System and method for subsea cooling a wellhead gas to produce a single phase dew-pointed gas

Abstract

A system and method for subsea cooling a wellhead gas containing components separable by dewpoint condensation to produce a single phase dew-pointed gas for pipeline transport is disclosed. The system includes a first cooling apparatus configured in use to cool the wellhead gas in direct or indirect heat exchange relation with ambient seawater to a first temperature marginally above ambient seawater temperature to condense liquids comprising one or more hydrocarbons other than methane and at least partially condense water in the wellhead gas. The system also includes a first separator to separate the condensed liquids and water from the cooled gas and a means to add a hydrate inhibitor into the separated cooled gas. The system further includes a second cooling apparatus configured to cool the separated cooled gas to a second temperature below the first temperature, wherein the second temperature is below the ambient seawater temperature to condense the remaining water and produce a single phase dew-pointed gas; and a second separator to separate the condensed remaining water from the single phase dew-pointed gas.

I claim: 
     
         1 . A system for subsea cooling a wellhead gas containing components separable by dewpoint condensation to produce a single phase dew-pointed gas for pipeline transport, said system comprising:
 a first cooling apparatus configured in use to cool the wellhead gas in direct or indirect heat exchange relation with ambient seawater to a first temperature marginally above ambient seawater temperature to condense liquids comprising one or more hydrocarbons other than methane and at least partially condense water in the wellhead gas;   a first separator to separate the condensed liquids and water from the cooled gas;   a means to add a hydrate inhibitor into the separated cooled gas;   a second cooling apparatus configured to further cool the separated cooled gas to a second temperature below the first temperature, wherein the second temperature is below the ambient seawater temperature to condense the remaining water and produce a single phase dew-pointed gas; and,   a second separator to separate the condensed remaining water from the single phase dew-pointed gas.   
     
     
         2 . The system according to  claim 1 , wherein the first temperature is above the hydrate formation temperature and the second temperature is below the hydrate formation temperature. 
     
     
         3 . The system according to  claim 1 , wherein the first cooling apparatus comprises a conduit for passage of the wellhead gas therethrough, the conduit being arranged in direct heat exchange relation with ambient seawater. 
     
     
         4 . The system according to  claim 1 , wherein the first cooling apparatus comprises a plurality of conduits configured in a parallel network, said network of conduits being arranged in direct heat exchange relation with ambient seawater. 
     
     
         5 . The system according to  claim 1 , wherein the first cooling apparatus comprises a first subsea heat exchanger in heat exchange relation with a cooling medium fluid from one or more subsea cooling modules. 
     
     
         6 . The system according to  claim 1 , wherein the first cooling apparatus comprises a first subsea heat exchanger in heat exchange relation with a cooling medium fluid comprising seawater directly pumped from surrounding ambient seawater. 
     
     
         7 . The system according to  claim 1  wherein the second cooling apparatus comprises a gas-gas heat exchanger in serial combination with an expander, whereby the separated gas exiting the expander is used as a cooling medium in the gas-gas heat exchanger. 
     
     
         8 . The system according to  claim 7 , wherein a second subsea heat exchanger is configured upstream of the gas-gas heat exchanger. 
     
     
         9 . The system according to  claim 8 , wherein the second subsea heat exchange is in heat exchange relation with the cooling medium fluid from one or more subsea cooling modules. 
     
     
         10 . The system according to  claim 9 , wherein the cooling medium fluid is cooled in the one or more subsea cooling modules by heat exchange with ambient surrounding seawater. 
     
     
         11 . The system according to  claim 7 , wherein the gas-gas heat exchanger, the expander and the second separator are closely positioned with respect to one another or directly coupled to one another in serial combination. 
     
     
         12 . The system according to  claim 1 , wherein the means to add the hydrate inhibitor into the separated cooled gas comprises an injector adapted to inject a fluid comprising the hydrate inhibitor into a flowpath of the separated cooled gas. 
     
