Offshore structure

Abstract: A braced offshore structure (10, 12) can be constructed on land on a single unit in a compact bundle before being transported on a barge to an installation site. After the compact unit is launched, it is unfolded and ballasted before being installed upon the ocean floor. The structure (10) comprises an elongate central tower (16) having a plurality of braces (18) pivoted (20) thereto. The braces (18) are initially positioned generally parallel to the central tower (16) within recesses in the central tower (16) but after the braces (18) are unfolded, they extend at an angle to the central tower (6). Struts (26) mounted on slide assemblies (30) can be provided between the braces (18) and the tower (16).

Description: The invention relates to an offshore structure. As is well known, fabricating structures on land is less expensive and risky than doing the same work offshore. However, during the construction of offshore structures, some offshore assembly time is generally required. To minimize costs, structures are often subdivided into a few large components. One such component of an offshore structure is the substructure, which generally extends from the sea floor to an elevation above the water surface. The substructure is normally fabricated on its side onshore, skidded onto a barge, towed to the site, launched from the barge, upended to a vertical orientation while floating and set on the bottom by ballasting. Piling is driven into the sea floor through features in the substructure after which the substructure is secured to the driven piling by grouting, welding, or other mechanical means. In most cases, the substructure is a cantilever tower, but there are some circumstances, however, when a braced tower is more efficient since it can be a lighter structure. In order to realize savings from the lighter weight of the braced tower, a low risk, economical method of fabrication and installation is required. Methods already have been developed which involve assembling a braced tower at an offshore site. Under these methods, the central tower and the braces, which are each independently fabricated onshore, are separately transported to the offshore site. After launching, each brace is connected to the tower thereby completing the substructure. Assembling the substructure in this fashion is both expensive and risky because of the length of time required and because a storm can strike at any time. Patent Specification FR-A-1 450 410 discloses a submersible construction according to the features of the preamble of claim 1 and comprising a support column with perimeter braces pivotally joined thereto and positional alongside the column for transport to an installation site, operating means being provided to pivot the braces to positions angled away from the column. According to the invention there is provided an offshore substructure to support a superstructure above a waterline comprising: an elongate central tower extending from a mudline, where it is anchored in place, to above the waterline, and upon which the superstructure is supported; a plurality of perimeter braces each pivotally connected to the tower and extending to and anchored to the mudline; and operating means to pivot the braces from a bundled position generally parallel to the tower to an unbundled position at an angle to the tower; wherein the central tower terminates above the mudline, thereby forming a gap between the central tower and the mudline; skirt piles secure the tower to the mudline across the gap between the central tower and the mudline; and the braces nestle alongside or are folded into recesses in the central tower, so that in the bundled position the tower and the braces form a unitary compact bundle. Such a braced tower can be fabricated entirely onshore in a compact bundle, transported to an offshore installation site, placed in the water, and opened to its final configuration while floating. An absolute minimum of offshore assembly and/or construction time need be required. The structure can be a compact substructure that can be towed to the site and launched without requiring elaborate reconfiguration of the transport vessel. The structure can provide sufficient clearances for piling to be driven to anchor the unfolded structure in place. The cross section of the tower with the recesses into which the braces are folded, can provide that no elements of the bundle protrude into space during fabrication and transportation of the bundle which would hinder such fabrication and transportation. The invention is diagrammatically illustrated by way of example in the accompanying drawings in which:- Figure 1 is an pictorial view of an offshore structure according to the invention secured to the ocean floor; Figure 2 is a plan view, without a superstructure, of the offshore structure of Figure 1; Figure 3 is a plan view of a strut of the offshore structure of Figure 1 taken on line 3-3 of Figure 8; Figure 4 is a sectional view of the strut taken on line 4-4 of Figure 3; Figure 5 is a plan view of a hinged connection taken on line 5-5 of Figure 9; Figure 6 is a sectional view, partially cut away, taken on line 6-6 of Figure 5; Figure 7 is a sectional view, partially cut away, taken on line 7-7 of Figure 6; Figure 8 is a partial elevation view (only one brace is shown) taken on line 8-8 of Figure 2 and showing the