Patent Publication Number: US-2013252493-A1

Title: Rigid Hull Gas-Can Buoys Variable Buoyancy

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     FIELD OF INVENTION 
     The present invention is directed to a variable buoyancy gas-can module for use with a Self Supporting Riser (SSR). Further, the present invention is directed to the construction of a gas-can buoy, specifically to a flexible liner that is a barrier to isolate the gas from the water in the gas-can buoy especially at significant depths. 
     BACKGROUND OF THE INVENTION 
     It has been the practice to use gas-can buoys for near surface buoys; however, when used at greater and greater depths in seawater the efficiency of the prior art buoys decreases. This is particularly true when the buoy must be partially ballasted to change the buoyancy. Seawater dissolves gas. Near surface seawater water tends to be saturated with gas due to its contact with the atmosphere where surface water is mixed by wave action. Below the wave zone there is little opportunity for water to have direct contact with the atmosphere so the water is essentially isolated from any potential source of additional gas. Further, as expressed by Henry&#39;s law, water under higher pressure must dissolve more gas to reach equilibrium so the quantity of gas needed for saturation increases with increasing depth in the ocean. Water deep in the ocean is typically water that has sunk from the surface due to density difference. Water that is saturated with gas near the surface and then sinks to greater depth is exposed to higher pressure without the opportunity to dissolve more gas. Water deep in the ocean therefore typically has far less gas dissolved than needed for saturation and therefore quickly dissolves gas that is exposed to it. Gas charged variable buoyancy for use below the near surface mixing zone, and particularly at greater depth, therefore requires an impermeable or very low permeability liner barrier between ambient water and the gas in order to avoid loss of gas (loss of buoyancy) that would result from contact between the gas and ambient water. 
     An object of the present invention is to provide an apparatus and method whereby gas/water isolation and variable buoyancy can be achieved without the need for precision machined sealing surfaces while maintaining the advantages of rigid hull gas-can buoyancy modules. A further object of the present invention is to provide a buoyancy module for a Self Supporting Riser (SSR) as fully described in U.S. application Ser. No. 12/714,919, filed Mar. 1, 2010, entitled “Riser Technology”. The large dimensions of the buoyancy module(s) of a deepwater SSR make it impractical to provide the precision machined surfaces required for conventional sliding seals between the hull and a barrier. Further the hull of a gas-can buoy for an SSR is subject to flexure due to load variations from current and other forces so the distance between the hull walls changes. An impermeable boundary or barrier between the gas and water is required. Still further, variable buoyancy is desired and therefore, this boundary or barrier must be movable in the hull to allow increase or decrease of gas volume and of buoyancy (the greater the gas volume—the greater the water displaced from the gas-can—the greater the buoyancy). 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus and method directed to a variable buoyancy gas-can buoyancy module or buoy having a flexible barrier between a variable volume gas chamber in the gas-can hull and water in the hull. More specifically, the present invention is directed to a variable buoyancy module for a Self Supporting Riser (SSR) wherein the tension in the SSR may be increased/decreased by increasing/decreasing the variable volume of a chamber formed by a flexible liner that provides a barrier between the variable volume gas chamber in the gas-can hull and water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of one embodiment of a variable buoyancy rigid hull gas-can buoyancy module or buoy of the present invention; 
         FIG. 2  is a schematic view of another embodiment of a variable buoyancy rigid hull gas-can buoyancy module or buoy of the present invention; 
         FIG. 3  is a schematic view of a variable buoyancy rigid hull gas-can buoyancy module or buoy of the present invention with a fill/vent structure for increasing/decreasing the volume of a variable volume gas chamber from either the bottom or the top of the hull and with typical control elements; 
         FIGS. 4 and 5  are schematic views to illustrate a multi-chamber variable buoyancy rigid hull gas-can buoyancy module or buoy of the present invention; and 
         FIG. 4A  is a schematic view of the multi-chamber variable buoyancy rigid hull gas-can buoyancy module or buoy of  FIG. 4 , illustrating that the center column in each of the chambers may be the structure for holding the flexible liner. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION 
     Referring to  FIG. 1 , a rigid gas-can hull  10  is preferably a cylindrical can with a cylindrical side surface  12  and a top surface  14 . Hull  10  has a bottom  16  with vent openings, a screen (not shown) or an open lower end  16 . In this embodiment a flexible cylindrical hull liner  20 , the height of which is approximately equal to the height of the hull side surface or wall  12 , is attached to the hull  10  near the top of side surface  12  or the top surface  14  and attached to an inner structure, a floating barrier  22  to bridge a clearance gap  21  (the distance between the side surface  12  of hull  10  and the floating structure  22 ) and provide a barrier between a variable volume gas chamber  19  in the gas-can hull and water, seawater that enters through lower end  16  in the hull. 
