly does not move relative to the vessel. In the locked position, a metal-to-metal high pressure seal, with a secondary and tertiary seal controls the pressure in the riser. In the unlocked position, a hydraulically operated dynamic low pressure seal is used.

Abstract: An intervention riser for an oil or gas well, which forms a connection between the well head and a floating surface vessel, is sufficiently flexible to accommodate the relative vertical movement between and the well head and a floating surface vessel, is attached to a compensating deck on the vessel which is free to move vertically relative to the vessel, and has winching means having a winch line attached to the riser such that the winching means may pull upon the riser. The compensating deck comprises a deck surface supported on at least one vertically arranged gantry support. The riser system comprises a lower riser section which extends substantially vertically and is essentially rigid, being maintained under tension, and a flexible upper riser section to accommodate movements in the surface vessel.

Abstract: A deepwater riser assembly is disclosed for a riser connecting subsea equipment to a surface wellhead. A buoyancy can assembly provides an open ended buoyancy can tube which surrounds the upper end of the riser and an upper seal which effectively closes the annulus between the riser and the buoyancy can tube. A load transfer connection in the buoyancy can assembly then connects the riser to buoyancy can tube. A pressure charging system communicates with the annulus between the riser and the buoyancy can tube below the upper seal. Injecting gas into this annulus then provides support to the riser.

Abstract: A slide joint for a riser between a well and a floating petroleum installation such as an oil platform includes an outer pipe and an inner pipe which are adapted to move telescopically relative to one another to compensate for changes in the distance between the sea bed and the platform. The inner pipe is connected to a piston, which piston is responsive to actuation by hydraulic pressure in order to provide tractive force on the riser. The outer diameter of the inner pipe is adapted to the diameter of the outer pipe so as to form an annulus between the pipes. The piston is fixedly connected to the inner pipe at or near the downward oriented end thereof, said annulus above the piston being subjected to hydraulic pressure. Below the piston is provided a protective sleeve, which is slidably disposed within the outer pipe.

Abstract: A subsea wellhead assembly located at the sea floor that includes a subsea wellhead and a stationary blowout preventer mounted to the subsea wellhead for controlling a well during drilling. A drill pipe defines an annulus within the stationary blowout preventer. The drill pipe extends to a rotational drive mechanism on a drilling rig at the surface. A drilling head forms a seal against a drill pipe with a rotatable inner portion. The rotatable inner portion rotates with the drill pipe and prevents drilling fluid returns from flowing up the annulus around the drill pipe and prevents drilling fluid returns from discharging onto the seabed. The drilling head is mounted on top of the stationary blowout preventer. A riser extends upward from the drilling head and extends to a floating drilling rig. The riser has a lower end that connects into the drilling head below the drill pipe seal for receiving mud-returns flowing up around the annulus of the drill pipe.

Abstract: There are disclosed two types of valves connectible in a subsea drilling riser to permit the opening and closing of side ports therein. One such valve is automatically opened when the hydrostatic subsea pressure is greater by a predetermined amount than that of drilling mud in the riser. The other valve is installed in the lower end of the riser and is adapted to be opened to discharge drilling fluid therefrom.

Abstract: A single bore riser system is disclosed which is based on a dual bore completion tree. An eccentric offset sub is coupled to the top of the tree and which can be sealed by BOP pipe rams. The completion tree has a main bore and an auxiliary bore with ball valves located in the respective bores. A remotely actuatable annulus plug is located in the bottom of the annulus bore and is moveable between a first position allowing fluid flow through the annulus bore and a second position sealing the annulus bore to provide control of various operations. Embodiments of the invention are described.

Abstract: An apparatus for performing workover operations on a subsea well that has a subsea production tree with a production passage and an annulus access passageway. The subsea production tree is secured to a lower riser portion. The lower riser portion connects to the tree and has a production passage which registers with a production passage of the tree. The apparatus also has an annulus access passage which registers with the annulus access passage of the tree. The apparatus further includes a monobore riser which extends downward from the surface. A string of tubing, preferably coiled tubing, extends through the monobore riser and defines an inner riser passage and an annular riser passage. A circulation plug is secured proximate a lower end of the coiled tubing and is landed in the lower riser portion of the subsea well. The circulation plug communicates the inner riser passage with the annulus access passage.