offshore structure of the invention in its unfolded position; Figure 9 is a partial elevation view (only one brace is shown) illustrating the offshore structure of the invention in its folded position; Figure 10 is a sectional view of the offshore structure on a transport barge and in its folded position taken on line 10-10 of Figure 9; Figure 11 is a sectional view of the offshore structure on the transport barge and in its folded position taken on line 11-11 of Figure 9; Figure 12 is an elevation view, partially cut away, taken on line 12-12 of Figure 10; Figure 13 is an enlarged detail view, partially cut away, of a portion of Figure 12. Figure 14 is an enlarged detail view, partially cut away, of a portion of Figure 12; Figure 15 is a sectional view, partially cut away, taken on line 15-15 of Figures 13 and 14; Figure 16 is a schematic view, partially cut away, taken on line 16-16 of Figure 14; Figure 17 is a schematic view of the operation of a slide assembly of the offshore structure of Figure 1; Figure 18 is an enlarged detail view, partially cut away, of a portion of Figure 3; Figure 19 is a sectional view of the offshore structure on the transport barge and in its folded position taken on line 19-19 of Figure 9; Figure 20 is a section view of the offshore structure on the transport barge and in its folded position taken on line 20-20 of Figure 9; Figure 21 is an enlarged detail view, partially cut away, of a portion of Figure 19; Figure 22 is an enlarged detail view, partially cut away, of a portion of Figure 19; Figure 23 discloses an alternate cross-section of a bundled offshore substructure according to the invention; and Figure 24 discloses another alternate cross-section of a bundled offshore substructure according to the invention. As can be seen from the drawings, a substructure 10 is a braced structure that is constructed as a compact bundle for towing purposes but which is unfolded while floating in the water before being installed on the ocean bottom. The advantage of being able completely to assemble such a braced structure on land should not be overlooked in view of the fact that earlier braced structures required a considerable amount of offshore construction. An additional feature is the ability of this compact bundle to be loaded onto a transport barge such that is rests upon parallel rows of transport barge ways. Since the ways are in parallel rows, they can guide and/or support the structure as it slides by during the launching operation (presuming, or course, the structure is not lifted into place). Instead, if the ways are not in parallel rows, they will interfere with the launching operation because they are unable to support the structure as it moves by (i.e. they would no longer be aligned with the support points on the structure). A further feature is the capacity of individual braces to nestle alongside or be folded into recesses in the central tower (this capability being a factor of the geometry of the central tower). These recesses provide room and sufficient clearance so that the braces do not extend outward from the compact bundle to such a degree that they interfere with the loading of the substructure 10 upon the transport barge. Referring now initially to Figures 1 and 2, there is shown the substructure 10 supporting a superstructure 12 above a waterline 14. The substructure 10, in this embodiment, comprises a central tower 16 and three outwardly extending braces 18. The braces 18 support the tower 16 by restraining the tower 16 in place against the effects of any external forces incurred by either the substructure 10 or the superstructure 12. Both the tower 16 and the braces 18 can be open trusses as illustrated herein. Each brace 18 has one end secured to the tower 16 by means of a hinged connection 20 while the other end of each brace 18 is angled away from the tower 16 when in the unfolded position. This end engages a mudline 22 where skirt piles 24 anchor each of the braces 18 in place. As shown, the tower 16 is also anchored in place by skirt piles 24. As shown in this embodiment (but which is not necessary in every embodiment), struts 26 are provided to assist in the support of the superstructure 12 and to increase the strength of both the tower 16 and the braces 18. Each strut 26 (which is shown as an open truss in Figures 3 and 4) extends generally horizontally between a respective brace 18 and the central tower 16. It connects at an elevation intermediate the hinged connection 20 and the mudline 22 of each of the tower 16 and the braces 18. The struts 26 effectively decrease the unbraced length of both the tower and the braces 18 such that their design may be minimized (a longer unbraced length requires more material thereby making the structure heavier and bulkier). Figures 5, 6 and 7 show in greater detail the specifics of the hinged connection 20 between each of the braces 18 and the tower 16. Other configurations are also possible, depending upon the forces involved, the sizes of the braces and the configuration of tower 16. In this embodiment, the tower 16 is configured having a hexagonal cross-section, but other cross-sectional shapes are equally likely. In theory, any cross-sectional shape that can accommodate a series of the perimeter braces 18 is workable. One important factor is the ability of each such brace 18 to be constructed in its folded position