FIELD OF THE INVENTION

The instant invention relates to foundations and their methods of manufacture.

BACKGROUND OF THE INVENTION

When installing a foundation for an off-shore structure, such as off-shore platforms or the like, it has been frequent to prepare the ground to receive the foundation. Preparation includes extensive work to provide a surface as planar as possible to receive the foundation.

Since such preparation has been cost intensive and work intensive, with sometimes unsuitable results, it has been tried to get rid of this operation.

The invention relates to a foundation for an offshore structure.

JP 1-174727 describes a foundation which is provided as a hollow structure which comprises an envelope made of a top rigid structure and a bottom net. The top rigid structure has a base which is placed approximately horizontal. The net will be used to accommodate the space below the level of the base. The whole envelope is filled with dense fluid matter such as concrete.

However, the volume of concrete to be brought on site to fill the envelope is huge. Furthermore, this prior art still requires that the base be placed horizontal, which is not always possible, or requires ground preparation .

JP 7-158082 attempts to solve the second problem by pre-levelling a base part using cylinders which define a temporary position for the base part. A colonnade of concrete is then provided below and around the base part, which acts as a formwork for a later filling with concrete.

Yet, providing such a colonnade is very difficult and no industrial realisation of this document is known so far. This is because the colonnade is not well suited to adapt to irregular ground and receive concrete. Furthermore, this solution requires two concrete injections, with impact on installation duration as the second injection cannot be done prior to first concrete solidification and hardening.

The instant invention has notably for object to mitigate those drawbacks.

SUMMARY OF THE INVENTION

To this aim, according to the invention, it is provided a foundation for an off-shore structure comprising :

a hollow gravity caisson filled with water, and having a bottom surface,

- at least one bag, attached to the caisson, extending below the caisson and filled with a dense fluid element ,

the at least one bag comprising a flexible envelope having a flexible bottom and flexible peripheral walls extending from the bottom to the caisson.

With these features, the envelope has a flexible bottom which allows it to adapt to any irregularity of the ground. Furthermore, the envelope has flexible peripheral walls which can adapt more easily to the simultaneous flow of concrete or other dense fluid material therein.

According to an embodiment the bag(s) is/are provided over an area that can be optimized and not necessarily over the entire surface of the caisson bottom.

This allows to appropriately accommodate the concrete in order to reduce its volumeby reducing the location and size of the envelope, the amount of concrete to be poured on-site can be dramatically reduced. Unlike in the prior art, the concrete is not poured only to provide weight, but mainly to adapt the gravity caisson to the irregular ground. Hence, it provides an adaptable footing for the gravity caisson.

In some embodiments, one might also use one or more of the following features :

said at least one bag is a plurality of bags, at least two bags having adjacent peripheral walls ;

the at least one bag extends below a radially outward section of the caisson ;

the at least one bag forms a radially outward ring extending below the caisson ;

- the foundation further comprises a flexible liner attached to the caisson, extending below the caisson, and which surrounds the at least one bag ;

the flexible liner surrounds the at least one bag radially inward and/or radially outward of the at least one bag ;

the flexible liner comprises a barrier comprising a plurality of flexible straps spaced apart from one another along a longitudinal direction, and each extending along a transverse direction, and a flexible envelope attached to said plurality of straps ;

the foundation further comprises a pre- stabilizing device, comprising a plurality o jacks attached to the caisson and each jack can be actuated along at least one degree of freedom between a rest position and an active position where the jack projects below the bottom surface of the caisson ;

the stabilizing device comprises at least 3 jacks provided around the periphery of the caisson, and radially outward with respect to the bag ;

- the bag has no other openings than openings designed to receive an inflow of dense fluid element ;

the at least one bag defines an empty space below the caisson.

According to another aspect, the invention relates to a method of manufacturing a foundation for an off-shore structure comprising:

providing a hollow gravity caisson tillable with water, and having a bottom surface,

providing at least one bag, attached to the caisson, extending below the caisson, comprising a flexible envelope having a flexible bottom and flexible peripheral walls extending from the bottom to the caisson, and adapted to be filled with a dense fluid element,

sinking the caisson close to ground by filling with water,

filling the at least one bag with a dense fluid element until it contacts the ground, the flexible bottom of the envelope adapting to irregularities of the ground .

According to some embodiments:

the method of manufacturing further comprises: before filling the at least one bag, preliminarily levelling the caisson with a pre-stabilizing device ;

- before providing, determining a geometrical pattern for the at least one bag.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will readily appear from the following description of some of its embodiments, provided as non- limitative examples, and of the accompanying drawings.