     The liner  20  is made of a flexible material that is highly impermeable to gas and water, such as metalized Mylar, a product of TEKRIA Corporation, or polyethylene film. The inner structure or floating structure  22  of this embodiment may be made from materials such as syntactic foam and epoxy bonded fiber glass to float on the water in or below hull  10 . The inner structure  22  is free to move up and down inside the hull  10 , and is kept aligned by either guides, which may be on a central column  24 , or by the sliding sealed sleeve  26  around a central column  24 . The relatively small dimensions of a central column  24  make it practical to maintain a conventional sliding seal between the floating structure  22  and the column  24 . When the floating structure  22  is high on the column  24  there is slack in the liner  20 . This slack is stored in a slack loop  27  (shown in  FIG. 1  as a U-shape between ends of the liner  20  connected to the top of hull  10  and the outer end of structure  22 ) which is tended or maintained by weight such as sand or metal balls  28  to keep the slack or slack loop  27  in a known location. The loop  27  and weights  28  help ensure that the liner  20  is applied evenly to the wall  12  of the hull as the floating structure  22  goes down the column  24 . If the liner were applied to the wall with wrinkles, the slack might all be used before the floating barrier reached the bottom of the hull. The upper surface of the floating structure  22  is sloped to help ensure that sand or balls  28  displaced onto the floating structure fall back down into the slack loop  27  of the liner  20 . The specific structure of this embodiment is to deal with a phenomenon that must be dealt with in a high ambient pressure environment, i.e. the increase in friction between non lubricated surfaces. An analogy is a toy suction cup providing an example of ambient pressure holding a flexible surface tight against another surface. The friction force that must be overcome to slide the suction cup can be calculated as the coefficient of friction times the force holding the two surfaces together, which is ambient pressure times the surface area. With one atmosphere ambient pressure the friction force between a toy suction cup and the surface to which it is attached can be readily overcome. At a depth of just over 300 feet in the ocean, ambient pressure is approximately 10 times as much so the friction force is 10 times as great. With increasing depth, particularly over a large surface area, this friction force soon exceeds either the force available to slide the cup or flexible material or the strength of the flexible material. Free gas or liquid between the two surfaces minimizes or eliminates this friction force, as can be demonstrated by sliding the toy suction cup over a crack that provides access for ambient air to get between the suction cup and the surface to which it was engaged. 
     Preferably the liner  20  is a composite material that includes a layer of felt or open weave material attached to one or both sides of the gas and water impermeable layer of the liner so that free water is always permitted or wicked into the pores of the open weave material in a manner that maintains continuity of fluid to the ambient seawater. This helps ensure that when gas is introduced into the liner  20 , as through line  30  in the top surface  14  (that includes a control box  7 ), the floating structure  22  moves down the column  24  or when gas is removed or vented from the liner  20 , the structure  22  moves up the column  24  while the relatively impermeable barrier is maintained. These features provide a method and apparatus whereby variable buoyancy gas-cans have a rigid hull for protection and a liner between the water and the gas, and the volume of the enclosed gas chamber  19  can be changed in a way that does not require precision sealing surfaces, avoids sticking when sliding one material surface on another in the presence of high ambient pressure, and can include a method to reduce the friction so that the liner material can be held on the side surface  12  or removed without damage. 