Abstract: A riser arrangement increases the stability and/or decreases sensitivity to vortex induced vibration of risers of a riser system including steel tubular lines, SCRIBS and flexible hose risers leading to a floating storage/production vessel. A cross-link is placed between two or more steel tubular lines in order to enhance the stability of the riser system. Devices are coupled to the steel tubular lines for increasing their tension in order to increase the natural frequency of vibration in order to reduce sensitivity to vortex induced vibration.

Abstract: An offshore oil or gas well riser system which forms a connection between the well head and a surface vessel. The system has a first riser section which extends vertically and is essentially rigid and a second riser section which is made of a flexible material to accommodate movements of the surface vessel. The first riser section has buoyancy means attached to its upper end to support at least partly the weight of the first riser section, and the first and second riser sections have corresponding first and second bores which provide a continuous path between the surface vessel and the well head. The profile of the flexible second riser section is variable to provide a smooth continuous path through its corresponding second bore and such that the path does not extend below the upper end of the first riser section.

Abstract: A movable diverter housing including a guide adapted to be mounted to the underside of a drill floor on a drilling rig and a diverter housing adapted to move in engagement with the guide.

Abstract: An offshore system is provided of the type that includes riser pipes (30) extending up from the seafloor (44) to a tall and narrow caisson (12) at the sea surface, with the caisson moored by mooring lines (34) extending to the seafloor and anchored thereat, which minimizes bending of the upper portion of the riser pipes when the caisson drifts in severe weather. Although the caisson has a Ballasted lower end and buoyant upper end to keep its axis (20) vertical, a device is provided for applying a horizontal force (54) to a location along the caisson that is vertically spaced from the upper ends of the mooring lines, to tilt the caisson so the axis of the caisson is parallel to portions (82) of the riser pipes lying immediately below the caisson. In one arrangement, a second set of mooring lines (60) is provided, that have upper ends coupled to second locations (64) along the caisson that are vertically spaced from the upper ends of the first mooring lines.

Abstract: An undersea telescopic joint for a riser system is connected to a drilling vessel with a plurality of tensioners. The joint has a bearing with inner and outer annular mating members. A cap seals the outer member to the joint. The outer member closely receives and is axially movable relative to the inner member. A flat thrust bearing is located in a chamber between the two members. The members are sealed to one another with upper and lower swivel seals. A passage communicates hydraulic fluid to the chamber. The bearing has a pressure gage which registers with a passage that extends between the swivel seals. The chamber is filled with hydraulic fluid so that the two members are separated and the drilling vessel may rotate easily. The gage is used to detect whether the primary swivel seal is leaking.

Abstract: A variable-buoyancy marine riser and a method for providing and adjusting the buoyancy for such a riser. The riser is comprised of a main riser conduit on which a plurality of cans are affixed. A slurry of buoyant material, e.g. small, hollow spheres, is pumped into the cans to displace the seawater therein and give buoyancy to the riser. When it is desired to reduce the buoyancy of the riser, the spheres are removed by merely opening the cans and allowing the surrounding seawater to displace the spheres from the cans.

Abstract: A drilling and/or production riser includes on at least part of the length thereof, a stiffener suited to withstand strains induced by the stresses transmitted by the marine environment, those induced by said supporting and guide device, and the strains due to the weight of the loads supported by said stiffener. System intended for production of petroleum effluents from a production site, such as one or more production wells, to a floater having at least one assembly allowing transfer of the effluents from at least one of the production wells to the floater. The assembly includes a production riser equipped on at least part of the length thereof with a float and with at least one stiffener situated above the float and suited to withstand strains induced by the stresses transmitted by the marine environment, those induced by said supporting and guide device and the strains due to the weight of the loads supported by said stiffener.

