TECHNICAL FIELD 
     This invention relates to the general subject connecting a low or negative pressure accumulator to the low pressure side of the pistons operating blowout preventer rams in a high pressure subsea environment to increase the shearing force. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     REFERENCE TO A “MICROFICHE APPENDIX” 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     The field of this invention is that operating blowout preventers in deep water operations to seal the well bore and protect the environment in emergency situations when an obstruction is in the well bore. 
     Blowout preventer systems are major pieces of capital equipment landed on the ocean floor in order to provide a conduit for the drill pipe and drilling mud while also providing pressure protection while drilling holes deep into the earth for the production of oil and gas. The typical blowout preventer stacks have an 18¾ inch bore and are usually of 10,000 psi working pressure. The blowout preventer stack assembly weighs in the range of five hundred to eight hundred thousand pounds. It is typically divided into a lower blowout preventer stack and a lower marine riser package. 
     The lower blowout preventer stack includes a connector for connecting to the wellhead at the bottom on the seafloor and contains several individual ram type blowout preventer assemblies, which will close on various pipe sizes and in some cases, will close on an open hole with what are called blind rams. Characteristically there is an annular preventer at the top, which will close on any pipe size or close on the open hole. 
     The lower marine riser package typically includes a connector at its base for connecting to the top of the lower blowout preventer stack, it contains a single annular preventer for closing off on any piece of pipe or the open hole, a flex joint, and a connection to a riser pipe which extends to the drilling vessel at the surface. 
     The purpose of the separation between the lower blowout preventer stack and the lower marine riser package is that the annular blowout preventer on the lower marine riser package is the preferred and most often used pressure control assembly. When it is used and either has a failure or is worn out, it can be released and retrieved to the surface for servicing while the lower blowout preventer stack maintains pressure competency at the wellhead on the ocean floor. 
     The riser pipe extending to the surface is typically a 21 inch O.D. pipe with a bore larger than the bore of the blowout preventer stack. It is a low pressure pipe and will control the mud flow which is coming from the well up to the rig floor, but will not contain the 10,000-15,000 psi that the typical blowout preventer stack will contain. Whenever high pressures must be communicated back to the surface for well control procedures, smaller pipes on the outside of the drilling riser, called the choke line and the kill line, provide this function. These will typically have the same working pressure as the blowout preventer stack and rather than have an 18¾-20 inch bore, they will have a 3-4 inch bore. There may be additional lines outside the primary pipe for delivering hydraulic fluid for control of the blowout preventer stack or boosting the flow of drilling mud back up through the drilling riser. 
     The blowout preventers are operated or closed in response to an electric signal from the surface to an electro-hydraulic control valve which directs fluid stored under pressure in accumulator bottles to the operating cylinders on the blowout preventer. Any number of events can prevent this sequence from occurring such as failure in the surface controls to send the signal, failure in the connecting lines from the surface to depth as great as 12,000′, failure of the electro-hydraulic valve to close, and absence of fluid stored under pressure. 
     All subsea blowout preventers have 100% redundant control systems to minimize the risk of non-operation. They are very characteristically called the yellow system and blue system and represent primary and secondary means to operate any function on the blowout preventer stack. 
     When all else fails, it is not necessary to have emergency operation of multiple components in the subsea blowout preventer stack. A single component—the blind shear rams can immediately secure an uncontrolled flow of oil or gas from the well. A flat faced gate from each side will meet at the middle to seal off the bore. If a pipe of any sort is in the bore at the time, it will simply shear the pipe in half and then seal. The blind shear ram is the ultimate safety device, but it must operate. Unfortunately, contemporary rams will not shear every kind of pipe in half, but are rather limited to shearing the smaller drill pipe bodies. Larger cross section and higher strength materials provide limitations on contemporary devices, providing situations in which the safety devices simply will not close. 
