Patent Publication Number: US-6907933-B2

Title: Sub-sea blow case compressor

Description:
TECHNICAL FIELD 
   The present invention relates to compressor apparatus for gas production from underwater wells. In particular the invention relates to a compressor for gas production from an underwater well. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a submersible compressing station for a well producing gas and oil. The invention has the greatest applicability to offshore oil and gas production, although it may be employed in lakes and bays as well. In such production, wells are drilled from a platform or a semi-submersible vessel, or a drill ship, etc. on the surface of the water into the subsea formations. The well bore is drilled into a petroleum producing formation and the well is completed, i.e. put in condition for producing gas and oil. Many times oil present in a hydrocarbon reservoir contains dissolved gas and the capability of the oil to hold such gas decreases as the pressure decreases and temperature increases. 
   Once a well is placed in production the raw material flowing from the well may be transported to the surface through a tubing string or riser, or may be transported to the shore through a sub-sea pipeline. Frequently a liquid/gas separator is employed to separate the gas from the oil and water which can be produced by the well. It is often desirable to operate oil and gas production separators at low pressures to improve the well productivity and recovery. When the pressure of the separated gas from the liquid/gas separator is too low to flow to its destination, a gas compressor is usually employed to boost its pressure. 
   Sub-sea production separators, i.e. separators located on the sea bed, have been used. When sub-sea separators are utilized then the gas compressor must also be located on the sea bed. The disadvantages of mechanical gas compressors include that they often require more power than is practical to supply sub-sea, they have a complex construction, and they are complex to operate and difficult to maintain. 
   Thus, there has been a need for a reliable sub-sea gas compressor having a robust construction which is simple to maintain and operate. The present invention has the advantages over mechanical gas compressors by utilizing a simple system which consumes less power, and is simpler to operate and maintain. 
   SUMMARY OF THE INVENTION 
   In one embodiment the invention relates to a compressor system suitable for use underwater. While the invention may be used in freshwater or seawater its greatest application will be found in offshore applications. 
   In one embodiment the present invention relates to the submersible compressing apparatus which is attached to a gas/liquid separator. The gas/liquid separator has an opening for connection to the well head, a gas exit opening and a production liquid exit. A separator gas conduit is connected to the gas exit opening of the gas/liquid separator and is connected to a gas valve. Connected to the liquid/gas separator is a first compressor tank which has first and second openings. The first opening of the compressor tank is connected to the gas valve by a production conduit and the production conduit also has a gas production valve connected to it. The gas production valve is connected to a conduit for carrying the gas to a desired location, such as to the surface or to the shore. Connected to the second opening of the compressor tank is a liquid conduit. The liquid conduit is connected to a inlet valve and a tank valve. The tank valve is connected to a evacuation conduit which is connected to a pump. In operation, raw material from the well is separated into liquid and gas phases in the liquid/gas separator. The inlet valve to the compressor tank is opened and water from the environment is allowed to flood the tank. The inlet valve is then closed and the tank valve opened. The pump is started and water is pumped from the compressor tank. As a result the pressure in the compressor tank is decreased and the gas valve is opened allowing gas to flow from the liquid/gas separator into the compressor tank. Once the desired amount of gas has flowed into the compressor tank the tank valve is closed and the pump stopped. Thereafter, the gas valve to the liquid/gas separator is closed and the gas production valve is opened. Then the inlet valve to the compressor tank is opened allowing water to again fill the tank. The hydrostatic head of the water surrounding the compressing apparatus in the environment provides pressure to compress and push the gas out of the first compressor tank. When a desired amount of water has entered the compressor tank the inlet valve is closed and the process is repeated. 