     Now referring to  FIG. 2 , in this embodiment the phenomenon of high ambient pressure environment and the resulting increase in friction between non lubricated surfaces is addressed without need for the floating barrier  22 . In this embodiment the liner  20  is removed from the side surface  12  of the hull  20  by moving it at a right angle from the surface. An analogy is removing tape from a surface. The tape will easily overcome the friction and adhesion without damage to the tape if pulled at a right angle to the surface. The flexible cylindrical hull liner  20 , which provides a moveable barrier between air and water, is attached to the hull  10  near the top of side surface  12  or to the top surface  14  and attached to an inner structure, in this embodiment a central column  24 , as by a ring  3 . Flexible liner  20  provides a gas/water barrier between central column  24  and the hull side surface or wall  12  for any volume of variable volume gas chamber  19  in the gas-can hull. The volume of gas chamber  19  is increased by adding gas to the chamber  19  and the result is added buoyancy. The length of liner  20  is the sum of three dimensions; L 1  the length held to wall  12 ; L 2  the length that is in the gap between the wall  12  and column  24 ; and L 3  the length held to column  24 . It is noted that L 2  remains constant and essentially horizontal to maintain the liner  20  at right angles to both the wall  12  and the column  24 . When gas is introduced in line  30 , the barrier across L 2  is moved downward stripping a length of liner from the column  24 , the increase of L 1  being equal to the decrease in length of L 3 . That the liner  20  is stripped from column  20  at a right angle allows the liner to move without ripping or damage. Likewise, the volume of chamber  19  may be reduced by venting gas from line  30 . The barrier across L 2  as it moves upward strips a length of liner from wall  12 , the decrease of L 1  being equal to the increase in length of L 3 . 
     In a preferred embodiment, a joint  13  extends through central column  24  to produce a buoyancy module  15  for a Self Supporting Riser (SSR) as fully described in U.S. application Ser. No. 12/714,919, referred to above. The joint  13  illustrated is a conventional box and pin joint that has a shoulder  9  that fits to corresponding fitting  11  on the top surface  14  of can  10 . However, load shoulder  9  may be the bottom of the box as illustrated in  FIG. 1  or if the joint has flanges to connect the joints, the flanges may provide the load shoulder  9  for the variable buoyancy module  10 . The SSR when in use is attached to seafloor structure such that when the buoyancy is varied or adjusted there is a corresponding change in lift or tension in the SSR. The SSR is made up of joints and specialty joints, such as the buoyancy module  15 , as described more fully in U.S. application Ser. No. 12/714,919. 
     Referring to  FIGS. 3 , another embodiment of a gas-can hull  10  has the gas added by a line  31  from the bottom of hull  10 . In line  31  is a control element  32  that may include a valve and electronics to regulate the flow and/or to prevent overfill or under-fill so that the buoyancy can be varied safely in service. Line  31  has a vertical portion  6  that may be in the column  24 , as shown, or in a groove in the side of column  24 . The vertical portion  6  ends in a space  5  at or near the highest point in the chamber  19 . Alternately a vent line  30  at the top of hull  10  may also terminate in space  5 . A control element  7  allows filling and venting in a controlled manner to regulate the flow and/or to prevent overfill or under-fill so that the buoyancy can be varied safely in service. 
     Referring now to  FIGS. 4 and 5 , configurations of multiple chamber rigid gas-can hulls  10  is illustrated. In  FIG. 4 , illustrated are four cylindrical chambers A-D in the hull  10 , each of which may have details of structure as illustrated in the embodiments above.  FIG. 5  illustrates that the cylindrical hull  10  may have four quadrants W-Z. The advantage of multiple chambers is redundancy. 
     Referring to  FIG. 4A , each flexible cylindrical hull liner  20 , the height of which is approximately equal to the height of the hull side surface or wall  12 , is attached to the hull  10  near the top of side surface  12  or the top surface  14  and attached to an inner structure, in this embodiment a center column  34 , to provide a barrier between a variable volume gas chamber  19  in the gas-can hull and water. 
     The volume of gas chamber  19  is increased by adding gas to the chamber  19  and the result is added buoyancy. Gas line  31  may enter the top of the hull as shown at the left of  FIG. 4A  or at the bottom of hull  10  as shown at the right of  FIG. 4A .