Patent Publication Number: US-2012043090-A1

Title: Improved subsea riser system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation-in-part of U.S. application Ser. No. 12/785,221, filed May 21, 2010, entitled “IMPROVED SUBSEA RISER SYSTEM,” the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to systems for fluid transportation in deepwater environments. Specifically, the present invention relates to a subsea riser system for the transportation of fluids from, for example, a sea floor to a floating vessel or from the floating vessel to the seafloor. 
     BACKGROUND OF THE INVENTION 
     Within various industries, pipes are used to transport fluids from one location to another. In the petroleum industry, for example, pipes are used to transport crude oil and gas from wells on the seafloor to the sea surface, and to a distribution network at least for some distance between the fluid&#39;s source and its destination. Proper design of piping systems is important to ensure the transportation of fluids in a safe and environmentally friendly manner. Specifically, a piping system has to be designed so that it maintains its integrity when put in use in its particular application. For example, piping systems for use on land have to be designed to take into account parameters such as the pressure of the fluid being transported, the corrosiveness of the fluid being transported, the environment in which the piping system will be located and seismic activity at the location, to name a few. Designers of piping systems for use in water must contend with such parameters and additional parameters such as hydrostatic pressure (the force exerted by the water due to gravity) and hydrodynamic forces (forces due to the motion of the water). 
     Hydrostatic and hydrodynamic forces become increasingly more relevant for piping systems as the water depth in which the piping system is installed increases. In the case of offshore petroleum production, pipes, known as risers, extend from the seafloor to sea surface for transporting, for example, oil and gas from a wellhead on the sea floor to a surface facility. Risers in deepwater systems are subjected to significant internal and hydrostatic pressure and hydrodynamic forces. Consequently, designing risers to withstand the internal pressures, hydrostatic pressures and hydrodynamic forces of deep water can be challenging. This challenge is exacerbated when the surface facility to which the riser is connected is a floating platform because movement of the floating platform due to the wave, wind and sea currents can transmit significant stress to the riser. Continuous application of stress to the riser causes fatigue and eventually could rupture the riser. 
     Close to the surface of a deep body of water, the hydrostatic pressure is low while the hydrodynamic forces are high due to the wind, waves and associated currents. Below the surface currents, there may be submerged currents that cause vortex induced vibrations. For example, in the Gulf of Mexico, the surface currents are typically in the first 200 feet of water depth and the submerged currents can exist in about 1,000 feet of water depth. 
     In the deeper zones of the water, the hydrostatic pressure is higher and the hydrodynamic forces lower than the zones close to the surface. Taking into account the different forces existing at different depths, one type of riser system includes a flexible conduit in the upper turbulent zone of the body of water. Because the flexible conduit is limited in its ability to withstand hydrostatic pressures and axial tension capacity, the flexible conduit is connected to a catenary riser located in the deeper zone of the water (the catenary riser normally curves gently upward from the sea floor). The catenary riser, often made of steel, is able to withstand the hydrostatic pressures at deeper zones of the body of water. The connection between the flexible conduit and the catenary riser is typically located below that zone in the water where the hydrodynamic forces are high. In some riser systems, a buoy is used to support the catenary riser by attaching the riser to the buoy. However, because the flexible conduit is in the upper zone of water, i.e. the first 200 feet of water depth in the Gulf of Mexico, it moves with the currents and this movement causes stress on the catenary riser because the moving flexible conduit is attached to the catenary riser. 
     What is more, the demands on riser systems are changing, in part, because drilling is increasingly occurring in deeper and more hostile water depth locations. This development has made it more challenging to provide cost effective riser systems because of the corresponding increase in hydrostatic pressure and hydrodynamic forces as riser systems are deployed in deeper and more hostile water depth locations. An additional challenge in designing current riser systems is a need to accommodate subsea systems that permit the size of gas and oil risers to be on the order of 16 inches in diameter and larger. 
     As noted above, some current riser systems address the hydrodynamic forces in the turbulent zones close to the surface of a body of water by connecting one end of a flexible conduit to a surface vessel. The other end of the flexible conduit is then connected to a catenary riser made of less flexible material. In order to make the conduit flexible enough to withstand the hydrodynamic forces in the turbulent zone, it comprises several thin layers of steel and elastomeric material (i.e. a composite flexible conduit). The layers of steel and elastomeric material imposes limits on the conduit&#39;s bore size and the pressure and temperature it can withstand. 
     In view of the bore size limitation, it should be appreciated that any change in internal diameter between the catenary riser and one or more flexible conduits connected to the catenary riser makes pigging a complex operation. Pigging involves inserting a device (a pig) into a pipeline and using a fluid to push the pig through the pipeline. As the pig moves through the pipeline, it performs functions such as cleaning the pipeline and, for specialized pigs, inspecting the pipeline. Pigging, in some operations, may need to be done as often as once per week. As the complexity of the pigging operation increases, so does a riser&#39;s operational costs. 
     Though it is possible to pig composite flexible conduits having an internal diameter less than the catenary riser, such an operation adds complexity. For instance, if the catenary riser has an internal diameter of 18 inches and the composite flexible conduit has an internal diameter of 14 inches, current systems provide for a pig that will jump in diameter from 14 inches to 18 inches. It should be noted, however, that pigs usually cannot have a jump in diameter above four inches. What is more, pigs that jump in diameter usually do not work as efficiently as pigs that maintain a constant diameter. 
     Composite flexible conduits are susceptible to high temperature production fluids. As such, a composite flexible conduit is usually the component that limits a riser system&#39;s ability to handle such fluids. 
     In addition to hydrodynamic forces due to wind, waves and associated currents, described above, the conduits in a riser system are subject to movements caused by a change in the products that pass through the conduits. For example, a flexible conduit and a catenary riser will be installed in saltwater. Subsequently, oil is used to displace the saltwater. In turn, the oil may be later displaced by natural gas. These different fluids have different densities. Thus, as the fluid composition in the conduits changes, the weight of the contents in the conduits and the load on the conduits change. Indeed, it is possible that submerged conduits that initially contained a liquid, which is replaced with a gas, will float up towards the surface of the water. Therefore, as the contents of the different conduits change, the relative loads exerted by the conduits against each other change and cause fatigue of components of the riser system. 
