GB2116234A - Construction of offshore platform structures - Google Patents

Abstract

A method of constructing an offshore platform structure having a plurality of cast concrete supporting legs 2 which are included inwardly towards each other in an upwards direction, may take place entirely in a dry dock, or optionally a foundation 3, 9 may be constructed in the dry dock and then be towed out to a deep water site where the structure is completed. In all cases the platform foundation which includes a plurality of foundation footings 3 and the lower ends of the supporting legs 2 are built in the dry dock. The legs 2 are cast in vertical positions enabling conventional slip forms to be used and then, after the legs have been cast vertically, they are tilted towards each other and are permanently interconnected at their upper ends as shown at 5. At their lower ends the legs 2 are then permanently fixed to the foundation footings 3. Remaining sections of the legs, if any, may then be cast in any conventional manner. preferably using slip forming. <IMAGE>

Description

SPECIFICATION Construction of offshore platform structures The present invention relates to methods of constructing offshore platform structures having supporting legs which are inclined inwards towards each other in an upward direction.

Concrete platform structures having inclined supporting legs are commonly known. U.S.

specification No. 4,043,138 discloses an open frame offshore foundation structure of concrete which is firmly but removably found on the sea bed. The structure comprises a lower set of hollow columns inclined towards each other in an upward direction, a ballastable intertie box structure rigidly receiving the tops of the inclined columns and an arrangement of upper vertical columns, the lower ends of which are rigidly connected to the inter-tie box structure and the upper ends of which are intended to support a deck superstructure above sea level.

Both the lower set of inclined columns and the upper vertical columns are cast using slip forming, the lower set of inclined columns being cast using inclined slip forms.

Inclined slip forming is substantially more complicated and expensive than casting vertical columns using slip forms. Further, when using the conventional slip forming technique for casting inclined columns it is not possible to vary the diameter of the column as casting proceeds. It should further be appreciated that with conventional slip forming techniques for casting inclined legs, the inclination cannot exceed approximately 200 from the vertical. In particular, but not exclusively, such platform structures are suitable for sea depths exceeding 100 metres. As platform structures in general are constructed for increasing water depths, the lengths of the inclined legs are increased correspondingly.

However, since legs formed by the conventional methods of construction are rigidly fixed to the foundation even during the construction stage, temporary supports or stiffeners must be used during the construction phase in order to reduce stresses imposed on the foundation to an acceptable level.

Since the inclination of the legs is limited, the forces appearing in any cross-section of the legs when installed on the site will be excessive unless ballast is added to improve the stability of the structure. Consequently, both construction and installation works become complicated and more costly. If the inclination of the legs is increased, the area of the dry dock in which construction of the platform starts must be increased correspondingly. The same considerations apply to platform structures designed for greater depths of water.

The main object of the present invention is to provide a method of constructing offshore platform structures, especially structures for use in great depths of water, in which the disadvantages and limitations described above are overcome.

According to the present invention, we provide a method of constructing an offshore platform structure having a plurality of cast concrete supporting legs, which are inclined inwards towards each other in an upward direction, each leg having a foundation footing, which may be common to the legs, the supporting legs carrying a deck superstructure at their upper ends, or supporting one or more vertical columns which carry the deck superstructure, the construction starting in a dry dock and being completed either in the dry dock or at a deep water site, wherein the legs are cast in vertical positions using vertical slip forms and after casting the legs are tilted inwardly towards each other and are then interconnected either directly or indirectly at least at their upper ends.

The entire structure may be constructed in the dry dock. Optionally, only the lower section of the platform structure is constructed in a dry dock, and the structure is then completed at a deep water site. It should be appreciated that in both cases the legs are cast in vertical positions by means of slip forming.

After tilting, the lower ends of the legs are preferably further fixed to the foundation footing or footings by providing a rigid interconnection.

Further vertical legs may be constructed on top of the interconnected inclined legs, if required, preferably using slip forming, although any other conventional techniques may be used.

The bottom portion of each support leg may be provided with a curved cradle-shaped plate or surface which bears against its footing and has a suitable radius of curvature, which preferably corresponds to the distance from the plate or surface to the centre of gravity of the leg.

Alternatively, each leg and its footing may be interconnected at their lower end by means of intertying rigid structures, each leg and its footing being hinged to the rigid structure during the construction stage and being rigidly fixed subsequent to the tilting operations.

According to a further embodiment of the present invention the legs may be cast at least in two separate steps, adjusting the verticality between the steps in order to provide inclined legs consisting of at least two portions having different inclinations when the legs are completed and tilted.

The inclined legs may be interconnected by means of an intertying structure arranged in the middle region of the completed platform structure.