Abstract: A dual bore riser system is described which includes a conventional monobore riser (12) for production access and an independent coil tubing (14) disposed in parallel with the monobore riser (12) for providing annular access. The monobore riser includes discrete A joints of tubing, casing or drill pipe and the coil tubing riser may be any suitable size but is normally between 23/8" and 27/8" outside diameter. The standard monobore riser (12) and coil tubing riser (14) is fed from a coiled tubing reel (78), which is conventional, via a sheave (80) and straightening rollers into the well with the tubing riser. The coiled tubing is clamped to the tubing riser by clamps (16) at intervals along its length corresponding to a joint every 30 ft. The upper end of the landing spool adaptor (28) receives the tubing (14) and also contains a termination for the coiled tubing which is typically a swage device (32). The 5".times.

Abstract: The present invention relates to an apparatus and to a method for facilitating the connection of underwater flexible risers coming from the sea bed at any point located above the sea surface on a surface structure. Two principal components are used, namely a guide device (1,51), and a connection device (8), which is connected to an underwater flexible riser (14) which it is desired to connect to the structure.

Abstract: There is disclosed an assembly including a protective sleeve spaced about an intermediate pipe of a riser which is adapted to extend through an opening in the bottom of a vertical compartment of a offshore rig for use in drilling or completing a subsea well, with a ball shaped portion on the upper end of the sleeve is closely received by ball shaped surfaces of the upper portion of the riser pipe, while a ball shaped part on the lower portion of the riser pipe is so received within the lower end of the sleeve to permit them to swivel as well as to move vertically with respect to one another.

Abstract: A system and a method are provided for managing the weight of an underwater riser assembly. The system includes a blocking mechanism for selectively blocking the bottom end of the riser assembly so that heavy drilling mud is retained within the riser assembly. Upper and lower flooding valves are located in the riser assembly above the blocking mechanism and are spaced apart. The valves can be opened so that an annulus in the riser assembly is in fluid communication with surrounding water. Drilling fluid can be introduced to or removed from the annulus during deployment or disconnect conditions using the upper and lower flooding valves. For the blocking mechanism, an elongated cylindrical tool can fill the annulus at the bottom end of the riser assembly to block fluid flow in the annulus.

Abstract: A fill-up valve is provided for a riser having a longitudinal axis that is connected at one end to a subsea wellhead and at the other end to a platform of an offshore oil or gas well. The fill-up valve is formed from a gate valve that mounts to an exterior of the riser and occupies a position extending less than the entire transverse periphery of the riser. The gate valve has a transverse profile projecting outward from the exterior of the riser that allows passage of the riser and fill-up valve through confined areas, such as through a rotary table of the well. A flow passage of the gate valve communicates with the interior of the riser to allow water to flow into the interior of the riser.

Abstract: A travel joint provides economical manufacture, installation, service, and operation thereof. The travel joint substantially reduces the number of parts potentially exposed to corrosive fluid, eliminates cavities wherein such corrosive fluid may become trapped, and substantially reduces the number of dynamic and static seals required for its operation. Additionally, a torque transmitting member is utilized to prevent torquing through threaded service breaks in the travel joint. In a preferred embodiment, a travel joint has a tubular upper case, a torque collar rotationally and axially coupled to the upper case, a tubular mandrel axially slidably received in the upper case and torque collar, and rotationally coupled to the torque collar, and an adapter which axially and radially inwardly retains the torque collar.

Abstract: An end of a composite cylinder is formed about an end connection assembly having two longitudinally arranged pieces having a contour which prevents the withdrawal of the end connection assembly from the cylinder. The two pieces of the end connection assembly are relatively forced apart and a snap ring having a certain rest diameter is reduced in diameter and inserted into the end connection assembly to a position between the pieces. The snap ring then assumes its rest diameter positioned between the pieces, forcing them to maintain intimate contact with the inner surface of the cylinder, thus removing any residual gap from the assembly process and/or preloading the interface between the end of the composite cylinder and the end assembly prior to application of an axial load.

Abstract: The invention relates to a guide arrangement 1 for a riser assembly 10 including a length of a second riser tube 6 running from a well head 2 to a surface vessel 6. First buoys 8 are provided which permit the evacuation and refilling of water to adjust the buoyancy effect and comprise a system to admit a controlled amount air or water and which are connected to the lower end of the riser assembly 10 by a first line 14 via a pulley 16 such that as the first buoys are raised the riser assembly 10 is pulled downwards towards the well head. Second buoys are provided to form a rigid frame work for the guide arrangement 1 when air has replaced the water contained therein. Support buoys 22 are connected to the first and second riser tubes 4, 8 by air line tubes 13 which enable air to be removed from and added to the support buoys 22 to adjust the amount of buoyancy they provide.

