EP0146468B1 - Gravity concrete base for an offshore platform - Google Patents

Description

The present invention relates to a ballastable concrete base for an offshore platform.

Ballastable concrete bases of offshore platforms are known which consist of solid concrete walls. These bases can be suitable for platforms intended for cold seas because they have sufficient strength to withstand the pressure of the ice which can be very high but it has the disadvantage of having a very high weight. We tried to reduce this weight by using light concrete but this solution is very expensive and does not give any satisfaction.

The present invention aims to provide a base which can be made of normal concrete, which offers maximum resistance and which, however, is of reasonable weight.

The base of the present invention essentially consists of a volume formed by a rigid three-dimensional lattice of concrete bars assembled into concrete nodes, certain nodes being connected to each other by clamping cables which pass outside the bars and which optionally pass through intermediate nodes, these cables performing a three-dimensional prestressing of the entire trellis, the base comprising means for making the sides and the bottom of the trellis waterproof.

We know the concept of a three-dimensional concrete lattice but we have not, until now, used such a lattice for the specific application envisaged above in combination with prestressing cables of the entire lattice and in combination with waterproof sides and bottom.

On the other hand, we do not know, until now, an industrial technique making it possible to produce a concrete trellis under acceptable conditions and an object of the invention is also to provide such a technique and to apply it as well to the realization of a platform base than to any other realization.

• la figure 1 est une demi-coupe verticale d'une embase de plate-forme conforme à l'invention ;
• la figure 2 est un ensemble de plans de coupe horizontaux de l'embase à différents niveaux ;
• la figure 3 est un schéma d'un bloc de constitution du treillis de l'embase ;
• la figure 4 est un schéma d'assemblage de deux bras pour réaliser une barre du treillis ;
• la figure 5 est un schéma d'une pyramide de fond de l'embase,
• la figure 6 est un schéma d'une partie de façade latérale de l'embase ;
• la figure 7 est une perspective d'une variante de réalisation d'un bloc préfabriqué et d'une partie d'embase constituée au moyen de ces blocs ;
• la figure 8 représente une autre variante de réalisation d'un bloc préfabriqué conforme à l'invention,
• la figure 9 est une perspective d'une partie d'embase selon une variante de réalisation de l'invention sur laquelle on a représenté une partie de façade, et
• la figure 10 est un schéma illustrant des câbles de précontrainte de l'embase.

According to one embodiment of the invention, the lattice consists of an assembly of prefabricated blocks by molding, each block comprising a core and a plurality of arms which radiate from the core, each arm having at least one longitudinal housing opening out to the free end of the arm, the arms being assembled in pairs in alignment to form the lattice bars, the housings of the assembled arms being aligned and containing a common metal frame, the junction zone of the assembled arms being surrounded by a sleeve waterproofing, said housings being filled with hardened mortar, and said trellis being clamped by prestressing cables which pass outside the bars of the trellis and which are fixed to nodes of the trellis. Embodiments in accordance with the present invention will be described below, with reference to the figures in the accompanying drawing in which:

• Figure 1 is a vertical half-section of a platform base according to the invention;
• Figure 2 is a set of horizontal sectional planes of the base at different levels;
• FIG. 3 is a diagram of a building block of the trellis of the base;
• Figure 4 is an assembly diagram of two arms for making a lattice bar;
• FIG. 5 is a diagram of a bottom pyramid of the base,
• Figure 6 is a diagram of a side front portion of the base;
• Figure 7 is a perspective of an alternative embodiment of a prefabricated block and a base portion formed by means of these blocks;
• FIG. 8 shows another alternative embodiment of a prefabricated block according to the invention,
• FIG. 9 is a perspective view of a base part according to an alternative embodiment of the invention in which a front part has been shown, and
• Figure 10 is a diagram illustrating the prestressing cables of the base.

The platform base shown in Figures 1 and 2 is a hexagonal base whose edge is 72 m. This base consists of a trellis which is provided with a sealed front and which has a sealed bottom. According to the invention, the trellis consists of concrete bars assembled in nodes and the lateral facade of this trellis constitutes inclined planes on which the facade of the base rests.

It is convenient to constitute the trellis by an assembly of regular tetrahedra, the nodes constituting the vertices of the tetrahedra and the bars being arranged according to the edges of the tetrahedra.

In this assembly of tetrahedrons, we can distinguish inclined planes where the bars form a mosaic of equilateral triangles and inclined planes where the bars form a mosaic of squares.

We can also distinguish horizontal planes where the bars form a mosaic of equilateral triangles.

In the embodiment shown, the lattice bars form squares in planes inclined at 50-60 °, form equilateral triangles in planes inclined at 65-75 ° and form equilateral triangles in horizontal planes.

Preferably, the sides of the trellis alternately comprise planes where the bars of the trellis form equilateral or isosceles triangles and planes where the bars of the trellis form squares or rectangles.

