Field of the invention

The invention concerns the field of slipform casting. More specifically, the invention concerns a concrete support structure, and a method of constructing a concrete structure, as set out by the preambles of claims 1 and 6.

Background of the invention

Slipforming is well known in the casting of concrete structures. In general, a slipform comprises two rigid plate elements (referred to as "form halves" or "form sides") that constitute mould surfaces between which concrete is continuously poured and shaped to form structures (straight, circular, curved, etc.). The slipform sides are moved synchronously, usually by means of a yoke and a lifting arrangement. As the poured and shaped concrete hardens and cures, it forms the foundation for fresh concrete being poured on top of it, thus forming a monolithic structure.

Slipform casting is commonly used to make structures such as land-based concrete structures and cylindrical and conical towers, floating concrete support structures, and columns and platform foundations (so-called gravity base structures; GBS) that are subsequently placed on a seabed and serve as foundations for e.g. wind turbine generators or offshore oil and gas platforms.

A common feature of concrete structures that are intended as offshore support foundations (floating or fixed) is that their cross-section is not constant, but is the narrowest in the region of the nominal water line. The structures flare outwards to wider cross-sections above and below the water line. In other words, the structure is in the shape of a cone segment below the water line and an inverted cone segment above the water line. Such cone segment is commonly referred to a frustum or a frusto-conical body, irrespective of its orientation.

The structure below the water line may also be cylindrical.

In the prior art, frusto-conical concrete structures for offshore use are made by means of slipform casting, as it is desirable to have water-tight structure with as few casting joints as possible. When towers, columns, etc., having such frusto-conical, circular or polygonal structures are being cast, the slipform sides form the inner and outer contours of the inclined concrete walls. These inclined walls may have varying thicknesses with varying vertical radii of curvature. The concrete wall may also have varying horizontal radii of curvature.

When the slipform is moved upwards (e.g. is lifted), the interconnected slipform sides slide upwards and shape inner and outer contours of the wall that is in the process of being cast. This happens during the first part of the curing process. When the concrete emerges from below the slipform sides, the wall contour has been shaped and the concrete has cured sufficiently for it to support its own weight (and the fresh concrete being poured on top of it) and retain its shape without support from the slipform. A common slipform height is 1 meter.

When the slipform is moved upwards and moves in relation to the fresh concrete (which is in the process of being cured) between the upper parts of the slipform sides, friction occurs between the concrete and the slipform sides. In conventional slipform casting, this friction force is less than the weight of the concrete that is affected by the friction. When, however, an inclined (e.g. frusto-conical) section is cast, and the slipform sides are correspondingly inclined, the lower slipform side will be subjected to a greater friction force than a conventional slipform side, caused by fresh concrete. This friction force increases as the inclination (i.e. deviation from the vertical) increases.

For inclined, frusto-conical walls having constant or varying vertical radii of curvature, partly, or over their entire heights, the straight and rigid slipform sides will always form a chord to the theoretically curved wall contour. In both cases, a cavity will be formed between the chord defined by the straight and rigid slipform sides and the vertical curve to be formed. When the slipform is lifted, the fresh and still not sufficiently cured concrete will flow into this cavity and tend to fill it. The friction between the slipform sides and the concrete wall will increase substantially at the lower edge of the straight slipform when the slipform sides are moved further upwards to pass the partly cured concrete in the cavity. The increased friction may in turn pull along some of the fresh concrete being present between the slipform sides, and all the way into the already assembled reinforcement of horizontal and inclined bars. This movement will, together with the flow of fresh concrete, result in a combined relative movement of the still uncured concrete between the slipform sides and the already assembled lattice of horizontal and inclined reinforcement bars. The movement will result in bond behaviour of the concrete to the reinforcement bars being reduced, and visible lifting fissures and cracks may appear in the concrete surface. Such reduced bond behaviour may appear even when using conventional slipforms, caused by uncontrolled slipform movements. Reference is made to "Deutscher Ausschuss fur Stahlbeton, Heft 378", published by the Technical University of Munich.

One objective of the present invention is to obtain a solution for slipform-cast concrete structures that have inclined sides, e.g. a frusto-conical shape (i.e. increasing or decreasing cross-section with height), and producing concrete structures that have a similar strength (or better) as structures being cast using conventional slipforming techniques.

