Method and water submerged device for generating a force under water. The device includes a low pressure recipient configured to contain a volume of a first fluid at a low pressure volume, an inlet connected to the low pressure recipient and configured to exchange a second fluid with an external enclosure, and a valve connected to the external enclosure and the inlet and configured to separate a pressure source in the external enclosure from the low pressure recipient. When the valve is open, such that there is a flow communication between the external enclosure and the low pressure recipient, a pressure imbalance occurs in the external enclosure which generates the force and the second fluid from the external enclosure enters the low pressure recipient and compresses the first fluid.

BACKGROUND 
     1. Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for generating a subsea force. 
     2. Discussion of the Background 
     During the past years, with the increase in price of fossil fuels, the interest in developing new production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. 
     The existing technologies for extracting the fossil fuel from offshore fields use a system  10  as shown in  FIG. 1 . More specifically, the system  10  includes a vessel  12  having a reel  14  that supplies power/communication cords  16  to a controller  18 . A Mux Reel may be used to transmit power and communication. Some systems have hose reels to transmit fluid under pressure or hard pipe (rigid conduit) to transmit the fluid under pressure or both. Other systems may have a hose with communication or lines (pilot) to supply and operate functions subsea. However, a common feature of these systems is their limited operation depth. The controller  18 , which will be discussed later, is disposed undersea, close to or on the seabed  20 . In this respect, it is noted that the elements shown in  FIG. 1  are not drawn to scale and no dimensions should be inferred from  FIG. 1 . 
       FIG. 1  also shows a wellhead  22  of the subsea well and a production tubing  24  that enters the subsea well. At the end of the production tubing  24  there is a drill (not shown). Various mechanisms, also not shown, are employed to rotate the production tubing  24 , and implicitly the drill, to extend the subsea well. 
     However, during normal drilling operation, unexpected events may occur that could damage the well and/or the equipment used for drilling. One such event is the uncontrolled flow of gas, oil or other well fluids from an underground formation into the well. Such event is sometimes referred to a “kick” or a “blowout” and may occur when formation pressure exceeds the pressure applied to it by the column of drilling fluid. This event is unforeseeable and if no measures are taken to prevent it, the well and/or the associated equipment may be damaged. 
     Another event that may damage the well and/or the associated equipment is a hurricane or an earthquake. Both of these natural phenomena may damage the integrity of the well and the associated equipment. For example, due to the high winds produced by a hurricane at the surface of the sea, the vessel or the rig that powers the undersea equipment starts to drift resulting in breaking the power/communication cords or other elements that connect the well to the vessel or rig. Other events that may damage the integrity of the well and/or associated equipment are possible as would be appreciated by those skilled in the art. 
     Thus, a blowout preventer (BOP) might be installed on top of the well to seal it in case that one of the above events is threatening the integrity of the well. The BOP is conventionally implemented as a valve to prevent the release of pressure either in the annular space between the casing and the drill pipe or in the open hole (i.e., hole with no drill pipe) during drilling or completion operations.  FIG. 1  shows BOPs  26  or  28  that are controlled by the controller  18 , commonly known as a POD. The blowout preventer controller  18  controls an accumulator  30  to close or open BOPs  26  and  28 . More specifically, the controller  18  controls a system of valves for opening and closing the BOPs. Hydraulic fluid, which is used to open and close the valves, is commonly pressurized by equipment on the surface. The pressurized fluid is stored in accumulators on the surface and subsea to operate the BOPs. The fluid stored subsea in accumulators may also be used to autoshear and/or for deadman functions when the control of the well is lost. The accumulator  30  may include containers (canisters) that store the hydraulic fluid under pressure and provide the necessary pressure to open and close the BOPs. The pressure from the accumulator  30  is carried by pipe or hose  32  to BOPs  26  and  28 . 
     As understood by those of ordinary skill, in deep-sea drilling, in order to overcome the high hydrostatic pressures generated by the seawater at the depth of operation of the BOPs, the accumulator  30  has to be initially charged to a pressure above the ambient subsea pressure. Typical accumulators are charged with nitrogen but as precharge pressures increase, the efficiency of nitrogen decreases which adds additional cost and weight because more accumulators are required subsea to perform the same operation on the surface. For example, a 60-liter (L) accumulator on the surface may have a useable volume of 24 L on the surface but at 3000 m of water depth the usable volume is less than 4 L. To provide that additional pressure deep undersea is expensive, the equipment for providing the high pressure is bulky, as the size of the canisters that are part of the accumulator  30  is large, and the range of operation of the BOPs is limited by the initial pressure difference between the charge pressure and the hydrostatic pressure at the depth of operation. 
