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TECHNICAL FIELD 
       [0001]    This invention relates to the general subject of providing a pressurized working fluid for the operation of subsea equipment, especially in very deep waters. 
       CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0002]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0003]    Not applicable 
       REFERENCE TO A “MICROFICHE APPENDIX” 
       [0004]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0005]    The field of this invention is that of deepwater control systems for the purpose of providing a supply of pressurized working fluid for the control and operation of equipment. The equipment is typically blowout preventers (BOP) which are used to shut off the well bore to secure an oil or gas well from accidental discharges to the environment, gate valves for the control of flow of oil or gas to the surface or to other subsea locations, hydraulically actuated connectors and similar devices. The fluid to be pressurized is typically an oil based product or a water based product with added lubricity and corrosion protection. 
         [0006]    The working fluid for such control systems typically comes from accumulators. Currently accumulators have historically come in three styles which operate on a common principle. The principle is to precharge them with pressurized gas to a pressure at or slightly below the anticipated minimum pressure required to operate equipment. Fluid can be added to the accumulator, increasing the pressure of the pressurized gas and the fluid. The fluid introduced into the accumulator is therefore stored at a pressure at least as high as the precharge pressure and is available for doing hydraulic work. 
         [0007]    The accumulator styles are bladder type having a balloon type bladder to separate the gas from the fluid, the piston type having a piston sliding up and down a seal bore to separate the fluid from the gas, and a float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of gas. 
         [0008]    Accumulators providing typical 3000 p.s.i. working fluid to surface equipment can be of a 5000 p.s.i. working pressure and contain fluid which raises the precharge pressure from 3000 p.s.i. to 5000 p.s.i. 
         [0009]    As accumulators are used in deeper water, the efficiency of conventional accumulators is decreased. In 1000 feet of seawater the ambient pressure is approximately 465 p.s.i. For an accumulator to provide a 3000 p.s.i. differential at 1000 ft. depth, it must actually be precharged to 3000 p.s.i. plus 465 p.s.i. or 3465 p.s.i. 
         [0010]    At slightly over 4000 ft. water depth, the ambient pressure is almost 2000 p.s.i., so the precharge would be required to be 3000 p.s.i. plus 2000 p.s.i. or 5000 p.s.i. This would mean that the precharge would equal the working pressure of the accumulator. Any fluid introduced for storage would cause the pressure to exceed the working pressure, so the accumulator would be non-functional. 
         [0011]    Another factor which makes the deepwater use of conventional accumulators impractical is the fact that the ambient temperature decreases to approximately 35 degrees F. If an accumulator is precharged to 5000 p.s.i. at a surface temperature of 80 degrees F., approximately 416 p.s.i. precharge will be lost simply because the temperature was reduced to 35 degrees F. Additionally, the rapid discharge of fluids from accumulators and the associated rapid expansion of the pressurizing gas causes a natural cooling of the gas. If an accumulator is quickly reduced in pressure from 5000 p.s.i. to 3000 p.s.i. without chance for heat to come into the accumulator (adiabatic), the pressure would actually drop to 2012 p.s.i. 
         [0012]    A more recent solution to this problem has been what is referred to as constant differential accumulators as is illustrated in U.S. Pat. No. 6,202,753. These accumulators use a double piston looking like a barbell which acts as mechanical summing relay. On the top side of the top piston is the gas charge similar to the more conventional accumulators. On the lower side of the upper piston is the pressurized working fluid. The lower piston is connected to the upper piston by a connecting rod. Seawater pressure is vented onto the top side of the lower piston, pushing it down and therefore pulling the upper piston down harder onto the working fluid. A vacuum is on the lower side of the lower piston and so offers no support. The net effect is that the working fluid pressure is generally equal to the sum of the nitrogen pressure plus the seawater pressure. In other words its pressure is always higher than the ambient pressure by the amount of the nitrogen pressure. This provides a good solution irrespective of depths, but provides a relatively costly construction. 