Since the construction is based on slip forming vertical legs, the cross sectional areas of the legs may be dimensioned to meet the requirements of sufficient water displacement volume, stability in their floating state and sufficient wall thickness and cross sectional area to accommodate shear stress and bending moments which occur in service, in an economic way, without it being necessary to consider bending moments which occur during slip forming if the slip forming is inclined. Since no restrictions exist with respect to the maximum inclination of the legs, water may be used as ballast in the structure thus avoiding the dependency on ballast in the solid form with a large specific weight, such as for instance sand.

Consequently, the structure will be less complicated, and the economy of the structure as a consequence thereof will be improved.

Another major improvement is achieved by the invention in that the inclination of the legs may be changed during the slip forming stage, if required or deemed necessary, without complicating the construction work to any substantial degree.

Some examples of methods in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a plan view of one example of a platform structure intended to be installed on sites with water depths less than approximately 1 50 metres; Figure 2 shows two stages of construction of the platform structure of Figure 1 one stage being shown in dotted lines; Figure 3a shows in more detail a side elevation of a footing and the lower end of a leg of a second example of a platform structure; Figure 3b is a plan view of three footing towers of the second example cast in a dry dock; Figure 3c shows three connecting braces intended to interconnect the footing towers shown in Figure 3b, the braces being cast in the dry dock, preferably together with the footing towers;; Figure 4 is a side elevation of two footing towers, interconnected by a connecting brace, a raft being shown in a floating state, anchored to the sea bed by means of anchor chains; Figure 5 is a side elevation showing two stages of construction of the example shown in Figures 3a-3c and 4, one stage being indicated by dotted lines; Figure 6 is a side elevation showing two constructional stages of a third example of the structure, one stage being indicated by dotted lines; and, Figure 7 is a plan view of a completed platform structure in accordance with the example shown in Figure 6.

The example shown in Figures 1 and 2 is particularly suitable for water depths of up to approximately 1 50 metres; the example shown in Figures 3-5 for depths of up to 250 metres, in particular in the region of from 150 m-250 m, while Figures 6 and 7 show an example intended for even greater depths, such as up to 400 metres.

Figure 1 shows schematically a horizontal elevation of an offshore platform structure having three foundation legs 2, each of which is terminated at its lower end by means of a footing structure 3. The foundation legs 2 are inclined towards each other in an upward direction, the inclined legs 2 being interconnected at their upper portions. The interconnected portions coincide with the axis of symmetry of the platform structure, forming an upper portion 4 comprising vertical columns 5. The three footing structures 3 are designed so as to enable them to float when towed out of a flooded dry dock, in which the structure is built.If the entire deck is constructed in the dry dock, the footing structure and, optionally, lower intertying portions 9 are given sufficient buoyancy to allow the platform to be towed in an upright position out of the dry dock, with a draft less than the minimum depth available.

Figure 2 shows two steps of the method according to the present invention for constructing the structure shown in Figure 1. The bed of the dry dock is denoted by the reference numeral 6.

Firstly, the footing structures 3 are constructed, using any known construction techniques per se.

The footing structures may be prebuilt and transported into the required position in the dock or the footing structures may be cast in situ. The intertying, lower structure 9 is then constructed and built-in in the footing structure 3. Said intertying structure may also either be prebuilt and transported to its proper position between the footing structures, or said ties may be cast in situ.

The footing structures 3 are provided with a centrally arranged open topped section. The supporting legs 2 are cast in an upright, vertical position, depending preferably on slip forming. The supporting legs are in situ, the legs 2 being positioned in said centrally arranged open topped section of the footing structure 3. Each supporting leg is at its bottom portion provided with a cradleshaped body 8 of steel or concrete or a combination thereof. Said cradle-shaped body 8 has a curved surface, the radius of curvature of which corresponds to the distance between the curved surface and the center of gravity of supporting leg 2. When the portions of the leg intended to be inclined are completed, the legs 2 are tilted inwardly till they meet centrally.Since the radius of curvature of the lower surface corresponds to the distance between said curved surface and the center of gravity the weight of each leg will at any time during tilting phase act on the portion of the curved surface being in contact with the supporting bottom surface of the open topped section of the footing structure. In the tilted position the lower end of each leg is rigidly affixed to the footing structure, preferably by prestressing cables and convention reinforcement and subsequently by pouring concrete into the voids around the lower ends of the legs. The construction of the vertical column portion 5 on top of the inclined legs may now be carried out in any conventional manner. The stage where the legs are in their untilted, vertical position is shown by the undivided lines, while the inclined state is indicated by dotted lines. Further, only one, respectively two columns or legs 2 are shown in Figure 2.