On the drawings :

Fig. la is a schematic view of a step of a method of manufacturing a foundation according to one embodiment,

Fig. lb is a bottom view of a foundation as shown on Fig. la,

Fig. lc is an enlarged view of a part of Fig. la,

- Figs. 2a-2f are views similar to Fig. lc, showing different steps of a method of installing a foundation,

Figs. 3a, 3b and 3c are detailed views of a flexible formwork for the foundation of Figs. 2a-2f,

- Fig. 4 is a view similar to Fig. 2f for a variant embodiment, and

Figs. 5, 6 and 7 are schematic views similar to Fig. lb of various embodiments.

On the different Figures, the same reference signs designate like or similar elements.

DETAILED DESCRIPTION

Fig. la schematically shows a step of installing foundation for an offshore structure according to an embodiment of the invention.

An assembly 1 which comprises a bottom portion 2 and a top portion 3 is made to float in the sea 4. The bottom portion 2 is to be used as a foundation for an offshore structure, and a top portion 3 can be used as the structure itself, or only a footing on which the structure would be assembled after installation of the assembly. The assembly 1 is brought to the suitable location by any suitable means, such as a tug boat 5. At the location where the offshore structure is to be based, the submarine ground 6 might comprise irregularities 7. The ground may or may not (such as in shown example) be globally horizontal at this location.

As can be seen on Fig. lb and lc, the bottom portion 2 can comprise a hollow caisson 8. Please note that the top portion is not shown on the following figures. The caisson 8 is sufficiently massive to be used as a gravity foundation when filled with water or other ballast material. The empty caisson weighs more than 1000 tons, for example at least 4000 tons, and is much more heavy than the top structure. The caisson is divided in compartments 9a and 9b, in order to allow a sequential ballasting. The compartments comprise openings 10 which can be closed, or opened to let sea water 4 fill the compartments, in order to sink the caisson (these openings are not shown on the following figures) .

The caisson 8 has a top surface 8a which receives the top portion 3, and an opposed bottom surface 8b which is designed to face the ground 6.

Whatever the configuration of the caisson 8 is, it can be arbitrarily dealt in a central portion 11 and a peripheral portion 12.

As can be seen on Fig. lc, one or more bags 13 are provided below the bottom surface 8b of the caisson 8. Only a reduced portion of the bottom surface 8b, for example, less than 60% of this surface, is provided with a bag below.

As can be seen on Fig. lb, the bag(s) 13 can be provided along a peripheral ring provided in the peripheral portion 12 of the caisson. In the described example, the foundation comprises an outer barrier 14, an inner barrier 15, which surround a plurality of bags 13 provided between the two barriers 14 and 15. This will be described in more details below. Each bag 13 has an envelope 16 provided with a single opening 17 connected to a duct 118 through which a dense fluid material can be injected into the envelope.

The foundation further comprises a pre-stabilizing device, such as plurality of hydraulic or pneumatic or mechanical jacks 18. The hydraulic or pneumatic or mechanical jacks 18 can be provided radially outward of the caisson 8, for example regularly disposed along the periphery of the caisson 8. In the present example, four hydraulic or pneumatic or mechanical jacks are provided. The hydraulic or pneumatic or mechanical jacks 18 may each comprise a cylinder 19 which is fixed to the caisson 8, and plunger 20 which is movable with respect to the cylinder, and can project below the bottom surface 8b level of the caisson 8.

Turning now to Fig. 2a, once a suitable location for the foundation is reached, the hydraulic or pneumatic or mechanical jacks are set so that their plungers 20 extend below the bottom surface 8b of the caisson. Some of the compartments 9a, 9b are filled by water, whereas others remain empty. Hence, the caisson 8 is partly filled with seawater, as shown on Fig. 2a, so as to sink the assembly 1 below the seawater level 4a.

The envelopes 16 are provided in a suitable material like natural or synthetic rubber, suitable to receive therein concrete or another dense fluid element made of aggregate, cement and synthetic resin. Hence, the envelope 16 has a flexible bottom 21 and flexible lateral sides 22, and is otherwise attached to the caisson 8.

Similarly, the inner and outer barriers 14, 15 are provided in a flexible material like polyurethane, which will allow them to adapt to the deformation due to the neighbouring envelopes being filled with concrete, and also to adapt to irregularities of the ground.

In a specific embodiment, as shown in Fig. 3a, an example of an inner barrier 15 is provided. A similar example could be applied to the outer barrier 14. The barrier 15 will be provided as a plurality of flexible straps 23 which are spaced apart along the peripheral direction of the caisson 8. Each strap 23 extends along its longitudinal direction between two ends 23a and 23b attached to the caisson 8. Using different straps 23 along the longitudinal direction will allow adapting to the irregularities of the ground along this direction as shown on Fig. 3a. Further, as shown on Fig. 3b, the bottom portion 23c of each strap will also be able to adjust to the irregularities of the ground in the other direction (Y) . Another example is shown on Fig. 3c. A flexible envelope 24 is provided to join the straps 23 together, and is stretchable and flexible in order to allow for different geometries of the neighbouring straps 23.

