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.

Description

This is a reissue of Patent No. 3,641,774 dated Feb. 15, 1972. assembles assemblies 112. These eyelet assemblies 112 at the outer end edges of a given end section 74 facilitate selective connection of one or more cable lines to the outer end edges thereof so as to enable transporting of the device to the offshore site such as by a service vessel, not shown, or placement or folding of the device about a predetermined part of the extended structure for selectively engaging and isolating the same from the water as will now be discussed.

At the offshore site, device 22 is lowered from the surface vessel (not shown) into the water in an appropriate manner. Upon lowering of the device into the water, pontoons 76 connected thereto are selectively and controllably flooded until the device is suitably floated in the water, as indicated in FIG. 5. Although device 22 is illustrated in FIG. 5 in an extended position substantially on top of the water, it is to be understood that in actual practice the cofferdam device upon being lowered into the water will probably be partially submerged by selective flooding of the tanks or pontoons 76. As the result of the partial submergence of the device at the offshore site, end sections 74 thereof may tend to pivot upwardly and inwardly in opposite directions relative to each other towards the top or channel-shaped recess of intermediate section 72 whereby the device will be usually in a somewhat collapsed rather than the extended position shown in FIG. 5 when presented to the elongated structure 14.

Various methods can be employed for emplacing partially submerged device 22 about the parts forming a given joint J₁ of structure 14. To this end, various chambers in the chambered pontoons 76 of the device are selectively flooded and/or evacuated by way of the valves 78 in order to first sink the device 22 to a level below the bottom of joint J₁ of structure 14. It is to be understood of course that device 22 will also have being advanced through the water in a suitable manner until it can be disposed beneath the section parts making up joint J₁. A series of cables 114, usually four, suspended from a hoist (not shown) and mounted on a barge (also not shown) is disposed on opposite sides of adjoining legs 24 at the abutting ends thereof which form the joint so that the lower ends of the four cables can be connected to eyelets 112. When cables 114 are hoisted upwardly upon operation of the hoist (not shown), submerged device 22 can be correspondingly hoisted upwardly until it fits about the abutting ends of adjoining legs 24 at joint J₁, in the manner illustrated in FIG. 6.

During the maneuvering of the submerged device toward and about the outer peripheral portions of the legs in the area of the selected joint of the structure by means of cables 114 of the hoist (not shown) in order to isolate the joint area from the surrounding water pontoons 76 can be selectively evacuated to increase the buoyancy of the device. If necessary, a plurality of circumferentially spaced U-shaped brackets 116 can be provided on the adjoining legs 24 of structure 14 and a plurality of U-shaped brackets 96 can be attached to outer surface portions of various flanges of sections 72 and 74 of device 22 whereby cables 117 as indicated by dotted lines in FIG. 6 can extend between brackets 116 and 96 so as to further tiedown the device relative to structure 14 during placement of the device thereabout. As indicated in FIGS. 5-6, device 22 is advantageously of such a size whereby while partially embracing the extended structure at joint J₁ it will also extend well above the top surface of the water.

L-shaped bracket assemblies 118 may be provided for anchoring the cofferdam device 22 to the structure when the device is in engagement therewith, as illustrated in FIG. 7. Cables 119, as indicated by dotted lines in FIG. 6, can extend between eyelet assemblies 112 so that device 22 is securely held in watertight engagement about the periphery of the structure in the area of the joint. The open-ended chamber C defined by opposed portions of device 22 and structure 14 can be evacuated of water by an appropriate pump device (not shown) whereby the operator can enter the water-free chamber in order to weld the section 10 together by way of fillet welds 120 in the joint J₁.

As indicated in FIG. 8, the abutting edges of adjoining legs of a given joint J₁ of structures 10 can be advantageously provided with aligned beveled edge portions 121 preferably on opposite sides of the abutting edges of adjoining legs in the area of the joint J₁. The beveled edge portions define opposed depressions or valleys for receiving individual deposits of welding material during formation of weldments 120 on opposed sides of the abutting edges of adjoining legs 24 at joint J₁.

A preferred method of welding the abutting ends of adjoining legs of the joint together will now be set forth. The operator by an appropriate arc-welding tool (not shown), selectively first makes spaced tack welds on abutting beveled edge portions 121 on either the inside or outside of adjoining legs of the structures at a joint J₁ thereof after the abutting edges of the opposing legs have been disposed in the desired opposed positions. After making spaced tack welds, so as to temporarily lock the various elements together, the operator can next completely form a weld bead 120 about the entire circumference of abutting ends of adjoining legs of the structures on opposite sides thereof as shown in FIGS. 6 and 8. Prior to tack welding the abutting edges of adjacent legs of structures 10, overlapping longitudinal edges of guide elements 56 may be tack welded or otherwise seam welded, as indicated at 122 in FIG. 6, to the overlapped leg of adjoining legs of structures 10 at the joint thereof thereby enabling guide elements 56 to additionally function as splice plates during the final precision welding of the joint. In order that proper engagement is assured between abutting edges of adjacent ends of adjoining legs of structures 10, an extensible actuator 72 of one or more clamping devices 21 is maintained at a predetermined pressure by a control circuit (not shown) despite any separation or compression forces acting across the joint at the offshore site during formation of the structure. Thus, the guide elements 56 in conjunction with one or more clamping devices 21 across a given joint guarantee formation of a precise sound and generally uniform welding of the entire joint that will meet the overall strength requirements of an extended structure 14 as ultimately used at an offshore site.

After final welding of the adjoining legs at each joint in the fashion aforedescribed of all legs and structures 10 and disconnection of device 22 from structure 14, extended structure 14 is made up of two or more sections 10 can be upended and lowered into the body of water at the offshore site upon selective flooding of legs 24 thereof. The lower end of the upended structure can be anchored to the earth's surface beneath the body of water in a manner well known in the art.

In another advantageous embodiment of the instant invention, a slightly modified form of extended structure 14' having two joints J₁ and J₂ is generally comprised of a pair of structural sections 10 and 10' each of different cross-sectional configuration as shown in FIG. 9. Reinforcing elements 124 extending between and interconnected to certain portions of the pair of sections 10 and 10' can be provided to increase the overall strength of the extended structure upon fabrication thereof at the offshore site in accordance with the teachings of the instant invention.

Although the polyhedrally shaped structural sections of extended structure 14 or 14' have been of generally triangular configuration in transverse section, they could be of any suitable polyhedral shape in transverse section such as rectangular, square, hexagonal, etc.

Advantageous embodiments of the instant invention have been disclosed and described. It is obvious that various changes and modifications may be made therein without departing from the appended claims, wherein:

Claims

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.

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.

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.

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.

22. The cofferdam element of claim 19 further comprising pontoon means for controlling the overall buoyancy of said element during flotation thereof.

23. The cofferdam element of claim 19 further comprising means for anchoring said cofferdam element to said modular platform section.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.