   In a preferred embodiment, the present invention relates to a submersible compressing apparatus which contains two or more compressor tanks and preferably more than two compressor tanks. Use of at least two compressor tanks is preferred because production into the compressing apparatus can be more continuous than in the single compressor tank configuration which operates in an interruptible fashion. In a preferred embodiment, there is a first compressor tank which has a first and second opening. A first production conduit is attached to the first opening of the first compressor tank. Also connected to the first production conduit is a first production gas valve for connection to a riser, and a first gas valve for connection to a liquid/gas separator. Connected to the second opening is the first liquid conduit which has connected to it a first inlet valve and a first tank valve. Connected to the first tank valve is an evacuation conduit. The second tank and any additional compressor tanks have a similar construction. The evacuation conduit which is attached to the tank valve of the first compressor tank and to the tank valve of the second compressor tank is connected to a pump. This compressing apparatus operates in a fashion similar to the above described methodology. However, in this embodiment as one compressor tank is being flooded with water to compress the gas for transport to the surface the other compressor tank(s) are having water evacuated from it in order to draw in gas from the liquid/gas separator. The rate at which a tank is flooded and the rate at which water is pumped from it are proportioned preferably such that a nearly continuous intake of gas to the compressor apparatus can be achieved. More than two compressor tanks can be employed in the apparatus if desired. These additional compressor tanks can be utilized to enhance continuous flow or can be employed as reserve units in the event one of the primary tanks fails. 
   The compressing apparatus may be operated in an open circuit mode in which ambient water is allowed to flow into the compressor tanks and is then pumped out of the compressor tanks into the sea. Alternatively, return conduits and valves can be provided such that there is a closed system in which water or other incompressible liquid is pumped from one of the compressor tanks to the other compressor tank so as to provide a closed system in which the fluid is repeatedly transferred from one tank to the other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood with reference to the figures in conjunction with the detailed description of preferred embodiments. 
       FIG. 1  is a schematic view of a pumping apparatus with a single compressor tank; 
       FIG. 2  is a schematic view of a pumping apparatus having two compressor tanks in a first cycle of operation; 
       FIG. 3  is a schematic view of the pumping apparatus of  FIG. 2  having two compressor tanks in a second cycle of operation; 
       FIG. 4  is a schematic view of another pumping apparatus having two compressor tanks and return lines for the water in a first cycle of operation; 
       FIG. 5  is a schematic view of the pumping apparatus of  FIG. 4  having two compressor tanks and return lines for the water in a second cycle of operation; 
       FIG. 6  is a schematic view of another embodiment of the invention utilize multiply compressor tanks; 
       FIG. 7  is a schematic view of another embodiment of the invention; 
       FIG. 8A  is a schematic view of a single compressor tank and a table showing the single acting compressing cycle; and 
       FIG. 8B  is a schematic view of two compressor tanks and a table showing the double acting compressing cycle. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates one embodiment of a submersible compressing apparatus of the present invention. It is believed that the invention will have the greatest application to offshore production, and thus, the preferred embodiments will be discussed in relation to that environment. It being understood that the invention can also be employed in other water environments. A well bore  10  has been drilled through the seabed  11  into an offshore petroleum reservoir  12  and has a well head  14  on the sea bed  16 . At the surface  18  of the sea is platform  20 . The well head  14  is connected to a liquid/gas separation system  5 , enclosed by the dashed box. The primary component of the liquid/gas separation system  5  is the liquid/gas separator  22 . The liquid/gas separator  22  is connected to the well head  14  by well head conduit  24 . Interposed in well head conduit  24  is well head valve  26  which controls flow of the raw material produced by the well into liquid/gas separator  22 . Liquid/gas separator  22  has a raw material opening  28  connected to the well head conduit  24  and has a gas opening  30  and a production liquid opening  32 . A gas conduit  34  is connected to the gas opening  30  at one end and at the other end to a first gas valve  36 . 
   The compressing apparatus includes a first compressor tank  38  which has a first opening  40  and a second opening  42 . First opening  40  is connected to production conduit  44 . Production conduit is connected to first gas valve  36  opposite the gas conduit  34 . A first production gas valve  46  is also connected to the first production gas conduit  44 . Attached to the second opening  42  is first liquid conduit  48 . First liquid conduit  48  is attached to a first inlet valve  50  and a first tank valve  52 . Attached to the first tank valve  52  opposite the first liquid conduit  48  is evacuation conduit  54 . The other end of evacuation conduit  54  is attached to pumps  56 . Attached to this first inlet valve  50  opposite the first liquid conduit  48  is inlet conduit  58  that is open to the ambient sea. 
