WO2002095139A1

WO2002095139A1 - Foundation in water

Info

Publication number: WO2002095139A1
Application number: PCT/FI2002/000442
Authority: WO
Inventors: Esa Eranti
Original assignee: Esa Eranti
Priority date: 2001-05-22
Filing date: 2002-05-22
Publication date: 2002-11-28
Also published as: DK1402119T3; EP1402119B1; EP1402119A1; FI20011074A0; FI20011074A

Abstract

The object of the invention is an offshore foundation structure, the shell structure (1) of which is thin in relation to the diameter of the structure, and the shell structure (1) is filled with soil (7), and that the braced ring footing (2) is the plane of the bottom of the foundation structure, the braced connection plate (6) for the structures above the water surface (4), and longitudinal and/or transverse bracings (5) for the shell structure (1) are connected to the shell stucture (1).

Description

FOUNDATION IN WATER

The invention relates to a foundation structure to be built offshore. More closely defined, the object of the invention is a shell structure of a foundation structure to be built, operating together with the soil inside it.

Solutions are previously known, in which the offshore foundation structure is made by pile-driving the framework structure submerged in water to the bottom. Such solutions have been shown, for example, in the patent specifications US 3 832 857, US 3 638 436 and US 5 127 767. The use of one large pile anchored or submerged to the bottom is known as a different alternative. Such a solution has been disclosed, for example, in the patent specification US 3 677 113 and the patent application WO 00/28153. It can be used both on a soft or hard bottom. On the rock bottom, a hollow is formed to the rock, to which the large steel pile is soldered. A third alternative is to transport to the site or to manufacture on site a massive foundation struc- ture (of steel and/or concrete), onto which a construction, for example a wind power station, to be joined to the foundation structure and extending either partly below the water surface or being totally above the water surface, is installed. Such constructions that are disclosed, for example, in the patent specifications U 3 793 840 and US 5 613 808 can be either fixed or movable. Also this alternative is suitable both for a soft and hard bottom. When a different bottom is concerned, the installation only requires slightly different manufacturing procedures for the installation.

The problem with the previously used solutions has been the heavy special equipment needed in the transport of the foundation structures, which is available to a very limited extent. In addition, the use of special equipment is very expensive, es- pecially when preparing the foundation structures in offshore circumstances, in which the weather windows suitable for working are short. In the Northern conditions, the season that is best suitable for working, only lasts the summer months. In all bottom conditions that come to question, the foundation piles are rather massive. The time needed for the driving of the piles is long in relation to the weather win- dow that can be predicted, even in ideal conditions. In difficult bottom circumstances, the length of the drive time increases and becomes very difficult to predict. The required bottom work, such as for example, the loosening of moraine by exploding that has to be performed now and then may considerably increase the time. The fast and even hard changes in weather may interrupt the foundation project, and even force to demobilise and remobilise expensive equipment. If several foundation structures are to be installed to the same area, for example when building a wind park, it is extremely difficult to work out a fixed schedule, and advantages of serial production are lost.

When rock bottom is concerned, the preparing of the rock hollow for the caisson pile is very expensive and time-consuming. It involves risks related with rock conditions and the same schedule and weather risks that have been mentioned above. When the rock bottom is covered by a thick soil layer, the use of any kind of pile solution is generally out of the question.

Concrete caissons are also used as offshore foundation structures. There is a limited number of dockyards used for their building, and their use in connection of other productional activities is expensive and difficult. Also the reservoir to be separately built for the manufacture of caissons is in practice often an expensive and time- consuming solution. Weather and schedule risks are involved in the transport of caissons by floating or by heavy equipment and in the ballasting. The use of the caisson is made more difficult, because it cannot be installed to a very uneven bottom. In addition, the caisson solution in its entirety is expensive.

The object of the present invention is to avoid the drawbacks mentioned above that are related to expensive foundation solutions, the use of special equipment, and the predictability of schedules. When preparing several foundation structures to the same offshore area, for example, when a wind park is concerned, one has to strive for well programmed serial work, in which the time required by expensive marine work operations is short and the schedule risk related with them can be controlled. The equipment to be used should be moderately-priced standard equipment. Work requiring expensive special equipment, such as the installation of shaft and mill, has to be performed as serial work so that one single work stage offshore would preferably last less than 24 hours, in which case the weather risk can be controlled. In this way, the entire wind park could be installed and implemented during one summer.