     In this regard,  FIG. 2  shows the accumulator  30  connected via valve  34  to a cylinder  36 . The cylinder  36  may include a piston (not shown) that moves when a first pressure on one side of the piston is higher than a second pressure on the other side of the piston. The first pressure may be the hydrostatic pressure plus the pressure released by the accumulator  30  while the second pressure may be the hydrostatic pressure. Therefore, the use of pressured canisters to store high-pressure fluids to operate a BOP make the operation of the offshore rig expensive and require the manipulation of large parts. 
     Still with regard to  FIG. 2 , the valve  34  may be provided between the accumulator  30  and the cylinder  36  in order to control the timing for applying the supplemental pressure from the accumulator  30 . The supplemental pressure may be generated by the accumulator  30 , according to an exemplary embodiment, by providing, for example, 16 300-L bottles, each carrying nitrogen under pressure.  FIG. 3  shows such an example of a bottle  50 .  FIG. 3  shows that a bottle  50  has a first chamber  52  that includes nitrogen under pressure and a second chamber  54 , separated by a bladder or piston  56  from the first chamber  52 . The second chamber  54  is connected to the pipe  32  and includes hydraulic fluid. When the controller  18  instructs the accumulator  30  to release its pressure, each bottle  50  uses the nitrogen pressure to move the bladder  56  towards the pipe  32  such that the supplemental pressure is provided via pipe  32  to the cylinder  36 . 
     Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, i.e., low efficiency, safety issues related to the surface high precharge pressures, large size and weight of the accumulator, etc. 
     SUMMARY 
     According to one exemplary embodiment, there is a water submerged device for generating a force under water. The device includes a low pressure recipient configured to contain a volume of a first fluid at a low pressure volume; an inlet connected to the low pressure recipient and configured to exchange a second fluid with an external enclosure; and a valve connected to the external enclosure and the inlet and configured to separate a pressure source in the external enclosure from the low pressure recipient. When the valve is open, such that there is a flow communication between the external enclosure and the low pressure recipient, a pressure imbalance occurs in the external enclosure which generates the force and the second fluid from the external enclosure enters the low pressure recipient and compresses the first fluid. 
     According to another exemplary embodiment, there is a method for generating a force by moving a piston inside an external enclosure of a water submerged device, the piston dividing the external enclosure into a closing chamber and an opening chamber and the opening chamber communicating with a low pressure recipient via a pipe having a valve, the valve separating a pressure source in the opening chamber from the low pressure recipient, and the low pressure recipient containing a volume of a first fluid. The method includes applying a first pressure to the closing and opening chambers, wherein the first pressure is generated by a weight of the water at a certain depth of the device; applying a second pressure to the first fluid of the low pressure recipient, the second pressure being lower than the first pressure; opening the valve between the opening chamber and the low pressure recipient such that a second fluid from the opening chamber moves into the low pressure recipient and compresses the first fluid; and generating the force by producing a pressure imbalance on the piston. 
