IE41551B1 - A gravity platform and a method used in its construction - Google Patents

Abstract

1512025 Offshore structures; gravity platforms BOUYGUES SA 2 June 1975 [25 June 1974] 23808/75 Heading E1H A gravity platform comprises a deck 2 which is supported by a plurality of vertical reinforced concrete columns P and a base I which includes two walls 5, 6 defining an annular space in which lower cylindrical portions 7 of the support columns P and further reinforced concrete columns A are located, the lower cylinder portions of the columns being integrally formed with the walls 5, 6. The platform is built by first constructing a lower portion of the base comprising a horizontal slab 3 externally of the portions 7 and extending between the portions 7 and the walls 5, 6, lower parts of the walls 5, 6 and cylinder portions 7 in a dry dock and then floating out this lower portion of the base for subsequent completion afloat. Alternatively sections of this lower base portion are built separately in a dry dock and then progressively connected together afloat. The base is completed afloat by further casting concrete and then frusto portions 9, 10 of the support columns P and frusto conical portion 12 and upper portion 11 of the further columns A are built onto the floating base 1. The further columns A are sealed at their upper ends and carry floatation collars 19. The deck 2 is built in a dry dock and has upper portions 18 of the support columns P subsequently fixed temporarily in passages 14. The base 1 is submerged and the deck 2 is located above it so that the support column portions 10, 18 can be permanently connected together. The deck 2 is moved relative to the column portions 18 by either submerging the base 1 further or raising the deck 2 and, when in the desired position, it is then permanently connected to the columns P.

Description

The present invention relates to a gravity platform, and a method used in its construction.

Gravity platforms are normally used for offshore work at sea.

More particularly the invention relates to a gravity platform made of reinforced concrete and having a base to be grounded on the bottom beneath a water surface, the upper part of the platform including a deck located above the surface.

The gravity platform comprises a deck supported by a plurality of hollow reinforced concrete columns having lower cylindrical portions which form part of the reinforced concrete base.

In use the base is normally at a depth of 100 to 200 metres with the deck approximately 30 metres above the water surface.

According to the invention a gravity platform comprises a deck supported by a plurality of hollow support columns of reinforced concrete and a reinforced concrete 2o base having txjo vertical walls which define an annular space therebetween in which lower cylindrical portions of the hollow columns are located, the lower cylindrical portions being integrally formed with the two walls.

Preferably, the base further comprises a lower horizontal slab externally of said lower portions and - 3 extending therebetween and said walls, and an upper horizontal slab which extends across the whole of the annular gap between the walls.

Preferably each column is closed at its lower end by an internal transverse wall which is recessed with respect to the lowermost open end of the cylindrical portion of the column.

One method used in the construction of the gravity platform comprises manufacturing in a dry dock, a first part comprising the lower slab of the base, lower portions of the said vertical walls, and lower portions of said cylindrical portions, and then floating this first part of the platform out of the dry dock and completing the construction of the floating base by casting concrete Another method comprises manufacturing, in dry dock sections of a first part of the platform, each section comprising a lower portion of a said cylindrical portion integral with lower portions of parts of the two vertical walls and part of said lower slab, floating each section out of the dry dock, connecting the floating sections together, and then completing the construction of the floating base by casting concrete.

Preferably the base further comprises lower cylindrical portions of hollow further columns which do not support the deck, said further lower cylindrical portions being interposed between the first mentioned lower cylindrical portions.

A platform formed according to the invention and its method of construction will now be described by way of example with reference to the accompanying drawings, in which:a * ts w J- 4 Figure 1 is a plan view of a gravity platform formed according to the invention; Figure 2 is a diagrammatic section on the line II-II of Figure 1; g Figures 3 to 5 show a method of constructing the lower part of the base of the platform, this lower part being shown in plan in Figure 3, in diagrammatic vertical section in Figure 4 taken on the line IV-IV of Figure 3, and afloat in Figure 5; Figures 6 to 10 show an alternative method of constructing the lower part of the base of the platform. Figures 6 and 7 illustrating respectively an initially fabricated section of the lower part of the base in plan view and in diagrammatic section on line VII-VII of Figure 6, Figure 8 illustrating several assembled sections in plan view and Figures 9 and 10 illustrating respectively diagrammatic vertical sections on the lines IX-IX and X-X of Figure 8; Figure 11 is an enlarged and diagrammatic frag20 mentary vertical section on the line XVIII-XI in Figure 19, of the base during a further step in its manufacture when afloat; Figure 12 is a similar view to Figure 11 showing the completed base of the platform afloat; Figures 13 and 14 are fragmentary and diagrammatic vertical sections, on the line XVIII-XI in Figure 19, showing the construction of the columns on the floating base. Figure 14 showing the height to which the columns are built before being connected to the deck; Figures 15 to 17 show respectively the deck of the platform when viewed from beneath, in vertical section oil l ine XVI-XVI of Figure; 15, mid‘in vi-iLl e.-ij occtiou when I Io.iliig·’ Figure 18 is a diagrammatic section on the line XVIII-XVIII of Figure 19 showing a late stage in the assembly of the support columns and the connection of the deck to the base; Figure 19 is a plan view of the deck and base shown in Figure 18; and Figures 20 to 23 show in diagrammatic vertical section various stages in the final positioning of the deck oh the support columns.

