Patent Publication Number: US-7213532-B1

Title: System and method for managing the buoyancy of an underwater vehicle

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the mechanical arts and methods that embody underwater work methods. More particularly, it relates to devices and methods for improving the productivity of a remotely operated vehicle (ROV) engaged in underwater maintenance and construction work. 
   2. Description of Related Art 
   Conventional underwater work techniques often include the use of remotely operated vehicles (ROV&#39;s). A surface support vessel and its associated personnel support and operate the ROV. The ROV may be deployed directly from the support vessel or from the surface via a tether management system (cage). When deployed directly from the surface, the ROV is connected to its control and powering components on the support vessel with an umbilical cable. When deployed from the surface in a cage, the cage and ROV are lowered to a location near the worksite on a similar umbilical cable. Thereafter, the ROV may be maneuvered from the cage to the worksite while coupled to a tether extending between the ROV and the cage. 
   Regardless of the method employed to deploy the ROV to the worksite, ROV&#39;s are designed so that they are essentially neutrally buoyant (they neither float nor sink). Therefore, addition or removal of payloads (weight) to/from the ROV requires that the ROV have either excess thrust capacity or the ability to add or remove buoyancy or ballast to compensate for the addition or removal of weight. 
   ROV operations include use at an underwater worksite to manipulate various payloads. Supporting payloads with a specific gravity (SG) greater than unity tends to make the ROV sink. Supporting payloads with a SG less than unity tends to make the ROV float. Because of this, the ROV must be able to compensate for or manage its buoyancy when on-loading or off-loading a payload. 
   A typical ROV utilizes fixed buoyant volumes such as syntactic foam or fixed air voids in combination with its vertical thruster&#39;s capacity to manage its buoyancy relative to the ROV equipment&#39;s weight or negative buoyancy. When large packages are added to the ROV, the package&#39;s buoyancy is typically compensated for via fixed buoyant volumes or ballast tanks added to the package at the surface, thereby enabling the ROV to manage the package&#39;s buoyancy. The ballast tank may be filled with gas or liquid or a combination of both. Replacing liquid with gas in the ballast tank makes the ROV rise while replacing gas with liquid tends to make the ROV sink. Typically, the gas is air and the liquid is water. 
   When on-loading a dense payload (SG&gt;1) the ROV&#39;s buoyancy may be adjusted by replacing liquid with gas (deballasting) in the ballast tank. To compensate for off-loading the dense payload, the ROV&#39;s buoyancy may be adjusted by replacing gas with liquid (ballasting) in the ballast tank. Conversely, to compensate for on-loading a scant payload (SG&lt;1), the ROV&#39;s buoyancy may be adjusted by replacing gas with liquid. The ROV&#39;s buoyancy may be adjusted to compensate for offloading the scant payload by replacing liquid with gas. 
   The ROV consumes compressed gas from an integral (onboard) gas storage system each time it performs the deballasting operation. When the integral gas storage supply is depleted, it must be replenished. The ROV must return to the surface for gas replenishment. A remote operator maneuvers the vehicle back to the surface, either directly or via the cage, where surface vessel resources replenish its integral gas storage system. Redeployment of the ROV is in either case accomplished by reversing the recovery operations. 
   ROV productivity is significantly reduced when it is employed to repetitively move payloads from one location to another. Repeated on-loading and off-loading of payloads requires repeated gas recharge operations which deplete the ROV&#39;s integral gas storage supply. The ROV is therefore required to make frequent trips to the surface to replenish this supply. Such trips to the surface consume time and are inefficient, regardless of how the ROV is deployed. 
   Accordingly, there has existed a need for improved ROV buoyancy control systems. There is a still further need for improved ROY work methods. The present invention satisfies these and other needs, and provides further related advantages. 
