Mitigating hydrate formation during a shutdown of a deep water fpso

Abstract

The present invention relates to a system and method for avoiding hydrate formation during a shutdown of a deep-water floating production, storage and offloading unit with a long distance tie back. In one embodiment the system is configured such that during a shutdown at least a portion of fluids in the production flow loop are delivered to the one or more wash tanks in the hull. Advantageously, this may allow one to avoid having to oversize the expensive top-side equipment.

What is claimed is: 
     
         1 . A subsea production system for managing hydrates during a shutdown, the system comprising:
 a production flow loop configured to deliver produced fluids from one or more remote subsea production wells with a long distance tie-back (LTDB) to an offshore production facility;   an offshore production facility comprising a floating production, storage and offloading (FPSO) unit;   wherein the FPSO unit comprises (1) top side equipment comprising at least one separation unit for separating oil, gas, and water in the produced fluids and (2) a hull comprising one or more wash tanks and one or more cargo tanks configured for storage of crude oil;   wherein the system is configured such that during a shutdown at least a portion of fluids in the production flow loop are delivered to the one or more wash tanks in the hull under conditions such that a minimal amount of hydrates form in the production flow loop.   
     
     
         2 . The system of  claim 1 , further comprising one or more circulation pumps configured to deliver one or more displacement fluids from the FPSO to the production flow loop during a shutdown. 
     
     
         3 . The system of  claim 1 , wherein said displacement fluid comprise a degassed oil. 
     
     
         4 . The system of  claim 1 , wherein the top side equipment comprises one or more of a high pressure separator, a low pressure separator, a flash drum/oil treater, and a stabilizer. 
     
     
         5 . The system of  claim 4 , wherein the top side equipment is oversized relative to the production and fluid processing rates associated with the production flow loop and one or more remote subsea production wells. 
     
     
         6 . The system of  claim 4 , wherein the top side equipment size is smaller relative to the production and fluid processing rates associated with the production flow loop and one or more remote subsea production wells and wherein said top side equipment size is made smaller because the system is configured such that during a shutdown at least a portion of produced fluids are delivered to one or more wash tanks in the hull instead of being delivered to top side equipment. 
     
     
         7 . The system of  claim 1 , wherein the system is configured such that fluids in the production flow loop that are delivered to the one or more wash tanks in the hull during shutdown comprise a gas to liquid ratio that is lower than that of fluids in the production flow loop delivered topside during normal operation. 
     
     
         8 . A method for managing hydrates in a subsea production system during a shutdown wherein said method comprises:
 producing hydrocarbon fluids from one or more remote subsea production wells through a production flow loop to an FPSO comprising top-side equipment and a hull comprising a wash tank and a crude oil cargo tank;   interrupting the production of hydrocarbon fluids from the one or more remote subsea production wells through the production flow loop;   pumping a displacement fluid from the topside equipment, the hull, or both into the production flow loop to transfer at least a portion of the hydrocarbon fluids in the production flow loop to the top-side equipment, the wash tank, or both;   dehydrating and degassing at least a portion of the transferred hydrocarbon fluids in the top-side equipment, the wash tank, or both.   
     
     
         9 . The method of  claim 8 , wherein the step of dehydrating and degassing the liquid hydrocarbon phase during shutdown comprises:
 separating additional aqueous phase and gaseous phase material from the liquid hydrocarbon phase at a reduced pressure;   further dehydrating and degassing the liquid hydrocarbon phase in a holding vessel.   
     
     
         10 . The method of  claim 8 , further comprising using at least a portion of the dehydrated and degassed produced hydrocarbon fluids as at least a portion of the displacement fluid. 
     
     
         11 . The method of  claim 8 , further comprising using crude oil in the cargo tank as at least a portion of the displacement fluid. 
     
     
         12 . The method of  claim 8 , further comprising using crude oil in the cargo tank as at least a portion of the displacement fluid and using at least a portion of the dehydrated and degassed produced hydrocarbon fluids as at least a portion of the displacement fluid. 
     
     
         13 . The method of  claim 9 , wherein the holding vessel is the wash tank in the hull of the FPSO. 
     
     
         14 . The method of  claim 8 , wherein the displacement fluid is supplied to the production flow loop under conditions such that a minimal amount of hydrates form in the production flow loop. 
     
     
         15 . The method of  claim 8 , wherein the transferred hydrocarbon fluids are maintained at a temperature in the range from about 25° C. to about 50° C. and for a period of up to 24 hours in a wash tank for dehydrating and degassing. 
     
     
         16 . The method of  claim 8 , wherein the displacement fluid is supplied to the production flow loop at a temperature of greater than 20° C. 
     
     
         17 . The method of  claim 8 , wherein the displacement fluid is supplied to the production flow loop at a temperature of from about 40° C. to about 70° C. 
     
     
         18 . The method of  claim 8 , wherein after interrupting the production of hydrocarbon fluids the displacement fluid is supplied to the production flow loop before hydrate formation. 
     
     
         19 . The method of  claim 8 , wherein the displacement fluid is supplied to the production flow within no more than about  4  hours after interrupting the production of hydrocarbon fluids.