alongside the central tower 16 with sufficient clearance between the adjacent braces for the substructure 10 to be supported upon a transport barge 28. It should here be noted that Figures 5 illustrates the hinged connection 20 in its folded position which is how it will appear during fabrication onshore and upon the transport barge 28 during the towing operation. Referring now to Figures 8 and 9, one possible embodiment of the installation procedure of the substructure 10 is disclosed. Figure 8 illustrates the substructure 10 in its unfolded or installed position while Figure 9 shows the tower 16, one of the braces 18 (the others would operate identically) and the respective strut 26 in their folded or towing and launching operation. As can be surmised, after launching and while the substructure 10 is floating, each of the braces 18 is unbundled and pivoted away from the tower 16 via the respective hinged connection 20. The accomplish this, each strut 26 is moved from its folded position parallel to the tower 16 to its unfolded position generally perpendicular to the tower 16. This repositioning of the strut 26 is made possible by means of a slide assembly 30 secured along the tower 16 and by means of a hinged connection to the respective brace 18. The details of the slide assembly 30 are shown in Figure 10 to 17 while the hinged connection between the brace 18 and the strut 26 is shown in Figure 18. Shown more specifically in Figure 8, the bottom of the tower 16, while anchored to the mudline 22 via the skirt piles 24, does not generally come into contact with the mudline 22. Instead, the tower 16 is supported just slightly above the mudline 22 thereby creating a gap between the two. This is to allow the braces 18 fully to engage the ocean bottom without any hindrance from the central tower 16. As shown in this embodiment, the slide assembly 30 comprises an open truss 32 spanning between adjacent legs 34 of the tower 16. The truss 32 is generally a rectangular planar structure that slides within two parallel slide rails 36 which are attached to the legs 34. Each of the slide rails 36 extends partially along each of the legs 34 with a series of plates 38 securing each slide rail 36 to its respective leg 34. Four slide blocks 40, one secured to each corner of truss 32, are configured to slide within the slide rails 36 thus, as the slide blocks move within the rails 36, so does the truss 32. Immediately below the truss 32 and also secured to the lower blocks 40 is a hinge 42 which connects between these lower blocks 40 and one end of the strut 26 (Figures 3 and 14). In this fashion, as each truss moves downward along the rails 36, the strut 26 is pivoted from a position generally parallel to the tower 16 to a position generally perpendicular to the tower 16 via the hinge 42 and the hinged connection between it and its respective brace 18. Such movement of the strut 26 consequently pivots this brace 18 away from the tower 16 about the respective connection 20 as can be expected to occur. In addition to moving the strut 26 downward, the slide assembly 30, being confined between the slide rails 36, prevents the strut 26 from twisting during this unfolding operation. The operation of the slide assembly 30 is shown diagrammatically in Figure 17. While only one method of operation is shown herein, other similar methods may be contrived to achieve the same result, such as by the use of hydraulic cylinders. In fact, in some cases, there will be no need for either struts 26 or slide assemblies 30 for operation. In these instances, the unfolding operation will occur simply by rotating the substructure 10 while it is floating upright in the water and by selectively de-ballasting the braces 18 one at a time thereby causing each such brace 18 to pivot upwards towards the waterline 14. After the desired angle is achieved, the brace can simply by locked in place before the substructure 10 is flooded and installed on the ocean floor. However, in the embodiment illustrated in these drawings, one end of a cable 44 is secured to the upper block 40 while the other end of the cable 44 is secured to the respective lower block 40. Intermediate these connections, the cable 44 is routed through a series of turning sheaves 46 and a capstan 48. Since there are three braces 18 with each brace having it own slide assembly 30 and with each slide assembly 30 requiring two cables each, there is needed a total of six cables 44 for operation. These six cables 44 are ultimately coupled to a single lifting cable 50 that extends above waterline 14. Thus, as the lifting cable 50 is pulled upward by means such as a winch, this upward movement causes all of the cables 44 to move the three slide assemblies 30 downward simultaneously. This in turn uniformly pivots the struts 26 outward thereby forcing the respective braces 18 also to pivot outward about the respective hinged connection 20 thereby unfolding all of the braces 18 in unison. Additionally, once each strut 26 is completely unfolded and in a position generally perpendicular to the tower 16, it acts as a lock to prevent the respective brace 18 from refolding or pivoting back against tower 16. Upon completion of the unfolding operation, the substructure 10 is ballasted to settle on the ocean bottom, and then anchored in place via the skirt piles 24. It should here be noted that the cross-section