   SUMMARY OF THE INVENTION 
   According to the invention, a system and method is provided for managing the buoyancy of a ROV working at an underwater site. The ROV includes an integral ballast tank, a fluid connector in fluid communication with said ballast tank, and optionally an integral pressurized gas storage tank in fluid communication with the fluid connector. A second mating fluid connector is located proximate to the underwater site. One or more pressurized gas storage tanks are in fluid communication with said second connector. The gas storage tanks are separate from said ROV. Interconnection of the first and second mating fluid connectors provides for gas transfer to the ROV. The gas transfer may refill said integral gas storage tank, recharge the ballast tank, or both. 
   The ROV is adapted to engage and disengage payloads. Neutral buoyancy of the ROV is restored in conjunction with following a payload on-load or off-load by adjusting the ROV ballast. Gas consumed by the ROV during these buoyancy adjustments is supplied/replenished by the gas transfer operations. 
   An underwater workstation may be located proximate to the underwater site. The payload(s), second fluid connector, and one or more of the gas storage tanks may be mounted on the workstation. While adjacent to the payload at a first location, the ROV may exchange a payload, perform gas transfer, and adjust buoyancy as needed. Subsequently the ROV may transport the payload to a second location where payload exchange and buoyancy adjustment activities may be repeated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described with reference to the accompanying figures. In the figures, like reference numbers indicate identical or functionally similar elements. The accompanying figures, which are incorporated herein and form a part of this description, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and enable a person skilled in the relevant art to make and use the invention. 
       FIG. 1  is an isometric view of a ROV spread complete with a cage deployed at a typical worksite. 
       FIG. 2  is an isometric view of a ROV spread excluding a cage deployed at a typical worksite. 
       FIG. 3  is a schematic view of the gas supply and refill components located on the ROV and workstation. 
       FIG. 4  is a schematic view of the gas supply, refill, and liquid transfer components located on the ROV. 
       FIG. 5  is a schematic view of a ROV and a worksite. 
       FIG. 6  is a tabular description of a ROV project. 
       FIG. 7  is a schematic showing operations that comprise the ROV work task. 
       FIG. 8  is a flowchart showing steps that comprise the payload transfer activity. 
       FIG. 9  is a flowchart showing steps that comprise the gas transfer activity. 
       FIG. 10  is a flowchart showing steps that comprise the buoyancy adjustment activity. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Introduction 
   The present invention provides time saving work methods and systems applicable to the operation of a ROV. ROV systems operated according to the present invention have specific features and advantages, including, but not limited to, increased productivity and reduced operating risk. These features and advantages are especially evident when the ROV is repetitively moving payloads from one location to another. 
   As noted above, a ROV may advantageously employ the present invention to support or to carry out underwater work, including maintenance, repair, and construction work. The system and methods described enable a ROV to replenish its gas supply proximate to the worksite. These and other features and advantages of the present invention will now be described in detail with reference to the accompanying drawings. 
   Improved ROV Work Methods 
   In an embodiment,  FIG. 1  shows a ROV spread  100  with a deployed ROV  116  mobilized at an underwater worksite  130 . The worksite typically comprises a ROV  116 , an underwater workstation  120 , a stationary structure  108 , and a payload destination area  134 . The workstation is lowered to the worksite by the station winch  132  and boom  104  on support vessel  102  using a station umbilical means  118 . The ROV is coupled by a ROV tether  114  to a cage  112  that includes a tether management system. The cage is suspended above the worksite using an umbilical means shown as cage umbilical  110 . The cage umbilical is deployed from a cage winch  106  located on the support vessel. 
   The support vessel  102  provides a deck  128  where the aforementioned winches, booms, tethers, and umbilical means are mounted. The support vessel also provides dry storage locations on the deck for the ROV  116 , cage  112 , and workstation  120 . 
