Abstract:
The method of providing a piston type accumulator with a controlled depth liquid shield on the top of a piston with seals separating a pressurized gas from the seals sealing the pressurized liquid comprising providing a portion of the gas in a chamber portion above said piston and a portion of the gas in a chamber portion not above said piston such that liquids accumulating in the chamber above the piston can be vented into the chamber not above the piston for venting to a location outside said chambers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK 
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     BACKGROUND OF THE INVENTION 
     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 lubricity and corrosion protection. 
     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. 
     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. 
     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. 
     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 455 p.s.i. or 3465 p.s.i. 
     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-functonal. 
     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. 
     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. 
     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. 
     The pressure compensated type has exhibited two disadvantages. First it has required a relatively high pressure seal between the nitrogen chamber and the working fluid chamber. Very smooth seal surfaces are required to seal the nitrogen at relatively high pressures, and nitrogen still will tend to leak past the seals during dynamic movement. Secondly, there is some chance that the liquids will go past the seals and into the nitrogen chamber on one end and into the vacuum chamber on the opposite end and prevent effective performance of the accumulator. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of this invention is to provide a pressure compensated accumulator for deepwater ocean service which does not require a high pressure gas seal between a nitrogen chamber and an oil chamber. 
     A second object of the present invention is to provide a pressure compensated accumulator for deepwater ocean service which can prevent the accumulation of liquids in the vacuum chamber. 
     A third object of the present invention is to provide a pressure compensated accumulator for deepwater ocean service which can prevent the accumulation of liquids in the nitrogen chamber. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a partial section thru a subsea blowout preventer stack showing applications of principles of this invention. 
     FIG. 2 is a half section of an accumulator of the present invention. 
     FIG. 3 is a partial section of the top portion of the accumulator of this invention. 
     FIG. 4 is a partial section of the accumulator of this invention showing means to exhaust accumulated liquids from the nitrogen chamber. 
     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 
     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. 
     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. 
     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 . 
     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 . 
     Second cylinder  54  is empty except for pressurized gas and a valve assembly  90  near the bottom. Third cylinder  55  is empty except for pressurized gas. 
     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 . 
     Chambers  120  and  121  contain sea water and the resultant sea water pressure which comes in thru ports  122  and  123 , respectively. 
     Chambers  130  and  131  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. 
     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 top 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 . 
     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 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. 
     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  54  might be expelled thru port  141  into second cylinder  54 . 
     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 . 
     Referring now to FIG. 4 a lower portion of second cylinder  54  is shown. When 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 move 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 . 
     Referring now to FIG. 5, a partial section of the bottom of cylinder  53  is shown. In this case a check valve  190  is provided with a spring  191 . If the piston  192  is simply lowered to the bottom of the stroke by the pressure of the gas from the top of the upper piston  74 , 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  122 ), any liquids in chamber  130  will be expelled past check valve  190 . 
     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. 
     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. 
     SEQUENCE LISTING 
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