Methods of establishing and/or maintaining flow of hydrocarbons during subsea operations

Abstract

Methods are disclosed of establishing and maintaining flow of hydrocarbon-bearing fluid from a subsea source while controlling or limiting hydrocarbon gas hydrate formation in a riser and a collection tool fluidly connected to a distal end of the riser during subsea positioning of the riser and tool. Riser sections are connected at or near the sea surface, and a collection tool may be connected to the distal end of the riser at the surface or picked up subsea by the riser. The riser and tool are deployed subsea near a subsea source of hydrocarbons. A low-density fluid is forced down the riser and tool, and then the riser and tool are positioned to collect hydrocarbons from the subsea source of hydrocarbons. Flow of a low-density fluid is gradually reduced to initiate flow of hydrocarbons up the tool and riser.

1 . A method comprising:
 deploying a collection tool connected to the distal end of a riser subsea upstream from a plume of hydrocarbons from a subsea source of hydrocarbons;   displacing seawater from the riser and tool by forcing low-density fluid into the riser and tool;   positioning the tool to collect hydrocarbons from the source of hydrocarbons while the riser and tool remain filled with the low-density fluid;   establishing flow of hydrocarbons up the tool and riser by ceasing the forcing of the low-density fluid into the riser and tool; and   collecting hydrocarbons up the tool and riser.   
     
     
         2 . The method according to  claim 1 , wherein the deploying step comprises:
 deploying the riser from a surface vessel subsea upstream from the plume of hydrocarbons from the subsea source of hydrocarbons;   deploying the collection tool on the seabed; and   stabbing a distal end of the riser into the tool and picking up the tool from the seabed using the riser.   
     
     
         3 . The method according to  claim 1 , wherein the ceasing of the forcing of the low-density fluid into the riser and tool comprises gradually reducing the amount of low-density fluid being forced into the riser and tool. 
     
     
         4 . The method according to  claim 1 , further comprising controlling hydrocarbon gas hydrate formation by limiting contact of liquids or solids with inner surfaces of the riser and tool during at least the positioning of the riser and tool subsea. 
     
     
         5 . The method according to  claim 1 , further comprising pumping a functional fluid into the tool, the functional fluid selected from the group consisting of wax inhibitor, asphaltene inhibitor, hydrocarbon gas hydrate inhibitor, and combinations thereof. 
     
     
         6 . The method according to  claim 1 , wherein a dispersant chemical is mixed with hydrocarbons in the subsea source of hydrocarbons before the hydrocarbons escape the subsea source or just after some hydrocarbons escape from the subsea source of hydrocarbons and are not collected by the riser and tool. 
     
     
         7 . The method according to  claim 2 , wherein the tool is a top hat, the vessel is a mobile offshore drilling unit (MODU), and the deploying and positioning comprise:
 moving the MODU upstream of a plume of hydrocarbons emanating from the subsea source;   establishing flow of the low-density fluid inside the riser and top hat until it bubbles out of the lower end of the top hat, limiting ingress of water or hydrocarbon gas hydrates into the riser and thus limiting plugging of the riser and top hat;   moving the MODU so that the top hat is a few meters above the subsea source, with the top hat in the hydrocarbon plume, and the low-density fluid still bubbling out of the lower end of the top hat and limiting hydrates from forming in inside surfaces of the top hat;   landing the top hat on the subsea source, while the top hat remains filled with low-density fluid, keeping seawater out of the top hat;   initiating flow of a liquid hydrate inhibitor, and optionally wax and asphaltene inhibitors, through surface control or subsea valves; and   optionally initiating flow of a dispersant that mixes with any non-collected hydrocarbons.   
     
     
         8 . The method according to  claim 7 , wherein the top hot makes no positive seal with the subsea source, the method further comprising:
 wherein the ceasing of flow of the low-density fluid comprises gradually reducing the flow of low-density fluid and gradually opening a choke, causing hydrocarbons to move up the riser as the low-density fluid pressure is pushed up by the heavier seawater and/or by well pressure if a seal with the subsea source has been established, and controlling the choke opening so that entrance of seawater into the top hat is at least mitigated.   
     
