GB2477217A

GB2477217A - Actively stabilised floating caissons

Info

Publication number: GB2477217A
Application number: GB201101313A
Authority: GB
Inventors: Gerard Van Dam; Russell Thornton
Original assignee: BUTTERROW OFFSHORE DEVELOPMENTS Ltd
Current assignee: BUTTERROW OFFSHORE DEVELOPMENTS Ltd
Priority date: 2010-01-26
Filing date: 2011-01-26
Publication date: 2011-07-27
Also published as: GB201001302D0; GB201101313D0

Abstract

A floating caisson 2 comprises a mooring line 12 attached to the floating caisson at two vertically spaced points 14 and 16, means (40, Fig 5) for detecting tilt, and means (44, Fig 5) to adjust the tension or length of the mooring line dependent on the detected tilt. The floating caisson may comprise three mooring lines that are equally circumferentially spaced. The floating caisson may further comprise control means (42, Fig 5) arranged to receive measurement signals from the tilt detection means and in response to these send control signals to the adjustment means such that they adjust the tension and/or length of the mooring lines. Also disclosed is an array of floating caissons, comprising a plurality of such actively stabilised floating caissons.

Description

Floating Caissons This invention relates to floating caissons for supporting offshore structures. It relates in particular to the stabilisation of floating caissons in order to keep offshore structures upright.

Offshore structures have been provided for a number of years for a variety of uses, for example oil and gas drilling, and to house the associated infrastructure, e.g. living quarters, helipads, etc. More recently offshore structures have been built to provide other uses such as wind power generation.

Although some stability is afforded to deep water floating platforms from either or both of tension legs and tethering cables, a lot of the stability of such platforms comes from the physical mass of the structure, i.e. a very large ballast at the bottom of the structure. A typical floating caisson for a spar platform has a mass of 25,000 tonnes which gives the platform a large amount of inertia against the forces exerted upon it out in the open seas, e.g. prevailing winds, currents, tides and waves.

From a first aspect the invention provides a floating caisson comprising: a mooring line attached to the caisson at two vertically spaced points; means for detecting tilt; and means to adjust the tension or length of the mooring line dependent on the detected tilt.

It will therefore be appreciated by the skilled person that by providing a floating caisson with a mooring line whose tension and/or length can be adjusted in response to the tilt detection, the floating caisson can be actively stabilised against deviations from vertical caused by the motion of the sea and wind. Furthermore having the mooring line attached to the floating caisson at two vertically spaced points greatly increases the vertical stability of the floating caisson compared to a situation whereby there is only one attachment point which creates a pivot point for the floating caisson therefore allowing it to swing about this point, creating deviations from the vertical. Thus it can be seen that providing two mooring line attachment points and having the ability to adjust the tension and/or length of the mooring line allows a stable floating caisson to be provided without necessarily having to provide a large ballast to keep it stable.

The floating caisson of the present invention can be used individually to support a structure or a plurality of floating caissons can be used together to support a larger structure. For example, a single floating caisson is suitable for supporting a wind turbine, and an array of floating caissons would be suitable for supporting an airstrip.

Where a single caisson is provided (i.e. one not attached to another caisson) it is preferred to provide at least three mooring lines in accordance with the invention, i.e. each attached at two respective vertically spaced points with active management of tension or length of the line, as this allows the caisson to be moved or for a force to be exerted on the caisson in any direction which is not possible with just two such mooring lines. Preferably the mooring lines are equally circumferentially spaced. It is not essential for all of the mooring lines attached to a particular caisson to be attached at two vertically spaced points or to be actively controlled; one or more passive moorings could be provided in addition.

When a plurality of floating caissons are used together in an array to support a structure, they are preferably connected together as well as being attached to at least one mooring line which is secured to the sea bed. Preferably the floating caissons are connected together at a point towards the top of the floating caisson and at a point towards the bottom of the floating caisson. The connection towards the top of the floating caisson could be a cable but preferably is the structure that the array of floating caissons is supporting, e.g. a platform. The connection towards the bottom of the floating caisson is preferably a cable. This could attach to the floating caisson at the same height as the lower attachment point for the mooring line.

