Abstract:
The method of providing an accumulator for the storage of pressurized liquids by the use of a pressurized gas, comprising providing for the storage of said pressurized gas, providing for the storage of said pressurized liquids which are pressurized by said pressurized gas, moving said accumulator to a location of lower environmental temperatures, and increasing the temperature of said pressurized gas to increase the pressure of said gas and therefore to increase the pressure of said pressurized liquid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Ser. No. 10/314,361 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     N/A  
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK  
       [0003]     N/A  
       BACKGROUND OF THE INVENTION  
       [0004]     The field of this invention is that of deepwater accumulators for the purpose of providing a supply of pressurized working fluid for the control and operation of equipment. Typical equipment includes, but is not limited to, 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, or hydraulically actuated connectors and similar devices. The fluid to be pressurized is typically an oil based product or a water based product with additives to enhance lubricity and corrosion protection.  
         [0005]     Currently accumulators come in three styles and 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.  
         [0006]     The accumulator styles are 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.  
         [0007]     Accumulators at the surface typically provide 3000 p.s.i. working fluid maximum pressure. 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.  
         [0008]     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.  
         [0009]     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.  
         [0010]     A fourth type accumulator has been developed which is one which is pressure compensated for depth, and is illustrated in the U.S. Pat. No. 6,202,753. This style operates effectively like a summing relay to add the nitrogen precharge pressure plus the ambient seawater pressure to the working fluid. This means that irrespective of the seawater depth (pressure), the working fluid will always have a greater pressure available for work by the amount of the nitrogen precharge.  
         [0011]     This “pressure compensated” style has numerous advantages in addition to the pressure compensation. It allows lower gas pressures with associated safety, eliminates the need to recharge the system for differing operational depths, and eliminates expensive mistakes in setting the charge pressures.  
         [0012]     Although the pressure compensated type has advantages over the other types of accumulators, it is still impacted by the change in temperature as the seawater and therefore nitrogen becomes colder with depths.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The object of this invention is to provide a pressure compensated accumulator for deepwater ocean service which compensates for the decrease in temperature due to operating in ocean depths.  
         [0014]     A second object of the present invention is to provide an accumulator which can have the gas of the accumulator cooled at the surface to the temperature of the subsea environment to allow realistic operational tests at the surface without having to change the charge of the accumulator before deploying it to ocean depths. 
     
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0015]      FIG. 1  is a partial section thru a subsea blowout preventer stack showing applications of principles of this invention.  
         [0016]      FIG. 2  is a half section of an accumulator of the present invention.  
         [0017]      FIG. 3  is a partial section of the top portion of the accumulator of this invention.  
         [0018]      FIG. 4  is a partial section of the accumulator of this invention showing means to exhaust accumulated liquids from the nitrogen chamber.  
         [0019]      FIG. 5  is a partial section of the accumulator of this invention showing the lower portion of the vacuum portion of the accumulator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     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  to the surface are attached for communicating drilling fluids to and from the surface.  
         [0021]     Blowout preventer  16  shows that an accumulator  40  of this invention being connected to one of the outer cavities  41  thru line  42  and valve  43 . If the valve  43  is opened, fluid pressure from accumulator  40  will move the ram  45  toward the center of the vertical bore (and seal against an opposing ram similarly moved). Accumulator  40  can be any of the types described in the description above.  
         [0022]     Referring now to  FIG. 2 , accumulator  50  has an upper plate  51 , a lower plate  52 , a first cylinder  53 , a second cylinder  54 , a third cylinder  55 , a fourth cylinder  56 , connecting bolts  57 , connecting nuts  58 , and lifting eye  59 .  
         [0023]     First cylinder  53  has an upper bore  70 , a lower bore  71 , a bulkhead  72 , a cylinder rod  73 , an upper piston  74 , and a lower piston  75 . Fourth cylinder  56  has an upper bore  80 , a lower bore  81 , a bulkhead  82 , a cylinder rod  83 , an upper piston  84 , and a lower piston  85 .  
         [0024]     Second cylinder  54  is charged with a pressurized gas, has a valve assembly  90  near the bottom, and a heating element  130 . Third cylinder  55  is charged with a pressurized gas and has a heating element  131 .  
         [0025]     Chambers  100 ,  101 ,  102 , and  103  are pressurized with a gas such as nitrogen or helium. Chambers  115  and  116  contain a working fluid accessible thru ports  117  and  118 .  
         [0026]     Chambers  120  and  121  contain sea water or oil at seawater pressure and the resultant sea water pressure which comes in thru ports  122  and  123  respectively. Chambers  125  and  126  contain a vacuum or may simply be allowed to have atmospheric pressure at the surface at assembly which will effectively be a vacuum in deep water.  
         [0027]     Electric heating elements  130  and  131  have terminals  132 ,  133 ,  134 , and  135  which penetrate the upper plate  51 . Electric heating elements  130  and  131  are suspended within second cylinder  54  and third cylinder  55  respectively. These chambers house the majority of the nitrogen gas which acts as the energy storage “spring” to give the accumulator a pressure drive.  
         [0028]     If an accumulator has a precharge of 3000 p.s.i. and the temperature of the accumulator is dropped from 84 to 34 degrees F., the pressure will drop by 275 p.s.i. to 2725 p.s.i., giving an automatic loss of efficiency of 29.5%.  