   In operation of this compressing apparatus raw material is feed to liquid gas separator  22  and is separated into gas and liquid phases. As a starting configuration for discussion it will be assumed that compressor tank  38  is empty. The valves on all the conduits to the first compressor tank  38  are closed. Then the first inlet valve  50 , that has an inlet  58  which is open to the sea, is opened. When valve  50  is opened, seawater flows into first compressor tank  38  and is allowed to fill first compressor tank  38  to a desired level. At that point valve  50  is closed. First tank valve  52  is then opened and pumps  56  started and water is pumped from the first compressor tank  38 . First gas production valve  46  remains closed and first gas valve  36  is opened. As water is pumped from tank  38 , gas is drawn from liquid/gas separator  22  into first compressor tank  38 . When the desired amount of liquid has been withdrawn from compressor tank  38 , valve  52  is closed and pump  56  is stopped. Gas valve  36  is then closed and first gas production valve  46  is opened. Thereafter, first inlet valve  50  is opened allowing seawater to again flow in and fill compressor tank  38 . The hydrostatic head of the water is used to compress the gas in first compressor tank  38  and cause it to flow through production conduit  44  and first gas production valve  46  into riser  49  then to the surface. 
   While  FIG. 1  has shown the present invention with the gas being transported to the surface of the sea, the production gas can also be compressed to any desired location, such as a sub-sea pipeline and transported to a shore facility through the pipeline, or exhausted into the sea. 
   Production liquid from the gas/liquid separator  22  flows through production liquid conduit  60  and liquid valve  62  and is pumped by pump  64  to the surface or other desired location through conduit  66 . 
   In the figures, like reference numbers refer to the same or similar items. 
     FIG. 2  illustrates a preferred embodiment of the present invention. In  FIG. 2  the dashed box shows submersible compressor  70  of the present invention. Submersible compressor  70  is comprised of two tank units. A first compressor tank unit  38  is provided with related conduits and valves as discussed with reference to FIG.  1 . In this embodiment a second compressor tank  72  is provided which has a first opening  74  and a second opening  76 . Connected to the first opening  74  of the second compressor tank  72  is a second production gas conduit  78 . Attached to the second production conduit  78  is second gas valve  80  and second gas production valve  82 . Second gas valve  80  on the side opposite of the second production conduit  78  is connected to gas conduit  34 . The side of the second gas production valve  82  is connected to riser  49 . The second opening  76  is connected to second liquid conduit  84 . A second inlet valve  86  is connected to the second liquid conduit  84 . Also connected to the second liquid conduit is second tank valve  88 . Connected on the opposite side of the second tank valve  88  is first evacuation conduit  54 . The evacuation conduit  54  is attached to pump  56 .  FIG. 2  illustrates the phase of the compressing operation in which water is being drawn out of the first tank  38  thereby creating a low pressure area which draws gas into the first compressor tank  38 . At the same time inlet valve  86  is open and water is flowing into second compressor tank  72  forcing gas out of the second tank  72  through production gas conduit  78  and the second gas production valve  82  and into riser  49 . 
     FIG. 3  shows the valve configurations for compressor  70  in the second cycle where gas is being pulled into the second tank  72  and compressed and expelled from tank  38 . As can be seen in this cycle first tank valve  52 , first gas valve  36 , second inlet valve  86  and second production gas valve  82  are closed, while first inlet valve  50 , first production gas valve  46 , second tank valve  88  and second gas valve  80  are open. 
