WO2010143976A2

WO2010143976A2 - A foundation, a method of manufacturing the foundation, and a method of installing the foundation on a seabed

Info

Publication number: WO2010143976A2
Application number: PCT/NO2010/000222
Authority: WO
Inventors: Sigurd Ramslie; Karel Karal
Original assignee: Seatower As; Karal, Eva
Priority date: 2009-06-10
Filing date: 2010-06-10
Publication date: 2010-12-16
Also published as: NO20092241L; NO331114B1; WO2010143976A3

Abstract

A foundation (1) for installation on a seabed (B) below a body of water (W), having a bottom part (6), a main body (5) and a connection portion (15) for a support structure and/or an equipment unit and comprising a first ballast compartment (13; 13b) and a second ballast compartment (16; 13a); a portion (20') of the first ballast compartment (13; 13b) in a region of its upper end having an opening (19) between the first and second ballast compartments; a channel (20) extending between a region above the second ballast compartment (16) and into the first ballast compartment (13); and a conduit (18) extending between the second ballast compartment (16; 13a) and the exterior of the foundation (1). The invention also comprises a method of assembling the foundation and a method of installing the foundation ion a seabed.

Description

A foundation, a method of manufacturing the foundation, and a method of installing the foundation on a seabed

Field of the invention

The present invention relates to structures for supporting offshore wind turbines and similar equipment. More specifically, the invention relates to a foundation for installation on a seabed below a body of water, as specified in the introduction to the independent claims.

Background of the invention

The increasing demand for exploitation of renewable energy sources enhances the demand for offshore wind power generation, as offshore wind conditions are more favorable than onshore conditions and the environmental impact is much less. There is an increasing need for structures that safely and reliably can support heavy wind turbines in a significant height over the sea level. The support structure generally comprises a of shaft, or tower, fixed to the seabed either directly by means of a foundation, or the structure is floating and connected to the seabed by means of a mooring arrangement. The present invention relates to the former type, namely the fixed support structures.

Typical fixed support structures for wind turbines presently in use, planned for use, and/or patented and described in publicly accessible sources are, in general terms characterized by the following:

1. Due to feasibility constraints the entire support structure is divided into two parts, namely a foundation and a tower, and the tower is in-situ mounted on the pre-installed foundation.

2. The foundation is fixed to the seabed by driven or drilled piles (either using multiple piles or large diameter mono-piles) or the foundation is deployed directly onto an artificial gravel layer through which the loads from the wind tower structure are transferred into the seabed.

Existing techniques for delivery of foundations from some delivery locations cannot meet the demand for high manufacture and installation rate, i.e. number of units to be manufactured and installed in one installation season. Existing solutions using the gravity force to fix the foundation to the seabed instead of piles, are known for their considerable limits of application related to their weight, water depth at the installation site as well as water depth at the load-out locations and along the transport route. These solutions use the gravity as the sole reaction to the operational and environmental loads that are transferred via the structure-soil horizontal contact area into the seabed. As a rule, the top soil layers at the contact area are weak and therefore need to be replaced by better materials such gravel. Moreover, as the seabed is often uneven and may also be inclined, the need for seabed preparation (leveling and flattening) is further enhanced.

EP 1 429 024 discloses a support structure for an offshore wind turbine, comprising a caisson supported by several columns embedded in the seabed and subjected to tension and pressure loads. Selected columns are piled at an inclined angle with respect to the vertical. The caisson is supported below the water surface but above the seabed.

WO 03/080939 discloses a foundation structure for a wind turbine tower or similar, for installation on the seabed. The foundation structure can be manoeuvred to its offshore position using a vessel and separate (and removable) buoyancy means.

These buoyancy elements must be rather large in order to maintain stability. When in position, the structure is lowered to the seabed and a pumping mechanism is used to sink a lower portion of the structure (e.g. skirts) into the seabed. When the foundation structure has been anchored (or piled) in position on the seabed, it is capable of supporting the wind turbine tower.

By their nature, the above solutions tend to yield high overall capital investment costs, i.e. the total costs for fabrication, load-out, transport,¹ seabed preparations and installation.

Summary of the invention It is provided a foundation for installation on a seabed below a body of water, having a bottom part, a main body and a connection portion for a support structure and/or an equipment unit, characterized by a first ballast compartment and a second ballast compartment; a portion of the first ballast compartment in a region of its upper end having an opening between the first and second ballast compartments; a channel extending between a region above the second ballast compartment and into the first ballast compartment.

In one embodiment, the second ballast compartment is arranged above the first ballast compartment, and a portion of the first ballast compartment extends a first distance into the second ballast compartment. In one embodiment a conduit extends between the second ballast compartment and the exterior of the foundation.

In one embodiment, the second ballast compartment comprises a central ballast compartment and the first ballast compartment comprises a ballast compartment surrounding the second ballast compartment, preferably in the shape of an annular compartment around the central compartment. In one embodiment, a lower channel portion is connected to the channel via an articulated joint, whereby the lower channel portion extending into the first ballast compartment. Preferably, the foundation comprises controllable means for moving the lower channel portion within the first ballast compartment. The conduit comprises in one embodiment a valve and a first opening outside of the foundation and a second opening in the second ballast compartment, at a second distance above the first opening.

In one embodiment, the foundation comprises a spreader element arranged below a lower channel portion on the channel such that ballast material coming out of the lower channel portion impacts the spreader element. The spreader element may comprise a conical surface and a plurality of plates arranged around the conical surface, a portion of each plate being raised above the conical surface.

The bottom part, preferably around its periphery, comprises a skirt element extending downwards thereby defining a chamber underneath the bottom part, wherein the skirt element is adapted for at least partial penetration into the seabed, the foundation further comprising orifices between the chamber defined by the skirt and the exterior of the foundation.

