Subsea structure and methods of construction and installation thereof

Abstract

A structure ( 200 ) for subsea installation has buoyancy distributed substantially along its length in order to float said structure in horizontal orientation to a point of installation. When said structure is upended, said distributed buoyancy maintains said structure in a substantially vertical orientation and operational state, without the need for further, non-distributed buoyancy (for example, without substantial tension being provided by a top buoy or surface vessel ( 100 )). The structure may comprise a flexible or rigid riser conduit for hydrocarbons. Buoyancy modules may be fixed with spaces between them, for example to achieve a structure similar to a SLOR, or COR, but without a top buoy. The buoyancy modules may alternatively be free to slide longitudinally along the structure, so as to impart their buoyancy force through one another to the top of the structure.

1 . A structure for subsea installation, the structure comprising at least one rigid riser conduit extending substantially the full length of the structure for transfer of fluid between the seabed and sea surface, the elongate structure having buoyancy distributed substantially along its length, the quantity and distribution of said buoyancy being sufficient both to float said structure in horizontal orientation to a point of installation, and when said structure is upended, to maintain said structure in a substantially vertical orientation and operational state.  
   
   
       2 . A structure as claimed in  claim 1 , wherein said distributed buoyancy is provided by buoyancy modules provided at regular intervals along said subsea structure.  
   
   
       3 . A structure as claimed in  claim 2 , wherein said buoyancy modules are fixed with spaces between them.  
   
   
       4 . A structure as claimed in  claim 3 , wherein said buoyancy modules are spaced substantially regularly along said subsea structure to enable said structure to be floated to the deployment site in a substantially horizontal orientation prior to deployment.  
   
   
       5 . A structure as claimed in  claim 1 , wherein said buoyancy modules are free to slide longitudinally along the structure, so as to impart their buoyancy force through one another to the top of the structure, rather than being transmitted to the structure at points along the length thereof.  
   
   
       6 . A structure as claimed in any one of claims  1  or  2 , wherein said structure in its installed state is connected to at least one flexible jumper or similar flexible portion at its top end to connect with said riser conduit a source or destination of fluid.  
   
   
       7 . A structure as claimed in claims  6 , wherein said flexible jumper is provided with distributed buoyancy, whose buoyancy contributes to the tension in the structure in its installed state.  
   
   
       8 . A structure as claimed in  claim 7 , wherein only a lesser proportion of said flexible jumper has distributed buoyancy.  
   
   
       9 . A structure as claimed in  claim 6 , wherein said buoyancy is distributed in such a way to give said flexible portion a “steep wave” shape, in order to apply tension co-linear with the structure, thereby avoiding large bending moments in the structure.  
   
   
       10 . A structure as claimed in  claim 6 , wherein the distribution of buoyancy about the length of the structure is selected to provide substantially normal orientation of said flexible portions as they attach to said structure, such that any bending moment induced by tension in said flexible portions is minimise.  
   
   
       11 . (canceled)  
   
   
       12 . A structure as claimed in any one of claims  1  or  2  comprising a plurality of spaced risers surrounding a central core.  
   
   
       13 . A structure as claimed in  claim 12 , wherein said also serves as a riser conduit.  
   
   
       14 . A structure as claimed in  claim 12 , wherein said core comprises a support for said riser conduit.  
   
   
       15 . A structure as claimed in any one of claims  1  or  2 , wherein said distributed buoyancy is of a permanent type, such as syntactic foam.  
   
   
       16 . A method of installing an elongate subsea structure comprising a riser conduit for transfer of fluid between the seabed and sea surface and having buoyancy distributed substantially along its length in order to float said subsea structure to the point of installation, wherein when said structure is upended and anchored, either directly or indirectly, to the seabed, said distributed buoyancy is sufficient to maintain said structure in a substantially vertical orientation without a substantial further separate buoyant structure.  
   
   
       17 . A method of installing an elongate subsea structure as claimed in  claim 16 , wherein said riser conduit in its installed state is connected to at least one flexible jumper or similar flexible portion at its top end for connection to a source or destination of fluid.  
   
   
       18 . A method of installing an elongate subsea structure as claimed in  claim 17 , wherein said flexible jumper is provided with distributed buoyancy, whose buoyancy contributes to the tension in the structure in its installed state.  
   
   
       19 . A method of installing an elongate subsea structure as claimed in  claim 18 , wherein only a lesser proportion of said flexible jumper is provided with distributed buoyancy.  
   
   
       20 . A method of installing an elongate subsea structure as claimed in any one of  claims 17  to  19 , wherein said buoyancy is distributed in such a way as to give said flexible portion a steep wave shape, in order to apply tension co-linear with the structure, thereby avoiding large bending moments in the subsea structure.  
   
   
       21 . A method of installing an elongate subsea structure as claimed in any of  claims 17  to  19 , wherein the distribution of buoyancy about the length of the riser is selected to provide substantially normal orientation of said flexible portions as they attach to said structure, such that any bending moment induced by tension in said flexible risers is minimised.  
   
   
       22 . A method of installing an elongate subsea structure as claimed in any one of  claims 16  to  19 , wherein adjustments to said distributed buoyancy are made temporarily by the addition of subsequent modules.  
   
   
       23 . A method of installing an elongate subsea structure as claimed in any one of  claims 16  to  19 , wherein adjustments to said distributed buoyancy are made by air-filling then flooding parts of the structure at different stages in the installation process.  
   
   
       24 . A method of installing an elongate subsea structure as claimed in any one of  claims 16  to  19 , wherein dense material is used to fill the structure during installation in order to compensate for the extra buoyancy required to perform said method of installation.  
   
   
       25 . A method of installing an elongate subsea structure as claimed in  claim 24 , wherein said dense material comprises seawater.  
   
   
       26 . A method of installing an elongate subsea structure as claimed in  claim 24 , wherein said dense material comprises a material denser than the ambient seawater.  
   
   
       27 . An installed structure as claimed in any of  claims 16  to  19 , whereupon said structure has been upended and said distributed buoyancy maintains said structure in a substantially vertical orientation and operational state, without the need for further, non-distributed buoyancy.