     FIG. 4  shows another preferred embodiment of the present invention, a closed circuit compressor  90  indicated by the dashed box. Many of the components of the closed circuit compressor and the open circuit compressor  70  illustrated in  FIGS. 2 and 3  are the same with the exception of the piping and valve arrangement for controlling water or liquid flow. In this embodiment, the closed circuit compressor has a first inlet valve  50  which is connected to the evacuation conduit  54 . The first liquid conduit  48  is connected to first tank valve  52  and first return valve  94 . The second liquid conduit  84  is connected to second tank valve  88  and second return valve  96 . The return conduit  92  is connected to first return valve  94  and second return valve  96  and to the output end of pump  56 . This construction allows water to be pumped into either compressor tank. In a preferred embodiment the return conduit has an exhaust valve  98  connected to it which can be opened to pump water/liquid into the sea or reclaimed through the liquid/gas separation system  5 , more fully described in FIG.  1 . 
   If the compressor  90  is placed in position with both tanks empty, then by opening first inlet valve  50  one of the tanks may be filled with water to the desired level. This may be done in two manners. For example, both the first and second return valves  94  and  96  are closed and either the first the second tank valves  52  or  88  is closed. The tank valve which is not closed is opened so that the tank connected to the open inlet valve  50  will be filled. The second manner of making the initial charge of water is to close both the first and second tank valves  52  and  88  and either one of the first or second return valves  94  and  96 , the other return valve is opened. When first inlet valve  50  is opened seawater can be allowed into the evacuation conduit and through pump  56  and the open return valve and may fill either first tank  38  or second tank  72  depending upon which return valve  94  or  96  is open. Alternatively, either valves  52  or  88  can be opened allowing water to flow into selected tanks under the force of the hydrostatic head. Once one of the tanks  38  or  72  is filled with the desired amount of water, first inlet valve  50  is closed. Now that one of the tanks is filled the compressing mode is achieved by repeatedly transferring water from one compressor tank to the other compressor tank. In the illustrated phase of the compressing in  FIG. 4 , the first compressor tank  38  was previously filled with water and is now being evacuated in order to draw gas from the liquid/gas separator  22  into the first compressor tank  38  in a manner similar to that described with reference to  FIGS. 2 and 3 . This is done by opening valve  52  and starting pump  56 . In contrast to the open system of  FIGS. 2 and 3 , pump  56  pumps the water into return conduit  92 . Return conduit  92  has attached to it a first return valve  94  which is connected to the first liquid conduit  48  and has a second return valve  96  connected to second liquid conduit  84 . In the illustration, the first return valve  94  is closed preventing the water from flowing back into the first compressor tank  38 . Second return valve  96  is open and the water is pumped into the second compressor tank  72 , thereby expelling the gas from second compressor tank  72  through open second production gas valve  82  and into the gas riser  49 . 
   In the second cycle water is pumped from second compressor tank  72  into first compressor tank  38  as illustrated in FIG.  5 . In  FIG. 5 , first inlet valve  50 , first tank valve  52 , first gas valve  36 , second return valve  96  and second production gas valve  82  are closed. First gas production valve  46 , first return valve  94 , second gas valve  80 , and second tank valve  88  are open allowing pump  56  to pump water from the second compressor tank  72  into first compressor tank  38 . 
   As illustrated in  FIG. 4 , the compressor  90  can include an exhaust valve  98  in the return conduit  92 . In this manner both compressor tanks can be completely or partially filled with water at the surface prior to being submerged. After submersion and installation, water from one of the compressor tanks is pumped into the sea through the exhaust valve in the initial start-up operation. Thereafter, the exhaust valve is closed and the water is transferred from the remaining tank into the first tank in a cyclic function to achieve the compression as described above. This has the advantage that the compressor tanks can be preloaded on the surface with deaerated water or fresh water containing corrosion inhibitors which would be less corrosive to the compressor than utilizing seawater. In this embodiment it is also beneficial to have the inlet valve  50 . This will allow use of seawater in the event that all of the freshwater is inadvertently exhausted into the sea or to make up for loss through evaporation. In another alternative, a compressor tank could be preloaded with an incompressible liquid other than water such as hydraulic fluid. 
   Further, in the closed circuit embodiment neither the first inlet valve  50  nor the exhaust valve  98  is required. One of the compressor tanks can be filled with water or other incompressible liquid at the surface. Thereafter, the compressor  90  can be submerged and installed. Liquid can then be pumped from one tank to the other. This embodiment is considered less desirable as it limits the ability to take corrective action or make repairs without retrieving the compressor to the surface. 