Abstract: An end of a composite cylinder is formed about a multipiece end assembly having a contour which prevents the withdrawal of the end assembly from the cylinder. The pieces of the end assembly are relatively forced apart and a spacer is positioned between the parts to maintain the pieces in intimate contact with the inner surface of the cylinder thus preloading the end prior to application of an axial load or removing any residual gap from the manufacturing process. The cylinder may have end assemblies at both ends and to which may be coupled external connectors such as flange members, for construction of a marine riser.

Abstract: The present invention relates to an improved apparatus and method for minimizing vortex induced vibrations and hydrodynamic drag of a drilling riser. Vortex induced vibrations and hydrodynamic drag are minimized by installing on a drilling riser streamlined faring sections. The fairing sections are installed on and removed from a riser through the use of one or more door panels on a rounded front portion that have a latch mechanism which can be easily opened and closed. The fairing sections are configured so they can nest one inside the other for easy storage. A tapered back or tail section of each fairing section has an attachment receptacle for engagement by a handling mechanism with a telescoping arm for grasping the fairing section. The handling mechanism is designed to move the fairing sections between a rack, where they are stored, and a position adjacent to the riser.

Abstract: A catenary riser support tube that receives the riser and is attached to the floating offshore structure. The support tube is attached to the floating offshore structure at an angle from the vertical so as to be in line with the natural angle that the installed catenary riser would assume on a calm day. The support tube is attached to the floating offshore structure at different points along the length of the support tube and thus flexes about its attachment points to the floating structure. This minimizes stresses on the catenary riser.

Abstract: A drilling, production, test, and oil storage caisson for use in deep water offshore well operations. Separate low pressure and high pressure drilling risers are independently supported on buoyancy modules. Multiple drilling and production risers are left in the water and connected to the drilling rig and well(s) as needed to prevent the need for the raising and lowering of different risers during the various steps involved in beginning and completing wells. Surface and lower BOP stacks are utilized. Means for controlling the accelerations and velocity of the drilling riser buoyancy modules in the event of riser failure is provided. A subsea tree with dual master valves allows for production directly through a vertical production riser, flowline, to the production manifold on the caisson. A twisted tubing production riser is formed from three strings of tubing that are used for the flowline, annulus, and conduit for control lines.

Abstract: A process comprising: organizing the top of the risers; organizing the bottom of the risers; and twisting the risers to impose tension in the risers. A system comprising: an organizer of the top of the risers; an organizer of the bottom of the risers; and a twister the risers to impose tension in the risers.

Abstract: A deep water support platform, suitable for use as a hydrocarbon exploration or production facility in very deep waters, and a method of constructing the same are shown. The platform is positioned on top of one or more flexible, buoyant piles made of large diameter, high strength steel tubing. A watertight bulkhead is located within each pile and the portion of the pile below is filled with seawater, while the portion above the bulkhead is substantially empty and in communication with the atmosphere. The bulkhead is positioned to cause the pile to have a predetermined net buoyancy so that the portion below the bulkhead, which is anchored to the seabed, is in tension. Adjacent piles are joined at their tops by rigid bending members to prevent rotation of the tops under wind and wave conditions, so that the platform will remain level.

Abstract: A tube assembly for servicing a well head injects an inner continuous coil tubing into an outer continuous coil tubing which can be connected at one end to a well head of an oil or gas reservoir. The inner coiled tubing can be connected to a tool string for servicing the well.

Abstract: A riser for great water depths includes a main tube, peripheral lines, and a base at the lower end of the main tube. The main tube has devices for holding the peripheral lines in position with respect to the main tube. The lower end of each of the peripheral lines is connected with another device placed on the base allowing axial relative movement of at least one of the peripheral lines with respect to the main tube. Also, the riser is provided with a damping device.

