GB1561010A

GB1561010A - Floating concrete platforms for use at offshore locations

Info

Publication number: GB1561010A
Application number: GB32037/77A
Authority: GB
Original assignee: Dyckerhoff and Widmann AG
Current assignee: Walter Bau AG
Priority date: 1976-07-31
Filing date: 1977-07-29
Publication date: 1980-02-13
Also published as: NO153486C; NO153486B; DE2634622B1; JPS5327988A; FR2360462A1; FR2360462B1; NO772622L; DE2634622C2

Description

(54) FLOATING CONCRET PLATFORMS FOR USE AT OFFSHORE LOCATIONS (71) We, DYCKERHOFF & WIDMANN AKTIENGESELLSCHAFT, a German Body Corporate, of Sapporobogen 6,8000 Munich 40, German Federal Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- The present invention relates to floating concrete platforms, and more particularly to a floating concrete platform having a number of buoyant float members monolithically connected to the underside of a platform member.

At offshore locations, it is often a problem to provide a working or a trafic platform. Seaworthy platforms are necessary, for example, to sink boreholes for the exploration and extraction of oil and/or gas at offshore facilities which may be just off the coastline or at locations on the high seas. These offshore platforms are often provided with facilities for processing the raw materials and, in the case of natural gas, with apparatus for liquefying the gas so that it can be shipped. Such platforms usually include docking facilities for a ship and also landing facilities for airplanes, as well as capacity for storing the raw materials preferably in a quantity at least equal to the capacity of a large cargo vessel. Additionally, the storage facility should include the necessary transfer equipment for loading the raw materials into a vessel. Finally, shelters are required for the personnel working on the platform.

There are three principal types of offshore platforms. One type of platform which is constructed similarly to a pontoon has a buoyant platform member which floats on the water surface. Another type of platform is a so-called semi-submersible floating platform, where the platform member is supported on float members which are semisubmersed, i. e. they extend deep below the water surface so that they are not influenced to any marked extent by wave movement. The third type of platform is an artificial island which is supported on the sea bed itself.

In a pontoon-like platform which floats on the water surface, the surface area of the platform at the water line, that is, the base arqa of the platform member at the water level, is large compared to its general base area. As a result, the platform is vulnerably exposed to wind and wave movements which tend to cause vibrations to be generated within the platform. Such vibrations are unfavourable for the installation of the platform and for its operation.

In a semi-submersible floating platform on the other hand, which has a relatively deep draft, the platform member is connected to float members located deep below the water surface by relatively slender columns, so that the waves at the surface act directly only on the columns. The surface area of the platform at the water line is small compared to the base area of the float members themselves and accordingly, the influence of the waves on the floating stability of the semi-submersible floating platform is less than in the case of the pontoon-like platform described above. Semi-submersible floating platforms, however, have the disadvantage that they must be trimmed very accurately and they tend to react very sensitively to varying load distributions. Also, their manufacture is complex and expensive because the relatively deep draft required for the float members causes problems both in the manufacture and operation of the semi-submersible floating platform.

All of these types of off-shore platforms used in the past have the common characteristic of being extremely elaborate with regard both to their costs of manufacture and the type of manufacture involved. Due to the size of these installations and their draft, elaborate docking installations are usually required for their manufacture or at least locations along a shoreline having a great depth of water are needed. As a result, the installations cannot be built at any location and, in addition, they require a long period of construction.

Therefore, the present invention is concerned with providing a floating platform which, like the semi-submersible floating platforms is not exposed, or is exposed only to a minor extent, to vibrations caused by wave movements, yet which, unlike the semisubmersible floating platforms, has a relatively shallow draft so that it has much smaller material costs and can be built in a shorter period of time than a semi-submersible platform at locations where the water depth is relative shallow.

In accordance with the present invention, there is provided a floating concrete platform suitable for use at offshore locations, comprising a floating concrete platform suitable for use at offshore locations, comprising a platform member formed by a concrete slab or shell which is prestressed in two orthogonal horizontal directions, and a plurality of reinforced concrete float members arranged in an array below the platform member and monolithically connected thereto in order to support the platform member, each float member having an external shape constituted by at least a part of a closed body of revolution having a curved generatrix.