     The need to be able to send a single command which will quickly secure the well bore against discharges to the environment has long been known in the industry as indicated by a test demonstration of shearing a drill collar at the Offshore Technology Conference in Houston more than 20 years ago. Since this demonstration of the desire for this to be accomplished, manufacturers have not accomplished this, but rather have settled back in a mode of building systems which in some cases will shear only the drill pipe body and the tool joint, and in some cases the products offered will only shear the drill pipe body and will not shear the drill pipe tool joint. The need for this level of safety has long been known, and industry has simply not figured out how to practically achieve this. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of this invention is to provide a method of using the ambient subsea pressure to increase the force available for shearing pipe or other objects in the well bore. 
     A second object of this invention is to provide a method of connecting a vacuum tank to the low side of the pistons operating shear rams to increase the force on the shear rams. 
     A third object of this invention is to provide a solution which can be added to the systems presently in the field rather than solely depending on long term obsolescence of present systems and upgrades on new system only. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a deepwater drilling system such as would use this invention 
         FIG. 2  is a schematic of a portion of a blowout preventer stack illustrating how the yellow and blue control pods direct operating fluids from pressurized accumulators to the function to be actuated, illustrating various items which might be in the well bore when closure is needed, and illustrating shear rams which are intended to cut the items in the well bore. 
         FIG. 3  is a schematic similar to  FIG. 2  showing the negatively charged accumulator of this invention added to the system. 
         FIG. 4  is a schematic similar to  FIG. 3  showing the negatively charged accumulator of this invention having assisted in shearing a tool joint of the drill pipe in the well bore. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a view of a complete system for drilling subsea wells  20  is shown in order to illustrate the utility of the present invention. The drilling riser  22  is shown with a central pipe  24 , outside fluid lines  26 , and cables or hoses  28 . 
     Below the drilling riser  22  is a flex joint  30 , lower marine riser package  32 , lower blowout preventer stack  34  and wellhead  36  landed on the seafloor  38 . 
     Below the wellhead  36 , it can be seen that a hole was drilled for a first casing string, that string  40  was landed and cemented in place, a hole drilled through the first string for a second string, the second string  42  cemented in place, and a hole is being drilled for a third casing string by drill bit  44  on drill string  46 . 
     The lower Blowout Preventer stack  34  generally comprises a lower hydraulic connector for connecting to the subsea wellhead system  36 , usually 4 or 5 ram style Blowout Preventers, an annular preventer, and an upper mandrel for connection by the connector on the lower marine riser package  32 . 
     Below outside fluid line  26  is a choke and kill (C&amp;K) connector  50  and a pipe  52  which is generally illustrative of a choke or kill line. Pipe  52  goes down to valves  54  and  56  which provide flow to or from the central bore of the blowout preventer stack as may be appropriate from time to time. Typically a kill line will enter the bore of the Blowout Preventers below the lowest ram and has the general function of pumping heavy fluid to the well to overburden the pressure in the bore or to “kill” the pressure. The general implication of this is that the heavier mud will not be circulated, but rather forced into the formations. A choke line will typically enter the well bore above the lowest ram and is generally intended to allow circulation in order to circulate heavier mud into the well to regain pressure control of the well. 
     Normal drilling circulation is the mud pumps  60  taking drilling mud  62  from tank  64 . The drilling mud will be pumped up a standpipe  66  and down the upper end  68  of the drill string  46 . It will be pumped down the drill string  46 , out the drill bit  44 , and return up the annular area  70  between the outside of the drill string  46  and the bore of the hole being drilled, up the bore of the casing  42 , through the subsea wellhead system  36 , the lower blowout preventer stack  34 , the lower marine riser package  32 , up the drilling riser  22 , out a bell nipple  72  and back into the mud tank  64 . 
     During situations in which an abnormally high pressure from the formation has entered the well bore, the thin walled drilling riser  24  is typically not able to withstand the pressures involved. Rather than making the wall thickness of the relatively large bore drilling riser thick enough to withstand the pressure, the flow is diverted to a choke line or outside fluid line  26 . It is more economic to have a relatively thick wall in a small pipe to withstand the higher pressures than to have the proportionately thick wall in the larger riser pipe. 