The section plane of Figure 1 is a vertical plane and the view represents one half of the section plane.

In Figure 2, there are shown horizontal section planes. In fact, Figure 2 is divided in six portions which each represent a fraction of horizontal section at a certain level. For example, marks 1, 2, 3, 4, 5, 6 represent section planes at levels 0 m, 5 m, 10 m, 15 m, 20 m, 25 m approximately. On the fraction of the figure representing the section plane at level 0, there is the lower plane of the trellis which is made up of a mosaic of equilateral triangles A, B, C whose sides are formed by bars of the trellis and whose vertices are formed by lattice nodes.

On the fraction of the figure relating to a section through a horizontal plane at the level + 5 m approximately, there is shown, by hatching, the part of the side facade which is located below the section plane. The same was done for the cutting plane at the level around + 10 m and for the cutting plane at the level around + 25 m.

It will be noted that the section plane in FIG. 1 is a plane along the line A-A in FIG. 2. The trellis is produced by any suitable technique and the following technique is preferably used.

In this technique according to the invention, blocks comprising a central core and arms which radiate from the core are molded by injection molding in a closed mold. The core is intended to constitute a knot of the trellis and each arm is intended to constitute a part of a trellis bar.

Assembling two by two an arm of a block and an arm of another block, the two arms being arranged end to end to together constitute a lattice bar. This makes the trellis step by step. In a preferred embodiment, part of the lower level of the trellis is first produced, then, in degraded form, part of the level placed above, and so on to the upper level, the devices necessary for the installation of the blocks that can roll on the ground from the place where the assembly is made. We then complete step by step each level. It will be noted that it is possible to prefabricate all the blocks in the workshop, which is particularly advantageous in the case of a marine platform of the ballastable type where it is usually necessary to carry out the manufacture in dry dock.

The invention makes it possible to carry out a large part of the work outside the dry dock, since only the assembly of the blocks is carried out in dry dock.

To assemble two arms, any suitable means can be used and, preferably, the two arms are provided, at prefabrication, with two housings which will open out opposite one another when the arms are placed end to end, these two housings being each provided with a passage making it possible to introduce mortar into the housings and, simultaneously, to empty the air from the housings. For mounting, a common frame is placed in the two housings, the junction of the two arms is surrounded by a sleeve and mortar is introduced into the two housings until it sets. The sleeve provides lateral sealing. Preferably, a sleeve made of heat-shrinkable material is used.

It will be noted that the mortar which fills the housings can constitute between the opposite faces of the two arms a more or less thick joint acting as a jack between the two arms. This more or less thick joint J makes it possible to position the two prefabricated blocks on demand one with respect to the other and it is the same step by step for all the blocks.

We can easily adjust the positions of the nodes, which is a very important advantage of the technique of the invention.

Figure 4 is a diagram explaining the technique of assembling two arms, as described above. In this diagram, the arms are marked 14, 14 ', the corresponding cores 15, 15', the corresponding housings 16, 16 ', the passages of the housings 17,17', the sleeve 18 and the frame 19.

In a typical example, the arms are bars whose cross section is part of a circle with a diameter of 20 to 100 cm and which have a length of 2 to 10 meters. Preference is given to bars of circular section whose diameter is in the range 30-80 cm and preferably used for the assembly of the arms, a mortar having a high compressive strength, of the order of 600 to 1,000 bars.

Preferably, each arm constitutes a half bar.

This preferred choice, which allows extremely rational manufacturing is not limiting and, in alternative embodiments, the arms constitute fractions of bars different from one half.

On the other hand, one can join two arms by an intermediate element instead of joining them directly. For example, if each arm constitutes a third of a bar, the two arms are united by an intermediate element which itself constitutes a third of a bar.

The entire trellis is clamped by cables which carry out a three-dimensional prestress. These cables are fixed at their ends to nodes of the trellis.

In a typical example, a cable alternately and repeatedly meets a trellis bar which it crosses substantially in the middle and orthogonally, and a node of the trellis which it crosses.

Figure 3 of the accompanying drawing shows, by way of example only, a block consisting of a core'1 from which radiate 12 arms (2-13) which each constitute a half-bar of the trellis.

In the trellis of Figures 1 and 2, there are thus blocks with eight arms, blocks with nine arms and blocks with twelve arms.

The blocks located on the end planes of the trellis, that is to say the planes which constitute the bottom, the sides and the top of the trellis, have a lower number of arms, as it is understood.

The bottom and sides of the base are sealed.

The waterproof bottom is preferably made up by a mosaic of pyramids, which allows it to sink on demand into the underlying soil at the final location of the platform.

Figure 5 is a perspective showing an element of the pyramid in one of the lattice tetrahedra.

The pyramid and the tetrahedron have a common base DEF but the vertex G of the tetrahedron is above the vertex H of the pyramid. To build the pyramid, it is convenient to arrange that a part of each face of the pyramid comes from molding with the corresponding knot of the trellis. For example, one half of the DHE face will come from molding with node D and the other half of the DHE face will come from molding with node E.