     
         13 . The system according to  claim 1 , wherein the system further comprises a gas-liquid heat exchanger either disposed upstream or downstream of the second cooling apparatus in an arrangement whereby the condensed liquids separated by the second separator are used as a heat exchange fluid in the gas-liquid heat exchanger. 
     
     
         14 . The system according to  claim 1 , wherein the second separator comprises a dual phase separator vessel in fluid communication with a dehydration column. 
     
     
         15 . The system according to  claim 1 , wherein the second separator comprises a dual phase separator having an upper section thereof configured as a dehydration column. 
     
     
         16 . The system according to  claim 1 , wherein the second separator comprises a dual phase separator vessel in fluid communication with a dehydration column section. 
     
     
         17 . The system according to  claim 1 , wherein the second separator comprises a dual phase separator vessel having an upper section thereof configured as a dehydration column section. 
     
     
         18 . A method of subsea cooling a wellhead gas containing components separable by dewpoint condensation to produce a single phase dew-pointed gas for pipeline transport, said method comprising:
 a) cooling the wellhead gas to a first temperature marginally above ambient seawater temperature to condense liquids comprising one or more hydrocarbons other than methane and at least partially condense water in the wellhead gas;   b) separating the condensed liquids and water from the cooled gas;   c) adding a hydrate inhibitor to the gas separated in step b);   d) cooling the gas from step c) to a second temperature below the first temperature, wherein the second temperature is below ambient seawater temperature, to condense the remaining condensable liquids and produce a single phase dew-pointed gas; and,   e) separating the condensed liquids from the single phase dew-pointed gas.   
     
     
         19 . The method according to  claim 18 , wherein the first temperature is above the hydrate formation temperature and the second temperature is below the hydrate formation temperature. 
     
     
         20 . The method according to  claim 18 , wherein the first temperature is above the hydrate formation temperature and a wax formation temperature. 
     
     
         21 . The method according to  claim 18 , wherein cooling the gas from step c) comprises passing said gas through a gas-gas heat exchanger and expanding said gas exiting the gas-gas heat exchanger, wherein the gas from step e) is used as a cooling medium in step d) in heat exchange relation with the hydrate inhibitor-gas mixture. 
     
     
         22 . The method according to  claim 18 , wherein cooling the wellhead gas to the first temperature comprises passing the wellhead gas through a first subsea heat exchanger in heat exchange relation with a cooling medium fluid from one or more subsea cooling modules. 
     
     
         23 . The method according to  claim 18 , wherein cooling the wellhead gas to the first temperature comprises passing the wellhead gas through a conduit arranged in heat exchange relation with ambient seawater. 
     
     
         24 . The method according to  claim 18 , wherein cooling the wellhead gas comprises passing the wellhead gas through a plurality of conduits configured in a parallel network, said network of conduits being arranged in direct heat exchange relation with ambient seawater. 
     
     
         25 . The method according to  claim 18  wherein the hydrate inhibitor comprises methanol, monoethylene glycol (MEG), or similar hydrate inhibitors, or a combination thereof. 
     
     
         26 . The method according to  claim 18 , wherein the hydrate inhibitor is added in an amount sufficient to suppress formation of hydrates. 
     
     
         27 . The method according to  claim 18 , wherein prior to performing step c), the cooled gas is passed through a second subsea heat exchanger in heat exchange relation with a cooling medium fluid from one or more subsea cooling modules. 
     
     
         28 . The method according to  claim 27 , wherein the cooling medium fluid is cooled in the one or more subsea cooling modules by heat exchange with ambient surrounding seawater. 
     
     
         29 . The method according to  claim 21 , wherein prior to passing said gas through the gas-gas heat exchanger, the process comprises passing said gas through a gas-liquid heat exchanger, wherein the condensed liquids separated in step e) are used as a heat exchange fluid in the gas-liquid heat exchanger. 
     
     
         30 . The method according to  claim 21 , wherein prior to expanding the gas from step c) through the expander, the process comprises passing said gas through a gas-liquid heat exchanger disposed downstream of the gas-gas heat exchanger, wherein the condensed liquids separated in step e) are used as a heat exchange fluid in the gas-liquid heat exchanger.