Method of locating a subsea structure for deployment

Abstract

A method of locating a subsea structure beneath a floating deployment vessel for deployment to a seabed including at least the steps of: (a) transporting the subsea structure on a floating transporting vessel near to the deployment vessel; (b) lowering the transporting vessel to allow the subsea structure to float; (c) relocating either the transporting vessel or the subsea structure to allow the subsea structure to be separate of the transporting vessel; and (d) locating the subsea structure beneath the deployment vessel using one or more buoyancy elements with variable buoyancy.

1 . A method of locating a subsea structure beneath a floating deployment vessel for deployment to a seabed comprising at least the steps of:
 (a) transporting the subsea structure on a floating transporting vessel near to the deployment vessel;   (b) lowering the transporting vessel to allow the subsea structure to float;   (c) relocating either the transporting vessel or the subsea structure to allow the subsea structure to be separate of the transporting vessel; and   (d) locating the subsea structure beneath the deployment vessel using one or more buoyancy elements with variable buoyancy.   
     
     
         2 . A method as claimed in  claim 1  wherein the floating transporting vessel is moveable in step (b) between a sea-faring position and a semi-submerged position. 
     
     
         3 . A method as claimed in  claim 1  wherein the subsea structure is located in step (a) directly on or next to a deck of the floating transporting vessel. 
     
     
         4 . A method as claimed in  claim 1  further comprising the step of providing at least one buoyancy element with variable buoyancy on or connected to the subsea structure prior to or during step (b). 
     
     
         5 . A method as claimed in  claim 4  wherein the at least one buoyancy element is on or connected to the subsea structure onshore and/or prior to step (a). 
     
     
         6 . A method as claimed in  claim 4  wherein the at least one buoyancy element is connected to the subsea structure onshore, and remains associated with the subsea structure during steps (b) and (c) for use in step (d).  
     
     
         7 . A method as claimed in  claim 1  wherein
 the one or more buoyancy elements with variable buoyancy are secured to the subsea structure in or at a position above the subsea structure. 
 
     
     
         8 . A method as claimed in  claim 1  wherein each buoyancy element comprises one or more chambers able to contain at least a proportion of a buoyancy fluid. 
     
     
         9 . A method as claimed in  claim 8  wherein one or more of the chambers are formed from a flexible or elastic material to permit the volume thereof to vary and accommodate the volume of buoyancy fluid contained therein. 
     
     
         10 . A method as claimed in  claim 8  wherein the buoyancy fluid comprises a wholly or substantially incompressible fluid having a density less than that of sea water. 
     
     
         11 . A method as claimed in  claim 10  wherein the buoyancy fluid is a low molecular weight hydrocarbon such as methanol. 
     
     
         12 . A method as claimed in  claim 8  wherein the buoyancy fluid includes glass microspheres. 
     
     
         13 . A method as claimed in  claim 8  wherein the buoyancy fluid has a density of between 500 and 600 kg/m 3 . 
     
     
         14 . A method as claimed in  claim 8  wherein the buoyancy fluid is a low viscosity gel. 
     
     
         15 . A method as claimed in  claim 8  further comprising the step of varying the volume of buoyancy fluid within the or each buoyancy element to vary the overall buoyancy of the subsea structure. 
     
     
         16 . A method as claimed in  claim 8  further comprising providing an external reservoir of buoyancy fluid and transferring buoyancy fluid between the external reservoir and the or each buoyancy element. 
     
     
         17 . A method as claimed in  claim 8  wherein the or each chamber includes a bladder or diaphragm to allow the chamber(s) to simultaneously comprise buoyancy fluid and water, preferably to allow the proportions of the buoyancy fluid and water in such chamber(s) to be varied, without allowing the two components to mix. 
     
     
         18 . A method as claimed in  claim 1  comprising
 varying the buoyancy of the or each buoyancy element associated with the subsea structure by transferring of one or more of the elements: air, water, preferably seawater, and buoyancy fluids; 
 in, out, within, between or any combination thereof; the one or more of the buoyancy elements. 
 
     
     
         19 . A method as claimed in  claim 1  wherein prior to step (b), the buoyancy element(s) are fully vented of air and replaced with either buoyancy fluid and/or water. 
     
     
         20 . A method as claimed in  claim 1  comprising the further transporting of the floating subsea structure at or near sea-level between steps (c) and (d). 
     
     
         21 . A method as claimed in  claim 1  including the steps of:
 providing at least one buoyancy element on or connected to the subsea structure, said at least one buoyancy element comprising one or more chambers containing a buoyancy fluid comprising a substantially incompressible fluid having a density less than that of sea water; 
 providing a reservoir for said buoyancy fluid at a location remote from said subsea structure; 
 providing fluid communication between said reservoir and said one or more chambers of said at least one buoyancy element; 
 transferring said buoyancy fluid between said reservoir and said one or more chambers of said at least one buoyancy element to vary the volume of buoyancy fluid within the at least one buoyancy element and thus vary the overall buoyancy of the subsea structure to control the rate of descent of the subsea structure to beneath the deployment vessel. 
 
     
     
         22 . A method as claimed any  claim 1  wherein
 the subsea structure is subsequently deployed to an installation site on the seabed beneath the floating deployment vessel. 
 
     
     
         23 . A method as claimed in  claim 1  wherein
 the subsequent deployment of a subsea structure to the seabed involves the one or more of the buoyancy elements used in step (d), preferably by or involving at least partly the replacement of buoyancy fluid in the buoyancy element(s) by water. 
 
     
     
         24 . A method of deploying a subsea structure to a seabed comprising the steps of:
 (a) transporting the subsea structure on a floating transporting vessel near to a deployment vessel;   (b) lowering the transportation vessel to allow the subsea structure to float;   (c) relocating either the transporting vessel or the subsea structure to allow the subsea structure to be separate of the transporting vessel;   (d) locating the subsea structure beneath the deployment vessel using one or more buoyancy elements with variable buoyancy; and   (e) deploying the subsea structure from beneath the deployment vessel to the seabed.    
     
     
         25 . A method as claimed in  claim 24  wherein step (e) comprises replacing buoyancy fluid in one or more of the buoyancy elements with water.