     According to yet another exemplary embodiment, there is a blowout preventer activation device. The device includes a low pressure recipient configured to contain a volume of a first fluid at a low pressure volume; an inlet connected to the low pressure recipient and configured to exchange a second fluid with an external enclosure; a valve connected to the external enclosure and the inlet and configured to separate a pressure source in the external enclosure from the low pressure recipient; and at least one of a ram preventer including connected to a piston of the external enclosure and configured to receive the force and close rams to shear a pipe between the rams, and an annular blowout preventer connected to a piston of the external enclosure and configured to receive the force to seal a wellbore. When the valve is open, such that there is a flow communication between the external enclosure and the low pressure recipient, a pressure imbalance occurs in the external enclosure which generates the force and the second fluid from the external enclosure enters the low pressure recipient and compresses the first fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a schematic diagram of a conventional offshore rig; 
         FIG. 2  is a schematic diagram of a water submerged device for generating a force based on an accumulator; 
         FIG. 3  is a schematic diagram of a canister for producing supplemental pressure; 
         FIG. 4  is a schematic diagram of a water submerged device for generating a force without an accumulator according to an exemplary embodiment; 
         FIG. 5  is a graph illustrating a dependence of a pressure relative to a volume of a fluid inside the submerged device according to an exemplary embodiment; 
         FIG. 6  is a schematic diagram of a water submerged device illustrating various pressures acting on the device; 
         FIG. 7  is a schematic diagram of a water submerged device for generating a force based on an accumulator according to an exemplary embodiment; 
         FIG. 8  is a graph illustrating various pressure dependences with volume according to exemplary embodiments; 
         FIG. 9  is a schematic diagram of a water submerged device for generating a force according to an exemplary embodiment; 
         FIG. 10  is a schematic diagram of a water submerged device for generating a force according to another exemplary embodiment; 
         FIGS. 11A  and B are schematic diagrams of a valve connecting the BOP to the water submerged device for generating the force; and 
         FIG. 12  is a flow chart illustrating steps performed by a method for generating a force according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of BOP systems. However, the embodiments to be discussed next are not limited to these systems, but may be applied to other systems that require the supply of force when the ambient pressure is high such as in a subsea environment. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     As discussed above with regard to  FIG. 2 , the accumulator  30  is bulky because of the low efficiency of nitrogen at high pressures. As the offshore fields are located deeper and deeper (in the sense that the distance from the sea surface to the seabed is becoming larger and larger), the nitrogen based accumulators become less efficient given the fact that the difference between the initial charge pressure to the local hydrostatic pressure decreases for a given initial charge of chamber  52 , thus, requiring the size of the accumulators to increase (it is necessary to use 16 320-L bottles), and increasing the price to deploy and maintain the accumulators. 
     According to an exemplary embodiment, a novel arrangement, as shown in  FIG. 4 , may be used to generate the force F.  FIG. 4  shows an enclosure  36  that includes a piston  38  capable of moving inside the enclosure  36 . The piston  38  divides the enclosure  36  into a chamber  40 , defined by the cylinder  36  and the piston  38 . Chamber  40  is called the closing chamber. Enclosure  36  also includes an opening chamber  42  as shown in  FIG. 4 . 
     The pressure in both chambers  40  and  42  may be the same, i.e., the sea pressure (ambient pressure). The ambient pressure in both chambers  40  and  42  may be achieved by allowing the sea water to freely enter these chambers. Thus, as there is no pressure difference on either side of the piston  38 , the piston  38  is at rest. 
     When a force is necessary to be supplied for activating a piece of equipment, the rod  44  associated with the piston  38  has to be moved. This may be achieved by generating a pressure imbalance on two sides of the piston  38 . 
     Although the exemplary embodiment, which is shown in  FIG. 4 , describes how to generate the undersea force without the use of the accumulators, however, as will be discussed later, according to another exemplary embodiment, the accumulators still may be used to supply the supplemental pressure.  FIG. 4  shows the enclosure  36  (which may be a cylinder) that includes the piston  38  and a rod  44  connected to piston  38 . The opening chamber  42  may be connected to a low pressure storage recipient  60 . A valve  62  may be inserted between the opening chamber  42  and the low pressure recipient  60  to control the pressures between the opening chamber and the recipient  60 . The low pressure recipient  60  may include a piston  61  that is placed in the low pressure recipient  60  to slide inside the low pressure recipient  60  to divide a compressible fluid, inside the low pressure recipient  60 , from the enclosure  36 . The low pressure recipient  60  may include a bladder or a sealing element instead of the piston  61 . The compressible fluid (first fluid) may be, for example, air. 
     The low pressure storage recipient  60  may have any shape and may be made of steel, or any material that is capable of withstanding seawater pressures. However, the initial pressure inside the low pressure recipient is about 1 atm or lower to improve the efficiency, when the recipient is at the sea level. After the recipient is lowered to the sea bed, the pressure inside the recipient may become higher as the sea level exerts a high pressure on the walls of the recipient, thus compressing the gas inside. Other fluids than air may be used to fill the low pressure recipient. However, the pressure inside the recipient  60  is smaller than the ambient pressure P amb , which is approximately 350 atm at 4000 m depth. 