Figures 3, 4 and 5 show various components of the footing structure 3 and the intertying members 1 9 according to a second preferred embodiment of the present invention. The Figures 4 and 5 show further two different stages in the construction procedure. The embodiment disclosed in Figures 3-5 is particularly, but not exclusively, suitable for greater depths, such as down to approximately 250 metres. According to this embodiment the foundation footings 13 are constructed separately and independently in the dry dock.Figure 3a shows a side view of a footing structure 13, while Figure 3c shows a horizontal view of the three intertying sections 1 9 in the form girders etc., intended in the final stage to rigidly interconnect the three footing structures 1 3. Figure 3b shows a horizontal view of the footing structures.

Each footing structure 13 is provided with a horizontal base plate 20 of concrete and an abutment portion 21 affixed to the base plate 20.

The axis of symmetry 23 of the abutment portion 21 is inclined with respect to the base plate, the indication corresponding to the final indication of the legs 12 when in completed tilted position. The abutment portion is at its upper end provided with a top surface 22 perpendicularly arranged with respect to said said of symmetry 23.

The girders 1 9 shown in Figure 3c may optionally be provided with ends having a ball joint shape 24, while the footing structures may be provided with corresponding spherical or cylindrical ball-shaped recesses or depressions for the ball joints 24 or optionally for cylinder joints 24.

Subsequently, upon the completion of the construction work in the dry dock the raft is towed to a deep water site where the remaining part of the structure is constructed. Optionally, the footing structures and the intertying girders are towed separately out of the dry dock and interconnected at the deep water site, ball joint ends of the intertying girders being arranged in the corresponding recesses or depressions 25 in the foundation footings, cfr. Figure 3a. The footing structures which are anchored to the sea bed, and which now are interconnected by means of the girders, are trimmed by adding and/or removing ballast from their ballast cells, providing tilting the footing structure with respect to the girders, the ball joint 24, 25 serving as a hinge. The footing structures are tilted until the axis of symmetry 25 of the abutment portion becomes vertical.The legs may now be constructed in vertical direction, preferably using vertical slip forming. Such construction stage is shown in Figure 4. The girders 1 9 may either have sufficient buoyancy to be self-floating, or they may be temporarily supported by barges or corresponding buoyancy means.

Once the required height of the inclined legs has been achieved the vertical legs are tilted towards each other until they meet in a central position and then rigidly interconnected. During the tilting stage, the ball joint between the footing structure 20 and the girders 1 9 once more serve as a hinge allowing such relative motion.

The tilting effect is achieved preferably by manipulating the buoyancy of each leg, adding and/or removing ballast. Subsequent to the tilting operation the footing structures 20 are rigidly interconnected to the girders 19 by means of prestressing cables and reinforcement and by addition of grouting material in the voids around said ball joints. Pipes for supplying said grouting material have been installed in advance.

In Figure 5 the structure shown in fully drawn lines shows the supporting legs 1 2 in their vertical constructional position I while the dotted lines show the supporting legs 12 subsequent to the tilting operation in their inclined position II. Once the legs are rigidly interconnected the upper special column portion 15 of the structure may be constructed.

Figures 6 and 7 illustrate a third platform embodiment particularly designed for depths down to 400 meters. The foundation footings 33, support legs 34, 35 and girders 39 are constructed as described above with reference to Figures 3-5. Further, a buoyancy structure 40 is constructed in the dry dock, preferably simultaneously with the casting of the girders 39 and/or the footings 33, the buoyancy structure being intended to serve as an intertying structure intertying the upper end of the inclined legs 34, 35. The purpose of said buoyancy structure will be described in further details below.

In case of very high platform structures intended for deep waters the stability of the structure during towing to the offshore site is a main concern since the maximum towing draft is limited, at least during touring inshore. For example, a preferred towing route in Norway for towing platform structure out from the construction site to the offshore site, the maximum allowable touring draft is limited to approximately 220 metres. In case of tall platform structures and in particular for structures being provided with a deck superstructure on which substantial pay load is arranged, extra temporarily arranged or permanently arranged buoyancy tanks are required in the vicinity of the touring water line during tow out.

According to the embodiment shown in Figures 6 and 7 said intertying buoyancy tank 40 serves the purpose of the additional buoyancy described in the paragraph above. In addition said tank 40 serves as an intermediate intertying member, intertying the legs 34. The tank 40 has a polygonal cross sectional area and is provided with recesses, the position and dimensions of which corresponding to the position and dimensions of the legs 34 in the touring water line. The legs may either be given a shape as shown in Figure 5 or a shape as shown in Figure 6.

According to the embodiment shown in Figure 6, the lower portion of the leg 34 having one inclination up to the region for the touring water line and a second different inclination above the touring water line. The lower portion is cast in vertical position, denoted as position I, whereafter the leg is partly tilted to an intermediate position II for further casting in vertical direction of the upper section 34b of the legs. In this manner the leg is provided with a bend or knee section 34a which preferably is positioned at a level corresponding to the position of the buoyancy tank 40 when the legs subsequently are tilted to their ultimate position Ill where the legs are rigidly interconnected. Subsequently to the establishing of a rigid interconnection at their upper end and in the intermediate position, the girders are rigidly affixed to the footing structures.The casting of the upper vertical column portion may now be done.

