Method and apparatus for fabricating an off-shore structure

Abstract

Improved method and apparatus for assembling and completing the fabrication of a multilegged structure of extended length at an offshore site wherein separate multilegged structural modules are selectively floated, disposed end-to-end, and aligned relative to each other at the offshore site such that the ends of aligned legs of the individual sections can be drawn and clamped together and then welded while in a clamped condition whereby the several welded-together sections can form an elongated unitary structure. .Iadd.

What is claimed is: 
     
       1. The method of fabricating a modular offshore structure comprising the steps of transporting at least a pair of individual structural modules each of which is made up of interconnected and floatable watertight legs from an onshore site to a preselected offshore site, then while the said pair of modules are partially submerged in water at the offshore site selectively controlling the ballast in the watertight legs of each module of the pair of modules and maneuvering said pair of modules relative to each other until the legs of one module are fully aligned with the legs of the other and opposed module, progressively drawing said aligned legs of the pair of opposed modules together until they are abutted, and while holding the abutted legs of the modules firmly clamped together rigidly securing one leg of a pair of abutted legs to the other leg of said pair of legs. 
     
     
       2. A method as set forth in claim 1 wherein the legs of said pair of legs are rigidly secured together by first tack welding the legs of said pair of legs together and subsequently fully welding the said pair of legs together. 
     
     
       3. A method as set forth in claim 1 including the step of isolating the said pair of abutted legs of opposed modules from the surrounding water prior to rigidly securing said pair of legs together. 
     
     
       4. A method as set forth in claim 3 wherein the legs of the said pair of abutted legs are rigidly secured together by arc welding the same together. 
     
     
       5. A method as set forth in claim 3 wherein the step of isolating the abutted pair of legs from the surrounding water is effected by surrounding the abutted legs with a collapsible cofferdam in the area of the joint between the legs, drawing the pairs of said cofferdam together about the abutted legs and then evacuating the water from the compartment formed by said cofferdam. 
     
     
       6. A method as set forth in claim 1 wherein said abutted legs are clampingly held together by firmly engaging each of said legs with opposing parts of a clamping device and then drawing said opposing parts of the device together. 
     
     
       7. A method as set forth in claim 1 wherein opposed modules are maneuvered and drawn together at least in part by drawing upon cables attached to each module of said pair of modules. 
     
     
       8. A method as set forth in claim 1 including the step of locking adjustable clamping elements to each of said modules of said pair of modules in order to clamp the abutted legs of the modules together. 
     
     
       9. A method as set forth in claim 1 including the step of fillet arc welding the abutted legs of opposed modules together after clamping said abutted legs to each other. 
     
     
       10. A method as set forth in claim 1 including the step of floating said modules to the offshore site. 
     
     
       11. A method as set forth in claim 1 wherein the aligned legs of the pair of modules are drawn together by drawing up on cables attached to each of the opposed modules while at the same time exerting module separating forces on the modules which are of less magnitude than the forces exerted on the modules by the cables. 
     
     
       12. The method as set forth in claim 1, including the step of installing the modular structure at the preselected offshore site. 
     
     
       13. A collapsible and floatable cofferdam device for engaging and isolating a pair of abutted legs on adjacent modular platform sections in the area of the joint between the pair of legs from the surrounding water at an offshore site, said device comprising a plurality of generally channel-shaped and interconnected elements, opposed edges of adjoining web portions of adjacent elements of the plurality of elements being hingedly interconnected together so as to effect opening and closing of the plurality of elements from an extended condition to a collapsed condition and vice versa, the opposed side edges of adjoining flange portions of adjacent elements being adapted to be separated from each other in order to define a groove therebetween when adjacent elements of the plurality of elements are in an extended condition, said opposed side edges at the adjoining flange portions of said elements being adapted to be disposed in abutting and sealing engagement with each other and the platform sections upon adjacent elements of the plurality of elements being in a collapsed condition, and pontoon means connected to certain of said elements for controlling the overall buoyancy of the plurality of elements during flotation and collapse of the plurality of elements about the pair of abutted legs at an offshore site. 
     
     
       14. A collapsible and floatable cofferdam device as set forth in claim 13 including means on said cofferdam device for anchoring said device to said modular platform sections. 
     
