WO2011115504A2

WO2011115504A2 - Device for improving floating stability and floating ability of floating structures

Info

Publication number: WO2011115504A2
Application number: PCT/NO2011/000088
Authority: WO
Inventors: Sigurd Ramslie; Karel Karal
Original assignee: Seatower As; Karal, Eva
Priority date: 2010-03-18
Filing date: 2011-03-17
Publication date: 2011-09-22
Also published as: NO20100398A1; WO2011115504A3

Abstract

A buoyancy element (6, 7) for releasable connection to a structure (1) floating in a body of water (W) is characterized by a lower support element (19) and compartments (11 -14) fluidly interconnected via openings (60). One of said compartments is a central compartment (11) which via a control means (11b) and a duct (11a) is selectively fluidly connected with water surrounding the buoyancy element, whereby the buoyancy element (6, 7) is connected to the structure (1) only by means of the hydrostatic pressure exerted by the water onto the buoyancy element (6, 7). A device (8) for increasing the buoyancy and stability of a structure (1) floating in a body of water (W), for releasable connection to said structure is characterized by a wall (24a, 24b) encircling a portion (2a) of the structure and extending upwards from a support region (3; 30) on the structure to a distance above the water and thereby defining a compartment (25) between the wall section and the portion of the structure, whereby the water plane area and buoyancy for the combined device and structure are enhanced. A buoyant compounded structure, capable of being transported in a semi- submersible state in a body of water, comprises a plurality of buoyant individual structures (l a-c) having respective individual water plane areas and connection means (30; 31, 30a-c, 33) and abutment regions (9, 34, 35, 34', 35 '). The connection means are releasably connected between each of said individual structures at a vertical distance from said abutment regions, whereby the compounded structure comprises one, substantially rigid, body having a floating stability which is better than the floating stability of the individual structures when such individual structures are disconnected from each other.

Description

Device for improving floating stability and floating ability of floating structures

Field of the invention

The present invention relates to floating structures that are designed for wet tow to the installation site and for deployment on the seabed by increasing weight of the structure. In most practical applications the invention relates to support structures for offshore wind turbines where installation of multiple structures is normally required.

More specifically, the invention relates to a buoyancy element for releasable connection to a structure floating in a body o water, as set out in the introduction to the independent claim 1 ; a device for increasing the buoyancy and stability of a structure floating in a body of water, as set out in the introduction to the

independent claim 12; and a device for controlling and connecting a plurality of structures, as set out in the introduction to the independent claim 16.

Background of the invention

There is an increasing need for wind turbine supporting structures that can be installed without the use of crane vessels, as disclosed in e.g. WO 2009/154472. Two major cost drivers are associated with design and size of such structures, i.e.

(a) Draft of the structure when floating in non-ballasted condition during load out and wet tow to deeper waters: Limits for admissible draft given by natural conditions at available construction or load-out sites drive the cost of such structure up as they set constraints for optimization of the structure and implies increased dimensions of bottom part of the structure with negative consequences on the area and volume exposed to current and wave loads in installed state hence calling for increase of the foundation part transferring the loads into the seabed.

(b) Floating stability in all phases of the installation until supported by seabed:

Requirements for adequate floating stability dictates both position of the center of gravity, centre of buoyancy and size of water plane area with the results of increased dimensions, material consumption and modified shape compared to a design disregarding the floating stability requirements.

It is therefore a need for devices which overcome the disadvantages of the prior art.

Summary of the invention

It is thus provided a buoyancy element for releasable connection to a structure floating in a body of water, characterized by a lower support element and compartments fluidly interconnected via openings, of which one of said

compartments is a central compartment which via a control means and a duct is selectively fluidly connected with water surrounding the buoyancy element, whereby the buoyancy element is connected to the structure only by means of the hydrostatic pressure exerted by the water onto the buoyancy element.

In one embodiment, the central compartment is located in the buoyancy element's centre of mass. In one embodiment, the central compartment is located in the buoyancy element's geometrical centre.

Preferably, the other compartments are located symmetrically around the central compartment. In one embodiment, a first series of compartments are located radially around the central compartment, a second series of compartments are located radially around the first series of compartments, and a third series of compartments are located radially around the second series of compartments.

In one embodiment, the buoyancy element further comprises an upper support element, whereby the plurality of compartments are enclosed.

The wall sections of the individual compartments are configured for interaction with a portion of the structure whereby the plurality of compartments are enclosed when the buoyancy element is connected to the structure, and hydrostatic loads are transferred from the lower support element to the portion of the structure.