   The words “umbilical means,” as used herein, refers to one or more lines and/or conductors that may be grouped into one or more bundles. The umbilical means is generally flexible and may be spooled on a winch or otherwise coiled. The umbilical means may include load bearing line(s) (a metallic cable is typical), electrical cables(s), fiber optic cable(s), and fluid transport line(s). The word “tether” as used herein, refers to an umbilical means extending from the ROV to the cage for the potential supply of load bearing, electrical, fiber optic, and fluid transport connectivity. 
   With continued reference to  FIG. 1 , the underwater worksite includes an offshore structure  108 . Submerged metallic portions  124  of offshore structures are frequently protected from corrosion by cathodic protection systems such as anodes  122 . One object of the present invention is to provide an improved ROV work-method for servicing anodes. 
   Referring now to  FIG. 2 , a second embodiment of the present invention is illustrated that excludes the cage  112 . In this embodiment, a cageless ROV spread  200  with a deployed ROV  116  is shown. The ROV is coupled by an umbilical  114  to the support vessel  102 . The umbilical is deployed from a winch  106 . With the cageless ROV spread, the ROV is deployed directly from the support vessel  102  rather than from an underwater cage. The cageless ROV spread may be used for worksites in relatively shallow water (generally less than 150 feet in depth). 
   Referring now to  FIG. 3  and the features of the present invention adapted to gas supply and replenishment, a gas refill system  300  is shown. Selected gas refill system equipment is integrated with the ROV  116  and with the workstation  120 . Gas transfer between the ROV and the workstation is enabled when a ROV connector  318  and a workstation connector  350  are mated. In an embodiment, the ROV and the workstation each have one or more connectors  318  and  350  respectively (one of each is shown). At least one ROV connector  318  and one mating workstation connector  350  are capable of exchanging fluids. Any of these connectors may also be capable of exchanging one or more of electrical signals, optical signals, and mechanical loads with mating connectors. 
   Still referring to  FIG. 3 , the ROV equipment comprising a part of the gas refill system includes a ROV gas supply system  302  and a ROV connector  318 . The gas supply system includes a ROV gas transfer module  306  and a ROV gas storage tank  304  (optional). The gas transfer module may include valve(s) and control(s) for managing gas flow and may be partially or wholly integrated with the connector. An optional ROV gas transfer line  322  fluidly connects the gas transfer module and the gas storage tank. Gas flow in this transfer line is bi-directional as shown by the flow arrows to  308  and from  310  the gas transfer module. 
   Still referring to  FIG. 3 , the ROV gas inlet line  316  fluidly connects the gas transfer module and the connector. Gas flow in the inlet line is toward the gas transfer module as shown by a flow arrow  330 . A gas recharge line  314  fluidly connects the gas transfer module  306  and the ballast tank  312 . Gas flow in this recharge line is bi-directional as shown by the flow arrows to  328  and from  326  the gas transfer module. Finally, a vent line  324  fluidly connects the gas transfer module and the underwater environment  333 . Flow in the vent line is toward the underwater environment as indicated by a flow arrow  332 . 
   In an embodiment of  FIG. 3 , the ROV equipment comprising a part of the gas refill system  300  excludes the optional ROV gas storage tank  304  and the ROV gas transfer line  322 . This embodiment relies on gas storage means separate from the ROV  116  to supply gas to the ballast tank  312  for gas recharge. 
   In another embodiment of  FIG. 3 , the ROV equipment comprising a part of the gas refill system  300  includes the optional ROV gas storage tank  304  and the ROV gas transfer line  322 . This embodiment may rely on gas storage means integral to and/or separate from the ROV  116  for gas recharge. 
   With continued reference to  FIG. 3 , workstation equipment comprising a part of the gas refill system  300  includes a station gas supply system  354  and a station connector(s)  350  designed to mate with the ROV connector(s)  318 . The gas supply system includes a station gas transfer module  358  and an optional station gas storage tank  356 . The gas transfer module includes valve(s) and control(s) for managing the gas flow and may be partially or wholly integrated with the connector. A station gas transfer line  360  fluidly connects the gas transfer module and the optional gas storage tank. Gas flow in this transfer line is bi-directional as shown by the flow arrows to  364  and from  362  the gas transfer module. 