In one embodiment, the foundation comprising a plurality of load resisting and leveling means arranged around, and extending from, the foundation. The load resisting and leveling means comprise in one embodiment outer skirt elements arranged at regular intervals around the bottom part periphery and extending downwards, thereby defining respective separate chambers underneath each outer skirt element, wherein the outer skirt elements are adapted for at least partial penetration into the seabed and each outer skirt element further comprising respective orifices between the chamber defined by the outer skirt and the exterior of the foundation, whereby a filler material may be injected and direct contact between water in the skirt elements and the porous seabed sediment is prevented.

In one embodiment, the foundation comprises a plurality of arrester means arranged around the periphery of the bottom part and selectively movable from a retracted position where the arrester means do not extend below the bottom part, to an extended position where the arrester means extend below the bottom part.

In one embodiment, the bottom part comprises a bottom plate having a peripheral and upward extending wall, said bottom plate and upwardly extending wall being made substantially of concrete or a similar castable and heavy material, while the remainder of the foundation generally is made of steel or a similar metallic material. It is also provided a method of providing a plurality of the foundation according to the invention at an onshore, inshore or atshore assembly site, characterized by the steps of: a) pre-fabricating modules, optionally at one or more locations other that at said site; b) assembling the modules at said site in order to produce a desired number of foundations; c) optionally, storing the completed foundations in a floating state at an inshore location in preparation for installation; d) towing one or more foundations to an offshore installation site; and e) installing said foundations on a seabed.

In one embodiment, the assembly of said modules comprises: a) extending a circumferential lower wall from the bottom plate to form a foundation bottom part, said lower wall having a vertical extension dimensioned according to the buoyancy requirements for the completed support structure; b) placing the bottom part in a floating position on the surface of the body of water; c) further assembling the foundation by successive assembly of prefabricated units. It is also provided a method of installing the foundation according to the invention on a seabed below a body of water, characterized by the following steps: a) enabling water from the body of water to flow into the second ballast compartment through the conduit until a portion of the foundation at least partly is penetrating into the seabed, whereby water inside the compartment defined by the portion is forced out via outlets; c) activating selected ones of the leveling means, whereby the foundation is placed on the seabed in a substantially level state; and d) deploying ballast material into the main ballast compartment and optionally into the second ballast compartment.

In one embodiment, step b) of the installation method is repeated until a target embedment depth is reached and a landing surface on the foundation has attained contact with the seabed and sinking of the foundation has ceased. In one embodiment of the installation method, step a) is preceded by the step of moving the arrester means from a retracted position to a locked state in the extended position. Optionally, before step a), a pre-determined amount of particulate ballast material is deployed into the main ballast compartment, such ballast material comprising dry or naturally moist materials such as sand, gravel or iron ore. In one embodiment, step c) of the installation method comprises the at least partial filling of selected ones of the outer skirt compartments with grout. In one embodiment, step a) of the method of installation is terminated when water in the second ballast compartment has reached a level where it flows via an internal conduit into the first ballast compartment.

In one embodiment, step a) of the method is terminated when water in the first ballast compartment has reached a level where it flows via an internal conduit into the second ballast compartment.

If deemed necessary, the installing method comprises moving the foundation into a substantially level state during embedment of skirts into the seabed by exerting a rectifying load/moment onto the foundation or structure, hence possible seabed unevenness, sloping seabed or non-uniform soil conditions can be counteracted. The moment/load is produced by different water pressures in individual skirt compartments. The skirt design and the associated installation method enables also providing sufficient stability of the foundation for a desired time span until entrapped water in the skirt compartment above mudline is expelled by grout.. By providing a number of optional solutions the wind farm development project with its particular conditions may be efficiently optimized for e.g. the lowest price of installed units, for largest manufacture and installation rates, for the use of available vessels, etc.

The present invention introduces a number of parameters and structural compatibility by using different material types that can be applied for optimizing the supply of ready-for-operation structural supports for offshore wind farms. The following advantageous aspects are achieved:

1. Large degree of completion, commissioning and load-out work can be done at the fabrication site instead at the offshore installation site 2. Optional embodiment allowing integration of the tower to the foundation, cabling work and similar

3. Wider material selection and range of structural dimensions

4. Transport to the site on deck of barges and vessels is eliminated or significantly reduced 5. Separate buoyancy elements during tow-out are not required, also in optional embodiment

6. Deployment into the position (transfer from the transport position to the operation position) by adding ballast, not by lifting

7. No seabed preparation or seabed soil replacement 8. No piling, except in weak soil

9. Design and outfitting for removal can be easily implemented

10. Need for large offshore cranes is avoided

In addition to lower overall costs the present invention resolves shortcomings associated with the known solutions by:

1. Enabling delivery of the foundations from assembly sites allowing work in moderately sized areas and operation of shallow draft vessels thus widening the selection of fabrication sites

2. Increasing manufacturing and installation rate 3. Reducing risks from adverse weather conditions on the progress of installation

4. Reducing needs for specialized vessels

5. Allowing superstructure (tower, wind generator, etc) to be fitted to foundation structure at or near shore, prior to tow to installation location

6. Allowing foundation structure to be leveled during or following installation on seabed, to prevent unpredictable inclination of the installed support

7. Resistance to heavy ice loads Brief description of the drawings

These and other characteristics of the invention will be clear from the following description of a preferential form of embodiments, given as a non-restrictive examples, with reference to the attached figures wherein:

Figure 1 is a schematic side view of an embodiment of the foundation according to the invention, installed on a seabed;

Figure 2 is a top view of the foundation shown in figure 1 ;

Figure 3 is a vertical section through the foundation shown in figure 1 , along the section line A-A in figure 2, illustrating ballast chambers and outfitting for solid ballast deployment;

Figures 3a and 3b are top and side views, respectively, of an embodiment of a spreader element;

