Patent Publication Number: US-2011052328-A1

Title: Apparatus and method for performing an intervention in a riser

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the use and maintenance of tubular flowlines and, more particularly, to an apparatus and method for performing an intervention in a flexible riser, e.g., to remove a blockage in the riser. 
     2. Description of Related Art 
     In the production of fluids from a subsea hydrocarbon reservoir, a riser is typically used to provide a fluid conduit for production fluids to flow from a subsea location to a topside location. For example, fluids produced from such a reservoir can be delivered to the riser at the seafloor from one or more subsea wells via a subsea pipeline or flowline that connects each well to the riser. The production fluid, which can include crude oil, natural gas, water, as well as other materials, can flow upward through the riser to the topside location, where the fluid can be stored, processed, offloaded, or otherwise handled. For example, a floating, production, storage, and offloading vessel (FPSO) can receive the fluid at the topside location. 
     Hydrates or other materials can sometimes build in the riser and block the riser, partially or entirely, thereby interfering with the delivery of the production fluid and reducing the performance and output of the reservoir. Once formed, a blockage can be difficult to remove. 
     In some cases, a blockage can be removed using a remotely operated vehicle (ROV) that is used to access the subsea end of the riser and either change the pressure in the riser or reverse the flow of fluid therein back and forth in an effort to wash out the blockage. In other cases, such as where the riser is a steel catenary riser, it may be possible to insert a steel tube into the top of the riser and down through the riser to the blockage so that chemicals can be delivered through the steel tubing to break up the blockage. Such steel tubing is provided from a large spool or coil and is typically referred to as steel coiled tubing. The steel coiled tubing and equipment associated with its handling is typically too large and too heavy for deepwater applications. Further, steel coiled tubing may retain a slightly nonlinear shape when passed into the riser and may damage the inside of the riser. Steel catenary risers, which are typically made up of a number of consecutively joined, rigid steel pipe sections, may not be affected by the passage of steel coiled tubing therethrough; however, steel coiled tubing typically cannot be used in flexible risers, which may be damaged by the steel coiled tubing. 
     Thus, a need exists for an improved apparatus and method for performing an intervention in a riser. The apparatus and method should be compatible with flexible risers and risers used in deepwater applications. 
     SUMMARY OF THE INVENTION 
     The embodiments of the present invention generally provide an apparatus and method for performing an intervention in a flexible riser. The method generally includes the use of a composite tube, i.e., a tube formed of a composite material, that can be inserted into a flexible riser, e.g., to deliver a chemical or other material into the riser to remove a blockage in the riser. 
     According to one embodiment of the present invention, a method is provided for performing an intervention in a flexible riser extending between topside and subsea locations. The method includes providing a composite tube defining a leading end, the composite tube being formed of a composite material. The leading end of the composite tube is inserted into the flexible riser so that the composite tube extends through the flexible riser from the topside location toward the subsea location, and so that the composite tube is configured to deliver a material therethrough and into the flexible riser. 
     The composite tube can be formed of a composite material, e.g., a laminate reinforcement material impregnated with a matrix material. In particular, the composite tube can be formed of a composite material comprising carbon fiber impregnated with a polymer. In some cases, the composite tube has an internal diameter of between 0.5 inch and 1 inch. 
     According to one embodiment, the leading end of the composite tube is inserted to a touchdown point of the riser so that the composite tube extends from the topside location to the subsea location. The composite tube can be inserted through at least 1000 feet of the riser so that the tube extends at least 1000 feet below the topside location, e.g., 10,000 feet or more in some cases. In some cases, a material is delivered through the composite tube from the topside location to the leading end of the composite tube so that the material exits the composite tube at the leading end in the riser. Such delivery of material can include delivering a chemical to the flexible riser to remove a blockage in the riser. For example, the leading end of the composite tube can be inserted to a blockage in the riser so that the chemical is provided to the blockage and removes the blockage. 
     According to another embodiment of the present invention, there is provided an apparatus that is configured to perform the above methods. For example, the apparatus can include a device for performing an intervention in a flexible riser by providing a composite tube and inserting a leading end of the composite into the flexible riser so that the composite tube extends through the flexible riser from the topside location toward the subsea location and the composite tube is configured to deliver a material therethrough and into the flexible riser. 