     
       15. A collapsible and floatable cofferdam device as set forth in claim 13 including a flexible and resilient gasket element carried by the upper edge of a flange portion of a given element of said device for sealing the upper edge of the flange portion of the given element to a leg of said pair of legs. 
     
     
       16. A collapsible and floatable cofferdam device as set forth in claim 13 including a flexible and resilient gasket element carried by the side edge of a flange portion of a given element for sealing the side edge of the flange portion of a given element to the opposed side edge of an adjoining flange portion of an adjacent element with the plurality of elements are in a collapsed condition. 
     
     
       17. A collapsible and floatable cofferdam device as set forth in claim 13 including a flexible and resilient gasket element carried by an edge of the web portion of a given element for sealing the edge of the web portion of the given element to the opposed edge of an adjoining web portion of an adjacent element when the adjacent and given elements are in a collapsed condition. 
     
     
       18. A collapsible and floatable cofferdam device as set forth in claim 13 including means operatively associated with each one of the plurality of said elements for drawing the upper edge of a flange portion of a given element into sealing engagement with selected surface portions of a given leg of the pair of abutted legs. .Iadd. 19. A cofferdam element for a collapsible and floatable cofferdam device used for isolating from the surrounding water at an offshore site a pair of abutted legs of adjacent modular platform sections in the area of the joint between abutted legs, said cofferdam element comprising a web portion having first, second, third and fourth web edges, said first and third web edges and said second and fourth web edges, respectively, being in opposed relationship to one another, first and second hinge means located separately along said first and third web edges for hingedly connecting said cofferdam element with a pair of flanking cofferdam elements, first and second upstanding flange portions attached in watertight relationship to said web portion along said second and fourth web edges, each of said flange portions comprising a first and second flange edge each originating respectively at said first and third web edges and terminating at a top edge of each of said upstanding flange portions, at least a portion of each of said top edges being configured to closely conform with the cross-sectional configuration of at least a portion of the underside of the legs of the tower section, web edge sealing means positioned along said first and third web edges to sealingly engage with web edges of said flanking cofferdam elements, flange edge sealing means positioned along said flange edges to sealingly engage with flange edges of said flanking cofferdam elements, and top edge sealing means positioned at said top edge to sealingly engage with the outer surface of said tower leg. .Iaddend..Iadd. 20. The cofferdam element of claim 19 further comprising means for drawing said top edge sealing means into pressure engagement with said outer surface of said tower leg. .Iaddend..Iadd. 21. The cofferdam element of claim 19 wherein said web portion is substantially planar and is adopted to be substantially horizontally oriented when in engagement with said tower legs. .Iaddend. .Iadd. 22. The cofferdam element of claim 19 further comprising pontoon means for controlling the overall buoyancy of said element during flotation thereof. .Iaddend. .Iadd. 23. The cofferdam element of claim 19 further comprising means for anchoring said cofferdam element to said modular platform section. .Iaddend. .Iadd. 24. A cofferdam element for a collapsible and floatable cofferdam device used for isolating from the surrounding water at an offshore site a pair of abutted legs of adjacent modular platform sections in the area of the joint between abutted legs, said cofferdam element comprising a web portion having first, second, third and fourth web edges, said first and third web edges and said second and fourth web edges, respectively, being in opposed relationship to one another, hinge means located along said first web edge for hingedly connecting said cofferdam element with another flanking cofferdam element, first and second upstanding flange portions attached in watertight relationship to said web portion along said second and fourth web edges, each of said flange portions comprising a first flange edge originating at said first web edge and terminating at a top edge of each of said upstanding flange portions, at least a portion of each of said top edges being configured to closely conform with the cross-sectional configuration of at least a portion of the side of the legs of the tower section, each of said flange portions further comprising a second flange edge originating at said third web edge and extending substantially perpendicularly to said web portion and terminating at said top edge, web edge sealing means positioned along said first and third web edges to sealingly engage with web edges of said flanking cofferdam elements, flange edge sealing means positioned along said first flange edges to sealingly engage with flange edges