Abstract:
An accumulator for hydraulically actuating subsea equipment includes a hydraulic fluid chamber and a gas chamber. The hydraulic fluid chamber is in fluid communication with the subsea equipment and comprises a hydraulic piston slidably received, at least partially, within the hydraulic chamber. The gas chamber comprises a charge piston slidably received within the gas chamber, the charge piston dividing the gas chamber into a first portion and a second portion. The first portion of the gas chamber is configured to receive ambient hydrostatic pressure therein, and the second portion of the gas chamber is configured to receive precharge gas therein.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/003,150, filed on Jan. 7, 2011, which is a 35 U.S.C. §371 national stage application of PCT/US2009/052709 filed Aug. 4, 2009, which claims the benefit of U.S. Provisional Patent Application No. 61/086,029 filed Aug. 4, 2008, all of which are incorporated herein by reference in their entireties for all purposes. 
     
    
     BACKGROUND 
       [0002]    Deepwater accumulators provide a supply of pressurized working fluid for the control and operation of subsea equipment, such as through hydraulic actuators and motors. Typical subsea equipment may include, but is not limited to, blowout preventers (BOPs) that 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, or hydraulically actuated connectors and similar devices. 
         [0003]    Accumulators are typically divided vessels with a gas section and a hydraulic fluid section that operate on a common principle. The principle is to precharge the gas section with pressurized gas to a pressure at or slightly below the anticipated minimum pressure required to operate the subsea equipment. Hydraulic fluid can be added to the accumulator in the separate hydraulic fluid section, increasing the pressure of the pressurized gas and the hydraulic fluid. The hydraulic 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. 
         [0004]    Accumulators generally come in three styles—the 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 the 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 the charging gas. A fourth type of accumulator is pressure compensated for depth and adds the nitrogen precharge pressure plus the ambient seawater pressure to the working fluid. 
         [0005]    The precharge gas can be said to act as a spring that is compressed when the gas section is at its lowest volume/greatest pressure and released when the gas section is at its greatest volume/lowest pressure. Accumulators are typically precharged in the absence of hydrostatic pressure and the precharge pressure is limited by the pressure containment and structural design limits of the accumulator vessel under surface ambient conditions. Yet, as accumulators are used in deeper water, the efficiency of conventional accumulators decreases as application of hydrostatic pressure causes the gas to compress, leaving a progressively smaller volume of gas to charge the hydraulic fluid. The gas section must consequently be designed such that the gas still provides enough power to operate the subsea equipment under hydrostatic pressure even as the hydraulic fluid approaches discharge and the gas section is at its greatest volume/lowest pressure. 
         [0006]    For example, accumulators at the surface typically provide 3000 psi working fluid maximum pressure. In 1000 feet of seawater the ambient pressure is approximately 465 psi. For an accumulator to provide a 3000 psi differential at 1000 ft. depth, it must actually be precharged to 3000 psi plus 465 psi, or 3465 psi. 
         [0007]    At slightly over 4000 ft. water depth, the ambient pressure is almost 2000 psi, so the precharge would be required to be 3000 psi plus 2000 psi, or 5000 psi. This would mean that the precharge would equal the working pressure of the accumulator and any fluid introduced for storage may cause the pressure to exceed the working pressure and accumulator failure. 
         [0008]    At progressively greater hydrostatic operating pressures, the accumulator thus has greater pressure containment requirements at non-operational (no ambient hydrostatic pressure) conditions. 
         [0009]    The accumulator design must also take into account human error contingencies. For example, removal of the external ambient hydrostatic pressure without evacuating the fluid section of the accumulator to reestablish the original gas section precharge pressure may result in failure due to gas section pressures exceeding the original precharge pressures. 
         [0010]    As shown in  FIGS. 1 and 2 , accumulators may be included, for example, as part of a subsea BOP stack assembly  10  assembled onto a wellhead assembly  11  on the sea floor  12 . The BOP stack assembly  10  is connected in line between the wellhead assembly  11  and a floating rig  14  through a subsea riser  16 . The BOP stack assembly  10  provides emergency fluid pressure control of fluid in the wellbore  13  should a sudden pressure surge escape the wellbore  13 . The BOP stack assembly thus prevents damage to the floating rig  14  and the subsea riser  16  from fluid pressure exceeding design capacities. 