         [0029]     The electricity it takes to heat one gallon of nitrogen gas 1 degree at approximately 2000 p.s.i. is about 2.08 watt-hours. For a 100 gallon system to raise the temperature 50 degrees F., it will take about 2867 watt-hours total. At the 480 volts in a typical deepwater drilling control system, this means a total of approximately 358 amp-minutes. If a typical system can send 200 amps down the dual control and power cables, this means that it will take about 2 minutes to heat the gas to compensate for the temperature differential.  
         [0030]     This means when a substantial withdrawal occurs from the accumulator banks, the power umbilicals can be utilized to restore the equivalent of the surface temperature within a couple of minutes to give the full operating capacity back to the accumulators. After the operations are completed, the temperature will return slowly to the deepwater ambient temperature (34 deg. F.) as the accumulators are trickle charged back to their full capacity.  
         [0031]     Referring now to  FIG. 3 , upper plate  51  has port  140  communicating the top of first cylinder  53  with second cylinder  54 , port  141  communicating fourth cylinder  56  with second cylinder  54 , and port  142  communicating third cylinder  55  with second cylinder  54 . As the top of all four cylinders are interconnected, the volumes of the four cylinders are combined to provide a gas spring on the top of the two pistons  74  and  84 . Pistons  74  and  84  contains seals  152  and  153  respectively to seal between the gas chamber  100  and  103  and the working fluid chambers  115  and  116 .  
         [0032]     Recesses  160  and  161  on the upper sides of pistons  74  and  84  serve to hold fluid  165  and  166 . The retention of the fluid  165  and  166  in the recesses  160  and  161  serves to prevent the pressurized gas at  100  and  103  from contacting and thereby tending to leak past the seals  152  and  153 . As liquids are characteristically easier to seal than gasses, the insurance of liquids on both sides of the seal will improve the quality of the sealing.  
         [0033]     If not for the recess, as piston  74  goes to the top of the stroke of cylinder  53 , all of the liquid might be expelled thru port  140  and dumped into second cylinder  54 . Likewise the liquid in the top portion of fourth cylinder  56  might be expelled thru port  141  into second cylinder  54 .  
         [0034]     Alternately, if during the service life of the accumulator, an excess amount of liquid from chamber  115  passes by seal  152  into chamber  100 , the excess amount of liquid will be expelled into the second chamber  54  and excess liquids from fourth cylinder  56  will also be expelled into second cylinder  54 .  
         [0035]     Referring now to  FIG. 4 a  lower portion of second cylinder  54  is shown. When an excess amount of fluid is vented into second cylinder  54 , float  170  is raised pulling pin  171 , link  172 , and pin  173  up while pivoting up on shoulder  174 . As pin  173  is pulled up valve  175  moves up and opens against spring  177 . At this time the high gas pressure in chamber  101  pushes the excess liquid out until the float  170  lowers and allows the valve  175  to close. The excess liquid moves out through check  180  to vent out port  182  to the ocean. The check  180  will then be closed by spring  181 . In this way, a single valve assembly  90  can remove any excess fluids which may be vented past the seals on either piston  74  or  84 .  
         [0036]     Referring now to  FIG. 5 , a partial section of the bottom of cylinder  56  is shown. In this case a check valve  190  is provided with a spring  191 . If the piston  85  is simply lowered to the bottom of the stroke by the pressure of the gas from the top of the upper piston  84 , a high pressure will be generated in any liquid trapped at the bottom of the cylinder. The pressure will approximately be the sum of the pressure of the seawater entering port  122  plus the pressure of the gas in chamber  100 . As the total pressure will exceed the seawater pressure (i.e. at port  123 ), any liquids in chamber  126  will be expelled past check valve  190 .  
         [0037]     In this way, the manufacturing convenience of a four cylinder accumulator bank is complimented with the ability to remove any collection of liquids by a single valve assembly  90 , and each of the lower vacuum chambers can be purged by a simple check valve assembly.  
         [0038]     In addition to the ability to bring the temperature of the pressurized gas back to the temperature at the surface, a considerable efficiency can be obtained by increasing the temperature of the gas to elevated temperatures.  
         [0039]     By selecting appropriate gasses, when the gas reaches a temperature like 34 deg. F. at the bottom of the ocean, it may become a liquid and functionally collapse in volume, allowing the chamber of the gas to become smaller. This can cause a chamber of liquids to become larger, or in other words can recharge the accumulator with liquid. When the gas chamber is then reheated, the gas in the liquid state can be evaporated and thereby repressurized. In this manner a relatively simple system for recharging an accumulator can be located subsea.  
         [0040]     The heating coils of this preferred embodiment illustrated can likewise be replaced by cooling elements. The cooling elements can cool the temperature of the gas at the surface to the temperature of the seawater in ocean depths (typically 34 deg. F.). This will allow gas to be charged to the full pressure of 3000 p.s.i. at the surface for testing and then be normally operational at 3000 p.s.i. when it reaches subsea.  
         [0041]     The foregoing disclosure and description of this invention are illustrative and explanatory thereof, and various changes in the size, shape, and materials as well as the details of the illustrated construction may be made without departing from the spirit of the invention.