A floating platform according to the present invention is thus kept afloat on the water surface by the plurality of float members, which are preferably at least partspherical in form. The float members are partially immersed in the water, and due to the curvature of the generatrix of the body of revolution to which their shape corresponds, the surface of the float members at the water surface is considerably smaller than the base area of the platform itself viewed in plan. With such an arrangement, the force of the waves striking the floating platform is less than in the case of a onepart float member, that is, a pontoon-like platform of similar base area. If a platform with a base area of 150 X 150m is carried by 16 part-spherical float members, each having a diameter of 26m and a wall thickness of 0.20m, the area of the float member at the water surface is only about 38% of the base area of the platform and, accordingly, the force of the waves striking the floating platform is only about 38% of what it would be if a single pontoon of the same area as the olatform were used as the float member.

Preferably, the float members are connected directly with the lower surface of the slab or shell, the external shape of each float member having the geometrical shape of a sphere from which a minor segment has been removed to form an upper edge for the float member by which it is connected to the underside of the platform member.

Alternatively, each of at least some of the float members has a short cylindrical extension at its upper edge, which extension interconnects the upper edge of the part-spherical float member with the lower surface of the concrete slab or shell.

In one embodiment of the invention, the platform member may comprise a planar hollow slab having continuous upper and lower slab members. The interior of the hollow slab may be divided by bulkhead partitions into a number of hollow chambers. The partitions are preferably arranged in a narrow egg crate-like arrangement relative to the size of the platform.

In another embodiment, the platform member may comprise a shell which is curved in one or two orthogonal horizontal directions, that is, the longitudinal and transverse directions of the platform. Preferably in this case, the platform is in the form of a spherically shaped shell open at its upper side. Preferably the open space within the shell is divided by ribs into a number of individual compartments. The ribs may be arranged to extend radially and/or circumferentially of the centre of the platform, when the circumferential periphery of the platform is circular or polygonal.

The upper edges of the ribs may be arranged in a stepped arrangement extending outwardly from the centre of the platform giving the appearance of an amphitheatre.

In one embodiment, the periphery of the platform member is rectangular in shape, preferably square, and its float members are disposed in parallel rows. The platform member may instead have a polygonal periphery or, preferably, it can be circular, in which case the floats may be arranged in radially spaced concentric circles.

It is preferable to dimension the base area of the platform member to correspond substantially to the length of the greatest waves it is likely to experience at a particular offshore location at which it is to be used.

Both the float members themselves and the hollow spaces when provided within the platform slab, may be adapted for use as tanks for storing liquids, such as crude oil or liquid gas, or for holding ballast. Further, the platform member may be adapted for use as a route for traffic or as a runway as described below.

The floating platform can be used in any desired manner. Accordingly, it may be used as a support for boring frames and for plants for processing the raw materials and for the collection and liquefaction of natural gas. Further, the platform may also be used as a berth for tankers and other transport vessels. At the same time, however, as stated above the float members may be used as tanks for the intermediate storage of oil and natural gas and the safety of such an arrangement is greater than in comparable known installations, because the tanks thus formed by the float members can be separated from the installations on the platform by a solid concrete slab. If the platform has the dimensions given above, i. e. a base area of 150x150m, and carried by 16 part-spherical float members, each of a diameter of 26m and a wall thickness of 0.20 m, the platform may carry equipment, for example of 10, 000t for the liquefaction of gas, and 70,000t of crude oil or 140 cu m of liquid gas. This amount permits loading a tanker having a low capacity of 125,000cu m of liquid gas.

The platform may be constructed with a working surface or slab of any desired length. With such a slab, the platform may be used as a floating runway as stated above for airplanes which can be used at deep water locations, and which can be turned readily into the direction of the wind.

Another advantage of a floating platform according to the present invention is that it can be constructed without the use of a boat yard or drydock facilities in a simple manner and without any elaborate auxiliary equipment. Accordingly, the invention also concerns a method of producing a floating concrete platform according to the invention as defined above, comprising the steps of forming said plurality of the float members, arranging the float members in a predetermined array, and casting a platform member in the form of a concrete slab or shell over the upper ends of the float members so that the slab or shell is integrally connected to the upper ends of the float members thereby to form a floating platform.