Therefore in a preferred embodiment the floating caissons are attached together in an array which supports a structure on top of them. The array is then actively stabilised by a plurality of mooring lines attached to some or all of the floating caissons in the same manner as an individual floating caisson is actively stabilised.

Each of the floating caissons in the array can have a mooring line attached to it, but preferably the mooring lines are attached to the floating caissons which are at the vertices of the array, e.g. for a triangular array of three floating caissons each of the floating caissons would have a mooring line attached, but a rectangular array of six floating caissons would only have four mooring lines attached to the array on the four floating caissons which form the corners of the rectangular array.

The floating caissons of the present invention are generally vertical elongate structures whose upper portions are arranged to provide a support for a further structure to be mounted upon them. Examples of structures which can be mounted on top of the caisson include a helipad, a drilling platform, a pontoon, a viewing platform, an aircraft landing strip, a supplies storage, living quarters, a scientific research base, offshore hotel and leisure facilities, a humanitarian relief base, and military versions of all the above, but in one set of preferred embodiments the structure mounted on top of the floating caisson(s) is a wind turbine.

A floating caisson is particularly suitable to be used to support a wind turbine. As floating caissons can be used in very deep water this allows wind turbines to be situated in deep water locations, i.e. well away from coastlines from where they are not visible but such that they are able to generate power that can be carried back to land via an underwater cable. Alternatively the wind turbines can be used to generate power for other offshore installations, e.g. drilling platforms.

This approach allows for the position of the caisson to take advantage of the open ocean wind strength which is stronger nearer the water surface, the more predictable and less variable prevailing wind direction, and allows it to circumvent limitations due to the water depth and the nature of the seabed.

In use, the caisson would generally be partially submerged, i.e. the upper portion of the caisson being above water and the lower portion of the caisson being below water. Preferably the majority of the caisson is submerged below water. The caisson is usually a hollow structure, preferably comprising a ballast and/or buoyancy. Preferably the ballast is arranged in the lower portion of the caisson.

The preferred arrangement, with the ballast at the bottom and buoyancy at the top gives the caisson the most stability while still allowing it to have a relatively small mass.

Preferably the mooring lines are catenary mooring lines, i.e. fixed at each end and freely hanging in between. The mooring lines can be manufactured from any suitable material which can resist the forces and weathering exerted upon it, e.g. metal cable, metal chain, natural or synthetic fibre rope, or any combination thereof.

The ends of the mooring lines distal from the floating caisson could be attached to neighbouring floating caissons or buoys, but preferably the mooring lines are attached to fixed points beneath the water, e.g. sea bed or ocean floor. The mooring lines may be attached to the bed using known attachment means, e.g. clump weights.

Preferably the end of the mooring line proximal to the floating caisson comprises an upper section and a lower section connected respectively to the two vertically separated points on the caisson. This allows the mooring line to exert a tension on the floating caisson at these two points which increases its stability. The point at which the mooring line splits into the upper and lower sections can be variable, e.g. the mooring line could comprise a main section on which there is a sliding attachment point from which the lower section starts and the upper section is defined from the point of the attachment to the floating caisson, but preferably the mooring line splits into the upper and lower sections at a fixed point.

Preferably the upper section of the mooring line attaches to the upper portion of the floating caisson. In the preferred embodiment in which the upper portion of the floating caisson is above the water level, preferably the upper section of the mooring line attaches to the floating caisson above the water level. Preferably the lower section of the mooring line attaches to the lower portion of the floating caisson. In the preferred embodiment in which the lower portion of the floating caisson is submerged below the water level, preferably the lower section of the mooring line attaches to the floating caisson below the water level. The stabilisation of the floating caisson is maximised if the upper and lower sections of the mooring line are spaced apart from each other by a substantial distance, i.e. if the upper and lower sections of the mooring line are attached to the upper and lower portions of the floating caisson respectively.

In some embodiments each mooring line can, in the portion close to the floating caisson, split horizontally into two sections attached to the floating caisson at two horizontally spaced points. Such horizontal splitting is employed for different purposes to the vertical splitting. Where the mooring line comprises upper and lower sections, the horizontal split may occur closer to the floating caisson than the point of the vertical split, or it may occur further away from the floating caisson.

Alternatively the horizontal and vertical splitting could occur at the same point.