The buoyancy element is adapted for insertion into a cavity underneath the structure. In one embodiment, the buoyancy element comprises a circular, disk-like, shape.

It is also provided a device for increasing the buoyancy and stability of a structure floating in a body of water, for releasable connection to said structure, characterized by a wall encircling a portion of the structure and extending upwards from a support region on the structure to a distance above the water and thereby defining a compartment between the wall section and the portion of the structure, whereby the water plane area and buoyancy for the combined device and structure are enhanced.

In one embodiment, the wall comprises a frusto-conical body. In one embodiment, the wall comprises a substantially straight-walled, circular, body, substantially aligned with the structure's axis of symmetry, and having a lower region with a first diameter and an upper region with a second diameter which is less than the first diameter, an intermediate transition step between the two sections and at a distance below the surface of the water. The wall may comprise at least two segments interconnected by connecting means and provided by sealing means.

It is also provided a buoyant compounded structure, capable of being transported in a semi-submersible state in a body of water, characterized in that it comprises a plurality of buoyant individual structures having respective individual water plane areas and connection means and abutment regions, said connection means being releasably connected between each of said individual structures at a vertical distance from said abutment regions, whereby the compounded structure comprises one, substantially rigid, body having a floating stability which is better than the floating stability of the individual structures when such individual structures are disconnected from each other.

In one embodiment, the connection means are releasably connected between each of said individual structures at individual regions at a vertical distance from said abutment regions, whereby a horizontal load is exerted at each point and oppositely directed reactive forces of the same magnitude are generated in the abutment regions, thus preventing an inadvertent separation in the compounded structure and the individual structures comprise one, substantially rigid, body.

In one embodiment the connection means comprise cables or chains, each one connected at first end to a respective connection region and at a second end to a connection element which is located at or near the plurality of individual structures' vertical axis of symmetry, and a tensioning mechanism is connected between a first end of one of said cable or chain, and a connection region.

In one embodiment the connection means comprise a cable or chain which at a first end is connected to a region on a first individual structure, via a tensioning mechanism, and which at a second end also is connected to said first individual structure, and intermediate portions of the cable or chain are movably connected to the remaining connection regions. In one embodiment, the connection means comprise a rigid frame structure connected to respective connection regions on each of the individual structures.

In one embodiment the connection regions are such located on the structures that they are in or near the water line for the individual structures when the rigid frame structure is disconnected from the compounded structure. The rigid frame structure may comprise a buoyant element.

In one embodiment, the connection regions are located at or near the top end of each of the individual structures. The tensioning means may be arranged

substantially horizontally. Preferably, the individual structures are arranged substantially symmetrically about a common vertical axis defining a vertical axis of symmetry for the compounded structure, and further comprise at least one ballast compartment in each of the individual structures, arranged at a distance from said structures' common axis of vertical symmetry.

Preferably, the individual structures are substantially identical bodies and preferably comprise circular horizontal cross-sections. In one preferred embodiment, the connection means comprise tensioning means. The abutment regions may comprise releasable locking means. Preferably, the connection means are arranged above the body of water, while the abutment regions are arranged in the body of water. In one embodiment, each of the individual structures comprises a buoyancy element according to the invention.

The present inventions introduce a number of related solutions for increasing floating stability and decreasing required draft of offshore structures designed for wet tow to the installation site followed by seawater enhanced descent to the seabed.

The solutions provide means for temporary:

• utilization of space confined by skirt for creating air support cushions for

increasing buoyancy and thus decreasing draft;

• augmentation of water plane area for increased floating stability and for

increased buoyancy thus decreased draft; · assembly of several structures into one rigid body exercising increased floating stability; and

• combinations of these inventions.

The inventive solutions utilize the effects of several measures introduced to overcome the above mentioned negative aspects. These can preferably be used in appropriate combinations or they can be, with less net improvements, used individually.

Brief description of the drawings

These and other characteristics of the inventions will be clear from the following description of preferential forms of embodiment, given as non-restrictive examples, with reference to the attached schematic figures wherein:

Figure 1 is a vertical section of a floating structure;

Figure 2 is a vertical section of the floating structure shown in figure 1 , provided with a floating ability enhancing temporary bottom;

Figure 3 is a vertical section of the floating ability enhancing bottom shown in place with a floating structure in figure 1 ;

Figure 4 is a top view of the floating ability enhancing bottom shown in figure 3; Figure 5 is a vertical section of the floating structure shown in figure 1 , provided with a floating ability enhancing temporary device below the structure formed as a buoyancy element;