     As explained herein, the apparatus and method of the present invention are compatible with flexible risers and risers used in deepwater applications, e.g., so that interventions can be performed in situations where steel coiled tubing could not be used. In some cases, the apparatus can be smaller and/or lighter than conventional equipment used for performing interventions with steel coiled tubing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a schematic view illustrating a hydrocarbon production system having a flexible riser undergoing an intervention according to one embodiment of the present invention; 
         FIG. 2  is a partial cutaway view of the flexible riser of  FIG. 1 , illustrating a composite tube inserted in the flexible riser for an intervention according to one embodiment of the present invention; and 
         FIG. 3  is an enlarged view of the leading end of the composite tube of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     Referring now to the drawings and, in particular, to  FIG. 1 , there is shown a hydrocarbon production system  10  with a flexible riser  12  that can be treated with an intervention according to one embodiment of the present invention. The system  10  is generally used to receive fluids from a subsea hydrocarbon reservoir  14  and deliver the production fluids to an offshore platform or other surface structure  16  at a topside location  18 . In particular, the system  10  can include one or more subsea wells  20  from which a production fluid is delivered, typically including crude oil, natural gas, water, and/or other fluids. The production fluid can be delivered from the well  20  through a subsea flowline  22  to other subsea equipment  24 , such as a subsea manifold, pump, or processing equipment. Fluids from multiple wells can be combined and/or processed at the sea floor. 
     The riser  12  provides a fluid connection from the subsea location  26  of the equipment  24  to a topside location  18 . In the illustrated embodiment, the riser  12  is a flexible riser, i.e., a flexible, tubular device that is typically formed of multiple layers of materials. For example, the flexible riser  12  can be formed as a flexible tubular structure with a laminar wall including one or more layers of steel, polymers, and/or other materials such that the resulting structure can communicate high pressure fluids in deepwater ocean environments. Unlike a steel catenary riser or other subsea lines formed of rigid steel sections, the flexible riser  12  can undergo significant flexibility during use. Accordingly, flexible risers may be provided with minimum bend radii of less than 50 feet, e.g., 25 feet or less, whereas the typical minimum bend radius of a comparable steel riser made of rigid sections may be 250 feet or more. 
     The flexibility of a flexible riser  12  can be a factor of its laminar construction. In some cases, the flexible riser  12  can include a number of thin layers, each of which is designed to allow for flexing. For example, the flexible riser  12  can have a steel carcass with an inner layer made of corrugated steel. Braided and/or spiraled layers made of steel or other metals (e.g., helical armor layers) can be provided outside the carcass. The braided and/or spiraled layers can be separated and/or covered by polymer (e.g., thermoplastic) sheaths or other intermediate layers. 
     Production fluids are delivered through the riser  12  to the topside location  18 , e.g., to the surface structure  16 . The surface structure  16  can be fixed to the seafloor by a rigid structure. More typically in deepwater applications, the surface structure  16  is either a floating structure or a moored structure. For example, in some cases, the riser  12  can deliver the production fluids to an FPSO at the topside location  18 , where the fluid can be stored, processed, offloaded, or otherwise handled. The flexible riser  12  may undergo significant flexing during operation, e.g., due to water movement and/or movements of the surface structure  16 . 
     A buildup of materials in the flexible riser  12  can occur, potentially interfering with the flow of the production fluid through the riser  12 . For example, in some cases, hydrates, wax, scale, or other materials can build in the riser  12 . Such buildups can occur as deposits that form over time, during transition periods such as during startup or shutdown of a riser  12 , as a result of different materials occurring in the production fluids, as a result of temperature changes, and the like. These materials can build on the inner surface of the riser  12  to partially block the riser  12  and thereby restrict the flow of fluids. For example, hydrates or other materials can sometimes build in the riser  12  and block the riser  12 , thereby interfering with the delivery of the production fluid and reducing the performance and output of the reservoir  14 . Such a blockage can be partially, i.e., such that production fluid can still be communicated through the riser  12  at a reduced rate. As shown in  FIG. 2 , the blockage  30  can entirely block the riser  12  such that production fluid is prevented from passing through a particular portion of the riser  12 . 