         [0011]    The BOP stack assembly  10  includes a BOP lower riser package  18  that connects the riser  16  to a BOP package  20 . The BOP package  20  includes a frame  22 , BOPs  23 , and accumulators  24  that may be used to provide back up hydraulic fluid pressure for actuating the BOPs  23 . The accumulators  24  are incorporated into the BOP package  20  to maximize the available space and leave maintenance routes clear for working on the components of the subsea BOP package  20 . However, the space available for other BOP package components such as remote operated vehicle (ROV) panels and mounted controls equipment has become harder to establish due to the increasing number and size of the accumulators  24  required to be considered for operation in deeper water depths. Depending on the depth of the wellhead assembly  11  and the design of the BOPs  23 , numerous accumulators  24  must be included on the frame  22 , taking up valuable space on the frame  22  and adding weight to the subsea BOP stack assembly  10 . The accumulators  24  are also typically installed in series where the failure of any one accumulator  24  prevents the additional accumulators  24  from functioning. 
         [0012]    The inefficiency of precharging accumulators under non-operational conditions requires large aggregate accumulator volumes that increase the size and weight of the subsea equipment. Yet, offshore rigs are moving further and further offshore to drill in deeper and deeper water. Because of the ever increasing envelop of operation, traditional accumulators have become unmanageable with regards to quantity and location. In some instances, it has even been suggested that in order to accommodate the increasing demands of the conventional accumulator system, a separate subsea skid may have to be run in conjunction with the subsea BOP stack in order to provide the required volume necessary at the limits of the water depth capability of the subsea BOP stack. With rig operators increasingly putting a premium on minimizing size and weight of the drilling equipment to reduce drilling costs, the size and weight of all drilling equipment must be optimized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more detailed description of the embodiments, reference will now be made to the following accompanying drawings: 
           [0014]      FIG. 1  is a schematic of a subsea BOP stack assembly connecting a wellhead assembly to a floating rig through a subsea riser; 
           [0015]      FIG. 2  is a perspective view of a BOP package of the BOP stack assembly of  FIG. 1 ; 
           [0016]      FIG. 3  a cross-section view of an accumulator in accordance with one embodiment of the claimed subject matter; and 
           [0017]      FIG. 4  is a cross-section view of an accumulator in accordance with one embodiment of the claimed subject matter. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    In the drawings and description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present invention is susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. Any use of any form of the terms “connect”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
         [0019]    In  FIG. 3 , an accumulator  300  includes an accumulator body  301  with a hydraulic fluid portion  304  and a charge fluid portion  309 . The hydraulic fluid portion  304  partially forms a hydraulic fluid chamber  305  and the charge fluid portion  309  partially forms a precharge gas chamber  310 . An end cap  330  having a hydraulic fluid port  335  seals off an end of the hydraulic fluid portion  304  at one end of the accumulator  300 . Another end cap  340  having a hydrostatic pressure port  345  seals off an end of the charge fluid portion  309  at the other end of the accumulator  300 . 
         [0020]    A hydraulic piston  315  is slidably and sealingly mounted in the hydraulic fluid portion  304 . The hydraulic fluid chamber  305  is defined in the hydraulic fluid portion  304  between the hydraulic piston  315  and the end cap  330 . A charge piston  320  is slidably and sealingly mounted in the charge fluid portion  309 . The precharge gas chamber  310  is defined in the charge fluid portion  309  between the charge piston  320  and the hydraulic piston  315 . 
         [0021]    At the surface before installation on the sea floor, a precharge gas, such as nitrogen, is provided into the precharge gas chamber  310  and pressurized according to a predetermined depth at which the accumulator will operate and the pressure needed to operate the subsea equipment, such as the rams of the BOPs. A precharge pressure port (not shown) may be, for example, in the side of the accumulator body  301  or in the charge piston  320 . During pressurization of the precharge gas chamber  310 , the hydraulic piston  315  moves towards the end cap  330 . After placement on the seafloor, hydraulic fluid is pumped into the hydraulic fluid chamber  305 , which moves the hydraulic piston  315  towards the opposing end of the hydraulic fluid portion  304  until contacting a shoulder  316 . The hydraulic fluid may be any suitable hydraulic fluid and may also include performance enhancing additives such as a lubricant. The accumulator  300  is then ready to provide pressurized hydraulic fluid to operate the rams of the BOPs. 
         [0022]    In normal operation, the force of the precharge gas acting against the hydraulic piston  315  is sufficient to operate the subsea equipment with the hydraulic fluid stored in the hydraulic fluid chamber  305 . However, in case additional force is needed, the accumulator  300  further includes a valve  350 , which communicates ambient hydrostatic pressure through the port  345  when open. That hydrostatic pressure acts against the charge piston  320  and increases the pressure within the precharge gas chamber  310 . The increased pressure of the precharge gas in turn acts against the hydraulic piston  315  to increase the pressure of the hydraulic fluid. As hydraulic fluid is forced out of the hydraulic fluid chamber  305  by movement of the hydraulic piston  315 , the charge piston  320  will move in the same direction with hydrostatic pressure continuing to act against the charge piston  320 . Because hydrostatic pressure acts against the charge piston  320 , the effective increase in pressure of the hydraulic fluid is increased proportional to the difference in piston diameters, giving a multiplier effect to the hydrostatic pressure upon the hydraulic piston  315 . The hydrostatic pressure provides a boost in the force acting on the subsea equipments, such as hydraulic rams of a blowout preventer, which may be useful in an emergency situation. As the hydraulic rams close and the hydraulic fluid exits the accumulator  300 , seawater will flow into the accumulator to apply the constant hydrostatic pressure. Thus, the force applied by the hydraulic rams remains constant between the fully opened and fully closed positions. 