     The current technology of suspending an SCR directly from a host facility is limited due to motions caused by ultra deepwater host facilities. The use of flexible pipe directly suspended from the host to the seafloor has different limitations due to its own weight, collapse pressure and temperature restrictions. Thus, a need exists to decrease the limitations for fluid conduits extending the entire length between the seafloor and the host. There are many different types of host facilities, each having different associated hull designs and motions. There is a need for a single system solution that has the versatility to adapt for a broad range of hosts facilities including Floating Production Storage and Offloading (FPSO), SPAR, Tension Leg Platforms (TLP), Semisubmersible (SS), Floating Storage and Offloading (FSO) and any other type of floating deepwater facility. In sum, a need exists for an improved riser system that can address the current demands being placed on riser systems. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to an improved riser system and method of installation. Embodiments of the invention reduce the transmission of forces from one portion of the riser system to another through a connector and use a buoy system that provides fixed and variable buoyancy. 
     One embodiment of the invention includes an improved riser system for use in a deep body of water. The riser system includes two conduits. The first conduit has a first end that is attached to, or is a continuation of, a pipeline located on the sea floor. A connector is connected to the second end of the first conduit. The connector is also connected to a first end of the second conduit and the first and second conduits are coupled together to permit fluid communication between the first and second conduits. The second end of the second conduit is located proximate the surface of the body of water. The connector is configured to reduce transmission of forces from one conduit to the other. The improved riser system includes a mooring system for mooring the connector to the sea floor and a buoy system for supporting the connector, and corresponding portions of the first and second conduits. In embodiments of the invention, the buoy system is attached to the connector and is configured to provide a fixed buoyancy. The buoy system also provides variable buoyancy for adjustment of the buoyancy requirement for the installation method and during the life of the riser. The buoy system is connected to the connector so as to provide vertical support and lateral restraint. 
     Another embodiment of the invention is a method of installing a riser system in a body of water. The method includes preparing a riser assembly above the surface of the body of water. The preparation of the riser assembly includes connecting a first conduit and a second conduit to a connector so that the first and second conduits are in fluid communication with each other. The preparation of the riser assembly also includes connecting a mooring line to the connector and connecting a buoy system to the connector via a flexible member. When the buoy system is connected to the connector it may be at least partially ballasted. This embodiment of the invention further includes lowering the riser assembly into the body of water to a depth below the surface, and at this point the mooring line is attached to a seabed foundation. While lowering the riser assembly, the first and second conduits can be flooded to provide a slight negative buoyancy, and the mooring line is fixed to the sea floor. After the mooring line is fixed to the sea floor, at least a portion of the buoy system can be deballasted, allowing the connector to stabilize at a second predetermined depth. 
     In a further embodiment of the invention, the flexible conduit is made of titanium. Due to titanium&#39;s strength, low density and elasticity, the flexible conduit may be manufactured out of titanium instead of several layers of steel and elastomeric material. Because of the strength and elasticity of titanium, the wall of a titanium flexible conduit is relatively thin yet strong enough to meet the pressure rating and withstand the hydrodynamic forces required for conduits used in turbulent sections of a body of water. 
     Further yet, embodiments of the invention involve a two stage installation process of a riser system. The two stage installation process includes assembling two major sections of the riser system above the surface of the water and installing these sections at separate times in the body of water. Each of the major sections includes a buoy apparatus and portion of a connector. The portions of the connector are connected, under the surface of the body of water in which they are deployed, to form the riser system. 
     Another embodiment of the invention includes a system for pigging a riser. The system includes a pig launching station connected to a first conduit. At least a portion of the first conduit is located on a floor of the body of water. The system also includes a pig receiving station connected to the first conduit. The pig receiving station is configured to receive a pig and liquid displaced from the first conduit by the pig. The first conduit is connected to a second conduit and the second conduit has an internal diameter different from the first conduit. 
     A further embodiment of the invention includes a system that provides components of a riser system to pivot around a certain point of a connector. For example, embodiments of the invention include a riser system in a body of water having a first conduit with first and second ends. The first end of the first conduit interfaces the seafloor. The riser system also includes a connector that has a pivoting device. The connector is connected to the second end of the first conduit. The riser system also includes a second conduit having first and second ends. The first end of the second conduit is connected to the connector. The first and second conduits are coupled together and are in fluid communication with each other. The riser system also includes a mooring system for mooring the connector to the seafloor. The mooring system includes a tendon connected to the pivoting device. The pivoting device is adapted to allow any one of, or a combination of, the tendon, the first conduit and the second conduit to pivot about the pivoting device when a load is applied to any one of the tendons, the first conduit and the second conduit. 
     Further yet, embodiments of the invention include a method of installing a riser system. The riser system has a system structure that includes a first conduit connected to a frame, which has a pivoting device. The system structure also includes a buoy having a tubular configuration with a lumen and a lift line passing through the lumen. The method of installing comprises connecting at least one mooring tendon to the frame and deploying the riser system structure in a body of water. The method of installing also includes connecting the at least one mooring tendon to a floor of the body of water. Further, the method includes positioning a second conduit via the lift line passing through the buoy&#39;s lumen and connecting the second conduit to the first conduit so that the second and the first conduit are in fluid communication. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It will be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It will also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an illustration of a riser system according to one embodiment of the invention; 
         FIGS. 2A-2C  are illustrations of a connector as it is used in a riser system, according to one embodiment of the invention; 
         FIGS. 3A and 3B  illustrate an installation process for a riser system according to one embodiment of the invention; 
         FIGS. 4A-4G  illustrate an installation process for a riser system according to one embodiment of the invention; 
         FIG. 5  is an illustration of a riser system according to one embodiment of the invention; 
         FIGS. 6A-6G  illustrate an installation process for a riser system according to one embodiment of the invention; 
         FIG. 7  is an illustration of a pigging system according to one embodiment of the invention; 
         FIGS. 8A-8E  are illustrations of a connector in a riser system, according to embodiments of the invention; 
         FIGS. 9A-9D  illustrate an installation process for a riser system according to one embodiment of the invention; 
         FIG. 10  is an illustration of a buoy according to one embodiment of the invention; and 
         FIG. 11  is an illustration of a buoy according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an illustration of a riser system according to one embodiment of the invention. Riser system  100  may be for the transportation of oil from a pipeline connected to a wellhead assembly located on seafloor  103  to a floating production, storage and offloading vessel (FPSO)  108 . It should be noted that in embodiments of the invention, riser system  100  may be used to transport other types of fluids, such as water and natural gas, and to different types of export and surface facilities, such as a floating LNG facility. Moreover, in addition to the transportation of fluids from the seafloor to a surface facility, riser system  100  may transfer fluids from the surface facility to the seafloor, for example, for production enhancement of a seafloor reservoir. 