of each brace 18 does not remain constant throughout its length. Instead as indicated in Figures 10 and 11, provisions are made to accommodate the configuration of the struts 26 so that the substructure 10 can be folded into a compact shape for transportation. Figures 10 and 11 illustrate the strut 26 nestled against the brace 18 as the substructure 10 is in its folded position upon the transport barge 28. Referring now to Figures 19 to 22, there is shown additional sectional views through the folded substructure 10. In Figure 19, the position and orientation of the substructure 10 upon the transport barge 28 is illustrated. Also shown are details of how the braces 18 are confined in the folded position by cables 52 and spacer stubs 54. After the substructure 10 is placed in the water, the cables 52 are cut thereby enabling the braces 18 to pivot outward. As can be seen, this compact arrangement provides sufficient clearance between any part of the substructure 10 and the transport barge 28. Figure 20 discloses various skirt pile sleeves 56 attached to both the central tower 16 and the braces 18 and through which the skirt piles 24 are driven into the ocean bottom. As also indicated, this portion of the substructure 10 extends beyond the stern of the transport barge 28 thereby being cantilevered out over the water so as to accommodate the extending skirt piles sleeves 56. As stated earlier, the cross-section of the central tower 16 need not be hexagonal as illustrated in the above embodiment. It can, if desired, be of a cruciform shape (Figure 23) or it can be octagonal (Figure 24) or it can be of some other shape. In any event, the braces 18 are pivotally secured to the central tower 16 so that upon launching, they can be unfolded and secured to the ocean bottom. Furthermore, by the configuration of the folded substructure 10, the transport barge 29 is provided with parallel launch ways 58 which makes loadout and launching considerably easier than would be possible if the structure had non-parallel legs 34. In conclusion, then, one important feature is a central tower 16 having a cross-section that includes recesses into which braces 18 may be folded thereby forming a compact bundle for the fabrication and transportation operations. Furthermore, the bundle preferably has no protrusions that will hamper fabrication and/or transportation.

Claims: An offshore substructure (10) to support a superstructure (12) above a waterline (14) comprising: an elongate central tower (16) extending from a mudline (22), where it is anchored in place, to above the waterline (14), and upon which the superstructure (12) is supported; a plurality of perimeter braces (18) each pivotally connected to the tower (16) and extending to and anchored to the mudline (22); and operating means to pivot the braces (18) from a bundled position generally parallel to the tower (16) to an unbundled position at an angle to the tower (16); characterised in that the central tower (16) terminates above the mudline (22), thereby forming a gap between the central tower (16) and the mudline (22); skirt piles (24) secure the tower (16) to the mudline (22) across the gap between the central tower (16) and the mudline (22); and the braces (18) nestle alongside or are folded into recesses in the central tower (16), so that in the bundled position the tower (16) and the braces (18) form a unitary compact bundle. An offshore substructure according to claim 1, further comprising skirt pile sleeves (56) secured to the lower end region of each of the braces (18) and the tower (16), through which skirt pile sleeves (56) the skirt piles (24) extend to anchor the substructure (10) in place. An offshore substructure according to claim 1, further comprising at least one respective strut (26) pivotally secured to each of the braces (18) and the central tower (16), the strut (26) being positioned intermediate the mudline (22) and the pivotal connection (20) between the tower (16) and the brace (18). An offshore substructure according to claim 3, wherein the operating means comprise slide means (30) secured to the tower (16) to slide each of the struts (26) along the tower (16) thereby to move the respective strut (26) from a folded position which is generally parallel to the tower (16) to an unfolded position which is generally perpendicular to the tower (16). An offshore substructure according to claim 4, wherein each of the braces (18) and its respective strut (26) is an open truss configured to nestle together alongside one another when the brace (18) is in its bundled position within the recess. An offshore substructure according to claim 5, wherein the operating means further comprise a cable (44) secured to the slide means (30) to move the slide means (30) thereby pivoting its respective strut (26) and hence its respective brace (18) outward. An offshore substructure according to claim 6, wherein each strut (26), when in its unfolded position generally perpendicular to the tower (16), effectively locks its respective brace (18) in its unbundled position thereby preventing the brace (18) from subsequently pivoting back towards the tower (16).

Inventors: HANEY JAMES ALLAN, MENARD STAFFORD JOSEPH, HANEY, JAMES ALLAN, MENARD, STAFFORD JOSEPH
Applicants: MCDERMOTT INT INC, MCDERMOTT INTERNATIONAL, INC.
Application year: 1991
Main classification: E02B17
IPC codes: E02B17