The invention also concerns apparatus for carrying out this method comprising means for forming said plurality of reinforced concrete float members, means for arranging said float members in a predetermined array comprising a plurality of elongate basins arranged side by side and divided by jetties, the basins being at least in part below a free water surface so that said float members can be floated thereinto and then ballasted so as to be correctly located therein in said array. the basins being divided by jetties, and scaffolding which is movable over said basins, being supported by said jetties for use in casting said concrete slab or shell over the upper ends of the float members. Conveniently, said means for forming said float members comprises a moulding construction arranged adjacent a shoreline and a plurality of shaped depressions therein, each depression corresponding to the external shape of the bottom of one of the float members, climbing formwork which when constructed over each depres- sion constitutes a mould in which a float member can be cast, and lifting means for raising a finished float member from said mould transporting it to said shoreline, and lowering it into the water so that it can float therein.

Such apparatus has the advantage that the platforms, regardless of size, may be produced in a series manner in a short period of time with a relatively low cost compared to the construction of a ship of a similar volume. The cost advantage compared to a ship results from the small amount of building materials used, the low investment costs, and the possibility of producing the part-spherical float members in a series operation using simple and mechanized equipment, for example, climbing formwork, the float members themselves are formed as shells of reinforced concrete which are only stressed in compression.

Reference will now be made to the accompanying drawings which illustrate, by way of example, various embodiments of the present invention, by way of example, and of which: Figure 1 is a plan view with a portion broken away and shown in section, of one embodiment of a floating platform according to the present invention, the platform member thereof being in the form of a hollow slab; Figure 2 is a cross sectional view taken along the line II-II in Figure 1; Figure 3 is a plan view of another embodiment of a floating platform according to the present invention, with a platform member which has a square-shaped periphery in plan but which is part-spherical; Figure 4 is a cross sectional view taken along the line IV-IV in Figure 3 showing the curvature of the platform member in one direction, the left-hand side of the Figure 4 showing the platform floating when unloaded, the right-hand side showing it floating when fully loaded; Figure 5 is a plan view of still another floating platform embodying the present invention, of which the platform member is part-spherical and has a circular periphery in plan; Figure 6 is a cross sectional view taken along the line VI-VI in Figure 5; Figure 7 is an elevational view, partly in section, displaying a floating runway comprising a plurality of floating platforms embodying the present invention; Figure 8 is an underside view of the floating runway illustrated in Figure 7; Figure 9 is a diagrammatic plan view of apparatus for use in manufacturing float members for incorporation in the platforms shown in the above figures; Figure 10 is a cross sectional view taken along the line X-X in Figure 9 ; Figure 11 is a cross sectional view taken along the line XI-XI in Figure 9; Figure 12 is a diagrammatic plan view of apparatus for holding the float members in a predetermined array and for the construction of the platform over the float members; and Figure 13 is a cross sectional view iaken along the line XIII-XIII in Figure 12.

Referring to Figures 1 and 2, in Figure 1, a platform 1 comprises a platform member in the form of a planar hollow slab 2 having an upper slab member 3 and a lower slab member 4 which are interconnected and reinforced by bulkhead partitions 5.

A portion of the upper slab member 3 is broken away in Figure 1 for clarity. The upper slab member 3, the lower slab member 4 and the bulkhead partitions 5 are all constructed of concrete which is pre-stressed in two orthogonal horizontal directions i. e. in the longitudinal and transverse directions of the platform. Within the slab 2 are formed spaces or prestressed chambers 6 which can be used for various purposes, for example, for storage or for ballasting the platform with water.

The hollow slab 2 has a square periphery and is supported on a total of sixteen float members 7. Each float member 7 is in the form of a thin-walled hemisphere of re inforced concrete, and is monolithically, i. e. integrally connected at its upper dge 8 to the underside of the lower slab member 4 of the hollow slab 2. Suitable openings are provided in the hollow slab 2 so that the chambers 6 and also the chambers formed in the float members 7 themselves may be filled as desired, a typical opening 9 in the upper slab member 3 is identified in Figure 2 by way of example.