Obviously, it is feasible to tilt the legs 34 shown in Figure 6 to position II, rigidly interconnect the floating body 40 to the legs 34 are their upper ends and then cast the remaining portions of the legs in vertical directions, projecting vertically up from the floating tank 40 in its ultimate shape. The legs 34 will then not meet in a central position, but form separate columns. The buoyancy body will serve as an intertying rigid stiffening body.

The operational procedure described in conjunction with the above disclosed embodiments, may be varied or combined in order to obtain the most efficient and economic method adapted to the local conditions of the construction site and the towing route. In the embodiment shown in Figure 2 it may be possible to perform the tilting operations in a dry dock. In such a case the dry dock should be filled with water so as to allow the footing structures to tilt with respect to the girders, manipulating with the buoyancy of the footing structures.

If the platform is designed for shallow waters, it may further be feasible to produce the tilting by means of conventional mechanical means. Still further, the footing structure may preferably comprise a plurality of contiguous cells. Several of said cells may be designed as as to function as a support for the ends of the separately constructed intertying girders, cfr. Figures 5-7.

According to the embodiment shown in Figures 6 and 7, the upper inclined portions of the support legs may optionally be cast using inclined slip forms, if deemed desirable.

Claims (13)

1. A method of constructing an offshore platform structure having a plurality of cast concrete supporting legs, which are inclined inwards towards each other in an upward direction, each leg having a foundation footing, which may be common to the legs, the supporting legs carrying a deck superstructure at their upper ends, or supporting one or more vertical columns which carry the deck superstructure, the construction starting in a dry dock and being completed either in the dry dock or at a deep water site, wherein the legs are cast in vertical positions using vertical slip forms and after casting the legs are tilted inwardly towards each other and are then interconnected either directly or indirectly at least at their upper ends.

2. A method according to Claim 1, in which the legs support one or more vertical columns and the vertical column or columns are cast using vertical slip forms after the legs have been interconnected.

3. A method according to Claim 1 or Claim 2, wherein the legs are completed in the dry dock, the legs being cast in an open top section extending from their footing or footings and being secured in their correct positions in horizontal directions and the legs are then tilted relative to their footing or footings after which the legs are rigidly fixed to their footing or footings.

4. A method as claimed in Claim 3, wherein each leg is provided with a curved cradle-like supporting surface over which it bears against its footing.

5. A method according to Claim 4, in which each supporting surface has a radius of curvature which corresponds to the distance between the surface and the centre of gravity of the leg.

6. A method according to Claim 1, wherein separate foundation footings and transverse braces which connect the footings to each other are cast in a dry dock, the footings being provided with abutment portions for the support legs, the abutment portions having abutment surfaces, and wherein the dock is then flooded and the end portions of the braces are mounted pivotally on the foundation footings and the footings are adjusted so that their abutment surfaces are substantially horizontal whereupon the support legs are cast in vertical positions integrally with the footings and are subsequently tilted inwardly towards each other, after which the braces are fixed to the footings either inside or outside the dock.

7. A method according to Claim 6, wherein the footings are provided with tilt bearings for the end portions of the braces.

8. A method according to Claim 6 or Claim 7, wherein the support legs are cast in two or more parts at an inclination to each other and wherein the inclination of the legs is adjusted between the casting of the parts so that all the parts are cast vertically before the legs are tilted towards each other and are interconnected at their upper ends.

9. A method according to any one of Claims 6 to 8, wherein a buoyant body is cast in the dock and is located between the support legs to increase the buoyancy of the support leg structure and subsequently the buoyant body is permanently fixed to the support legs.

10. A method according to Claim 9, wherein the buoyant body is provided with abutment means and control means for temporarily supporting the support legs during casting.

11. A method according to Claim 10, wherein portions of the support legs which extend upwardly from the buoyant body are cast using sloping slip forms.

12. A method according to Claim 1, comprising continuing the casting of a support structure from the legs upwards to a platform deck after the legs have been interconnected and comprising the steps of mounting a stiffening collar forming a buoyant body between the supporting legs when they are partly completed, tilting the legs towards the collar, joining the legs integrally with the collar to interconnect the legs and subsequently casting upper portions of the legs upwards to a common point of engagement or upwardly to the platform deck.

13. A method according to Claim 1, substantially as described with reference to Figures 1 and 2, or Figures 3a to 3c, 4 and 5, or Figures 6 and 7, of the accompanying drawings.