Buoyancy device for marine structures

Abstract

Disclosed is a buoyancy device for tensioning an elongate vertical (or quasi vertical) subsea structure at a point below the sea surface, anchored at its bottom. The device comprises as its main buoyancy elements a plurality of hollow elongate objects, and further comprises at least one auxiliary buoyancy element comprising a solid buoyant material. Ideally, the solid buoyant material provides enough uplift to maintain the device afloat should the main buoyancy elements be flooded. A riser apparatus comprising such a buoyancy device is also described.

1 - 19 . (canceled) 
     
     
         20 . A buoyancy device for tensioning an elongate subsea structure, wherein the elongate subsea structure is adapted to be disposed substantially vertically and under a water surface in use, at a point below the sea surface, wherein said buoyancy device comprises as main buoyancy elements a plurality of floodable hollow objects, and further comprises at least one auxiliary buoyancy element substantially comprising a solid buoyant material. 
     
     
         21 . A buoyancy device as claimed in  claim 20  wherein said hollow objects are hollow elongate objects. 
     
     
         22 . A buoyancy device as claimed in  claim 21  wherein said hollow objects are cylindrical. 
     
     
         23 . A buoyancy device as claimed in  claim 22  wherein said hollow elongate objects comprise sections of pipe, capped at both ends. 
     
     
         24 . A buoyancy device as claimed in  claim 21  wherein said hollow elongate objects are arranged evenly and vertically around a central core, such that they are parallel with a vertical axis of the buoyancy device. 
     
     
         25 . A buoyancy device as claimed  claim 24  comprising a supporting structure to support said hollow elongate objects around said central core. 
     
     
         26 . A buoyancy device as claimed in  claim 20  wherein said at least one auxiliary buoyancy element comprises syntactic foam. 
     
     
         27 . A buoyancy device as claimed in  claim 20  wherein said at least one auxiliary buoyancy element provides sufficient uplift to keep the buoyancy device afloat when all of said hollow objects are flooded. 
     
     
         28 . A buoyancy device as claimed in  claim 20  wherein said plurality of hollow objects are arranged around the periphery of said at least one auxiliary buoyancy element. 
     
     
         29 . An elongate subsea structure, adapted to be disposed substantially vertically and underneath a sea surface in use, the elongate subsea structure comprising:
 an anchor at the bottom of the elongate subsea structure to anchor the elongate subsea structure to a seabed; and   a buoyancy device for tensioning the elongate subsea structure, the buoyancy device being attached at, or near to, the top of the elongate subsea structure in use; wherein the buoyancy device comprises as main buoyancy elements a plurality of floodable hollow objects, and further comprises at least one auxiliary buoyancy element substantially comprising a solid buoyant material.   
     
     
         30 . An elongate subsea structure as claimed in  claim 29  wherein the floodable hollow objects are cylindrical and elongate. 
     
     
         31 . An elongate subsea structure as claimed in  claim 29  comprising one or more rigid conduits. 
     
     
         32 . An elongate subsea structure as claimed in  claim 31  comprising a structural core. 
     
     
         33 . An elongate subsea structure as claimed in  claim 32  wherein said one or more rigid conduits are arranged around said structural core. 
     
     
         34 . An elongate subsea structure as claimed in  claim 32  wherein some or all of said one or more rigid conduits are located inside said structural core. 
     
     
         35 . An elongate subsea structure as claimed in  claim 32  wherein said structural core is a tubular core used as a conduit. 
     
     
         36 . An elongate subsea structure as claimed in  claim 31  further comprising the same number of flexible conduits as rigid conduits such that a flexible conduit connects each rigid conduit to a surface structure. 
     
     
         37 . An elongate subsea structure as claimed in  claim 29 , wherein the elongate subsea structure is a riser tower for conveying production fluids from the seabed to the surface.