Method and apparatus for recovering methane from hydrate near the sea floor

Abstract

Method and apparatus are disclosed for recovering natural gas from a hydrate deposit near or at the sea floor. A rigid dome structure, supported by a floating vessel and movable along the sea floor, is placed over the hydrate deposit. Heated sea water may be pumped from the floating vessel through headers and nozzles in the rigid dome structure. The heated sea water preferably fluidizes the marine sand containing hydrate, releasing gas in the hydrate. The gas is collected in the rigid dome structure and moved through a pipe to the floating vessel, where it may be processed through a turbo-expander and liquefied.

1 . A method for recovering methane from a hydrate associated with a marine sand in a selected area on the sea floor, comprising:
 placing a floating vessel at a selected location on the surface of the sea;   using the vessel, placing a dome structure in proximity to the sea floor and over the hydrate;   pumping sea water from the vessel through a water pipe to below the dome structure to contact the marine sand;   capturing gas released from the hydrate under the dome structure; and   flowing the gas released from the hydrate to a selected destination through a gas pipe.   
     
     
         2 . The method of  claim 1  wherein the heated water is pumped to below the dome structure to contact the marine sand through a plurality of spray jet nozzles. 
     
     
         3 . The method of  claim 1  wherein the flow rate of the gas through the gas pipe is controlled by a valve in the gas pipe. 
     
     
         4 . The method of  claim 1  wherein the selected destination is the floating vessel. 
     
     
         5 . The method of  claim 1  wherein the gas released from the hydrate is further flowed through a turbo-expander. 
     
     
         6 . The method of  claim 5  wherein the gas flowed through the turbo-expander is further cooled to form liquefied gas. 
     
     
         7 . The method of  claim 6  wherein the liquefied gas is stored on the vessel. 
     
     
         8 . The method of  claim 5  wherein the turbo-expander is mechanically coupled to an electrical generator to generate electrical power. 
     
     
         9 . The method of  claim 1  wherein the dome structure is propelled along the sea floor by directing a portion of the sea water pumped through the water pipe through a nozzle outside the dome structure. 
     
     
         10 . The method of  claim 2  wherein the sea water is pumped through the water pipe and the plurality of spray jet nozzles at a rate greater than 10,000 gal/min. 
     
     
         11 . The method of  claim 1  wherein the sea water is heated by a heater on the vessel to a temperature to allow the temperature of the water contacting the marine sand to be at a temperature of about 300 deg F. or more. 
     
     
         12 . A method for recovering gas from a gas seep at the sea floor, comprising:
 placing a floating vessel at a selected location on the surface of the sea;   using the vessel, placing a dome structure in proximity to the sea floor and over the gas seep;   capturing gas released from the gas seep under the dome structure;   flowing the gas released from the gas seep to a selected destination through a gas pipe; and   flowing the gas from the gas pipe through a turbo-expander.   
     
     
         13 . Apparatus for recovering methane from a hydrate associated with a marine sand in a selected area on the sea floor, comprising:
 a rigid dome structure adapted to be supported by a floating vessel and having a connection for a gas pipe and a valve to control flow through the gas pipe;   a spray header for water within the dome structure; and   spray jet nozzles connected with the spray header.   
     
     
         14 . The dome structure of  claim 13  wherein the rigid dome structure comprises an array of domes connected by a connecting material. 
     
     
         15 . The dome structure of  claim 13  further comprising an outer skirt. 
     
     
         16 . The dome structure of  claim 13  wherein the rigid dome structure comprises stainless steel plate. 
     
     
         17 . The dome structure of  claim 13  further comprising apparatus for propel the dome structure along the sea floor. 
     
     
         18 . Apparatus for liquefying gas from a hydrate associated with a marine sand, comprising:
 a floating vessel;   a dome structure adapted for placing in proximity to the sea floor and over the hydrate;   a pump on the vessel for pumping sea water from the vessel through a water pipe to below the dome structure to contact the marine sand;   a gas pipe from the dome structure to the vessel;   a valve for controlling flow through the gas pipe; and   a turbo-expander connected to the gas pipe.   
     
     
         19 . The apparatus of  claim 18  further comprising a heater on the floating vessel for heating the sea water. 
     
     
         20 . The apparatus of  claim 18  further comprising apparatus on the vessel for cooling gas from the turbo-expander to liquefy the gas.