         [0013]    Subsea drilling has been done for about 60 years and during that time drilling has occurred in progressively deeper and deeper water. The deeper water is associated with colder temperatures making the deepwater use of accumulators especially difficult. Substantial and ongoing research has been done to try to make conventional accumulators operational in waters in depths of greater than 6,000′. From there it only gets more difficult as drilling is now happening in depths as great as 12,000′. This has resulted in very high nitrogen precharges simply to be higher than the pressure at these ocean depths along with concerns about liquefying the nitrogen charge gas. As industry and standards societies have pursued the difficulties of making conventional accumulators work in conventional situations, a better solution is needed. 
         [0014]    The problem being discussed here is that the environment in which the accumulators are working is changing. The different pressure and temperature combinations of various have been a problem for the industry for many years, and is only exaggerated as the drilling depths continue to be deeper and deeper. 
       SUMMARY OF THE INVENTION 
       [0015]    The object of this invention is to provide a control system for deepwater ocean service which allows the equipment to be operated as if it were in a constant environment. 
         [0016]    A second object of this invention is to provide a control system for deepwater ocean service which does not lose its operating differential across subsea working pistons due to high deep sea ambient pressures. 
         [0017]    A third object of the present invention is to provide a control system for deepwater ocean service which operates with similar characteristics when deep sea and during surface testing. 
         [0018]    Another object of the present invention is to provide a control system which operates in conjunction with conventional accumulators rather than requiring constant differential accumulators. 
         [0019]    Another object of this invention is to provide a system which does not require high gas precharge pressures so that they will have a differential above ambient pressures at sea depths. 
         [0020]    Another object of this invention is to provide a system which does not present a concern with gas pressures high enough and temperatures low enough to provide the possibility of liquefying the compressed gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a partial section of a system of subsea equipment utilizing the control system in the mode of pushing the blowout preventer rams forward to seal across the bore. 
           [0022]      FIG. 2  is a partial section of a system of subsea equipment utilizing the control system in the mode of blocking the movement of the blowout preventer ram. 
           [0023]      FIG. 3  is a partial section of a system of subsea equipment utilizing the control system in the mode of retracting the blowout preventer rams from the bore 
           [0024]      FIG. 4  is a partial section of a system of subsea equipment utilizing the control system in the mode of emptying the low pressure reservoir of control fluids. 
           [0025]      FIG. 5  is a partial section of a system of subsea equipment utilizing the control system in the mode of flooding the low pressure reservoir with sea water to eliminate any accumulated gas. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    Referring now to  FIG. 1 , a blowout preventer (BOP) stack  10  is landed on a subsea wellhead system  11 , which is supported above mudline  12 . The BOP stack  10  is comprised of a wellhead connector  14  which is typically hydraulically locked to the subsea wellhead system  11 , multiple ram type blowout preventers  15  and  16 , an annular blowout preventer  17  and an upper mandrel  18 . A riser connector  19 , and a riser  20  which extends to the surface are attached for communicating drilling fluids to the surface. 
         [0027]    Blowout preventer  16  includes a body  30 , rams  32  and  34  for moving into the vertical bore  36 , connecting rods  38  and  40 , pistons  42  and  44 , outer chamber  46  and  48 , and inner chambers  50  and  52 . 
         [0028]    When lines  60  and  62  are pressured, the pistons  42  and  44 , connecting rods,  38  and  40 , and rams  32  and  34  move toward the centerline of the bore  36  to seal off the bore  36  when appropriate. When lines  64  and  66  are pressured, the components are retracted from bore  26 . 
         [0029]    Control valve  70  is a 3 position valve which is utilized to operate the blowout preventer  16  and is illustrative of dozens of valves which become part of a subsea control system. In the position as shown the control valve  70  receives fluid along line  72  from accumulator  74  and delivers it along line  76  and in turn to lines  60  and  62  to move the rams  32  and  34  towards the bore  36 . Accumulator  74  can be any of the conventional accumulators as indicated in the background of the invention. 
