Marine subsea riser systems and methods

Abstract

A riser system connects a subsea source of hydrocarbons to a collection vessel. The system includes a riser, a lower end of the riser fluidly coupled to a seal stem, the seal stem in turn fluidly attached to a lower riser assembly through a polished bore receptacle. The upper end of the riser is connected to the collection vessel, the riser being maintained in a near vertical position. Methods of installing and using the riser systems for killing and cementing wells are described.

1 . A riser system connecting a subsea source to a surface vessel, said system comprising:
 a near-vertical riser comprising a lower end and an upper end, the upper end of the riser mechanically and fluidly connected to the surface vessel;   a seal stem, a lower end of the riser fluidly and mechanically connected to the seal stem, the seal stem comprising one or more exterior elastomeric sealing elements;   a lower riser assembly comprising a member having a longitudinal bore, a lower end, an upper end, and an external surface, the member comprising sufficient intake ports extending from the external surface to the bore to accommodate flow of hydrocarbons from a hydrocarbon fluid source, at least one of the intake ports fluidly connected to the subsea source; and   a polished bore receptacle comprising a polished bore, a lower end of the PBR fluidly and mechanically connected to the upper end of the member;   the exterior elastomeric sealing elements of the seal stem sealingly engaging the polished bore to create a pressure-tight flow path through the polished bore receptacle, seal stem, and riser.   
     
     
         2 . The riser system according to  claim 1 , wherein the riser comprises a plurality of riser joints. 
     
     
         3 . The riser system according to  claim 1 , wherein the vessel comprises a dynamic positioning system, and wherein the riser is maintained in the near-vertical position by the dynamic positioning system. 
     
     
         4 . The riser system according to  claim 1 , wherein the member comprises a subsea wellhead housing having a lower end and an upper end, the lower end capped with an end forging that is attached to a foundation in the seabed. 
     
     
         5 . The riser system according to  claim 1 , wherein the seal stem comprises a latch ring that allows reduction of travel of the seal stem in the polished bore receptacle. 
     
     
         6 . The riser system according to  claim 1 , wherein the subsea source is fluidly connected to one of the intake ports via a flexible conduit and a gooseneck assembly. 
     
     
         7 . The riser system according to  claim 1 , wherein the lower riser assembly further comprises one or more hot stab ports for ROV intervention and/or maintenance. 
     
     
         8 . The riser system according to  claim 4 , the wellhead housing further comprising one or more ports allowing pressure and/or temperature monitoring. 
     
     
         9 . The riser system according to  claim 1 , wherein the riser upper end is connected to a drill ship or drilling rig on the vessel. 
     
     
         10 . The riser system according to  claim 1 , the lower end of the lower riser assembly connected to a subsea mooring, and further comprising one or more structural supports for the polished bore receptacle extending from the subsea mooring to a point about midway up the polished bore receptacle. 
     
     
         11 . The riser system according to  claim 10 , wherein the subsea mooring is a suction pile. 
     
     
         12 . The riser system according to  claim 4 , wherein the polished bore receptacle is threaded in to the wellhead housing. 
     
     
         13 . The riser system according to  claim 1 , wherein at least some portions of the riser comprise sections of pipe joined by threaded joints. 
     
     
         14 . The riser system according to  claim 1 , wherein the riser joints are constructed using high strength steel tubulars using threaded coupled connectors. 
     
     
         15 . The riser system according to  claim 1 , wherein an upper end of the polished bore receptacle comprises a guide funnel. 
     
     
         16 . The riser system according to  claim 6 , wherein the subsea flexible conduit comprises a lazy wave flexible jumper with at least one distributed buoyancy module connected from the base of the riser to a subsea manifold on the seafloor, the manifold fluidly connected to the subsea source. 
     
     
         17 . The riser system according to  claim 1 , wherein the lower riser assembly is fluidly connected to an active subsea wellhead via one or more flexible conduits. 
     
     
         18 . The riser system according to  claim 1 , wherein the lower riser assembly comprises one or more ROV hot-stab ports allowing a flow assurance fluid to flow into both the lower riser assembly and the riser, the flow assurance fluid selected from the group consisting of nitrogen or other gas phase, heated seawater or other water, and one or more organic chemicals. 
     
     
         19 . The riser system according to  claim 1 , wherein the seal between the polished bore receptacle and the seal stem is such that the riser and seal stem may be disconnected from the polished bore receptacle, allowing the polished bore receptacle and lower riser assembly to be disconnected from the surface vessel in an either an emergency or planned event. 
     
     
         20 . The riser system according to  claim 1 , the lower riser assembly further comprising an additional assembly or sub fluidly connecting the lower riser assembly to a source of a flow assurance fluid. 
     
     
         21 . The riser system according to  claim 6 , wherein the gooseneck assembly comprises, in order starting at the generally cylindrical member, an API flange, a section of tubing, a high pressure subsea connector, a subsea API connector and API flange, and a bend restrictor. 
     
     
         22 . The riser system according to  claim 1 , the member comprising a forged, high-strength steel intake spool fluidly connected to a gooseneck assembly, the gooseneck assembly fluidly connected to a flexible conduit, the member also comprising a connector allowing connection to a source of a flow assurance fluid. 
     
     
         23 . The riser system of  claim 1 , wherein the subsea source is a malfunctioning subsea BOP, the system further comprising one or more umbilicals, one of the umbilicals fluidly connected to locations on the subsea BOP selected from the group consisting of a kill line of the subsea BOP, a choke line of the subsea BOP, and both the kill and choke lines of the subsea BOP. 
     
     
         24 . The riser system of  claim 1 , wherein the subsea source is a malfunctioning subsea BOP, the system further comprising one or more umbilicals, one of the umbilicals fluidly connected to a subsea BOP stack manifold. 
     