     Referring still to  FIG. 1 , riser system  100  includes two conduits, steel catenary riser (SCR)  102  and flexible conduit  106 . In this configuration, SCR  102  is in fluid communication with a pipeline  109  on seafloor  103  that in turn connects to wellhead assembly  110 . Further, SCR  102  can be coupled to flexible conduit  106  at connector  104  so that SCR  102  and flexible conduit  106  are in fluid communication. Thus, fluid from the wellhead assembly  110  may flow through pipeline  109 , SCR  102 , flexible conduit  106  to FPSO  108 . SCR  102  and pipeline  109  are able to withstand the hydrostatic pressures in the deeper portions of body of water  101  and may be made of material such as carbon steel and other alloys, the like and combinations thereof. 
     Flexible conduit  106  is able to withstand the hydrodynamic forces of the upper levels of body of water  101  and, in embodiments of the invention, is designed to be flexible. Flexible conduit  106  may be made of materials such as steel, alloys and synthetic material the like and combinations thereof. 
     Connector  104 , in embodiments of the invention, is configured to reduce the transmission of forces emanating from the movement of flexible conduit  106  to SCR  102 . As such, connector  104  can reduce the overall stress and strain to which SCR  102  is exposed over time. 
     Connector  104  is preferably moored to seafloor  103  by mooring line  105  and a fastening device  112 . Fastening device  112  comprises a suction pile, gravity weight, the like or combinations thereof. Mooring line  105  comprises a synthetic fiber tendon. Mooring lines must be able to accommodate high loads. Consequently, mooring lines have traditionally been made from materials such as wire ropes and chains. Over time, however, the development of synthetic fibers has brought about the use of mooring lines made from synthetic tendons. These synthetic fiber tendons have the advantage of being lighter than wire ropes and chains but able to accommodate as high loads as wire ropes and chains do. Therefore, the use of synthetic fiber tendons as mooring lines allows the riser system as a whole to be lighter than when other mooring equipment is used, particularly in the deeper water of current production activity. Mooring line  105  may comprise materials such as Polyester, Aramid (aromatic polyamid), LCAP (Liquid Crystal Aromatic Polyester), the like, and combinations thereof. 
     Riser system  100  includes buoy system  107  for vertically supporting the submerged weight of connector  104 , mooring line  105 , flexible conduit  106  and SCR  102 . Buoy system  107  can include a variable buoyancy buoy. As such, buoy system  107  may be unitary or may comprise two or more buoys. Accordingly, buoy system  107  may include fixed buoyancy buoy  107 A and variable buoyancy buoy  107 B. For example, in one embodiment of the invention, variable buoyancy buoy  107 B may be positioned at a fixed depth of about 150-200 feet below the surface of body of water  101  and the fixed buoyancy buoy  107 A may be positioned at a fixed depth below variable buoyancy buoy  107 B. Because buoy system  107  may be capable of providing variable buoyancy, buoy system  107  facilitates the placement of connector  104  at a desired water depth for the attachment of mooring line  105  to fastening device  112 , which fastens mooring line  105  to seafloor  103 . Additionally, buoy system  107 , when connected to connector  104 , is preferably configured to provide only vertical support and thereby lateral restraint to connector  104 , mooring line  105 , flexible conduit  106  and SCR  102 . Therefore, by reducing the transmission of forces from flexible conduit  106  to SCR  102 , and providing preferably vertical support only to connector  104  by buoy system  107 , the service life of SCR  102  may be improved. Connector  104  is preferably connected to flexible member  111  which is connected to fixed buoyancy buoy  107 A. In this manner, connector  104  is suspended from buoy system  107 . Thus, flexible member  111  provides the vertical support for connector  104  and to some extent laterally restrains connector  104 . However, preferably flexible member  111  does not transfer forces from buoy system  107  to SCR  102  and flexible conduit  106  through the connector  104 . 
     Referring to  FIG. 2A , the process for connecting flexible conduit  106  to SCR  102  includes first connecting flexible conduit  106  to connector  104  above the surface of the water. In this embodiment of the invention, flexible conduit  106  is connected to connector  104  by placing flexible conduit  106  on curved support  204 . Fastener  205 , which may be a circular or loop shape, is used to secure flexible conduit  106  at one end of curved surface  204 . Fastener  205  prevents flexible conduit  106  from being dislodged from curved surface  204  but has a large enough diameter to allow flexible conduit  106  to be pulled along curved support  204 , as will be described below. After flexible conduit  106  is secured on curved surface  204 , connector  104  may be placed in the water. While connector  104  is underwater, SCR  102  may then be pulled into frame assembly  201  using pull lines. After SCR  102  is pulled into frame assembly  201 , SCR  102  is locked into frame assembly  201  with a latch mechanism (not shown but well known to those skilled-in-the-art). At this point, there may be gap  203  between SCR  102  and flexible conduit  106 . 
     Referring now to  FIG. 2B , to close gap  203  and provide fluid communication between SCR  102  and flexible conduit  106 , flexible conduit  106  is pulled down onto SCR  102 . Mechanisms known in the art, such as pull lines and hydraulic systems may be used to pull or push flexible conduit  106  onto SCR  102 . Once gap  203  is closed, flexible conduit  106  and SCR  102  may be coupled together. In embodiments of the invention, flexible conduit  106  and SCR  102  may be coupled together by a coupling that comprises a Retlock® connector or other such coupling well known to those skilled-in-the-art. In embodiments of the invention, the coupling may comprise the pull-in mechanism for pulling flexible conduit  106  onto SCR  102 , (in direction x as shown in  FIG. 2B ). Further, it should be noted that in a variation of the invention, flexible conduit  106  may first be locked to frame assembly  201 , then SCR  102  may be pulled up to and coupled to flexible conduit  106 . SCR  102  may then be secured to frame assembly  201 . The top portion of frame assembly  201  can be connected to flexible member  111  while the bottom portion of frame assembly  201  is attached to mooring line  105 . Referring now to  FIG. 2C , this diagram shows how curved support  204  keeps flexible conduit  106  in a bent configuration. Because flexible conduit  106  is in a bent configuration, a force applied to flexible conduit  106  in direction “y,” for example, would bend flexible conduit  106  upwards and pull it away from curved support  204  but not transmit that force to SCR  102 . Conversely, a force in the opposite direction of “y” may bend flexible conduit  106  against curved surface  204  but not transmit that force to SCR  102 . 
       FIGS. 3A and 3B  illustrate an installation process for riser system  100  according to one embodiment of the invention.  FIG. 3A  illustrates an aspect of the installation process that may occur above the surface of the water, for example, on installation vessel  301 . To begin the process, mooring line  105  may be cast from installation vessel  301  into body of water  101 . Mooring line  105  may be connected to connector  104 . Additionally, SCR  102  and flexible conduit  106  may be connected to connector  104  so that SCR  102  and flexible conduit  106  are in fluid communication with each other. 