The two halves are then assembled by any suitable method and, for example, by a technique analogous to that which allows the assembly of two arms to form a bar. Thus, the pyramids at the bottom of the base are mounted at the same time as the nodes of the lower level of the trellis are mounted.

The facade of the base is preferably an embossed concrete facade. To make this facade (FIG. 6), it is convenient to prefabricate concrete dihedra made up of two planes P1 P2 and to join these planes to the bars of the surface of the lattice against which the facade is applied. It is therefore advantageous that in this surface the bars of the trellis constitute rectangles which extend in the height of the surface of the trellis and the walls P1, P2 are tightly joined with the bars b located along the long sides of the rectangle. and so step by step.

Figures 7 to 10 show alternative embodiments according to the invention.

In FIG. 7, the molded block consists of a spherical central core 15 from which radiate cylindrical arms 14. A part of a lattice made up of these blocks is shown to the right of the block and we see on this lattice sleeves 18 which are mounted on the lattice bars.

In Figure 8, there is shown another alternative embodiment of a lattice block.

In FIG. 9, part of a trellis is shown. The bars of the trellis which are in the planes underlying the facade are arranged along the sides of squares Q and along the sides of triangles T which, possibly, draw trapezoids. These provisions are not limiting and are given only by way of example. In FIG. 9, part of the side façade has also been shown. In this example, this side facade is made up of facade portions. In fact, each portion of the facade is integral with one of the tetrahedrons of the lattice and the various portions of the facades are made contiguous step by step by mortar or added concrete.

FIG. 10 is a simplified view schematically showing two prestressing cables 20, 21.

The prestressing cable 20 is straight and its ends are fixed to two nodes 22, 23 of the trellis. The cable crosses several bars of the trellis such as bars 24, 25 but remains outside the bars. The prestressing cable 21 is also attached at its two ends to nodes 26, 27 of the trellis, but this cable is not straight and it is deflected by nodes of the trellis such as nodes 28, 29. The node 28 is provided a groove 30 and the node 29 is provided with an internal passage 31 for deflecting the cable 21. Only part of the arms of these nodes is shown in the drawing.

The invention is not limited to a specific geometrical arrangement of the bars but, preferably, the bars of the lateral faces of the trellis are arranged along the sides of equilateral or isosceles triangles and / or along the sides of rectangles or squares.

In the embodiment shown, the side faces are planes inclined with respect to the vertical but, in other embodiments, they are vertical.

The sides and bottom of the trellis are sealed by any suitable means. Preferably, the seal is obtained by a plurality of concrete walls which are fixed in a sealed manner or which are in one piece with the bars of the lattice which are located in the lateral faces and in the bottom face of the lattice. Preferably, the walls which seal one side of the trellis are arranged in a pleated arrangement, which reduces the effects of the temperature difference between the part of this side which is in the water and the part of this side which is in the water. - above the water. Such a temperature difference, which in cold seas can be 50 ° C or more, could cause destructive stresses if the side walls were flat. The term "pleated must be understood as a global term which includes all configurations and all dimensions of pleats.

The invention is particularly applicable to the production of structures at sea, including to form reservoirs.

Claims (10)

1. A ballastable concrete base for an offshore platform characterized in that the base is essentially constituted by a volume formed by a three-dimensional rigid lattice of concrete bars (2-13) interconnected at concrete nodes (1), some of the nodes (22, 23 ; 26, 27) being interconnected by stressing cables (20, 21) passing outside the bars and optionally past intermediate nodes, the cables three-dimensionally prestressing the lattice as a whole, and the base further including means for making watertight the sides and the bottom of the lattice.

2. A base according to claim 1, characterized in that the bars of the lattice are disposed along the sides of regular tetrahedra.

3. A base according to claim 2, characterized in that the sides of the lattice is constituted by planes in which the bars of the lattice form equilateral triangles, isosceles triangles, squares or rectangles.

4. A base according to claim 1 or 2, characterized in that the means for making watertight the sides of the lattice are watertight walls which are sealingly fixed to or integral with some bars of the lattice and which are disposed according to a corrugated pattern.

5. A base according to claim 4, characterized in that the said walls which are making watertight a side of the lattice are pairs of plane walls (P_1' P₂) disposed at an angle to each other and defining a base rectangle which coincides with the bars of the said side of the lattice.

6. A base according to claim 1, characterized in that the bottom of the lattice is made watertight by a mosaic of hollow pyramids (DEFH).

7. A base according to claim 6, characterized in that the said pyramid have triangular bases and the vertices (D, E, F) of the triangles are constituted by nodes of the lattice.

8. A base according to claim 1 or 2, characterized in that the said cables include rectilinear cables (20).

9. A base according to claim 1 or 2, characterized in that some of the said cables are deviated by some nodes (28, 29) of the lattice.

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