     
         25 . The riser system of  claim 1 , further comprising one or more umbilicals, wherein one of the umbilicals is fluidly connected to a subsea manifold. 
     
     
         26 . The riser system of  claim 1 , further comprising the seal stem extending into the member a distance sufficient to create upper and lower seals between the member and the seal stem, wherein the intake ports are between the upper and lower seals, the seal stem further comprising one or more inlet ports positioned between the upper and lower seals. 
     
     
         27 . A method of installing a subsea marine riser system, the method comprising:
 attaching a first end of a member to an end forging, a first end of a polished bore receptacle to the member, the polished bore receptacle comprising a polished bore and a guide funnel on an end opposite the first end, and attaching the end forging to a subsea foundation so that the polished bore receptacle is substantially vertical;   directing a drill string riser toward the guide funnel, the drill string comprising a seal stem comprising one or more elastomeric seal elements; and   stabbing the seal stem into the polished bore receptacle and establishing a pressure-tight seal between the elastomeric seal elements and the polished bore.   
     
     
         28 . The method of  claim 27 , further comprising:
 connecting a subsea flexible conduit and gooseneck assembly to the member and to a subsea source.   
     
     
         29 . The method of  claim 27 , wherein the steps of directing and stabbing are performed using a mobile offshore drilling unit. 
     
     
         30 . The method of  claim 27 , further comprising assisting the directing and/or the stabbing steps using one or more ROVs. 
     
     
         31 . The method of  claim 27 , comprising constructing the drill string riser using high strength steel tubulars using threaded coupled connectors. 
     
     
         32 . The method of  claim 27 , further comprising supporting the polished bore receptacle using structural supports extending from the subsea foundation to a point approximately midway up the polished bore receptacle. 
     
     
         33 . A method of producing a fluid from a subsea source, the method comprising:
 deploying subsea from a surface vessel a lower riser assembly (LRA) comprising a member having a longitudinal bore, a lower end, an upper end, and an external surface, the member comprising sufficient intake ports extending from the external surface to the bore to accommodate flow of hydrocarbons from a hydrocarbon fluid source, the lower riser assembly having attached thereto a polished bore receptacle comprising a polished bore, a lower end of the polished bore receptacle fluidly and mechanically connected to the upper end of the member;   fluidly connecting at least one of the intake ports to the subsea source using a flexible conduit;   lowering a riser from the surface vessel, the riser comprising a lower end and an upper end, the upper end of the riser mechanically and fluidly connected to the surface vessel, the riser being maintained in an erect substantially vertical position by dynamic positioning of the vessel, the riser comprising a seal stem fluidly and mechanically connected to its lower end, the seal stem comprising one or more exterior elastomeric sealing elements;   stabbing the seal stem into the polished bore receptacle, the exterior elastomeric sealing elements of the seal stem sealingly engaging the polished bore to create a pressure-tight flow path through the polished bore receptacle, seal stem, and riser; and   initiating flow from the subsea source through the subsea flexible conduit, the lower riser assembly, the polished bore receptacle, the seal stem, and the riser.   
     
     
         34 . A method of killing a well producing a fluid from a subsea source, the method comprising:
 deploying subsea from a surface vessel a lower riser assembly comprising a member having a longitudinal bore, a lower end, an upper end, and an external surface, the member comprising sufficient outtake ports extending from the bore to the external surface to accommodate flow of a kill density fluid from the surface vessel to a hydrocarbon fluid source, the lower riser assembly having attached thereto a polished bore receptacle comprising a polished bore, a lower end of the polished bore receptacle fluidly and mechanically connected to the upper end of the member;   fluidly connecting at least one of the outtake ports to the subsea source using a flexible conduit;   lowering a riser from the surface vessel, the riser comprising a lower end and an upper end, the upper end of the riser mechanically and fluidly connected to the surface vessel, the riser being maintained in an erect substantially vertical position by dynamic positioning of the vessel, the riser comprising a seal stem fluidly and mechanically connected to its lower end, the seal stem comprising one or more exterior elastomeric sealing elements;   stabbing the seal stem into the polished bore receptacle, the exterior elastomeric sealing elements of the seal stem sealingly engaging the polished bore to create a pressure-tight flow path through the riser, seal stem, and polished bore receptacle; and   initiating flow of kill density fluid from the surface vessel through the riser, seal stem, polished bore receptacle, lower riser assembly, and subsea flexible conduit.   
     
     
         35 . A method of cementing a subsea wellbore using a surface marine vessel, the method comprising:
 deploying subsea from a surface vessel a lower riser assembly comprising a member having a longitudinal bore, a lower end, an upper end, and an external surface, the member comprising sufficient outtake ports extending from the bore to the external surface to accommodate flow of a cementing fluid from the surface vessel to a hydrocarbon fluid source, the lower riser assembly having attached thereto a polished bore receptacle comprising a polished bore, a lower end of the polished bore receptacle fluidly and mechanically connected to the upper end of the member;   fluidly connecting at least one of the outtake ports to the subsea source using a flexible conduit;   lowering a riser from the surface vessel, the riser comprising a lower end and an upper end, the upper end of the riser mechanically and fluidly connected to the surface vessel, the riser being maintained in an erect substantially vertical position by dynamic positioning of the vessel, the riser comprising a seal stem fluidly and mechanically connected to its lower end, the seal stem comprising one or more exterior elastomeric sealing elements;   stabbing the seal stem into the polished bore receptacle, the exterior elastomeric sealing elements of the seal stem sealingly engaging the polished bore to create a pressure-tight flow path through the riser, seal stem, and polished bore receptacle; and   initiating flow of a cementing fluid from the surface vessel through the riser, seal stem, polished bore receptacle, lower riser assembly, and subsea flexible conduit.