   Still referring to  FIG. 3 , an optional station gas supply line  366  fluidly connects the station gas transfer module  358  and an optional auxiliary gas storage tank  372  that is separate from the ROV  116 . Gas flow in this supply line is toward the gas transfer module as shown by a flow arrow  370 . A station discharge line  352  fluidly connects the station gas transfer module and the station connector  350 . Gas flow in the discharge line is toward the station connector as shown by a flow arrow  368 . Either one of or both of the optional gas storage tanks  356  (including transfer line  360 ) and  372  (including supply line  366 ) are included in the workstation equipment comprising a part of the gas refill system  300 . 
   In still another embodiment of  FIG. 3 , the workstation equipment comprising a part of the gas refill system  300  includes the optional auxiliary gas storage tank  372  and the station gas supply line  366 ; it excludes the optional station gas storage tank  356  and the station gas transfer line  360 . This embodiment provides a gas storage means that may be either integral or external to the workstation  120 . 
   In another embodiment of  FIG. 3 , the workstation equipment comprising a part of the gas refill system  300  includes the optional station gas storage tank  356  and the station gas transfer line  360 ; it excludes the auxiliary station gas storage tank  372  and the station gas supply line  366 . This embodiment provides a gas storage means that is integral to the workstation  120 . 
   In yet another embodiment of  FIG. 3 , the workstation equipment comprising a part of the gas refill system  300  includes the optional auxiliary gas storage tank  372 , the station gas supply line  366 , the station gas storage tank  356 , and the station gas transfer line  360 . This embodiment provides a gas storage means that is integral to the workstation  120  and that may also be external to the workstation. 
   Referring again to  FIG. 3 , workstation features adapted for manipulating payloads  374  are shown. Workstation  120  may provide one or more storage racks  378  for holding one or more payloads  374 . The payload(s)  374  is fitted with a handle  376  suitable for engagement with the ROV manipulator  320 . 
   In  FIG. 4 , a ROV buoyancy management system  400  is shown. Selected buoyancy management equipment is integrated onto the ROV  116 . The buoyancy management equipment functions include varying the liquid fraction in the ballast tank  312 . Minimum ROV buoyancy is achieved when the tank is full of liquid, the liquid fraction is 100%, and the gas fraction is 0%. Maximum ROV buoyancy is achieved when the tank is full of gas, the liquid fraction is 0%, and the gas fraction is 100%. 
   Still referring to  FIG. 4 , the ROV  116  has a ballast tank  312 . In an embodiment, the ROV gas supply system  302  is fluidly connected to the ballast tank via a gas recharge line  314 . The ballast tank is also in fluid communication with the ROV underwater environment  333  via a liquid transfer line  410 . When gas is exchanged for liquid in the ballast tank (increases ROV buoyancy), a gas flow arrow  326  indicates gas flow into the ballast tank while a liquid flow arrow  406  indicates the corresponding liquid flow from the ballast tank. When liquid is exchanged for gas in the ballast tank (decreases ROV buoyancy), liquid flow arrow  404  indicates liquid flow into the tank while gas flow arrow  328  indicates the corresponding gas flow from the tank. Ballast tank  312  may include pumps, valves, and controls that facilitate ballast exchange operations. 
     FIG. 5  is a view  500  of the ROV  116  adjacent to a payload transfer structure  502 . In the present example, the payload transfer structure is a portion of an underwater structure and the payload is an anode. The transfer structure provides one or more interfaces  506  for receiving the payload. The transfer structure also provides one or more optional connectors  504  for mating with the ROV connector(s)  318 . Connector(s)  504  may exchange electrical signals, optical signals, or mechanical loads with ROV connector(s)  318 . Those skilled in the art will recognize that the features and benefits of the present invention are adaptable to many underwater worksites and to payloads associated with those worksites. 