Figure 4 is an enlargement of a portion of figure 3, illustrating the ballast spreading device;

Figure 5 is a top view of the ballast spreading device shown in figure 4; Figures 6 - 11 are side views illustrating main steps in the assembly of the foundation according to the invention;

Figure 12 is a side view of a temporary mooring arrangement for a plurality of foundations; Figure 13 is a top view of the mooring arrangement shown in figure 12;

Figures 14 - 20b are sectional side views along the section line A-A in figure 2, illustrating main steps in an installation procedure on the seabed of the foundation according to the invention;

Figures 21 and 22 are side views illustrating an optional assembly and load-out method of the bottom section of the foundation using a submersible barge;

Figures 23 and 24 are side views of the foundation according to the invention, in figure 23 placed on one embodiment of a stability device floating in the water, and in figure 24 the foundation being detached from the stability device;

Figure 25 is a top view of the foundation and stability device shown in figure 23; Figure 26 is a side view illustrating a second embodiment of a floating stability device, connected to the foundation;

Figure 27 is a side view of the foundation and stability device shown in figure 26, illustrating the assembly method of the floating stability device onto the foundation;

Figure 28 is a top view of one element of the floating stability device shown in figure 27, connected to a foundation; and

Figures 29a-j illustrate an optional assembly method for the foundation. Detailed description of a preferential embodiment

Referring first to figures 1 and 2, these figures illustrate an embodiment of the foundation according to the invention, generally denoted by the reference numeral 1 , installed on a seabed B having an uneven surface (often referred to as a

"mudline") M, below a body of water W. Although not mandatory for the invention, it is advantageous to give the foundation 1 a circular cross-section that can efficiently resist environmental loads in various directions and phases during fabrication, transport and operation; typically hydrostatic water pressure and wave loads.

The foundation 1 comprises a main body 5 and bottom part 6. The bottom part 6 comprises a bottom plate 4, a circumferential skirt 7 around the bottom part, and a plurality of outer skirt compartments 8a,b,c connected to the bottom part at regular intervals around the bottom part periphery. Figure 3 shows three such outer compartments 8a-c. A plurality of optional fins 9a-f may be arranged at intervals around the bottom part periphery.

The bottom part 6 also comprises a number of extendable dowels 1 Oa,b,c, arranged at intervals around the bottom part periphery. In figure 1 , the dowels are shown in an extended position, protruding into the seabed B below the lower tip of the skirt 7. Figure 1 also illustrates how the skirt 7, the outer skirt compartments 8a-c and the optional fins 9 are embedded into the seabed B.

The outer skirt compartments 8a-c represent significant improvement of the known technology as these provide the following: (a) Means for leveling of the foundation 1 during installation on the seabed, as will be described in more detail below;

(b) In-situ stability in the temporary phase between settlement of the foundation 1 onto the seabed B and until the foundation has been supported under the entire bottom area, e.g. by grouting, as will be explained below; (c) Resistance to torque moments, e.g. induced by operation of a wind turbine placed on a structure resting on the foundation;

(d) Resistance to horizontal loads in any phase (e.g. transportation, installation, operation); and

(e) Additional resistance to overturning moments. It should be noted that the resistance to torque moments and resistance to horizontal loads (items c and d above) are uncoupled. Hence they are not additive (i.e. one compartment cannot be fully utilized for resisting torque and at the same time to resisting horizontal load).

The main body 5 comprises a lower circumferential and substantially vertical wall 11, connected to the bottom plate 4, followed by - in an upward direction - of one or several frustro-conical sections 12a,b,c, and a column 14 having a connection portion 15 to which the tower (not shown) for carrying a turbine (not shown) may be eventually connected.

Figure 3 is a sectional drawing along the section line A-A in figure 2, and illustrates additional details of the foundation 1 and its outfitting for transportation and installation. The main body 5 comprises internal ballast compartments 13, 16. In the illustrated embodiment, the vertical wall 11 and the first frustro-conical section 12a define a main ballast compartment 13, and the second and third frustro-conical section 12b,c and the column 14 define a secondary ballast compartment 16. The secondary ballast compartment 16 is situated above the main ballast compartment 13, separated by a bulkhead 17. A conduit (pipe or similar) 18 extends between a first opening 18a on the outside of the foundation (preferably at a lower part, as shown in figure 3) and a second opening 18b inside the secondary ballast compartment 16 (preferably near the bulkhead 17, as shown in figure 3). A channel (chute or similar) 20 for filling solid ballast into the main compartment 13, extends from an upper region of the main body, preferably a funnel shaped element 2 in the column 14, through the secondary compartment 16 and into the main compartment 13. As illustrated by figure 3, an inner wall 16' of the secondary ballast compartment 16 defines a cavity 20' (in figure 3 a cylindrical, elongate cavity). An overflow opening 19 in the inner wall 16', hence between the secondary compartment 16 and the main compartment 13, is arranged at a suitable distance above the bulkhead 17.

When the foundation is lowered to the seabed during installation, water flows from the body of water W via the pipe 18 into the secondary ballast compartment 16. The geometry of the secondary ballast compartment will permit only a small water plane area, such that the ballast water will be not impede the floating stability during the foundation's descent to the seabed. The pipe 18 is provided with a valve (not shown) which e.g. may be remotely operated, or preferably from a Remotely Operated Vehicle (ROV). As the water continues to flow into the secondary ballast compartment during descent, the water will eventually (designed to be in a final stage of the descent to the seabed), reach the overflow opening 19 and flow into the main ballast compartment 13 via the pipe cavity 20'. When water starts to flow into the main ballast compartment 13, both the draft of the foundation and the stabilizing arm of floating stability have increased so much that the large water plane area being created in the permeable ballast material in the main ballast compartment 13 does not compromise the floating stability of the foundation. Alternatively, the foundation will touch down on seabed B prior to the water starting flowing into the main ballast compartment 13, thereby providing support so that the floating stability is not compromised. As described above, the foundation 1 comprises an arrangement for filling of solid ballast over the top of the column 15 into the main ballast compartment 13, via a chute 20. In some applications it is advantageous to fill ballast, at least partly, into the main compartment 13 before tow to the installation site commences. In such a case it is important to deploy the ballast so that its centre of gravity is substantially in the centre of the structure 1 and so that possible shifts of the centre of gravity that may be caused by wave induced motions of the foundation are as small as possible.

