Method and system for subsea processing of multiphase well effluents

Abstract

A method and system are disclosed for subsea processing multiphase well effluents comprising natural gas and liquid from a subsea hydrocarbon containing formation, which system ( 1 ) comprises: —a fluid separation vessel ( 6 ) which is connected to a downstream end of a multiphase well effluent transportation conduit ( 7 ); —a liquid level transmitter assembly ( 21,22 ) for monitoring the gas liquid interface ( 23 ) in the fluid separation vessel ( 6 ); —a liquid enriched fluid transportation flowline ( 14 ) connected at or near the bottom ( 13 ) of the fluid separation vessel ( 6 ) and a gas enriched fluid transportation flowline ( 12 ) connected at or near a top ( 11 ) of the fluid separation vessel ( 6 ); —a pump ( 17 ) connected to an electric motor ( 18 ); and —a fast acting variable speed drive system, which is coupled to the liquid level controller which adjusts the pump and motor speed setpoint within 2 seconds to maintain the liquid level in the vessel at a predetermined setpoint.

1 . A method of subsea processing multiphase well effluents produced from an underwater hydrocarbon containing formation, the method comprising:
 arranging a fluid separation vessel at or near the seabed;   inducing the multiphase well effluents to flow from the formation through a well and an underwater multiphase well effluent transportation conduit to the fluid separation vessel;   separating the mixture in the fluid separation vessel into gas and liquid enriched fluid fractions;   transferring the liquid and gas enriched fluid fraction to surface crude oil gas processing facilities via separate flowlines;   boosting the pressure in the flowline for transferring the liquid enriched fluid fraction by a pump of which the pump speed is adjusted in response to variation of a gas liquid interface level in the separation vessel;   wherein the pump is driven by an electric motor of which the speed is controlled by a fast acting variable speed drive system which is coupled to a liquid level controller, which monitors the liquid level in the separation vessel and which increases the pump speed within 2 seconds if the liquid level rises above a predetermined liquid level setpoint and decreases the pump speed within 2 seconds if the liquid level drops below the predetermined liquid level setpoint.   
     
     
         2 . The method of  claim 1 , wherein the fast acting variable speed drive increases or decreases the pump speed in a cascade arrangement within 1 to 2 seconds if the liquid level is below the predetermined minimum level or above the predetermined maximum level. 
     
     
         3 . The method of  claim 1 , wherein the pump is a gas tolerant multiphase pump. 
     
     
         4 . The method of  claim 3 , wherein the pump is a positive displacement pump. 
     
     
         5 . The method of  claim 4 , wherein the pump is a twin screw pump. 
     
     
         6 . The method of  claim 1 , wherein the fluid separation vessel is a gravity separation vessel and the flowline for transferring the gas enriched fluid fraction is connected at or near the top of the gravity separation vessel and the flowline for transferring the liquid enriched fluid fraction is connected at or near the bottom of the gravity separation vessel. 
     
     
         7 . The method of  claim 1 , wherein the fluid separation vessel is a compact, for example cyclonic, separation vessel and the flowline for transferring the gas enriched fluid fraction is connected at or near the top of the gravity separation vessel and the flowline for transferring the liquid enriched fluid fraction is connected at or near the bottom of the gravity separation vessel. 
     
     
         8 . The method of  claim 6 , wherein the flowline for transferring the gas enriched fluid fraction is equipped with a liquid overflow valve, which is closed if liquid enters the valve. 
     
     
         9 . The method of  claim 8 , wherein the separation vessel and liquid valve overflow valve are arranged in a separator module which is retrievably mounted on a pump station base at the seabed. 
     
     
         10 . The method of  claim 9 , wherein the pump and the electric pump motor are mounted in a pump module, which is retrievably mounted on the pump station base. 
     
     
         11 . The method of  claim 1 , wherein an inlet of a fluid recirculation conduit is connected to the liquid enriched fluid transportation conduit at a location downstream of the pump and an outlet of the fluid recirculation conduit is connected to the multiphase well effluent transportation conduit, which conduit is equipped with a valve, which is opened during start up of the pump and which is subsequently closed. 
     
     
         12 . The method of  claim 11 , wherein the liquid overflow and pump recycle valves are slow acting valves, which is configured to be moved between a fully opened and a fully closed position over a time interval of typically around 20 seconds. 
     
     
         13 . A system for subsea processing multiphase well effluents comprising natural gas and liquid from an underwater hydrocarbon containing formation, the system comprising:
 a fluid separation vessel which is connected to a downstream end of a multiphase well effluent transportation conduit;   a liquid level transmitter assembly for monitoring the gas liquid interface in the fluid separation vessel;   a liquid enriched fluid transportation flowline connected at or near the bottom of the fluid separation vessel and a gas enriched fluid transportation flowline connected at or near a top of the fluid separation vessel;   a pump connected to an electric motor;   a fast acting variable speed drive system, which is coupled to the liquid level controller which provides the pump and motor speed setpoint in a cascade arrangement to control the level to a predetermined setpoint, and which is configured to bring the pump and motor speed to the designated speed setpoint within 2 seconds.   
     
     
         14 . The system of  claim 13 , wherein the fast acting variable speed drive system is configured to increase and decrease the pump and motor speed in a cascade arrangement within 1 to 2 seconds, to achieve the speed setpoint provided by the level controller.