     Referring now to  FIG. 3B , fixed buoyancy buoy  107 A may be connected to connector  104  by flexible member  111 . Variable buoyancy buoy  107 B may be connected to fixed buoyancy buoy  107 A, via flexible member  107 C, to form buoy system  107 . Fixed buoyancy buoy  107 A may be a syntactic foam buoy. Variable buoyancy buoy  107 B may be a buoyancy tank in, and from, which water may be pumped to vary its buoyancy. Once fixed buoyancy buoy  107 A is connected to connector  104 , connector  104 , SCR  102 , flexible conduit  106  and the remaining portion of mooring line  105  are lowered into body of water  101 . Because mooring line  105  comprises synthetic fiber, which is relatively light, fixed buoyancy buoy  107 A is able to handle loads in deeper zones of body of water  101 , as compared to riser systems that use heavier mooring equipment. SCR  102  and flexible conduit  106  are flooded for the riser system  100  to achieve negative buoyancy when buoy system  107  is placed in body of water  101 . It should be noted that although buoy system  107  is shown as having two buoys, in embodiments of the invention, buoy system  107  may comprise more than two buoys. 
     Referring still to  FIG. 3B , in embodiments of the invention, fixed buoyancy buoy  107 A is designed to partially support the weight of mooring line  105 , connector  104 , SCR  102  and flexible conduit  106 . In this scenario, the riser system  100  is negative buoyant. To continue the installation operation, flexible member  107 C and variable buoyancy buoy  107 B may be deployed and allowed to sink to a predetermined depth in body of water  101 . Variable buoyancy buoy  107 B may be deployed in a fully ballasted or partially ballasted mode so that riser system  100  as a whole still has a negative buoyancy. That is, riser system  100  continues to sink but may be supported from a crane located on installation vessel  301 . As riser system  100  sinks, seafloor  103  can support more of the submerged weight of riser system  100  as more of SCR  102  rests on seafloor  103 . 
     When the top of variable buoyancy buoy  107 B reaches a desired depth, mooring line  105  may be connected to fastening device  112  which in turn may be fastened to seafloor  103 . The connection of mooring line  105  to fastening device  112  may be done with the assistance of a Remote Operated Vehicle (ROV). Indeed, any of the operations disclosed herein, in particular those that take place below the surface of body of water  101 , may be done with the assistance of an ROV. In embodiments of the invention, once variable buoyancy buoy  107 B is at the desired depth and mooring line  105  is connected to fastening device  112 , variable buoyancy buoy  107 B is deballasted until it exerts an upward force large enough to counteract the weight of riser system  100  and thereby suspend riser system  100  in body of water  101  at a fixed depth. At this point in this embodiment of the invention, riser system  100  is installed and variable buoyancy buoy  107 B is positioned vertically above fixed buoyancy buoy  107 A so that buoy system  107  provides only vertical support and lateral restraint to connector  104 , mooring line  105 , flexible conduit  106  and SCR  102 . 
     Typically, riser systems are installed by laying a pipeline from an end location, such as a wellhead, to the SCR location with the end of the pipeline furthest from the wellhead forming the SCR. The SCR is usually located proximate to the expected location of the FPSO. However, in instances where the FPSO is already moored at its final location, it may be desirable to install the riser system so that the installation process begins at the riser location and proceeds towards the wellhead. Referring now to  FIGS. 4A-4G , embodiments of the invention that may implement this variation of the installation process may include partially assembling the riser system, which may comprise connecting the pipeline to a connector. The preparation of the partial riser assembly includes connecting a mooring line to the connector with a gravity weight suspended from the connector. A buoy system is then connected to a connector via a flexible member, as discussed above in relation to  FIGS. 2A-2B . The gravity weight  112  A may then be placed on the seabed onto a suction pile. The magnitude of buoyancy provided by the buoy system may be sufficient to accommodate the weight of the pipeline/SCR when the pipeline/SCR installation begins. 
     Referring to  FIG. 4A , some embodiments of the invention may include a mooring line that comprises synthetic fiber tendons. Mooring lines made from synthetic fiber tendons usually stretch up to 30% of their original length when a load is applied. If a mooring line stretches after it is installed in a riser system, that stretching may change the whole configuration of the riser system. To prevent this problem, installation of a riser system that includes mooring lines made from synthetic fiber tendons preferably includes stretching mooring line  105  prior to installing it. The stretching process may begin by attaching gravity weight  112 A to lowering line  113  and then lowering gravity weight  112 A into the water with the lowering line  113 . In other words, lowering line  113  may be used to provide support to, and suspend, gravity weight  112 A in body of water  101 . One end of mooring line  105  is attached to gravity weight  112 A prior to placing gravity weight  112 A in body of water  101  and the other end secured to installation vessel  301 . By increasing the length of lowering line  113  so that it is longer than mooring line  105  (assuming both lowering line  113  and mooring line  105  are suspended from installation vessel  301  at the same level), the load of gravity weight  112 A may be transferred from lowering line  113  to mooring line  105 . This transference of load to mooring line  105  may stretch mooring line  105  to a desired length. If the desired length is not at first achieved, the process may be repeated to achieve the desired stretching of mooring line  105 . 
     Referring now to  FIG. 4B , after stretching mooring line  105 , lowering line  113  is detached from gravity weight  112 A and flexible conduit  106 , buoy system  107  and mooring line  105  are connected to connector  104  above the surface of the water and then lowered into the water. It should be noted that though buoy system  107  is shown as including fixed buoyancy buoy  107 A and variable buoyancy buoy  107 B, buoy system  107  could be a composite buoy, as discussed further below. 
     Referring now to  FIG. 4C , the installation of the riser system may include the use of a ramp on installation vessel  301  to assemble pipeline  302  and thus installation vessel  301  may act as a pipe laying vessel. Pipeline  302  may be connected to the riser assembly after the riser assembly has been immersed in body of water  101 . In embodiments of the invention, the riser assembly includes a fastening device  112 , which may be gravity weight  112 A. However, in some situations, for example, when seafloor  103  is sloped, it may be necessary to add suction pile  112 B, which provides gravity weight  112 A with horizontal stability. In this variation of the invention, gravity weight  112 A is lowered onto suction pile  112 B. Such devices  112  are well known to those skilled-in-the-art. 