     When higher pressures are to be contained, one of the annular or ram Blowout Preventers are closed around the drill pipe and the flow coming up the annular area around the drill pipe is diverted out through choke valve  54  into the pipe  52 . The flow passes up through C&amp;K connector  50 , up pipe  26  which is attached to the outer diameter of the central pipe  24 , through choking means illustrated at  74 , and back into the mud tanks  64 . 
     On the opposite side of the drilling riser  22  is shown a cable or hose  28  coming across a sheave  80  from a reel  82  on the vessel  84 . The cable or hose  28  is shown characteristically entering the top of the lower marine riser package. These cables typically carry hydraulic, electrical, multiplex electrical, or fiber optic signals. Typically there are at least two of these systems for redundancy, which are characteristically painted yellow and blue. As the cables or hoses  28  enter the top of the lower marine riser package  32 , they typically enter the top of control pod to deliver their supply or signals. When hydraulic supply is delivered, a series of accumulators are located on the lower marine riser package  32  or the lower Blowout Preventer stack  34  to store hydraulic fluid under pressure until needed. 
     Referring now to  FIG. 2 , portion of the complete system for drilling subsea wells  20  is shown in greater detail for better clarity and shows a conventional dual pod (yellow and blue) control system. Connector  100  at the bottom is hydraulically operated to provide a connection between the lower blowout preventer stack  34  and the subsea wellhead system  36  as shown in  FIG. 1 . Ram type blowout preventers are shown at  102  and  104  and an annular blowout preventer is shown at  106 . An annular blowout preventer is basically a ring of rubber which is pushed into the bore to seal the bore or on anything in the bore, but is not presently under consideration. 
     Ram type blowout preventer  104  has pistons  110  and  112  connected to rams  114  and  115  respectively. Ram  114  has seal element  116  and shear blade portion  117 . Ram  115  has seal element  118  and shear blade portion  119 . When pressure and flow are introduced into line  120 , the pistons and rams move toward one another and sealingly engage in the center of the bore  122 . When rams  114  and  115  are appropriately constructed, they will shear pipe which is within bore  122  and then seal across the bore. When pressure and flow are introduced into line  124  the pistons  110  and  112  along with rams  114  and  115  move away (retract) from each other until the bore  122  is unobstructed. 
     The yellow pod control system  130  is shown with a single valve  132 , pressure supply from accumulator  134 , and control wire or umbilical  136  going to the surface vessel. The blue pod control system  140  is an exact duplicate for the yellow pod control system  132 , except for the color. It shows a single valve  142 , pressure supply from an accumulator  144 , and control wire or umbilical  146  going to the surface. Control valves  132  and  142  are illustrative of dozens of similar valves in each of the control pods for various functions. 
     When control valve  132  is shifted to the right and pressure line  148  communicates with line  150 , it supplies pressure and flow to shuttle valve  152 , moving the internal ball  154  opposite the position as shown directing the fluid to line  124  to push rams  114  and  115  into the bore  122  to shear pipe in the well and seal across the bore. When control valve  132  is shifted to the left and pressure line  148  communicates with line  156 , it supplies pressure and flow to shuttle valve  158 , moving the internal ball  160  to the position opposite the position as shown directing the fluid to line  124  to retract rams  114  and  115  out of the center of bore  122 . 
     Similarly, when control valve  142  is shifted to the right and pressure line  170  communicates with line  172 , it supplies pressure and flow to shuttle valve  152 , moving the internal ball  154  to the position as shown directing the fluid to line  124  to push rams  114  and  115  into the bore  122  to shear pipe in the well and seal across the bore. When control valve  142  is shifted to the left and pressure line  170  communicates with line  174 , it supplies pressure and flow to shuttle valve  158 , moving the internal ball  160  to the position as shown directing the fluid to line  124  to retract rams  114  and  115  out of the center of bore  122 . 