     
         9 . The method according to  claim 1 , further comprising an auxiliary method to increase flow through the riser selected from the group consisting of pumping the hydrocarbon up the riser using a subsea pump, employing gas lift with a non oxidizing mixture of collected gas with or without nitrogen added through an umbilical or coiled tubing employing a compressor, and employing gas lift using a flue gas with or without nitrogen added through an umbilical or coiled tubing employing a compressor. 
     
     
         10 . The method according to  claim 1 , comprising flowing a dispersant chemical near and/or into the subsea source of hydrocarbons where hydrocarbons are escaping into seawater. 
     
     
         11 . The method according to  claim 1 , wherein the deploying step comprises:
 deploying the riser and the collection tool to a distal end of the riser from a surface vessel subsea upstream from the plume of hydrocarbons emanating from the subsea source of hydrocarbons.   
     
     
         12 . The method according to  claim 11 , wherein the deploying step comprises assembling riser sections at or near the sea surface to form the riser, wherein the riser sections comprise one or more tubulars selected from the group consisting of drill pipe sections, reel pipe, flexible pipe, composite pipe, and hose. 
     
     
         13 . The method according to  claim 11 , wherein the low-density fluid consists essentially of nitrogen. 
     
     
         14 . The method according to  claim 11 , wherein the ceasing of the forcing of the low-density fluid into the riser and tool comprises gradually reducing the amount of low-density fluid being forced into the riser and tool. 
     
     
         15 . The method according to  claim 11 , comprising controlling hydrocarbon gas hydrate formation by limiting contact of liquids and solids with inner surfaces of the riser and collection tool during at least the positioning of the riser and tool subsea. 
     
     
         16 . The method according to  claim 11 , wherein a functional fluid is pumped into the tool, the functional fluid selected from the group consisting of wax inhibitor, asphaltene inhibitor, hydrocarbon gas hydrate inhibitor, and combinations thereof. 
     
     
         17 . The method according to  claim 11 , wherein a dispersant chemical is mixed with hydrocarbons in the subsea source of hydrocarbons before the hydrocarbons escape the subsea source or a dispersant chemical is mixed with hydrocarbons just after some hydrocarbons escape from the subsea source of hydrocarbons and are not collected by the riser and tool. 
     
     
         18 . The method according to  claim 11 , wherein the tool is a top hat, the vessel is a mobile offshore drilling unit (MODU), and the deploying and positioning comprise:
 moving the MODU upstream of a plume of hydrocarbons emanating from the subsea source;   establishing flow of the low-density fluid inside the riser and top hat until it bubbles out of the lower end of the top hat, limiting ingress of water or hydrocarbon gas hydrates into the riser and thus preventing plugging of the riser and top hat;   moving the MODU so that the top hat is a few meters above the subsea source, with the top hat in the hydrocarbon plume, and the low-density fluid still bubbling out of the lower end of the top hat and limiting hydrates from forming in inside surfaces of the top hat;   landing the top hat on the subsea source, while the top hat remains filled with low-density fluid, keeping seawater flow out of the top hat;   initiating flow of a liquid hydrate inhibitor, and optionally wax and asphaltene inhibitors, through surface control or subsea valves; and   optionally initiating flow of a dispersant that mixes with any non-collected hydrocarbons.   
     
     
         19 . The method according to  claim 18 , wherein the top hat makes no positive seal with the subsea source, the method further comprising:
 wherein the ceasing of flow of the low-density fluid comprises gradually reducing the flow of low-density fluid and gradually opening a choke, allowing hydrocarbons to move up the riser as the low-density fluid pressure is pushed up by the heavier seawater and/or well pressure if a seal with the subsea source has been established, controlling the choke opening so that entrance of seawater into the top hat is at least mitigated.   
     