Figure 6 is a vertical section of the floating ability enhancing device shown in place with a floating structure described in figure 5;

Figure 7 is a top view of the buoyancy device explained in figure 6, shown inserted into the space confined by skirts;

Figure 8 is a side view of a floating structure shown in figure 1, provided with a floating ability and floating stability enhancing temporary device formed as a cofferdam;

Figure 9 shows a vertical section through a floating structure and through a temporary device as illustrated in figure 8, that increases the water plane area for the assembly and increases buoyancy;

Figure 10 is a horizontal section along the line A-A in figure 9;

Figure 1 1 is as vertical section through another embodiment of a floating ability and floating stability enhancing temporary device;

Figure 12 is a horizontal section along the line A-A in figure 1 1 ;

Figure 13 is a vertical section of three similar floating structures shown in figure 1 interconnected into a body with enhanced floating stability compared to structures floating individually;

Figure 14 is a side view of the assembly of structures in figure 13, shown with additional details;

Figure 15 is top view of the assembly of structures in figure 13, shown with additional details;

Figure 16 is a vertical section of structures connected into rigid assembly showing eccentrically located ballast compartments;

Figure 17 is a horizontal section of structures connected into rigid assembly showing eccentrically located ballast compartments and the lower connecting elements;

Figure 18 is vertical section of bottom connections between two structures, aligned for entering into position;

Figure 19 is vertical section of engaged connection; Figure 20 is a side view showing three individually floating structures rigged for establishing connection between them;

Figure 21 is a side view of alternative embodiment of the connection illustrated in figure 13;

Figure 22 is a top view of the embodiment illustrated in figure 21 ;

Figure 23 is a top view illustrating an alternative connection;

Figure 24 is a top view illustrating yet an alternative connection; and

Figure 25 is a sectional drawing illustrating a combination of the invented buoyancy and stability enhancing devices.

Detailed description of preferential embodiments

Figure 1 is a schematic sectional drawing of a typical structure 1 , floating in a body of water W and comprising a superstructure 2a and a foundation base 2b. Only features which are necessary for describing the invention are shown; chambers inside the superstructure, valves, etc., have been omitted for the sake of clarity of illustration.

The foundation base 2b comprises a bottom slab 3 and a circumferential sheet element 4 which is protruding downwardly from the bottom slab 3, thus defining an open cavity 5. This so-called skirt 4 is designed to penetrate into the seabed S in an installed state to ensure sufficient bearing capacity for the superstructure and the equipment that may be erected from it (e.g. wind turbine tower). Deeper skirts often require smaller bottom slab 3 and smaller foundation base and superstructure, with reduced fabrication costs as a consequence. However, safe operations with floating structures require a minimum clearance between the seabed S and the lower tip of the skirt 4 and yields a maximum allowable draft denoted by letter a, which prevents the optimization. Therefore it is a need for reducing the draft of the structure 1.

Figure 2 shows an innovative buoyancy device in the form of a temporary bottom 6 assembled onto the skirt 4 and having wall segments extending into the cavity 5. Figures 3 and 4 illustrate the temporary bottom 6 in more detail. The position of the skirt 4, when the temporary bottom is assembled onto the structure 1 , is indicated by dashed lines in both figures.

The temporary bottom 6 comprises a bottom plate 19 and a plurality of concentric ballast compartments 1 1 , 12, 13, 14, one of which being a central ballast

compartment 1 1. These ballast compartments are defined by circular vertical walls 15, 16, 17, extending inside the cavity 5, from the bottom plate 19 and up to the lower face of the bottom slab 3 when the temporary bottom 6 is assembled on the structure 1. These vertical walls are designed for transferring hydrostatic loads from the bottom plate 19 to the bottom slab 3. When submerged, e.g. as shown in figure 2, water pressure holds the temporary bottom 6 in contact with the skirt 4, and also holds the compartment walls in contact with the bottom slab, by exerting a vertical load that lifts the structure higher up, to float with reduced draft, indicated by letter b in figure 2. The vertical force holds the temporary bottom 6 in place. Hence, no additional connection means are necessary to keep safely the temporary bottom 6 in place when the structure floats in water. Water tightness on the interface between the temporary bottom and the structure, i.e. between the skirt 4 and the bottom plate 19 and between the vertical walls and the bottom slab 3, may be ensured by e.g. a rubber gasket (not shown), or by a circumferential mould 18 creating space 20 that can be filled with a suitable material such as concrete or plastic clay (not shown).