Another embodiment of a floating platform 11 is displayed in Figures 3 and 4, in which the platform member 12 is formed as a shell, rather than a slab as in the previous embodiment. The platform shell 12 is part-spherical, being curved in both the longitudinal and transverse directions, the periphery which is square, being provided with a reinforced border 13. The curvature of the platform shell 12 provides greater strength over a planar platform member with the rigidity of the floating platform being increased, while the material used is reduced. This particular platform is especially suitable for locations which are liable to experience great wave heights. The platform shell 12 is formed of concrete which is prestressed in both the longitudinal and transverse directions. Supporting the shell 12 are a plurality of float members 14 each having the shape of substantially tores quarters of a sphere, that is, each partspherical float member 14 as viewed in Figure 4 has the geometrical shape of a sphere of which a minor segment has been removed, the minor segment subtending an angle of about 45 . Each float member 14 can float by itself since the centre of gravity of a three quarters spherical ball is below its metacentre, a factor which is useful in the manufacture of the platform as described hereinafter. To assure that the bottom of each of the floats is at the same level, despite the curved configuration of the platform shell 12, the outer float members are each connected integrally to the shell 12 by a short vertically extending cylindrical shaft 15. The float members 14 are formed of reinforced concrete. In this embodiment the inner chambers of float members 14 can be filled with liquid, for instance, crude oil or liquid gas.

The left-hand side of Figure 4 shows the location of the water level 16 with respect to the float members 14 when the floating platform is unloaded whilst the water level 17 on the right-hand side of Figure 4 indicates the loaded state of the platform.

Any desired superstructure can be positioned on the platform, such as a derrick, a liquefying plant for natural gas, helicopter landing pad, or shelters for operating personnel. Furthermore, the floating platform can be provided with driving engines 18 permitting it to be positioned or moved.

Figures 5 and 6 represent another embodiment of the invention, having a circular dish or bowl shaped platform member 21 in the form of a part-spherical shell 22. The shell 22 is constructed of prestressed concrete, being prestressed in both of two orthogonal directions in the horizontal plane, and its peripheral edge is defined by a vertical edge beam 23 forming a lateral cylindrical wall. A bending resistant connection between the part-spherical, reinforced concrete float members 24 and the shell 23 is afforded by short vertically extending, hollow, reinforced concrete cylinders 25.

To provide an improved arrangement for supporting a refinery or natural gas liquefy- ing plant on the floating platform, which equipment is not illustrated for clarity's salSe, the shell 22 is provided with an intersecting array of radially extending ribs 26 and circumferentially extending ribs 27 the ribs 26,27 extending upwardly within the open upper portion of the shell 22. The upper edges 28 of the ribs 26 and 27 rise from the centre of the shell outwardly toward the beam 23 in the manner of an amphitheatre, (see Figure 6). These ribs 26 and 27 also provide stiffening for the shell 22.

Compared to the floating platform 1 shown in Figure 1, which has a slab-like platform member, the floating platforms 11 and 21 of Figures 3 and 5 are constructed as shells, since a solid planar slab is only suitable for use when relatively small waves are to be encountered, e. g. wave heights of up to about 4m, because of the limited bending resistance of a slab platform member. A planar slab can be used, for example, as a runway in relatively calm water. Such an embodiment is shown in Figures 7 and 8 where a floating platform 31 consists of a planar slab 32 formed of prestressed concrete and supported on partspherical, reinforced concrete float members 33. The floating platform or runway has a width of about 150m and a length of about 3200m and the slab has a thickness of about 0.60m. The floating platform may be constructed of individual sections, each similar to the floating platform illustrated in Figure 1, separated by expansion joints 34, (see Figure 8).

Figures 9 to 13 illustrate installations for constructing a plurality of float members and then a platform member thereover in order to construct a floating platform in accordance with the invention.

For the series-production of the float members, a concrete slab 41 with reinforcing abutments 42, 43 is provided along a shoreline at a level slightly above the adjacent water level, note Figures 10 and 11.

As can be seen in Figure 9, the concrete slab 41 extends over a number of float member construction stations with the rumber depending on how many float members are to be produced at one time. In Figure 9, eight such stations A, B, C, D, E. F. G and H are disposed in side-by-side relation.

Centrally of each station, the slab 41 is provided with a rounded recess or trough 44 which serves as a form for constructing the lowermost portion of a spherically shaped float 45. Extending outwardly into the water on each side of each station are jetties 46, the upper edge 47 of each ietty lying in the same plane as the upper surface of the slab 41 and the unper surfaces of the reinforcing abutments, 42, 43.