Another objective is to obtain a geometrical configuration for the slipform-cast concrete structure such that it is easier to reinforce. In the prior art construction of frusto-conical structures having varying vertical radii of curvature, varying wall thicknesses, and possibly also varying horizontal radii of curvature, both horizontal and vertical reinforcement bars must be cut individually, pre-bent and shaped such that each bar fits into the contours of the concrete wall. Such geometrical changes may require that a given reinforcement bar needs to have different radii of curvature over the length of the bar.

Another objective is to being able to pre-fabricate parts of the reinforcement bars, for later assembly into the wall being cast.

Yet another objective is to eliminate the limitation of the prior art frusto-conical slipforms, i.e. a maximum wall surface inclination of 15° to 17° (from the vertical) and a minimum vertical radius of curvature of 30 metres to 40 metres.

Summary of the invention

The invention is set forth and characterized in the main claims, while the dependent claims describe other characteristics of the invention. It is thus provided a method of constructing a concrete structure having inclined sides, characterized by

a) producing one or more support members and rotatably supporting the support members via hinge means on support means;

b) rotating the support members in the hinge means such that the support members assume an angle of inclination; and

c) securing the support members and placing at least one interlocking object on top of the support members.

The plurality of support members in step a) are preferably produced by slipform casting.

In one embodiment, a shaft is cast by slipform casting, and the support members are arranged around the shaft periphery, thereby forming a frusto-conical structure when step b has been completed.

The shaft and the support members are preferably cast simultaneously in step a), by slipform casting. The hinge means are locked during step a) and then unlocked.

It is also provided a support structure, comprising one or more support members supported by support means and configured for carrying an object, characterized in that the support members are rotatably supported by the support means via hinge means, whereby the support members may be tilted an angle of inclination and thus assume an inclined orientation before the object is placed on the support members.

In one embodiment, the support structure further comprises an interlocking object arranged and configured to lock the support member in the angle of inclination.

The support members preferably comprise channels for tension cables.

In one embodiment, the support structure comprises a shaft, and a plurality of support members are arranged around the shaft and form a frusto-conical shape when the support members are tilted the angle of inclination. The support members are preferably concrete elements, made by slipform casting.

The present invention provides a method and a device for casting frusto-conical structures with comparably large inclinations and small and varying horizontal and vertical radii of curvature. The concrete structure is cast using conventional slipforming equipment in casting support members that subsequently may be tilted to the desired angle of inclination in order to form the frusto-conical structure. The need for the complex, inclined slipforming methods and equipment of the prior art is eliminated. The present invention also provides a method and a device for casting structures having inclined support members.

Brief description of the drawings

These and other characteristics of the invention will become clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached schematic drawings, wherein:

Figure 1 is a side view of an embodiment of the invented concrete structure, shown in a first state;

Figure 2 is a sectional view in the plane defined by the section line A-A in figure i;

Figure 3 is a sectional view in the plane defined by the section line B-B in figure i;

Figure 4 is a side view of the structure of figure 1, shown in a second state; Figure 5 is a sectional view in the plane defined by the section line C-C in figure

4;

Figure 6 is a side view of the structure of figures 1 and 4, shown in a third state; Figure 7 is a sectional view in the plane defined by the section line D-D in figure

6;

Figure 8 is a sectional view of a part of the invented structure, and illustrates the support member in a tilted position;

Figure 9 is a side view of another embodiment of the invented structure, shown in a first state; and

Figure 10 is a side view of the structure of figure 9, shown in a second state. Detailed description of a preferential embodiment

The following description may use terms such as "horizontal", "vertical", "lateral", "back and forth", "up and down", "upper", "lower", "inner", "outer", "forward", "rear", etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader's convenience only and shall not be limiting.

Figure 1 shows a structure having a foundation 1 (only partly shown) that is configured to be submerged below a water surface 5. For example, the foundation 1 may be configured for installation on a seabed (not shown). Referring additionally to figures 2 and 3, the foundation supports a plurality of support columns 9 upon which is arranged a horizontal circular concrete girder 11 and an upright circular shaft 8. The columns 9 and shaft 8 may be cast using conventional slipforming techniques and be integrally formed (as shown in figure 8).

A plurality of upright support members 3 are arranged around the shaft 8 circumference, and placed in conjunction with the support columns 9. The support members 3 are in the illustrated embodiments in the shape of columns, and may also be in the shape of wall elements. The support members 3 may be cast using conventional slipforming techniques, simultaneously with the shaft 8. However, the support members may also be prefabricated elsewhere and lifted into the position illustrated in figures 1-3 when the shaft has been cast.

