Method and a system for drain liquid collection and evacuation in a subsea compression system

Abstract

A method for drain liquid collection and evacuation in a subsea compression system using a compressor, wherein an external drainage tank is arranged in direct flow communication with the compressor sump via a sump evacuation valve, the tank is set under compressor discharge pressure via a tank pressurizing line arranged to connect the tank to a compressor discharge flow upstream of a throttle valve, a tank evacuation line is arranged to connect the tank to the compressor discharge downstream of the throttle valve, drain liquid evacuation is accomplished through generation, by adjusting the throttle valve, of a flow restriction in the compressor discharge. The tank is set under compressor discharge pressure by opening of the pressure valve and the drain liquid flow is permitted into a reduced compressor discharge pressure downstream the throttle valve by opening an evacuation valve in the evacuation line.

What is claimed is: 
     
         1 . A method for drain liquid collection and evacuation in a subsea compression system using a compressor, wherein the compressor comprises a compressor inlet, a compressor discharge and a compressor sump for collecting liquid from wet gas, the method comprising:
 arranging an external drainage tank in direct flow communication with the compressor sump via a sump evacuation valve;   arranging a tank pressurizing line by which the external drainage tank is set under a compressor discharge pressure by operation of a pressure valve;   arranging a tank pressure relief line by which the external drainage tank is set under a compressor inlet pressure by operation of a relief valve;   arranging a tank evacuation line by which liquid is evacuated from the external drainage tank into the compressor discharge by operation of an evacuation valve;   arranging the tank pressurizing line to connect the external drainage tank to the compressor discharge upstream of a throttle valve; and   arranging the tank evacuation line to connect the external drainage tank to the compressor discharge downstream of the throttle valve,   wherein evacuation of drain liquid from the external drainage tank is accomplished through generation of a flow restriction in the compressor discharge by adjusting the throttle valve, setting the external drainage tank under the compressor discharge pressure by opening the pressure valve, and permitting the drain liquid to flow into a reduced compressor discharge pressure downstream the throttle valve by opening the evacuation valve.   
     
     
         2 . The method of  claim 1 , wherein the evacuation of the drain liquid is performed intermittently during production by generation, temporarily, of a pressure drop over the throttle valve while the drainage tank is isolated from the compressor sump. 
     
     
         3 . The method of  claim 2 , wherein a drainage sequence during production comprises the steps of:
 closing the sump evacuation valve;   opening the pressure valve and the evacuation valve;   adjusting the throttle valve to generate a pressure drop in the compressor discharge, and after the accomplishment of the evacuation of the drain liquid:   closing the pressure valve and the evacuation valve;   opening the throttle valve, the relief valve, and the sump evacuation valve; and   closing the relief valve.   
     
     
         4 . The method of  claim 1 , wherein by adjusting the throttle valve a pressure difference is generated between inlet and outlet sides of the external drainage tank, during the evacuation of the drain liquid. 
     
     
         5 . The method of  claim 1 , wherein the evacuation of the drain liquid is performed during start-up of the subsea compression system by temporarily pressurizing the external drainage tank from a separate pressure source or a gas accumulator. 
     
     
         6 . The method of  claim 1 , wherein a first drainage tank and a second drainage tank are arranged in parallel, each of the first and the second drainage tanks individually connected to the compressor sump, the compressor inlet, and the compressor discharge via separate lines and valves, respectively, and the evacuation of the drain liquid comprises:
 alternately evacuating liquid from one of the first and the second drainage tanks into the compressor discharge while the other of the first and the second drainage tanks is configured to collect liquid from the compressor sump.   
     
     
         7 . The method of  claim 6 , wherein the first and the second drainage tanks are arranged in series one after the other in a flow direction from the compressor sump, the first drainage tank emptying into the second drainage tank via an on/off tank isolation valve, and the evacuation of the drain liquid comprises:
 evacuating liquid from the second drainage tank while being isolated from the first drainage tank.   
     
     
         8 . A subsea compression system using a compressor, wherein the compressor comprises a compressor inlet, a compressor discharge, and a compressor sump for collecting liquid from wet gas, the subsea compression system comprising:
 an external drainage tank arranged in direct flow communication with the compressor sump via a sump evacuation valve;   a tank pressurizing line comprising a pressure valve configured to set the drainage tank under a compressor discharge pressure;   a tank pressure relief line comprising a relief valve configured to set the drainage tank under a compressor inlet pressure; and   a tank evacuation line comprising an evacuation valve configured to evacuate liquid from the external drainage tank into the compressor discharge,   wherein the tank pressurizing line connects the external drainage tank to the compressor discharge upstream of a throttle valve, the tank evacuation line connects the external drainage tank to the compressor discharge downstream of the throttle valve, the throttle valve is configured to generate, by adjustment of the throttle valve, a flow restriction in the compressor discharge while the external drainage tank is set under the compressor discharge pressure via an open pressure valve, and the liquid is permitted to flow into a reduced compressor discharge pressure via an open evacuation valve.   
     
     
         9 . The subsea compression system of  claim 8 , further comprising a separate pressure source or a gas accumulator connectable to the external drainage tank. 
     
     
         10 . The subsea compression system of  claim 8 , further comprising a first drainage tank and a second drainage tank, wherein the first drainage tank and the second drainage tank are connectable in parallel or in series to the compressor sump.