The platform comprises a base 1 (Figure 2) of reinforced concrete, a deck 2 and columns which partially form the base at their lower ends and support the deck at their upper ends.

The base comprises two vertical, circular, concentric walls 5 and 6 between which are disposed twelve hollow cylindrical portions 7 closed at their lower ends by internal transverse walls 8 having convex undersides, a lower slab 3 at the level of the walls 8 and extending between the portions 7 and the walls 5 and 6, and an upper slab 4 extending across the whole of the annular gap between the wall 5 and the wall 6. The cylindrical portions 7 are formed integral one with another and with the walls 5 and 6, and the slabs 3 and 4 are also formed integral with the cylindrical portions 7 and the walls 5 and 6.

The cylindrical portions 7 are the lower ends of twelve vertical columns of which eight columns P support the deck and four further columns A serve firstly to increase the mass of the immersed part of the platform and secondly serve as a support for floats during the construction of the platform as will be described - 6 41551 hcreinaftc r.

In the drawings, the diameter of the columns at their upper ends is approximately one third of the diameter of the cylindrical portions 7.

Each support column P comprises the lower cylindri cal portion 7, an upper cylindrical portion 18 and inter mediate frustoconical portions 9 and 10 between the two end portions 7 and 18.

Each further column A comprises the lower cylindri cal portion 7, an upper cylindrical portion 11 and an intermedia te frustoconical portion 12 between the lower portion 7 and the upper portion 11.

To give an example of the size that the off-shore platform can be, the base 1 can have a height of about metres, the diameter of the cylindrical portions 7 can be in the range 17 to 22 metres, the frustoconical portions 9 and 10 each can have a height of about 36 metres, the frustoconical portion 12 can have a height of about 44 metres, the upper cylindrical portion 11 can have a height of about 40 metres, and the upper cylindrical portion 18 can have a height of about 80 metres.

Two methods of constructing the base will now be described with reference to Figures 3 to 10.

In a first method shown in Figures 3 to 5, the lower part of the platform's base constituted by the lower slab 3 of the base, by lower wall portions of the vertical walls 5 and 6, by the lower ends of the cylindrical portions 7 of the columns, and by the inter30 nal transverse walls 8 is manufactured in a dry dock D. The cylindrical portions 7 are integral one with another as indicated at 7'. 41S51 This lower part of the base can have a height of about 10 metres, for example.

The dry dock D is is injected into spaces of the columns in order base buoyant. then flooded and compressed air 13 under the internal walls 8 to render the lower part of the In an alternative to the first method of construction shown in Figures 6 to 10, sections of the lower part of the platform's base are manufactured in dry dock D, each section comprising the lower end of a cylindrical portion 7 integral with adjoining sections of the lower wall portions of the vertical walls 5 and 6 and portions of the lower slab 3 as shown in Figures 6 and 7. The dry dock D is flooded and compressed air is injected, under the internal wall 8 of the section, in the space 13 in order to float out the section. The progressive assembly of the floating sections is carried out by conventional means, firstly by connecting together the portions of the lower slab 3 and secondly the adjacent portions of the walls 5, 6 as well as the portions 7 arranged side-byside as shown in Figure 8 After the first part of the base has been constructed by one of the above methods, construction of the base is continued afloat by the subsequent casting of concrete into sliding shuttering, the base sinking as the concrete is cast. Stability is ensured by the cylindrical portions 7 which, since they are hollow and closed at the bottom, form floats. Figure 11 illustrates a stage reached in the operation of continuing the construction of the base, and Figure 12 diagrammatically shows the finished base after the upper covering slab 4 is formed.

When the base has been built, the lower frusto51 conical parts 9, 12 of the columns are constructed on top (Figures 13 and 14). On these lower parts 9 and 12, construction of the upper frustoconical parts 10 of the support columnsand·the upper portions 11 of the further columns is performed.

The upper ends of the columns A are permanently closed and those of the support columns are provisionally closed.

The deck is manufactured in a dry dock D as shown in Figures 15 and 16, with either through or initially blind vertical passages 14, and with a recess 15 in the underside.

The passages 14 are intended to receive the support columns P and the recess 15 is intended to facilitate the injection of compressed air for rendering the deck buoyant.

The upper portions 18 for the columns P are tempo rarily connected to the floating deck, only one portion 18 being shown in Figure 18, and are kept in place in the passages 14 of the deck by any appropriate means providing temporary connections.

To connect the deck to the base, the base is submerged by appropriately ballasting the portions of the columns on the base and by providing the further columns A with floats 19 in order to regulate the depth of submersion.

