Abstract:
An accumulator for use in deepwater operational and control systems which uses a differential between a high pressure ambient pressure source such as sea water pressure and a low pressure source such as a chamber holding vacuum or atmospheric pressure to provide storage and delivery of hydraulic power for operation of equipment.

Description:
BACKGROUND OF THE INVENTION 
     The field of this invention of that of deepwater accumulators 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. 
     Currently accumulators 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. 
     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-functional. 
     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. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide an accumulator for deepwater ocean service which does not lose its precharge differential relative to ambient pressures due to high ambient pressures. 
     A second object of the present invention is to provide an accumulator for deepwater ocean service which does not lose its precharge relative to ambient pressures due to low ambient pressures. 
     A third object of the present invention is to provide an accumulator which has a relatively constant discharge pressure relative to ambient pressure irrespective of the ambient pressure. 
     Another object of the present invention is to provide for actuation of subsea equipment by taking advantage of the inherent pressure of deepwater seawater in relationship to a vacuum. 
    
    
     BRIEF DESCRIPTION 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 section thru a first accumulator style which provides no gas precharge, but rather takes all energy from the seawater pressure. 
     FIG. 3 is a section thru a second accumulator style that provides a nitrogen precharge plus taking energy from seawater pressure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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  19   a  to the surface are attached for communicating drilling fluids to and from the surface. 
     Blowout preventer  15  includes a body  20 , rams  21  and  22  for moving into the vertical bore  23  for sealing, rods  24  and  25 , pistons  26  and  27 , outer chamber  30  and  31 , and inner chambers  32  and  33 . Lines  34  and 35 vent the outer chambers  30  and  31  to the seawater. Lines  36   a  and  36   b  communicate the inner chambers  32  and  33  with low pressure chambers  39   a  and  39   b  thru valves  37  and  38 . If the valves  37  and  38  are opened, the differential pressure between the seawater pressure in outer chambers  30  and  31  and the low pressure in inner chambers  32  and  33  will be available to move the rams  21  and  22  toward each other to close off the vertical bore  23 . 
     Alternately, blowout preventer  16  shows that an alternate accumulator  40  of this invention being connected to one of the outer cavities  41  thru line  42  and valve  43 . The inner chamber  44  is shown communicating with the seawater pressure. 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). 
     Referring now to FIG. 2, accumulator  50  has a body  51  with a smaller bore  52 , a large bore  53 , an annular bulkhead  54 , and a bulkhead  55 . Ram  60  has a smaller diameter  61 , a large diameter  62 , and annular bulkhead  63 , and an end  64 . 
     Assume smaller bore  52  and large bore  53  are sized in a diametrical ratios of 0.707/1 which results in the larger bore piston area  70  having twice the area of the smaller bore piston area  71 , and therefore the annular piston area  72  being the difference between the other two bores has a area equal to the smaller bore piston area  71 . 
     Larger bore piston area  70  is responsive to the seawater pressure. Smaller bore piston area is responsive to the pressure in chamber  80  which can be a very low pressure or a vacuum. Annular piston area  72  is responsive to pressure in annular chamber  81 . 
     Assume that the accumulator is in 10,000 feet of seawater. The seawater pressure is 10,000*0.465 p.s.i./ft. or 4650 p.s.i. The pressure in chamber  81  is twice the seawater pressure or 9300 p.s.i., or 4650 p.s.i. above the deep sea ambient pressure of 4650 p.s.i. 
     Assuming a vacuum in chamber  80 , the pressure in chamber  81  remains at 4650 p.s.i. above ambient for the full discharge of fluids from that chamber. 
     This type of accumulator has no precharge and no output pressure at the surface, but utilizes the inherent pressure of deep sea water to generate an operational pressure differential with respect to a vacuum. 
     Referring now to FIG. 3, accumulator  100  has a body  101 , a smaller bore  102 , an upper annular bulkhead  103 , a lower annular bulkhead  104 , an upper larger bore  105 , a lower larger bore  106 , and upper bulkhead  107 , and a lower bulkhead  108 . Piston means  110  has an inner shaft  111 , an upper piston  112 , a lower piston  113 , an upper annular shoulder  114 , a lower annular shoulder  115 , an upper bulkhead  116 , and a lower bulkhead  117 . 
     Upper chamber  120  is filled with a nitrogen charge such as 3000 p.s.i., which pressure is increased as the bulkhead  116  is moved up to reduce the size of chamber  120 . Chamber  121  is filled with fluid which will be sent to other equipment such as the blowout preventer devices as discussed in FIG.  1 . Chamber  122  is vented thru line  123  and balloon  124  to the sea water pressure. Chamber  125  is filled with a low pressure or a vacuum. 
     Assuming that the area of the inner shaft  111  is  10  percent of the area of the large bore  105 , the pressure in chamber  121  will be intensified by  10  percent over the precharge in chamber  120 , irrespective of the sea water depth of the accumulator application. If the accumulator would be placed in seawater slightly more than 6000 feet deep, the ambient pressure would be 3000 p.s.i. and the pressure in chamber  121  would be 3000 p.s.i., making the accumulator ineffective. This paragraph has described the operation of a conventional accumulator, irrespective of whether it is a bladder type, piston type, or float type. 
     Now if the ambient pressure of the sea water is introduced into chamber  122  and pulls the inner shaft  111  down with the lack of resistance from a vacuum in chamber  125 . The pressure in the chamber  121  will be increased exactly as the pressure in chamber  122  is increased. By this means of automatically increasing the pressure in the chamber  121  according to the increases in ambient pressure, a 3000 p.s.i. initial pressure at the surface will be a 3000 p.s.i. pressure differential at 6000 feet of sea water. This style accumulator closely maintains a constant pressure differential with respect to the ambient pressure, irrespective of the actual depth in sea water of the accumulator. 
     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.