   Another potential function of exhaust valve  98  is to facilitate the reclaiming of any condensate that may be produced in the compressor system. Over time, heavier hydrocarbons or other constituents in the inlet gas can condense in the compressor tanks  38  and  72 . It may then be desirable to reclaim the condensate by routing the fluid through pump  56  and exhaust valve  98 , and a conduit not shown, to the liquid/gas separation system  5 . Once the reclaimed condensate is in the liquid/gas separation system  5 , it can be commingled with the production liquid from the well and transported to the surface or other desired location through conduit  66 . 
     FIG. 6  illustrates a compressor  100  which is an open circuit compressor. Compressor  100  differs from compressor  70  shown in  FIGS. 2 and 3  in that it contains a third compressor tank  102  (n+2) having a first opening  104  and a second opening  106 . Connected to the first opening  104  of the third compressor tank  102  is production gas conduit  108 . Production gas conduit is connected to third gas valve  110  and a third gas production valve  112 . The second opening  106  of the third compressor tank  102  is connected to third tank conduit  114  which is connected to third tank valve  116  and third inlet valve  118 . The other side of the tank valve  116  is connected to evacuation conduit  54 . This embodiment can be useful to provide a unit which has a spare compressor tank so that if one of the compressor tanks springs a leak it can be closed off and compressing continued with the other two. Alternatively, all three tanks can be operated in a three-phase cycle. 
   The compressor of the invention can have any number of additional compressor tanks (n, n+1, n+2, n+3, etc.), each having a similar arrangement of conduits as explained above. Use of multiple tanks can be beneficial in that the sequencing of the tanks can be timed such that the fluid pump  56  runs continuously, and to smooth out the pressure and gas flow from the liquid/gas separation system  5 , and into the gas export user  49 . 
     FIG. 7  shows yet another embodiment of the present invention. In this embodiment separate openings for inflow and outflow from each of the compressor tanks are provided. One opening for a gas inlet, another opening for gas exit, an opening for water inlet, and an opening for water outflow. This embodiment is considered less preferable because of the additional openings in the tank and the additional piping. This embodiment has a first compressor tank  130  with a gas inlet opening  132  and a gas exit opening  134 , a liquid inlet  136  and a liquid exhaust opening  138 . Connected to gas inlet opening  132  is first gas inlet conduit  140 , connected to the gas exit opening  134  is a first production gas conduit  142 . A first liquid inlet conduit  144  is connected to the first liquid inlet  136 , and a first liquid exit conduit  146  is connected to a first liquid exhaust opening  138 . A second compressor tank  150  is provided with a gas inlet opening  152  and a gas exit opening  154 , a liquid inlet opening  158  and a liquid exhaust opening  156 . Connected to gas inlet opening  152  of the second compressor tank is second gas inlet conduit  160 , connected to the gas exit opening  154  of the second compressor tank is second production gas conduit  162 . A second liquid inlet conduit  164  is connected to the second liquid inlet  158 , and a second liquid exit conduit  166  is connected to the second liquid exhaust opening  156 . 
   Connected to the first gas inlet conduit  140  is first gas valve  170  which is connected on the other side to gas conduit  34  from the liquid/gas separator  22 . A first production gas valve  172  is connected to the first production gas conduit  142 . A first inlet valve  174  is connected to the first liquid inlet conduit  144  and a first exhaust valve  176  is connected to the first liquid exit conduit  146 . The opposite side of first exhaust valve  176  is connected to evacuation conduit  178  which is connected to pump  180 . A similar construction is used with respect to the second compressor tank  150 . Connected to the second gas inlet conduit  160  is second gas valve  190  which is connected on the other side to gas conduit  34  from the liquid/gas separator  22 . A second production gas valve  192  is connected to the second production gas conduit  162 . A second inlet valve  194  is connected to the second liquid inlet conduit  164  and a second exhaust valve  196  is connected to the second liquid exit conduit  166 . The opposite side of second exhaust valve  196  is connected to evacuation conduit  178  which is connected to pump  180 . 