Radial bulkheads 21 and 22 divide the concentric compartments created by the cylindrical walls 15, 16 and 17 into smaller compartments thus reducing the free surface area during ballasting.

The vertical walls 15, 16, 17 are provided with a number of overflow openings 60, preferably located high up, away from the bottom plate 19. The overflow openings 60 allow fluid communication between the compartments and may be simple cutouts in the vertical walls; in a quantity and of a dimension which is suitable for the actual application. Alternatively, overflow between the compartments may be achieved by making selected ones of the vertical walls a little shorter than the majority of the vertical walls, thus providing a gap between the vertical wall and the bottom slab when the temporary device is installed in the cavity.

When the cavity 5 - and thus the compartments 1 1 , 12, 13, 14 - is filled with air, the hydrostatic pressure exerts a vertical force acting upwards. This vertical force improves the floating ability (buoyancy) and reduces the draft of the structure 1 to a reduced draft, denoted b in figure 2, that is less than the minimum achievable draft a (figure 1 , without the temporary bottom 6).

When the structure is at location with sufficient water depth along a towing route, the temporary bottom 6 can be removed. Removal of the temporary bottom 6 is achieved by letting seawater flow into the concentric ballast compartments 1 1 , 12, 13, 14 in a controlled manner, via fluid inlet 1 la and valve l i b, starting with the central compartment 1 1. The entrapped air is vented out of the ballast compartment and into open sea where it ascends to the surface. (Some of the required inlet/vent piping, valves, etc. have been omitted for clarity of illustration and as these features are readily understood by the skilled person.) When the central compartment 1 1 is filled, ballast water flows through the overflow openings 60 in the vertical wall 15 and into the adjacent compartment 12. When this compartment has been filled, ballast water flows through the overflow opening in the vertical wall 16, and so on into the next compartments 13 and 14, until the net hydrostatic pressure on the temporary bottom 6 becomes equal to or larger than the submerged weight of the bottom 6, and the temporary bottom 6 releases itself from the structure 1.

The compartmentalization of the temporary bottom 6 and the successive flooding from the central compartment 1 1 is introduced for achieving consistent floating stability throughout the ballasting. As a result of flooding of the compartments the bottom 6 releases itself from the contact with structure and tends to move down from the floating structure. When suitable for further operations the bottom is prevented from further descent and retrieved for e.g. reuse.

The temporary bottom 6 is conveniently installed in the cavity 5 during fabrication, i.e., the structure 1 is erected over the temporary bottom.

Figures 5, 6 and 7 show an alternative embodiment of the inventive buoyancy device, in the form of a temporary buoyancy element 7, inserted into the cavity 5 defined by skirts 4 and the bottom slab 3.

The temporary buoyancy element 7 is in the illustrated embodiment a disk-like buoyancy element inserted in the cavity 5. The position of the skirt 4, when the temporary bottom is assembled onto the structure 1 , is indicated by dashed lines in figures 6 and 7. As in the first embodiment described above, the temporary buoyancy element 7 comprises a bottom plate 19 and a plurality of concentric ballast compartments 1 1 , 12, 13, 14, one of which being a central ballast compartment 1 1. These ballast compartments are defined by circular vertical walls 15, 16, 17, extending from the bottom plate 19 and up to a top plate 23. These vertical walls are - together with a circumferential wall 62 - designed for transferring hydrostatic loads from the bottom plate 19 to top plate 23, which is bearing against the underside of the bottom slab 3 when the temporary buoyancy element 7 is installed in the cavity 5. The temporary buoyancy element 7 thus comprises an enclosed buoyancy element. (Some of the required inlet piping, valves, etc. have been omitted for clarity of illustration and as these features are readily understood by the skilled person.).

It may be advantageous, both from technical and price points of view, to construct the bottom plate 19, or both the bottom plate 19 and top plate 23 of reinforced concrete. Potential advantages are lowering of the center of gravity, hence increase of floating stability of the assembly comprising of the structure (indicated by the circumferential skirt 4) and the inserted temporary buoyancy element 7. The division into compartments 1 1 , 12, 13, 14 by walls 15, 16, 17 and further subdivided by means of bulkheads 21 , 22 (as in the first embodiment) may also serve for the construction of the structure's bottom slab during construction of the bottom slab, when it may serve as the work area and support for the pre-assembled reinforcement "cage" for the bottom part of the structure and for carrying weight of concrete during casting and curing.