Abstract: A riser tensioner system 10 is provided for use on offshore oil drilling platforms to support a riser 12 relative to a floating platform. The riser tensioner system 10 uses either elastomeric pads 32 or helical metal springs 100 as an energy absorbing medium, rather than hydraulic, or pneumatic systems. The elastomeric pads 32 or helical metal springs 100 are maintained in compression within a shock absorber housing 76 by a plunger 70 which is compressed by the weight of the riser 12. If the riser tensioner system 10 is arranged to place the shock absorber 20 in tension, the shock absorber plunger 70 is inverted in the housing 76 to maintain the elastomeric pads 32 or helical metal springs 100 in compression. To achieve varied response characteristics, nested inner and outer elastomeric pads 50, 52 or compression springs 102, 106 are arranged to act together.

Abstract: A riser tensioner system for applying a substantially constant tensioning force to a riser and allowing a floating platform to move within a given range along a longitudinal axis of the riser. The system includes a plurality of tensioner assemblies each of which are coupled to the riser and to the platform. Each of the tensioner assemblies includes an upper member, a lower member, a connecting member coupled to the upper and lower members, and intermediate members coupled to the upper and lower members at a point intermediate the ends of the upper and lower members. At least one of the upper member, the lower member, and the intermediate members are adapted to provide a constant tensioning force. The arrangement of the upper member, lower member, connecting member, and intermediate members further provide a linkage whose centerline in angularly spaced from the longitudinal axis of the riser by a substantially constant amount throughout the range of motion of the linkage.

Abstract: A system for transporting hydrocarbon fluid produced from an offshore well using an offshore production platform and a hydrocarbon fluid storage space in fluid communication with the offshore well via a conduit is provided. The system comprises at least one hydrocarbon fluid export system including a shuttle tanker provided with positioning apparatus capable of limiting movement of the tanker away from a location at a selected distance from the production platform, a flowline for transferring the hydrocarbon fluid to the tanker, at least pan of the flowline being a continuous flexible flowline at one end provided with a connector for releasably connecting the flowline to the tanker so as to allow fluid transfer from the flowline to the tanker, and with a valve for opening or closing the flowline, the flexible flowline extending along the seabed and from the seabed in the form of a riser to the tanker when the flowline is connected to the tanker.

Abstract: An ocean riser assembly for conveying fluids between the seabed and the surface of the sea includes a fluid conveying pipe formed into a helical configuration or into a planar cyclically undulating configuration, such as a sinusoid, that extends longitudinally from the bottom of the sea to the surface. One or more flexible and preferably elastic tension members extend in the longitudinal direction of the riser assembly and are secured to the pipe at spaced intervals, so that they absorb tension forces applied between the ends of the riser assembly. The helical or undulating configuration of the riser pipe decouples axial stresses in the pipe caused by internal fluid pressure from stresses caused by external tension forces acting on the riser assembly, the external tension forces being converted to torque and bending moments acting on the pipe itself. This arrangement permits substitution of carbon steel pipe for flexible pipe, at substantial cost savings.

Abstract: A connector employs a segmented nut and a seal. The connector includes a housing with a bore having a bowl in it. A shaft extends through the bore, the shaft having exterior threads. Segments are carried on the bowl and move between a upper retracted position to an engaged position. A drive system slides the segments down and rotates them until threads from the segments align with the shaft threads. Each segment has a resilient seal which also has a portion of a thread for sealing against the threads of the shaft. An actuator will move the seals from an outboard position which occurs during rotation to a sealing position after rotation.

Abstract: A fixed, non-compliant, non-floating offshore platform subject to lower wave, wind, current, and rig loadings resulting from a configuration that supports the portion of the well casings that extend from near the seafloor to a deck level above the water surface using tension instead of intermediate lateral supports. Only the smallest one or two well casings among all those that comprise the well casing system are extended back from the seafloor, which further reduces the lateral loadings on the structure.

Abstract: A compression element having a deflectable elastomeric member is disclosed. The compression element exhibits a decreasing axial spring rate in response to axial movement and compression of the deflectable member. The compression element may be used alone or in combination with other compression elements in riser tensioner systems to maintain a substantially constant upward tensioning force on a riser as it moves axially relative to a floating platform.