Turning now to figure 8, each support member 3 is supported by the columns 9 via a hinge arrangement comprising a first hinge part 12 and a second hinge part 13

(collectively identified as a hinge connection 4 in figure 1). The first hinge part 12 is connected (preferably cast via anchoring bars, not shown) to the lower portion of the support member 3. The second hinge part 13 is connected (preferably cast via anchoring bars, not shown) to the upper portion of the column 9. In the illustrated embodiment, the hinge parts 12, 13 have circular and complementary shapes, allowing the support member to be rotated with respect to the fixed second hinge part 13. Suitable friction- reducing agents (e.g. grease) may preferably be applied in the second hinge part 13. It should be understood that other hinge configurations are conceivable. In figure 8, the support member 3 is illustrated in the vertical state (dotted lines), which is the state and orientation in which it is being cast (i.e. corresponding to figure 1). During casting, the hinge parts are preferably temporarily interlocked by suitable means (not shown) in order to prevent inadvertent rotation.

The support member 3 also comprises, at the level of the circumferential tension cable channels 18, buckling supports 20. In radial direction, preferably by a concrete casted plate (dotted lines) between the support member 3 and the shaft 8 and circumferential preferably also by a concrete casted plate 19 (dotted lines) between the support members 3.

When the support member is tilted to its final, inclined, position, a supplement unit 21 with anchoring bars (not shown) is erected to accomplish the 180° force transferring area of the second hinge part 13. The supplement unit 21 is connected to the shaft 8 and the top of column 9. The connection is preferably concrete cast into pre erected block- outs with anchoring bars (not shown) in the slipformed shaft 8 and top of the column 9.

Figure 8 also shows the completed support member 3 (solid lines) having been tilted an angle V by rotating the hinge parts. This is the state corresponding to figure 4 and figure 6. In this tilted state, the support member 3 is secured by a rod 2 (see figure 4) in order prevent further rotation. Both the wall section 3 and the support column 9 comprise channels 14 in which tension cables 16 may be installed when the wall section 3 has been tilted to its inclined position. Figure 8 shows an anchoring fixture 15 connected to an embedded portion 17 in the support column 9. The other end of the tension cable is connected to the upper end of the support member or to a structure (not shown in figure 8) being supported by the wall section. The wall section also comprises circumferential channels 18 that allow for installation of circumferential tension cables (not shown).

The invention thus makes it possible to construct a frusto-conical concrete structure by

(i) making the structure as described above with reference to figures 1-3 and 8, preferably by slipform casting;

(ii) tilting the wall sections 3 a predetermined angle V as shown in figures 4, 5 and securing each wall section by rods 2;

(iii) constructing and installing a ring girder 6, e.g. casting in concrete, on top the tilted wall sections 3, preferably installing tension cables as described above, and placing a topsides structure 7 on top of the ring girder 6, as shown in figures 6, 7. The topsides structure 7 may be a platform deck, processing facility, wind turbine platform, etc.

While the embodiment described above produces a structure where the horizontal cross- section increases with increasing height (i.e. an inverted frustum), figures 9 and 10 illustrate an embodiment where the cross-section decreases with increasing height. This configuration could be useful for supporting a topsides structure T in the form of a tower (carrying e.g. a wind turbine). Figure 9 corresponds essentially to figure 1 except that the shaft 8' is narrower, allowing the support members 3 to be tilted towards the shaft. This state is illustrated in figure 10, also showing a ring girder 6' installed on top of the support members 3.

For both embodiments, when the support members 3 have been tilted to the angle V, further wall sections (not shown) may be cast in between the support members 3, in order to reinforce the finished structure against buckling loads. Thus, the support members and wall sections together form a polygonal frusto-conical structure.

One advantage with the present invention is that all reinforcement bars in the slipform (except the horizontal reinforcement in the circular shaft 8 and the bent bars in the support members 3) may be straight. The time-consuming and costly process of fitting the reinforcement bars to a geometrically complex structure has been eliminated.

Although the invention has been described with reference to a frusto-conical structure, it should be understood that the invention is equally applicable for constructing pyramidal structures, or any concrete structure that comprises an inclined wall portion or support member.