Turning now to Fig. 2b, the next step of providing the foundation would be discussed. When reaching the irregular ground, the first part of the assembly which touches the ground are the plungers 20 of the hydraulic or pneumatic or mechanical jacks 18. At this stage, the hydraulic or pneumatic or mechanical jacks 18 can be commanded so that the caisson 8 is stabilized to be approximately horizontal (the top portion should be placed about vertical) . As shown in Fig. 2c, after actuation of the hydraulic or pneumatic or mechanical jacks 18, the caisson 8 is about horizontal. Further, the inner and outer barriers 15, 14 are in contact or close to contacting the irregular ground 6. This of course depends on the irregularities 7 of the ground. The inner and outer barriers 14 and 15 and the ground 6 serve as a formwork for the concrete to be injected or poured into the envelopes 16. At this stage, other compartments 9a, 9b can be opened, to be filled with water, to improve stability of the caisson, while still other compartments remain empty.

As shown on Fig. 2d, concrete 25 is injected simultaneously into the envelope 16 through the openings 17 and ducts 118 shown on Fig. lc. As can be seen on Fig. 2e, the bottom surface 21 will adapt to the shape of the ground 6 below it. The lateral sides 22 will also deform to receive the concrete poured into the envelope.

When outer and inner barriers 14, 15 are used such as in the present case, they act as a formwork, thereby preventing the lateral sides 22 from being deformed too much under concrete pouring. Even in the event that the lateral sides 22 or bottom surface 21 would be torn during the pouring of the concrete, the barriers might be used as a liner, i.e. an additional external envelope preventing the concrete to be poured boundless below the caisson and/or around. As can be seen on Fig. lb, this applies to the inward and outward lateral sides 22. For the peripheral lateral sides, it can be provided with a plurality of envelopes 16 adjacent to one another, and thus having adjacent peripheral walls. In such case, neighbour envelopes being poured simultaneously with concrete can be used also as a formwork for one another.

As will be appreciated from the above Figures, the above operation provides a footing for the caisson 8. In other words, an efficient link between the caisson 8 and the ground 6 is provided. The poured concrete 25 is used to fill in the space between the bottom surface 8 of the caisson and the irregular ground 6, at suitable locations to offer stability of the caisson 8. These locations can be precisely pre-determined before hand. Upon precise determination of the geometrical pattern for the bag(s), the volume of poured concrete can amount only to a few percents of the overall volume of the foundation, for example less than 15%, or even less than 5%. Further an empty space 27 is provided below the casing. Hence, the bags are provided to let an empty space 27 between the caisson 8 and the ground 6. This empty space 27 can have any suitable shape as dictated by the specificities of the installation. After some time, the concrete may have set and hardened, and the caisson 8 might be totally filled with seawater, such as shown on Fig. 2f . Overall, the weight of water is of the order of the weight of the empty caisson, for example about 70%-130% of the weight of the empty caisson. At this stage, the hydraulic or pneumatic or mechanical jacks might be considered useless, and can be removed, if needed. It is provided with a foundation which is gravity based, and has a very efficient link to any kind of irregular grounds 6.

Fig. 4 now shows another embodiment of the invention. Compared to the embodiment described above, the various envelopes may be provided along one or more longitudinal axis of the bottom portion 2 rather than as a ring as shown on Fig. lb. On these drawings, it is more clearly seen how the flexible bottom 21 of the envelopes adapt to the irregular ground 6, and how the flexible lateral sides 22 of adjacent envelopes cooperate with one another. As can be seen on this embodiment, one does not necessarily use barriers 14 and 15. If necessary, it can be provided with an additional outer envelope 26 which will surround all of the envelopes 16 which receive concrete. This outer envelope 26 will be attached to the caisson 8 and will act as a liner, i.e. will prevent concrete from pouring into the sea in case of unexpected tearing of one of the envelopes 16. It may therefore surround the envelope 16 below their bottom and around their outmost sides .

As can be understood, any shape and arrangement of caissons, and envelopes and hydraulic or pneumatic or mechanical jacks can be used in variants. As shown on Fig. 5, the caisson could be polygonal, with hydraulic or pneumatic or mechanical jacks 18 provided at each side. The envelopes 16 may be individual envelopes provided at each corner of the polygon.

As shown on Figs. 6 and 7, the caisson 8 might be cylindrical as explained before. As shown on Fig. 6, only three hydraulic or pneumatic or mechanical jacks 18 are regularly spaced around the periphery of the caisson 8. Envelopes 16 are provided below the caisson, each corresponding to a hydraulic or pneumatic or mechanical jack 18. Additional envelopes 16 can be provided, such as a central envelope 16.

Fig. 7 basically corresponds to the embodiment of Fig. 6, with the difference that each bag is provided as a plurality (here three) of envelopes 16 adjacent to one another.