Either or both of the upper and lower sections of the mooring line can be split horizontally, thus resulting in, in these embodiments, either three or four attachment points for each mooring line on the floating caisson. By differentially adjusting the tension and/or length of each end of the horizontal splitting of the mooring line, this allows the floating caisson to be rotated. This could, for example, be used to rotate the floating caisson in order to direct a wind turbine into the wind.

The mooring lines can be attached directly to the floating caisson, but in some embodiments it has been found advantageous to attach the mooring lines to structures attached to the floating caisson, e.g. each attachment point for the mooring line or section of the mooring line could be on a truss. This, for example, gives greater leverage to rotate the floating caisson in the embodiments which have a horizontally split mooring line.

The tension and/or length of the mooring line can be adjusted by any of a variety of adjustment means. The adjustment means could be located at any point along the mooring line, e.g. at the point where the mooring line attaches to the sea bed or part way along the mooring line, but preferably the adjustment means are located at the point at which the mooring line attaches to the floating caisson. The adjustment means could comprise a moveable attachment point on the floating caisson, e.g. that moves vertically to adjust the effective length of the mooring line, but preferably the attachment point of the mooring line to the floating caisson is fixed and the adjustment means comprises a device such as a winch to adjust the tension and/or length of the mooring line. In the embodiments that comprise a vertically split mooring line, adjustment means can be provided for both the upper and lower sections of the mooring line, or just for one of either the upper or lower sections. In the embodiments that comprise a horizontally split mooring line, each section of the mooring line is preferably provided with adjustment means.

The tilt sensing means is arranged to sense the angular deviations of the floating caisson from vertical. It could be arranged simply to sense deviation above a threshold or series of threshold, or could provide a quantitative measurement. The output from the tilt detection means can be used by the adjustment means to adjust the tension and/or length of the mooring lines in order to keep the floating caisson vertical. Therefore preferably the caisson comprises control means arranged to receive deviation measurement signals from the tilt sensor, the control means being further arranged to send control signals to the adjustment means dependent on the deviation measurement signals, the control signals being used by the adjustment means to adjust the tension and/or the length of the mooring lines.

The tilt sensor could comprise any of a variety of different devices which are able to measure the angular deviations of the floating caisson from vertical, e.g. a gyroscope, a gyrocompass, a tiltmeter, or an inclinometer. Additionally the floating caisson could comprise a position sensor, e.g. a global positioning system (GPS) receiver, so that the tension and/or length can be adjusted to alter the position of the floating caisson, e.g. to move it back to its original position. If a position sensor is used, the same control means as for the tilt sensor could be used to transmit the position deviation measurements from the position sensor to the adjustment means, or alternatively a separate control means could be used.

Thus it can be seen that the floating caisson is actively stabilised to keep it vertical by the output of the tilt detecting means being fed into the adjustment means which then adjusts the tension and/or length of the mooring lines as necessary. This active stabilisation can work on a variety of different timescales. For example, long timescales, e.g. hours, are needed to actively stabilise the floating caisson to compensate for the prevailing wind and water current, e.g. from tides or an ocean current. However the active stabilisation is also needed on short timescales, e.g. 15 to 60 seconds, to stabilise the floating caisson on a wave by wave basis. This is especially important in heavy seas.

In some embodiments in which a wind turbine is mounted on top of the floating caisson the control means could be powered by tapping off some of the power generated by the wind turbine. In other embodiments the power could be provided by any of a variety of different means such as a battery, solar cell. In one set of preferred embodiments the caisson comprises or is connected to a wave power generator. This enables the floating caisson to generate power from the continuous source of the waves.

Preferably the wave power generator comprises a plurality of hollow tubes arranged vertically around at least part of the circumference of the floating caisson. The plurality of hollow tubes can be used to direct water through them in order to turn a turbine to generate power. The plurality of hollow tubes can be arranged at any point along the length of the floating caisson, but as the displacement of the water due to the waves is greatest at the surface, preferably the hollow tubes are arranged around at least part of the circumference of the upper portion of the floating caisson. Preferably the hollow tubes are arranged on the upper portion of the floating caisson so that they are, on average, partially submerged. This arrangement creates a head of air at the top of the hollow tube which as the water moves up and down the hollow tube (open at the bottom) due to the wave motion, the air can be arranged to be forced through a duct at the top of the hollow tube in order to drive a turbine to generate power. There can be a separate turbine and generator for each hollow tube, but preferably the wave power generator comprises a header pipe which connects the air ducts passing out of the top of the hollow tubes in order to drive a central generator.