For achieving this, a thin shell structure, preferably of steel, is prepared, which is filled with soil after embedding, for example, with natural non-cohesive soil, crushed stone or mixed blasted stone. A conical structure located in the water line and a structural braced ring footing form a part of the shell structure. The conical structure improves the suitability of the foundation structure for demanding conditions, but in stable ice conditions and on marginal ice areas or iceless areas the shell structure can be a straight cylinder. Such a thin shell structure can be lifted and transported to the installation location by using conventional equipment, in which case the costs are considerably reduced. The assembling of the thin steel shell requires no extensive investments, but it can be done on quayside or in a workshop near the installation area. Compared with a rock hollow to be done, for example, by exploding, the bottom work required by the foundation structure can be performed fast, at low cost and with moderate tolerance.

The above-mentioned advantages are achieved with the solution of the invention, which is characterised in what is disclosed in the enclosed patent claims.

The invention is next described in more detail, referring to the enclosed drawings, in which

Figure 1 is a cross-section of a foundation structure manufactured of a thin-walled, rotationally symmetrical shell structure,

Figure 2 is a cross-section of a foundation manufactured of a thin-walled, rotationally symmetrical shell structure, in which the conical structure has been inverted,

Figure 3 presents an embodiment to be used in shallow waters, with the foundation structure cross-sectioned,

Figure 4 presents an embodiment to be used in deep waters on top of a soft bottom layer, with the foundation structure cross-sectioned, and

Figure 5 presents an embodiment for installing the foundation structure onto rock bottom.

Figure 1 shows an advantageous embodiment of the offshore foundation structure of the invention. The foundation structure advantageously consists of the rotationally symmetrical shell structure 1 of steel and of the braced ring footing 2 in the plane of foundation, attached to the shell structure. The shell structure 1 can also be of some other form besides rotationally symmetrical. For example, it can be a polygon. The shell structure 1 contains the conical area 3, with which the dynamic ice loads directed to the structure are reduced, and above all, the intensity of ice induced vibrations is reduced by an order of magnitude. The intensity of vibrations is extremely significant especially if the foundation structure is used as the foundation for a wind power station. Their reduction improves the operation and durability of the wind power station. The conical area 3 is located substantially on the level of the water surface 4. The conical area 3 improves the suitability of the foundation structure to demanding conditions, but in stable ice conditions and on marginal ice areas or ice- less areas, the shell structure 1 can be a straight cylinder. The conical structure 3 is preferably braced with horizontal and/or vertical bracings attached to the surface of the thin shell structure 1 so that the connection plate 6, closing the shell structure partly or entirely from above, can be more firmly connected to the shell structure. A braced steel plate or a reinforced concrete plate can preferably be used as the connection plate 6. With the help of the connection plate 6, a structure above the water surface 4, such as a wind power station, a fixed navigation mark, a lighthouse, or some other structure, is attached to the foundation structure. A shell-structured foundation can also be built without the connection plate 6 so that the structures above the water surface 4 are directly connected to the shell structure 1, for example, by welding. The shell-structured 1 foundation can further be used, for example, for quays, dolphins, oil loading structures, oil drilling structures, or as ice-resistant bridge piers. In this case, the form of the shell structure 1 can also differ from the rotationally symmetrical form. The diameter of the shell structure 1 is preferably 4 - 40 m and the thickness preferably 6 - 40 mm. In the water line, also a steel plate with a stronger thickness can be used for reinforcing the structure.

The structural braced ring footing 2 according to Figure 1 on the plane of foundation ensures the cooperation of the shell structure 1 and the fill 7, consisting pref- erably of soil, in relation to dynamic loadings. Without the ring footing 2 there is the risk that the foundation structure will gradually tilt by the action of the dynamic loadings. In the structure, the ring footing 2 works as an anchorage in relation to dynamic forces. The ring footing 2 can be provided with a uniform or segmented skirt/skirts 21, penetrating into the soil. The skirt/skirts 21 improve the stability of the foundation structure. Before embedding the shell structure 1 to the installation location, the installation location is levelled with the soil layer 8, when required. Thus, as good a base as possible is produced for the foundation structure to be made. At the final stage of the installation of the foundation structure, soil material 9 is brought outside the shell structure 1 for covering the ring footing 2 for the ex- ternal part of the shell structure 1 for protecting the structure from erosion and for improving stability.

The conical structure 3 can also be inverted, as in Figure 2, which facilitates the control of wave loads. By manufacturing the connection plate 6 to have a bigger diameter than the overlying structure to be installed to it, the free area remaining for the connection plate can advantageously be used as an entrance/working plane. When required, the building of the foundation structure of the invention begins by preparing the bottom 10 of the installation area. This step can comprise, for example, the transport of soil to the site by barge. The filling of shell by soil is performed by using standard equipment (for example, a combination of barge and bucket loader), and it is thus relatively fast and inexpensive. In some cases, the bottom 10 can be ready for shell placement without any additional measures. The thin steel shell 1 can be simultaneously assembled in the vicinity of the water area in a workshop and on quay from prefabricated segments without expensive special preparations.