     As shown in  FIG. 4 , when there is no need to supply the force, the pressure in both the closing and opening chambers is P amb  while the pressure inside the recipient  60  is approximately P r =1 atm. When a force applied to the rod  44  is required for actuation of a piece of equipment in the rig, the valve  62  opens such that the opening chamber  42  may communicate with the low pressure storage recipient  60 . The following pressure changes take place in the closing chamber  40 , the opening chamber  42  and the recipient  60 . The closing chamber  40  remains at the ambient pressure as more seawater enters via pipe  64  to the closing chamber  40  as the piston  38  starts moving from left to right in  FIG. 4 . The pressure in the opening chamber  42  decreases as the low pressure P r  becomes available via the valve  42 , i.e., seawater (second fluid, which may be incompressible) from the opening chamber  42  moves to the recipient  60  to equalize the pressures between the opening chamber  42  and the recipient  60 . Thus, a pressure imbalance is achieved between the closing chamber  40  and the opening chamber  42  and this pressure imbalance triggers the movement of the piston  38 . 
       FIG. 5  shows a graph of the pressure versus volume for the closing chamber  40  and the recipient  60 . The pressure of the closing chamber  40  remains substantially constant (see curve A) while the volume of the closing chamber  40  expands from a small initial volume, V 1 , to a larger final volume, V 2 , while the pressure in the recipient  60  slightly increasing from approximately 1 atm due to the liquid received from the opening chamber  42 , as shown by curve B. 
     Thus, according to an exemplary embodiment, a large force F is achieved without using any canister charged with nitrogen at high pressure. Therefore, the system shown in  FIG. 4  advantageously provides a reduced cost solution to generating a force as the low pressure recipient  60  is filed with, for example, air at sea level surface. In addition, the device for generating the force may have a small size as the size of the low pressure recipient is smaller compared to the existing accumulators. In one exemplary embodiment, the low pressure recipient may be a stainless steel container having a 250 l volume. Another advantage of the device shown in  FIG. 4  is the possibility to easily retrofit the existing deep sea rigs with such a device. 
     According to an exemplary embodiment shown in  FIG. 6 , a numerical example is provided for appreciating the effectiveness of the low pressure recipient  60 . The example shown in  FIG. 6  is not intended to limit the exemplary embodiments but only to offer to the reader a better understanding of the force generated by the low pressure recipient  60 .  FIG. 6  shows the enclosure  36  including the piston  38  with the various pressures acting on it. More specifically, the pressure in the closing chamber  40  is P AMB , the pressure in the opening chamber is P ATM , when the opening chamber  42  communicates with the low pressure recipient  60 , and the pressure acting on rod  44  is P MUD , which is the column pressure or wellbore pressure depending on the application. The net force F NET , which is calculated in this example, is constant along the entire stroke of the piston. This is different from conventional devices in which the force decreases as the piston in the accumulator moves due to the lost pressure as the nitrogen gas expands. Preferably, a constant pressure would ensure enough pressure/force to cut the drill pipe when needed. 
     Assuming that P AMB  is 4,500 psi, P ATM  is 14.5 psi, P MUD  is 15,000 psi, D 1  is 22 in, and D 2  is 5,825 in, the net force F NET  is given by:
 
 F   NET   =P   AMB (π/4)( D 1) 2   −P   ATM (π/4)[( D 1) 2 −( D 2) 2   ]−P   MUD (π/4)( D 2) 2 =1,298,850 lbf.
 
Assuming that P ATM  is 4,500 psi, the net opening force F NET  is −284,639 lbf. According to an exemplary embodiment, the ambient pressure (high pressure) may be between 200 and 400 atm and the P ATM  (low pressure) may be between 0.5 and 10 atm.