It can be seen that the floating caisson of the present invention allows a much smaller, and therefore less expensive, structure to be built which can also withstand the weathering and forces that such structures are exposed to out in the sea or ocean, is suitable for mooring in depths of water that previously only larger structures were capable of doing, and can remain stable because of its arrangement of the mooring cables to provide active stabilisation.

It will readily be appreciated that being able to provide a smaller, lighter floating caisson affords a number of advantages, primarily the cost and ease of construction and installation. If a caisson is lighter then it requires less raw material to be used to fabricate it which is therefore cheaper. Being smaller and lighter makes the caisson easier to assemble and install as, for instance, it could be assembled on land and then towed into place to be installed.

It is estimated that floating caissons embodying the present invention could have a typical lifetime of 20-25 years of use in the open sea or ocean and that they can be preferably arranged to limit its pitch and roll to a maximum of 2 degrees.

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 shows a side view of an embodiment of the invention; Fig. 2 shows a schematic plan view of an embodiment of the invention; Fig. 3 shows a side view of a floating caisson and one mooring line in accordance with the invention; Figs. 4a, 4b, 4c and 4d show an example of a clump weight for use with the invention; Fig. 5 shows a schematic representation of the tilt sensor, the control system and the winches for the mooring lines; Fig. 6 shows a plan view of one embodiment of the invention with horizontally split mooring lines; Figs. 7a and 7b show details of an example of a power generator for use with the invention; Fig. 8 shows a side view of a connected array of floating caissons; and Figs. 9a, 9b, 9c and 9d shows examples of different arrangements for arrays of floating caissons.

Fig. 1 shows a side view of a floating caisson 2 which is installed in position out at sea. The floating caisson 2 has a ballast region 4 at the bottom with a buoyancy region 6 above the ballast region 4 so that it floats in the water 8 with the majority of its body submerged beneath the water level. The floating caisson 2 supports a wind turbine 10 mounted on top of it.

Three mooring lines 12 are each attached to the floating caisson 2 at respective vertically spaced points 14 at the top and at the bottom of the floating caisson via winches (not shown). The end of each mooring line 12 distal from the floating caisson 2 is secured to the sea bed 18 with a clump weight 20.

Fig. 2 shows a schematic plan view of the floating caisson 2 and its three mooring lines 12. The three mooring lines 12 are arranged so that there is an equal angle (120°) between them, thus ensuring that each mooring line 12 has an equal effect on the floating caisson 2.

Fig. 3 shows the floating caisson 2 and one of the mooring lines 12 in more detail.

The structure of the floating caisson 2 has the same components as in Fig. 1 but it can be seen more clearly that the mooring lines 12 split at a fixed point 26 into a lower section 22 and an upper section 24. The lower section 22 is attached at a point 16 to the bottom of the floating caisson 2. The upper section 24 is attached at a point 14 at the top of the floating caisson 2 which is spaced from the main body of the floating caisson 2 by means of a truss 28. This allows the mooring line to have greater leverage. Also shown in Fig. 2 at the level of the water on the floating caisson 2 are a plurality of hollow tubes 30 which are each connected to a generator (not shown) to generate power for the active stabilisation system.

Figs. 4a, 4b, 4c and 4d show different views of a typical clump weight 20 which can be used to secure the distal end of the mooring line 12 to the sea bed 18. Fig. 4a shows a side. In a vertical cross section the clump weight 20 shown here has a trapezoid body with a plurality of teeth 32 depending from its lower edge that help to secure the clump weight 20 to the sea bed 18 in a fixed position. The clump weight also has pad-eyes 34 attached to its upper surface to allow the mooring line 12 to be attached to it.

Fig. 4b shows a plan view of the clump weight 20 which in this particular example happens to have a square footprint but this is not essential.