The first of these two requirements is achieved by providing the chute 20 with a lower pipe section 22 which is pivotally connected to the chute 20 by means of an articulated joint 21, as illustrated by figure 3. The lower pipe section 22 has a cross- sectional dimension which, compared to the cross-sectional dimension of the cavity 20', allows a movement of the pipe section 22 - substantially in a horizontal plane - within the cavity 20'. Figure 4 illustrates these relative differences in dimensions and shows how the lower pipe section 22 has been moved out of its central position and to an extreme position in contact with the inner wall 16'. Figures 4 and 5 illustrate suitable means for moving the lower pipe section 22, in the form of hydraulic cylinders 23. The cylinders may be remotely controlled and operated to move the lower pipe section in a suitable manner for guiding the deposition of solid ballast into the main ballast compartment as desired.

The second requirement is achieved by spreading the solid ballast in the main compartment 13 without creating high heap(s) with sides sloping at angles close to the angle of internal friction of the solid ballast. Such deposition of the solid ballast is accomplished by a spreader 24 that may consist of a number of plates 24b inclined in different angles and radially arranged in the horizontal plane to resemble a fan. Figure 3 a is a top view of an embodiment of a spreader element 24, having a conical surface 24a and raised surfaces or plates 24b. Figure 3b is a side view of the embodiment shown in figure 3 a, illustrating how the plates 24b are raised above the conical surface 24a. When the falling solid ballast hits these plates, the falling trajectory is changed at several angles so that distribution of the ballast material is achieved. Model tests performed with a spreader indicated also that due to the velocity of the ballast material particles and inclined trajectories, the angle of deposition of the material in the outwards directions tends to be lower than the angle of internal friction thus further improving the resistance of the deployed solid ballast against shifts of the centre of gravity in case of motions in waves.

The inventive fabrication, transport and installation procedure will now be described with reference to figures 6 to 20.

Figure 6 shows the initial phase of assembly where the bottom part 6 is being assembled on a quay 26 and resting on the supports 27a,b,c. The sections making up the main body 5, which may have been prefabricated elsewhere, are brought to the assembly site and efficiently assembled into a suitable large part of the entire structure. The bottom part 6 is designed for lifting and with floating ability. In a preferred embodiment the structure is designed as a single or in parts also as a double steel shell. In Figure 7 the assembled bottom part 6 is lifted off the supports on the quay 26, e.g. by means of suitable lifting arrangement L, and about to be placed in a floating state ting in the body of water W.

Figure 8 shows, illustrated by arrows C, concrete casting onto the bottom plate 4 of the bottom part 6. The concrete may stiffen the bottom plate and provide heavy ballast in the lowest part of the foundation, thus ensuring a lower center of gravity and improved floating stability.

Figure 9 shows lifting and assembling of the lower sections 12a,b, as one unit, onto the bottom part 6. The sections 12a,b may also be assembled individually, or include additional sections up to the upper part of the foundation.

Fig 10 shows the lower sections 12a,b integrated with the bottom part 6, and the upper section 12c, the column 14 and the connection portion 15, being lifted as one unit on top of the completed part of the structure. The elements may also be assembled individually. Figure 11 shows a completed foundation 1 floating in water and ready for tow out.

During construction, assembly and tow-out of the foundation 1 , the dowels 10a-c are in a retracted position, i.e. not extending below the lower edge of the skirt 7. This is shown in e.g. figures 6 - 11. When the foundation is about to be installed in its intended position on the seabed, the dowels are lowered and locked in a position where they extend beyond the lower edge of the skirt 7, as illustrated e.g. in figures 1 and 15, and which will be explained in more detail below.

In situations where a number of foundations 1 are to be supplied, it may be advantageous to prefabricate a number of foundations and store them at-shore or in comparably sheltered waters until beginning of the installation campaign. The storage may be achieved in comparably shallow water thus widening the selection of possible sites for the storage. Figure 12 illustrates this situation and shows a series of completed foundations la,b,c moored in an area protected from wind, waves and hazards. Each foundation is moored via mooring lines 30a-f, e.g. towing bridles, connected to buoys 31a-d, which in turn are connected via mooring lines 32a-f to anchors 33a-d and finally to the seabed B.

Figure 13 shows this mooring arrangement in a top view. Each mooring line 32a-c is provided with anchor 33a-d. The mooring arrangement shown in the figure is advantageous from several reasons: it occupies small area per each moored structure, the structures can be attached to and released from the mooring in any sequence, the structures la-f are attached to the mooring by lines 30a-f that serve also in the towage operations. This mooring arrangement allows any one of the foundations la-f to be connected to or disconnected from the string of foundations. For example, foundation Ib may be removed without altering or in any way compromising the moorings of any of the other foundations in the mooring arrangement. Also, the use of towing bridles 30a-f instead of ordinary mooring lines between each foundation and the buoys, reduces the quantity of required lines and provides for faster preparation for tow-out. The respective mooring lines are used when towing the foundation to the installation location. When the foundation 1 is due for installation, the first operation is deployment of ballast in a quantity derived from the available draft along the towage route to the installation site. However, this step may be omitted when the foundation has sufficient floating stability during offshore ballasting and at the same time the installed structure exhibits sufficient in place stability in the temporary phase between the installation and the filling of all voids between the bottom slab and the seabed. The latter requirement may be relaxed if the installation and the filling is performed in one continuous operation. The foundation 1 in figure 14 is secured in position at the ballasting location. Unless the main ballast compartment 13 is fitted with a water drainage system, the ballasting is performed by solid ballast such as sand, gravel, iron ore etc, that is not mixed with water (i.e. the materials are dry or only naturally moist). Ballast 35 is lifted to top 34 or the foundation 1 by means of conveyer belt, crane or similar (not shown). Ballast 35 falls down through the chute 20 and its movable lower pipe 22, further impacting on the spreader 24 and finally it is deposited in a layer 36 in the main ballast compartment 13. The layer 36 is shown schematically to indicate that the deposited ballast will not heap in large cones that would be prone to sliding, thus undesirable center of gravity shift and consequently heel of the floating structure.