 
     According to another exemplary embodiment, the low pressure recipient  60  may be used in conjunction with nitrogen based accumulators as shown in  FIG. 7 . The closing chamber  40  of the enclosure  36  is connected not only to the seawater via pipe  64  but also to the accumulator  30  that is capable of supplying supplemental pressure. When appropriate conditions are reached, a valve  66  may close the sea water supply to the closing chamber  40  and valve  46  may open to allow the supplemental pressure from the accumulator  30  to reach the closing chamber  40 . According to an exemplary embodiment, the hydraulic liquid from accumulator  30  mixes with the seawater from the closing chamber  40 . According to another exemplary embodiment, another piston (not shown) separates the hydraulic liquid of accumulator  30  from the seawater inside the closing chamber  40 . Optionally, the valve  66  opens when the pressure in the accumulator  30  becomes less than a preset threshold. The variation of pressure as a function of volume for the accumulator  30  is illustrated by shape C in  FIG. 8 . Thus, the supplemental pressure (curve C) decreases as the piston  38  moves, producing a diminishing supplemental force on the rod  44 . The profile of curve C is given by an appropriate equation of state for the particular gas used in the accumulator  30 , depending on whether the temperature or heat transfer is considered to be constant or negligible, i.e., whether the change of state for the gas is isothermal or adiabatic, respectively. 
     However, as one of ordinary skill in the art knows, the product of pressure and volume of an ideal gas is proportional to the gas temperature, as illustrated by curve C in  FIG. 8 . Thus, in a conventional accumulator, when the pressure of the canisters is released to a specific device, the pressure decreases as the volume increases. On the contrary, the pressure in the closing chamber  40  does not change inversely proportional with the increase of volume of this chamber as shown by curve A in  FIG. 5 , i.e., the pressure stays substantially constant when the volume of the closing chamber  40  increases. 
     However, when the supplemental pressure from accumulator  30  is combined with the low pressure of the low pressure recipient  60 , the pressure exerted on the piston  38  from the closing chamber  40  has the profile shown by curve D in  FIG. 8 , i.e., a high pressure that slightly decreases with the movement of the piston  38 . According to an exemplary embodiment, the pressure from accumulator  30 , P AC , may be released after the low pressure storage recipient  60  becomes activated, thus producing the pressure profile shown by curve E in  FIG. 8 . It is noted that according to this profile, the pressure in the closing chamber is P amb  after valve  62  has been opened and increases to P amb +P AC  when the supplemental pressure from the accumulator  30  is made available. 
     The spike in pressure shown in  FIG. 8  in profile E may be advantageous as discussed next. Returning to  FIG. 1 , the BOP is shown to include two elements  26  and  28 . Element  28  may be an annular blowout preventer while element  26  may be a ram blowout preventer. The annular blowout preventer  28  is a valve, that may be installed above the ram preventer  26  to seal the annular space between the pipe and the wellbore or, if no pipe is present, the wellbore itself. The annular blowout preventer does not cut (shear) the lines or pipes present in the wellbore but only seals the well. However, if the annular blowout preventer fails to seal the wellbore or is not enough, the ram preventer may be activated. 
     The ram preventer may use rams to seal off pressure on a hole that is with or without pipe. If the hole includes a pipe, the ram preventer needs enough force to shear (cut) the pipe and any cords that might be next or inside the pipe such that the well is completely closed, to prevent a pressure release to the atmosphere. 
     Thus, the force providing devices discussed in the exemplary embodiments may be used to provide the necessary force to the annular blowout preventer, the ram preventer, both of them, etc. Other applications of the force providing exemplary embodiments may be envisioned by one skilled in the art, such for example, applying the force to any subsea valve on the BOP stack or production trees. 
     Various valves and pilots may be added between each chamber and the low pressure recipient  60  and/or accumulator  30  as will be appreciated by those skilled in the art. Two exemplary diagrams showing the implementation of the low pressure recipient  60  are shown in  FIGS. 9 and 10 . However, these examples are intended to facilitate the understanding of the reader and not to limit the exemplary embodiments.  FIG. 9  shows the cylinder  36  connected to the pipe  64  and the low pressure recipient  60  via the valve  62 . Valve  62  is connected to a plunger valve  68  that is connected to a pilot accumulator  70 . The pilot accumulator  70  may be, for example, a 2.5-L recipient. The pilot accumulator  70  may be connected, via a coupler  72  to an autoshear valve pilot  74  and an autoshear arm pilot  76 . A port I is provided to connect line  64  to seawater and a port II is connected to coupler  72  and to an auto-shear disarm pilot. In another exemplary embodiment shown in  FIG. 10 , the plunger valve  68  is substituted with a valve that is connected to the valve pilot  74 . 