     Referring still to  FIG. 4C , after the riser assembly has been placed in body of water  101  at a final predetermined depth, pipeline  302  is payed out into body of water  101 . As one end of pipeline  302  reaches the vicinity of connector  104 , pipeline  302  is connected to pull lines  303 , which in turn runs through connector  104  to winches located on vessel  305 . Referring now to  FIG. 4D , pull lines  303  can be adjusted in length in order for pipeline  302  to conform to a curvature consistent with a permissible stress level in pipeline  302 . Referring now to  FIG. 4E , if pull lines  303  are reduced in length by the winches on vessel  305 , the end of pipeline  302  will move upward and give pipeline  302  more of a curved configuration. 
     Referring still to  FIG. 4E , installation vessel  301  may continue assembling and paying out pipeline  302  while vessel  305  continues to shorten pull line  303  and thereby pull pipeline  302  towards connector  104 . Pull line  303  pulls pipeline  302  into connector  104  and then pipeline  302  is locked onto frame assembly  201  of connector  104 . Then pipeline  302  may be connected to flexible conduit  106 , similar to the procedure discussed with respect to  FIGS. 2A-2B . After pipeline  302  is locked into connector  104  and connected to flexible conduit  106 , pull lines  303  may be disconnected from vessel  305  and connector  104 . As discussed above with respect to  FIGS. 2A-2B , pipeline  302  may be connected to flexible conduit  106  using a coupling suitable for the purpose and this coupling may comprise a Retlock® connector, which is well known to those skilled-in-the-art. As pipeline  302  is payed out with one of its end locked onto connector  104 , pipeline  302  sinks and bends into a catenary configuration. 
     Referring back to  FIG. 1 , riser system  100 , especially one where the FPSO is moored prior to installation of pipeline  302 , may require that the section of pipeline  302  extending from connector  104  touches down or intersects with seafloor  103  at a particular point—a desired touchdown point. To illustrate, this concept, the touchdown point is labeled T.P. in  FIG. 1  and the desired touchdown point is labeled DTP in  FIGS. 4E and 4F . In embodiments of the invention, a preferred method of achieving the DTP is to use connecting lines  306  and  307  to establish the touchdown point. Connecting lines  306  and  307  may be made from wire rope. Referring to  FIGS. 4E-4F , connecting line  306  may be attached to pipeline  302  and connecting line  307  may extend from and run through channel  309  in fastening device  112 . 
     Referring now to  FIG. 4F , as pipeline  302  approaches seafloor  103 , connecting line  306  may be joined to one end of connecting line  307  using an ROV. The other end of connecting line  307  may then be pulled through fastening device  112  up to vessel  305 . Connecting line  307 , in this configuration, may be used as a hauling line by vessel  305  to ensure that the DTP of pipeline  302  is achieved. Specifically, vessel  305  may apply a pulling force on connecting line  307  in one direction. Connecting line  307  may have a stopper  308 , which is too large to go through channel  309 . The configuration of connecting lines  306  and  307  (including the position of stopper  308 ) is such that when connecting lines  306  and  307  are joined and stopper  308  rests against gravity weight  112 , the touchdown point will be the intersection of line  306  with pipeline  302 . In other words, the distance of line  306 / 307  from stopper  308  to the end of line  306 / 307  that intersects with pipeline  302  determines the desired touchdown point. 
     Referring now to  FIG. 4G , pipeline  302  may be installed at one end location, for example, to wellhead assembly  110 , and connecting line  306  and  307  may be severed. In its installed position, pipeline  302  comprises SCR  302 A and sea floor pipeline  302 B. In this configuration, pipeline  302 B lies on seafloor  103  and provide fluid communication between wellhead assembly  110  and SCR  302 A, which in turn is in fluid communication with flexible conduit  106 . 
     The installed parameters of riser system  100  may vary depending on the body of water in which it is installed and the depth of that body of water. For example, in the Gulf of Mexico, riser system  100  may be installed so that fixed buoyancy buoy  107 A is located below submerged currents which typically means greater than 1,000 feet below the surface. Concurrently, the variable buoyancy buoy  107 B is located below upper currents and turbulent wave action which typically is about 200 feet below the surface. 
     The installation methods described above with respect to  FIGS. 3A and 3B  include performing significant portions of the installation process on an installation vessel. For example,  FIGS. 3A and 3B  show that connector  104 , flexible conduit  106  and SCR  102  are connected together on vessel  301  and then deployed in a body of water. This type of installation can be complex and requires concurrent operation of different types of equipment on vessel  301 . Major challenges for installers of riser systems in this type of operation include (1) concurrently managing major aspects of the installation process in limited space on an installation vessel; (2) meeting the time limits set for the installation process; and (3) reducing safety hazards on the installation vessel. 
     To understand these challenges, it should be noted that some installation processes require at least three different reels on the installation vessel. A first reel is used to hold tendon  105 . The length of tendon  105  needed depends on the depth of the water. A second reel is required for holding flexible pipe. A riser system installation typically requires between several hundred feet to 2,000 feet of flexible pipe. A third reel is required to hold the SCR  102 /pipe  109 . In the installation processes described in  FIGS. 3A and 3B , tendon  105 , flexible conduit  106 , connector  104 , buoy  107  and SCR  102  are deployed at the same time, which is demanding on the installation crew and equipment. 
     Referring now to  FIGS. 6A-6G , a two stage installation process is shown that involves the consecutive installation of two major parts of the riser system. The first stage begins on vessel  615  with the assembling of foundation system  600   a.  The assembling process includes attaching connector portion  604   a  to a mooring system that will be used to moor the riser system to the seafloor  603 . The mooring system includes tendon  605 , which extends from connector portion  604   a  to fastening device  612 . The assembling process also includes connecting buoys  613  to connector portion  604   a.  Once foundation system  600   a  is assembled, it is deployed in water body  601 , as shown in  FIG. 6B . As foundation system  600   a  descends in body of water  601 , fastening device  612  is used to fasten mooring line  605  to seafloor  603  by plugging fastening device  612  into device  616 , as shown in  FIG. 6C . Buoys  613  suspends connector portion  604   a  in an area in the water where connector portion  604   a  will be connected to the other part of the riser system. 
     Once foundation system  600   a  is installed, the second stage of the installation of the riser system begins. Referring to  FIG. 6D , the second stage includes assembling riser structure  600   b  on installation vessel  615 . Assembling riser structure  600   b  includes connecting connector portion  604   b  to SCR  602 . Connector portion  604   b  is configured to mate with, and couple to, connector portion  604   a  forming connector  604  (shown in  FIG. 6G ). Assembling riser structure  600   b  also includes connecting flexible conduit  606  to SCR  602  and connecting buoy  607  to connector  604   b.    