Figure 15 shows the foundation 1 with a desired amount of ballast 36 upon arrival at the installation site, and upon the dowels 10a,b,c have been released from a

(retracted) transport position to an (extended and locked) installation position and they are now protruding below the lower tip of the skirt 7.

Figure 16 shows the foundation 1 descending to the seabed B by increasing the weight of the structure. A valve 18c controlling the intake 18a to the pipe 18 is open, hence water flows into the pipe 18 and further into the secondary (upper) ballast compartment 16 through the opening 18b. At reaching an elevation above the seabed B which is suitable for positioning, the valve controlling the water intake 18a into the pipe 18 is closed thus the descent is interrupted and the foundation is moved by towing vessels (not shown) into the required horizontal position and orientation. Upon reaching the required position, the ballast water intake 18a is opened again allowing more water flow into the secondary ballast compartment 16; thus the foundation starts to descend again due to the increasing weight. The foundation may experience wave induced motions that will be gradually impeded and subsequently stopped as the dowels 10a,b,c gradually start penetrating into the surface M and further into the seabed B. The dowels are sized so that before the lower tip of the skirt 7 touches the seabed B, the motions either have been stopped or reduced to small and acceptable amounts.

In Figure 17 is shown the foundation 1 in a stage of installation in which the skirt 7 has partly penetrated into the seabed B due to the weight of additional water that has flowed into the secondary ballast compartment 16, filled it up, and is now flowing over the overflow opening 19 into the main ballast compartment 13. The increasing weight of the foundation makes the skirt 7 penetrate into the seabed B thus creating sealed skirt compartments. The entrapped water is pressed out thought vents 42a,b between the region enclosed by the skirt 7 and the water outside the foundation. The vents are conveniently provided with check valves (not shown).

In instances where the seabed's upper layer consists of granular, hence rather permeable material such sand, silty sand or gravel, in contrast to the impermeable sediments such as clay and similar sediments, it is necessary to prevent direct contact between the entrapped water entrapped in the outer compartments and water in the seabed. This is to prevent seepage of water due to pressure gradients generated for achieving the leveling function described below. The sealing effect can be achieved by e.g. membrane or bags separating water inside and outside from a direct contact. In the preferred embodiment the sealing effect is achieved by grout, that may typically be a mixture of water, cement and sodium silicate. Figure 18 shows the final stage of embedding the skirt 7 into the seabed B where the outer skirt compartments 8a-c (only 8a,b is shown) are being filled with grout 46 in order to separate the entrapped water in the compartments from water in the porous soil. Again, the entrapped water is displaced through outlets 44a,b out of the compartment until the water has been fully replaced by grout 46 as shown in the skirt compartment 8b and the outlets 44 have been closed. Thereafter the ballasting by free flow of seawater into the foundation through inlet 18a is resumed. Although ballast water increases the weight of the foundation, no penetration takes place until the outlets 44a,b have been opened again and some of the grout, still in fluid phase, is being pressed out of the outer skirt compartments 8a-c by the excess weight of the foundation out of the outer skirt compartments 8a-c. Should at this point in the installation sequence the foundation have an unacceptable deviation from the horizontal, the deviation is easily rectified by restricting grout flow from the appropriate one(s) of the outer skirt compartments 8a-c. By this manner the levelness of the foundation can be achieved with high accuracy. When the desired penetration has been reached, the grout outlets 44 are closed and the penetration into seabed ceased even in the event that not all ballast water has flowed into the ballast chamber in the foundation. During the penetration process, water entrapped in the main skirt compartment 39 is being displaced through the outlets 43 and 42a,b, by the weight of the foundation reduced by the penetration resistance to the foundation's members that are being embedded into the seabed, such as the dowel, main skirt and the skirts of the outer skirt compartments. In case of hard soil layers the weight of the foundation with all ballast water in it may not be sufficient to penetrate the skirts and dowels into the soil. In such a case the embedment is enhanced by pumping water out of the main skirt compartment 39 though a suction outlet 43 thus creating a vertical downwards acting load on the .foundations that — overcomes the penetration resistance exerted on the skirts. Target embedment depth is reached when the landing surface 100 has attained contact with the seabed and sinking of the foundation has ceased. In case the embedment is enhanced by suction, reaching of target depth may be identified by a sudden increase of the suction pressure. When the landing surface solution is not used, visual observation of penetration marks or detectors of proximity to the mudline are used to identify the completed embedment.

Upon this there is a water filled cavity 45 between the mudline M and bottom plate 4 that is suitable for grouting as illustrated in figure 19. At the end of this phase when the outer skirt compartments 8 are filled with grout the foundation has capacity of resisting of substantial wave and current loads thus can wait until the next phase - the grouting. This feature represents significant practical, cost and logistics implications; reducing duration of required weather window thus increasing the probability of its occurrence, and allowing to perform grouting in another weather window. Figure 19 shows the grouting in progress where the grout 46 is pressing the entrapped water out of the skirt compartments 39 thought the vents 42b.