     Valve  62  is discussed in more details with regard to  FIGS. 11A  and B.  FIG. 11A  shows the enclosure  36  connected to the low pressure recipient  60  via a a shuttle valve  67  and the valve  62 . The shuttle valve  67  may be a spring bias type to prevent seawater ingress and to maintain the correct position to vent. Valve  62  (which is produced by Hydril, Houston, Tex., US) may be a 3-way 2-position valve that is spring loaded to maintain its position. As shown in  FIG. 11A , the opening chamber  42  is connected to a vent port  62   a  in the valve  62  that is always open to seawater. However, the port  62   b  of valve  62 , which is connected to the low pressure recipient  60 , is blocked to maintain the low pressure in the low pressure recipient  60 . When functioned by an external pilot (not shown), an internal spool of the valve moves compressing spring  62   c , blocking the vent port  62   a , and opening the opening chamber  42  to the low pressure recipient  60 . After valve  62  is piloted by the external pilot it looks as shown in  FIG. 11B , in which a free communication is allowed between the opening chamber  42  and the low pressure recipient  60 . Element  62   e  shown in  FIG. 11A  blocks the vent port  62   a  in  FIG. 10B . 
     According to an exemplary embodiment, illustrated in  FIG. 12 , there is a method for generating a force by moving a piston inside an external enclosure of a water submerged device, the piston dividing the external enclosure into a closing chamber and an opening chamber and the opening chamber communicating with a low pressure recipient via a pipe having a valve, the valve separating a pressure source in the opening chamber from the low pressure recipient, and the low pressure recipient containing a volume of a first fluid. The method includes a step  1200  of applying a first pressure to the closing and opening chambers, wherein the first pressure is generated by a weight of the water at a certain depth of the device, a step  1210  of applying a second pressure to the first fluid of the low pressure recipient, the second pressure being lower than the first pressure, a step  1220  of opening the valve between the opening chamber and the low pressure recipient such that a second fluid from the opening chamber moves into the low pressure recipient and compresses the first fluid, and a step  1230  of generating the force by producing a pressure imbalance on the piston. 
     According to an exemplary embodiment, one or more pressure sensors may be inserted into the low pressure recipient  60  to monitor its pressure. When the pressure sensor determines that the pressure inside the recipient  60  is far from 1 atm, the operator of the rig is informed of this fact such that the operator may rely on other force generator for closing the ram preventer in case of an emergency or for replacing the recipient  60 . Alternatively, the recipient  60  may be provided with a hydraulic equipment (not shown) which starts pumping the water out of the recipient when the sensor senses that the pressure inside the recipient is above a certain threshold. In another exemplary embodiment, the hydraulic equipment may pump out the water from the recipient  60  after the valve  62  has been opened and the ram preventer has closed. It is noted that after the recipient  60  is filled with water it cannot be used to generate the force unless the low pressure is reestablished inside the recipient  60 . 
     According to another exemplary embodiment, more than one recipient  60  may be used either simultaneously or sequentially, or a combination thereof. Further, at least one recipient  60  may be connected to a device that empty the recipient  60  of the seawater after the valve  62  has been opened and the seawater entered the recipient. Thus, according to this embodiment, the recipient  60  may be reused multiple times. 
     According to another exemplary embodiment, the pressure difference between (i) the sea water pressure at 2000 to 4000 m in the closing chamber and (ii) the atmospheric pressure inside the recipient  60  generates an appropriate force for closing the ram preventer. However, if the seabed is deeper than 4000 m from the sea level, adapters (for example, pressure reducing valves) may be used to reduce the pressure difference such that the ram preventer is not damaged by the excessive pressure difference. On the contrary, if the sea bed lies at less than 2000 m from the sea surface, the pressure difference might not be enough to create enough force to close the ram preventer. Thus, according to an exemplary embodiment, accumulators may be used to supplement the hydrostatic pressure. However, even if no accumulators are used, the force may be generated as long as there is a pressure difference between the opening chamber and the low pressure storage recipient. 
     The disclosed exemplary embodiments provide a system and a method for generating a force undersea with a reduced consumption of energy and at a low cost. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other example are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.