Fig. 4c shows a cross-section through the clump weight 20 showing the hollow and inside a valve 35 that closes a hole 38 in it. This allows for easier installation as the hollow clump weight 20 can be made on land, fitted with the valve 35 and then, while floating, towed out to its installation point. Once the installation point has been reached, the valve 35 can be opened to flood the hollow inside of the clump weight 20 with water. This sinks the clump weight 20 to allow it to reach the position in which it is going to be secured to the sea bed. -10-

Fig. 4d shows the next stage of installation once the clump weight 20 has been sunk into position. The inside of the clump weight is then filled with rocks 36 through the hole 38 in the top of the clump weight 20, which displaces the water and provides a heavier weight to provide extra ballast.

The form of the clump weight shown is not essential and many alternative shapes and configurations could be employed.

Fig. 5 shows a schematic representation of the active stabilisation system for the floating caisson. The tilt sensor 40 is the input to the system, providing a measurement of the deviation of the floating caisson from vertical (and therefore also the deviation from vertical of the structure, e.g. wind turbine, mounted on top of the wind turbine). This measurement is then fed to a control system 42 which uses the deviation measurement to work out by how much the tension and/or length of the mooring lines should be altered in order to minimise the deviations of the floating caisson from vertical. The control system 42 then sends control signals to one or more of the winches 44 which are attached to the end of the mooring lines 12 to control the tension and/or length of the mooring lines 12.

Fig. 6 shows a plan view of an embodiment of the floating caisson 2 in which the mooring lines 12 each split horizontally into two sections 50,52 for attachment to floating caisson 2 via respective winches (not shown). The tension and/or length of the horizontally split sections 50,52 can be adjusted differentially to allow the floating caisson 2 to be rotated about its vertical axis. For example, as shown in Fig. 6 if the right-hand sections 50 (from the perspective of looking towards the floating caisson 2 along the mooring line 12) of the mooring lines 12 are shortened and the left-hand sections 50 of the mooring lines 12 are lengthened, this results in the floating caisson 2 being rotated clockwise about its vertical axis (when viewed from above).

Figs. 7a and 7b show details of the power generation system aboard the floating caisson 2 which allows power to be generated to drive the winches which alter the tension and/or length of the mooring cables, and the control system. As Fig. 7a shows, a plurality of hollow tubes 30 are arranged around the outer circumference of the floating caisson 2 at the level of the water. The top of each of the hollow tubes 30 is connected to a common header pipe 54 which runs around the outer circumference of the floating caisson 2. The enlarged sectional view of one of the tubes in Fig. 7b shows the header pipe 54 which is connected to a turbine 56 that drives a generator 58. The rise and fall of the water in the hollow tubes 30 due to the associated motion of the sea decreases and increases respectively the amount of air in the top of the hollow tubes 30. This creates a flow of air through the header pipe 54 and therefore through the turbine 56 which drives the generator 58 resulting in power being generated for use on the floating caisson 2.

In a typical example, the hollow tubes are 8m in height and have a diameter of 1.1 m. A typical height over which waves rise and fall is 3m, with the waves having a time period of about 15s. This gives a rate of volume change of 0.7m3/s in each hollow tube. Assuming that the air is compressed by a factor of 2, this results in a work input of 0.1 5kW per hollow tube. A floating caisson of diameter 11 m could have 24 hollow tubes arranged around its outer circumference giving a total power output of 4kW to drive the winch motors and active stabilisation control system.

Fig. 8 shows an example of an arrangement of multiple floating caissons 2 tethered together to provide support for a larger structure, e.g. a floating platform 60. In this example there are three floating caissons 2 grouped together which each have the same features as shown in the previous figures, e.g. they are attached to the sea bed by mooring lines 12 which have upper and lower sections 24,22, except that only one mooring line 12 is attached to each floating caisson 2 which results in three mooring lines 12 being attached to the group of floating caissons 2 at the three corners of the group. This means that the group can then be actively stabilised by altering the tension and/or length of the mooring lines in the same manner as a single floating caisson 2. The three floating caissons 2 are connected together at the top by the platform 60 mounted on them and at the bottom by cables 62 which attach at the same point as the lower section 22 of the mooring line 12 would normally attach to the floating caisson 2.