In Figure 20a the voids between the bottom plate and seabed has been filled by grout 46 in the previous operation and now the structure is under deployment of final ballast. It is shown that the main ballast compartment is already filled up and the ballast material is being deployed in the upper ballast compartment 16. In the shown arrangement deployment of the ballast material 35 is done by means of a temporary piping 48 that is routed over the top 34 of the foundation 1 and that is made of a rigid pipe section 49, flexible pipe section 50 and coupling 51. A standard dredging boat (not shown) is connected with its own pipes to the coupling 51 and pumps a slurry of water and solid ballast material through the temporary piping 48 into the funnel 2 on top of the chute 20. In the first phase the slurry of water and ballast material runs in the main ballast compartment 13. In the figure is shown a stage when the main compartment 13, the chute and funnel 20 have been filled up and the slurry is running over the rim of the funnel into the secondary (upper) ballast compartment 16. Hence, the entire interior of the foundation can be filled up by ballast material. Excessive water from the slurry freely runs out of the foundation through suitable piping, e.g. the conduit 18 described above, terminating at the outlet 18a. As the retention time is long for the water in the foundation, the displaced water, flowing through the outlet 18a, contains a reduced number of suspended solid particles. This is advantageous both from operational and environmental points of view.

The solid particles of the ballast slurry deposit in the foundation while the excessive water freely flows through the conduit 18 water outlet 18a out of the foundation. At the end of ballasting the foundation is filled with a desired volume of solid ballast and voids between the particles of the solid ballast are water filled up to the sea water level 52 outside the foundation 1. Finally, the valve controlling the outlet/inlet 18a is closed, the temporary piping 48 removed and installation of the structure is now completed. Figure 20b shows an optional embodiment of the foundation 1 ' and an associated ballasting system. The interior of the foundation 1 ' is divided by means of a bulkhead 17b into a ballast compartment 13 a, which is located centrically in the foundation, and a ballast compartment 13b, which has a shape of an annulus. Upon opening a valve at the water inlet/outlet 18a, ballast water flows into the center ballast compartment 13a via a conduit 18. Dimensions of the center compartment are designed so that the free surface of ballast water does not compromise floating stability of the foundation. When the water level in the central ballast compartment 13a has reached the top of the open conduits 18d and 18e, water flows into the annular ballast compartment 13b. Eventually, the entire interior fills with water until level inside equalizes with level of water W outside the foundation. In course of the final ballasting by sand slurry the filling may be done via a temporary piping as shown in figure 20a. Ballast filling of the compartments follows the same pattern as that of water filling described above.

If in the embodiment illustrated in figure 20b, the bottom plate 4 and the vertical wall 11 are made of concrete or a similar castable and heavy material while the remainder of the foundation 1 ' is made of steel, an inherently inert and comparably low centre of gravity is achieved for the foundation. It may therefore not be necessary to use sand ballast in the foundation when towing it to the offshore installation site, and the ballast spreader element and the associated parts described above with reference to the first embodiment are unnecessary.

Figure 21 illustrates an optional method for assembly of the bottom part 6 of the foundation 1, which is based on the use of a submersible barge 53. The barge is moored by means of a mooring 54, preferably to a quay 26. Prefabricated sections are assembled into the bottom section 6 on deck of the barge 53. Figure 22 illustrates load out for completed bottom parts 6a,b into floating.

Depending on draft conditions, both local and along the towing route to storage, it is preferable to perform as much assembly work as possible before the load out. By dashed lines it is indicated that conical section 28 and also the column section 29 can be mounted while the structure is on deck of the barge if the draft permits that. The barge 53 is shown submerged and resting on the seabed 3 and bottom parts 6a,b have floated off the deck and can be towed away thus allow the barge de-ballasted and prepared for assembly of next foundations.

In some instances, depending on the design, there may be an intermediate draft range where the foundation requires additional stabilizing to maintain sufficient floating stability. Figure 23 shows a side view of a floating stability device 82 supporting foundation 1 in a phase when the foundation has been floated in position between the columns 83a-d of the ballasted floating stability device 82 to a sufficient draft, and thereafter the floating stability device has been de-ballasted so that the foundation 1 is resting on the lower section 84 of the floating stability device 82. In this position, the foundation is prepared to be ballasted hence transferred from shallow draft to deep draft for towage to the installation site.

In the side view in figure 24, the foundation 1 is shown upon completion of ballasting and upon the foundation has reached desirable weigh and draft, followed by ballasting of the floating stability device 82 until sufficient clearance between the foundation 1 and the lower part 84 for float off of the foundation has been reached. In the position shown in figure 24, the foundation is ready to be towed away and the floating stability device 82 can be reused for aiding the next foundation. Figure 25 is a top view of the floating stability device 82 and the foundation 1 shown in figures 23 and 24. The preferred application of this optional design is to support the foundation during transfer from shallow to deep draft condition while being ballasted thus eliminating the need of designing the foundation for floating stability during such vertical transfer induced by adding weight to the foundation, hence savings from reducing the size of the body of the foundation 1. Another optional use of the floating stability device 82 is to use the device both for the entire assembly of the foundation thus replacing the use of the barge 53 and also for the transfer to deep draft position.

Figures 26 - 28 illustrate a modular floating stability device 90.

Figure 26 shows a side view of the floating foundation 1 to which the modular floating stability device 90 has been connected. The connection between the modular device 90 and the foundation may e.g. be achieved by a flange element 91 on the lower part of the foundation, against which the modular device 90 is abutting, thus supporting the foundation. The modular device 90 comprises a number of columns 93, extending above the water surface. As seen in the figures, the modular device 90 comprises a first section 90a and a second section 90b, interconnected by means of a hook-joint, key or similar. Each module is provided with a respective hook element 92a,b, where one hook element 92a is facing downwards while the other hook element 92b is facing upwards. The modular device 90 is connected to the foundation 1 by first moving the first section 90a into abutment with the foundation, preferably below the flange element 91, as illustrated in figure 27. Then, the second section 90b is also moved against the foundation in a ballasted condition, whereupon the second section is de-ballasted, causing it to rise into abutment with the flange element 91 and whereupon the hook elements 92a,b interlock, thus forming a rigid support structure for the foundation. This embodiment allows connecting and disconnecting of the floating device 90 to and from the foundation 1 while it is resting on the bottom of a seabed.