     Referring now to  FIG. 6E , after assembling riser structure  600   b,  it is deployed in the water. As SCR  602 / 609  descends in the water, flexible conduit  606  is connected to FPSO  608 , as shown in  FIG. 6F . Further, an ROV may be used to move connector portion  604   b  closer to connector portion  604   a.  As connector portion  604   b  approaches connector portion  604   a,  guide cone  614 , which is attached to connector portion  604   a,  guides element  615  of connector portion  604   b  so that connector portions  604   b  and  604   a  are properly aligned. Once connector portions  604   a  and  604   b  are properly aligned, they are connected. 
     Referring now to  FIG. 6G , connector portions  604   a  and  604   b  form connector  604  which functions in a manner similar to connector  104  described above with respect to FIGS.  1  and  2 A- 2 C. The connection of connector portions  604   a  and  604   b  may be done by various means well known in the art such as welding and mechanical latching. For example, latch sections l 1  and l 3  are configured to mate and couple l 2  and l 4 . Thus, latches l 1 /l 2  and l 3 /l 4  connect portions  604   a  and  604   b,  which thereby connect foundation system  600   a  to riser structure  600   b.  Referring still to  FIG. 6G , an installed riser system  600  is shown whereby foundation system  600   a  and riser structure  600   b  are connected. 
     This two stage installation process has several advantages. First, the two-stage installation process is less complex as the crews install the foundation system and the riser structure at different times. 
     Second, the two-stage installation process is more easily managed on vessels with limited space, thereby creating a safer working environment. Essentially, the fewer major processes the installation crew has to perform at any one time, the safer the working environment. 
     Third, the two-stage installation process allows more installation vessels to install riser system  600 . Referring to  FIGS. 6A-6G , installation vessel  615  will be required to have a lifting capacity sufficient to raise the entire riser system. However, the two stage installation process described herein reduces the maximum load that the installation vessel needs to support at any one time. This reduction in lifting capacity means more installation vessels are suitable. 
     Fourth, the two-stage installation process requires less space. There has to be enough deck space on a surface vessel to accommodate the activity. In the two-stage method disclosed herein, all the components do not have to be handled at the same time. Thus, the deck space required on the installation vessel is less. 
     Referring now to  FIG. 5 , a riser system  500  is shown for the transportation of fluid from a pipeline connected from a wellhead assembly  510  on seafloor  503  to an FPSO  508 . Riser system  500  includes two conduits, SCR  502  and flexible conduit  506 . In this configuration, SCR  502  is in fluid communication with a pipeline  509  on seafloor  503  that in turn connects to wellhead assembly  510 . Further, SCR  502  is coupled to flexible conduit  506  at connector  504  so that SCR  502  and flexible conduit  506  are in fluid communication. SCR  502  and pipeline  509  are able to withstand the hydrostatic pressures in the deeper portions of water  501  and may be made of material such as carbon steel and other alloys, hybrids, composite materials and combinations thereof. 
     As noted above, typical composite flexible conduits usually have thick walls of steel and elastomeric material. Further, composite flexible conduits have greater limitations in terms of combined pressure, temperature and inner diameter relative to catenary risers to which they are attached. Consequently, riser systems having composite flexible conduits may require a plurality of flexible conduits for a single catenary riser to achieve equivalent flow, require complex and expensive pigging operations and have fluid temperature limitations based on the temperature limitation of the elastomeric material. 
     To address these issues, the present invention may have flexible conduits made of titanium. Referring still to  FIG. 5 , flexible conduit  506  may be made of titanium. Since titanium is strong, has low density and is elastic, flexible conduit  506  can be made entirely of titanium as compared with several layers of different material used for composite flexible conduits. Since flexible conduit  506  is made of titanium, it does not have the limitations composite flexible conduits have with respect to internal diameter, temperature and pressure. As such, flexible conduit  506  can be sized to correspond with the design criteria of the SCR  502 . For instance, the need to have multiple smaller internal diameter flexible conduits connected to a larger internal diameter catenary riser is eliminated. A catenary riser having substantially the same internal diameter as the flexible conduit means that the catenary riser may be pigged using a pig whose diameter does not need to be changed. Because flexible conduit  506  has the same or substantially the same internal diameter as SCR  502 , it is possible to pig both SCR  502  and flexible conduit  506  in one pigging operation. 
     The use of titanium to make flexible conduit  506  presents further advantages. For instance, with flexible conduit  506  made of titanium, riser system  500  is able to withstand temperatures higher than riser systems that use composite flexible conduits. The temperature limitation of the conduits in a riser system is becoming increasingly significant as the temperatures of produced fluids increase. For example, and in general, composite flexible conduits are not ideal for temperatures above 250° F. (depending on other design parameters this temperature can be significantly less), while a flexible conduit  506  made of titanium can withstand temperatures significantly higher. 
     Referring still to  FIG. 5 , connector  504  is preferably moored to seafloor  503  by mooring line  505  and fastening device  512 . Fastening device  512  may comprise a suction pile, gravity weight, the like or combinations thereof, all of which are well known to those skilled-in-the-art. Mooring line  505 , in some embodiments of the invention, may comprise a synthetic fiber tendon. Riser system  500  includes buoy system  507  for vertically supporting the submerged weight of connector  504 , mooring line  505 , flexible conduit  506  and SCR  502 . Connector  504  is preferably connected to flexible member  511 , which is connected to fixed buoyancy buoy system  507 . In this manner, connector  504  is suspended from buoy system  507 . Thus, flexible member  511  provides the vertical support for connector  504 . However, preferably flexible member  511  does not transfer motions (such as vortex induced vibrations) from buoy system  507  to SCR  502  and flexible conduit  506  through the connector  504 . Buoy system  507  includes components  507   a  - 507   c  which operate similarly to components  107   a - 107   c  described above with respect to  FIG. 1 . 
     It should be noted that though the titanium flexible conduit  506  has a greater bend radius relative to composite flexible conduits, it is still less than that of a steel pipe. Accordingly, referring to  FIGS. 1 and 5 , typically distances R 5  and D 5  of a riser system  500  made of titanium are greater than distances R 1  and D 1  of a system  100  that includes a composite flexible conduit. 
     In some embodiments of the invention, flexible conduit  506  is made of both steel and titanium sections. For example, for sections of flexible conduit  506  that have the most curvature or exposure to significant stress, titanium may be used. Sections  506   a  and  506   f,  for example, may be tapered stress joints and subjected to significant loads due to the movement of FPSO  508 . As such, sections  506   a  and  506   f  may be made of titanium and typically are about  30  feet in length. Similarly, since section  506   d,  known as the dip or sag bend, has a higher curvature than other sections, it may be made of titanium. On the other hand, where strength or elasticity is not critical, steel may be used. Thus, for sections  506   b  and  506   e,  which are relatively straight and are not subject to high stress, steel may be preferable. Another possible reason for using steel is cost. Different sections of titanium and steel may be joined by methods known in the art such as via welding, mechanical flanges and the like. 