     Once formed, a blockage  30  can be difficult to remove. In some cases, hydrates may be removable by changing the pressure of the fluid and/or direction of flow, but such a process typically requires using an ROV to access the subsea end of the riser  12 . Hydrates may also be removable by a process of heating or providing certain chemicals to the blockage  30 . In some cases, it is not desirable to insert steel coiled tubing into a riser  12 . Steel coiled tubing can retain a “memory” of its coiled shape even after being uncoiled from a roll and straightened for insertion into a riser  12 , and portions of the steel coiled tubing can bend during use, thereby putting the steel coiled tubing in contact with the inner surface of the flexible riser  12  and possibly damaging the riser  12 . Moreover, the steel coiled tubing can contact corrugations or other variations on the inner surface of the flexible riser  12 , such that the steel coiled tubing may catch, puncture, tear, abrade, or otherwise damage the flexible riser  12  during use, especially when the steel coiled tubing is inserted into the riser  12 . 
     According to one embodiment of the present invention, the blockage  30  can be removed by an intervention process that includes providing a composite tube  32  into the riser  12 . As shown in  FIG. 2 , the composite tube  32  can be provided from a reel  34  or other storage device. For example, the composite tube  32  can be wound and stored on the reel  34  and unwound during the intervention process as the tube  32  is needed. A leading end  36  of the tube  32  can be inserted into the riser  12  at an entry point  38 , typically at the topside end of the riser  12 . The composite tube  32  can be controlled by an insertion device  40  that grips the composite tube  32  and selectively advances the tube  32  into the riser  12  and withdraws the tube  32  from the riser  12 . During insertion of the tube  32 , the leading end  36  of the composite tube  32  is typically pushed into the flexible riser  12  from the topside location  18  toward the subsea location  26  such that the composite tube  32  extends through the flexible riser  12  from the topside location  18  toward the subsea location  26 . 
     The composite tube  32  defines at least one passage  42  therethrough. In this way, the composite tube  32  can be configured to deliver a material therethrough and into the flexible riser  12 . In particular, the leading end  36  of the tube  32  can be inserted into the riser  12  and advanced to the portion of the riser  12  where the blockage  30  exists. A material for dispersing the blockage  30  can be communicated through the passage(s)  42  defined by the tube  32  so that the material is provided to the blockage  30 . The appropriate material for removing a blockage  30  typically depends on the type of blockage  30  that occurs. For example, in the case of a hydrate blockage  30 , the material provided through the composite tube  32  may be (or include) a hot fluid for melting the hydrate, a solvent for dissolving the hydrate, a relatively light weight fluid (e.g., lighter than the fluid in the riser) for washing out the heavier fluid in the riser above the blockage to thereby reduce the hydrostatic head pressure above the blockage  30 , or the like. 
     It is appreciated that the composite tube  32  used in the methods of the present invention can be formed of a variety of composite materials, such as thermoplastic or thermoset polymers used with reinforcements materials. In particular, the composite tube  32  can be formed of a reinforcement material such as glass fibers, carbon fibers, Kevlar, nylon, polyester, or the like, which is impregnated with resin, such as polyester, polyvinylidene fluoride, polypropene, polyethylene, polyaniline, epoxies, or phenolics. The make-up and structure of the composite tube  32  can be designed according to the required characteristics for the intervention process. For example, if continuous reinforcement fibers are used, the fibers can be wound or laid in a direction or configuration to provide strength in the directions required for the application. If a particular characteristic is required (such as heat resistance, chemical resistance, electrical resistance, or the like), the materials can be selected accordingly. In addition, internal and/or external layers can be provided, such as layers of a polymer that is the same polymer used as the reinforced resin. For example, as shown in  FIGS. 2 and 3 , the composite tube  32  can be a thermoplastic composite tube  32  that is formed of continuous glass fibers impregnated with polyvinylidene fluoride and internal and/or external layers of polyvinylidene fluoride can be provided inside and/or outside the impregnated fiber layer. Alternatively, the composite tube  32  can be made of polypropylene-impregnated carbon fiber, which can be oriented in a spirally-wrapped or other configuration. An additional inner and/or outer layer (or liner) of polypropylene can be provided, and additional carbon fibers can be welded on the side(s) of the tube  32  along the longitudinal direction of the tube  32  to increase the tensile strength to any required specification. 