   Operation 
   Referring to  FIG. 6 , a typical underwater project  600  utilizing the present invention is outlined in tabular format. The project employs a ROV  116  to move multiple payloads  374  between a first submerged location  136  and a second submerged location  134 . Project tasks include ROV deployment  604 , ROV work  606 , and ROV recovery  608 . These tasks comprise operations that are more fully described below. 
     FIGS. 1 and 2  illustrate alternative operations for deploying the ROV. In  FIG. 1 , the first deployment alternative  610  involves the use of a ROV cage  112 . The ROV is lowered in the cage from the surface vessel  102  to a location  136  near the worksite  130  and is then maneuvered from the cage to the worksite. In  FIG. 2 , a second deployment alternative  612  does not involve use of a ROV cage. Here, the ROV is deployed directly from the surface vessel and is maneuvered from the surface  126  to the worksite  130 . Once the ROV deployment task is completed, the ROV work task may begin. 
     FIG. 7  illustrates sequential operations comprising an exemplary ROV work task. From the ROV work task start  701 , the methodology progresses to the workstation operation  614  which includes payload transfer. A transport operation  616  follows where the payload is moved to a second location  134 . The methodology then progresses to a destination operation  618  where the ROV off-loads the payload. A decision step  710  follows. If the ROV work task cycle is to be repeated, a first cycle alternative  712  is chosen; the methodology then proceeds to a return operation  620  where the ROV is maneuvered back to the first payload transfer location  136 . If the ROV work task operation is to be discontinued, a second cycle alternative  714  is chosen and the methodology then proceeds to a ROV work task end-step  716 . The end-step  716  may be followed by ROV recovery task  608 . These operations comprise activities that are further described below. 
   Referring again to  FIG. 6 , the workstation operation  614  is further described by tabulated activities. In particular, the figure shows that the workstation operation  614  comprises activities including payload transfer  626 , gas transfer  628 , and buoyancy adjustment  630 . These activities are described below and illustrated in the flowchart form of  FIG. 8 . 
   Referring to  FIG. 8 , the payload transfer activity  626  is further illustrated by flowcharted steps. The methodology begins at step  801  and progresses to step  802  where the ROV is located adjacent to a payload transfer location  136 ,  134 . Step  804  follows where during on-loading the ROV engages (disengages during off-loading) the payload handle(s)  376  with its manipulator  320  to form manipulator connection(s)  704 . The methodology may also include the optional step  806  where the ROV connector(s)  318  is engaged with the workstation connector(s)  350  to form a ROV/workstation connection(s)  702 . The manipulator connection provides a mechanical connection between the ROV and the payload for transporting the payload. The ROV/workstation connection(s) may provide any one or more of fluid, electrical, optical, and mechanical connections between the ROV and the workstation. In an embodiment; mechanical connection(s) provided by the ROV/workstation connection(s) stabilize the ROV during payload transfer and or periods of non-neutral buoyancy. End-step  808  follows steps  804  and  806 . While adjacent to the workstation, the ROV may receive buoyancy compensation gas from the workstation as described below. 
   Referring to  FIG. 9 , exemplary gas transfer activity is further illustrated by flowcharted steps. The methodology begins at step  901  and progresses to step  902  where the ROV is located adjacent to the payload transfer location  136 . Step  904  follows where the ROV engages its connector(s)  318  with the workstation connector(s)  350  forming a ROV/workstation connection(s)  702 ; at least one of the ROV/workstation connection(s) is a fluid connector. Decision step  908  follows to check for the presence of a ROV gas storage tank  304 . If a tank is present the methodology proceeds along flowchart branch  910  to step  912  where gas is selectively transferred to one or both of the ROV gas storage tank and the ballast tank  312 . If a ROV gas storage tank is not present then the methodology proceeds along flowchart branch  914  to step  916  where gas is selectively transferred to the ballast tank. Steps  912  and  916  lead to Step  917  where the ROV/workstation connection(s) is disconnected. End-step  918  follows step  917 . 