Figure 7 is a top view of the temporary buoyancy element 7 inserted into the space confined by the skirts 4. Dashed lines indicate inner compartments 1 1 , 12, 13, and 14, and division walls 15, 16, 17, 21 and 22. The compartmentalization is again necessary for achieving sufficient floating stability during flooding when water inside the device has free surface. Removal of the temporary buoyancy element 7 from the structure in depth in which the structure can float with assistance of the device is performed in the same manner as for the first embodiment, described above, i.e. by letting water flow into the central compartment 1 1 via fluid inlet 1 1 a and valve 1 lb. The air entrapped in the compartments 1 1 -14 is compressed as water is flowing into the compartments. Depending on the operational circumstances, this entrapped air may or may not be vented out of the element via outlet piping (not shown). Thus, the design of the temporary buoyancy element 7 can achieve an optimal submerged weight for the removal and retrieval operations.

The temporary buoyancy element 7 is conveniently installed in the cavity 5 during fabrication, i.e., the structure 1 is erected over the temporary buoyancy element.

Figure 8 shows the structure 1 fitted with an innovative temporary device 8 that increases the water plane area with the effect of enhancing both the floating ability and floating stability of the structure 1. Because of the increased water plane area and the increased buoyancy volume, the floating stability is increased and the draft of the structure is reduced to a reduced draft b, compared to the draft a shown in figure 1.

Figure 9 shows a vertical section through structure 1 and through the temporary device 8, and figure 10 is a horizontal section at level A-A in figure 9.

The temporary device 8 comprises a conical wall (frustum) 24a having a lower portion resting on a suitable contact area on the structure, in this embodiment on the foundation base 2b, hence providing additional water plane area for enhanced stability and creating empty space 25 furnishing additional buoyancy. In sidewise direction the wall 24a is secured in position by a rim 26, or by other means such a radial rib, downwards protruding dowels (not shown) into the base 2b. In a vertical direction, the wall 24a is held in tight contact with the case by means of is weight (the entire dry weight of the wall is transferred onto the contact area. As the wall 24a is conical and inclined inwards as shown in figure 9, water pressure enhances the contact force. In figure 9, the water pressure p is decomposed into a horizontal component pn and py to indicate how the vertical contact force increases with the inclination of the wall 24a. Water tightness at the contact area may be ensured by e.g. a rubber gasket (not shown), or by a suitable material such as concrete or plastic clay (not shown). Contrary to the known buoyancy elements, the temporary device 8 does not require a water-tight and load-bearing bottom, as the temporary device is capped it its lower part by the structure supported by the element to form a water-tight buoyancy volume.

In this embodiment, the wall 24a comprises two segments 27a,b provided with connecting means 28a,b and gaskets 29a,b. Hydrostatic water pressure acting on the wall 24 exerts a comparatively large load with a horizontal component pn (see figure 9) which forces the two sections together, hence no operational loads need to be transferred through the connecting means 28a,b. A redundant connection may be added for the case of accidental loads. The circumferential rim 26 prevents the device 8 from uncontrolled sliding due large accidental sideways loads.

When the structure 1 has either been lowered to a depth when it has sufficient stability or after it has been deployed onto the seabed S, the void space 25 can be flooded by seawater, e.g. via piping and valves (not shown) in a conventional manner. Due to its tailored buoyancy the temporary device 8 floats off the structure and upon disconnecting the wall segments 27a,b is removed.

In the event that the amount of the required temporary increase of water plane area and buoyancy allow reduced dimensions of the temporary device 8, the temporary device 8 can be attached to the structure 1 at a higher elevation than described above. One such example is shown in figure 12. This reduces the dimensions of the device 8, which is in figure 1 1 is shown with vertical circular walls 24b, having a stepped change in diameter at a predetermined elevation E where the required properties of the device are still within acceptable limits. At the elevation E where the diameter change occurs, a horizontal surface is attracting vertical pressure py, pushing the device 8 towards the foundation base 2b in order to improve contact with the structure 1 as explained above.

Figure 12 is a horizontal section at level A-A in figure 1 1 , through the assembly of the structure 1 and temporary buoyancy device 8, showing two segments 27'a,b connected by connecting means 28a-d and comprising gaskets 29a,b.

Figure 13 shows three similar structures l a,b,c, innovatively connected into a rigid body with three individual water plane areas. The distances between the structures' water plane areas add to the stabilizing moment for the cluster of structures, resulting in improved stability of this temporary assembly, compared to the stability of each foundation floating individually. The connection is achieved by a first connector 9 releasably connected to the upper part of the structures l a,b,c, e.g. on top of the structures. The bottom part of the structures l a,b,c are releasably connected to each other by means of second connectors 10, which will be described in more detail in the following. Referring to figures 14 and 15, the first connector 9 comprises cables 30a,b,c, each of which at one end is connected to a respective structure la-c via respective attachment points 32a,b,c and at the respective other end connected to a central element 31. One of the cables 30a is in addition furnished with a tensioning mechanism 33. By operating the tensioning mechanism 33, all three cables 30a-c are exposed to the same load, as they are separated in equal (120°) segments.