Figs. 9a, 9b, 9c and 9d show plan views of different arrangements of groups of floating caissons 2. Fig. 9a shows an array of three floating caissons 2 forming a triangle, as in the example shown in Fig. 8. A platform 60 is mounted on top of the floating caissons 2 which could, for example, serve as a helipad. Fig. 9b shows an -12-array of twelve floating caissons 2 which are arranged in two lines of six floating caissons 2. Such an arrangement could be used, for example, to mount a aircraft landing strip on top of the floating caissons 2. Fig. 9c shows an array of five floating caissons 2 arranged at the each of the corners of a regular pentagon, and Fig. 9d shows an array of four floating caissons 2 arranged at each of the corners of a square.

In all of these examples showing arrays of floating caissons 2, a number (but not necessarily all) of the floating caissons 2 would have mooring lines attached to them in the manner as has been previously described to provide active stabilisation for the group of floating caissons 2 as a whole. The individual floating caissons 2 are also connected together, as has been described for the group of three floating caissons 2 shown in Fig. 8.

In operation a floating caisson 2 according to the present invention is installed by assembling the main body of the caisson on land and then towing it out to its position in the open sea 8 and mooring it in place with the mooring lines 12 that are secured to the sea bed 18 using clump weights 20. The mooring lines 12 each split into upper and lower sections 24,22 which are attached respectively to the floating caisson on a truss 28 attached to the top of the floating caisson 2 and a padeye 16 at the bottom of the floating caisson 2. The attachment of the mooring line 12 at the top of the floating caisson 2 is via a winch 44 which is able to control the tension and length of the mooring line.

Once installed in position, the floating caisson 2 is actively stabilised using information from its tilt sensor 40. External forces such as wind, waves and sea currents cause the floating caisson 2 to pitch and roll away from its naturally vertical position. These forces can vary over a short time scale, e.g. less than a minute, for individual waves striking the floating caisson 2, or a long time scale, e.g. hours, for changes in winds and tides. The deviations of the floating caisson 2 from vertical are detected by a tilt sensor 40 which sends this information to a control system 42.

The control system 42 uses the measurements of the tilt sensor 42 to determine by how much the tension and/or length each of the mooring lines 12 needs to be changed in order to restore the floating caisson 2 to its vertical position. The control system 42 then sends control signals to the winch motors which act to operate the -13-winches 44 in order to change the tension and/or length of the mooring lines 12 as instructed by the control system 42 which therefore moves the floating caisson 2 back into its vertical position.

For a floating caisson with horizontally split mooring lines 12 the tilt sensor is used to detect rotational deviations of the floating caisson 2. The control system 42 uses these measurements to determine by how much the tension and/or length of either section 50,52 of the horizontally split mooring lines 12 needs to be changed in order to restore the floating caisson 2 to its original rotational position. As before, the control system 42 then sends control signals to the winch motors which act to operate the winches 44 in order to change the tension and/or length of the mooring lines 12 as instructed by the control system 42 which therefore moves the floating caisson 2 back into its original rotational position.

The power, e.g. electricity, needed to drive the winch motors 44 and the control system 42 is generated by the power generation system attached to the floating caisson 2. Water rising and falling in the hollow tubes 30 attached around the external circumference of the floating caisson 2 force air into and out of a header pipe 54 which runs around the top of the hollow tubes 30. The header pipe 54 directs the compressed air into a turbine 56 which is turned by the movement of the air such that the turbine 56 drives a generator 58 which generates electricity that can then be used by the winch motors 44 and control system 42 on the floating caisson 2.

The embodiments that have been described are not limiting and many variations and modifications are possible within the scope of the invention. For example a number of different locations for connection points for mooring lines could be envisaged, and this could be more than two. A mooring line could therefore be attached to the floating caisson at three or more vertically spaced points. This might therefore mean that the mooring line splits vertically into three or more sections. The mooring line could also split into more than two sections horizontally.