   Referring also to  FIG. 3 , the gas transfer activities of steps  912  and  916  require a source of gas external to the ROV. External gas supplies include a gas storage tank integral to the workstation  356 , a gas storage tank separate from the workstation  372 , or a combination of both. 
   During the workstation operation  614 , the payload transfer activity  626  is typically associated with a buoyancy adjustment activity  630 . Referring to  FIG. 10 , the buoyancy adjustment activity is illustrated by flowcharted steps. The methodology begins at step  1001  and progresses to step  1002  where the ROV either on-loads or off-loads a payload. A decision block  1004  follows. If the ROV tends to sink as a result of the payload transfer, then the methodology proceeds along flowchart branch  1006  to step  1008 . In step  1008 , gas is exchanged for liquid (deballasting) in the ballast tank to restore neutral buoyancy to the ROV. Conversely, if the ROV tends to rise after the payload transfer, then the methodology proceeds along flowchart branch  1010  to step  1012 . In step  1012 , liquid is exchanged for gas (ballasting) in the ballast tank to restore neutral buoyancy to the ROV. From either step  1008  or  1012  the methodology progresses to step  1014  where it ends leaving the ROV in a neutrally buoyant state. 
   Referring also to  FIGS. 3 and 6 , when the ROV  116  has an integral gas storage tank  304 , the buoyancy adjustment activity  630  and the gas transfer activity  628  need not occur simultaneously; for that case the ROV may perform one or more gas recharge steps  1008  prior to carrying out a gas transfer. Conversely, if the ROV lacks an integral gas storage tank, the gas recharge step  912  does require a simultaneous gas transfer to the ROV. When the workstation operation  614  is completed, the transport operation  616  follows. 
   Referring to  FIGS. 6 and 7 , the transport operation  616  is further described by tabulated activities. In particular, the figure shows that the transport operation comprises the ROV moving with the payload  632  from a first location  136  to a second location  134 . When the transport operation is completed, the destination operation  618 , which includes off-loading, follows. 
   Referring to  FIGS. 6 ,  7 , and  8 , the destination operation  618  is further described by tabulated activities. In particular, the figures show that the destination operation comprises activities including the payload transfer activity  626  and the buoyancy adjustment activity  630 . The payload transfer activity, including on-loading and off-loading, has been described above in connection with  FIG. 8 . The buoyancy adjustment activity has also been described above in connection with  FIG. 9 . When the destination operation is completed a continuation decision  710  elects either a first cycle alternative  712  or a second cycle alternative  714 . 
   Referring again to  FIGS. 6 and 7 , the first cycle alternative  712  is followed by the return operation  620  that continues the ROV work task  606 . In particular, the figures show that the return operation comprises the ROV move without the payload  638  from the second payload transfer location  134  back to the first payload transfer location  718 . When the return operation is completed the ROV is ready to begin another cycle of the work task. 
   Referring to  FIGS. 6 and 7 , the second cycle alternative  714  reflects the choice to discontinue the current ROV work task  606 . The ROV  116  may be recovered  608  at this time.  FIGS. 1 and 2  illustrate alternative operations for recovering the ROV  116  when the second cycle alternative is chosen. In  FIG. 1 , a first recovery alternative  622  involves the use of a ROV cage  112 . The ROV is maneuvered from the second payload transfer location  134  into the cage at location  136  near the worksite. The cage is then recovered to the surface vessel  102 . In  FIG. 2 , a second recovery alternative  624  does not involve use of a ROV cage. Here, the ROV is recovered directly from the second payload transfer location to the surface vessel  102 . Those skilled in the art will recognize that in lieu of immediate recovery, the ROV might be employed on other similar tasks prior to being recovered to the surface.