The sections 90a,b of the modular floating stability device 90 are designed as hollow bodies, preferably in a shape embracing the foundation along its periphery, e.g. circular as seen in the figures. The body of each section 90a,b comprises of a lower section 94 resembling a pontoon and a number of vertical columns 93. The first and second sections 90a,b of the modular floating device 90 are designed for free floating conditions (i.e. not attached to the foundation) in a stable vertical position. In this stage the lower section 94 is flooded with ballast water. Fine adjustments of draft, required for mating with the foundation and detaching for it, are performed by changing the amount of ballast water in dedicated vertical column(s) 83. The lower section 94 may be designed so that either it is flooded or empty, thus there is no free water surface during the operations. Further by means of its design the wave induced motions are greatly reduced. Figure 28 is a top view of the foundation 1 attached to the first section 90a, similar to what is shown in figure 27. The purpose of the modular floating stability device 92 is to provide additional water plane area to the floating foundation 1 and hence make it stable during tow-out and installation.

The invention is particularly suitable for suitable for supplies from assembly site with limited water depth at the assembly site or along the towing route.

Figures 29a-j illustrate an optional assembly method based on the use of a buoyancy device and making some parts of the foundation by structural concrete. In particular the bottom slab and the vertical wall, or part of it, can be advantageous to construct of normal or pre-stressed concrete. Fig. 29a is a top view of a pre-fabricated bottom part 110 of the foundation comprising of the circumferential skirt 7 with plurality skirt stiffeners 106, the outer skirt compartments 8a-c and dowels lOa-c. As explained above, the foundation may optionally comprise fins (not shown in figure 29a.). The main skirt compartment is at both ends open, while the outer skirt compartments 8a-c are preferably provided with a watertight upper cap. The circumferential skirt 7 may extend above the bottom of the future bottom slab in order to provide a form for concrete casting.

Fig.29b is a cross section of the prefabricated bottom part 110 of the foundation resting on supports 27a,b showing the circumferential skirt 7, an outer skirt compartment 8b with its watertight cap 107b. The prefabricated bottom part is ready for the next step described in Fig. 29d.

Fig. 29c illustrates a float 108 that serves two purposes in the further assembly of the foundation, namely as the support for fresh, non-hardened concrete within the circumferential skirt area and as additional buoyancy during the assembly process. The upper face 109 of the float 108 is flat and suitable interface with the concrete.

Fig 29d shows lowering of the prefabricated bottom section 110 onto the float 108, whereby the float 108 is accommodated within the circumferential skirt, as is illustrated in fig. 29e. The bottom sectionl 10 can alternatively be assembled from suitably sized parts on top of the float 108.

Fig. 29e shows vertical section of the float 108 and of the bottom section 110 resting on the float. It is seen that the float 108 is divided into compartments 11 la-d and 112. The former are designed for ballasting by seawater while the latter is always air- filled. Dimensions of the ballast able compartments 11 la-f are so small that the possible free water surface area does not reduce the floating stability below acceptable level.

In Fig. 29f the shown situation upon performing preparatory work, such as covering of the area outside the upper surface 109 of the float 108 and inside the extension of the circumferential skirt, providing the horizontal support surface 109 with a separation membrane that will facilitate removal of the float 108 in a next phase, and finally, installation of the steel reinforcement and other steel details that will be embedded in the concrete. Further, the figure shows casting of concrete in progress indicated by arrows C. In case that fresh concrete of rather fluidized consistency is used, the float 108 needs to be stabilized until the concrete has stiffened.

Fig. 29g illustrates completed bottom slab 112 and a section of the superstructure made of vertical wall 11 and a conical section 113 being lowered onto the bottom slab for integration of the two parts into one. Alternatively, the construction could proceed according to commonly known technique using concrete cast into temporary formwork.

Fig, 29h shows installation of another section 114.

Fig. 29i shows the completed foundation 1 floating in water and supported by the float 108 and prepared for removal of the float.

Fig29j shows the float 108 resting on the seabed and ballasted with seawater. Ballasting has made the float non-buoyant and therefore is has descended from the main skirt compartment onto the seabed.

The float 108 shown in figures 29a-j may be replaced with the submersible barge 53 shown in fig 21 and 22.

Claims

1. A foundation (1; 1') for installation on a seabed (B) below a body of water (W), having a bottom part (6), a main body (5) and a connection portion (15) for a support structure and/or an equipment unit, characterized by a first ballast compartment (13; 13b), a second ballast compartment (16; 13a), and means for filling the second ballast compartment with a ballasting agent; said second ballast compartment being configured such that the free surface of the ballasting agent in the compartment does not compromise the floating stability of the foundation; and means for filling the second ballast compartment (16; 13a) with a ballasting agent until a predetermined level has been reached; and means for subsequently filling the first ballast compartment (13; 13b) with a ballasting agent.

2 The foundation of claim 1, wherein the means for subsequently filling the first ballast compartment comprises at least one opening (19; 18d, 18e) between the first and second ballast compartments.

3. The foundation of claim 2, wherein the at least one opening between the first and second ballast compartments is arranged in the region of said predetermined level of the second ballast compartment, whereby ballasting agent is allowed to flow from the second ballast compartment into the first ballast compartment through said opening when the ballasting agent has reached the predetermined level.