   In one cycle of operation the first gas valve  170 , second production gas valve  192 , first exhaust valve  176  and second inlet valve  194  are closed, and second gas valve  190 , first production gas valve  172 , second exhaust valve  196  and first inlet valve  174  are opened. Pump  180  is started. The inflow of water through first inlet valve  174  and into the first compressor tank  130  causes gas to be compressed and expelled through the first gas production valve  172  and into riser  200 . The pump  180  withdraws water from the second compressor tank  150  through second exhaust valve  196  which causes gas to be drawn into the second compressor tank  150  from the liquid gas separator  22  through second gas valve  190 . The process is reversed in a similar fashion as described above to produce a second compressing cycle. 
   Other valving and piping arrangements may be utilized. The exact arrangement of the conduits and valves is not important. Thus, a conduit means for passage of gas into and out of the compressor tanks can be a single conduit as described in reference to  FIGS. 2 and 3 , or multiple conduits as described in reference to  FIG. 7. A  conduit means for passage of liquid into and out of the compressor tanks can be a single conduit as described in reference to  FIGS. 2 and 3 , or multiple conduits as described in reference to  FIG. 7. A  valve means to control inlet and outlet of gas from the compressor tanks can be connected to a common conduit as described in reference to  FIGS. 2 and 3 , or individual conduits as described in reference to FIG.  7 . Also a valve means for controlling the inlet and exit of liquid from the compressor tanks can be connected to a common conduit as described in reference to  FIGS. 2 and 3 , or individual conduits as described in reference to FIG.  7  and can include valve means to control the use of a return-conduit as described in reference to FIG.  6 . Finally the pump means is a pump which will either exhaust liquid to the surrounding sea or will recycle the liquid from one tank to the other. 
   In one embodiment the invention can be n compressor tank units (where n is an integer of 2 or more). Each unit has a compressor tank; with conduit means for passage of gas into and out of each of the n compressor tanks; with conduit means for passage of liquid into and out of each of the n compressor tanks, valve means to control inlet and outlet of gas from each compressor tank, valve means for controlling the inlet and exit of liquid from each compressor tank; and a pump means for exhausting liquid into the ambient surrounding or to transfer liquid from one compressor tank to another. Preferably n is 6 or less. 
   The compressor tanks of the present invention are preferably made of high strength material such as steel, titanium and stainless steel. Also it may be desirable to treat the surface of certain parts of the compressor with corrosion resistant layers. The pump to transfer water or other fluid in the compressor can be of suitable centrifugal or reciprosating design powered by an electric motor or other means. The valves may be of any suitable design and at certain valves may be check valves. 
   In operation the valves are sequenced and the water pump controlled based upon consideration of the following preferred operations: (1) during the compression process gas should not be allowed to back flow from the gas discharge piping into the separator; (2) during the intake process gas should not be allowed to back flow from the gas discharge piping into the compressor tank; (3) during the compression process water (or other liquid) should not be allowed to exit the compressor tank into the gas outlet conduit; (4) during the intake process gas should not be allowed to enter the water/liquid pump. Operations 1 and 2 can be satisfied by use of check valves, which open when the pressure across the valve in the direction of flow is positive, and closed to prevent back flow when pressure across the valve in the direction of flow is a negative. Alternatively, an actuated valve with the differential pressure instrument across the valve can be used instead of a check valve. In this situation, the sequencing system would open the valve when the pressure measured across the valve in the desired direction of flow is positive, and would close the valve when the pressure measured across the valve in the desired direction of flow is negative. The sequencing of the actuated valves can be dependent only on the differential pressure across the valve without regard to any other measurements. With regard to operations 3 and 4, they specify conditions at which the compression process and the intake process respectively should be stopped. 