Many different combinations of splitting of the mooring lines and how and where these are attached the floating caisson are envisaged within the scope of the invention. -14-

Claims

Claims 1. A floating caisson comprising: a mooring line attached to the floating caisson at two vertically spaced points; means for detecting tilt; and means to adjust the tension or length of the mooring line dependent on the detected tilt.
2. A floating caisson as claimed in claim 1, comprising at least three mooring lines.
3. A floating caisson as claimed in claim 2, wherein the mooring lines are equally circumferentially spaced.
4. A floating caisson as claimed in claim 1,2 or 3, comprising a vertical elongate structure whose upper portion is arranged to provide a support for a further structure to be mounted thereon.
5. A floating caisson as claimed in any preceding claim, arranged in use to be partially submerged.
6. A floating caisson as claimed in claim 5, wherein the majority of the floating caisson is arranged to be submerged below water.
7. A floating caisson as claimed in any preceding claim, comprising a hollow structure.
8. A floating caisson as claimed in any preceding claim, comprising a ballast and/or a buoyancy.
9. A floating caisson as claimed in claim 8, wherein the ballast is arranged in a lower portion of the floating caisson.
10. A floating caisson as claimed in claim 8 or 9, wherein the buoyancy is arranged in an upper portion of the floating caisson. -15-
11. A floating caisson as claimed in any preceding claim, wherein the mooring lines are catenary mooring lines.
12. A floating caisson as claimed in any preceding claim, wherein the mooring lines are attached to fixed points beneath the water.
13. A floating caisson as claimed in any preceding claim, wherein the end of the mooring line proximal to the floating caisson comprises an upper section and a lower section connected respectively to the two vertically separated points on the floating caisson.
14. A floating caisson as claimed in claim 13, wherein the mooring line splits into the upper and lower sections at a fixed point.
15. A floating caisson as claimed in claim 13 or 14, wherein the upper section of the mooring line attaches to the upper portion of the floating caisson.
16. A floating caisson as claimed in claim 15, arranged in use such that the upper portion of the floating caisson is above the water level, and wherein the upper section of the mooring line attaches to the floating caisson above the water level
17. A floating caisson as claimed in any of claims 13 to 16, wherein the lower section of the mooring line attaches to the lower portion of the floating caisson.
18. A floating caisson as claimed in claim 17, arranged in use such that the lower portion of the floating caisson is submerged below the water level, and wherein the lower section of the mooring line attaches to the floating caisson below the water level.
19. A floating caisson as claimed in any preceding claim, comprising adjustment means to adjust the tension and/or length of the mooring line.
20. A floating caisson as claimed in claim 19, wherein the adjustment means are located at the point at which the mooring line attaches to the floating caisson. -16-21. A floating caisson as claimed in claim 19 or 20, wherein the attachment point of the mooring line to the floating caisson is fixed and the adjustment means comprises a device to adjust the tension and/or length of the mooring line.22. A floating caisson as claimed in claim 19, 20 or 21, comprising control means arranged to receive deviation measurement signals from the means for detecting tilt, the control means being further arranged to send control signals to the adjustment means dependent on the deviation measurement signals, the control signals being used by the adjustment means to adjust the tension and/or the length of the mooring lines.23. A floating caisson as claimed in any preceding claim, wherein the floating caisson comprises or is connected to a wave power generator.24. A floating caisson as claimed in claim 23, wherein the wave power generator comprises a plurality of hollow tubes arranged vertically around at least part of the circumference of the floating caisson.25. A floating caisson as claimed in claim 24, wherein the hollow tubes are arranged around at least part of the circumference of the upper portion of the floating caisson.26. A floating caisson as claimed in claim 25, wherein the hollow tubes are arranged on the upper portion of the floating caisson so that in use they are, on average, partially submerged.27. An array of floating caissons for supporting a structure comprising a plurality of floating caissons, at least one of which comprises a floating caisson as claimed in any preceding claim.28. An array as claimed in claim 27, wherein the plurality of floating caissons are connected together. -17-29. An array as claimed in claim 28, wherein the floating caissons are connected together at a point towards the top of the floating caisson and at a point towards the bottom of the floating caisson.30. An array as claimed in claim 29, wherein the connection towards the top of the floating caisson comprises the structure that the array of floating caissons is supporting.31. An array as claimed in claim 29 or 30, wherein the connection towards the bottom of the floating caisson comprises a cable.32. An array as claimed in any of claims 28 to 31, comprising a plurality of mooring lines attached to the floating caissons which are at the vertices of the array.