Initially, the rounded bottom portion of a float member 45 is formed on the respective rounded base 44, the remainder of the float member being constructed by means of suitable climbing formwork which is not illustrated in the drawings. A completed part-spherical float member 45 is shown in Figures 10 and 11. Additional climbing formwork can also be used to form a cvlindrical extension 47 on the upper end of the float member, if necessary for its connection to a shell-shaped platform member such as that shown in Figures 3 and 4.

After the completion of each float member. it is lifted from the forming station and placed into the water by means of a supporting frame 48, which includes horizontal beams 49 supported on vertical columns 50. The float member 45 is sus. pended on the frame 48 as indicated in chain-dot lines in Figure 10 by means of suitable gripping means (not shown) which are inserted into the open upper end of the float member and then raised to lift the float member into the illustrated position. The support frame can then be moved by suitable sliding or driving means 51 outwardly along the top surfaces 47 of the jetties 46 on either side of the respective station into position above the free water surface between the jetties, whereafter the float member 45 is lowered onto the water where it floats (Figure 11) and from which position it can be transported for subsequent assembly with a superposed platform member.

When sufficient float members 45 have been constructed, they are assembled in a predetermined array using further equipment constructed along and outwardly from the shoreline. The equipment consists of four basins 61 separated from one another by jetties 62. Each basin is suitably shaped in cross section so that a constructed float member 45 can be floated into the basin, and floated with ballast water 66 (Figure 13) until the float member rests on and is supported by a pad 63 located in the bottom of the basin. When all the float members are thus assembled in the basins, a prepared scaffold 64 is located thereover, being supported on the jetties 62 for use as a form for the lower surface of the platform member 65, whether the latter is a solid or hollow slab, or a curved shell. The scaffold or formwork 64 is pressed against the exterior of the cylindrical extension 49 on the float members 45. Thereafter the concrete for the platform member 65 is poured in a single operation or in a number of successive operations so that it is monolithically, i. e. integrally, joined to the float members 45. After the concrete has set and the platform member has been prestressed in a suitable known manner, the completed floating platform is floated by pumping out the ballast water 66.

If the dimensions of a platform to be produced exceed the capabilities of available conventional lifting equipment, then the platform may be made in sections which are subsequently joined together. For example, where available lifting equipment has a total span of 150 metres, and a platform such as that shown in Figures 1 and 2 is to be produced which is 150 metres square which if made in one operation would necessitate the use of lifting equipment with a span of more than 150 metres, then the platform can be made in two halves each 150 metres X 75 metres. The first half platform is produced as described above, and is floated out of the basins, ready for the production of the second half platform, which once made may be likewise floated out of the basins. After suitable manoeuvring of the half platforms, possibly involving their rotation about a vertical axes, until they are positioned with the correct edges to be joined adjacent one another, they are suitably ballasted, to ensure their correct alignment, whereafter they are suitably joined using conventional techniques for joining two prestressed concrete members.

Figures 12 and 13 show a plant used for producing a square platform 65 with a total of sixteen float members 45. For the production of a longer platform, for example, a runway as shown in Figures 7 and 8, the finished part of the platform is kept at the corresponding level in the area immediately in front of the basins 61 and the jetties 62 by ballasting with water, and the following part of the platform is concreted in a monolithic manner onto subsequent float members.

For optimum performance of a platform according to the invention, it is preferable that the peripheral dimensions of the platform (e. g. the length of one side of slab 2 in Figures 1 and 2) is of the same order as, i. e. is approximately as large as, the maxi- mum length of the waves which the platform is likely to encounter.

Claims

WHAT WE CLAIM :

1. A floating concrete platform suitable for use at offshore locations, comprising a platform member formed by a concrete slab or shell which is prestressed in two orthogonal horizontal directions, and a plurality of reinforced concrete float members arranged in an array below the platform member and monolithically connected thereto in order to support the platform member, each float member having an external shape constituted by at least a part of a closed body of revolution having a curved generatrix.

2. A floating concrete platform as claimed in claim 1, in which each float member is at least partly spherical in shape.

3. A floating concrete platform as claimed in claim 2, in which the external shape of each float member has the geometrical shape of a sphere from which a minor segment has been removed to form an upper edge for the float member by which it is connected to the underside of the platform member.