   Referring now to  FIG. 8A , closing valve  50  when the water level in the compressor tank reaches a predetermined maximum amount the compression process is stopped. With no water entering the compressor tank, the water level in the compressor tank will not rise further, and water thus will not overfill the compressor tank. Closing valve  52  when the water level in the compressor tank reaches a predetermined minimum amount stops the intake process described in operation 4 above. With no water exiting the compressor tank, the water level will not decrease further and the water seal will be maintained to prevent exit of gas into the conduits to pump  56 . 
   A single acting sequence is shown in  FIG. 8A. A  single acting compressor cycle, using only compressor tank  38 , is illustrated. With a single compressor tank, the compression process is started immediately after the intake process is stopped and vice versa. Check valves can be used for the gas valves. A differential pressure instrument  200  connected at the desired minimum  202  and maximum  204  water levels in vessel compression tank  38  is used to infer the water level. The differential pressure will be at a maximum when the water level is at a maximum and the differential pressure will be at a minimum when the water level is at a minimum. Thus, when the differential pressure instruments  200  senses a minimum level, the intake process is stopped and the compression process is started, and when the differential pressure instrument  200  senses a maximum level, the compression process is stopped and the intake process is started. This is a preferred method for control. It will be appreciated by those skilled in the art that other sensing instruments other than differential pressure measuring instruments may be used to generate control signals. 
     FIG. 8   b  shows sequencing and multiple compressor tank apparatus. Each of the four processes is applied to each vessel in the multiple compressor tank compressor. Check valves or actuated valves ( 36 ,  46 ,  80 ,  82 ) can be used on the gas side of each compressor tank to satisfy the first and second operations. Closing the corresponding water valves ( 50 ,  52 ,  86 ,  88 ) to stop the compression process or to stop the intake process in each vessel will satisfy criteria 3 and 4. A preferred method of operation is to stop the intake process and start the compression process on a single tank simultaneously while starting the intake process on another vessel in an apparatus having 3 or more compressor tanks as each of the other tanks may be in different phases of compression or intake steps. It is considered best practice that starting the intake process on a compressor tank immediately on stopping the intake process on another compressor tank allows pump  56  to operate continuously and not to be stopped or started during the cycling. It is believed that continuous operation of the pump prevents or minimizes wear and tear caused by starting and stopping the pump and would increase pump life. An additional factor in a preferred method of operation is that it should be assured that the compression process has sufficiently advanced in the compressor tank for which the intake process will be started. If the level in the compressor tank performing the compression process is not increased so as to sufficiently compress the gas to open the gas outlet valve  46 , then stopping the compression process and starting the intake process will prevent compressed gas from being discharged in this compressor outlet piping. The cycle will then accomplish nothing more than to move water around. 
   To ensure the compressed gas is actually discharged, the water valves, and conduits and the pump should be sized with consideration to the water depth and other relevant factors for the environment of use such that water entering the compressor tanks from the ocean at a greater rate than the pump can pump water out of the compressor tanks. This assures that the compression process will always take less time to complete than the intake process. Thus, in multiple compressor tank configuration, it can be assured that when the intake process in one vessel is stopped, there will be another vessel for which the compression process has been stopped and is waiting to start the intake process. 
   A double cycle is illustrated in  FIG. 8B. A  differential pressure instrument  200 ,  204  is used to infer the water level in each compressor tank and water valves and the pump had been sized as described above so that the compression process requires less time than the intake process. Thus, while one vessel is performing the intake process, the other vessel is performing the compression process. The level in the compressor tank performing the compression process will reach a maximum before the level in the compressor tank performing the intake process reaches a minimum. When the level in the vessel performing the compression process reaches maximum, all the water valves for that vessel are closed and the vessel remains in a holding state with no gas or water entering or exiting until the intake process on the other vessel is stopped. When the intake process on the other vessel is stopped, the compressor tanks are switched to start the intake process on the compressor tank that was performing the compression process and vice a versa. This sequencing is illustrated in the table presented in FIG.  8 B. 
   While we have illustrated and described preferred embodiments of our invention, it is to be understood that these are capable of variations and modifications and we therefore do not wish to be limited to precise details set forth, but avail ourselves of changes and alterations as fall within the preview of the following claims.