4. The foundation of any one of claims 1 - 3, further comprising a channel (20) extending between a region above the second ballast compartment (16; 13a) and into the first ballast compartment (13; 13b).

5 The foundation of any one of claims 1 - 4, wherein the second ballast compartment (16) is arranged above the first ballast compartment, and a portion (20') of the first ballast compartment (13) extends a first distance (d) into the second ballast compartment (16).

6. The foundation of claim 7, further comprising a conduit (18) extending between the second ballast compartment (16) and the exterior of the foundation (1).

7. The foundation of any one of claims 1 - 4, wherein the second ballast compartment (13a) comprises a central ballast compartment and the first ballast compartment (13b) comprises a ballast compartment surrounding the second ballast compartment, preferably in the shape of an annular compartment around the central compartment.

8. The foundation of claim 7, further comprising a conduit (18) extending between the first ballast compartment (13b) and the exterior of the foundation (V)

9. The foundation of any one of claims 1-6, wherein a lower channel portion (22) is connected to the channel (20) via an articulated joint (21), whereby the lower channel portion (22) extending into the first ballast compartment (13).

10. The foundation of claim 8, further comprising controllable means (23) for moving the lower channel portion (22) within the first ballast compartment.

11. The foundation of any one of the preceding claims, wherein the conduit (18) comprises a valve (18c) and a first opening (18a) outside of the foundation (1) and a second opening (18b) in the second ballast compartment (16), at a second distance (e) above the first opening (18a).

12. The foundation of any one of claims 1 - 6 or 9 - 11, further comprising a spreader element (24) arranged below a lower channel portion (22) on the channel (20) such that ballast material coming out of the lower channel portion (22) impacts the spreader element (24).

13. The foundation of claim 12, wherein the spreader element comprises a conical surface (24a) and a plurality of plates (24b) arranged around the conical surface, a portion of each plate (24b) being raised above the conical surface (24a).

14. The foundation of any one of the preceding claims, wherein the bottom part (6), preferably around its periphery, comprises a skirt element (7) extending downwards thereby defining a chamber (45) underneath the bottom part, wherein the skirt element (7) is adapted for at least partial penetration into the seabed, the foundation further comprising orifices (42a,b) between the chamber (45) defined by the skirt and the exterior of the foundation.

15. The foundation of any one of the preceding claims, further comprising a plurality of load resisting and leveling means (8a-c) arranged around, and extending from, the foundation.

16. The foundation of claim 15, wherein the load resisting and leveling means comprise outer skirt elements (8a-c) arranged at regular intervals around the bottom part periphery and extending downwards, thereby defining respective separate chambers underneath each outer skirt element, wherein the outer skirt elements (8a- c) are adapted for at least partial penetration into the seabed and each outer skirt element (8a-c) further comprising respective orifices (44a-c) between the chamber defined by the outer skirt and the exterior of the foundation, whereby a filler material 46 may be injected and direct contact between water in the skirt elements and the porous seabed sediment is prevented.

17. The foundation of any one of the preceding claims, further comprising a plurality of arrester means (lOa-c) arranged around the periphery of the bottom part (6) and selectively movable from a retracted position where the arrester means do not extend below the bottom part, to an extended position where the arrester means extend below the bottom part.

18. The foundation of any one of the preceding claims, wherein the bottom part (6) comprises a bottom plate (4) having a peripheral and upward extending wall (11), said bottom plate (4) and upwardly extending wall (11) being made substantially of concrete or a similar castable and heavy material, while the remainder of the foundation generally being made of steel or a similar metallic material.

19. A method of providing a plurality of foundations according to any one of claims 1 - 19 at an installation site, characterized by the steps of: a) storing the foundations in a floating state or resting on a seabed at an inshore location in preparation for installation; b) towing one or more foundations to an offshore installation site; and c) installing said foundations on a seabed.

20. The method of claim 19, wherein, before step a, the plurality of foundations are pre-fabricated, optionally at one or more locations other than the site at which they are being assembled, and assembled at said site in order to produce a desired number of foundations;

21. The method of claim 19, wherein the assembly of said modules comprises: a) extending a circumferential lower wall (11) from a bottom plate (4) to form a foundation bottom part (6), said lower wall having a vertical extension dimensioned according to the buoyancy requirements for the completed support structure; b) placing the bottom part (6) in a floating position on the surface of the body of water; c) further assembling the foundation (1) by successive assembly of prefabricated modules (12a-c, 14, 15).

22. A method of installing the foundation of any one of claims 1 - 18 on a seabed below a body of water (W), characterized by the following steps: a) enabling water from the body of water (W) to flow into the second ballast compartment (16; 13a) through the conduit (18) until a portion (7) of the foundation

(1) at least partly is penetrating into the seabed (B), whereby water inside the compartment defined by the portion (7) is forced out via outlets (42a,b); b) activating selected ones of the leveling means (8a-c), whereby the foundation is placed on the seabed in a substantially level state; and c) deploying ballast material (34) into the first ballast compartment (13) or into the second ballast compartment (13a).

23. The method of claim 21, wherein step b) is repeated until a target embedment depth is reached and a landing surface (100) on the foundation has attained contact with the seabed and sinking of the foundation has ceased.

24. The method of claim 22, wherein step a) is preceded by the step of moving the arrester means (lOa-c) from a retracted position to a locked state in the extended position;

25. The method of any one of claims 22, wherein before step a), deploying a predetermined amount of paniculate ballast material into the first ballast compartment (13), such ballast material comprising dry or naturally moist materials such as sand, gravel or iron ore.

26. The method of claim 22, wherein step c) comprises the at least partial filling of selected ones of the outer skirt compartments (8a-c) with grout (46).

27. The method of claim 22, wherein step a) is terminated when water in the second ballast compartment (16; 13a) has reached a level where it flows via an internal conduit (19; 18d, 18e) into the first ballast compartment (13; 13b).