     The problem described above with respect to the pigging of a riser system having a SCR and a flexible conduit of different internal diameters may be solved by other methods. For example, in the case of a gas export riser in which liquid is periodically deposited in its pipelines, it is difficult to send a pig through the catenary riser section of the pipe when the catenary riser and the flexible conduit have different internal diameters. As mentioned above, one solution is to use flexible titanium conduits that have the same diameter as a catenary riser. 
     One solution to the problem of pigging different sized conduits is to locate a pig launching device either on the connector or on the seabed at the location of the Pipeline End Termination (PLET). Liquids that are to be displaced in pigging operations frequently accumulate in the valleys of the pipeline that rests ion the seabed since the seabed is not flat. As such, pigging need only be carried out on the section of the pipeline that is on the seafloor and not through the catenary riser section. Thus, the problem of pigging through the catenary riser and the flexible conduit having different diameters is avoided. 
     Referring now to  FIG. 7 , riser system  700  includes components  701 - 712  that operate similarly to components  101 - 112  of riser system  100 .  FIG. 7 , however, shows pipe  709  having curved sections  709   c  and  709   e  (valleys) and  709   a,    709   b  and  709   d  (peaks) due to the unevenness of seafloor  703 . Liquid condensate will accumulate in sections  709   c  and  709   e.  To clear the liquid condensate, riser system  700  has a pigging system that includes pig launching stations  713  and pig receiving station  714 . Pig launching and receiving stations are known in the art and are available from, for example, RHINO® Process Equipment. When displacing liquid from sections  709   c  and  709   e  is necessary, a pig is launched from pig launching station  713 . The pig is pushed by a fluid through line  709  until it reaches pig receiving station  714  where the pig and the displaced liquid are removed from line  709 . In this system, it is not necessary to have the pig traverse different diameters of flexible conduit  706  and pipe  709 . It should be appreciated that riser system  700  could be designed so that pig launching station  713  is located on connector  704  and pig receiving station  714  located on seafloor  703 , or vice versa. 
     Referring to  FIGS. 8A and 8B , a riser system  800  is shown which includes an SCR  802  in fluid communication with pipeline  809 . Pipeline  809  in turn connects to wellhead assembly  810 . SCR  802  is coupled to a flexible conduit  806 , at connector  804  so that SCR  802  and flexible conduit  806  are in fluid communication. Flexible conduit  806  is also connected to FPSO  808  as shown. Connector  804  functions as a tension frame. At the top end of connector  804  is a pivoting device  804   b.    
     Referring to  FIGS. 8B-8C  and  8 E, the pivoting device shown includes a hinge. However, other pivoting devices may be used such as a trunnion, as also discussed below. At the bottom end of connector  804  is crosshead  804   a,  which is a beam connected to the sides of frame struts  804   c.  Crosshead  804   a  supports SCR  802 . To moor riser system  800 , tendon  805  extends from hinge  804   b  to fastening device  812 . Buoyancy device  807  is connected to connector  804  to provide buoyancy support to riser system  800 . 
     Referring now to  FIG. 8B , a perspective view of connector  804  is shown. Flexible members  811  connects hinge  804   b  to buoy  807  (See  FIG. 8A ). Any number of flexible members may be used in embodiments of the invention. For example, it may be desirable to have more than one flexible member  811  as a safety feature. Similarly, the mooring system may include one or more tendons  805 . A plurality of tendons may provide more stability and safety benefits. 
     Referring again to  FIG. 8A , angle A is found between frame strut  804   c  and flexible conduit  806 , angle B between frame strut  804   c  and tendon  805 , and angle C between tendon  805  and flexible conduit  806 . After installation, angles A, B and C tend to change as a result of, for example, loads resulting from a change in the density of the contents of flexible conduit  806  and SCR  802 . Replacing the liquid in flexible conduit  806  and SCR  802  with a less dense gas may cause flexible conduit  806  to move in direction “x” and SCR  802  to move in direction “y.” Movements similar to those indicated in directions “x” and “y” in a conventional riser system bend the flexible conduit and the SCR. In contrast, in the present invention of riser system  800 , the forces exerted in directions “x” and “y” would cause either, or both of, flexible conduit  806  and SCR  802  to pivot about pivoting device  804   b.  In other words, when certain loads are exerted on flexible conduit  806  and SCR  802 , riser system  800  utilizes the pivoting device to allow riser system  800  to move to a new state of equilibrium. Thus, riser system  800  is configured to have loads pass through the pivoting device, allowing riser system  800  to adjust automatically to angular variations between components of the riser system without inducing and storing bending loads on the components, such as flexible conduit  806 , SCR  802  and tendons  805 . 
     Referring to  FIGS. 8C-8E , different configurations for attaching flexible conduit  806  to connector  804  are shown.  FIG. 8C  shows flexible conduit  806  running above hinge beam  804   b.  It should be appreciated that because connector  804  includes a pivoting device, flexible conduit  806  could be a metal pipe. The pivoting device would, at least partially compensate for the inflexibility of the metal pipe when loads are applied. 
       FIG. 8D  shows flexible conduit  806  passing through the center of trunnion  804   d.  In this case, pipe  806   a  is a part of flexible conduit  806 . It should be noted, however, that instead of connecting flexible conduit  806  directly to SCR  802 , pipe  806   a  may be a different pipe from flexible conduit  806 . In this instance, pipe  806   a  is used to connect flexible conduit  806  and SCR  802  at point “P.” The benefit of the design shown in  FIG. 8D  is that loads applied to flexible conduit  806  are transmitted directly to the center of trunnion  804   d.    
       FIG. 8E  shows a design where flexible conduit  806  is connected to bent pipe  813  having “a gooseneck shape.” Bent pipe  813  is in fluid communication with flexible conduit  806  and SCR  802 . Bent pipe  813  is supported by triangular plate  814 . In this configuration, a load applied to flexible conduit  806 , in direction “z,” is transferred by triangular plate  814  to hinge beam  804   b.  Likewise, because the pivoting device is connected to buoy  807 , upward pull loads from buoy  807  are directed to the pivoting device. 
     In sum, as illustrated in  FIGS. 8A-8E , embodiments of the invention seek to have all the major loads exerted on riser system  800  transmitted through a pivoting device (e.g. hinge beam  804   b  or trunnion  804   d ). The loads transmitted to hinge beam  804   b  or trunnion  804   d  will then cause the components, such as flexible conduit  806  and SCR  802  to pivot around hinge beam  804   b  or trunnion  804   d.    