of said flanking cofferdam element, and top edge sealing means positioned at said top edge to sealingly engage with the outer surface of said tower leg. .Iaddend..Iadd. 25. The cofferdam element of claim 24 further comprising means for drawing said top edge sealing means into pressure engagement with said outer surface of said tower leg. .Iaddend..Iadd. 26. The cofferdam element of claim 24 wherein said web portion is substantially planar and is adapted to be substantially vertically oriented when in engagement with said tower legs. .Iaddend. .Iadd. 27. A floatable module which is floatable to an offshore assembly site in horizontal orientation to be attached while horizontal to at least one other said module and then upturned to vertical orientation and emplaced, each said module comprising a series of laterally spaced floatable watertight leg elements and a plurality of brace elements interposed therebetween and interconnected to the series of spaced leg elements to form a module of polyhedral configuration, guiding and aligning means adapted to facilitate the abutting and joining of the ends of said leg elements of adjacent of said modules while said leg elements are horizontally oriented and floating in the water, said guiding and aligning means comprising a plurality of guide strip elements spaced about the outer peripheral portion of at least one of said leg elements at the end thereof, said guide strip elements extending outwardly beyond said leg end generally horizontally to receive therebetween the leg end of an adjacent of said modules to be brought into abutting relationship therewith, said guide strip elements being of such length as to guide and align the leg end of the adjacent leg elements during at least the final portion of movement of said leg end into abutting relationship, and an upstanding lug attached near the end of a leg in opposed relationship to a leg on an adjacent one of said modules to facilitate the attachment of a clamping device between said legs when the adjacent ends of the legs are drawn into abutment to form an extended structure. .Iaddend..Iadd. 28. The structure of claim 27 further comprising attachment means on said module for permitting attachment of means for pulling together adjacent of said modules from positions spaced from one another to a position wherein said leg elements are substantially in abutting relationship with one another. .Iaddend. .Iadd. 29. A module as set forth in claim 27 in which said module is comprised of a series of three laterally spaced legs to form a module of triangular shape in transverse section. .Iaddend. .Iadd. 30. A module as set forth in claim 27 in which said opposed lugs are interposed between adjacent strip elements of said series of strip elements associated with the pair of adjacent and longitudinally aligned leg elements. .Iaddend..Iadd. 31. A module as set forth in claim 27 in which adjacent and abutted leg ends of adjoining modules are rigidly secured together upon forming a weldment between abutted leg ends thereof. .Iaddend. .Iadd. 32. A floatable module which is floatable to an offshore assembly site in horizontal orientation to be attached while horizontal to at least one other said module and then upturned to vertical orientation and emplaced, each said module comprising a series of laterally spaced floatable watertight leg elements and a plurality of brace elements interposed therebetween and interconnected to the series of spaced leg elements to form a module of polyhedral configuration, guiding and aligning means adapted to facilitate the abutting and joining of the ends of said leg elements of adjacent of said modules while said leg elements are horizontally oriented and floating in the water, and attachment means on said module for permitting attachment of means for pulling together adjacent of said modules from positions spaced from one another to a position wherein said leg elements are substantially in abutting relationship with one another. .Iaddend..Iadd. 33. A floatable module which is floatable to an offshore assembly site in horizontal orientation to be attached while horizontal to at least one other said module and then upturned to vertical orientation and emplaced, each said module comprising a series of laterally spaced floatable watertight leg elements and a plurality of brace elements interposed therebetween and interconnected to the series of spaced leg elements to form a module of polyhedral configuration, guiding and aligning means adapted to facilitate the abutting and joining of the ends of said leg elements of adjacent of said modules while said leg elements are horizontally oriented and floating in the water, and means to move adjacent of said modules, while the leg elements are in horizontally oriented position, from positions spaced from one another to a position wherein the leg elements are substantially in abutting relationship with one another. .Iaddend. .Iadd. 34. A module as set forth in claim 33 in which said module is comprised of a series of three laterally spaced legs to form a module of triangular shape in transverse section. .Iaddend..Iadd. 35. A module as set forth in claim 33 in which adjacent and abutted leg ends of adjoining modules are rigidly secured together upon forming a weldment between abutted leg ends thereof. .Iaddend.