4. A floating concrete platform as claimed in claim 3, in which each of at least some of the float members has a short cylindrical extension at its upper edge, which extension interconnects the upper edge of the part-spherical float member with the lower surface of the concrete slab or shell.

5. A floating concrete platform as claimed in any of claims 1 to 4, in which said platform member comprises a planar hollow concrete slab having continuous upper and lower slab members vertically spaced from one another.

6. A floating concrete platform as claimed in claim 5, in which a plurality of intersecting partitions divide the space between said upper and lower slab members into a plurality of hollow chambers.

7. A floating concrete platform as claimed in claim 6, in which said partitions form a rectangular network of hollow chambers with said chambers being small in size compared to said concrete slab.

8. A floating concrete platform as claimed in any of claims 5 to 7, in which said concrete slab has a rectangular outer periphery and said float members are disposed in said array in a number of parallel rows.

9. A floating concrete platform as claimed in any of claims 1 to 4, in which said platform member comprises a concrete shell which is curved in at least one of said two orthogonal horizontal directions.

10. A floating concrete platform as claimed in claim 9, in which said concrete shell has a rectangular outer periphery, and said float members are disposed in said array in a number of parallel rows.

11. A floating concrete platform as claimed in any of claims 1 to 4, in which said platform member comprises a concrete shell which is part-spherical and which is open at the top.

12. A floating concrete platform as claimed in claim 11, in which said concrete shell has a periphery which is polygonal and said float members are disposed in said array in radially spaced concentric circles.

13. A floating concrete platform as claimed in claim 11, in which said concrete shell has a circular periphery and said float members are arranged in said array in radially spaced concentric circles.

14. A floating concrete platform as claimed in any of claims 11 to 13, in which vertically extending ribs arranged in an intersecting network extend upwardly from the upper surface of said shell.

15. A floating concrete platform as claimed in claim 14, in which said ribs are formed in two groups, one group of said ribs extending radially of the centre of said concrete shell-and the other group of said ribs extending in concentric circles so that they intersect said radially extending ribs.

16. A floating concrete platform as claimed in claim 15, wherein the upper edges of both groups of said ribs are stepped upwardly from the centre of said concrete shell toward the radially outer periphery thereof, rising in the manner of an amphitheatre.

17. A floating concrete platform as claimed in any of claims 1 to 16, wherein the interior of each of said float members is formed as a storage tank for holding liquids such as crude oil or liquid gas.

18. A floating concrete platform as claimed in any of claims 5 to 8, in which said concrete slab is elongated in one of said orthogonal directions relative to the other orthogonal direction as to be adapted to form at least a part of an elongated traffic route such as a runway.

19. A method of producing a floating concrete platform according to claim 1, comprising the steps of forming said plurality of the float members, arranging the float member in a predetermined arrav, and casting a platform member in the form of a concrete slab or shell over the upper ends of the float members so that the slab or shell is integrally connected to the upper ends of the float members thereby to form a floating platform.

20. Apparatus for constructing a floating concrete platform according to claim 1, comprising means for forming said plurality of reinforced concrete float members means for arranging said float members in a predetermined array comprising a plurality of elongate basins arranged side by side and divided by jetties, the basins being at least in part below a free water surface so that said float members can be floated thereinto and then ballasted so as to be correctly located therein in said array, the basins being divided by jetties for use in casting said concrete slab or shell over the upper ends of the float members.

21. Apparatus as claimed in claim 20, in which said means for forming said float members comprises a moulding construction arranged adjacent a shoreline and having a plurality of shaped depressions therein, each depression corresponding to the external shape of the bottom of one of the float members climbing formwork which when constructed over each depression constitutes a mould in which a float member can be cast, and lifting means for raising a finished float member from said mould transporting it to said shoreline, and lowering it into the water so that it can float therein.

22. A floating concrete platform substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2, or Figures 3 and 4, or Figures 5 and 6, or Figures 7 and 8 of the accompanying drawings.

23. A method of constructing a floating concrete platform according to claim 1, substantially as hereinbefore described with reference to and as illustrated in Figures 9 to 13 of the accompanying drawings.

24. Apparatus for constructing a floating concrete platform according to claim 1, substantially as hereinbefore described with reference to and as illustrated in Figures 9 to 13 of the accompanying drawings.