Control valve timing

Abstract

A method for lifting drilling mud from subsea to a drilling vessel which a pump having a body with a chamber, and a bladder in the chamber. The bladder attaches to the body and defines water and mud sides in the chamber. A mud inlet valve allows mud into the mud side of the chamber; which moves the bladder into the water side and urges water in the water side from the chamber and through a water exit valve. Pressurized water enters the chamber through a water inlet valve, which in turn pushes the bladder and mud from the chamber through a mud exit valve. The bladder separates the mud and water as it reciprocates in the chamber. Water and/or mud inlet valves are opened before associated exit valves to reduce pressure fluctuations in the fluid lines.

What is claimed is: 
     
         1 . A method of lifting drilling mud from a subsea wellbore comprising:
 directing an inlet flow of fluid to mud pumps that are disposed subsea;   controlling portions of the inlet flow of fluid to individual mud pumps with inlet valves; and   opening a one of the inlet valves while closing another one of the inlet valves so that individual mud pumps respectively associated with the one of the inlet valves and the another one of the inlet valves are at the same time in communication with the inlet flow of fluid, and so that a flow rate of the inlet flow of fluid remains above a threshold value.   
     
     
         2 . The method of  claim 1 , wherein the inlet flow of fluid comprises drilling mud from the subsea wellbore, and opening and closing of the inlet valves is staggered to maintain the flow rate of the inlet flow of fluid at a substantially constant value. 
     
     
         3 . The method of  claim 1 , wherein the inlet flow of fluid comprises water for driving the mud pumps, and opening and closing of the inlet valves is staggered to maintain the flow rate of the inlet flow of fluid at a substantially constant value. 
     
     
         4 . The method of  claim 1 , wherein the mud pumps comprise a first, a second, and third mud pumps that reciprocate between a mud intake cycle to a mud discharge cycle, and wherein the one of the inlet valves controls flow to the first mud pump and wherein the another one of the inlet valves controls flow to the second mud pump. 
     
     
         5 . The method of  claim 4 , wherein the first mud pump changes from the mud intake cycle to the mud discharge cycle when the second pump changes from the mud discharge cycle to the mud intake cycle. 
     
     
         6 . The method of  claim 5 , wherein the second mud pump changes from the mud intake cycle to the mud discharge cycle when the third pump changes from the mud discharge cycle to the mud intake cycle, and wherein a third inlet valve controls flow to the third mud pump, and wherein the third inlet valve begins to open prior to the another one of the inlet valves closing. 
     
     
         7 . The method of  claim 1 , wherein the another one of the inlet valves comprises a mud inlet valve, and wherein the mud pumps are driven by selectively flowing pressurized water to water spaces in the mud pumps through water inlet valves, and discharging water from the pumps through water discharge valves, the method further comprising opening a one of the water discharge valves in a one of the mud pumps that is associated with the mud inlet valve. 
     
     
         8 . A method of lifting drilling mud from a subsea wellbore comprising:
 directing an inlet flow of drilling mud from the subsea wellbore;   controlling a first portion of the inlet flow of drilling mud to a first mud pump with a first mud inlet valve;   controlling a second portion of the inlet flow of drilling mud to a second mud pump with a second mud inlet valve; and   maintaining a flow rate of the inlet flow of drilling mud above a threshold value by opening the first mud inlet valve prior to closing the second inlet valve.   
     
     
         9 . The method of  claim 8 , further comprising controlling a third portion of the inlet flow of drilling mud to a third mud pump with a third mud inlet valve, and opening the second inlet valve prior to closing the third inlet valve. 
     
     
         10 . The method of  claim 8 , wherein the first mud inlet valve begins to open at least about 0.5 seconds prior to when the second mud inlet valve is closed. 
     
     
         11 . The method of  claim 8 , wherein the first and second mud pumps reciprocatingly discharge mud through first and second mud discharge valves, and wherein pressurized water is selectively delivered to a water side of the first and second mud pumps for discharging mud from the first and second mud pumps. 
     
     
         12 . A method of pumping drilling mud from a subsea wellbore comprising:
 providing first, second and third mud pumps subsea, and that each comprise a housing, a water space in the housing, a mud space in the housing, and a membrane in the housing that defines a barrier between the mud space and water space;   directing first, second, and third portions of a total flow of drilling mud respectively through first, second, and third mud inlet lines and to the first, second and third mud pumps;   controlling the first, second, and third portions of the total flow of drilling mud in the first, second, and third mud inlet lines with first, second, and third mud inlet valves that are respectively disposed in the first, second, and third mud inlet lines;   sequencing the first, second, and third portions of the total flow of drilling mud in the first, second, and third mud inlet lines by closing the first mud inlet valve when opening the second mud inlet valve;   selectively directing a flow of pressurized water to the water spaces in the housings of each of the first, second, and third mud pumps to urge drilling mud from the first, second, and third mud pumps; and   maintaining the total flow of drilling mud above a threshold value by beginning to open the second inlet valve prior to fully closing the first inlet valve.   
     
     
         13 . The method of  claim 12 , further comprising beginning to open the third inlet valve prior to fully closing the second inlet valve. 
     
     
         14 . The method of  claim 12 , further comprising discharging water from the water space of the second mud pump prior to beginning to open the second mud inlet valve.