         [0030]    Conventionally, the return fluid coming out of line  64  and  66  through line  78  are vented to the subsea environment. At a 10,000 ft. depth, this subsea environment is at a 10,000×0.465 p.s.i./ft.=4650 p.s.i. This is extremely hard work to do for a conventional accumulator, with the only workable solution being the more expensive constant differential accumulators as described in the background of this application. 
         [0031]    In this embodiment, the flow out of lines  64  and  66  goes through line  78 , through control valve  70 , through line  80 , through check valve  82  and into reservoir  84 . Reservoir  84  is simply an empty bottle at or near atmospheric pressure which will withstand the external pressures of the sea water. If the gas pressure in accumulator  74  is 3000 p.s.i. and the pressure in reservoir  84  is zero, the operating differential pressure across the pistons  42  and  44  is 3000 p.s.i., irrespective of depth. It operates exactly the same at 10,000 ft. as it does at the surface. 
         [0032]    Three position control valve  70  is shown with two opposing electric actuators  90  and  92  along with centering springs  94  and  96 . As actuated with electricity sent electric actuator  92 , program section  98  is active and delivers fluid from the accumulator  74  to the outer chambers  46  and  48 . As the pistons  42  and  44  move forward, the fluid in inner chambers  50  and  52  is flushed out to the reservoir  84 . 
         [0033]    Hydraulic line  104  directs a supply of hydraulic control fluid from the surface through check valve  102  and into accumulator  74  to keep the accumulator  74  charged with pressurized control fluid. Electric line  100  is illustrative of control wires coming from the surface to do tasks such as operating control valve  70 . 
         [0034]    Referring now to  FIG. 2 , the electric signal has been removed from the electric actuator  92  and the two springs  94  and  96  have centralized the valve on program section  106 . In this case the flow is blocked and the rams  32  and  34  will remain stationary in their present position. 
         [0035]    Referring now to  FIG. 3 , an electric signal is sent to electric actuator  90  and has moved program section  110  to the active position. In this position control fluid will be directed from the accumulator  74 , through line  78 , through lines  64  and  66  to the inner chambers  50  and  52 . This will push the pistons  42  and  44  away from the bore and thereby move the rams  32  and  34  away from the bore. 
         [0036]    Referring now to  FIG. 4 , when control fluid collects in reservoir  84  during operations, electric motor  120  drives pump  122  and pumps the control fluids out of line  124  to the ocean as indicated by arrow  126 . The control fluids will be environmentally friendly. Alternately, the fluid can be returned to a hose back to the surface, such as hose  104 . 
         [0037]    Referring now to  FIG. 5 , if there is any gas entrained in the control fluids, they will tend to accumulate as a gas in the low pressure reservoir  84 . Over time, a collection of gas in reservoir  84  can impede the performance of the system. Higher gas pressure in reservoir  84  reduces the pressure differential from accumulator  74 . If motor  120  and therefore pump  122  is reversed, reservoir  84  will be completely filled with seawater up to flowing out of check valve  132  as indicated by arrow  132 . When reservoir  84  is completely filled with water, the entrained gas will be pushed out check valve  130  also. At that time the motor  120  and pump  122  can be returned to the normal pumping direction and remove the water from the reservoir  84 , as is seen in  FIG. 4 . By this procedure a low pressure gas or vacuum can be maintained in reservoir  84 . 
         [0038]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

Summary:
The method of providing a pressurized control fluid for the operation of subsea equipment of providing an accumulator with control fluid pressurized by compressed gas, supplying the control fluid to a control valve for the purpose of operating a function, receiving a return flow of the control fluid from the function to a control valve, directing the return flow of the control fluid to a low pressure chamber whose pressure is substantially unaffected by the subsea environmental pressure, and maintaining the low pressure level in the low pressure chamber.