Methods of installing offshore platforms

Abstract

A tension leg subsea platform (TLSP) ( 1 ) comprises a buoyant, submersible hull anchored to the seabed by tendons ( 3 ). A jack-up platform ( 6 ), either for drilling operations, production operations or any combination of these, is located such that its legs ( 8 ), when lowered, rest on the TLSP. In an alternative arrangement (FIG. 3 ), legs or columns are pre-installed on the TLSP, and the working platform lifted or floated onto them. In either case, the two components can be fabricated and travel separately to the field location. The effective depth of the TLSP may be 30 m to 100 m, while the sea itself is substantially deeper than 100 m. The working platform may be re-used at other locations. A drilling platform may be exchanged for a production platform while the TLSP remains moored on location, and the production platform may in turn be exchanged for a decommissioning platform.

1 . A method of installing a working platform in a depth of water, the method comprising: 
 providing a buoyant, submersible hull for subsea installation, the submersible hull being adapted to provide a foundation for a working platform standing above the surface of the sea    anchoring the hull to the seabed at a desired depth and location by means of tendons, thereby to form a tension leg subsea platform (TLSP), and    subsequently mounting a working platform on the TLSP.    
     
     
         2 . A method as claimed in  claim 1 , wherein said working platform is a jack-up platform comprising one or more supporting columns and a working deck mounted on said column(s) for vertical travel thereon, said mounting being performed subsequently to anchoring of the TLSP by lowering the columns from the working platform until they are planted on the TLSP.  
     
     
         3 . A method as claimed in  claim 1  or  2 , wherein said working platform is operated as a drilling platform.  
     
     
         4 . A method as claimed in  claim 1  or  2 , wherein said working platform is operated a production platform.  
     
     
         5 . A method as claimed in  claim 1  or  2 , wherein said working platform is operated as a decommissioning platform.  
     
     
         6 . A method as claimed in any preceding claim, wherein said working platform is exchanged for another working platform while the TLSP remains moored.  
     
     
         7 . A method as claimed in any preceding claim, wherein said working platform is one which has operated at another location prior to installation on said TLSP at its present location.  
     
     
         8 . A method as claimed in any preceding claim, wherein the effective depth of the TLSP is in the range 30 m to 100 m, while the sea at said location is substantially deeper than 100 m.  
     
     
         9 . A method as claimed in any preceding claim, wherein the TLSP carries wellheads for connecting the working platform to seabed wells.  
     
     
         10 . A method as claimed in any preceding claim, wherein the working platform is coupled with the TLSP by means of members movable to engage and disengage formations on the TLSP.  
     
     
         11 . A method of installing a working platform in a depth of water, the method comprising: 
 providing a buoyant, submersible hull for subsea installation, the submersible hull being adapted to provide a foundation for a working platform standing above the surface of the sea    anchoring the hull to the seabed at a desired depth and location by means of tendons, thereby to form a tension leg subsea platform (TLSP), and    mounting a first working platform on the TLSP    and wherein said working platform is exchanged for at least one other working platform while the TLSP remains moored.    
     
     
         12 . A method as claimed in  claim 11 , wherein said first working platform is mounted subsequently to anchoring and submerging the hull to form said TLSP.  
     
     
         13 . A method as claimed in  claim 11  or  12 , wherein each of said working platforms is of ajack-up platform type comprising one or more supporting columns and a working deck mounted on said column(s) for vertical travel thereon, said mounting being performed subsequently to anchoring of the TLSP by lowering the columns from the working platform until they are planted on the TLSP.  
     
     
         14 . A method as claimed in  claim 11 ,  12  or  13 , wherein one of said working platforms is operated as a drilling platform.  
     
     
         15 . A method as claimed in any of  claims 11  to  14 , wherein one of said working platforms is operated a production platform.  
     
     
         16 . A method as claimed in any of  claims 11  to  15 , wherein one of said working platform is operated as a decommissioning platform.  
     
     
         17 . A method as claimed in any of  claims 11  to  16 , wherein said working platform is one which has operated at another location prior to installation on said TLSP at its present location.  
     
     
         18 . A method as claimed in any of  claims 11  to  17 , wherein the effective depth of the TLSP is in the range 30 m to 100 m, while the sea at said location is substantially deeper than 100 m.  
     
     
         19 . A method as claimed in any one of  claims 11  to  18 , wherein the TLSP carries wellheads for connecting the working platform to seabed wells.  
     
     
         20 . A method as claimed in any one of  claims 11  to  19 , wherein each working platform couples with the TLSP by means of at least one retractable dog.  
     
     
         21 . An offshore platform installed by a method as claimed in any preceding claim.  
     
     
         22 . An offshore platform comprising: 
 a tension leg subsea platform (TLSP) comprising a buoyant, submersible hull for subsea installation and tendons anchoring the hull to the seabed at a desired depth and location, and    a working platform standing above the surface of the sea, the submersible hull providing a foundation for the working platform, said working platform being one which has operated at another location prior to installation on said TLSP at its present location.    
     
     
         23 . An offshore platform comprising: 
 a tension leg subsea platform (TLSP) comprising a buoyant, submersible hull for subsea installation and tendons anchoring the hull to the seabed at a desired depth and location, and    a working platform standing above the surface of the sea, the submersible hull providing a foundation for the working platform, said working platform being one which has replaced another working platform installed previously on said TLSP at its present location.