The deck is next positioned with respect to the sibmerged base as shown in Figure 18 and the base is then raised to introduce the upper ends of the column portions 10 of the support columns into the passages 14, after which these ends are connected to the lower ends of the column portions 18 as shown in Figure 20.

Finally the temporary connections between the column portions 18 and the deck 2 are released and the deck is moved upwards along column portions 18 to its desired height with respect to the base. Figure 21 shows one stage of this operation and Figure 22 shows the deck in its final position.

Alternatively, the same result is achieved by submerging the base by appropriate ballasting in the column portions associated therewith.

The deck 2 and column portions 18 are permanently connected together when the deck is at the desired position on the columns.

Once the floating platform has been brought to its final location, it is grounded on the bottom by appropriately ballasting the columns as shown in Figure 23.

When the platform is in its final position, concrete may be injected between the base of the columns and the ground to consolidate the foundations.

The invention is not limited to a specific number of columns P and A as long as there are sufficient support columns to adequately support the deck 2, but preferably the number of columns is between six and twelve, of which between two to four are further columns A.

The two vertical walls 5 and 6 are preferably circular, but this shape is in no way limiting provided that they define an annular space.

Claims (21)

1. CI AIMS:1. A gravity platform comprising a deck supported by a plurality of hollow support columns of reinforced concrete and a reinforced concrete base having two vertical 5 walls which define an annular space therebetween in which lower cylindrical portions of the hollow columns are located, the lower cylindrical portions being integrally formed with the two walls.

2. A platform according to claim 1, in which said 10 base further comprises a lower horizontal slab externally of said lower portions and extending therebetween and said walls.

3. A platform according to claim 1 or 2, in which said base comprises an upper horizontal slab which ex15 tends across the whole of the annular gap between the walls.

4. A platform according to any one of claims 1 to 3, in which each column is closed at its lower end by in internal transverse wall which is recessed with respecs 20 to the lowermost open end of the cylindrical portion of the column.

5. A platform according to claim 4, in which the underside of each transverse wall is convex.

6. A platform according to any one of claims 1 25 to 5, in which said two vertical walls are both circular.

7. A platform according to any one of claims 1 so 6, in which said base further comprises further lower cylindrical portions of hollow further columns which 30 do not support the deck, said further lower cylindrica. portions being interposed between the first mentioned lower cylindrical portions.

8. Λ platform according to claim 7, comprising six to twelve columns of which two to four are further columns.

9. A platform according to any one of claims 1 to 8, in which the deck is manufactured of reinforced concrete and then fitted as a single unit on the support columns.

10. A method used during the construction of a gravity platform according to claim 2 or any of claims 3 to 9 when appended to claim 2, comprising manufacturing in a dry dock, a first part comprising the lower slab of the base, lower portions of the said vertical walls, and lower portions of said cylindrical portions, and then floating this first part of the platform out of the dry dock and completing the construction of the floating base by casting concrete.

11. A method used during the construction of a gravity platform according to claim 2 or any of claims 3 to 9, when appended to claim 2, comprising manufacturing in a dry dock, sections of a first part of the platform, each section comprising a lower portion of a said cylindrical portion integral with lower portions of parts of the two vertical walls and part of said lower slab, floating each section out of the dry dock, connecting the floating sections together, and then completing the construction of the floating base by casting concrete.

12. A method according to claim 10 or 11, of constructing a gravity platform according to claim 7 or 8, 4155 1 - 12 or claim 9 when appended to claim 7 comprising building the support and further columns by casting concrete on the portions of the columns of the completed base, until the further columns are completed and until the support 5 columns are at a height lower than that of the further columns and then closing off the upper ends of the columns.

13. A method according to claim 12, comprising t'ie positioning of floats on the further columns. IO

14. A method according to claim 12 or 13, comprising the separate manufacture in a dry dock of a buoyant deck having passages for receiving the support columns.

15. A method according to claim 14, comprising the installation of upper portions of the support columns 15 in said passages, after the deck has been floated out of the dry dock, and while the deck is flbating.

16. A method according to claims 13 and 14, further comprising the steps of immersing the base until the upper ends of the support columns are submerged, 20 floating the deck into a position above the base to bring the passages in the deck adjacent to and above these upper ends, raising the base and passing said upper ends into said passages, connecting said upper ends to said upper portions of the support columns, displacing the 25 deck relative to the completed support columns, and connecting the deck to the support columns when the deck has reached its desired position With respect to the base.

17. A method according to claim 16, in which 30 said relative displacement is obtained by immersing said base.

18. A method according to any one of claims 10 to 17, comprising the injection of compressed air under the transverse walls of the columns in order to render buoyant the base or part or parts thereof, and/or the 5 injection of compressed air under the deck in order to render the deck buoyant.

19. · A method according to any one of claims 10 to 18, comprising the use of the hollow columns as ballast chambers for varying the level of the base in 10 the water.

20. A gravity platform, substantially as hereinbefore described with reference to the accompanying drawings.

21. A method of constructing a gravity platform, 15 substantially as hereinbefore described with reference to the accompanying drawings.