Abstract: A mechanical device for centralizing a subsea riser in an opening in a floating drilling vessel. The centralizing device includes at least three arms that are pivotally connected at one end to the vessel and pivotally connected at the other end to a rotating collar with the riser extruding through the collar.

Abstract: A marine riser section which has an elongated light-weight composite shells is provided with a protective liner of elastomer, epoxy resin or metal. The choice of elastomers includes nitrile rubber, fluorinated rubber or chlorosulfonated polyethylene; the choice of resins includes an aromatic amine cured epoxy with an added styrene curing system; and the metals include steel or titanium alloys or electroless nickel coating.

Abstract: A method of tensioning casing between a subsea well and a surface wellhead employs an adjustment sub. The adjustment sub has upper and lower portions connected together by a threaded section. The running tool locates within the adjacent sub and is employed to secure the lower end of the casing to a tieback latch within the subsea housing. Continued rotation shears out a lower portion of the running tool, causing the threaded section to rotate relative to at least one of the other portions. Rotation of the threaded section brings a hanger on the upper sub portion downward into engagement with a load shoulder in the surface housing.

Abstract: A process for manufacturing a line of variable stiffness over part of the length thereof and the associated element which allows the line to have the variable stiffness thereof is disclosed. The element is integrated during the manufacturing of the line or deposited thereafter around the line; the element consists of composite material underlayers in which the directions of the fibers have at least one component making a zero or a low-value angle with the axis of the pipe, and the layers end in a graded way. The process has application to the manufacturing of a riser base.

Abstract: A deep water platform, suitable for use as a hydrocarbon exploration or production facility in very deep offshore waters, and a method of constructing the same are shown. The platform is positioned on top of flexible, buoyant piles made of large diameter, high strength steel tubing. A watertight bulkhead is located within the pile and the portion of the pile below is filled with seawater, while the portion above the bulkhead is substantially empty and in communication with the atmosphere. The bulkhead is positioned to cause the pile to have a predetermined net buoyancy so that the portion below the bulkhead, which is anchored to the seabed, is in tension. Bending members (trusses) are joined to the top of the pile and are connected to a plurality of neutral buoyancy tendons anchored to the sea floor. The tendons may be driven into the sea floor using a pile driver located within the tendons.

Abstract: A tensioned riser deepwater platform suitable for offshore oil and gas applications is disclosed having a foundation secured to the ocean floor, a topside facility above the ocean surface, and a vertically extending tower jacket secured to the foundation and supporting the topside facility. A plurality of vertically extending risers provide fluid communication between the wells and the topside facility. These risers are connected to riser supports near their upper ends. The riser supports provide the principal load transfer between the risers and the tower jacket and the conductor guides and attendant horizontal bracing are substantially eliminated from the design.

Abstract: A tension leg platform for supporting an oil rig above a seabed includes a deck, a buoyant hull and a single tubular tether. The tether has a lower end penetrating into the seabed and an upper end that is secured to the hull to hold the hull down against its own buoyancy. The deck is secured to the hull and adapted to support an oil rig. Multiple casing strings extend through the tether from the deck to the seabed. Supplemental buoyancy, tanks can be provided between the hull and the tether or within the tether itself.

Abstract: A concentric riser joint comprising inner and outer tubular assemblies each having a longitudinal bore therethrough. An annular space is formed between the inner and outer tubular assemblies. The outer tubular assembly comprises a tubular element having a pin end coupling connected to a first end and a box end coupling connected to a second end of the tubular element. A cylindrical retainer plate is received in the pin end coupling. The cylindrical retainer plate has an inner bore for receiving the inner tubular assembly therethrough. The retainer plate further includes a plurality of longitudinal bores extending therethrough. A seal sub is connected to one end of the inner tubular assembly. The seal sub is partially received in the box end coupling and includes a plurality of longitudinal passageways extending therethrough. A threaded nut connects the outer tubular assembly to an adjoining concentric riser joint.

Abstract: A method to reduce vortex-induced vibrations in subsea risers is provided wherein a flexible fairing is provided surrounding the riser. The flexible fairing will reduce displacements caused by vortex induced vibrations regardless of the direction of the current and will be tolerant of marine life growth.