     Within bore  122  a drill string  46  is shown with bit  44  at the bottom. Drill pipe body  180  is illustrative of what the majority of the drill string and will typically be of high grade steel of 5.5 inch O.D. and 0.5 or 0.6 wall thickness. All conventional shear rams will shear the drill pipe body  180 . Tool joint  182  is a threaded section connecting  30  foot sections of drill pipe body together. The tool joint  182  is always thicker in cross section and is frequently of higher strength steel. Some conventional shear rams will shear a tool joint and some will not. Due to the relative length of the drill pipe body sections and the length of the tool joints, there is about 1 chance in 30 of hitting a tool joint. In calm times the footage of the pipes in the well bore can be calculated to minimize the risk. In emergency situations, these calculations may not be able to be made and the operator must simply close hoping to miss a tool joint. 
     Drill collars  184  immediately above the bit  44  are 30 foot long sections of small I.D. and large O.D. tubes for the purpose of concentrating weight on the bit to enhance drilling. If the drill collars are in the way of the shear rams at the time of emergency closure, none of the conventional rams will shear the drill collars. 
     The primary reason for the inability to shear the thicker cross section is the limited force generated by the pressure in line  120  pushing on the piston area of the pistons  110  and  112 . The piston area is typically limited by the general geometry of the assembly. 
     Referring now to  FIG. 3 , a valve  190  has been introduced into line  124  dividing it into lines  124 A and  124 B. Negative accumulator  192  is connected to valve  190  by line  194 . It should be noted that negative accumulator  192  does not have the internal symbols of an accumulator indicating a division of the nitrogen gas  196  and control liquid  198  as is seen in accumulators  144  and  148 . Negative accumulator  192  can be simply an empty bottle with atmospheric pressure in it or can have an internal pressure higher or lower than atmospheric pressure, but less than the anticipated ambient pressure at the working depth. If we are drilling in 7000 foot seawater depth, the water (ambient) pressure is 7000*0.465 p.s.i./ft. or 3255 p.s.i. Relatively speaking, the negative accumulator has a pressure 3255 p.s.i. lower than subsea ambient, or −3255 p.s.i. 
     Referring now to  FIG. 4 , when valve  132  or valve  142  directs the pressure from accumulator  134  or  144  respectively to line  120  the rams  114  and  115  are pushed forward by the force of the fluid on sides  200  and  202  of the piston  110  and  112  respectively. The magnitude of the force is the 3000 p.s.i. differential of the fluid from the accumulator to ambient across the piston area. 
     If this force is not adequate to shear the pipe, valve  190  can be actuated to block line  124   a  and communicate with line  194  and therefore to negative accumulator  192 . The differential pressure across the pistons  110  and  112  will now be the 3000 p.s.i. from the accumulator plus the −3255 p.s.i. negative charge of accumulator  192  creating a differential pressure of 6255 p.s.i. Again, as the force is a function of the differential pressure times the piston area, the force available for shearing is now doubled. Portion  210  of tool joint  182  is seen as it would be bent over if the tool joint was restrained from falling out of the shear rams such as when the bit is landed on the bottom of the hole being drilled. The upper portion of the sheared tool joint is not shown as the drill string will typically and it will simply move upward. 
     There will continue to be extremely large objects going through the rams which cannot be sheared such as casing hangers and running tools, but doubling the force available for shearing will substantially improve the safety of the system by improving the chances that whatever is in the bore will be sheared. 
     The non-obviousness of this invention is clearly demonstrated by the need for enhanced safety in emergency situations, the extended period over which the need has been known, and the lack of recognition of this solution to the problem. 
     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.
The method of providing increased motive force for one or more rams of a subsea blowout preventer comprising providing a one or more pistons connected to the one or more ram, the pistons having a distal side and a proximate side with respect to the ram, providing a tank contain a first pressure less than the ambient pressure of seawater at the location of the subsea blowout preventer, and communicating the first pressure with the proximate side of the one or more pistons to cause or enhance the differential pressure across the one or more pistons to urge the rams toward the center of the bore of the subsea blowout preventer.