         [0023]    Referring now to  FIG. 4 , another accumulator  400  is shown that shares many of the same components as the accumulator  300  shown in  FIG. 3 . In the accumulator of  FIG. 4  however the hydraulic piston  315  is extended to form a piston body  401  that includes a hydraulic diameter portion  402  and a charge diameter portion  403 . The hydraulic diameter portion  402  slidably and sealingly engages the inside of the hydraulic fluid portion  304  of the accumulator body  301 , and the charge diameter portion  403  slidably and sealingly engages the inside of the charge fluid portion  309  of the accumulator body  301 . Although shown as a solid piston body, those having ordinary skill in the art will appreciate that the piston body  401  may be a single hollow piece or any assembly of cylinders that results in a mechanical connection between the hydraulic diameter portion  402  and the charge diameter portion  403 . 
         [0024]    The hydraulic fluid chamber  305  is partially defined by the hydraulic fluid portion  402  of the piston body  401  and the end cap  330 . A buffer chamber  405  is defined as the annular space between the outer diameter of the piston body  401  and the inner diameter of the charge fluid portion  309  of the accumulator body  301 . At the surface before installation on the sea floor, the precharge gas is provided into the precharge gas chamber  310  defined between the charge piston  320  and the charge diameter portion  403  of the piston body  401  and pressurized according to a predetermined operating depth and pressure. As shown, the charge diameter portion  403  of the piston body  401  is larger than the hydraulic diameter portion  402 . Thus, the necessary precharge pressure may be reduced proportional to the difference in effective piston area of the two portions of the piston body  401 . 
         [0025]    The pressure in the precharge gas chamber  310  at the surface causes the piston body  401  to move towards end cap  330 , which reduces the size of the buffer chamber  405 . Fluid, such as air, contained in the buffer chamber  405  may be vented through port  410 . If port  410  is closed after the piston body  401  has travelled fully towards the end cap  330 , the buffer chamber  405  will have a vacuum when the hydraulic fluid chamber  305  is filled with hydraulic fluid at the sea floor. By having a vacuum, none of the pressure in the precharge gas chamber  310  is counterbalanced by the buffer chamber  405 . If air in the buffer chamber  405  is not vented, actuation of the piston body  401  will compress the air in the buffer chamber  405 , thereby providing a pressure counterbalance to the precharge gas pressure. 
         [0026]    In normal operation, the force of the precharge gas acting against the hydraulic piston  315  is sufficient to operate the subsea equipment with the hydraulic fluid stored in the hydraulic fluid chamber  305 . However, in case additional force is needed, the accumulator  300  further includes a valve  350 , which communicates ambient hydrostatic pressure through the port  345  when open. That hydrostatic pressure acts against the charge piston  320  and increases the pressure within the precharge gas chamber  310 . The increased pressure of the precharge gas in turn acts against the charge diameter portion  403  of the piston body  401  to increase the pressure of the hydraulic fluid. As hydraulic fluid is forced out of the hydraulic fluid chamber  305  by movement of the hydraulic diameter portion  402  of the piston body  401 , the piston body  401  will move in the same direction with hydrostatic pressure continuing to act against the charge diameter portion  403  of the piston body  401 . Because hydrostatic pressure acts against charge diameter portion of the piston body  401  via the charge piston  320 , the effective increase in pressure of the hydraulic fluid is increased proportional to the difference in piston diameters, giving a multiplier effect to the hydrostatic pressure upon the hydraulic diameter portion  402  of the piston body  401 . The hydrostatic pressure provides a boost in the force acting on the subsea equipment, such as hydraulic rams of a blowout preventer, which may be useful in an emergency situation. As the hydraulic rams close and the hydraulic fluid exits the accumulator  300 , seawater will flow into the accumulator to apply the constant hydrostatic pressure. Thus, the force applied by the hydraulic rams remains constant between the fully opened and fully closed positions. 
         [0027]    While specific embodiments have been shown and described, modifications can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments as described are exemplary only and are not limiting. Many variations and modifications are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.