     Referring to  FIGS. 9A-9D , an installation method for a riser system having a connector  904  with a hinge beam  904   b  is shown. The installation begins with vessel  908   a  transporting buoy  907 , tendons  905  and SCR  902 /pipe  909  to a desired location in body of water  901 . On vessel  908   a,  buoy  907  and tendons  905  are connected to hinge beam  904   b.  Further, SCR  902 /pipe  909  is welded to, or otherwise connected, to crosshead  904   a.  Once installation vessel  908   a  is at the desired location, vessel  908   a  deploys buoy  907 , connector  904  and SCR  902 /pipe  909  in the water, as shown in  FIG. 9A . As connector  904  sinks in the water, tendons  905  are fastened to seafloor  903  by inserting plugs  912   a  into receiving devices  912   b,  well known to those skilled-in-the-art. Vessel  908   a  then unravels SCR  902 /pipe  909  from a reel and moves away from connector  904 . As SCR  902 /pipe  909  descends into the water towards seafloor  903 , it pulls connector  904  into a vertical alignment as shown in  FIG. 9B . 
     Referring still to  FIGS. 9A and 9B , vessel  908   a  continues to move away from connector  904  and completes laying SCR  902 /pipe  909  onto seafloor  903 . The curved portion  902  is the catenary riser and the portion that lays on seafloor  903  is pipe  909 . At this point, flexible conduit  906  is not yet connected to connector  904 . To make this connection, flexible conduit  906  is lowered into the water from installation vessel  908   b  by, for example, wires L 1  and L 2 , as shown in  FIG. 9C . Wires L 1  and L 2  support flexible conduit  906  up to the side of connector  904 . Another lift wire L 3  is lowered to the center of connector  904  through lumen  907   a  of buoy  907 . An ROV is then used to take hold of hook  916  located on line L 3 . The ROV moves hook  916  to aperture  915  at the top of triangular support  913 . L 3  is then rolled in to pull flexible conduit  906  towards connector  904  as shown in  FIG. 9D . 
     Wires L 1  and L 2  are then removed from flexible conduit  906  by an ROV, for example. Wire L 3  is then used to lower flexible conduit  906  onto SCR  902  for connection. Supporting flexible conduit  906  through lumen  907   a  makes it easier to lower flexible conduit  906  on top of SCR  902 . It should be appreciated that the installation process shown with respect to  FIGS. 9A to 9D  uses the flexible conduit configuration described with respect to  FIG. 8C . However, one skilled-in-the-art may apply the installation process to different flexible conduit configurations such as flexible conduit configurations shown in  FIGS. 8A and 8B . 
     Referring back to  FIG. 1 , embodiments of the invention may include a variable buoy buoyancy and a fixed buoy buoyancy as shown with respect to buoy system  107 . The design of buoy system  107  with fixed and variable buoyancy buoys, for installation in riser systems, has several advantages. First, this design makes it easier to install riser systems because it facilitates easy lowering of the riser system at a desired depth. Specifically, the ability to vary the buoyancy provides an ability to change the depth of installation. Second, this variable buoy system provides the ability to select preferred weight requirements. In other words, fixed buoyancy buoy  107 A may be selected such that it is still at a slight negative buoyancy at the final operating depth (approximately 1,500 feet in the Gulf of Mexico). Third, in the event variable buoyancy buoy  107 B looses buoyancy and sinks, fixed buoyancy buoy  107 A may still provide a positive vertical load to support riser system  100  after it sinks marginally, at which point it will reach an equilibrium state (remain suspended). Equilibrium is achieved within body of water  101  because as riser system  100  sinks, some of the weight of SCR  102  will be supported on seafloor  103  rather than by buoy system  107 . Fourth, the installation process as disclosed may be easily reversible and thereby facilitates repairs that may be performed above the surface of the water. Specifically, the buoyancy applied to the riser system may be varied and thus after installation, the upward force from the buoy system may be increased to allow the riser system to ascend and be easily removed from the water. 
     Referring to  FIG. 10 , an embodiment of the invention may include composite buoy  1007 . Composite buoy  1007  comprises fixed buoyancy portion  1007 A and variable buoyancy portion  1007 B. Fixed buoyancy portion  1007 A may comprise syntactic foam or other material providing a constant or fixed vertical load. Variable buoyancy portion  1007 B may comprise a tank, to and from, which water may be pumped or any other configuration for providing variable buoyancy. The configuration of composite buoy  1007  may be preferred in selected water depths, particularly if it is desirable to locate the sources of the fixed buoyancy and the variable buoyancy below anticipated upper and loop currents. 
     Referring now to  FIG. 11 , an embodiment of the invention may include buoy  1107 . Buoy  1107  comprises housing  1108 . Housing  1108  encloses syntactic foam buoy elements  1107 A, which are separate elements and may be separated by voids  1107 B. When syntactic buoy  1107  is deployed in water, its buoyancy effect may be increased by passing a gas, such as air, through, for example, pipe  1109 . Conversely, buoy  1107 &#39;s buoyancy may be decreased by releasing gas from housing  1108  through valve  1110 . 
     In embodiments of the invention, mooring line  105  includes several tendons, which may include synthetic fiber tendons. If one or more tendons break, in this configuration, an unbroken tendon could still maintain the installation in the desired location. Referring again to  FIG. 1 , if all the tendons break, fixed buoyancy buoy  107 A will rise to the surface of body of water  101  or rise to a higher level that is below the surface as it moves into an equilibrium state when the weight of the SCR  102  and pipeline  109  increases (because they are less supported by seafloor  103 ) to a point when the upward force from buoy system  107  equals the downward force from the weight of SCR  102  and pipeline  109 . That is, riser system  100  becomes suspended closer to the surface. In sum, failure of the components of riser systems comprising embodiments of the current disclosure will not cause a catastrophic failure of the whole riser system. 
     Riser systems according to embodiments of the invention may include several combinations of SCR  102 , connector  104  and flexible conduit  106 . For example, a first combination of SCR  102 , connector  104  and flexible conduit  106  may connect a first wellhead assembly to a manifold assembly on FPSO  108 . Concurrently, a second combination of SCR  102 , connector  104  and flexible conduit  106  may connect a second wellhead assembly to the same manifold assembly on FPSO  108 . Other configurations may also include different combinations of SCR  102 , connector  104  and flexible conduit  106  running from the same well head to the manifold on FPSO  108 . As one skilled in the art would recognize, such combinations are within the scope of the current invention. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.