The cables 30a-c are tensioned to a required tension, thus exerting a horizontal load PH on each of the structures l a-c. An oppositely acting force PH of the same magnitude acts in the bottom connection 10 as a reaction to the tension exerted by the cables 30a-c, thus preventing an inadvertent separation of the structures and keeping the assembly of structures as a rigid body. The tension force in the cables should preferably be higher than that which the predicted wave loads would induce, in order to prevent slack in the cables. The tension in the cables 30a-c imposes a heel angle a, as shown in figure 14 (the figure indicating twice the heel angle). When the assembly of structures l a,b,c is being lowered down to deeper water (e.g. for disconnection, in a manner known in the art), it is necessary to maintain the tension in the cables 30a,b,c in order for the assembly to remain as a rigid body. This is achieved by feeding ballast water into eccentric ballast compartments 40a,b,c in the interior of the structures l a,b,c; illustrated schematically by figures 16 and 17, thus adding eccentric weight. These compartments are designed and sized so that the weight of ballast water with its moment arm from the buoyancy center exerts the required tension in cables 30a,b,c so that no slack can occur in the cables.

Referring to figures 17, 18 and 19, the second (lower) connector 10 comprises in the illustrated embodiment first mating portions 34 and second mating portions 35 by which the bottom parts 43a-c are interconnected. Each bottom 43a-c part is furnished with a protrusion 34' and a recess 35 ', interlocking in a tongue-and- groove configuration. The connection must be rigid in all degrees of freedom except rotations in two perpendicular planes. However, the rotations - and corresponding heel angle a - are fairly small (for wind turbine foundations typically < 2°). On the other hand, the shear loads in these connections may be considerable and the dimensions of the mating portions should be adequate for resisting the loads with appropriate reserves. In the illustrated embodiment, the protrusion 34' and recess 35' comprise spherical segments.

As the structures may have somewhat different drafts and sideways misalignment when a connection of two floating structures la and l b is to be made, it is advantageous that either the first mating portion 34 or the second mating portion 35 are furnished with guides 36a, 36b, for guiding the structures into a connecting engagement. The guides 36a,b are shaped such that the expected misalignments can be accommodated. By moving the structures towards each other, one of the guides 36a,b slides over the corresponding of the guide supports 37a,b such that the misalignment is gradually reduced and eliminated when the tongue 34' is entering the groove 35' . Misalignments in the horizontal plane are corrected by a similar manner. It may be required to safeguard the connection from accidentally disconnecting. This may be achieved by standard locking mechanisms, e.g. in the form of a pad eye and a bolt, or a lever arm locked by a pin (not shown), which are easy to open as they do not transfer any loads but only in accidental situation.

Figure 19 shows a situation where the connection has been completed, i.e. the structures have been pulled together as explained above and the reaction force PH is exerted in the connection as the result of the tension in the upper tensioning arrangement 9, as described above. By properly designing the connection elements, no additional means for keeping the structure connected is needed. As the result of the moment due to load PH at bottom and due to the load PH at top, the structures are tilted with respect to one another, as indicated by the heel angle a in figure 19 (see also figure 14). In this state the assembly of structures is ready for towage to deeper water where it is ballasted down to a draft at which the individual structures 1 a-c are stable with the water ballast inside.

Additional buoyancy for reduction of draft of the assembly can be achieved by displacing some of water inside the skirt compartments by air. The amount of air must be less than the stability on the compressive air cushions allows and less than extreme tilt during operation would lead to loss of air resulting from escape below the inclined skirt. A minimum height of the water plug is therefore required.

Connecting of the structures into one rigid body is performed either onshore or in sheltered waters. The latter method is explained with reference to figure 20, in which three structures la,b,c are floating separately, each of them controlled by winches and lines from either vessels, barges, quayside (none of these is shown). Cables 30b,c have been connected to connection points on the structures lb,c and since they are slack this is a simple task. A pilot line 38 running from the

tensioning/winching mechanism 33 on top of structure l a to the corresponding end of the cable 30a pulls the cable 30a towards the tensioning mechanism. The structures are moving to position for making the bottom connection 10, enabled by gradual tensioning the cable 30a, synchronized with operation of the said winches and lines. When the mating portions 34, 35 have entered their connected position, the cables 30a,b,c are tensioned to the prescribed level.

