Apparatus and method for operating a subsea compression system in a well stream

Abstract

A method of operating a subsea compression system in a well stream, the subsea compression system comprising a compressor and an electromotor driven pump, wherein the compressor is configured to compress and the pump is configured to pump liquid from the compression system. The method comprises providing a gas return line connecting a downstream side of the compressor with an upstream side of the compressor, arranging a turbo-expander unit in flow connection with the gas return line, connecting the turbo-expander unit drivingly to a generator configured to charge a battery, operating the generator in response to recycling of compressed gas through the turbo-expander unit, and operating the pump on electrical power supplied from the battery.

What is claimed is: 
     
         1 . A method of operating a subsea compression system in a well stream, the subsea compression system comprising a compressor and an electromotor driven pump, wherein the compressor is configured to compress and the pump is configured to pump liquid from the subsea compression system, the method comprising:
 connecting a downstream side of the compressor with an upstream side of the compressor through a gas line;   connecting a turbo-expander unit with the gas return line in flow communications;   connecting the turbo-expander unit drivingly to a generator configured to deliver charging current a battery;   operating the generator in response to recycling of compressed gas through the turbo-expander unit; and   operating the pump on electrical power supplied from the battery.   
     
     
         2 . The method of  claim 1 , further comprising feeding gas through a gas feed line to the compressor from a vessel arranged in the well stream, upstream of the compressor. 
     
     
         3 . The method of  claim 2 , further comprising mixing the gas and the liquid in the well stream; and
 feeding the gas and the liquid as a homogenized wet gas to the compressor from the vessel, wherein the vessel is configured as a mixer.   
     
     
         4 . The method of  claim 2 , further comprising feeding the gas and the liquid in the well stream separately to the compressor and the pump, respectively, from the vessel, wherein the vessel is configured as a separator. 
     
     
         5 . The method of  claim 2 , further comprising draining the liquid from the compressor into the vessel. 
     
     
         6 . The method of  claim 2  further comprising operating the pump in response to a detected liquid level in the vessel or the compressor. 
     
     
         7 . The method of  claim 1 , wherein the liquid is drained from the compressor into a separate drainage tank and the pump is operated for pumping the liquid from the drainage tank in response to a measured liquid level in the drainage tank or the compressor. 
     
     
         8 . The method of  claim 1 , wherein the pump is operated on battery power for drainage of the subsea compression system before start-up of the compressor. 
     
     
         9 . A subsea compression system for a well stream, the subsea compression system comprising
 a compressor;   an electromotor driven pump, the compressor configured to compress gas and the electromotor driven pump configured to pump liquid from the subsea compression system;   a gas return line connecting a downstream side of the compressor with an upstream side of the compressor;   a turbo-expander unit in flow connection with the gas return line; and   a generator wherein the turbo-expander unit is drivingly connected to the generator, the generator configured to deliver charging current to a battery in response to recycling of compressed gas via the turbo-expander unit, wherein the electromotor driven pump is operable on electrical power that is supplied from the battery.   
     
     
         10 . The subsea compression system of  claim 9 , wherein a gas feed line is configured to connect the compressor with a vessel arranged upstream of the compressor. 
     
     
         11 . The subsea compression system of  claim 10 , further comprising mixing the gas and the liquid into a homogenized wet gas; and
 supplying the homogenized wet gas to the compressor.   
     
     
         12 . The subsea compression system of  claim 10 , wherein the vessel is a separator supplying gas and liquid separately to the compressor and the electromotor driven pump, respectively. 
     
     
         13 . The subsea compression system of any of  claim 10 , wherein the compressor is connected with the vessel for dumping drainage liquid from the compressor into the vessel via a drainage line. 
     
     
         14 . The subsea compression system of  claim 12 , further comprising detecting a liquid level in the vessel; and
 controlling the operation of the electromotor driven pump based on the liquid level.   
     
     
         15 . The subsea compression system of  claim 9 , wherein the compressor is configured in flow connection with a drainage tank and the drainage tank is configured to detect a liquid level, controlling the operation of the electromotor driven pump. 
     
     
         16 . The subsea compression system of  claim 9 , wherein the intake to the turbo-expander unit is connected to a compressed-gas discharge line between the compressor outlet and a liquid injection point on the compressed-gas discharge line, and the outlet from the turbo-expander unit is over a flow control valve connectable to a fluid line feeding wet gas to the compressor. 
     
     
         17 . The subsea compression system of  claim 16 , further comprising activating the flow control valve in response to a detected liquid volume level in the vessel. 
     
     
         18 . The subsea compression system of  claim 10 , wherein an outlet from the electromotor driven pump is connectable to the vessel through a flow control valve arranged in a liquid return loop. 
     
     
         19 . The subsea compression system of  claim 9 , wherein gas flow through the turbo-expander unit is activated in response to a detected surge condition in the compressor. 
     
     
         20 . The subsea compression system of  claim 9 , wherein the pump is a positive displacement pump. 
     
     
         21 . The subsea compression system of  claim 9 , wherein a reduction gear or speed reduction device is inserted between the turbo-expander unit and the generator. 
     
     
         22 . The subsea compression system of  claim 9 , wherein the battery is a battery set in an uninterrupted power supply system.