     
         20 . The method according to  claim 11 , further providing an auxiliary method to increase flow selected from the group consisting of pumping the hydrocarbon up the riser using a subsea pump, employing gas lift with a non oxidizing mixture of collected gas with or without added nitrogen added through an umbilical or coiled tubing employing a compressor, and employing gas lift using a flue gas with or without added nitrogen added through an umbilical or coiled tubing employing a compressor. 
     
     
         21 . The method according to  claim 11 , comprising flowing a dispersant chemical near and/or into and/or inside the subsea source of hydrocarbons where hydrocarbons are escaping into seawater. 
     
     
         22 . A method comprising:
 connecting riser sections at or near the sea surface to form a riser;   deploying the riser from a surface vessel subsea upstream from a plume of hydrocarbons from a subsea source of hydrocarbons;   deploying a collection tool on the seabed;   stabbing a distal end of the riser into the tool and picking up the tool from the seabed using the riser;   displacing seawater from the riser and tool by forcing low-density fluid into the riser and tool;   positioning the tool connected to a distal end of the riser to collect hydrocarbons from the source of hydrocarbons while the riser and tool remain filled with the low-density fluid;   establishing flow of hydrocarbons up the tool and riser;   controlling hydrocarbon gas hydrate formation by limiting contact of liquids and solids with inner surfaces of the riser and tool during at least the positioning of the riser and tool subsea;   ceasing the forcing of the low-density fluid into the riser and tool and collecting hydrocarbons up the tool and riser; and   pumping an inhibitor liquid into the tool during at least the collecting step.   
     
     
         23 . A method comprising:
 connecting riser sections at or near the sea surface to form a riser;   connecting a collection tool to the distal end of the riser;   deploying the riser and tool from a surface vessel subsea upstream from a plume of hydrocarbons from a subsea source of hydrocarbons;   displacing seawater from the riser and tool by forcing low-density fluid into the riser and tool;   positioning the tool connected to a distal end of the riser to collect hydrocarbons from the source of hydrocarbons while the riser and tool remain filled with the low-density fluid;   establishing flow of hydrocarbons up the tool and riser;   controlling hydrocarbon gas hydrate formation by limiting contact of liquids and solids with inner surfaces of the riser and tool during at least the positioning of the riser and tool subsea;   ceasing the forcing of the low-density fluid into the riser and tool and collecting hydrocarbons up the tool and riser; and   pumping an inhibitor liquid into the tool during at least the collecting step.   
     
     
         24 . A method comprising:
 connecting drill pipe riser sections at or near the sea surface to form a drill pipe riser;   deploying the drill pipe riser from a mobile offshore drilling unit subsea near a subsea source of hydrocarbons;   positioning a top hat connected to a distal end of the drill pipe riser to collect hydrocarbons from the source of hydrocarbons;   collecting hydrocarbons using the top hat and drill pipe riser;   wherein the deploying and positioning comprise:
 moving the MODU upstream of a plume of hydrocarbon in the water column emanating from the subsea source; 
 displacing seawater from the drill pipe riser and top hat by forcing a gas consisting essentially of nitrogen down the drill pipe riser at least during the positioning of the top hat and riser until the gas bubbles out of the lower end of the top hat, limiting ingress of seawater or hydrocarbon gas hydrates into the drill pipe riser and top hat thus preventing plugging of the drill pipe riser; 
 moving the MODU so that the top hat is a few meters above the subsea source, with the top hat in the hydrocarbon plume, and the gas consisting essentially of nitrogen still bubbling out and limiting hydrates from forming in the inside surfaces of the top hat; 
 landing the top hat on the subsea source, while the top hat remains filled with gas consisting essentially of nitrogen, limiting ingress of seawater into the top hat; 
   gradually reducing flow of the gas and gradually opening a choke to establish and maintain flow of collected hydrocarbons up the top hat and drill pipe riser; and   pumping an inhibitor liquid into the top hat and optionally wax and asphaltene inhibitors through valves on the roof of the top hat during at least the collecting step, limiting hydrocarbon gas hydrate formation during the collecting of hydrocarbons.