During descent to deeper draft it is necessary to maintain tension in the cables. This is achieved by filling ballast water to the eccentric ballast compartments 40a-c. Hence, with increasing amount of water inside the structure, the tension in cables can be kept within acceptable limits, thus preventing slack. Upon descent to required depth, disconnecting of the assembly is rapid and simple. Towlines are attached to all structures, the tensioning mechanism 33 releases the tension in the cable 30a, and the structure la is pulled away from the other structures. In the meantime, one of the other cables 30b,c has been released and the structures lb,c separated.

Figure 24 shows an alternative configuration of the first connector 9, comprising a continuous cable 30 which at a first end is attached to a first attachment point 32a on the first structure la, then movably connected (via sheaves, or similar) to respective connections points 32b, c on the corresponding structures lb,c, and at the second end connected to the first attachment point 32a via a tensioning mechanism 33.

Figure 23 shows a similar connection as in figure 24, but for an assembly of four structures la,b,c,d. In the alternative illustrated in figure 23, as in figure 24, the cable 30 forms a loop whose ends are connected to a fixed point 32a on the structure la and is supported on the other foundations l b,c,d by points 32b, c,d. The latter points can be formed as sheaves that can rotate and enable tensioning of the cable 30 by means of the tensioning mechanism 33. Tension in the cable 30 exerts horizontal loads pointing to the geometrical center of the assembly as indicated by dashed arrows in the figure. These loads ensure desirable compression loads in connection points 10a,b,c,d.

In some situations it may be advantageous to replace the cables 30a-c by a rigid frame. One such embodiment is illustrated in figure 21 , where a rigid frame 41 is designed to resist both tension loads and compression loads. It is furthermore advantageous to install the frame 41 between the structures at an elevation which is at or near the water level when the frame is disconnected and removed, and the frame is supported by its own buoyancy when it is not in use, e.g. during and after disconnection.

Figure 22 is a top view of the embodiment illustrated by figure 21 , and shows how the rigid frame 4 l is connected to the structures l a,b,c by means of rigid connections 42a,b,c. It is advantageous to size the frame 41 so that the structures l a,b,c have to be pulled together for making the connection. This ensures compression loads in the bottom connections 10 shown in figure 13 and at the same it reduces the design compression loads in the frame 41 thus allowing for reduction of weight, price and capacity of lifting gear used for installation of the frame.

In practical application of the inventions, each structure may be furnished with a buoyancy device 6, 7 (as described with reference to figures 1 to 7) and these structures may be connected into one rigid body (as described with reference to figures 13 to 24). This is illustrated by figure 25.

Claims

I . A buoyancy element (6, 7) for releasable connection to a structure (1 ) floating in a body of water (W), characterized by a lower support element (19) and compartments (1 1 -14) fluidly interconnected via openings (60), of which one of said compartments is a central compartment (1 1) which via a control means (l ib) and a duct (1 1a) is selectively fluidly connected with water surrounding the buoyancy element, whereby the buoyancy element (6, 7) is connected to the structure (1) only by means of the hydrostatic pressure exerted by the water onto the buoyancy element (6, 7).

2. The buoyancy element of claim 1 , wherein the central compartment (1 1 ) is located in the buoyancy element's centre of mass.

3. The buoyancy element of any one of the preceding claims, wherein the central compartment (1 1 ) is located in the buoyancy element's geometrical centre.

4. The buoyancy element of any one of the preceding claims, wherein the other compartments (12-14) are located symmetrically around the central compartment

(1 1).

5. The buoyancy element of any one of the preceding claims, wherein a first series (12) of compartments are located radially around the central compartment (1 1).

6. The buoyancy element of claim 5, wherein a second series of compartments

(13) are located radially around the first series of compartments (12).

7. The buoyancy element of claim 6, wherein a third series of compartments

(14) are located radially around the second series of compartments (13).

8. The buoyancy element of any one of the preceding claims, further comprising an upper support element (23), whereby the plurality of compartments are enclosed.

9. The buoyancy element of any one of the preceding claims, wherein wall sections (15-17) of the individual compartments are configured for interaction with a portion (3) of the structure (1) whereby the plurality of compartments are enclosed when the buoyancy element is connected to the structure, and hydrostatic loads are transferred from the lower support element (19) to the portion (3) of the structure.

10. The buoyancy element of any claim 9, wherein the buoyancy element is adapted for insertion into a cavity (5) underneath the structure.