     The composite tube  32  can be a generally cylindrical structure that defines one generally cylindrical passage such that a generally constant wall thickness is provided throughout the tube  32 . Alternatively, the tube  32  can define multiple parallel passages, e.g., for communicating different fluid flows and/or for housing different structures. In some cases, additional materials such as electrically, optically, or thermally conductive wires or fibers, can be provided in the tube  32 , either integrated into the material of the tube  32  or disposed in a passage defined by the tube  32 . In this way, the composite tube  32  can be configured to power an electrical device inserted into the flexible riser  12 , transmit optical images from the riser  12 , or perform other control or observation functions. 
     The composite tube  32  can be used in various types of risers. In particular, the composite tube  32  can be useful in situations where steel coiled tubing is inappropriate for intervention or other use. For example, the composite tube  32  can be inserted into flexible risers that are formed partially or entirely of non-metallic materials, such as risers formed of composite or thermoplastic materials. In some such situations, the insertion of conventional steel coiled tubing may pose a risk of damage to the riser  12  that is unacceptable and which may be avoided by using the composite tube  32 . 
     It is appreciated that the composite tube  32  can be inserted to any length or depth in the flexible riser  12 . In some cases, the tube  32  can be inserted until the leading end  36  extends to a touchdown point of the riser  12 , i.e., a point where the riser  12  meets the seafloor. In this way, the composite tube  32  can extend from the topside location  18  to the subsea location  26 . In other cases, the tube  32  can be inserted to a lesser extent in the riser  12 , e.g., until the leading end  36  of the tube  32  reaches a blockage  30  located intermediately in the riser  12  between the topside location  18  and the subsea location. The use of the composite tube  32  can be advantageous in deepwater applications, such as application where the riser  12  extends by a vertical distance of 1000 feet or more. In deepwater applications, the tube  32  can be inserted 1000 feet or more, e.g., more than 2000 or 3000 feet, through the riser  12  such that the tube  32  extends at least 1000 feet below the topside location  18 . In some deepwater applications, the composite tube  32  can be inserted to significantly deeper depths, e.g., to 10,000 feet or more, depths to which interventions using conventional tubing may be difficult or impossible. 
     The dimensions of the tube  32  can be designed to correspond to the riser  12  and type of intervention that is planned. Thus, the length of the tube  32  can be at least as long as the planned insertion length, e.g., 1000 feet or more. Similarly, the other dimensions of the tube  32  can be sized appropriately. For example, in some cases the composite tube  32  define a passage that has an internal diameter of 0-3 inches, e.g., between 0.5 inch and 1 inch, and the tube  32  can be used in risers  12  having internal diameters greater than the outer diameter of the tube  32 . In one particular embodiment, the composite tube  32  has an internal diameter of about 1 inch and an outer diameter of about 2 inches. 
     In some cases, the composite tube  32  and the related equipment for storing, inserting, and otherwise handling the composite tube  32  can be significantly smaller and/or more lightweight than conventional equipment for interventions. For example, a given length of the composite tube  32  can be lighter than a corresponding length of conventional steel coiled tubing. Further, the reel  34  and/or insertion device  40  can be rated for handling lighter, more flexible composite tube  32  and, hence, can also be smaller and/or lighter than conventional equipment used for storing and inserting conventional steel coil tubing. In some cases, the savings in weight and size can allow longer lengths of the composite tube  32  to be stored and inserted, potentially increasing the depth to which interventions can be performed. 
     The apparatus shown in  FIG. 2  can be configured to perform the above-described methods for inserting the tube  32  into the flexible riser  12  and, optionally, delivering chemicals or other materials into the riser  12 . More particular, the insertion device can include a pump or other pressure source for delivering a material through the composite tube  32  from the topside location  18  to the leading end  36  of the composite tube  32  such that the material exits the composite tube  32  at the leading end  36  in the riser  12 . For example, the material delivered through the tube  32  can be a chemical that is selected or formulated for removing a blockage  30  in the riser  12 . For example, the material delivered through the tube  32  can be a chemical that is selected or formulated for removing a blockage  30  in the riser  12 . In particular, the material can be a solvent or a relatively light hydrocarbon, e.g., a hydrocarbon that is lighter than the production fluid in the riser  12  that is being delivered therethrough. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.