When the bottom 10 is suitable for installation, the steel shell 1 is transported to site using standard transport equipment, such as a transport barge. No special equipment is needed, because the structure is notably light, compared with the solutions previously used. The steel shell 1 can be lifted from the transport equipment using standard crane equipment, and it is sunk to the bottom 10.

The filling of the steel shell 1 is carried out by using soil 7. It is also preferable to roll soil and blocks along the conical surface to the bottom of the steel structure onto the footing 2 so that, for this part, the soil 9 works as a protection from erosion and increases the total stability of the structure. The soil 7 installed inside the steel shell 1 is arching as in a silo, and the entity formed by the shell and the soil operates with certain preconditions almost like a solid block, both in relation to tilting and sliding. In addition, the soil 7 supports the thin-walled steel shell 1 , thus preventing the loss of stability of the shell under stress, the shell being extremely thin-walled in relation to its diameter. Further, the soil 7 supports the steel shell 1 also in relation to local ice loads and wave impacts. The internal filling 7 also efficiently suppresses the vi- brations of the structure. Because of the arching effect of the internal filling 7 of the steel shell 1 it is I possible to use normal piles 12 provided with pile caps 11 for supporting the structure, when building on a soft bottom 10, as is later shown in Figure 4. There is no need for an underwater bedplate in this case, either.

In Figure 3, there is shown an advantageous embodiment for a foundation structure to be made to shallow water. Here the height of the thin steel shell 1 is smaller and the diameter is bigger than those of a foundation structure to be made to deeper water. With this solution, the stability of the structure can be made better in relation to the wind moment exerted from structures above the water. Especially when a wind power station is concerned, it is required that the foundation structure to be made to shallow water has a wide conical part 3. Figure 4 presents a two-part foundation structure to be made to deep water, consisting of a wider lower part 13 and a narrower upper part 14, which is, for example, similar to the structure shown in Figure 1. The two-part solution makes it possible to minimise the use of soil 7 and facilitates the installation to big water depths. Figure 4 also shows the piling of a foundation structure to be made with capped piles 12 through soft bottom 10 to the harder bed material 15 below.

Figure 5 illustrates an installation to be made on uneven rock bottom 16. In the installation to be made on rock bottom 16, a ring footing 17 of concrete and an internal anchoring of the structure are preferably used; the foundation structure can be attached to the rock with the said anchoring without an expensive rock hollow, which is difficult to realise. The shell structure 1 is attached to the rock 16 through the bed casting 19 and the concrete footing 17 levelling the bottom, using groutable rock anchors 18. The stability of the shell structure 1 is at the installation stage ensured by installation bolts 20 to be attached to the concrete footing 17.

Filling the thin shell structure 1 internally with soil 7 offers the structure the mass required by its total stability, prevents the loss of stability of the thin-walled shell under stress, supports the thin-walled shell against local loads, and acts as a damp- ener in relation to structural vibrations.

It is obvious for one skilled in the art that the various embodiments of the invention are not restricted to the examples presented above, but they can vary within the scope of the enclosed claims.

Claims

1. Offshore foundation structure with a shell structure (1), characterised in that the shell structure (1) of the foundation structure is thin in relation to the diameter of the structure, and the shell structure (1) is filled with soil (7), and that the ring foot- ing (2) in the plane of foundation, acting as the anchorage in relation to dynamic forces, and longitudinal and/or transverse bracings (5) are connected to the shell structure (1).

2. Foundation structure according to claim 1, characterised in that it further comprises the braced connection plate (6) for structures above the water surface (4).

3. Foundation structure according to claim 1 or 2, characterised in that at least part of the shell structure (1) is shaped conical in relation to the vertical axis, and that the conical surface (3) is located at the height of the water surface (4).

4. Foundation structure according to claim 3, characterised in that the foundation structure is attached to the bottom (15, 16) of the water area with piles (12) or rock anchors (18).

5. Foundation structure according to claim 4, characterised in that the shell structure (1) of the foundation structure consists of two or several parts (13, 14).

6. Foundation structure according to claim 5, characterised in that the foundation structure has the ring footing (17) of concrete, levelling the bottom, and that it is internally anchored to the bottom (16) of the water area with rock anchors (18).

7. Foundation structure according to claim 6, characterised in that the braced ring footing (2) in the plane of foundation is covered with soil (9) for the external part of the shell structure (1).

8. Foundation structure according to claim 7, characterised in that the diameter of the shell structure (1) is 4 - 40 m and the thickness 6 - 40 mm.