I I . The buoyancy element of any one of the preceding claims, wherein the buoyancy element comprises a circular, disk-like, shape.

12. A device (8) for increasing the buoyancy and stability of a structure (1 ) floating in a body of water (W), for releasable connection to said structure,

characterized by

a wall (24a, 24b) encircling a portion (2a) of the structure and extending upwards from a support region (3 ; 30) on the structure to a distance above the water and thereby defining a compartment (25) between the wall section and the portion of the structure, whereby the water plane area and buoyancy for the combined device and structure are enhanced.

13. The device of claim 12, wherein the wall comprises a frusto-conical body (24a).

14. The device of claim 12, wherein the wall comprises a substantially straight- walled, circular, body (24b), substantially aligned with the structure's axis of symmetry, and having a lower region with a first diameter and an upper region with a second diameter which is less than the first diameter, an intermediate transition step (79) between the two sections and at a distance below the surface of the water.

15. The device of any one of claims 12-14, wherein the wall (24a, 24b) comprises at least two segments (27a,b; 27'a,b) interconnected by connecting means (28a,b) and provided by sealing means (29a,b).

16. A buoyant compounded structure, capable of being transported in a semi- submersible state in a body of water, characterized in that:

it comprises a plurality of buoyant individual structures (l a-c) having respective individual water plane areas and connection means (30; 31 , 30a-c, 33) and abutment regions (9, 34, 35, 34', 35'), said connection means being releasably connected between each of said individual structures at a vertical distance from said abutment regions, whereby the compounded structure comprises one, substantially rigid, body having a floating stability which is better than the floating stability of the individual structures when such individual structures are disconnected from each other.

17. The compounded structure of claim 16, wherein the connection means (30; 31 , 30a-c, 33) are releasably connected between each of said individual structures at individual regions (32a-d; 42a-d) at a vertical distance from said abutment regions, whereby a horizontal load (PH) is exerted at each point (32a-d) and oppositely directed reactive forces of the same magnitude are generated in the abutment regions, thus preventing an inadvertent separation in the compounded structure and the individual structures comprise one, substantially rigid, body.

18. The compounded structure of claim 16 or claim 17, wherein the connection means comprise cables or chains (30a-d), each one connected at first end to a respective connection region and at a second end to a connection element (3 1 ) which is located at or near the plurality of individual structures' vertical axis of symmetry, and a tensioning mechanism (33) is connected between a first end of one of said cable or chain, and a connection region.

19. The compounded structure of claim 16, or claim 17 wherein the connection means comprise a cable or chain (30) which at a first end is connected to a region (32a) on a first individual structure, via a tensioning mechanism (33), and which at a second end also is connected to said first individual structure, and intermediate portions of the cable or chain (33) are movably connected to the remaining connection regions (32b-d).

20. The compounded structure of claim 16, wherein the connection means comprise a rigid frame structure (41) connected to respective connection regions (42a-c) on each of the individual structures (l a-d).

21. The compounded structure of claim 20, wherein said connection regions (42a-d) are such located on the structures that they are in or near the water line for the individual structures when the rigid frame structure (41) is disconnected from the compounded structure.

22. The compounded structure of claim 20 or 21 , wherein the rigid frame structure (41 ) comprises a buoyant element.

23. The compounded structure of any one of claims 16-20, wherein said connection regions (32a-d) are located at or near the top end of each of the individual structures.

24. The compounded structure of any one of claims 16-23, wherein the tensioning means are arranged substantially horizontally.

25. The compounded structure of any one of claims 16-24, wherein the individual structures (la-d) are arranged substantially symmetrically about a common vertical axis defining a vertical axis of symmetry for the compounded structure.

26. The compounded structure of any one of claims 16-25, further comprising at least one ballast compartment (40a-d) in each of the individual structures (l a-d), arranged at a distance from said structures' (l a-d) common axis of vertical symmetry.

27. The compounded structure of any one of claims 16-26, wherein the individual structures (l a-d) are substantially identical bodies and preferably comprising circular horizontal cross-sections.

28. The compounded structure of any one of claims 16-27, wherein the connection means comprise tensioning means.

29. The compounded structure of any one of claims 16-28, wherein the abutment regions further comprises releasable locking means.

30. The compounded structure of any one of claims 16-29, wherein the connection means are arranged above the body of water, while the abutment regions are arranged in the body of water.

31. The compounded structure of any one of claims 16-30, wherein each of the individual structures further comprises a buoyancy element (6, 7) as set forth by claims 1-1 1.